Volume 13 - Corrosion Part 6 pdf

In corrosive atmospheres, a duplex system of galvanized steel top coated with paint has several advantages that make it an excellent system for corrosion prevention: • The life of the ga

Table 10 Some standard galvanized fasteners

specification

Grade or type

Transmission tower bolts A 394 Type 0, 1

Quenched-and-tempered alloy steel bolts A 354 Grade BC

Galvanized coatings in bearing-type connections, which develop shear resistance by allowing the bolts to bear on the plates, are not detrimental to performance and have along history of use for example, in electrical utility transmission line towers In friction-type connections, all loads in the plane of the joint are transferred by the friction developed between the connected surfaces The load that can be transmitted is determined from the clamping force applied to the bolts and the coefficients of friction of the faying surfaces Clean, galvanized mating surfaces have a coefficient of friction that is slightly lower than that of as-rolled steel The coefficient of friction of galvanized surfaces can be made equal to that of uncoated steel surfaces by wire brushing or light grit blasting (Ref 40, 41) Neither treatment should be severe enough to produce breaks or discontinuities in the galvanized coating The fatigue behavior of galvanized steel connections equals that of uncoated steel connections, regardless of whether the galvanized surfaces were wire brushed or gritblasted after galvanizing (Ref 40, 41, 42)

Welding. Zinc-coated steel can be satisfactorily welded by all common welding methods, but attention must be given to the possible generation of zinc fumes Adequate ventilation, operator respiration units, or a fume-extracting welding unit should be used to avoid potential harmful effects Extensive tensile, bend, radiographic, and fatigue tests show that the properties of sound metal inert gas or metallic are welds on galvanized steel are equivalent to those on uncoated steel (Ref 35)

Penetration of molten zinc into the weld metal is the primary factor in cracking of galvanized steel weldments The

crack begins at the root of the weld and may or may not extend to the surface Recommendations to avoid fillet weld cracking include (Ref 35):

• Treat the base metal to reduce the amount of available zinc by, for example, beveling the standing plate

in a tee joint at an angle of 15 to 45°

• Remove zinc from both faying surfaces by burning with an oxygen fuel gas torch or by shotblasting

• Provide a parallel gap of 1.6 mm ( 1

16 in.) between the weld elements

• Choose consumables that will give a low silicon content weld, for example, manganese silicate flux and 2% low manganese/low silicon electrodes for submerged arc welding For CO2 welding, low-silicon filler wire gives freedom from zinc penetration cracking, but causes a small amount of porosity For shielded arc welds, use a low-silicon electrode and a rutile covering

Weld porosity can occur because of the volatilization of zinc during welding Porosity can be reduced by making

adequate provision for the escape of gases evolved during welding Normal porosity does not reduce the static tensile strength below that specified for satisfactory low-carbon steel weld metal, but it will affect the fatigue strength of a fillet weld

Weld Damage Repair Galvanized materials damaged by field welding can be touched in with an organic or inorganic

zinc-rich paint Organic paint does not require a high degree of surface preparation and dries quickly The paint must have

a zinc dust content of 95% and should be applied in several coats to a thickness of three times the galvanized thickness to provide equivalent protection

Low-melting zinc-cadmium or zinc-tin-lead alloy rods can also be used for repair The rod is heated to about 330 °C (625

°F) with an oxyacetylene torch, and the melted alloy is rubbed and spread over the damaged area This gives equal protection, but is more expensive than paint touch up The damaged area can also be zinc metallized by thermal spraying, but the equipment requirements may prevent convenient field use Additional information on repair techniques is given in Ref 43

Painting Galvanized Steel

Galvanized coatings, when used without further treatment, offer the most economical corrosion protection for steel in many environments The galvanized coating makes an excellent base on which to develop a paint system Painting of galvanized steel is desirable for aesthetics, as camouflage, as warning or identification markings, to prevent bimetallic corrosion, or when the anticipated environment is particularly severe (see the article "Organic Coatings and Linings" in this Volume for supplementary information on painting for corrosion protection)

In corrosive atmospheres, a duplex system of galvanized steel top coated with paint has several advantages that make it an excellent system for corrosion prevention:

• The life of the galvanized coating is extended by the paint coating

• The sacrificial and barrier properties of the zinc coating are used if a break occurs in the paint film

• Undercutting of damaged paint coatings, a major cause of failure of paints on steel, does not occur with

a zinc substrate (Fig 13)

• Surface preparation of a weathered zinc surface for maintenance painting is easier than that for rusted steel

Fig 13 Illustration of the mechanism of corrosion for painted steel (a) and painted galvanized steel (b) (a) A

void in the paint results in rusting of the steel, which undercuts the paint coating and results in further coating degradation (b) A void in the coating of a painted galvanized steel is sealed with zinc corrosion products; this avoids the undercutting seen in (a) and prevents further deterioration of the painted coating

Synergistic Effects of Galvanized and Painted Systems. The galvanized coating prevents rusting of steel by acting as a barrier against the environment and by sacrificially corroding to provide cathodic protection Painting the galvanized coating extends the service life of the underlying zinc because the barrier property of the paint delays the reaction of zinc with the environment If a crack or other void occurs in the paint and exposes the galvanized coating, the zinc corrosion products formed tend to fill and seal the void; this delays further reaction

When painted steel is exposed to the environment, rust forms at the steel/paint interface Because rust occupies a volume several times that of the steel, the expansion resulting from rusting leads to rupture of the steel/paint bond Further, rust is porous; it accumulates moisture and other reactants, and this increases the rate of attack on the steel The result is undercutting, flaking, and blistering of the paint film, leading to failure of the paint coating (Fig 13) Zinc corrosion products occupy a volume only slightly greater (20 to 25%) than zinc; this reduces the expansive forces and conditions that lead to paint failure

A coating system consisting of painted galvanized steel provides a protective service life up to 1.5× that predicted by adding the expected lifetimes of the paint and the galvanized coating in a severe atmosphere (Ref 44) This is demonstrated in Table 11 The synergistic improvement is even greater for mild environments (Ref 45, 46)

Table 11 Synergistic protective effect of galvanized steel/paint systems

Galvanized steel Paint Galvanized plus paint

Type of

atmosphere

μm mils

Service life (a) , years

μm mils

Service life (a) , years

μm mils

Service life(a), years

Paint adhesion is the primary concern in painting galvanized steel The surface of the zinc is nonporous and does not allow mechanical adhesion of the paint Surface contaminants, such as oils, waxes, or postgalvanizing treatments, also effect adhesion A fresh zinc surface is reactive to certain paint ingredients, such as fatty acids; this can produce zinc soaps and disrupt the zinc-paint bond

Galvanized coatings can be successfully painted immediately after galvanizing or after extended weathering The deliberate use of weathering is not recommended, because weathering may not be uniform, the time required is long (6 to

12 months), hygroscopic impurities can form that may be difficult to remove, and there is exposure to atmospheric pollutants

Chemical etchants, such as acids or copper sulfate, should not be used The action of these chemicals is difficult to control, surface preparation may be nonuniform, and the galvanized coating could be damaged if allowed to remain in extended contact with the chemicals Long-term adhesion will suffer with this type of treatment, although initial adhesion may be obtained

Mechanical roughening of the zinc surface through the use of a light blast can provide a good surface for painting However, careful control of the blast pressure and flow rate must be exercised to avoid excessive removal of the galvanized coating

Initial adhesion of the paint can be achieved through the use of a pretreatment primer to provide an adequate base for further coating Long-term adhesion is obtained by the selection of top coat that is compatible with the primer and galvanized steel

Pretreatment Methods. As with all painting operations, the surface to be painted must be free of contaminants that could affect the adhesion of the paint or the appearance of the painted article Non-oily substances can be removed by brushing or scrubbing, then rinsing with water Oily materials should be removed with a solvent, such as naphtha, turpentine, or mineral spirits, followed by a final wiping with clean cloths and clean solvent, to avoid spreading oil films

on the surface

Two-component wash primers meeting Steel Structures Painting Council (SSPC) specification SSPC-Paint 27 are ideal pretreatments The primer is applied by spraying in thin coasts according to the recommendations of the manufacturer The freshly prepared primer should be applied to a dry film thickness of 7.6 to 13 μm (0.3 to 0.5 mils)

Finish Coating. The primed material should be finish coated as soon as possible after priming treatment Wash primers are moisture sensitive, and the vehicle may gel under high-humidity conditions and lose adhesion The primer usually dries in 15 to 30 min, and it is dry enough to recoat after 30 to 60 min Almost all paints will adhere to the wash primer

Direct Application Systems. A more convenient alternative to natural weathering or pretreatment of the galvanized surface is the use of a paint system directly compatible with the surface Alternatives are discussed below

Zinc Dust-Zinc Oxide Paints. Federal specification TT-P-641G describes three types of zinc dust-zinc oxide paints

All three contain the same pigmentation of 4 parts zinc dust to 1 part zinc oxide; the only variation is in the paint base material Types 1 and 2 (linseed oil and alkyd resin bases, respectively) are recommended for general use, and type 3 (phenolic resin base) is especially formulated for severe moisture exposure or underwater service All are useful as primers for adherence and are satisfactory as finish coats If color is required, the top coat can be pigmented Other compatible top coats may be used, but those with very strong solvent systems should be avoided, particularly if used before proper aging of the base film

Portland Cement in Oil Paints These paints are compatible with either fresh or weathered galvanized coatings

Although they tend to be brittle, adherence is excellent They are not as versatile as the zinc dust-zinc oxide paints and are usually limited to applications in which a high gloss is not required and an oil base is suitable They are available in a wide choice of colors

Other Direct Application Systems Newer paint systems that have been successfully used on direct application to

galvanized steel include epoxy resin based paints, chlorinated rubber based paints, vinyl copolymer based paints, coal tar epoxy paints, and acrylic latex emulsion paints

Economics of Hot Dip Galvanizing

Corrosion control results in a negative cash flow for the owner of any facility In selecting a system for corrosion control, the economic consequences of the selection should be determined Initial cost should not be the determining factor in selecting a system Instead, the desired service life of the project should be reasonably estimated, and the life cycle cost of several systems should be evaluated to determine the most economical system for the particular project

A number of models have been developed for the economic evaluation of corrosion protection systems (Ref 47, 48, 49) The projected structure life, inflation and discount (cost of capital) rates, number of years in the maintenance cycle, estimated costs for future maintenance, and original system costs must be determined based on discounted cash flow techniques In the private sector, tax and investment incentives must also be considered Table 12 illustrates a simple analysis that does not take tax or investment incentives into account Although the initial galvanizing cost is set at 25% above the paint cost, this is often not the case; in reality, the cost of galvanized steel is frequently lower than that of painted steel More information on the use of engineering economy is available in the article "Corrosion Economic Calculations" in this Volume

Table 12 Discounted cash flow analysis of galvanized versus painted steel

The time value of money is an important consideration when analyzing the economics of different coatings This example assumes an inflation rate of 4%, a discount rate of 10%, a repaint cycle of 10 years, and an expected service life of 50 years No tax or investment considerations are made A repaint cost of 75% of the original cost is assumed, and the galvanizing cost is 25% greater than the paint cost

Galvanizing Paint Year

Original cost

Total lifetime costs $1.25 $1.89

(a) NPW, net present worth of inflated future maintenance costs

Selected Applications of Hot Dip Galvanized Steel

Hot dip galvanized coatings are found in a wide variety of applications requiring long-term maintenance-free corrosion protection The examples in this section will demonstrate the scope of galvanized steel use

