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130 T.M Devine and J Wulff Cast vs Wrought Cobalt-Chromium Surgical Implant Alloys, J Biomed Mater Rest., Vol 9, 1975, p 151-167
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135 K Asgar and F.C Allan Microstructure and Physical Properties of Alloys of Partial Denture Castings, J Dent Res., Vol 47, 1968, p 189-197
136 K Asgar and F.A Peyton, Effect of Microstructure on the Physical Properties of Cobalt-Base Alloys, J Dent Res., Vol 40, 1961, p 63-72
137 Revised ANSI/ADA Specification No 14, Dental Base Metal Casting Alloys, J Am Dent Assoc., Vol
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138 H.F Morris and K Asgar, Physical Properties and Microstructure of Four New Commercial Partial
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139 H Mohammed and K Asgar, A New Dental Superalloy System, J Dent Res., Vol 53, 1973, p 7-14
140 J.F Bates and A.G Knapton, Metal and Alloys in Dentistry, Int Met Rev., Vol 22 (No 215), 1977, p
148 R.M German D.C Wright, and R.F Gallant, In Vitro Tarnish Measurements on Fixed Prosthodontic
Alloys, J Prosthet Dent., Vol 47, 1982 p 399-406
149 D.C Wright and R.M German, Quantification of Color and Tarnish Resistance of Dental Alloys J Dent Res., Vol 58A, 1979 IADR No 975
150 D.A Nitkin and K Asgar, Evaluation of Alternative Alloys to Type III Gold for Use in Fixed
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151 S Civjan, E.F Huget, and J Marsden, Characterization of Two High-Fusing Gold Alloys, J Dent Res.,
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152 J.F Bates and A.G Knapton, Metal and Alloys in Dentistry, Int Met Rev., Vol 22, 1982, p 39-60
153 S Civjan, E.F Huget, N.N Dvivedi, and H.E Cosner, Jr., Characterization of Two Au-Pd-Ag Alloys, J Dent Res., Vol 52, 1973, IADR No 46
154 E.F Huget, S.G Vermilyea, and J.M Vilca, Studies on White Crown-and-Bridge Alloys, J Dent Res.,
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155 P.F Mezger, M.M.A Vrijhoef, and E.H Greener, Corrosion Resistance of Three High Palladium Alloys,
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156 M.M.A Vrijhoef and J.M van der Zel, Oxidation of Two High-Palladium PFM Alloys, Dent Mater., Vol
1 1985, p 214-218
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158 J.J Tuccillo, Compositional and Functional Characteristics of Precious Metal Alloys for Dental
Restorations, in Alternatives to Gold Alloys in Dentistry, T.M Valega, Ed., Conference Proceedings
DHEW Publication (NIH) 77-1227, Department of Health, Education, and Welfare, 1977
159 P.J Cascone, Phase Relations of the Palladium-Base, Copper, Gallium, Indium Alloy System, J Dent Res., Vol 63, 1984, IADR No 563
160 M.M.A Vrijhoef, Oxidation of Two High-Palladium PFM Alloys, Dent Mater., Vol 1, 1985, p 214-18
161 Sumithra, T.K Vaidyanathan, S Sastri, and A Prasad, Chloride Corrosion of Recent Commercial
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162 S.M Paradiso, Corrosion Evaluation of Pd-Cu-Ga, J Dent Res., Vol 43, 1984, IADR No 43
163 P.R Mezger, M.M.A Vrijhoef, and E.H Greener, Corrosion Resistance of Three High-Palladium Alloys,
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164 Oden and H Hero, The Relationship Between Hardness and Structure of Pd-Cu-Ga Alloys, J Dent Res.,
Vol 65, 1986, p 75-79
165 J.R Mackert, Jr., E.E Parry, and C.W Fairhurst, Oxide Metal Interface Morphology Related to Oxide
Adherence, J Dent Res., Vol 63, 1984, IADR No 405
166 G Baron, Auger Chemical Analysis of Oxides on Ni-Cr Alloys, J Dent Res., Vol 63, 1984, p 76-80
167 D.L Menis, J.B Moser, and E.H Greener, Experimental Porcelain Compositions for Application to Cast
Titanium, J Dent Res., Vol 65, 1986, IADR No 1565
168 ANSI/ADA Specification No 32, New American Dental Association Specification No 32 for Orthodontic
Wires Not Containing Precious Metals, J Am Dent Assoc., Vol 95, 1997, p 1169-71
169 P.J Brockhurst, Base Metal Wires for Gold Alloy Soldering to Cast Cobalt-Chromium Alloy Partial
Dentures, Aust Dent J., Vol 15, 1970, p 499-506
170 M.R Marcotte, Optimum Time and Temperature for Stress Relief Heat Treatment of Stainless Steel Wire,
J Dent Res., Vol 52, 1973, p 1171-1175
171 C.J Burstone and J.Y Morton, Chinese NiTi Wire A New Orthodontic Wire, Am J Ortho., Vol 87,
1985, p 445-452
172 M Bergman, Combinations of Gold Alloys in Soldered Joints, Swed Dent J., Vol 1, 1977, p 99-106
173 C.E Janus, D.F Taylor, and G.A Holland A Microstructural Study of Soldered Connectors of Low-Gold
Casting Alloys, J Prosthet Dent., Vol 50, 1983, p 657-663
174 T.M Devine and J Wulff, Cast vs Wrought Cobalt-Chromium Surgical Implant Alloys, J Biomed Mater Res., Vol 9, 1975, p 151-167
175
H.J Mueller and B.C Marker, Effect of and Cl- Upon Product Deposition on NTD and Cupralloy,
J Dent Res., Vol 59, IADR N 279, 1980
176 H.J Mueller, SIMS and Colorimetry of In-Vitro Sulfided Crown and Bridge Alloys, in Fifth International Symposium on New Spectroscopic Methods for Biomedical Research, Battelle Laboratories and University
180 T.K Vaidyanathan and A Prasad, In Vitro Corrosion and tarnish Characteristics of Typical Dental Gold
Compositions, J Biomed Mater Res., Vol 15, 1981 p 191-201
181 J Brugirard, Baigain, J.C Dupuy, H Mazille, and G Monnier, Study of the Electrochemical Behavior of
Gold Dental Alloys, J Dent Res., 1973, p 838-836
Trang 6182 W Popp, H Kaiser, H Kaesche, W Bramer, and F Sperner, Electrochemical Behavior of Noble Metal
Dental Alloys in Different Artificial Saliva Solutions, in Proceedings of the 8th International Congress of Metallic Corrosion, Vol 1, DECHEMA, 1981, p 76-81
183 N.K Sarkar, R.A Fuys, and J.W Stanford, The Chloride Corrosion Behavior of Silver-Base Casting
