Environmental and Material Influences on the Stress-Corrosion Cracking of Steel in H2O–CO–CO2 Solutions

The stress-corrosion cracking of A516 pressure vessel steel was investigated by the use of slow strain-rate tests. The orientation of samples to the rolling direction was investigated, and it was found that samples machined longitudinal to the rolling direction showed a slightly increased sensitivity to stress corrosion. The temperature variation showed that for different gas mixtures, the maximum sensitivity to stress corrosion was in the region of 45° to 55°C for the 25% CO gas mixture, whereas with higher CO concentrations, this temperature region of maximum sensitivity moved to higher temperatures. Surface finish showed a slight increase in sensitivity to cracking with increased surface roughness. The most significant increase was found with increased total gas pressures and when samples have been exposed to the environment for an extended period. This was as a result of the inhibition of the corrosion reaction by the passivation of the carbon monoxide, which is a time-dependent process. 1. Introduction During coal gasification processes, a significant volume of water comes in contact with the combustion gas mixture, and the result of this is a contaminated, black water by-product that is transported and processed. Due to water shortages, the by-product water is used as a cooling agent. This water by-product then comes in contact with steel piping which leads to embrittlement thereof during exposure to the appropriate conditions. The water by-product consists of water with dissolved gasses mainly as carbon monoxide and carbon dioxide. Steel is susceptible to stress-corrosion cracking (SCC) in CO/CO2/H2O environments. Extensive research [1, 2] has indicated that SCC in this system is probably due to inhibition of iron corrosion in the H2O-CO2 system by the time-dependent adsorption of CO. Rupture of the inhibited passive surface by emerging slip steps then creates the highly localised active region on a passive surface necessary for the development of a sharp crack. Previous work done by various researchers [1–3] using a wide spectrum of mechanical tests and potentio-dynamic polarisation has indicated that SCC of steel in this system occurs over a wide range of CO partial pressures and temperatures which include the typical operating conditions for coal gasification plants. This indicated that high susceptibility to SCC has not always been found in the plant, from personal experience, where SCC is indeed limited to certain areas of the plant. Malik [4] performed an investigation on steel exposed to the vapour phase of H2O–CO–CO2 mixtures and found