2.15 Corrosion of Ceramic Materials

2.15Corrosion of Ceramic MaterialsMathias Herrmann and Hagen Klemm,Fraunhofer-Institut für Keramische Technologien und Systeme IKTS,(www.ikts.fraunhofer.de)Dresden,GermanyÓ2014Elsevier Ltd.All rights reserved.2.15.1Introduction 4132.15.2Corrosion in Gases 4152.15.2.1Oxidation 4162.15.2.2Reactions in Humid Atmospheres 4212.15.3Corrosion in Aqueous Solutions 4232.15.3.1Basic Processes 4232.15.3.2Corrosion of Different Ceramics in Aqueous Solutions 4272.15.4Final Remarks 440References 4412.15.1IntroductionThe term corrosion is de fined as localized or large-scale reaction of a material with the surrounding media,resulting in degradation or even in destruction of the material or component.Corrosion behavior is therefore a property not of the material,but of the system including the material,the media and conditions such as temperature,pressure,medium type and volume or flow rate,and tribological as well as mechanical loads.Therefore,a simple statement concerning corrosion resistance of a material cannot be given because the material may be stable under one set of conditions,but unstable under another set of conditions (Jacobson,1986;Klemm,2010;Nickel &Gogotsi,2000;Nickel,Quirmbach,&Pötschke,2010;Petzow &Herrmann,2002;Schmalzried &Schwetz,2010).Fundamentally two distinctly different types of corrosion can occur,namely,active or passive.If the corrosion products are removed from the surface,i.e.if species with high vapor pressures or compo-nents soluble in the corrosion media are formed,the corrosion reaction is characterized by removal of the corrosion products and hence reduction in size and mass of the ceramic material.This type of corrosion is called active corrosion .Active corrosion is usually characterized by a linear time dependence of the mass loss or the decrease in dimensions (Figure 1(a)).D x ¼Kt (1)where x is the mass or dimensions and K ,the rate constant.If the reaction product of the corrosion reaction is a solid or a high-viscosity liquid (e.g.a silicate)that is insoluble in the corrosion media,the corrosion is often controlled by the properties of the formed surface layer.If the surface layer is dense and the rate of diffusion of the reactants or reaction products through this layer is low,the formed layer acts as a corrosion barrier and the corrosion is controlled by this barrier.This case is called passive corrosion (Figure 1(b)).For times longer than the time necessary for the formation of the passive layer,the change in mass or di-mensions is proportional to the square root of time:D x ¼k ffiffit p þC 1;(2)where D x is the thickness of the layer and the constant k is the rate constant,C 1includes the processes up to formation of a stable layer.The layer thickness and mass change can be recalculated using the equation of the chemical reaction and the component densities.The equation has the physical meaning that the diffusion through the barrier is rate-controlling and as a result,the thickness of the barrier increases with ffiffit p .More complicated equations have been derived for short reaction times (formation of the stable oxide layer)(Nickel et al.,2010;Nickel &Gogotsi,2000).The above equation is valid for steady-state systems,i.e.in which the material surfaces and surface layer properties can be considered to be constant.If any of these parameters is Comprehensive Hard Materials,Volume 2/10.1016/B978-0-08-096527-7.00034-9413Figure 1Schematics of different types of corrosion:(a)active (b)and (c)passive with amorphous and crystallized oxide layer respectively,in silicon carbide ceramics (d)paralinear (active)behavior in boron carbide ceramics.414Corrosion of Ceramic Materialschanged,more complicated equations must be used to describe the process (Nickel et al.,2010;Nickel &Gogotsi,2000).It was shown that if the layer crystallizes and the diffusion through the crystals is slow,the equation changes to D x ¼A þB log (t ),where A and B are constants (Figure 1(c)).The active and passive corrosion mechanisms are shown schematically in Figure 1for the oxidation of SiC.A detailed derivation of the equations depending on the mechanism and the sample geometry is given in the literature (Hou &Chou,2009;Nickel &Gogotsi,2000;Nickel et al.,2010;Persson &Nygren,1994;Herrmann,2013).Besides large-scale homogeneous corrosion,more intense corrosion can take place locally (pitting).Pitting can be caused by inhomogeneities in the ceramic (pores,cracks or segregated secondary phases with lower corrosion resistivity)or by locally damaged corrosion layers (through gas bubble formation or impurities from the corroding atmospheres).Pitting is mainly responsible for the decrease in strength of ceramics due to corrosion (Figure 2).A material ’s corrosion behavior is often distinguished by the surrounding media:lCorrosion in gases l Corrosion in liquid media.This classi fication is based on the differences in media,transport mechanisms and experimental methods of investigation.Gas corrosion often occurs at higher temperatures and plays a signi ficant role in high-temperature applications,especially for nonoxide materials.Water-or organic-based systems play an important role in industrial processes in which ceramics and hard materials are used,e.g.as seals,bearings or valves.These processes take place near ambient temperature.Nevertheless,if the interaction with metal or glass melts is taken into account,high temperatures are also involved.However,corrosion in these systems is very speci fic and therefore beyond the scope of this chapter.This chapter is focused on corrosion in air,combustion gases and water vapor-containing gases as well as in acidic and basic solutions and under hy-drothermal conditions.2.15.2Corrosion in GasesThis chapter concentrates on corrosion in air,combustion gases and water vapor-containing gases due to the practical importance of these processes.Corrosion in halogenides and SO 2/SO 3is not treated (see Jacobson &Fox,1988;Jacobson,1993;Marra,Kreidler,Jacobson,&Fox,1988;Nickel &Gogotsi,2000;Nickel et al.,2010;Presser,Heon,&Gogotsi,2011;Readey,1998;Van der Biest,Barnes,Corish,&Norton,1987).Presser and co-workers (Presser et al.,2011)provide an overview of the reaction of different carbides with Cl 2.The halo-genides d e.g.Cl 2and HCl d extract the metal from the carbon,leaving a porous carbon material.The goal of the investigations of the interaction was to gain an understanding of the process of formation of porous carbon (carbon derived from carbides,or CDC materials),not of the corrosion behavior.For unknown systems,an initial estimate of the corrosion behavior can be made by means of the gas reactions and the associated thermodynamic data.A corrosion reaction generally becomes possible when the Gibbs free energy (D G reaction )of the corresponding reaction is negative.If condensed phases are formed,passive corrosion can occur depending on the properties of the formed phases.Active corrosion is likely if the thermodynamic calculations predict the formation of gaseous phases only.In this case,the reaction can also take place if D G reaction is positive.This only means that the partial pressure of the gaseous product hasaFigure 2Pits formed in Si 3N 4ceramics during oxidation (Klemm,Taut,&Wötting,2003).Corrosion of Ceramic Materials 415partial pressure of <1atm.For example,under gas turbine conditions a partial pressure of the reaction products of 10À7MPa results in a degradation rate of Si 3N 4ceramics of approximately 400m m per 1000h (Fritsch,2008).A 100times higher pressure would result in approximately 100times higher recession rate.For gas reactions (active corrosion),the lower the partial pressure of the reaction products,the lower the corrosion rate.The basic principles of the interaction of nonoxide ceramics with air or combustion gases will be outlined in the following chapter