Alloys, J Dent Res., Vol 58, 1979, p 1572-1577
184 D.C Wright, R.M German, and R.F Gallant, Copper and Silver Corrosion Activity in Crown and Bridge
Alloys, J Dent Res., Vol 60, 1981, p 809-814
185 T.K Vaidyanathan and A Prasad, In Vitro Corrosion and Tarnish Analysis of Ag-Pd Binary System, J Dent Res., Vol 60, 1981, p 707-715
186 N Ishizaki, Corrosion Resistance of Ag-Pd Alloy System in Artificial Saliva: An Electrochemical Study,
J Osaka Dent Univ., Vol 3, 1969, p 121-133
187 L.A O'Brien and R.M German, Compositional Effects on Pd-Ag Dental Alloys, J Dent Res., Vol 63,
191 J.M Meyer, Corrosion Resistance of Ni-Cr Dental Casting Alloys, Corros Sci., Vol 17, 1977, p 971-982
192 R.J Hodges, The Corrosion Resistance of Gold and Base Metal Alloys, in Alternatives to Gold Alloys in Dentistry, T.M Valega, Ed., DHEW Publication (NIH) 77-1227, Department of Health, Education, and
Welfare, 1977
193 N.K Sarkar and E.H Greener, In Vitro Corrosion Resistance of New Dental Alloys, Biomater Med Dev Art Org., Vol 1, 1973, p 121-129
194 H.J Mueller and C.P Chen, Properties of a Fe-Cr-Mo Wire J Dent., Vol ll, 1983, p 71-79
195 N.K Sarkar, W Redmond, B Schwaninger, and A.J Goldberg, The Chloride Corrosion Behavior of Four
Orthodontic Wires, J Oral Rehab., Vol 10, 1983, p 121-128
196 H.J Mueller, Silver and Gold Solders Analysis Due to Corrosion, Quint Int., Vol 37, 1981, p 327-337
197 D.L Johnson, V.W Rinne, and L.L Bleich, Polarization-Corrosion Behavior of Commercial Gold- and
Silver-Base Casting Alloys in Fusayama Solution, J Dent Res., Vol 62, 1983, p 1221-1225
198 A.D Vardimon and H.J Mueller, In Vitro and In Vivo Corrosion of Permanent Magnets in Orthodontic
Therapy, J Dent Res., Vol 64, 1985, IADR No 89
Trang 7Corrosion of Emission-Control Equipment
William J Gilbert and Robert John Chironna, Croll-Reynolds Company, Inc
Introduction
CORROSION PROBLEMS and material selection for emission-control equipment can be difficult because of the varied corrosive compounds present and the severe environments encountered Therefore, a number of the more common emission-control applications will be discussed More detailed information on the applications is available in the references cited at the end of this article
Flue Gas Desulfurization
By far the most common cleaning application for flue gases is flue gas desulfurization (FGD) This section will discuss the selection of materials of construction for FGD systems More information on corrosion in FGD systems is available in the section "Corrosion of Flue Gas Desulfurization Systems" of the article "Corrosion in Fossil Fuel Power Plants" in this Volume
These systems came into being in the late 1960s and early 1970s because of the tightening of restrictions on the release of sulfur emissions The oil shortage of the mid-1970s and subsequent oil price increases led to the reuse of coal in new and renovated power plants In virtually all cases, this meant the potential for increased sulfur emissions Many more FGD systems were needed
Fuel gas desulfurization systems typically use wet scrubbing units with lime or limestone slurries for sulfur dioxide (SO2) absorption Initially, it was thought that the relatively mild pH and temperature conditions found within most of these systems would not present a significant corrosion problem This was soon found not to be the case The fact that the FGD system could constitute up to 25% of the total capital and operating expenses of the power plant made it imperative to determine the reasons behind the failure of the material
Environment The gases encountered by the FGD system are hot and contain SO2 at significant levels, some sulfur trioxide (SO3) as a result of the oxidation of SO2 at high temperatures, and fly ash Initially, these gases may be sent to a dry-dust collector, such as an electrostatic precipitator of fabric filter baghouse, for fly ash removal The gases typically enter a wet scrubber (venturi with separator) and are quenched as SO2 is absorbed The components that often have the severest problems, however, are the outlet duct and stack Here the condensates are more acidic, the gases are highly oxygenated, and the presence of chlorides and fluorides, can cause serious corrosion problems Nevertheless, throughout the entire system, corrosion can occur to various degrees and because of various factors
Corrosion Factors Four basic factors affect the severity and type of corrosion that occurs They are discussed below
pH. The result of the reactions that take place within the scrubber is a slurry with a typical pH of 4 to 5 This is desirable, because it allows for good absorption of SO2 and is acidic enough to reduce scale formation Local pH values as low as 1 may exist from the concentration of chlorides entering the makeup liquid with contributions from fluorides The low-pH conditions with the presence of chlorides and fluorides limit the use of carbon steels, stainless steels, and a number of higher-nickel alloys (Fig 1)
Trang 8Fig 1 Minimum levels of chloride that cause pitting and crevice corrosion in 30 days in SO2 -saturated chloride solutions at 80 °C (175 °F) Source: Ref 1
Gas Saturation. The dry flue gas is not severely corrosive However, when the gas reaches its dew point, sulfuric (H2SO4) and sulfurous (H2SO3) acids can form In addition, hydrochloric acid (HCl) is produced because of the presence
of hydrogen chloride (formed at the elevated temperatures of combustion) plus the condensing water vapor Again, significant problems arise from the use of carbon or stainless steels
