Microstructural characterization of carbon steel corrosion in clay borehole water under anoxic and transient acidic conditions

• Duplex layer of corrosion products (CPs) for all samples.
• Inner layer of siderite, chukanovite, disordered Fe phyllosilicate for all samples.
• Presence of β-Fe2(OH)3Cl at the steel-CP interface of extensively corroded samples.
• Protective (siderite, chukanovite) layer at the surface of samples with little damage.
• Nature of outer CPS reflects the evolution of fluid composition in the test chamber.

The corrosion interface of low-alloy carbon steel (C-steel) coupons (P235, ferrite-pearlitic with a columnar microstructure) reacted in clay porewater seeping in a test chamber in situ in the Callovo-Oxfordian formation of the Meuse-Haute Marne Underground laboratory (France) was probed by microscopic and microspectrocopic techniques. Two series of samples were investigated, with extensive and limited corrosion, respectively. For extensively corroded coupons, several layers of inner corrosion products were observed. The innermost layer was made of Cl-rich Fe (hydr)oxide, later evolving to form a massive β-Fe2(OH)3Cl unit close to C-steel, and sometimes crossed by magnetite ribbons sticking to cementite lamellas, or by patches of green rust. The second inner layer was made of veinlets of Fe hydrated silicate with a sponge-like morphology, a molecular-scale structure reminiscent of clay minerals, and containing nodules of siderite and chukanovite. These (hydroxy)carbonate solids sometimes merged to form a massive layer close to the trace of the original surface. The layers external to this original surface were discontinuous and made of successively Ca-doped siderite (ankerite), Fe sulfide, S‐rich Fe silicate, and finally pyrite nodules and ankerite. For the samples with limited damage, no β-Fe2(OH)3Cl layer was observed, siderite or chukanovite were in close contact with metal, sometimes with interfacial magnetite, and the veinlets of Fe hydrated silicate were less developed. Only external layers of Fe sulfide and ankerite were observed. This suggests that these slightly corroding samples were in fact confined. Evidence for microbial activity was obtained in the form of a Fe sulfide fringe mixed with ankerite. Microbial activity may have induced some local heterogeneity in corrosion processes, e.g. by changing local pH conditions at mm-scale.