Bridges. The first all hot dip galvanized bridge in the United States was erected in 1966 at Stearns Bayou, Ottawa County, MI The bridge is 128 m (420 ft) long and has a 9.1-m (30-ft) wide roadway with a 1.5-m (5-ft) walkway on each side All the components of this bridge were hot dip galvanized To avoid possible effects from road salting, telescoping splash plates in the joints were installed to divert deck drainage away from the beams

When inspected in 1986, the average coating thickness on stringers and diaphragms was 112 μm (4.4 mils), which is enough to last another 60 to 100 years The bearing pads, the most deteriorated component of the bridge, showed a coating thickness of 90 μm (3.5 mils) The article "Corrosion in Structures" contains detailed information on the use of hot dip galvanized steel structural members

Pulp and paper mills contain a number of areas that, in the absence of corrosion protection, would rapidly deteriorate because of exposure to various chemicals A paper mill in the northwest United States used galvanized structural steel pipe supports, ladders, cages, and miscellaneous steel items in the stock tank, black liquor, lime kiln, and paper machine areas Galvanized steel was used during the original construction 19 years ago and for subsequent expansions All of the galvanized steel is located outside in a humid environment subject to salt spray When inspected in 1985, coating thicknesses ranged from 90 to 160 μm (3.5 to 6.5 mils) More information on materials of construction for pulp and paper mills is available in the article "Corrosion in the Pulp and Paper Industry" in this Volume

Recreation. The Gettysburg Observation Tower overlooks the historic Gettysburg battle-ground in Gettysburg, PA The structure was erected in 1974 with galvanized pipes and rolled-steel sections An inspection in 1984 revealed coating thicknesses of 100 to 150 μm (4 to 6 mils); this is above the specification for newly galvanized material Service life can

be expected to be 100 years or more, assuming the environment does not change appreciably

Utility Industry. A galvanized substation located near Knoxville, TN, and owned by the Tennessee Valley Authority was constructed in 1936 to handle 100,800 kW of electricity produced by a nearby dam Substation structures are of a bolted lattice construction The minimum coating thicknesses measured in a recent inspection were 70 μm (2.75 mils) on lattice angles and 50 μm (2.5 mils) on bolt heads Based on estimations of the original coating weight, this substation will last another 30 years before maintenance coating will be required

Other Applications. Hot dip galvanized steel coated by the batch process is used in oil refineries and petrochemical industries and for miscellaneous highway uses, such as guard rail, light and sign standards, and fencing The coating has found extensive use in water and wastewater treatment plants, both in atmospheric and immersion service Such applications of electrical utility transmission towers, microwave transmission stations, pole line hardware, cooling towers and cooling tube bundles, nuts, bolts, and various fasteners all involve extensive use of galvanizing

Galvanized reinforcement bars, ties, and lintels for concrete and masonry reinforcement and support provide long-term corrosion protection in vital areas that are not normally visible The use of galvanized materials in concrete reinforcement and masonry applications is ideal, because the normal pH of these materials before setting is about 12 to 12.5, which corresponds to the pH range in which corrosion of zinc is at a minimum

• A 153 "Standard Specification for Zinc Coating (Hot Dip) on Iron and Steel Hardware"

• A 384 "Standard Recommended Practice for Safeguarding Against Warpage and Distortion During Dip Galvanizing of Steel Assemblies"

Hot-• E 376 "Standard Recommended Practice for Measuring Coating Thickness by Magnetic-Field or Current (Electromagnetic) Test Methods"

4 F.M Reinhart, in Twenty-Year Atmospheric Corrosion Investigation of Zinc-Coated and Uncoated Wire and Wire Products, STP 290, American Society for Testing and Materials 1961

5 L.P Devillers and P Niessen, The Mechanism of Intergranular Corrosion of Dilute Zinc-Aluminum

Alloys in Hot Water, Corros Sci., Vol 16, 1976, p 243-252

6 S.E Hadden, Effect of Annealing on the Resistance of Galvanized Steel to Atmospheric Corrosion, J Iron Steel Inst., Vol 171, 1952, p 121-127

7 H.S Campbell et al., Effect of Heat Treatment on the Protective Properties of Zinc Coatings on Steel, J Iron Steel Inst., Vol 203, 1965, p 248-251

8 J.J Friel, Atmospheric Corrosion Products on Al, Zn, and Al-Zn Metallic Coatings, Corrosion, Vol 42,

1986, p 422-426

9 G Schikorr, Corrosion Behavior of Zinc, Vol 1, English ed., American Zinc Institute and Zinc

Development Association, 1965, p 72

10 L Kenworthy and M.D Smith, Corrosion of Galvanized Coatings and Zinc by Waters Containing Free

Carbon Dioxide, J Iron Steel Inst., Vol 70, 1944, p 463-489

11 "Method for Estimating the Service Life of Metal Culverts," Test Method 643-B, California Department of Public Works, 1963

12 M Romanoff, "Underground Corrosion," NBS 579.227 National Bureau of Standards, 1957

13 R.M Burns and W.W Bradley, Protective Coatings for Metals, 3rd ed., Reinhold, 1967, p 165

14 S.G Denner et al., Hot Dip Aluminizing of Steel Strip, Iron Steel Int., June 1975, p 241-252

15 G Eggeler et al., On the Influence of Silicon on the Growth of the Alloy Layer During Hot Dip Aluminizing, J Mater Sci., Vol 21, 1986, p 3348-3350

16 H.F Graff, Aluminized Steel, in Encyclopedia of Materials Science and Engineering, Pergamon Press,

1986, p 138-141

17 V.I Kelley, in Atmospheric Corrosion Investigation of Aluminum-Coated, Zinc-Coated, and Copper Bearing Steel Wire and Wire Products (A 12 Year Report), STP 585, American Society for Testing and

Materials, 1975

18 H.P Godard et al., The Corrosion of Light Metals, John Wiley & Sons, 1967, p 11

19 L Allegra et al., Resistance of Galvanized, Aluminum-Coated, and 55% Al-Zn Coated Sheet Steel to Atmospheric Corrosion Involving Standing Water, in Atmospheric Corrosion, W.H Ailor, Ed., John

Wiley & Sons, 1982, p 595-606

20 G.E Morris and L Bednar, Comprehensive Evaluation of Aluminized Steel Type 2 Pipe Field

Performance, in Transportation Research Record 1001, National Research Council, Transportation

Research Board, 1984, p 49-60

21 J.C Zoccola et al., Atmospheric Corrosion Behavior of Aluminum-Zinc Alloy Coated Steel, in Atmospheric Factors Affecting the Corrosion of Engineering Metals, STP 646, American Society for

Testing and Materials, 1978, p 165-184

22 H.E Townsend and J.C Zoccola, Atmospheric Corrosion Resistance of 55% Al-Zn Coated Sheet Steel:

13-Year Test Results, Mater Perform., Vol 18, 1979, p 13-20

23 H.E Townsend and A.R Borzillo, Twenty-Year Atmospheric Corrosion Tests of Hot Dip Coated Sheet

Steel, Mater Perform., to be published

24 J.H Payer, Electrochemical Methods for Coatings Study and Evaluation, in Electrochemical Techniques for Corrosion, R Baboian, Ed., National Association of Corrosion Engineers, 1977

25 J.B Horton et al., Corrosion Characteristics of Zinc, Aluminum, and Al-Zn Alloy Coatings on Steel, in Proceedings of the Sixth International Congress on Metallic Corrosion (Sydney, Australia), 1975

26 S.A Kriner, unpublished research, 1985

27 A.J Stavros, Galvalume Corrugated Steel Pipe: A Performance Summary, in Transportation Research Record 1001, National Research Council, Transportation Research Board, 1984, p 69-76

28 D Horstmann, Reaction Between Liquid Zinc and Silicon-Free and Silicon-Containing Steels, in

Proceedings of the Seminar on Galvanizing of Silicon-Containing Steels, International Lead-Zinc Research

31 "Standard Recommended Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized

Structural Steel Products and Procedure for Detecting Embrittlement, A 143, Annual Book of ASTM Standards, American Society for Testing and Materials

32 The Design of Products To Be Hot Dip Galvanized After Fabrication, American Hot Dip Galvanizers

Association, 1985

33 Recommended Details of Galvanized Structures, American Hot Dip Galvanizers Association, 1983

34 "Standard Recommended Practice for Providing High-Quality Zinc Coatings (Hot-Dip)," A 385, Annual Book of ASTM Standards, American Society for Testing and Materials

35 Welding Zinc Coated Steels, American Welding Society, 1973

36 R.M Burns and W.W Bradley, Protective Coatings for Metals, 2nd ed., Reinhold, 1955, p 128

37 S.K Coburn, C.P Larrabee, H.H Lawson, and G.B Ellis, Corrosiveness at Various Atmospheric Test

Sites as Measured by Specimens of Steel and Zinc, in Metal Corrosion in the Atmosphere, STP 435,

American Society for Testing and Materials, 1968, p 371-372

38 C.J Sunder and W.K Boyd, Zinc: Its Corrosion Resistance, 2nd ed., International Lead Zinc Research

Porcelain Enamels

Introduction

PORCELAIN ENAMELS are glass coatings that are applied primarily to fabricated sheet steel, cast iron, or aluminum parts to improve appearance and to protect the metal surface Porcelain enamels are distinguished from other ceramic coatings by their predominantly vitreous nature and the types of applications for which they are used, and they are distinguished from paint by their inorganic composition and the fusion of the coating matrix to the substrate metal Porcelain enamels of all compositions are matured at 425 °C (800 °F) or above Because they offer only barrier protection

to the metal substrate, porcelain enamel coatings must be free from defects and coating discontinuities to provide optimum protection

The most common applications of porcelain enamels include major appliances, water heater tanks, sanitary ware, and cookware Porcelain enamels are also used in a wide variety of applications ranging from chemical-processing vessels, heat exchangers, agricultural storage tanks, piping and pump components, and barbeque grills to architectural panels, signing, specially executed murals, and microcircuitry components Table 1 lists some additional applications for porcelain enamels Normally, porcelain enamels are selected for products or components where there is a need for one or more of the special service requirements that porcelain enamel can provide, such as chemical resistance, corrosion protection, weather resistance, specific mechanical or electrical properties, appearance or color needs, cleanability, or thermal shock capability

Table 1 Some applications for porcelain enamels

Industrial products

Chemical reactors

Commercial heat exchangers

Food-processing vessels

Induction heating coils

Ion gun parts

Jet engine components

Cooking and serving utensils

Home laundry equipment

Ranges, gas and electric

Refrigerators

Space heaters

Water heaters

Architectural

Awnings

Baseboards

Chalkboards

Exterior buildings panels

Floor, roof, and wall tiles

Types of Porcelain Enamels

Porcelain enamels for sheet steel and cast iron are classified as either ground-coat or cover-coat enamels Ground-coat enamels contain oxides that promote adherence of the enamel to the metal substrate Cover-coat enamels are applied over ground coats to improve the appearance and properties of the coating Cover coats can also be applied directly to properly prepared decarburized steel substrates The color of ground coats is limited to various shades of blue, black, brown, and gray Cover coats, which may be clear, semiopaque, or opaque, can be pigmented to take on a great variety of colors Colors can also be smelted into the basic coating material

For aluminum, neither ground coats nor adherence-promoting oxides are required Single-coat systems are used for most applications When two coats are desired, the first coat can be of any color Porcelain enamels for aluminum are usually transparent and can be pigmented and opacified inorganically to produced the desired appearance