Temperature. The problems caused by temperature excursions are primarily related to the lessening of the resistant properties of synthetic coatings, fiberglass-reinforced plastics (FRP), and thermoplastics, possibly to the point of complete destruction at high enough temperatures This affects metals to a lesser extent, but can make a bordering problem a serious one
corrosion-Erosion generally occurs as a result of fly ash within the gas impacting on a surface in a relatively dry area of the system
or the liquid slurry impinging upon a wetted surface In either case, areas susceptible to corrosion attack are produced
General Materials Selection An easily overlooked but critical aspect of materials selection is the ability of the
manufacturer to construct the equipment properly with correct fabrication techniques In particular, with regard to the use
of high-nickel alloys, the welding recommendations of alloy producers should be precisely followed to maintain the corrosion resistance of the materials (Ref 2) This is of course true for any type of fabrication The most careful materials selection process can be negated by poor fabrication practices
Metals. Where pH is neutral or higher, austenitic stainless steels (AISI types 304, 316, and 317, L grades preferred) perform well even at elevated temperatures If pH is as low as 4 and chloride content is low (less than 100 ppm) but temperatures are above approximately 65 °C (150 °F) then Incoloy 825, Inconel 625, Hastelloy G-3, and alloy 904L
Trang 9(UNS N08904) or their equivalents are usually acceptable Table 1 lists compositions of alloys commonly used in FGD systems
Table 1 Compositions of some alloys used in FGD systems
Composition, %(a) Alloy
C Fe Ni Cr Mo Mn Others Type 304L 0.03
5.0 max
5.5 bal 15.5 16.0 1.0
max
2.5 max Co, 0.03 max P, 0.03 max S, 0.08 max Si, and 0.35 max
V
INCO alloy 0.02 bal 25.5 21.0 4.5 2.0 1.5 Cu 1.0 max Si, 0.045 max P, and 0.035 max S
(a) Nominal composition unless otherwise specified
When chloride content is up to 0.1% and pH approaches 2, only Hastelloys C-276, G, and G-3, and Inconel 625 can be successfully used The other alloys mentioned above would be subjected to pitting and crevice corrosion If a region is encountered with pH as low as 1 and chloride content above 0.1%, one of the only successful alloys acceptable is reported
to be Hastelloy C-276 or its equivalent In terms of metals selection, the higher the molybdenum content in an alloy, the more severe the corrosive environment it can withstand in the FGD system (Ref 3)
Nonmetals. Fiberglass-reinforced plastics can be used in almost any application in which temperatures do not exceed
120 °C (250 °F) (preferably 95 °C, or 205 °F), regardless of whether there are high chlorides or low pHs The best choices would be premium grades of vinyl-ester and polyester resins Polypropylene(PP), chlorinated polyvinyl chloride (CPVC), and other thermoplastics can be used in such applications as mist elimination, in which temperatures are suitably low, for example, 80 °C (175 °F) for PP Rubber linings can also be used where temperatures are suitable and mechanical damage can be avoided
Waste Incineration
In a number of ways, the problems associated with materials for incinerator off-gas treatment equipment are similar to those used for FGD systems Depending on the wastes being burned, however, significantly higher gas temperatures as well as more varied and more highly corrosive compounds may be encountered Materials selection for waste incineration parallels that for FGD systems to some extent, but can often be more demanding
The importance of incineration for the treatment of domestic and industrial wastes has increased as the availability of sanitary landfills has lessened and their costs have escalated At the same time, environmental safety regulations have limited the use of deep below-ground and sea-disposal sites for untreated wastes
Incineration provides a viable, although not inexpensive, alternative that produces scrubbable gaseous and particulate contaminants from a myriad of waste products Incinerators are used to burn municipal solid wastes, industrial chemical wastes, and sewage sludge In general, the off-gases can be classified according to their corrosiveness in descending order
as follows: industrial chemical, municipal solid, and sewage sludge
Trang 10Industrial Chemical These gases are characterized by extremely high temperatures (1000 °C, or 1830 °F, is not
uncommon) and the presence of halogenated compounds In many cases, chlorinated hydrocarbons and plastics are burned, producing HCl, chlorine, hydrogen fluoride (HF), and possibly hydrogen bromide Some sulfur and phosphorus compounds may also be produced
The typical treatment systems uses a gas quench to saturate and cool the gases, a wet venturi scrubber (if particulates pose
a problem), a packed tower absorber, exhaust fan, ducting, liquid piping, and liquid recirculation pumps Figure 2 shows a standard system arrangement
Because of high temperatures, the presence of chlorides, and the fact that the gas becomes saturated with water vapor within the quench, very few materials can be successfully used for the quench construction The major problem is not uniform attack but local pitting and crevice corrosion of many metals In particular, chloride stress-corrosion cracking severely affects austenitic stainless steels
The materials that have found to perform very well are such high-nickel alloys as Hastelloy C-276, Inconel 625, and titanium for the highest-temperature cases and Hastelloy G and G-3 for slightly less severe cases These materials have been used in other critical areas of the treatment system, such as fan wheels, dampers, liquid spray nozzles, and piping Multiple-year service life histories have been reported with these alloys (Ref 4.)
