Corrosion processes of carbon steel in argillite: Galvanic effects associated with the heterogeneity of the corrosion product layer

Abstract

S235 carbon steel electrodes covered by a compacted layer of natural argillite were immersed for 15–20 days in a 0.01molL−1 NaHCO3+0.01molL−1 NaCl solution at 80°C and the resulting corrosion product layers were characterized by μ-Raman spectroscopy. In some cases, a heterogeneous layer could be obtained, the center of the electrode being covered by a (FeS+Fe3O4)/Fe2(OH)2CO3 bilayer and the rest of the surface by a (FeS+Fe3O4)/FeCO3 bilayer. Heterogeneous corrosion product layers could also be generated reproducibly by coupling an initially bare electrode (anode) with an electrode covered by a Fe3O4/FeCO3 bilayer (cathode). Three zones were generated on the anode, the nature of the corrosion product layer varying with the distance from the cathode. The galvanic couplings induced by heterogeneous corrosion product layers could be evidenced using the scanning vibrating electrode technique.