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This article presents a synthesis of recent studies focused on the corrosion product layers forming on carbon steel in natural seawater and the link between the composition of these layers and the corrosion mechanisms. Additional new experimental results are also presented to enlighten some important points. First, the composition and stratification of the layers produced by uniform corrosion are described. A focus is made on the mechanism of formation of the sulfate green rust because this compound is the first solid phase to precipitate from the dissolved species produced by the corrosion of the steel surface. Secondly, localized corrosion processes are discussed. In any case, they involve galvanic couplings between anodic and cathodic zones of the metal surface and are often associated with heterogeneous corrosion product layers. The variations of the composition of these layers with the anodic/cathodic character of the underlying metal surface, and in particular the changes in magnetite content, are thoroughly described and analyzed to enlighten the self-sustaining ability of the process. Finally, corrosion product layers formed on permanently immersed steel surfaces were exposed to air. Their drying and oxidation induced the formation of akaganeite, a common product of marine atmospheric corrosion that was, however, not detected on the steel surface after the permanent immersion period.

This article focuses on the corrosion processes resulting from the galvanic coupling between an unprotected carbon steel coupon and a “passivated” Zn‐coated carbon steel coupon. Monitoring of the galvanic current showed that the galvanized steel coupon acted as cathode, which confirmed the inversion of polarity due to Zn passivation. However, it remained cathode for only 6 months and behave as anode for the rest of the experiment (i.e., 5 years). At the end of the experiment, both coupons presented localized degradation. The local corrosion rates were estimated by residual thickness measurements. The highest (200 μm/year) was measured on the more corroded parts of the galvanized steel coupon, while the lowest (13 μm/year) corresponded to the cathodic zones of the same coupon, covered by passivated zinc. The phenomena could be interpreted via the thorough analysis of the corrosion product layers by X‐ray diffraction and μ‐Raman spectroscopy as the composition of these layers depended on the anodic/cathodic character and intensity of the metal surface underneath.

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An experimental study was conducted to determine the influence of temperature on crevice corrosion initiation for Alloy 625 (UNS N06625) in natural seawater. These tests showed that there was a critical potential-temperature-time relationship needed to initiate crevice corrosion. The potential necessary to cause crevice corrosion on Alloy 625 decreased (became less noble) when the temperature was increased from ambient to 40°C. The crevice initiation potential decreased from 300 mV for ambient temperature seawater to 100 mV for 40°C seawater. Crevice initiation potentials were essentially unchanged between 40°C and 65°C, while the time required to initiate crevice corrosion decreased as temperature increased. In a second aspect of this work, natural seawater exposure studies were conducted to determine if there is a mechanistic connection between ennoblement (the gradual elevation of corrosion potential that occurs during long-term continuous immersion in natural seawater) and crevice corrosion initiation. It was found that ennoblement produced corrosion potentials that exceed the crevice corrosion initiation potential in ambient temperature natural seawater. At 65°C, the open-circuit potential did not exceed the crevice initiation potential. However, temperature transients from ambient to elevated temperature created temporary conditions where the corrosion potential was substantially higher than the crevice initiation potential if ennoblement had previously occurred at ambient temperatures.