Refractory linings for the quench have also been used with some success This can sometimes prove to be a more economical alternative to the use of high-nickel alloys Problems do occur, however, because of attack on the binding substances employed and on the carbon steel base material, if exposed
Following the quench, where temperatures are typically less than 95 °C (205 °F), the major equipment (venturis, tower shells, sump tanks, fan housings, and pump bodies) can be constructed of FRP A premium polyester or vinyl-ester resin can withstand even the most severe corrosive atmospheres at these milder temperatures Even the presence of glass-attacking fluorides would not preclude the use of FRP, given the availability of synthetic veils used to replace glass veils within the resin layers closest to the internally exposed surfaces
The recirculating fluids, often alkaline because of the need to scrub acidic gases, can often be handled satisfactorily by FRP or such thermoplastics as CPVC and PP In this case, the alkalinity is not the problem Free chlorides and fluorides may be present even in the most carefully operated and maintained systems
Fiberglass-reinforced plastic ductwork is used to transport the gases in the milder-temperature areas of the system Because PP exhibits good resistance to most of the corrosives usually encountered, it is used for tower packing, mist eliminators, and spray nozzles It is a particularly good choice for environments having the potential for severe fluoride attack The use of rubberlined components can be successful, but the emergence of sound FRP construction has limited its popularity
Caution must be exercised when using plastics in the system following the quench If the quench loses its liquid and there are no safeguards, a major part of the downstream equipment may be destroyed Typically, temperatures are monitored so that an emergency cooling liquids source, possibly city water, is injected into the quench to prevent disastrous temperature excursions if the normal liquid source is lost
A more conservative approach that is implemented in many system designs would also use high-nickel alloy construction for the equipment directly downstream of the quench In any case, this question must be addressed during the design phase of any incineration project
Fig 2 Schematic of a general scrubber system arrangement
Trang 11Municipal Solid Waste The by-products of solid municipal wastes can be similar to those found in chemical
incineration The levels of the worst contaminants chlorides, for example are usually lower The nature of the requirements for burning these wastes, which contain large portions of cellulose, result in lower off-gas temperatures than those for chemical incineration
Nevertheless, corrosion problems are severe, and materials selection is not very different from that of industrial chemicals incineration Reference 5 provides a ranking of metals with respect to corrosion resistance on the basis of corrosion tests
in this service In addition, Ref 6 shows the results of corrosion tests for a very wide range of alloys in six distinct system zones
Sewage Sludge The burning of sewage sludge presents the least corrosive discharge of the three types under
discussion This can be attributed to limited halogen compounds in the gas and somewhat lower temperatures (typically
315 to 650 °C, or 600 to 1200 °F
Type 304 and 316 stainless steels are suitable for construction in most areas of the system, including the quenching area, whether as a separate quench or part of the wet scrubber Again, FRP, thermoplastics, and lined carbon steel can be used
in the cooler regions
The predominant contaminants in the environment are odorous sulfur compounds, both organic (mercaptans) and inorganic (hydrogen sulfide, H2S), and particulate Chlorides can exist, but they normally originate from the water used for makeup Their presence sometimes requires the use of high-nickel alloys for such components as fan wheels and pump impellers
Erosion can be a significant problem in any of these systems It can wear down critical moving mechanical components
and equipment walls at points of liquid and/or gas impingement and, perhaps more importantly, it can contribute to corrosion attack
The overall effect is not as severe as that found with FGD treatment equipment, but there are a number of areas of concern The venturi throat and spray nozzles can suffer some abrasion Fan wheels and pump impellers, however, usually the most critical areas in these systems with respect to potential problems
The use of high-nickel alloys at these points has been noted above Rubber lining can also be used, although generally not
on fan wheels Fiberglass-reinforced plastic can also be fabricated with silicon carbide impregnation for increased abrasion resistance for the internal surfaces
The selection of materials of construction does vary with the industry, but the dust handled is generally not severely corrosive Carbon steel is the most common material of construction
In selecting a dust collector, the most common construction for the vessel itself is steel, but the fabric used in the collection varies greatly The manufacturers of fabric filters will have the greatest experience with selection for a specific application Their expertise should be used in evaluating the relative initial maintenance cost for alternate fabrics Typically, most bulk-handling applications can be managed with the use of PP for the filter bags Currently, the cost of PP filter bags is close to that cotton filter bags The low initial cost of PP makes it a versatile material of construction for this application It does not rot when it becomes wet and offers relatively good corrosion resistance The primary limitation is temperature
Trang 12Fans and stacks located downstream from a fabric filter are normally constructed of carbon steel Because most of the dust has already been collected before it reaches this point, abrasion is not a major concern in the design of the downstream components
Where extremely high performance is required, an electrostatic precipitator can be applied to a bulk solids application This normally occurs in relatively dry services with inert particles As such, the materials of construction are typically carbon steel