using the examples of SiC and Si 3N 4ceramics due to the abundance of studies performed on them and their predominance in high-temperature applications.Table 1gives a summary of important literature in which the behavior of the materials is explained in detail.Presser and Nickel (2008)gives a comprehensive overview concerning the oxidation of SiC materials and single crystals.2.15.2.1OxidationAll nonoxide ceramics are thermodynamically unstable in air,exhibiting passive oxidation behavior up to high temperatures.Therefore,their stability is connected with the stability of the oxide layer and diffusion of theTable 1Overview of corrosion data for nonoxide ceramics (EN12923,2006)Material Mechanism/Remarks LiteratureOverview of corrosion of ceramics Jacobson,1993;Nickel &Gogotsi,2000;Nickel et al.,2010;Opila,2003Testing procedures Advanced,2006;dos Santos e Lucato,Sudre,&Marshall,2011;Fritsch,2008;Jacobson,1993;Nickel &Gogotsi,2000;Nickel et al.,2010;Opila,2003Si 3N 4Passive oxidation,in fluence of material properties Backhaus-Ricoult,Guerin,&Urbanovich,2002;Klemm et al.,2003;Klemm,2002,2010;Klemm,Herrmann,&Schubert,1998;Nickel &Gogotsi,2000;Nickel et al.,2010;Petzow &Herrmann,2002Gas turbine conditions Fritsch,2008;Fritsch,Klemm,Herrmann,Michaelis,&Schenk,2010;Klemm,2002;Klemm,2010;Opila,2003;Parthasarathy,Rapp,Opeka,&Kerans,2009SiC Passive/transition from passive to active Balat,1996;Balat,Berjoan,Pichelin,&Rochmann,1998;Charpentier,Balat-Pichelin,&Audubert,2010;Charpentier,Balat-Pichelin,Glénat,et al.,2010;Charpentier,Maître,Balat-Pichelin,et al.,2009;Costello &Tressler,1986;Courtright,1991;Dawi,Balat-Pichelin,Charpentier,et al.,2012;dos Santos e Lucato et al.,2011;Heuer&Lou,1990;Kim &Moorhead,1990a,1990b;More,Tortorelli,Walker,et al.,2003;Narushima,Goto,Yokoyama,et al.,1994a,1994b;Narushima,Goto,Yokoyama,Iguchi,&Hirai,1993;Nickel &Gogotsi,2000;Nickel et al.,2010;Opila &Serra,2011;Osada,Nakao,Takahashi,&Ando,2009;Presser &Nickel,2008;Presser,Loges,Hemberger,et al.,2009;Presser,Loges,Wirth,et al.,2009;Ramberg &Worrell,2001;Schmalzried &Schwetz,2010;Schneider,Guette,Naslain,et al.,1998;Vaughn &Maahs,1990;Wang,Zhang,Zeng,Vignoles,&Guette,2008Gas turbine conditions Fritsch et al.,2010;Fritsch,2008;Hisamatsu,Etori,&Yamamoto,2003;Jacobson &Farmer,1999;Jacobson,1993;Nickel &Gogotsi,2000;Opila &Myers,2004;Opila &Serra,2011;Opila,2003;Opila,Smialek,Robinson,Fox,&Jacobson,1999;Roode,2010EBC for Si 3N 4and SiC and oxide ceramics Environmental barrier coatings Fritsch &Klemm,2006;Fritsch &Klemm,2008;Fritsch,2008;Klemm,2002;Klemm,2010;Roode,2010BN/B 4C/borides Passive/active,in fluence of moisture Eichler &Lesniak,2008;Fahrenholtz,Hilmas,Talmy,&Zaykoski,2007;Hu,Zhang,Han,Guang,&Du,2010;Jacobson &Farmer,1999;Nickel &Gogotsi,2000;Nickel et al.,2010;Parthasarathyet al.,2009;Schmalzried &Schwetz,2010;Sciti et al.,2005AlNDutta,Mitra,&Rabenberg,1992416Corrosion of Ceramic MaterialsCorrosion of Ceramic Materials417 oxygen through the formed oxidefilms.At ambient temperatures,nanometer-thick or even thinner oxidefilms are sufficient for stabilizing the nonoxide material(passive oxidation).At higher temperatures,thefilm thickness increases due to faster diffusion and oxidation reactions.The diffusion coefficients of the oxides and the resulting typical parabolic oxidation rates for selected ceramics are given in Figure3.The data reveal that SiO2is superior to other oxides as an oxidation barrier.SiO2has an additional advantage in that it forms amorphous layers up to high temperatures,thus preventing the formation of grain boundaries(with accord-ingly higher diffusion coefficients)in the oxide scale and the destruction of the oxide scale due to the volume change during crystallization.The highly viscous character of the silica in the layer also allows the healing of cracks and,at least above the glass transition temperature,the relaxation of stresses between the oxide layer and the matrix.These