Electrochemical investigation on carbon steel behaviour in CO‐CO₂‐H₂O environment for the interpretation of the SCC mechanism

Some failures of carbon steel equipment which have occurred in industrial plants since 1963 were attributed to stress corrosion cracking (SCC) phenomena produced by carbon monoxide‐carbon dioxide‐water environments. In spite of the fact that this problem has been known for many years, its characterization from the electrochemical point of view has not been throughly investigated. In this paper, the electrochemical characterization of SCC phenomena experienced in industrial plants was performed on a laboratory scale by means of polarization curves; the tests were performed keeping as reference conditions those encountered in a gas tank sealed by water, located in a steel plant in the South of Italy, which was strongly affected by SCC. The polarization curves registered at high and low scan rates did not allow to identify the potential range for SCC probability. The potential range in which the carbon steel shows a tendency to passivation was identified by chronopotentiostatic tests. Several tests were performed in a carbon dioxide‐water environment which is not able to cause localized corrosion. The electrochemical tests were accompanied by Scanning Electron Microscope (SEM) observations which confirmed the obtained results.