Initial stages of atmospheric corrosion of steel in the Arabian Gulf

Energy dispersive X-ray micro-analysis, X-ray diffraction and fluorescence, Auger, X-ray photo-electron spectroscopy and Fourier transform infrared spectroscopy have been used to characterise corrosion products on carbon steel after atmospheric exposure for periods up to 12 months to an industrial environment near the west coast of the Arabian Gulf. The results indicate that atmospheric corrosion starts by the formation of small blisters at discrete locations on the metal surface, presumably the anodic sites. The blister covers are very rich in iron chlorides and contain iron oxyhydroxides, oxides, sulphates and possibly hydroxide. The formation of iron chlorides as the primary corrosion product is only limited to the early stages of blister formation due to the aggressive nature of chloride ions. Chloride formation during later stages may be partially impaired since it requires the inward transport of fresh chloride ions through the then thick rust layer. In contrast, the formation of iron sulphates at the rust-metal interface continues by the acid regeneration mechanism (which leads to the electrochemical mechanism); therefore it is less dependent on the supply of fresh sulphate ions from the surface electrolyte through the growing rust layer.