simple principles are responsible for the high oxidation resistance of silica-forming systems (SiC,Si3N4and MoSi2)(Figure3).The data also indicate that the oxidation resistance of transition metal carbides,nitrides and borides alike (ZrB2,TiC,ZrC,ZrN,and WC)is poor due to the high oxygen diffusion coefficients of the posites with SiC are often used to improve the oxidation behavior of these materials.SiO2forms as an oxidation product of SiC,followed by a protective layer which includes the oxides or silicates of the other components. This method is used for example to improve the stability of the so-called ultrahigh-temperature materials (Fahrenholtz et al.,2007;Hu et al.,2010;Parthasarathy et al.,2009;Sciti,Brach,&Bellosi,2005).The properties of the protective oxide scales also depend on factors other than the main component of the material.Secondary phases such as those found in liquid phase-sintered materials can react with the oxide scale, usually reducing the viscosity and increasing the diffusion coefficients in the oxide scale.This was investigated in detail for Si3N4materials.The oxidation stability of Si3N4ceramics with different sintering additives at tem-peratures above1200 C was found to increase in the following order:MgO,MgO/Al2O3,MgO/R2O3<R2O3/Al2O3<<R2O3(R¼rare-earth metal)(Petzow&Herrmann,2002).Oxidation at high temperatures does not take place only at the surface of the component if the diffusion along the grain boundaries is faster than the oxidation reaction.This is the case in liquid phase-sintered ma-terials that have amorphous grain boundaries that allow relatively fast process of oxygen transport into the bulk (Klemm et al.,1998;Klemm et al.,2003;Petzow&Herrmann,2002).The differences between the chemical potentials of the components in the surface oxide layer and the grain boundary in the bulk also result in the diffusion of the additives or impurities toward the surface and in diffusion of unreacted oxygen and oxides formed by oxidation into the material.These processes lead to damage in the bulk of the material due to segregation of the grain boundaries and pore formation and ul-timately to degradation of the material(Klemm et al.,2003).This can be alleviated by crystallization of the grain boundaries as well as at least partial crystallization of the formed oxides.The high oxidation resistance and remarkable increase in long-term stability of Si3N4/SiC and Si3N4/MoSi2composite materials result from in situ crystallization of Si2N2O in the near-surface area(Klemm et al.,1998;Klemm,2010).These processes are shown in Figure4.Solid phase-sintered materials such as SiC and Si3N4with minimal additive contents(Si3N4HIPed without additives),have a higher oxidation resistance than the analogous liquid phase-sintered materials due to the low extent of diffusion of oxygen into the bulk of these ceramics,oxidation taking place only on the outer surface (Figure4(a)).Small amounts of alkaline oxide orfluorine impurities strongly reduce the viscosity of the amorphous grain boundary phase and hence the oxidation and creep resistance in Si3N4materials.Therefore for high-temperature materials,pure raw materials must be used.Impurities in the gas phase can also change the diffusion coefficients in the oxide scale.Alkaline,sulphate and vanadium impurities in combustion gases have a significant negative effect on the oxidation stability of Si3N4and SiC(Jacobson,1993;Klemm,2010;Nickel et al.,2010;Nickel&Gogotsi,2000;Petzow&Herrmann,2002).Si3N4ceramics with nitrogen-rich grain boundary phases exhibit accelerated oxidation,so-called catastrophic oxidation,in the range of900–1100 C due to the absence of a dense oxide layer.Stresses caused by the volume increase during oxidation result in formation of microcracks and ultimately in rapid destruction of the material (Sciti et al.,2005;Petzow&Herrmann,2002).