A Micro-Raman Spectroscopic Study of Marine Atmospheric Corrosion of Carbon Steel: The Effect of Akaganeite

Micro-Raman spectroscopy was used to characterize the rust formed on 1008 carbon steel exposed to marine test sites in Hawaii. Besides lepidocrocite in the outer rust layers and goethite in the inner rust layers, akaganeite was detected on all samples due to the presence of Cl− and increased in quantity in the rust layer with increasing Cl− deposition rate. The presence of Cl− did not affect the corrosion rate of carbon steel when the Cl− deposition rate was lower than a threshold of approximately 75 mg/m2/day, but significantly increased the corrosion rate when the Cl− deposition rate was higher than the threshold. This is likely due to akaganeite being a sink for Cl− by incorporating it into its tunnel structure when salt deposition is below a certain level. Once the akaganeite is saturated with Cl− and the akaganeite cannot take up any more Cl−, free Cl− can be available to accelerate corrosion at anodic sites.