Wet scrubbers are often used where the solids being handled are more reactive For example, if there is a concern over the potential for an explosive mixture of the dust with air, the wet scrubber eliminates this problem Wet scrubbers are also versatile and can simultaneously remove dust and gas
Where scrubbers are applied, their low initial cost is partially by the need to recirculate a water-base solution Care must
be taken to ensure that this solution does not become corrosive or, if it does, to select the proper materials of construction for this specific case
Commonly, general nuisance dust that is collected by a scrubber does not cause a direct corrosion problem Instead, the problems arise because of the need to minimize the wastewater from the scrubber For example, if the inlet is at ambient conditions, the scrubber will evaporate 122 L/h (31.8 gal/h) for every 10,000 m3/h (5889 ft3/min) If the solids quantity requires only 10% of the evaporation rate as a liquid bleed rate, then the dissolved solids in the water will be concentrated
by a factor of ten Thus, 200 ppm of chloride would suddenly become 2000 ppm of chloride This would be sufficient to cause corrosion problems
In most cases, fiberglass has been considered as a material of construction where abrasion is not particularly severe In other cases, carbon steel is used, particularly for coal handling or other applications in which abrasion is definitely present
Spray-drying applications typically require stainless steel Either type 304 or 316 is used, depending on the particular compound being collected The use of stainless steel arises from the need for product purity Because the slurry is usually returned to the spray dryer, care must be taken to avoid any potential corrosion
Chemical and Related Processing Plants
Chemical process and related industries experience a wide variety of potential corrosion problems Many of the compounds used have severe effects on many materials of construction For air pollution control, the quantities of these compounds can be greatly reduced, but the same corrosion problems may still be encountered Obviously, it is important
to rely on the experience of the plant with its process equipment in selecting air pollution control equipment for exhaust ventilation and process vents
There are some specific differences, and the most important is the difference in operating pressure Typically, the ventilation systems of chemical reactors will be at atmospheric pressure By comparison, the reactor itself may be at several atmospheres of pressure This is important because more economical materials of construction can often be selected for the ventilation system, but they would not have the mechanical strength necessary to handle the pressure in the reactor
Because fiberglass can be used for atmospheric conditions, many of the clean-up systems used in chemical and process plants are fabricated from fiberglass The primary reason for using FRP is its low initial cost and good corrosion resistance in a wide variety of services The corrosion resistance of FRP is a function of both the resin content and the specific resin used in the laminate
Chloro-Alkali Plants The production of chlorine results in severe corrosion problems Quantities of chlorine in the
effluent gas are normally scrubbed using dilute caustic solutions The most common material of construction is fiberglass Although fiberglass can be used in chlorine service, specific types of resin must be employed for this very difficult application Vinyl-ester resins are most commonly used Numerous resins of the vinyl-ester type are available that can handle chlorine and chlorides In addition to using a high-performance resin, the inner glass reinforcement is usually replaced with a synthetic veil to provide additional protection and to avoid any attack by hypochlorite or caustic on the inner liner
Trang 13Fiberglass-reinforced plastic is typically used for the scrubbers handling chlorine removal, ductwork, fans, and stacks Even recycled pumps are manufactured of this material
Where heat exchanges are used in the recycled solution, fiberglass is obviously not a practical material because of its low heat transfer coefficient For dilute hypochlorite solutions, such alloys as Hastelloy C-276 or Inconel 625 have been used Graphite can be used if the proper binder is selected With the high heat transfer coefficients of plate heat exchangers, constructions of Hastelloy C-276 can be economical
Polyvinyl chloride or high temperature PVC (CPVC) is another material of construction that performs well in this service These materials are sometimes selected for small units or for ductwork construction
Nitric Acid Plants In nitric acid (HNO3) manufacture, stainless steel is the most common material of construction Concentrated HNO3 will affect many of the nonmetallic materials of construction; therefore, FRP is not as common or as easily accepted
For many ventilation systems, either type 304 stainless steel or CPVC could be used Fiberglass-reinforced plastic could also be used when the acid is being neutralized Relative costs are shown in Figure 3 Because the cost of stainless steel is still relatively high compared to that of FRP, this alternative should be considered where very dilute acid concentrations are involved Where concentration and return to the process are involved, stainless steel remains the best solution
Trang 14In many cases, HNO3 manufacture also produces oxides
of nitrogen Nitric oxide (NO) and nitrogen dioxide (NO2) would be handled by the same materials of construction Most commonly, these are removed from the air by using scrubbing systems.The recycled solution becomes a dilute HNO3 solution A special wet-phase catalyst has been developed for use on this service This material has properties very similar to those of stainless steel
In other HNO3 facilities, thermal reduction has been used
to eliminate the residual oxides of nitrogen from the air Where thermal reduction is used, there is no wet surface However, the possibility of condensation remains should the system shut down Therefore, the holders in such units are typically stainless steel The catalyst itself is normally