The basic material of the porcelain enamel coating is called frit Frit is a special glass of small friable particles produced

by quenching a molten glassy mixture Because porcelain enamels are usually designed for specific applications, the compositions of the frits from which they are made vary widely

Enamel Frits for Sheet Steel. Alkali borosilicates are often used as ground coats on sheet steel Their compositions vary with the intended application of the enameled product For example, acid resistance is obtained by the addition of titanium dioxide (TiO2) and a large increase in silicon dioxide (SiO2) with a corresponding decrease in boron trioxide (BO3) The resistance of the enamel to alkalies or water can be improved by adding zirconium oxide, usually as zircon, to the frit and by maintaining a high content of silicon dioxide Table 2 lists compositions of frits for a regular porcelain enamel and of alkali-, acid-, and water-resistant ground-coat enamels for sheet steel

Table 2 Melted-oxide compositions of frits for ground-coat enamels for sheet steel

Acid-resistant enamel

Weather resistance is usually a function of acid resistance Porcelain enamels for outdoor use are made from various types

of frits that produce the resistance and color desired Resistance to thermal shock and high temperature is obtained by controlling the expansion of the glass coating

Cover coats for sheet steel are applied over ground coats or directly to properly prepared decarburized steel Electrostatic dry-powder cover coats can be applied over an electrostatic dry-powder ground coat, and the entire system can be matured in a single firing

Cover-coat enamels made from titania-opacified frits are generally quite acid resistant; even in amounts too small to impart any opacity, titania imparts acid resistance For alkali resistance, zirconium oxide is a desirable constituent

Enamel Frits for Cast Iron. Compositions of frits for enamels for cast iron vary depending on whether the frit is applied by the dry process (the article to be coated is heated above the firing temperature, and the enamel is applied to the hot metal as a dry powder) or the wet process (an enamel slip is applied to the metal at ambient temperatures, dried, and fired) Dry-process enamels are commonly used for large cast iron fixtures because of their brilliance and ability to cover small surface irregularities Acid resistance is imparted to these enamels by reducing the alumina content, by increasing silica, and by adding up to about 8% titanium dioxide Dry-process enamels are seldom used in applications requiring resistance to severe thermal shock

Ground coats are usually necessary to fill surface voids in castings Ground coats for wet-process enamels are often mixtures of frit, enamel reclaim, and refractory raw material used at very low application weight Ground coats for dry-process enamels are applied by the wet process and are fused to thin, viscous coatings that protect the casting surface from excessive oxidations while it is heated to enameling temperature

Enamel frits for aluminum are usually based on lead silicate and on cadmium silicate, but they can also be based on phosphate or barium The high-lead enamels for aluminum have a high gloss, good acid and weather resistance, and good mechanical properties The phosphate enamels generally are not alkali resistant or water resistant, but may have good acid resistance They melt at relatively low temperatures and are useful in many applications The barium enamels are not as low melting as the lead or phosphate glasses, but they do have good chemical durability

Surface Preparation

The adhesion and appearance of porcelain enamel depend on closely controlled cleaning and roughening of the metal surface Complete removal of oil, sand, drawing compounds, weld oxide, and other surface contaminants is required

Steel can be prepared by chemical or mechanical procedures Special steels for porcelain enameling are discussed in the

article "Selection of Steel Sheet for Porcelain Enameling" in Volume 1 of the 9th Edition of Metals Handbook and in the article "Sheet Formability of Steels" in Properties and Selection: Irons, Steels, and High-Performance Alloys, Volume 1

of the ASM Handbook

A modification of the Fe2(SO4)3 system called oxy-acid is also used Oxy-acid etchant solution is a mixture of H2SO4,

Fe2(SO4)3, and ferrous sulfate (FeSO4) The reactions involved are the same as with the Fe2(SO4)3 system The advantage

of this system is that all of the reactions take place in one etching tank; two tanks are required for the Fe2(SO4)3 method

Metal preparation for "no nickel/no pickle" enameling requires at least the same amount of cleaning as conventional metal preparation does for conventional enamel There is, however, no acid etching or nickel deposition required with the "no nickel/no pickle" system One advantage of using this system is the reduction in wastewater treatment problems

Mechanical preparation consists of abrasive blasting with steel shot or steel grit Grit- or shotblasting is used on parts designed without pockets or crevices when configuration and thickness permit blasting without distortion The flat areas

of parts made with sheet steel thinner than 1.5 mm (0.06 in.) distort excessively when cleaned by this method

Abrasive blasting is especially useful for preparing hot-rolled steel and parts that are to be enameled on one side only The process is also used for preparing large parts and when enamels with poor bonding characteristics are used Before blasting, oil and drawing compounds are removed by alkaline cleaning or by heating at 425 to 455 °C (800 to 850 °F) to burn off the organic contaminants

Preparation of Cast Iron and Aluminum

Cast iron is prepared by blasting to remove adhering mold sand and the thin surface layer of chilled iron Because the surface contains more combined carbon than is present in the remainder of the casting, the surface layer must be removed

to prevent excessive evolution of gas during firing of the enamel

Quartz sand is commonly used for abrasive cleaning of cast iron; however, steel shot, steel grit, and chilled cast iron grit propelled centrifugally from rotating wheels are generally used for cleaning sanitary ware Zircon sand and fused alumina grit are used for special purposes

After blasting, the casting is inspected for cracks, sand holes, slag holes, blow holes, fins, and washes Cracks and larger holes are repaired by welding Fins and washes are removed by grinding, and the repaired casting is blasted a second time before enameling

Aluminum. The preparation of heat-treatable aluminum alloys for porcelain enameling involves the removal of soil and surface oxide and the application of a chromate conversion coating Final drying removes all surface moisture; drying must be accomplished without contaminating the cleaned surface of the aluminum Parts made of nonheat-treatable aluminum alloys require only the removal of soil, which can be done by alkaline cleaning or vapor degreasing

The Porcelain-Enameling Process

Several basic methods are used to apply porcelain enamels to the base metal These include dipping, flow coating, electrodeposition, manual spray, electrostatic spray, and dry-powder spray The best method of application for a particular part is determined by quantity and quality requirements, the type of material being applied, units produced per hour, capital investment, labor cost, and, ultimately, part cost

Application techniques can be manual or mechanized Mechanization is used for high-volume part requirements of the same or similar shape Hand application is necessary if a variety of parts must go through the same process system

Regardless of the method of application, good porcelain-enameling techniques must be used to ensure uniform coverage

in the areas requiring porcelain enamel protection Excessive thickness, beads, or pooling of the porcelain enamel reduce product quality, and the product is more prone to chipping Areas at which the coating is too thin do not receive the full protective and decorative capabilities of the porcelain enamel

Dipping is widely used as a method of applying porcelain enamel, particularly when both sides of the parts require coverage This method can be used for both ground-coat and cover-coat applications Dipping is performed by immersing the part in the prepared porcelain enamel slip (a suspension of finely divided ceramic material in liquid), then withdrawing it and allowing the excess material to drain from the part It is sometimes necessary to rotate, tilt, spin, or shake complex shapes to ensure uniform coverage In areas where excessive porcelain enamel slip is retained on the part after draining, the excess should be removed by siphoning or wiping before the part is dried Dip porcelain enamels are normally applied at thicknesses of 50 to 100 μm (2 to 4 mils) The AISI 300-series stainless steels are the preferred materials for dipping equipment

In flow coating, the porcelain enamel slip is flowed onto the surface of the part The process is applicable to volume continuous operations for parts requiring the same porcelain enamel In automatic flow coating, the parts are placed on hangers at the correct angle for draining and are carried by conveyor through the flow coating chamber To ensure complete coverage, the porcelain enamel slip is pumped at high volume and low pressure though nozzles that are directed at various areas of the part

high-Another version of automatic flow coating involves the use of a constant-head tank to supply slip at a constant velocity to headers and nozzles that flood parts with slip as they are conveyed though the flow-coat chamber The advantage of this system is that the flow to the nozzles is constant and not subject to variations present in pumped systems

Automated flow coating is favored over hand dipping because it offers higher rates of production, improved coating quality, and reduced cost of the applied film Control of the porcelain enamel slip and proper operation of the machine are important functions of flow coating It is common practice to check the specific gravity and pickup of the porcelain enamel slip three to four times each hour All parts of the flow-coating machine that contact the porcelain enamel slip should be constructed of 300-series stainless steel

Spraying of the porcelain enamel slip is done primarily for one-side coverage It is also used to reinforce enamel bisque (the dried, unfired coating) and to repair enameled surfaces Spraying is ideal for parts that are too large for hand or mechanical manipulation, particularly where service and appearance requirements do not permit drain lines, beading, or buildup of the porcelain enamel Spraying is the method that is most commonly used to apply porcelain enamels to aluminum; thicknesses of 65 to 90 μm (2.5 to 3.5 mils) are desirable

Wet electrostatic spraying of porcelain enamel is used to reduce losses in material by charging the porcelain enamel slip during atomization to a potential of 100,000 to 120,000 V The electrostatically charged droplets are attracted to the grounded parts being sprayed A well-operated electrostatic unit can deposit up to 85% of the sprayed material on the part

as compared to 30 to 50% in conventional spraying operations

Electrostatic powder spraying is another method that can be used when many parts are being produced that require the application of the same porcelain enamel The parts must be of a configuration that can be properly and evenly coated

by this process When these conditions are met, this is a very efficient method of applying porcelain enamel Up to 99%

of the material is used, with little or no direct labor required for the application operation Smooth-running conveyors are required with this method of application to prevent loss of powder prior to firing the parts

Powder is delivered to the spray guns from a feeder unit where it is diffused by clean compressed air into a fluidlike state The fluidized powder is then siphoned by the movement of high-velocity air flowing through a venturi and is propelled through powder feed tubes to the spray gun The powder feeder provides a steady, controlled flow of powder to the guns Independent control of powder and air volume ensures the proper ratios to provide the desired thickness coverage on the product

The powder is propelled toward the workpiece in the form of a diffused cloud A high-voltage low-amperage power unit supplies current to the charging electrode, and this causes the powder to seek out and attach itself to the grounded workpiece

The recovery equipment booth serves to collect and return the powder that is not held on the workpiece The powder moves through a closed-loop system with the use of filters and final filters; thus, none of the airborne powder escapes into the environment

Electrodeposition is another process that can be used to apply enamel to steel The process uses a series of tanks in which the parts are submerged, and the enamel is deposited electrophoretically This process is basically limited to direct-

on enameling, but can be considered for two-coat/one-fire applications The main advantages of this system are the very uniform appearance and the exceptionally thin enamel layers

Drying and Firing. Parts coated with porcelain enamels are dried (if the enamel was applied by a wet process) and fired Drying permits the application of an additional coating of enamel, if required, and allows the parts to be handled more easily Parts are either air dried or dried with radiant or convection dryers Convection drying consists of gradual heating of the parts to 120 °C (250 °F); cycle times range from 2 to 5 min The coating is still wet during the initial stages

of drying, so drying must be accomplished in an atmosphere free of dirt, scale, or dust The parts are then fired in either continuous or batch-type furnaces at temperatures of about 425 °C (800 °F) for steel parts and 525 to 550 °C (980 to 1020

°F) for aluminum More information on the porcelain-enameling process is available in the article "Porcelain Enameling"

in Surface Engineering, Volume 5 of ASM Handbook, formerly 9th Edition Metals Handbook

Process Variables

The thickness of the applied layer of porcelain enamel, the firing time, and the firing temperature markedly affect the properties of the coating Increasing the thickness of the coating increases the resistance to burn-off and produces truer colors; however, thin coatings have the greatest flexibility

