Corrosion of Ceramic and Composite Materials Part 14 pdf

Preoxidation under some conditions can form a protective oxide layer that will minimize or possibly eliminate continued corrosion [9.7].. In addition, impurities present, generally in th

vaporization of the water to extract heat from the refractory.

If the thermal gradient through the material becomes too steep, failure may occur (this depends upon the thermal expansion characteristics of the material)

Another method that has been used to lower the hot-face temperature is to place metal plates either within individual bricks or between them A large portion of the heat is thus conducted through the metal plate A similar technique has been used by manufacturing a product containing oriented graphite particles The steel industry has used many of these techniques in their blast furnaces The most common technique today is the use of water-cooled internal metal plates (or boxes) Another way to take advantage of increased cooling is initially to use a thinner material This will automatically cause

a thinner reaction layer to form on the surface In general, glass furnace basin wall linings should not be greater than 10–

12 in thick Anything greater than about 12 in does not normally increase overall life but adds an economic penalty in refractory cost per campaign The thickness at the flux-line generally is 9 in so that effective air cooling can be used In fact, most linings could probably be less than 10 in.; however, the thermal-mechanical environment will determine the ultimate thickness that should be used

If a refractory lining is insulated, a greater portion of the refractory will be at a higher temperature and corrosion will proceed at a faster rate In these cases, a balance must be obtained between service life and energy conservation Because

of the potential for increased corrosion of insulated linings, the properties of the lining material must be carefully evaluated before insulation is installed In many cases, the engineer may want to upgrade the lining material if it is to be insulated Historically, it has been recommended that flue gas temperatures be 20–30°C higher than the dew point [9.4] However, because of thermal efficiencies and the related cost, this has been lowered to 5–10°C higher than the dew point Once condensation has occurred, reevaporation of the water

or other volatile can concentrate corrosive species causing a

more severe corrosion problem; thus condensates should be removed as rapidly as possible

The addition of redox couples in photoelectrochemical corrosion of electronically conductive materials in acids acts

on the environment to minimize corrosion An example is the addition of cobalt as the redox couple to scavenge SO4- that is formed by the reaction of a positive hole with the sulfate ion [9.5] The positive hole is photogenerated in the valence band

of an illuminated titania semiconductor The reactions listed below act to minimize corrosion:

(9.2)

In the preservation of national monuments, engineers have tried various materials to fill the porous limestones, marbles, etc to slow corrosion Not only are the fillers used to eliminate open porosity, but also they are used to consolidate or strengthen friable portions of the structure The most important parameter

of the fillers is that they too must be corrosion-resistant Many materials have been used to provide this filler/strengthening characteristic from waxes to acrylic polymers to silicic ester-type products [9.6] According to Amoroso and Fassina [9.6], these materials must have the following basic characteristics:

1 Control the diffusion of water

2 Protect against atmospheric pollutants

3 Possess a low coefficient of thermal expansion

4 Be inert toward the parent structure

5 Not modify the original appearance

Although the preservation of national monuments around the world has been in progress for over 100 years, it has been only recently that true advances have been made in their preservation At first, it would seem that the preservation of monuments is not very much like the slowing or elimination

of corrosion of ceramics; however, the two areas are very much

(9 )1

alike The major difference is that for the monuments, one is concerned with their final appearance, whereas with ceramics,

in general, that is not the case

9.3 CRYSTALLINE MATERIALS—NONOXIDES

9.3.1 Property Improvement

Most of the items discussed earlier can also be applied to these materials The one property improvement that should be discussed a little further is that of porosity For example, Si3N4

is predominantly covalent and does not densify on heating as

do conventional ionic ceramics In applications such as turbine blades, a theoretically dense material is desired Only through special densification procedures can theoretically dense materials be obtained In the past, this could be accomplished for Si3N4 only through hot pressing with large amounts (up to

10 wt.%) of additives at very high temperatures and pressures SiC, in contrast, could be prepared in the fully dense state with only a few percent of additives Newer techniques have recently been developed using gas pressure sintering and much lower amounts of additives that allow the production of materials that are fully dense The additives in these processes cause a liquid phase to form at high temperatures, and therefore densification can proceed through liquid-phase sintering This liquid either crystallizes or forms a glass phase upon cooling Much work has been done in attempting to obtain either crystalline phases with higher melting points or glassy compositions with higher viscosities to improve the high-temperature properties The densification processes using lower amounts of additives (generally <2 wt.%) help to maximize the high-temperature properties

Improved corrosion resistance of porous materials can be obtained by impregnating with either a material of the same composition as the bulk or with a material that, in the case of SiC or Si3N4, is later exposed to a carbiding or nitriding treatment Other pore-filling materials can also be used, such

as nitrates or oxychlorides Decomposition reactions then produce pore-filling oxides Impregnation with organosilicon compounds will yield SiC as the pore filler