a ceramic material with a vanadium oxide or similar catalyst applied to the surface These materials are selected by the manufacturers and would be compatible with stainless steel components selected for ductwork, fans, and other auxiliaries
Sulfuric Acid Service Sulfuric acid mist is collected
by fiber bed mist eliminators Such units employ a glass mat held inside of a vessel operating at low velocities to remove submicron mists
Fiber bed unit shells for H2SO4 are either type 316 stainless steel or alloy 20Cb3 The relative economics suggest the use type 316 stainless steel, although it may suffer a small amount of attack
Alloy 20Cb3 can be used for most ranges of acid that would be encountered in air pollution control systems If the solution is weak enough, type 316 stainless steel can
be used Also, if the temperatures are low enough, FRP should be considered because of its low initial cost It is best to obtain coupons of the materials and to conduct some initial testing at dilute conditions before making a final decision The industry practice is to use more and more FRP on these inorganic acid applications because of low initial cost
Sulfur Dioxide Service Sulfur dioxide has similar
requirements even though the initial solution formed is usually a neutralized salt; that is, SO2 is normally absorbed using an alkali solution, such as lime or caustic This solution is a sodium or calcium sulfite/sulfate mixture It can be handled at low temperatures in fiberglass and at higher temperatures in type 316L stainless steel
Because most of the gas is removed in the air pollution control equipment, the downstream equipment can often handled using liners rather than expensive alloys This is particularly true for the fan because epoxy coatings can be applied to the fan housing The wheel itself is recommended in solid alloy construction because of high speeds involved The combination of an epoxy liner and a stainless steel wheel can cost as much as 25% less than a solid stainless steel fan
Discharge stacks are often treated on the same basis Where the stacks are large enough, a coating can be applied to a steel stack Of course, the first selection might be an FRP stack if the temperature is low enough because of its elimination
Fig 3 Relative costs of scrubber materials
Trang 15of maintenance More information on materials of construction for the chemical-processing industry is available in the article "Corrosion in the Chemical-Processing Industry" in this Volume
The Fertilizer Industry Several severe problems can occur in the manufacture of fertilizers Trace quantities of
fluorides in phosphates result in the formation of HF and tetrafluoride in the gas Although these can be scrubbed out, the resulting solution is extremely corrosive to most metals and fiberglass
The most common solution is the use of FRP, but with the substitution of synthetic veils for the inner glass lining The FRP resin itself is not affected by the HF, but the internal glass could be A small pinhole leading to the glass would result
in a catastrophic attack This is avoided by substituting a synthetic veil for the glass Fiberglass is used throughout the industry as a standard Polypropylene, PVC, and similar thermoplastics are also used Nitrates and urea products are typically handled by using type 304 stainless steel As noted above, concentrated HNO3 could attack FRP-type materials
Lime Kiln and Similar Kiln Operations Lime kilns are found in several applications, including the pulp and the
paper industry Lime and other kiln applications result in a hot gas that contains dust
Most kilns use a pollution control system When possible, a dry collection system is used, because it allows the material
to be collected in a form that can be returned directly to the kiln Some products are simply too reactive for this technique,
or the temperature of the kiln is too high Wet scrubbers are then used
Most of the scrubbers on kilns are manufactured of carbon steel The problem of corrosion resistance is usually minimal because the solutions tend to be alkaline or at least neutral The primary problem is usually abrasion resistance The basic collectors are manufactured of carbon steel, and high-wear areas are often made of stainless steel Some units use heat-treated stainless steel, which is hardened for the wear-resistant areas Another technique is to install liner to protect areas
of greater wear Fiberglass-reinforced plastic is typically not used in this application
Pulp and Paper Industry Most of the pollution control problems in the pulp and paper industry consist of either the
organic sulfur compounds produced from digesting the pulp or the chlorine-related oxidizing agents produced from bleaching the pulp The reduced sulfur compounds are generally handled in FRP construction Temperature limitations are not normally a factor, because most of these applications are at temperatures of 80 °C (175 °F) or less Chlorine or chlorine dioxide applications can be handled by materials of construction similar to those discussed in the section
"Chloro-Alkali Plants." More information on materials of construction for the pulp and paper industry is available in the article "Corrosion in the Pulp and Paper Industry" in this Volume
5 R.W Kirchner, Corrosion of Pollution Control Equipment, Chem Eng Prog., Vol 71 (No 3), 1975, p 58-63
6 H.D Rice, Jr and R.A Burford, "Corrosion of Gas-Scrubbing Equipment in Municipal Refuse Incinerators," Paper presented at the International Corrosion Forum, National Association of Corrosion Engineers, 19-23 March 1973
Selected References
• G.L Crow and H.R Horsman, Corrosion in Lime/Limestone Slurry Scrubbers for Coal-Fired Boiler
Flue Gases, Mater Perform., July 1981, p 35-45
• T.G Gleason, How to Avoid Scrubbers Corrosion, Chem Eng Prog., Vol 71 (No.3), 1975, p 43-47
• E.C Hoxie and G.W Tuffnell, A Summary of INCO Corrosion Tests in Power Plant Flue Gas
Trang 16Scrubbing Processes, in Resolving Corrosion Problems in Air Pollution Control Equipment, National
Association of Corrosion Engineers, 1976, p 65-71
• T.S Lee and R.O Lewis, Evaluation of Corrosion Behavior of Materials in a Model SO2 Scrubber
System, Mater Perform., May 1985, p 25-32