“Pest oxidation”of MoSi2is another example of incomplete for-mation of a protective oxidation scale at intermediate temperatures(electric heaters at temperatures<1000 C).Oxidation of nitrides and carbides also results in gaseous reaction products(N2and CO/CO2respectively).If the diffusion of these gases is much lower than the diffusion of oxygen through the layer,bubbles can be formed in the oxide layer,resulting in locally increased oxidation rates and pitting(Figure2).These pores/bubbles in the oxide layer were found in Si3N4materials at oxidation temperatures above1200–1300 C due to the low4.55.0 5.56.0 6.57.07.58.01.0 x 10–71.0 x 10–81.0 x 10–91.0 x 10–101.0 x 10–111.0 x 10–121.0 x 10–131.0 x 10–14HfO 2 10Y 2O 3P e r m i a b i l i t y , g O 2/(c m s e c )104/Temperature (K –1)Al 2O 3SiO 2Y 2O 3CaZrO 3ZrO 2 10Y 2O 318001600140012001000Temperature (°C)4.0 4.55.0 5.56.0 6.57.07.58.01 x 10 1 x 101 x 101 x 101 x 101 x 10O x i d a t i o n r a t e , (μm 2 h −1)104/Temperature (K –1)AlN Temperature (°C)SiCAl l A N Figure 3Oxygen permeability in oxides (Courtright,1991)and typical parabolic oxidation constants (Courtright,1991;Jacobson &Farmer,1999;Petzow &Herrmann,2002).418Corrosion of Ceramic Materialssolubility and low diffusion rate of the nitrogen formed as a reaction product of oxidation (Backhaus-Ricoult et al.,2002;Jacobson,1986;Klemm,2010;Klemm et al.,1998;).Bubbles can also be formed by impurities with higher oxidation rates (e.g.carbon in SiC)(Nickel et al.,2010;Nickel &Gogotsi,2000;Presser &Nickel,2008;Schmalzried &Schwetz,2010).Oxidation is strongly in fluenced by the porosity in the material.All open pores provide sites for oxidation and hence porous materials oxidize faster than dense materials.Depending on the pore structure and the oxidation rate,small pores can be closed by the oxidation product due to the volume increase usuallyobserved Figure 4SEM micrographs of cross-oxidized sections of at 1500 C.(a)Si 3N 4without sintering additives oxidized for 2500h (b)Si 3N 4material with Y 2O 3/Al 2O 3additives oxidized for 1000h.(c)Si 3N 4material with Y 2O 3and MoSi 2additives oxidized for 2500h.Corrosion of Ceramic Materials 419during oxidation,thus reducing or preventing internal oxidation(oxidation in the bulk).For example,the rate of oxidation of porous reaction-bonded Si3N4and of recrystallized SiC materials decreases with increasing temperature in the range between900and1200 C due to faster pore closure.This process is described in detail in the literature(Jennings,1983).Depending on the oxygen partial pressure and the temperature,the oxide layers can become unstable, resulting in a change from passive to active oxidation.The theory of active oxidation wasfirst developed by Wagner for Si and then modified by different authors for SiC and Si3N4(Table1).The idea behind the theory was that during active oxidation,theflux of the oxygen or water to the surface must be as high as thefluxes of the reaction products away from the surface(Figure1(b)) and the partial pressure of the reaction products must be lower than the equilibrium concentration necessary for the formation of the oxide layer.Based on these ideas,combining thermodynamic equilibrium and transport equations in the gas phase,the oxygen partial pressure at which the change from active to passive oxidation takes place can be calculated as a function of temperature.The transition boundary between active and passive oxidation for Si3N4and SiC in air is shown in Figure5.In the literature,different equilibria determining the critical partial pressure are proposed for SiC,e.g.Heuer and Lou(1990)proposed the condition where SiO2 smoke is formed near the surface:SiOþO20SiO2ðsmokeÞ(3) (For silicon-based ceramics,the main silicon-containing species in the gas phase in the region of active oxidation in dry atmospheres is silicon monoxide(SiO)(Heuer&Lou,1990)).Based on the consideration of zero weight change during oxidation,Wang et al.