Coating Thickness. The optimum thickness of porcelain enamel depends on the substrate metal and the service requirements of the part On aluminum, porcelain enamel is applied to produce a fired enamel thickness ranging from about 65 to 125 μm (2.5 to 5 mils) A tolerance of ±13 μm (±0.5 mil) is required in order to maintain uniform opacity for

a white enamel coating 115 μm (4.5 mils) thick

On sheet steel, a ground coat about 50 to 100 μm (2 to 4 mils) thick is used to promote adhesion To cover the ground coat, a very opaque white or pastel cover coat about 100 to 150 μm (4 to 6 mils) thick is required Thus, a two-coat system on these products has a thickness ranging from 150 to 255 μm (6 to 10 mils)

Coatings for cast iron products are much thicker than those for sheet steel or aluminum Dry-process coatings on cast iron products such as sanitary ware range from 1020 to 1780 μm (40 to 70 mils) in thickness Coatings applied by the wet process are thinner than dry-process coatings; wet-process coatings range in thickness from 255 to 635 μm (10 to 25 mils)

Coating thicknesses for hot-water tanks normally range from 150 to 230 μm (6 to 9 mils), with 150 μm (6 mils) a generally accepted minimum thickness Heat-exchanger surfaces, depending on end use, are sometimes double coated for added durability

The thickness of the porcelain enamel on a large part of simple configuration can be closely controlled when application

is by a mechanical spraying system that is adapted to the part For example, mechanically applied porcelain enamel on curved silo panels measuring 2 × 3 m (6 × 9 ft) can be maintained within ±13 μm (±0.5 mil); however, when application

is by hand spraying, the variation in enamel thickness is ±50 μm (±2 mils)

An enamel thickness of 65 to 180 μm (2.5 to 7 mils) is desirable for aluminum architectural panels When white or colored enamel is used, however, the enamel thickness ranges above 75 μm (3 mils) in order to produce acceptable opacity Two coats with a total thickness of about 125 μm (5 mils) result in more uniform opacity than one coat that is

light-125 μm (5 mils) thick Additional details are available in Ref 1

Firing Time and Temperature. Firing of porcelain enamel involves the flow and consolidation of a viscous liquid and the escape of gases through the coating during its formation Within limits, time and temperature are varied in a

compensating manner For example, similar properties and appearances develop when liners for household refrigerators are fired at 805 °C (1480 °F) for 2 1

2 min or at 790 °C (1450 °F) for 4 min In all cases, there is a minimum practical

temperature for the attainment of complete fusion, acceptable adherence, and desired appearance Most ground-coat enamels for high-production steel parts exhibit acceptable properties over a firing range of 55 °C (100 °F) at an optimum firing time However, control within 11 °C (20 °F) is ordinarily maintained to produce uniform appearance and allow interchangeability of parts As the combined effects of firing time and temperature increase, resulting in more thorough firing, up to a maximum, the following conditions occur:

• Colors shift dramatically, particularly reds and yellows In general, white and colors shift toward yellow Usually, furnace temperature is changed to achieve minor adjustments in color matching

• Gloss of the enamel coating increases

• Chemical resistance of the enamel coating increases

• Gas bubbles are eliminated

• Enamel coating becomes more dense and brittle and less resistant to chipping

• Maximum adherence is attained in the optimum portion of the firing range

Color Matching and Control. In color matching, coloring oxides are usually used Although preblended oxides for a specific color are available, they are more difficult to adjust In most cases, two or three oxides are sufficient to match any specific color Usually, the proper color intensity is obtained first; adjustment is then made for the desired color shade Cadmium-sulfoselenide pigments, for red and yellow, are generally used with cadmium-stabilized clear frits

Color stability can be adversely affected by improper mill additions However, a color with only fair stability can be improved by the proper mill additions, and minor color adjustments, particularly white, are possible

Sometimes, gum must be used to control bisque strength; sodium nitrite and urea are used to control tearing Because these additions have a marked effect on some colors, they should be used in initial color matching

Finer grinding reduces the intensity of the color It is imperative that the fineness of the milled color be controlled within specific limits Milling is usually stopped before completion to permit sample firing of the enamel and comparison with a color standard Adjustments to the mill can then be made Color can be controlled to some extent by variations in the fineness of the grinding The thickness of the fired enamel coating affects many colors In general, thick coatings produce lighter colors, and thin coatings result in darker colors

The set and specific gravity of a colored enamel slip are important to the finished results Mottling or color separation is possible if the colored enamel is applied too wet or too dry Color corrections of electrostatic dry powder cannot be made

The effect of cover-coat enamels on the configuration and flatness of porcelain-enameled parts can be pronounced as a result of low coefficients of expansion and one-side application The likelihood of distortion is greatly increased when multiple or thicker coats of cover-coat enamels are necessary on one surface Sometimes, cover coats must be applied to the back side of parts to equalize the stresses

Adjustments in the firing cycle can sometimes help to minimize distortion A cycle with relatively slow heating and cooling rates is preferable to rapid heating and cooling

Variations in the method of supporting the work during firing can often change the sagging characteristics to an appreciable degree Furnace supports and fixtures can be designed to distribute the load and equalize heating and cooling rates The design and fabrication of sheet steel parts for porcelain enameling are discussed in Ref 2

Process Control

Proper workability during the application of wet-process porcelain enamels depends on:

• Control of the procelain enamel slip, particularly with respect to stability of suspension

• Weight of enamel slip deposited and retained per unit area

• Specific gravity, consistency, and particle size of the enamel slip

• Stability upon aging at ambient temperature

Stability of Suspension, or the ability of the various mill additions to keep the milled frit in suspension, is determined

by both slip measurements and visual observation of any separation that occurs Stability of the suspension is a function

of many factors, but is usually controlled by the quantity of colloid, in the form of clay or bentonite, and electrolytes used

to deflocculate the clay Enamel slips for aluminum have a shorter shelf life than those for sheet steel

Pickup weight of enamel and enamel retained per unit area are measured by draining the enamel on a flat or

cylindrical shape of known weight and area and actually weighing the pickup of enamel in wet or dry form This is a very useful test, particularly for dipping enamels, and it closely simulates actual production operations During the test, the operator can observe any tendency toward sliding, excessively long or short drain time, and variations in setting time The pickup of an enamel is a function of specific gravity, colloid content, total salts content and type, and consistency These are controlled by varying water content, addition of salts, and fineness of grind

The specific gravity of enamel slips is measured either by weighing a known volume in comparison with the weight of

an equivalent volume of water or by the use of a hydrometer Control of specific gravity is almost entirely a function of the ratio of water to solids To ensure uniformity, testing for specific gravity is required for the preparation of all procelain enamel slips

The consistency of a porcelain enamel slip for spraying is commonly determined by the slump test In this test, a fixed volume of the porcelain enamel slip is allowed to flow out suddenly in a circular pattern on a calibrated plate, and the diameter of the resulting pool is measured immediately This is a simple and useful test for procelain enamels that will be sprayed because it indicates uniformity of slip conditions between various millings Other tests for consistency involve the use of viscosimeters of various types, including those that use the rotational, flow, and falling piston methods

For porcelain enamel slips applied by dipping, a measure of drain time is a useful test Drain time is the total elapsed interval between the time a standard size sample plate is removed from a container of well-stirred procelain enamel slip and the time at which the draining motion of the slurry on the sample has stopped

The particle size of the frit for porcelain enamel slips is commonly determined by standard screen analysis Reproducible measurements are easily obtained when a standardized shaking device is used The particle size of the frit is important to the suspension characteristics of the procelain enamel slip, and slight solubility of the frits shows a major change with variation in the size of the particles

Stability toward aging of porcelain enamel slips is measured by exposing a tested sample of the enamel to whatever temperatures are expected in normal service Exposure is for many hours and days, and retests of the critical properties are made at intervals during testing Aging usually has an effect on the stability of the suspension, pickup, setting time, and the consistency of the procelain enamel slip Aging causes bubbly glass and poor surface quality of the fired enamel Leaching of soluble elements such as sodium or boron from the frit is a cause of aging This problem is more frequently encountered with less water-resistant frits The effect is greater at higher temperatures

Porcelain Enamels

Corrosion Resistance of Porcelain Enamels

Porcelain enamels possess excellent resistance to corrosion in a variety of environments Enamels are formulated to have resistance to specific environments, along with ease of processing and minimum cost Table 3 lists corrosion applications for porcelain enamels

Table 3 Applications in which porcelain enamels are used for resistance to corrosive environments

Corrosive Environment

Temperature Application

175-230 350-450 1-2 Concentrated sulfuric acid, nitric acid, and hydrochloric acid

Home laundry equipment To 71 To 160 11 Water; detergents; bleach

Range exteriors 21-66 70-150 2-10 Food acids; cleaners

Range oven liners, conventional 66-315 150-600 2-10 Food acids; cleaners

Range burner grates 66-590 150-1100 2-10 Food acids; cleaners

Refrigerators -18 to 66 0-150 2-10 Food acids; cleaners

Kitchen sinks To 71 To 160 2-10 Food acids; water; cleaners

Water heaters To 71 To 160 5-8 Water

(a) Applications include coal- and oil-fired boilers and black liquor evaporators; corrosive media would include ash from coal-fired boilers, corrosive condensates, and exhaust from black liquor evaporators

Atmospheric Exposure. Porcelain enamels have excellent resistance in atmospheric exposure, including corrosive industrial atmospheres, gases, smoke, salt spray, and seacoast exposures The weather resistance of procelain enamels is usually measured by the degree to which the coating maintains its original color and gloss Enamels formulated for acid resistance usually have better atmospheric-corrosion resistance than other types These enamels have shown no appreciable change in appearance after 15 years of exposure

Waters. All porcelain enamels are completely resistant to water at room temperature Resistance decreases at higher temperatures Special porcelain enamels have been developed for hotwater storage tanks that can withstand continuous exposure to hot water for periods of 10 to 20 years Natural waters, because of their varying compositions, have varying effects on porcelain enamels Aerated water with a low dissolved solids content, for example, has been found to be more corrosive than hard water Freezing and thawing cycles can cause some procelain enamels to spall or disintegrate; however, properly formulated and applied enamels can withstand thousands of freezing and thawing cycles without failure Porcelain enamels can withstand salt spray tests (ASTM B 117) for days and even weeks without evidence of corrosion (Ref 3), and they can provide excellent service in intermittent or continuous exposure to seawater

Soils. Porcelain enamels formulated to withstand both acid and alkaline attack provide good service in soils

Acids. Resistance to acids varies widely, depending on composition and the application process used The degree of attack by acids appears to depend less on the type of acid (with the exception of hydrofluoric) than on pH Special formulations can provide excellent protection against aqueous solutions of most acids except hydrofluoric The highest degree of acid resistance is obtained by sacrificing resistance to other media, such as alkalies Figure 1 shows weight loss

of a porcelain enamel in boiling mineral acids and in boiling water

Fig 1 Corrosion of a porcelain enamel in boiling water and boiling mineral acids Source: Ref 4

Alkalies. Most porcelain enamels are unaffected by alkalies at room temperature Special formulations provide alkali resistance to solutions with a maximum pH of 12 at temperatures to 100 °C (212 °F)

Organics. Porcelain enamels are completely resistant to attack by common organic solvents, dyes, greases, and oils Enamels are not dissolved by these materials and do not absorb then Acid-resistant porcelain enamels are required for organic material that hydrolyzes upon contact with moisture to form acid solutions