Corrosion resistance can sometimes be improved by changing the processing method Chemical vapor deposition (CVD) is one of the most attractive methods to produce high purity dense materials because the sintering process is not required if a bulk material can be obtained directly from the raw vapors or gases Microstructures of CVD products are strongly dependent upon the deposition temperature and total gas pressure Chemical vapor deposition can produce materials with no grain boundary phases but which are highly oriented It is a well-known fact that CVD materials contain residual internal stresses At present, the effects of these stresses upon high-temperature strength and corrosion are not well known

Preoxidation under some conditions can form a protective oxide layer that will minimize or possibly eliminate continued corrosion [9.7] In addition, impurities present, generally in the form of sintering aids, may migrate toward the surface and become part of the protective oxide layer This layer can then be removed resulting in a purer material with subsequent improvement in mechanical properties

The development of nitride-based materials today has progressed to the point of studying materials in SiaMbOcNd systems, where M has been confined mostly to trivalent cations Most work has been in systems where M=Al, Y, and/or Be These materials form secondary grain boundary phases which are highly oxidation-resistant and thus provide a better material than conventional Si3N4 materials

Cemented carbide cutting tools made from WC wear rapidly due to local welding of the tool to the steel piece being cut To overcome this welding, additions of TiC were made to the WC

to form a TiO2 surface layer that protected the tool from rapid wear WO3 also formed, but it was volatile and produced no protective layer In addition, small amounts of TaC and NbC were added to increase the overall oxidation resistance by

increasing the melting temperature of the carbide solution formed

9.3.2 External Methods of Improvement

One method of minimizing corrosion not widely practiced is that of coating the ceramic with a layer of more resistant material Probably the best method to coat a ceramic is by a layer of CVD [9.8] or plasma-sprayed material of the same composition as the substrate [9.9] Chemical vapor deposition,

in general, provides a better coating than plasma-sprayed coatings since it is difficult to form pore-free coatings with uniform thickness using plasma spraying This provides a well-attached, pure, nonporous layer that has a good thermal expansion match with the substrate Coating conditions can

be varied to produce layers of amorphous material covered by crystalline material of the same composition This sometimes provides a more complex diffusion path that minimizes oxidation

Although plasma or flame spraying can be used to deposit most materials, control of the spraying parameters confines the coating to mainly oxides Other methods investigated have been cathode sputtering [9.8,9.10], glow-discharge cathode sputtering, electron beam evaporation, and detonation deposition These methods are not necessarily confined to the coating of nonoxides; oxides can also be coated

Wittmer and Temuri [9.11] in their work on oxidation of carbon-carbon composites have described a method of protection by coating first with a well-adhering solid oxygen barrier and then coating with a glass-forming material to seal any cracks that may develop from thermal expansion mismatch The carbon-carbon composite coating system used for the Space Shuttle nose cap is composed of an inner SiC layer covered by

a silicate glaze This is probably the most successful example

of the use of oxygen barrier coatings to protect carbon-carbon composites [9.12]

9.4 GLASSY MATERIALS

9.4.1 Property Optimization

The development of more resistant glasses has been predominantly through optimization of compositions Historically, small amounts

of alumina have been added to the basic soda-lime-silicate composition to improve durability In general, lowering the alkali content increases the durability This, however, has practical limits based upon melting temperatures, viscosities, softening points, and working ranges Borosilicate glasses are, in general, more are less resistant to alkali solutions than they are to acid solutions

of varying compositions

One technique of composition variation to improve durability that has not received much attention is that of incorporation of nitrogen into the glass structure Frischat and Sebastian [9.13] have shown that soda-lime-silica glasses containing 1.1 wt.% nitrogen exhibited considerable improvement toward leaching

by water at 60°C over compositions containing no nitrogen This improvement was attributed to a denser structure for the nitrogen-containing glass

Small changes in the chemistry of the glass can cause a significant change in the dissolution mechanism as shown by Lehman and Greenhut [9.14] They reported that 1 mol% P2O5 addition to a lead silicate glass caused the formation of lead phosphosilicate crystals on the glass surface when exposed to 1% acetic acid at 22°C They attributed the reduction in dissolution to the reduction

of the apparent average interdiffusion coefficient of lead by a factor

of 11.3 This is an example of changing the material chemistry to form an interface reaction product that reduces the diffusion rate

of the species being leached

9.4.2 External Methods of Improvement

The development of coating technology has provided a means

to improve corrosion resistance, abrasion resistance, and resistant than soda-lime silicate glasses In general, silicate glasses Table 6.1 of Chap 6 lists the corrosion resistance of many glasses

strength Combinations of coatings applied while the glass is hot and after it has cooled have been developed that form a permanent bond to the glass These coatings are not removed

by cooking or washing

The most commonly used metallic hot-end coatings are tin and titanium As the piece goes through the annealing lehr, the metal oxidizes, forming a highly protective ceramic coating Tin is easier to work with since a thicker coating can be applied before problems of iridescence occur These hot-end metallic coatings give the glass a high glass-to-glass sliding friction and thus a cold-end coating must be applied over these metallic coatings The cold-end coatings usually have a polyethylene

or fatty acid base

Another type of coating is one that reacts with the surface

of the glass to form a surface layer that is more corrosion-resistant than the bulk composition Chemically inert containers are needed to contain various beverages and pharmaceuticals