(2008)proposed another condition,according to the following reaction:SiCþ4=3O252=3SiO2ðcond:Þþ1=3SiOðgÞþCOðgÞ(4)Figure5Dependence of the transition from active to passive oxidation on temperature and oxygen partial pressure in air for Si3N4-and SiC ceramics.420Corrosion of Ceramic MaterialsCorrosion of Ceramic Materials421Nevertheless,all these equations result in similar values for the boundaries(e.g.oxygen partial pressure as a function of temperature).Additional uncertainty is introduced by the fact that the precise values of the diffusion coefficients of the gaseous species and the thickness of the diffusion layer are not known exactly.On the other hand,the experimental data concerning the transition point for the change from active to passive oxidation exhibit a certain amount of scatter,making it difficult for the most precise boundary conditions to be identified (Presser&Nickel,2008).The change in the activity of SiO2in the oxide scale by formation of silica melts due to diffusion of additives into the surface layer(rare earth,Al2O3in Si3N4or LPSSiC materials or impurities in the gas phase)also in-fluences the theoretical boundary(Charpentier et al.,2009;Klemm,2010).For Si3N4-based materials,the boundary depends on both the oxygen partial pressure and the nitrogen pressure in the system(increasing the nitrogen pressure suppresses the active oxidation process).For SiC-based materials,the CO/CO2pressure in the atmosphere has an effect on the boundary.A higher CO pressure sta-bilizes the oxide,thus suppressing the change to active oxidation.Kim and Moorhead(1990a)showed that the strength of SSiC decreases linearly with mass loss during active oxidation.The stability regime of the oxide layer also depends on the gas velocity,with a high gas velocity expanding the active oxidation temperature/pressure region.In addition,water vapor influences the decomposition process.Besides the described region of active oxidation,a second region exists for SiC.The transition from passive to active oxidation takes place at high temperature nearly irrespective of the oxygen pressure if the interaction of the SiO2surface layer with the SiC results in a partial pressure which is higher than or equal to the overall pressure.Therefore,the reactionSiCþ3=2O20SiO2þCO(5) results in the formation of large bubbles in the oxide layer on the surfaces of the SiC components and a transition from passive to active oxidation(active II)(Jacobson,1993;Narushima et al.,1994b)(Figure5).The same mechanism is also known for Si3N4ceramicsAbove the oxide scale decomposition temperature,simple decomposition and evaporation reactions of SiC into Si and carbon and of Si3N4materials into Si and N can also take place.The same active oxidation mechanism described here is also active in humid atmospheres,especially in combustion atmospheres with high water vapor pressures.2.15.2.2Reactions in Humid AtmospheresIn humid atmospheres,corrosion is usually accelerated even in the region of passive oxidation due to the higher reactivity of water vapor and higher diffusion coefficients.Water vapor can also react to form hydroxides which can evaporate at high temperatures.For the common boride ceramic materials,H3BO3or HBO2has the highest partial pressure(Figure6).Therefore boride,BN and B4C materials are moisture-sensitive much below1000 C.For boride systems, the oxidation law can be changed from parabolic for passive oxidation in dry air to paralinear,indicative of active oxidation(Figure1(d)).The mechanism behind this is that the oxidation process determined by the diffusion through the oxide layer has the same rate as the evaporation process and hence the thickness of the oxide layer remains constant with time,but the material dimensions decrease.Hydroxides of Si,Al,Mg and of some of the transition metals have low partial pressures even at high temperatures.In static(nonflowing) atmospheres,the evaporation of hydroxides is of little significance,but the situation