Long-term corrosion behaviour of carbon steel and stainless steel in Opalinus clay: influence of stepwise temperature increase

Carbon steel (C-steel; E24 and S235 grades) and stainless steel (316L) electrodes were corroded in situ in Opalinus clay under anoxic conditions in a vertical descending borehole. The electrodes were exposed at ambient temperature for two years, and then at 85 degrees C for five years. In situ electrochemical impedance spectroscopy showed that the instantaneous corrosion rate of C-steel decreased over time down to 1 mu m year(-1), followed by steady state. Microbial and chemical investigations showed that sulphate and thiosulphate reducing prokaryotes were present in the porewater and at the metal surface. Post mortem characterisation revealed contrasting corrosion aspects. The E24 corrosion interface was thick and contained magnetite, mackinawite, hydroxychloride and siderite, together with more oxidised species (goethite, greigite, elemental sulphur). The S235 corrosion interface was thinner and contained siderite and mackinawite. Corrosion damage of the 316L electrode was negligible, and the surface was covered by a thin fringe of pyrite.