• T.S Lee and B.S Phull, "Use of a Model Limestone SO2 Scrubber to Evaluate Slurry Chloride Level
Effects on Corrosion Behavior," Paper presented at the APCA/IGCI/NACE Symposium on Solving Problems in Air Pollution Control Equipment, Orlando, FL, Dec 1984
• B.S Phull and T.S Lee, " The Effect of Fly Ash and Fluoride on Corrosion Behavior in a Model SO2
Scrubber," Paper presented at the International Corrosion Engineers, 25-29 March 1985
• S.L Sakol and R.A Schwartz, Construction Materials for Wet Scrubbers, Chem Eng Prog., Vol 70
(No 8), 1974, p 63-68
Trang 17Corrosion Rate Conversion Guide
Introduction
CORROSION RATE is the corrosion effect on a metal (change or deterioration) per unit of time The type of corrosion rate used depends on the technical system and on the type of corrosion effect Thus, corrosion rate may be expressed as an increase in corrosion depth per unit of time (penetration rate, for example, mils/yr) or the mass of metal turned into corrosion products per unit area of surface per unit of time (weight loss, for example, g/m2/d) The corrosion effect may vary with time and may not be the same at all points of the corroding surface Therefore, reports of corrosion rates should
be accompanied by information on the type, time dependency, and location of the corrosion effect
Conversion of Corrosion Rates
Table 1 provides factors for converting among units commonly used for expressing corrosion rates Table 2 is a nomograph for conversion of corrosion rates
Table 1 Relationships among some of the units commonly used for corrosion rates
d is metal density in grams per cubic centimeter (g/cm3)
Factor for conversion to Unit
mdd g/m 2 /d μm/yr mm/yr mils/yr in./yr
Milligrams per square decimeter per day (mdd) 1 0.1 36.5/d 0.0365/d 1.144/d 0.00144/d
Grams per square meter per day (g/m2/d 10 1 365/d 0.365/d 14.4/d 0.0144/d
Microns per year (μm/yr) 0.0274d 0.00274d 1 0.001 0.0394 0.0000394
Millimeters per year (mm/yr) 27.4d 2.74d 1000 1 39.4 0.0394
Mils per year (mils/yr) 0.696d 0.0696d 25.4 0.0254 1 0.001
Inches per year (in./yr) 696d 69.6d 25,400 25.4 1000 1
Source: G Wranglén, An Introduction to Corrosion and Protection of Metals, Chapman and Hall, 1985, p 238
Trang 18Table 2 Nomograph for conversion of corrosion rates.
The example given is for type 304 stainless steel (density 7.87 g/cm3) and a corrosion rate of 30 mils/yr
• Method designed to approximate, in a short time, the deteriorating effect under normal long-term service conditions
• Atmospheric precipitation with a pH below 5.6 to 5.7 Burning of fossil fuels for heat and power
is the major factor in the generation of oxides of nitrogen and sulfur, which are converted into nitric and sulfuric acids washed down in the rain See also atmospheric corrosion
Trang 19• A highly substructured non-equiaxed ferrite formed upon continuous cooling by a mixed diffusion and shear mode of transformation that begins at a temperature slightly higher than the transformation temperature range for upper bainite It is distinguished from bainite in that it has a limited amount of carbon available; thus, there is only a small amount of carbide present
• See differential aeration cell
• Composite wrought product comprised of an aluminum alloy core having on one or both surfaces
a metallurgically bonded aluminum or aluminum alloy coating that is anodic to the core and thus electrochemically protects the core against corrosion
• A metal in group IA of the periodic system namely, lithium, sodium, potassium, rubidium, cesium, and francium They form strongly alkaline hydroxides, hence the name
Trang 20• (1) Having properties of an alkali (2) Having a pH greater than 7
• A material blended from alkali hydroxides and such alkaline salts as borates, carbonates, phosphates, or silicates The cleaning action may be enhanced by the addition of surface-active agents and special solvents
• (1) Pronounced wide cracking over the entire surface of a coating having the appearance of alligator hide (2) The longitudinal splitting of flat slabs in plane parallel to the rolled surface Also called fish-mouthing
• The electrode of an electrolyte cell at which oxidation occurs Electrons flow away from the anode in the external circuit It is usually at the electrode that corrosion occurs and metal ions enter solution Contrast with cathode
Trang 21• anode corrosion
• The dissolution of a metal acting as an anode
• The ratio of the actual corrosion (weight loss) of an anode to the theoretical corrosion (weight loss) calculated by Faraday's law from the quantity of electricity that has passed
• The effect produced by polarization of the anode in electrolysis It is characterized by a sudden increase in voltage and corresponding decrease in amperage due to the anode becoming virtually separated from the electrolyte by a gas film
Trang 22• Forming austenite by heating a ferrous alloy into the transformation range (partial austenitizing)
or above the transformation range (complete austenitizing) When used without qualification, the term implies complete austenitizing
• In electroplating, a supplementary anode positioned so as to raise the current density on a certain area of the cathode and thus obtain better distribution of plating
• An electrode commonly used in polarization studies to pass current to or from a test electrode It
is usually made from a noncorroding material
• B
• Material placed in a drilled hole to fill space around anodes, vent pipe, and buried components of
a cathodic protection system
• A metastable aggregate of ferrite and cementite resulting from the transformation of austenite at
temperatures below the pearlite range but above Ms, the martensite start temperature Bainite formed in the upper part of the bainite transformation range has a feathery appearance; bainite formed in the lower part of the range has an acicular appearance resembling that of tempered martensite
• Macroscopic progression marks on a fatigue fracture or stress-corrosion cracking surface that indicate successive positions of the advancing crack front The classic appearance is of irregular elliptical or semielliptical rings, radiating outward from one or more origins Beach marks (also known as clamshell marks or arrest marks) are typically found on service fractures where the part
