Localized corrosion of carbon steel in seawater: Processes occurring in cathodic zones

The composition of the corrosion product layer forming on carbon steel in seawater depends on the cathodic/anodic character of the metal surface underneath. This variability is mainly linked to that of interfacial pH, that decreases in anodic zones and increases in cathodic zones. To study the role of pH on these phenomena, two types of experiments were achieved. First, carbon steel coupons were immersed 10 days in artificial seawater in conditions favoring differential aeration cells and the resulting corrosion product layer was characterized by X-ray diffraction. Lepidocrocite (gamma-FeOOH) and aragonite (CaCO3) proved to be the major components of this layer in the anodic and cathodic zones, respectively, while the sulfate and carbonate green rusts, GR(SO42-) and GR(CO32-), were observed as minor components. Secondly, aqueous suspensions of GR(SO42-) and GR(CO32-) were prepared in the laboratory to study the equilibrium conditions where both compounds can co-exist. A pK value of 7.85 +/- 0.35 could be estimated, leading to an equilibrium pH of 8.24 +/- 0.35 in seawater, consistent with the observed predominance of GR(SO42-) in the anodic zones and GR(CO32-) in the cathodic zones.