Hydrothermal corrosion

Maslar, J.E. Hurst, W.S. Bowers, W.J. Hendricks, J.H. In Situ Raman Spectroscopic Investigation of Stainless Steel Hydrothermal Corrosion Corrosion 2002, 58, 739-747. 

The more common classification scheme is to divide the corrosive media into their state of aggregation, that is to subdivide into corrosion by solids, liquids and gases. While solid state corrosion is rarely dealt with, we have vast amount on literature on hot gas corrosion. The case of corrosion by liquids is commonly further subdivided into more specific cases, such as aqueous corrosion (e.g. acids and water), corrosion by glasses, metal melts and salt melts. The last case is for historic reasons known in the form of a rather misleading expression hot corrosion. A special case, which spans from the liquid into the gaseous state is given by the corrosion in hot water systems hydrothermal corrosion. 

Hydrothermal Corrosion In contrast to the good behavior of SiC at lower temperatures a strong attack can be obtained in supercritical water. At pressures of 100 MPa there is some indication from powder experiments that initial reactions start at 300°C [58,59], strong corrosion is observed at T > 500°C [60,61]. The lower temperature onset is supported by tribological studies [62]. 

S.6.2.2.2 Hydrothermal Corrosion and Corrosion in Water Vapor The interactions of boron carbide with water vapor starts at 250°C. The following reactions are possible ... 

Hydrothermal corrosion of B4C has been studied theoretically and experimentally [100]. Reaction (21) as well as reactions leading to the formation of CH4 and CO are possible. Reactions that lead to the formation of carbon oxides dominate only at low pressmes and carbide water ratios. Under all other conditions, the formation of methane has been predicted. HBO2, H3BO3 and H3B3O6 gas molecules, which were predicted by thermodynamic simulation under various temperatures and pressures, are very stable thermodynamically. They may condense to some type of boric add on cooling. 

According to thermodynamic simulation [100], at a carbide water ratio of 1 10, tungsten carbide should be completely oxidized producing H2WO4 or WO3 at lower temperatures and WO2 at higher temperatures. Carbon is oxidized to CO and CO2 in the case of a low WC H2O molar ratio. Formation of free carbon upon hydro-thermal corrosion of WC was predicted to be possible at a high WC water ratio upon hydrothermal corrosion of WC. The stability of WC increases with increasing pressure. 

Also, hydrothermal corrosion of WC-Zr02 composites has been studied, but at lower temperatures when less significant interaction of WC with water occurred [114,115]. 

It should be noted that hydrothermal corrosion should not be confused with eor-rosion by wet atmospheres and that the behavior changes at higher temperatures when N2 -H H2 is the product gas rather then NH3 [60]. 

Thermodynamic analysis of the chemical equilibria in water-carbide systems [100] indicated that carbon, corresponding oxides, CH4, CO2, CO, and H2 are the main products of hydrothermal corrosion of metal carbides in the temperature... 

Under hydrothermal corrosion at 270°C, a significant dissolution of the Si3N4 grains takes place. In materials with Y2O3/AI2O3 as sintering additives, the dissolu-... 

The weight loss due to hydrothermal corrosion depends linearly on time. Only materials with high corrosion rates (MgO-containing and Si02-rich materials) show an increase of the corrosion rate with time. 

Figure 34. Corrosion layer of Si3N4 materials a) Mg0/Al203-additive system, hydrothermal corrosion conditions 210 C, 200 h under at 250°C b) Y203,/Al203-additive system, hydrothermal corrosion conditions 270°C, 200 h (the grain-boundary phase solves less than the Si3N4 grains and...

Higher turbine inlet temperatures are one way of making turbine engines consume less fuel, which reduces both operating costs and emissions. Currently, however, turbine inlet temperatures are limited to values below which downstream components can survive. Typically, creep of metallic components at elevated temperatures and pressures limit the conditions at which turbines can be operated. Therefore, for many years, there has been a desire to introduce into turbine engine hot sections ceramic materials that are more resistant to oeep. Even after sufficient strength, creep behavior, and reliability had been demonstrated, however, the lifetime of the candidate materials was found to be below desired values due to corrosion in the presence of water vapor, which is referred to as hydrothermal corrosion. 

Extensive research has identified several oxide materials with low silica-activity that are relatively resistant to hydrothermal corrosion. These materials do not possess the strength, creep behavior and reliability required to act as the structural component, however they could be used as coatings for turbine engine hot section components. These oxides include ytterbium silicate... 

A method for making bond coat materials and EBC systems for protection from hydrothermal corrosion of silicon-based ceramics has been demonstrated. The method that has been developed allows for tailoring to specific substrate materials and specific EBC materials. Application of the coatings does not damage substrate materials. Coatings using the PDC bond coat have survived thermal cycling up to 1300°C. Additional development is required to verify performance with state of the art substrate and EBC materials. 

Y. Gogotsi, Y. Tanabe, E. Yasuda, and M. Yoshimura, Effect of oxidation and hydrothermal corrosion on strength of SiC fibres. In Advanced Materials 93, I/A Ceramics, Powders, Corrosion and Advanced Processing, N. Mizutani, Editor, Elsevier, Amsterdam, Netherlands, 1994. 

Y. Gogotsi and M. Yoshimura, Low temperature oxidation, hydrothermal corrosion and their effects on properties of SiC (Tyranno) fibers. J. Am. Ceram. Soc., 78, 1439-1450, 1995. 

B. G. Nair, Q. Zhao and R. F. Cooper, Geopolymer Matrices with Improved Hydrothermal Corrosion Resistance for High Temperature Applications, 7. Mater. Sci, 42,3083-3091, (2007). 

Recently, SiOC-based ceramic nanocomposites (SiOC, SiZrOC, SiHfOC) were investigated with respect to their hydrothermal corrosion behavior (Figme 13). Dense SiOC-based samples were corroded in water in subcritical conditions (sealed containers temperatures from 100 to 250 °C) (Linck, 2012). 

Figure 4. Surface specific mass loss values of SiOC, SiZrOC and SiHfOC upon hydrothermal corrosion at 250 °C. The insets show SEM micrographs of SiOC and SiHfOC after being corroded at 250 °C for 100 hours. Obviously, the corrosion resistance the surface of SiHfOC does not seem to be damaged whereas the surface of SiOC clearly indicates that corrosion occurred. (Linck, 2012)...

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