Electrochemical synthesis and characterization of corrosion products on carbon steel under argillite layers in carbonated media at 80 °C

• Various and significantly different corrosion processes are simulated by anodic polarization.
• Corrosion products layers generated by anodic polarization are similar to those forming at OCP.
• Sulfide-containing minerals present in the argillite have an influence on the behavior of steel.

S235 carbon steel electrodes covered by a compacted layer of natural argillite were polarized anodically (1–7 days) at current densities comprised between 20 and 500 μA cm−2 in NaHCO3 + NaCl solutions at 80 °C. μ-Raman spectroscopy was used to identify the electro-generated compounds. The corrosion process proved to depend on NaHCO3 concentration, applied current density and polarization time. In most cases magnetite formed an inner stratum under a layer of siderite. Mackinawite was also identified on the surface of the electrodes polarized at 20–50 μA cm−2, especially at the early stages (∼1 day) of polarization. This result confirms the role of pyrite FeS2 (∼1 wt.% in argillite) as a source of sulfide. Chukanovite Fe2(OH)2CO3 could be detected locally in various cases with 0.01 M NaHCO3 + 0.01 M NaCl and 20–50 μA cm−2. A localized corrosion phenomenon was systematically observed with 0.1 M NaHCO3 + 0.05 M NaCl for 20–50 μA cm−2 while the corrosion was uniform with 0.01 M NaHCO3 + 0.01 M NaCl whatever the current density.