To provide increased corrosion resistance, these containers are coated internally to tie up the leachable components Internal treatment with a fluoride gas provides a new surface that is more corrosion-resistant than the original and is more economical than the older sulfur treatment

Although not a true coating technique, the manufacturers of flat glass have, for many years, treated the surface of their glass with SO2 gas just prior to the glass being annealed to increase the weatherability of their products This surface treatment allows the sodium in the surface layers to react with the SO2 forming sodium sulfate The sulfate deposit that forms

on the surface due to this reaction is then washed off prior to inspection and packing The first step in weathering is then diminished due to the low alkali content of the surface

It has been shown by Harvey and Litke [9.15] that matrix dissolution of an aluminosilicate glass apparently does not occur if the leaching solution is saturated first with solution products of the same glass composition This technique is an example of how dissolution can be minimized by decreasing the driving force for corrosion by lowering the concentration

gradient between the material and leachant, thus minimizing

or eliminating the diffusion of cations and anions across the interfacial boundary Using a different approach to minimize dissolution of a predominantly soda-borosilicate glass, Buckwalter and Pederson [9.16] have shown that the sorption

of metal ions onto the glass surface and/or the buffering of the leachate solution caused by the corrosion of metal containers significantly lowered the rate of aqueous corrosion

REFERENCES

9.1 McCauley, R.A Evolution of flat glass furnace regenerators.

Glass Ind 1978, 59 (10), 26–28, 34.

9.2 Brezny, R.; Semler, C.E Oxidation and diffusion in selected pitch-bonded magnesia refractories J Am Ceram Soc 1984,

67 (7), 480–483.

9.3 Siljan, O.-J.; Rian, G.; Pettersen, D.T.; Solheim, A.; Schøning,

C Refractories for molten aluminum contact Part I:

thermodynamics and kinetics Refract Appl News 2002, 7

(6), 17–25.

mechanisms and solutions In Dewpoint Corrosion;

Holmes, D.R., Ed.; Ellis Horwood Ltd.: Chichester, UK,

1985 Chapter 2.

9.5 Harris, L.A.; Cross, D.R.; Gerstner, M.E Corrosion suppression on rutile anodes by high energy redox reactions.

J Electrochem Soc 1977, 124 (6), 839–844.

9.6. Amoroso, G.G.; Fassina, V Stone Decay and Conservation;

Elsevier: Amsterdam, 1983; 453 pp.

9.7 Lange, F.F.; Davis, B.I.; Metcalf, M.G Strengthening of polyphase Si 3 N 4 materials through oxidation J Mater Sci.

1983, 18 (5), 1497–1505.

9.8 Davies, G.B.; Holmes, T.M.; Gregory, O.J Hot corrosion behavior of coated covalent ceramics Adv Ceram Mater.

1988, 3 (6), 542–547.

9.9 Gogotsi, Yu.G.; Lavrenko, V.A Corrosion protection and

development of corrosion-resistant ceramics Corrosion of

High-Performance Ceramics; Springer-Verlag: Berlin, 1992;

151–162 Chp 7.

9.10 Gregory, O.J.; Richman, M.H Thermal oxidation of sputter-coated reaction-bonded silicon nitride J Am Ceram Soc.

1984, 67 (5), 335–340.

9.11 Wittmer, D.E.; Temuri, M.Z Thermochemical studies in selected metal-carbon-oxygen systems J Am Ceram Soc.

1991, 74 (5), 973–982.

9.12 Strife, J.R Fundamentals of protective coating strategies

for carbon-carbon composites In Damage and Oxidation

Protection in High Temperature Composites; Haritos,

G.K., Ochoa, O.O., Eds.; ASME: New York, 1991; Vol.

1, 121–127.

9.13 Frischat, G.H.; Sebastian, K Leach resistance of nitrogen-containing Na 2 O–CaO–SiO 2 glasses J Am Ceram Soc.

1985, 68 (11), C305-C307.

9.14 Lehman, R.L.; Greenhut, V.A Surface crystal formation during acid corrosion of phosphate-doped lead silicate glass.

J Am Ceram Soc 1982, 65 (9), 410–414.

9.15 Harvey, K.B.; Litke, C.D Model for leaching behavior of aluminosilicate glasses developed as matrices for immobilizing high-level wastes J Am Ceram Soc 1984,

67 (8), 553–556.

9.16 Buckwalter, C.Q.; Pederson, L.R Inhibition of nuclear waste glass leaching by chemisorption J Am Ceram Soc 1982,

65 (9), 431–436.

Cor·rode, v.t to eat into or wear away gradually, as by rusting or by the action of chemicals

WEBSTER’S NEW WORLD

DICTIONARY

Alteration The change or modification of a material through

interaction with its environment, generally by the formation

of a new phase This reaction need not be deleterious

Atmospheric Corrosion The degradation of materials by

natural atmospheric environments Atmospheric corrosion is

a term often used by the metallurgist, whereas an equivalent

term used by ceramists is weathering.