Localized corrosion of carbon steel in marine media: Galvanic coupling and heterogeneity of the corrosion product layer

• Heterogeneity of corrosion product layers leads to localized corrosion.
• Magnetite plays a major role on oxygen reduction in cathodic zones.
• The formation of sulfate green rust is favored in anodic zones.
• Magnetite formation is hindered in anodic zones and favored in cathodic zones.
• The galvanic effect is self-sustaining via its influence on the corrosion products.

Corrosion product layers formed on carbon steel coupons after 6–8 years of permanent immersion in natural seawater were characterized by X-ray diffraction and μ-Raman spectroscopy. Some of the coupons showed blisters of corrosion products on a surface covered elsewhere by a much thinner layer. In anodic zones, the formation of the sulfate green rust proved to be favored at the expense of that of magnetite. In contrast, the formation of magnetite was favored in cathodic zones, together with that of aragonite and carbonated corrosion products. The galvanic effects are then self-sustaining via their influence on the corrosion products.