is loaded randomly, intermittently, or with periodic variations in mean stress or alternating stress See also striation
• An electrode in an electrolytic cell that is not mechanically connected to the power supply, but is
so placed in the electrolyte, between the anode and cathode , that the part nearer the anode becomes cathodic and the part nearer the cathode becomes anodic Also called intermediate electrode
Trang 23• (1) Injection of air or water under high pressure through a tube to the anode area for the purpose
of purging the annular space and possibly correcting high resistance caused by gas blocking (2)
In connection with boilers or cooling towers, the process of discharging a significant portion of the aqueous solution in order to remove accumulated salts, deposits, and other impurities
• Brittleness exhibited by some steels after being heated to a temperature within the range of about
200 to 370 °C (400 to 700 °F), particularly if the steel is worked at the elevated temperature
• (1) Permanently damaging a metal or alloy by heating to cause either incipient melting or intergranular oxidation See also overheating (2) In grinding, getting the work hot enough to cause discoloration or to change the microstructure by tempering or hardening
• C
• A layer consisting of a mixture of calcium carbonate and magnesium hydroxide deposited on surfaces being cathodically protected because of the increased pH adjacent to the protected surface
• An electrode widely used as a reference electrode of known potential in electrometric measurement of acidity and alkalinity, corrosion studies, voltammetry, and measurement of the potentials of other electrodes See also electrode potential , reference electrode , and saturated calomel electrode
• Absorption and diffusion of carbon into solid ferrous alloys by heating, to a temperature usually above Ac3, in contact with a suitable carbonaceous material A form of case hardening that produces a carbon gradient extending inward from the surface, enabling the surface layer to be hardened either by quenching directly from the carburizing temperature or by cooling to room temperature, then reaustenitizing and quenching
Trang 24hardening; cyaniding; nitriding; and carbonitriding The use of the applicable specific process name is preferred
of elements from a higher to a lower valence state Contrast with anode
• The formation and instantenous collapse of innumerable tiny voids or cavities within a liquid subjected to rapid and intense pressure changes Cavitation produced by ultrasonic radiation is sometimes used to effect violent localized agitation Cavitation caused by severe turbulent flow often leads to cavitation damage
Trang 25• cavitation corrosion
• A process involving conjoint corrosion and cavitation
• The degradation of a solid body resulting from its exposure to cavitation This may include loss
of material, surface deformation, or changes in properties or appearance
• (1) A molecular structure in which a heterocyclic ring can be formed by the unshared electrons of neighboring atoms (2) A coordination compound in which a heterocyclic ring is formed by a metal bound to two atoms of the associated ligand See also complexation
• (1) An organic compound in which atoms form more than one coordinate bond with metals in solution (2) A substance used in metal finishing to control or eliminate certain metallic ions present in undesirable quantities
• A chemical process involving formation of a heterocyclic ring compound that contains at least one metal cation or hydrogen ion in the ring
• A protective or decorative nonmetallic coating produced in situ by chemical reaction of a metal
with a chosen environment It is often used to prepare the surface prior to the application of an organic coating
• In a thermodynamic system of several constituents, the rate of change of the Gibbs function of the system with respect to the change in the number of moles of a particular constituent
• A coating process, similar to gas carburizing and carbonitriding, whereby a reactant atmosphere gas is fed into a processing chamber where it decomposes at the surface of the workpiece, liberating one material for either absorption by, or accumulation on, the workpiece A second material is liberated in gas form and is removed from the processing chamber, along with excess atmosphere gas
Trang 26parts whose widths are considerably greater than their thicknesses The points of the chevrons can be traced back to the fracture origin
• Improving paint adhesion on aluminum or aluminum alloys, mainly aricraft skins, by treatment with a solution of chromic acid Also called chromodizing or chromatizing Not to be confused with chromating or chromizing
• A type of weld cracking that usually occurs below 205 °C (400 °F) Cracking may occur during
or after cooling to room temperature, sometimes with a considerable time delay Three factors combine to produce cold cracks: stress (for example, from thermal expansion and contraction), hydrogen (from hydrogen-containing welding consumables), and a susceptible microstructure (plate martensite is most susceptible to cracking, ferritic and bainitic structures least susceptible) See also hot cracking , lamellar tearing , and stress-relief cracking
• Deforming metal plastically under conditions of temperature and strain rate that induce strain hardening Usually, but not necessarily, conducted at room temperature Contrast with hot working
• A stress that causes an elastic body to deform (shorten) in the direction of the applied load Contrast with tensile stress
Trang 27• An electrolytic cell , the electromotive force of which is caused by a difference in concentration
of some component in the electrolyte This difference leads to the formation of discrete cathode and anode regions
• A compound with a central atom or ion bound to a group of ions or molecules surrounding it Also called coordination complex See also chelate , complexation , and ligand
• An accelerated corrosion test for some electrodeposits and for anodic coatings on aluminum
• The maximum repeated stress that can be endured by metal without failure under definite conditions of corrosion and fatigue and for a specific number of stress cycles and a specified period of time