Marine corrosion of carbon and micro-alloyed steel: comparison between seaport and laboratory conditions

Carbon steel and low alloy steel coupons were immersed for 7-8 months in natural marine medium (Les Minimes seaport – Atlantic Ocean, La Rochelle, France) and in laboratory conditions, but using natural seawater sampled in the seaport. The corrosion processes were studied by open circuit potential and linear polarization resistance measurements whereas the corrosion product layers were characterized by µ-Raman spectroscopy and X-ray diffraction. Various steel grades with increasing Cr content (from less than 0.05 wt.% to 1.1 wt.%) were considered, some of them also containing various amounts of Al, Cu, Ni and/or Si. Polarization resistance measurements showed that the resistance to corrosion was significantly improved in seaport conditions only for the low alloy steel containing 1.1 wt.% Cr and 0.55 wt.% Al. In laboratory conditions, the low alloy steel containing 0.8 wt.% Cr but no Al also showed improved resistance to corrosion. The analysis of the corrosion product layers revealed significant differences between seaport and laboratory conditions. In seaport conditions, besides the specific presence of FeS due to bacterial activity, the formation of the sulfate green rust (Fe6(OH)12SO4∙8H2O) was favored with respect to that of magnetite (Fe3O4). Finally, the improved resistance to corrosion was associated with corrosion product layers enriched with FeOOH phases and (CrIII,FeII-III) spinel-like oxides.