The corrosion of metals in multicomponent gases

The reaction of metals with gas mixtures such as CO/CO2 and SO2/O2 can lead to products in which the reaction of the oxygen potential in the gas mixture to form the metal oxides is accompanied by the formation of carbon solutions or carbides in the first case, and sulphide or sulphates in the second mixture. Since the most important aspects of this subject relate to the performance of materials in high temperature service, the reactions are referred to as hot corrosion reactions. These reactions frequently result in the formation of a liquid as an intermediate phase, but are included here because the solid products are usually rate-determining in the corrosion reactions. 

Aircraft turbines in jet engines are usually fabricated from nickel-based alloys, and these are subject to combustion products containing compounds of sulphur, such as S02, and oxides of vanadium. Early studies of the corrosion of pure nickel by a 1 1 mixture of S02 and 02 showed that the rate of attack increased substantially between 922 K and 961 K. The nickel-sulphur phase diagram shows that a eutectic is formed at 910 K, and hence a liquid phase could play a significant role in the process. Microscopic observation of corroded samples showed islands of a separate phase in the nickel oxide formed by oxidation, which were concentrated near the nickel/oxide interface. The islands were shown by electron microprobe analysis to contain between 30 and 40 atom per cent of sulphur, hence suggesting the composition Ni3S2 when the composition of the corroding gas was varied between S02 02 equal to 12 1 to 1 9. The rate of corrosion decreased at temperatures above 922 K. 

The thermodynamic activity of nickel in the nickel oxide layer varies from unity in contact with the metal phase, to 10 8 in contact with the gaseous atmosphere at 950 K. The sulphur partial pressure as S2(g) is of the order of 10 30 in the gas phase, and about 10 10 in nickel sulphide in contact with nickel. It therefore appears that the process involves the uphill pumping of sulphur across this potential gradient. This cannot occur by the counter-migration of oxygen and sulphur since the mobile species in the oxide is the nickel ion, and the diffusion coefficient and solubility of sulphur in the oxide are both very low. 

Rosner and H.D. Allendorf. Heterogeneous Kinetics at Elevated Temperatures, G.R. Belton and W.L. Worrell (eds), p. 231. Plenum Press, New York (1970). 

von Bogdandy and H.J. Engell. The Reduction of Iron Ores. Springer Verlag, Berlin (1971). 

Laboratory studies of some important industrial reactions 285 

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