High Temperatures. Porcelain enamels greatly reduce high-temperature oxidation of the base metal This ability is largely due to the fact that the enamels themselves are fully oxidized and do not suffer further oxidation at elevated temperatures They also form an effective barrier to the diffusion of oxygen into the metal Protection depends on the temperature at which the enamel begins to soften and become more fluid This temperature is normally about 205 °C (400

°F) below the firing temperature, but specially formulated enamels can provide oxidation protection to metals at temperatures to 1095 °C (2000 °F) Maximum service temperatures for porcelain enamels are shown in Table 4

Table 4 Maximum service temperatures for porcelain enamels

Service temperature

°C °F

Limiting conditions

425 800 Usual limit for enamels maturing at about 815 °C (1500 °F)

540 1000 Maximum for enamels maturing at about 815 °C (1500 °F), without reboil

760 1400 Operating limit for special high-temperature enamels

1095 2000 Refractory enamels useful for short periods for protection of stainless steels and special alloys

The resistance of an enamel to thermal shock varies inversely with its thickness Thermal shock failure occurs when the temperature gradient perpendicular to the surface is large enough to cause excessive differential shrinkage and tensile stress Thermal shock resistance also depends on the design and section thickness of the coated part Flexing of the metal due to localized thermal gradients parallel to the surface can produce bending and tensile stresses in the coating; therefore,

an increase in the strength or rigidity of the part increases the resistance of the coating to thermal shock Most porcelain enamels applied at conventional thickness can withstand abrupt temperature changes of 110 to 165 °C (200 to 300 °F)

Evaluation of Porcelain Enameled Surfaces

Specifications and quality control for porcelain enamel coatings require the evaluation of a range of properties, depending

on the intended service of the porcelain-enameled product Although material and process variables can be brought into approximate control by using small test panels, process control is maintained by the evaluation of finished parts, even though the mechanical and chemical tests entailed are destructive

Standard test procedures are available for most porcelain enamels Specific test methods for various properties are listed

in Table 5 Some of these properties and tests are discussed below (refer to Table 5 for the title of the test, specification,

or standard)

Table 5 Test methods, specifications, and standards for porcelain enamels

Designation (a) Title of test, specification, or standard

Adherence

ASTM C 313 Adherence of Porcelain Enamel and Ceramic Coatings to Sheet Metal

ASTM C 703 Spalling Resistance of Porcelain Enameled Aluminum

PEI T-29 Adherence of Porcelain Enamel Copper Coats Direct To Steel

Thickness

ASTM C 664 Thickness of Diffusion Coating

ASTM D 1186 Dry Film Thickness of Non-Magnetic Organic Coatings Applied on a Magnetic Base, Measurement of

ASTM E 376 Coating Thickness by Magnetic Field or Eddy-Current (Electromagnetic) Test Methods, Rec Practice for

Measuring

Color and gloss

ASTM C 346 45-deg Specular Gloss of Ceramic Materials

ASTM C 540 Image Gloss of Porcelain Enamel Surfaces

ASTM E 97 Reflectance Factor of Opaque Specimens by Broad-Band Filter Reflectometry, 45 deg, 0 deg Directional

ASTM C 347 Reflectivity and Coefficient of Scatter of White Porcelain Enamels

ASTM D 2244 Color Differences of Opaque Materials, Instrumental Evaluation of

ASTM D 1535 Color by the Munsell System, Specifying

ASTM C 538 Color Retention of Red, Orange and Yellow Porcelain Enamels

Chemical resistance and weather characteristics

ASTM C 282 Acid Resistance of Porcelain Enamels (Citric Acid Spot Test)

ASTM C 614 Alkali Resistance of Porcelain Enamels

ASTM C 756 Cleanability of Surface Finishes

ASTM C 283 Boiling Acid, Resistance of Porcelain Enameled Utensils to

ASTM D 1567 Detergent Cleaners for Evaluation of Corrosive Effects on Certain Porcelain Enamels, Testing

ASTM C 872 Lead and Cadmium Releases from Porcelain Enamel Surfaces

Chipping resistance

ASTM C 409 Torsion Resistance of Laboratory Specimens of Porcelain Enameled Iron and Steel

Abrasion resistance

ASTM C 448 Abrasion Resistance of Porcelain Enamels

Thermal shock

ASTM C 385 Thermal Shock Resistance of Porcelain Enameled Utensils

Tests related to preparation of coatings and substrates

ASTM C 374 Fusion Flow of Porcelain Enamel Frits (Flow-Button Methods)

ASTM C 539 Linear Thermal Expansion of Porcelain Enamel and Glaze Frits and Ceramic Whiteware Materials by the

Interferometric Method

ASTM C 285 Sieve Analysis of Wet-Milled and Dry-Milled Porcelain Enamel

ASTM C 839 Compressive Stress of Porcelain Enamels by Loaded-Beam Method

ASTM C 715 Nickel on Steel for Porcelain Enameling by Photometric Analysis

ASTM C 810 Nickel on Steel for Porcelain Enameling by X-ray Emission Spectrometry

ASTM C 632 Reboiling Tendency of Sheet Steel for Porcelain Enameling

ASTM C 694 Weight Loss of Sheet Steel During Immersion in Sulfuric Acid Solution

ASTM C 774 Yield Strength of Enameling Steels After Straining and Firing

ASTM C 660 Production and Preparation of Gray Iron Coatings for Porcelain Enameling

Tests related to continuity of coating

ASTM C 536 Continuity of Coatings in Glassed Steel Equipment by Electrical Testing

ASTM C 743 Continuity of Porcelain Enamel Coatings

Specifications

PEI S-100 Specification for Architectural Porcelain Enamel on Steel for Exterior Use

PEI ALS-105 Recommended Specifications for Architectural Porcelain Enamel on Aluminum for Exterior Use

PEI ALS-106 Recommended Specifications for Porcelain Enamel Finishes on Aluminum Cookware

WH-196-J Federal Specification heater, water, electric and gas fired residential (This covers resistance of porcelain

enamels to hot water under "Solubility of Glass Lining.")

Plumbing fixtures standards

ANSI A 112.19.4 Porcelain Enameled Formed Steel Plumbing Fixtures

112.19.1M

Enameled Cast Iron Plumbing Fixtures

(a) ASTM, American Society for Testing and Materials; PEI, Porcelain Enamel Institute; ANSI, American National Standards Institute

Adherence refers to the degree of attachment of enamel to the metal substrate Although none of the adherence tests in common use results in the quantified force per unit area required to detach the enamel by tensile force normal to the interface, various tests aimed at evaluating adherence are regularly used in the industry

The standard adherence tests for porcelain enamel on steel are ASTM C 313 and PEI Bulletin T-29 The ASTM C 313 test is applicable only to steel substrates between 0.4 mm (0.016 in.) and 2 mm (0.082 in.) in thickness The PEI T-29 test applies to direct-on cover coats on substrates with a thickness range of 0.7 mm (0.028 in.) to 1.3 mm (0.050 in.)

Both adherence tests for porcelain enamel on steel include deforming the metal and measuring the area from which the porcelain enamel is removed The indicator of adherence is the adherence index, which is the ratio of the porcelain enamel remaining in the deformed are to that in the same measured area before deformation

Enamels for cast iron pose a special problem because of the relatively greater thickness and rigidity of the metal substrate and the brittleness of the iron Simple, unstandardized impact tests are used in these cases

Resistance to spalling, a defect characterized by the separation of porcelain enamel from the base metal without apparent external cause, is the indicator used to measure the adherence of porcelain enamel on aluminum Spalling can result from the use of improper alloys or enamel formulations, incorrect pretreatment of the base metal, or faulty application and firing procedures

Two methods for determining resistance to spalling are outlined in ASTM C 703 Method A, which uses a 5% solution of ammonium chloride, requires a 96-h immersion of the tests specimen Method B uses a 1% solution of antimony trichloride and requires a 20-h immersion of the test specimen

The spall test is a pass/fail test Failure is determined by either of two criteria The first is the existence of spall areas of specified dimensions at specimen edges The second criterion is the existence on the specimen interior (away from the edges) of spots exceeding specified dimensions or a spot level exceeding a specified density, usually spots/m2 (spots/ft2)

Thickness. The specifications for architectural porcelain enamel (PEI S-100 for steel and PEI ALS-105 for aluminum), the specification for porcelain-enameled aluminum cookware (PEI ALS-106), and the standards for porcelain-enameled formed steel plumbing fixtures (ANSI A112.19.4) and porcelain-enameled cast iron plumbing fixtures (ANSI A112.19.1M) all require a specific thickness for the porcelain enamel coating

The procedure used for measuring the porcelain enamel thickness depends on the type of base metal used For on-steel products, enamel thickness is measured through procedures outlined in ASTM D 1186 For porcelain-on-

porcelain-aluminum products, coating thickness is measured according to test procedures specified in ASTM E 376 In some cases, primarily laboratory investigations, measurement of porcelain enamel thickness is accomplished by using the procedures specified in ASTM C 664

Color and Gloss. Porcelain enamel finishes are produced in literally hundreds of colors and many textures This capability provides the manufacturers of appliances, cookware, outdoor grills, architectural panels, signs, decorative products, and many other applications with unusual design versatility as well as desirable performance properties

The common method of specifying color is based on the capacity of the observed article to reflect light of different wavelengths (different colors) A physical standard, such as a plaque of porcelain-enameled steel, is provided as the color

to be matched within stated limits The difference in color between the control standard and the test specimen can be measured instrumentally by using the procedures specified in ASTM D 2244

Gloss is particularly desirable in such products as appliances However, high-gloss enamels capable of reflecting distinct images are not recommended for architectural porcelain enamel for exterior use

The gloss of porcelain enamel can be measured by following procedures from two standards: ASTM C 346 and ASTM C

540 The test chosen for evaluating gloss depends on the purpose for which the porcelain enamel is used

Acid Resistance. There are two standard tests for determining the acid resistance of porcelain enamels: ASTM C 282 (generally referred to as the citric acid spot test) and ASTM C 283 (generally known as the boiling acid test) The citric acid spot test is used as a testing and grading system for such porcelain enamel applications as appliances, plumbing fixtures, and architectural products Evaluation is based on visual examination following wet or dry rubbing and blurring-highlight tests The boiling acid test is designed primarily for cookware applications, and the results are expressed in weight loss per unit area

Alkali Resistance. Home laundry equipment, dishwashers, and other porcelain enamel applications in which the surface is normally exposed to an alkaline environment at elevated temperatures require an alkali-resistant coating The standard test for alkali resistance is ASTM C 614 This test covers the measurement of the resistance of a porcelain enamel to a hot solution of tetrasodium pyrophosphate Alkali resistance is expressed in terms of weight loss for the area exposed to the test solution

Weather Resistance. The long-term weatherability of porcelain enamel is of primary interest to specifiers and manufacturers of architectural porcelain enamel products intended for exterior use The actual weathering performance of the material has been documented in a series of on-site exposure tests conducted by the National Bureau of Standards in cooperation with the Porcelain Enamel Institute

The weathering of porcelain enamel is evaluated in terms of the changes in gloss and color that occur during outdoor exposure Changes in gloss and color are measured by using the procedures outlined earlier Weathering tests of up to 30 years show that porcelain enamels have considerable inherent gloss and color stability; rates of change are primarily influenced by enamel compositions, choice of colors (reds and yellows are most susceptible to color change), and the severity of the exposure site (seacoast and corrosive industrial environments have been found to have the most aggressive effect)