is completely different in atmospheres with high gas stream velocities and high water vapor pressures such as those found in gas turbine environments(Fritsch,2008;Hisamatsu et al.,2003;Jacobson,1993;Klemm et al.,1998;Klemm,2010; Opila et al.,1999;Opila,2003).In these conditions,even alumina,SiC and Si3N4materials can undergo severe degradation(Fritsch&Klemm,2008;Fritsch et al.,2010;Fritsch,2008;Hisamatsu et al.,2003; Jacobson&Farmer,1999;Jacobson,1993;Klemm et al.,1998;Klemm,2010;Opila&Myers,2004;Opila et al.,1999;Opila,2003).Table2gives the dependencies of the corrosion mechanism on theflow rate of the hot gas.Measured regression rates of different nonoxide and oxide materials are compiled in Figure7.Therefore,for application of these materials in gas turbine environments,environmental barrier coatings (EBCs)must be used.These coatings have the function of suppressing or preventing the evaporation of the hydroxides.Up to now the best results have been observed with compounds of the transition metals(ZrO2, HfO2,Y2O3and Yb2O3).Besides possessing this chemical function,these materials must have similar thermalexpansion coef ficients to that of the substrate and must be stable in relation to the oxide scale and the substrate material.In the ideal case,the coating forms directly from the bulk to enable yered systems are also being considered to achieve the complex functionalities required for these coatings.A detailed overview of the current status of EBCs for Si 3N 4and other nonoxides is given by Klemm (Fritsch &Klemm,2006,2008;Fritsch,2008;Hong-fei &Klemm,2011;Roode,2010).The functionality of an EBC in nonoxide (Si 3N 4)and oxide (Al 2O 3)materials is demonstrated in Figure 8.The micrograph show polished cross-sections of the surface region after hot gas testing.Corrosion was not evident in the surface region protected by the EBC,but material loss was found in the unprotected region.Table 2Principle categories of corrosion attack of structural ceramics in water vapor-rich hot gas environments(tests performed at 1200–1500 C,7.5–100%water vapor,1–18bar total pressure and 100–5000h exposure times;adetailed compilation of literature results is given in Fritsch et al.(2010))TypeTest equipment Flow rate (m s À1)Corrosion value K w (mg cm À2h À1)Passive corrosion/oxidationFurnace 10À4–10À2Si 3N 4,SiC w weight gain,Al 2O 3stable Paralinear corrosionThermogravimetry 10À2–10À1Si 3N 4,SiC,Al 2O 3w 10À3–10À2Active corrosion Burner rig/test turbine 20–300Si 3N 4,SiC,Al 2O 3w 10À2–10À1Figure 6Partial pressure of different hydroxides at 1bar pressure (0.1bar oxide pressure and 0.1bar water pressure)as a function of the temperature (Fritsch,2008and data calculated using SGTE database).422Corrosion of Ceramic Materials2.15.3Corrosion in Aqueous Solutions 2.15.3.1Basic ProcessesCeramic materials exhibit a unique combination of properties such as hardness,wear and corrosion resistance,enabling them to be used in pumps and seals (mainly SiC and alumina),ball bearings (mainly Si 3N 4ceramics),chemical apparatus,valves and a wide variety of other applications.Despite the high corrosion resistance incomparison with that of many metals,the suitable material for the given application must be chosen carefullywith respect to mechanical load,tribological conditions and corrosion.Therefore,a detailed understanding of the corrosion behavior is necessary.In contrast to metals,whichundergo redox reactions (oxidation of metal)during corrosion,ceramics undergo acid –base reactions in cor-rosive conditions:M x O y þ2y H þ0x Me ð2y =x Þþðaq :Þþy H 2O(6)Figure 7Ranking of ceramic materials in terms of hot-gas-corrosion recession with linear weight loss rate measured in a gas burnertest at 1450 C at flow rate of 100m s À1,P H 2O ¼0:28atm and overall pressure of 1atm (Fritsch et al.,2010).Figure 8Functionality of EBC on Si 3N 4ceramics after hot gas testing at 1450 C for 100h.Corrosion of Ceramic Materials 423。