Spalling of enamels during exposure to weather occurs if improper materials and processing procedures have been used The spalling resistance of weathered porcelain enamel on aluminum, for example, is best ascertained by using ASTM C

703

In exposure tests, good correlation has been observed between acid resistance, as determined by the citric acid spot test, and the color retention of steel enamels However, use of the acid spot test has shown an even stronger reliability in predicting color change in porcelain-on-steel The correlation between acid resistance and color retention in porcelain-on-aluminum is somewhat less definitive but still observable The best indicator for predicting the weatherability of red, orange, and yellow porcelain enamels is ASTM C 538 Because architectural porcelain enamel for exterior use is subjected to weathering for long periods of time, the specifications for porcelain enamel on aluminum (PEI ALS-105) and porcelain enamel on steel (PEI S-100) require compliance with specific levels of acid resistance, which are considered indicators of good weatherability

Chipping Resistance. Relatively thick layers of porcelain enamel cannot be subjected to severe bending or other substrate deformation without fracture However, coatings of 125 μm (5 mils) or less that are well bonded to a relatively thin metal substrate for example, 26 gage 0.4546 mm (0.0179 in.) or less can withstand the bending of the substrate to radii of curvature within its elastic limit and the bending to return to the original shape with little or no apparent damage Chipping of typical porcelain enamel on sheet iron occurs at about the strain required for permanent deformation of the base metal

Abrasion Resistance. The test for determining the resistance of porcelain enamel to various types of abrasion is ASTM C 448 The test consists of three parts The first determines the resistance to surface abrasion of porcelain enamels for which the unabraded 45 ° specular gloss is more than 30 gloss units Here the specular gloss of the specimens is measured before and after a specified abrasive treatment of the surface; the percentage of the original specular gloss that

is retained is the surface abrasion index

The second part of this test determines the resistance to surface abrasion of porcelain enamels for which the unabraded 45° specular gloss is 30 gloss units or less The weight loss by a specific abrasive treatment modified by an adjustment factor for each abrasive tester, lot of abrasive, and lot of calibrated plate glass standards used results in an adjusted weight loss value This is recorded as the index of resistance to surface abrasion

The final portion of the test measures the resistance of porcelain enamels to subsurface abrasion In this case, the scope of the linear portion of the abrasion time-weight loss curve is determined and then multiplied by an adjustment factor associated with each abrasion tester, lot of abrasive, and lot of calibrated plate glass standards used The adjusted scope is taken as an index of the resistance to subsurface abrasion

Thermal Shock Resistance. The standard used for evaluating the durability of porcelain-enameled utensils subjected

to thermal shock is ASTM C 385 In this test, cooking utensils are subjected to a series of dry heating and quenching cycles until the utensil fails by removal of the enamel from the utensil

Continuity of Coatings. Ensuring the continuity of the coating after manufacture is important in porcelain enamel or so-called glassed steel applications in which a prime purpose of the coating is to protect the substrate against corrosion

There are two principal test methods for determining either discontinuity of coverage or potential discontinuity through too-thin coverage For glassed steel equipment, the prescribed test procedure is ASTM C 536 For conventional porcelain-enameled products, such as appliances, plumbing fixtures, and architectural panels, the test most commonly used is ASTM C 743 Both tests essentially involve the use of electrical probes of relatively high voltage to discern either discontinuities in the coating or insufficient coverage for coating integrity in service use

Resistance to Hot Water. Federal specification WH-196-J specifies a solubility test for determining the resistance of porcelain enamels to hot water In this test, 89- × 89-mm (3.5- × 3.5-in.) sections cut from the outer wall of a water heater are tested at a rolling boil for 8 cycles of 18 h each in a special apparatus containing 400 mg of reagent grade sodium bicarbonate dissolved in 1 L of water The resistance of the porcelain enamel is specified in terms of weight loss in milligrams per square inch exposed area The weight loss allowed by WH-196-J is 2.3 mg/cm2 (15 mg/in.2)

3 "Standard Method of Salt Spray (Fog) Testing," B 117, Annual Book of ASTM Standards, American Society

for Testing and Materials

4 C.L Hackler and M Dinulescu, Porcelain Enameled Flat Plate Heat Exchangers Engineering and

Application, in Industrial Heat Exchangers, Proceedings of the 1985 Symposium on Industrial Heat Exchanger Technology, A.J Hayes et al., Ed., American Society for Metals, 1985

5 "Development of Porcelain Enamel Coatings," Bulletin E-6, Porcelain Enamel Institute

• Glass; Ceramic Whitewares; Porcelain Enamels, Vol 15.02, Annual Book of ASTM Standards, American

Society for Testing and Materials

• Glass Linings and Vitreous Enamels, Publication 6H160, National Association of Corrosion Engineers,

• "Preparation of Sheet Steel for Porcelain Enameling," Bulletin P-307, Porcelain Enamel Institute

• "Pretreatment of Alloys for Porcelain Enameling Aluminum," Bulletin P-403, Porcelain Enamel Institute

• "Properties of Porcelain Enamel; High Temperature Properties," Data Bulletin PEI 504, Porcelain Enamel Institute

• "Properties of Porcelain Enamel; Resistance to Corrosion," Data Bulletin PEI 503, Porcelain Enamel Institute

• "Quality Control Procedures for Porcelain Enameling Aluminum," Bulletin P-405, Porcelain Enamel Institute

• "Weathering of Porcelain Enamels on Aluminum, 12-Year Inspection, Exposure period 1964-1976," Department of Ceramic Engineering, University of Illinois at Urbana-Champaign, 1977

• "Weather Resistance of Porcelain Enamels, 15-Year Inspection of the 1956 Exposure Test," Building Science Series 50, National Bureau of Standards

Chemical-Setting Ceramic Linings

Gregory D Maloney, Saureisen Cements Company

Introduction

INORGANIC CHEMICAL-SETTING CERAMIC LININGS have become one of the most widely used construction materials in designing protective linings for industrial installations in which high temperatures, aggressive corrosive media, and complicated substrate geometry exist, such as floors, trenches, sumps, reaction vessels, tanks, scrubbers, ducts, chimneys, and other air pollution control equipment They are used in various industries, including power, steel- and metalworking, chemical, pulp and paper, refinery, waste treatment, and mining

Inorganic monolithic linings have proved themselves in these industries because of their chemical resistance to both high and low concentrations of strong acids and solvents, thermal insulation that protects the substrates from extremely high temperatures, temperature resistance to 870 °C (1600 °F), good compressive and flexural strength for environments in which stress and strain are factors, and abrasion resistance Monolithic linings can be applied by cast or gunite (shotcreting) methods over old and new steel or concrete as well as brick and mortar masonry This article will discuss the function of monolithic linings, the use of these materials, the types of applications in which these materials can be successfully used, and the limitations of these linings

History of Chemical-Setting Silicates

The progress of silicate cement development has changed over the years with growing technology to meet industry needs The first silicate cements were composed of a sodium silicate (Na2SiO3) liquid and a combination of fillers, such as silica flour, clay, silica aggregates, and barytes, and were formulated as chemical-resistant mortars for use in ambient or high-temperature acid lining construction This type of silicate cement was very slow in setting and had to be exposed to the open air or heat cured, which created construction delays Another problem was that these cements were not water resistant These problems were resolved by the use of acid washing, which helped set the cements and make them water resistant

In the early 1920s, Na2SiO3 mortars were introduced that used an acid catalyst to insolubilize the silica gel; this produced

a mortar that cured faster and was water resistant The physical properties of the silicate mortar were unchanged; that is, the acid, temperature, and solvent resistance were maintained Because the acid-catalyzed mortar was chemically activated upon mixing, it could set within 24 to 48 h This was a distinct advantage for the construction industry because

it allowed brick to be laid continuously without concern over the mortar being squeezed out of the joints or the brick sliding out of line Typical setting agents used are ethyl acetate (C4H8O2), zinc oxide (ZnO), sodium fluorosilicate (Na2SiF6), glyceryl diacetate, formamide (CH3ON), metallic polyphosphates, and other amides or amines

It was later determined that when Na2SiO3 mortars were exposed to sulfuric acid (H2SO4) the reaction product was sodium sulfate (Na2SO4), which is a salt that expands and grows through hydration This sodium salt can pick up as much

as 10 mol of water of crystallization with a resultant size increase of 150%, thus creating internal stresses in the structure

in which it was used As a result, monolithic linings and brick surfaces would sometimes crack or spall

As time passed, industry required silicate technologies that would meet changing applications and structural designs By the 1950s, potassium silicate (K2SiO3) cements were introduced using methods of insolubilizing the silicate The K2SiO3

cements possess enhanced corrosion resistance, particularly in H2SO4 environments Potassium silicate cements react with

H2SO4 to form potassium sulfate (K2SO4), which is not a growth salt This eliminates the problem of internal stresses when the mortar is exposed to H2SO4 solutions

The introduction of K2SiO3 cements was a positive step in producing a material that was more suited for application technology and corrosive environments The K2SiO3 cements offer improved physical properties, which are particularly beneficial in monolithic applications Furthermore, K2SiO3 cements provide better workability with less tackiness and longer working times than Na2SiO3 cements Additional advantages include greater resistance to strong acid solutions and sulfation, more refractoriness, and no efflorescence as with Na2SiO3 cements By this time, Na2SiO3 and K2SiO3 materials were not only being used as mortars but were also being introduced in new areas of monolithic application as both castable and gunite grades

Within the last decade, the modified silicates have been developed Modified silicates are manufactured by the addition of

a powder form of Na2SiO3 in conjunction with a proprietary ingredient added directly to the powder fillers This process eliminates the need to have both powder and liquid on the job site; water is the only required addition Although these products have been commercially available, high cost and potential for sulfation have limited their use The modified silicates do not have the chemical properties of the other silicates The only advantage appears to be their simplicity of mixing

Advantages and Disadvantages

The Na2SiO3 and K2SiO3 cements were developed because a material was needed that was acid resistant in strong concentrations and that offered higher temperature resistance than chemical-resistant organic materials The development of these silicate-base acidproof cements resulted in many advantageous characteristics, such as a 100%

dilute-to-K2SiO3-bonded system that had resistance to most solvents and acids over a pH range of 0 to 7; that was water and vapor resistant without special treatment; and that could withstand all concentrations of H2SO4, nitric acid (HNO3), hydrochloric acid (HCl), and phosphoric acid (H3PO4) In addition, the Na2SiO3- and K2SiO3-bonded cements can be applied over damp acid-attacked concrete or brick surfaces as long as these substrates have acid pH surfaces, and they cure chemically within 36 h, thus decreasing construction delays These cements can be applied by gunite or cast methods Detailed information on guniting (shotcreting) can be found in American Concrete Institute publication ACl 547R on refractory concrete

Certain disadvantages were encountered during the development of the acid-resistant silicate cements It was common knowledge that the silicate cements were not resistant to alkalies, hydrofluoric acid (HF), and fluoride salts As previously mentioned, the Na2SiO3 cements formed a growth salt when exposed to H2SO4 that put undue internal stresses on the structure of the material

Inorganic monolithic linings also have a certain amount of permeability compared to organic surfacing materials Over time, acid can penetrate the lining and eventually reach the surface of the substrate This problem is now being combated

by using a dual-lining system, which includes a chemically resistant elastomeric membrane applied to the surface of the substrate

Current Technologies

Monolithic and Membrane Dual Linings. Organic linings or coatings have been used in stacks and ducts, but they will usually deteriorate if temperatures exceed 260 °C (500 °F) in high-sulfur gases There are many types of lining materials for stacks and ducts; however, because of unpredicted environmental conditions within a stack or duct, it may

be difficult to select the best candidate material Stacks that are designed with heat recovery units and/or scrubbers, combined with one or more flues inside the shell that can control the temperature, should be considered for dual-lining applications

In recent years, the trend toward using the superior technology of dual linings has emerged and is being recommended where corrosion problems occur throughout industry Figure 1 illustrates the design of a typical membrane/monolithic system in the chemical industry

Fig 1 Schematic of a chemical-resistant dual-lining system that provides double protection to the substrate in

the form of a flexible membrane and a rigid surface layer The flexible corrosion-resistant membrane is applied

in direct contact with steel or concrete substrates It is then covered by the monolithic cement lining, which provides protection over a broad pH range as well as against high temperatures

There are numerous reasons for this new construction process Condensation occurs not only on the face of the resistant lining but can also penetrate and condense on the substrate to be protected Although acidproof monolithic linings offer the proper chemical resistance, they are inherently inelastic, or brittle In time, monolithic linings may tend

acid-to crack and absorb acids or acid gas condensate; therefore, it is advantageous acid-to have a backup membrane In many applications, the coefficient of thermal expansion of the monolithic lining may not match that of the substrate Therefore,

a flexible membrane will help accommodate stresses resulting from these differences in thermal expansion as well as other mechanically induced stresses

For many applications, the monolithic lining should not be bonded to the substrate, and the membrane acts as a bond barrier The concept of placing an impervious membrane between the substrate and inorganic linings has become a recommended practice Some corrosion-resistant gunite manufacturers have installation specifications for use with membranes

Membrane choices include asphaltics, resins, and synthetic elastomers The membrane selected should resist the maximum acid concentrations and temperature expected Organic linings can fail by disbonding, swelling, abrasion, and blistering from high temperature Protection of an organic membrane with an inorganic barrier will minimize thermal exposure and mechanical abrasion to the membrane as well as the corrosive media that reach the membrane An organic membrane compensates for some of the shortcomings of inorganic linings, such as cracking or spalling due to mechanical stresses from shrinkage, induced stresses, insufficient thermal expansion allowances, the anchoring system, vibration, and thermal or other stresses To provide an effective barrier, inorganic linings are applied much thicker than typical organic coatings They often contain fillers to act as reinforcement and to decrease shrinkage Fillers are also incorporated to provide wear resistance for abrasive conditions

The installation procedure for a dual-lining system is basically the same as if each component were being applied separately These procedures are established for both organic coatings and inorganic monolithic linings The condition of the working area and preparation of the surfaces follow standard practices used throughout the industry Anchors are usually installed on the steel or concrete substrates before the organic membrane is applied

Lightweight Insulating Materials. Industry has recently seen the development and application of improved density, lightweight, insulating corrosion-resistant lining materials The evolution of these products has followed the need

lower-to accommodate load limitations of structural supports and improved thermal protection of membrane coatings where substrate temperatures can range from 40 to 150 °C (100 to 300 °F) and even higher if process limits or gas cleanup systems experience frequent excursion or bypass conditions Membranes require adequate thermal protection to retain physical and chemical properties throughout their service life Because the membrane will also coat the studs, the inorganic coating must be thick enough to protect the membrane on the highest point of the studs Therefore the inorganic lining material must not only protect the membrane from physical abuse but must also protect it from thermal deterioration

When membranes are not used, condensation of chemicals from hot process streams should be designed to occur within a lining and not at the substrate However, when membranes are used, the condensation can occur at the membrane, thus possibly reducing the required lining thickness Therefore, inorganic silicate lining materials with lower thermal conductivity and density have been developed to meet these needs

Application Procedures. Through the use of current technology, such as the dual-lining system and the availability of

improved application techniques, corrosion protection practices have been greatly enhanced Observations of successes and failures in various applications have led to the development of the following parameters, which must be addressed to specify the proper material In any application, the primary consideration is the exposure environment This includes the types and concentrations of chemicals, physical abuse, and temperatures In this evaluation, it is important to remember that both the silicate monolithic lining and the membrane must resist the corrosive environment The next consideration is the thickness of the monolithic lining In high-temperature environments, the monolithic lining must be able to decrease the temperature through its thickness to a level that will not burn out the membrane applied to the substrate or the anchoring system Once the materials that can resist the corrosive environment have been selected, the physical structure must be considered

Steel When a steel substrate is being used, sources of variation in electrical potential on the metal surfaces must be

considered These include mill scale, metal impurities, localized strains on parts of the metal, junctures of dissimilar metals, movement of electrolytes, weld joints, concentration cells, and externally imposed currents Some or all of these differences in potential can occur on the surface and must be considered in the recommendation of a protective coating The protective coating must provide the following properties to protect the steel:

• Physical protection to surface

• Prevention of concentration cells

• Dielectric properties

• Lower surface temperatures

• Prevention of electrolyte flow

• Deprivation of free oxygen from the surface

• Stress relief

Stress relief is an important factor in the design of a system If a system does not compensate for relief in areas of high stresses, cracking will occur that can severely damage the entire system Stresses on the system of major concern could result from vibration, unsupported surfaces, changes in planes, and welds

The following five steps are application procedures that should be followed to ensure proper installation of a dual lining system First, all areas of high stress that will result in movement of the steel structure, such as oil canning or vibration, must be externally supported Second, the anchoring system should then be applied to the substrate Anchors, such as V-type or longhorn studs, should be installed on the proper centerlines in a diamond or staggered pattern with the tines randomly oriented Third, the steel surface must be stripped of all oil and grease by chemical cleaning The metal should then be sandblasted to a Steel Structures Painting Council SP 10 Near-White Blast with a nominal 64- m (2.5-mil) profile Welds should be ground to a smooth, rounded radius with no sharp edges before blasting Fourth, the appropriate membrane should be applied, ensuring that all of the studs are completely coated and that the system is free of pin holes

The final step is to apply the monolithic lining The two available methods of application should be considered A castable material will shrink more than the gunitable material; therefore, expansion joints must be planned into the system

in areas exceeding 6 × 6 m (20 × 20 ft) or where changes in planes will cause stresses to occur A gunitable material will shrink less and therefore does not require expansion joints Figure 2 illustrates the gunite application of K2SiO3 cement over steel ductwork in a fossil fuel power plant

Fig 2 Gunite application of 50 mm (2 in.) of K2 SiO 3 cement over steel ductwork in a coal-fired power plant

Concrete In applications over concrete, as with steel, the first considerations are the corrosive environment and the

thickness of the material Once the materials have been selected, the physical structure must be considered When the substrate is concrete, factors different from those associated with steel must be considered Inorganic monolithics can be applied over old or new concrete surfaces When concrete substrates are being used, the following questions should be answered:

• Is the concrete old acid-attacked concrete or new concrete?

• Is the concrete capable of supporting loads?

• Is the structure above grade or below grade?

• Does the concrete need to be waterproofed from the outside?

• Does the concrete structure have existing cracks?

• Is the concrete surface a high or low pH?

A combination of many of these items may apply and should be considered in the recommendation of the proper system The following are the criteria for a concrete surface before a system may be applied When working with old acid-attacked concrete, all loose and deteriorated concrete should be removed The surface should be firm, hard, and at a

minimum pH of 5, and all surfaces should be brought back to grade and the slopes reestablished When working with new concrete, the concrete should be firm and sound All structural cracks should be repaired, and it should be cured for a minimum of 7 days Slopes should be a minimum 3.2 mm (1

8 in.) to a maximum 6.4 mm (

1

4 in.) per linear foot and

should have attained a minimum compressive strength of 21 MPa (3000 psi)

The following five steps are application procedures that should be followed to ensure proper installation of a dual-lining system First, the concrete should meet the standards outlined above Second, the concrete should be cleaned of all oil, grease, and form release compounds by chemical cleaning, waterblasting, or scarifying Third, the anchoring system should be placed at the specified centerline distance in a diamond-shape pattern with a random orientation of anchor tines The anchoring system should consist of V-type or longhorn studs Fourth, the membrane should be applied to the recommended thickness, ensuring that all studs are completely coated and that the coating is free of pinholes

The final step is to apply the monolithic The castable and the gunitable methods should both be considered As stated above, when applying by the gunite method there is less shrinkage; therefore, there is no need for expansion joints in the monolithic However, the lining will require control joints in the normal manner Expansion joints are recommended with cast applications and should be installed at the perimeter of floor areas, equipment pads, and floor trenches; around floor drains, column bases, and protrusions; at changes in planes; and over expansion joints in concrete slabs not to exceed 6 m (20 ft) on centerlines

Brick A monolithic lining can be applied over acid-attacked masonry construction The first step is the surface

preparation of the brick Flyash and other contaminants should be removed by sandblasting or waterblasting All attacked

or unsound mortar should be removed from the joints Mortar joints should be cleaned to a depth of at least 13 mm (1

2

in.) to provide support for the monolithic lining If the joints cannot be cleaned to this depth, it will be necessary to set longhorn or V-type anchors in the joints The preferred method of application in this case is to gunite the material Guniting allows the material to be applied under pressure, thus enabling the material to be packed into the 13-mm (1

Wastewater treatment systems in the southern United States have experienced corrosion problems in areas of high temperatures, high humidity, and slow drainage Consequently, millions of dollars have been spent in replacing the corroded concrete infrastructures in these wastewater treatment systems in wet wells, grit chambers, sewer lines, and aeration basins This corrosion went unnoticed throughout the years because the corrosion mechanism was not understood Most of the corrosion was not caused by discharged chemicals but was microbiologically induced The concrete infrastructures are seldom completely filled and the combination of high temperatures and high humidity creates

a perfect breeding ground for bacteria, fungi, algae, and gaseous products form decomposing sewage

Carbon dioxide (CO2), combined with hydrogen sulfide (H2S) from the slightly warm and fermenting sewage, reacts with the damp concrete surfaces to form carbonates and calcium sulfate (CaSO4) As this process proceeds, the pH becomes more acidic Bacterial action results in the production of elemental sulfur from various sulfate reactions, with subsequent oxidation of elemental sulfur to H2SO4, which the bacteria secretes

The H2SO4 then directly attacks the underlying portland cement concrete substrate and causes destruction of the infrastructure There are documented cases of microbiologically induced corrosion having proceeded to the point at which structures collapsed in as few as 6 years or required major rehabilitation in as few as 4 years

A proven method of rehabilitating deteriorated concrete wastewater structures involves applying a urethane-base membrane to the properly prepared substrate, followed by the pneumatic application of a 100% K2SiO3-bonded acid-resistant concrete The urethane-base membrane is a two-component material with a high solids content and very low permeability To ensure pinhole-free coatings, the membrane is applied in two or more coats to a wet film thickness

ranging from 1.6 to 3.3 mm (63 to 130 mils) These membranes are applied by airless spray with a spray tip pressure of approximately 28 MPa (4000 psi) The high pressure further helps to ensure pinhole-free application of the membrane The substrate must be dry enough to accommodate the urethane membrane without reacting with the polyisocyanate catalysts usually used

Therefore, it is sometimes desirable to apply a moisture-tolerant primer, such as a suitable formulated epoxy, to the damp concrete or to apply a fresh layer of concrete first Following the proper application and cure of the urethane membrane, a veneer of a 100% K2SiO3 acidproof cement is applied over the membrane and anchors

The chemical industry can be the most difficult of all industries in which to design a protective lining The conditions within one plant can be staggering; there may be acids, alkalies, and solvents, along with extremely high temperatures and physical abuse Potassium silicate base cements satisfy most of these conditions while maintaining economy The combination of a wide range of chemical resistance, high physical strength, and refractoriness makes these cements leading candidates for many chemical plant applications

For example, at a major Midwest chemical plant, the action of HCl by-products and ambient moisture on floors, digester tank supports, trenches, and sumps in NiCl2 and NiSO4 processing units produced very corrosive conditions Restoration

of the existing structures was accomplished with a K2SiO3 cement and an appropriate membrane

In the NiCl2 unit, the existing concrete floor was chipped out, and new concrete was poured; the steel beam supports for the digester tanks were also encapsulated with regular concrete Both floor and digester foundations then received a trowel-applied asphaltic membrane The membrane provided a flexible acid-, water-, and alkali-resistant barrier between the concrete substrate and the monolithic surfacing The trowel-applied membrane system is applied at room temperature and offers the chemical resistance of hot-applied membranes for those areas where a hot membrane may not be practical

or advisable

The next step was to apply a K2SiO3 monolithic lining by casting into forms The lining was 38 mm (1 1

2in.) thick over

foot traffic areas near the digesters; 38 to 50 mm (1 1

2to 2 in.) thick around the encapsulated digester foundation, creating

a collar; and 50 mm (2 in.) thick in forklift truck aisles Finally, at the interface of the digester tank wall and the collar, an elastomeric urethane-base compound was used as a permanently flexible expansion joint

In the NiSO4-processing unit, the problem areas included trenches running among process equipment in two different buildings In addition, the various collection sumps into which these trenches drained required restoration As was done in the NiCl2 area, the deteriorated concrete was reconstructed with new concrete The asphaltic membrane was trowel applied A K2SiO4 monolithic lining was cast over the membrane to 25 mm (1 in.) in the trenches, 25 to 38 mm (1 to 1

Deposition Processes

The vapor deposition processes fall into two major categories physical vapor deposition (PVD) and chemical vapor deposition (CVD) which in turn comprise various subcategories Although these divisions often appear confusing and arbitrary, each has its own well-defined advantages and disadvantages, whether technical or economic This is also true of the materials used in vapor deposition Therefore, the need for a thorough understanding of both processes and materials

is essential in order to select the optimum combination

The major deposition processes that will be reviewed in this article are sputtering, evaporation, ion plating, and CVD These processes all have one characteristic in common: The deposition species are transferred and deposited in the form

of individual atoms or molecules In this respect, they are fundamentally different from particulate or liquid deposition processes, such as thermal spraying or electroplating (see the articles "Thermal Spray Coatings," and "Electroplated Coatings" in this Volume) A primary benefit of vapor deposition is that it is essentially nondamaging to the environment, because there are no solvents or electrochemicals to dispose of At a time when environmental-protection regulations are continually being tightened, this crucial factor favors vapor deposition over other processes that may be more harmful to the environment

Sputtering is the principal PVD process It involves the transport of a material from a source (target) to a substrate by means of the bombardment of the target by gas ions that have been accelerated by a high voltage Atoms from the target are ejected by momentum transfer between the incident ions and the target These ejected particles move across the vacuum chamber to be deposited on the substrate

Figure 1 shows a schematic of the sputtering process In its simplest form, the process occurs in an inert (noble) gas at low pressure (0.13 to 13 Pa, or 1 × 10-3 to 100 × 10-3 torr); in most cases, the gas is argon Argon has higher mass than other noble gases, such as neon or helium, and is easier to ionize Higher mass gives a higher sputtering yield, especially

if the mass of the bombarding particle is of the same order of magnitude or is greater than that of the target atom Other gases, such as oxygen or nitrogen, can also be used, but they may react chemically with the target The sputtering process begins when an electric discharge is produced and the argon becomes ionized The low-pressure electric discharge is known as glow discharge, and the ionized gas is termed plasma

Fig 1 Schematic of the basic sputtering process

The argon ions hit the solid target, which is the source of the coating material (and not to be confused with the substrate, which is the item to be coated) The target is negatively biased and therefore attracts the positively charged argon ions, which are accelerated in the glow discharge This attraction of the ions to the target (also known as bombardment) causes the target to sputter, which means that material is dislodged from the target surface because of momentum energy exchange The higher the energy of the bombarding ions, the higher the rate of material dislodgement

Sputtering developed very rapidly in the 1970s in the semiconductor industry, in which the technique is essential for mass production With the advent of the technique of magnetron, or magnetron-enhanced sputtering, two original primary disadvantages nonuniform coverage and low deposition rate have been largely eliminated Sputtering is making rapid inroads in such corrosion applications as high-chromium alloy coatings on turbine blades and many other new applications that require high-quality coatings with good adhesion

For the deposition of thin metal films (in the 1000- A

o

range), sputtering is the best technique Deposition is feasible in a controllable manner for both compound and elemental targets (the source of the coating material) Adhesion is good and can be further improved by sputter cleaning the substrate or by bias sputtering Large equipment is readily available, and sputtering has become a highly automated process The quality, structure, and uniformity of the deposit are excellent The disadvantages of sputtering include its thickness limitation and high cost It is also a line-of-sight process that requires special fixtures and shaped targets to coat uniformly; furthermore, the coating of compound curves, recesses, and holes is difficult

Evaporation was the first PVD process used on an industrial basis for the aluminum metallization of plastics and glass for decorative purposes Originally the most widely used process, evaporation has now been overtaken by sputtering The metallization process is relatively simple and well adapted to mass production

The basic evaporation process involves the transfer of material to form a coating by physical means alone, essentially vaporization Practically all metals can be evaporated, making evaporation a universal process with regard to metals Like sputtering, it is a line-of-sight process; therefore, to achieve efficiency of coverage and uniformity, it is necessary to use multiple evaporation sources and to rotate or move the substrate uniformly to expose all areas

Unlike other vapor deposition processes, evaporation is a low-energy process, with particle energy averaging 0.2 to 0.3

eV as compared to 10 to 40 eV for sputtering, 200 eV or more for ion plating, and 100 keV for ion implantation The term low energy means that adhesion of the particle to the substrate may be marginal In addition, the temperature of the substrate is not increased by the deposition, which means there is little or no diffusion Because no strong physical or chemical action takes place at the interface (as opposed to ion plating or CVD, for example), particular care must be taken

to remove all impurities in the chamber and to have a perfectly clean substrate The basic process (with an electron beam source) is illustrated in Fig 2

Fig 2 Schematic of the basic evaporation process

Very large evaporation equipment, which was developed for the optical industry, is commercially available Coatings of several microns in thickness can be rapidly deposited The disadvantages of this method are nonuniform coverage (limited

to line of sight) and relatively low adhesion unless glow discharge cleaning is used (glow discharge cleaning removes surface atoms from the substrate) Evaporation is very successful in applications in which adhesion and good structure are not critical, such as decorative and optical uses Aluminum is the principal evaporation material; chromium and stainless steel are also widely used Because improvements in technique have improved quality, evaporation is not used for critical corrosion applications

Ion plating is actually a hybrid concept based on the evaporation (or sputtering) mechanism coupled with a glow discharge (Fig 3) There are three possible deposition systems: resistance evaporation, electron beam evaporation, and sputtering All three systems general a plasma by a glow discharge, thus imparting a large increase in the energy of the deposition species

It is generally accepted that plasma plays the same part as high temperature in improving the adhesion and increasing the reactivity of the coating; an ion passing through a potential of 100 eV acquires an equivalent temperature of 106 K The simplest forms of ion plating are ion nitriding and ion carburizing, in which nitrogen or carbon ions are obtained from the ionization of nitrogen or

a hydrocarbon These react with a steel substrate to form nitrides or carbides If the deposition species is ionized in

a reactive gas, a compound is formed with some of the gas atoms, and deposition of the compound occurs Ion plating

is widely used and is gradually replacing pack cementation or diffusion

Because ion plating is a glow discharge process in which a plasma is formed, it is necessary to have at least a threshold gas pressure to maintain the plasma This is different from standard nonionized evaporation, in which the minimum pressure obtainable is preferred The net effect of this higher gas pressure is to increase the number

of collisions with gas molecules and therefore produce more scattering of the deposition species, which in turn diminishes the importance of the line-of-sight principle and improves coverage

The deposited species in ion plating have higher energy than those of evaporation or sputtering processes; this results in improved adhesion as well as a deposit with improved structure and fewer imperfections In addition,

if a sputtering system is used, it is easy to sputter the surface of the substrate to clean it before deposition

On the negative side, the equipment is more complicated and therefore more expensive than either evaporation or sputtering equipment, and the process is more exacting The level of ionization can be difficult to control, which may result in uneven deposition Although the process has not yet reached the total reliability necessary for automated operation and total acceptance, its advantages are such that it is rapidly gaining in popularity

Chemical vapor deposition is a well-established deposition process that is extensively used in the semiconductor and cutting tool industries It has the unique ability to deposit thick, dense, high-quality films (up to 6.4 mm, or 1

4in in

thickness or more) at a cost that is generally lower than that of PVD processes The high temperature (about 1000 °C, or

1830 °F) necessary for CVD promotes good adhesion but also considerably restricts the type of substrate that can be coated to ceramics, refractory metals, and special alloys

In most cases, steel will require heat treatment in a vacuum or protected atmosphere after CVD coating and will often change dimensions sufficiently to require postdeposition machining if tolerances are close Another constraint resulting from the high temperature of deposition is the stress in the deposit due to the difference in thermal expansion between substrate and deposit These stresses may be sufficient to cause cracking, spalling, and loss of adhesion The new technique of plasma CVD can be used at much lower temperatures in, for example, semiconductor applications, but the resulting films tend to incorporate impurities

The materials that can be easily deposited by CVD are more limited than with PVD Chemical vapor deposition is particularly useful for depositing compounds and refractory metals and lends itself very well to corrosion applications

Figure 4 shows a schematic of CVD laboratory equipment The reactive gases are metered in the mixing chamber They then pass into the deposition chamber and contact the heated part to be coated (sample); at this point, they react and deposition occurs By-products are removed through the vacuum system

Fig 3 Schematic of the basic ion plating process

Fig 4 Schematic of a laboratory CVD reactor

Chemical vapor deposition reactions can occur over the full range of pressure Low pressure increases boundary-layer diffusion and uniformity, often at the expense of deposition efficiency Each application and each reaction must be analyzed to determine the optimum conditions for deposition

Coating Materials and Applications

The list of materials that can be vapor deposited is extensive and covers almost any coating requirement: the most important materials are given in Table 1 Aluminum and aluminum alloys are among the most widely used deposition materials and are gradually replacing cadmium in many corrosion applications Sputtered chromium and stainless steel are also making great inroads in corrosion applications A promising material is titanium nitride It is hard, very stable, and refractory, and it has good lubricating characteristics and a pleasing gold appearance It is widely used in wear applications, such as cutting tools, and in decorative applications It is usually applied by sputtering or ion plating, and it holds potential for corrosion protection

Table 1 Some corrosion-resistant vapor-deposited materials Material Deposition process

Titanium diboride Evaporation, CVD

Silicon carbide Sputtering, CVD

Titanium carbide Sputtering, evaporation, CVD

Tungsten carbide Sputtering, CVD

Boron nitride CVD

Silicon nitride Sputtering, ion plating

Titanium nitride CVD, sputtering, ion plating

Aluminum oxide Sputtering, ion plating

Molybdenum silicide Sputtering, ion plating

Nickel-chromium alloys Sputtering