The morphology of atmospheric pitting corrosion in 304L stainless steel plate was analysed using MgCl(2) droplets in relation to changes in relative humidity (RH) and chloride deposition density (CDD). It was found that highly reproducible morphologies occur that are distinct at different RH. Pitting at higher concentrations, i.e. lower RH, resulted in satellite pits forming around the perimeter of wide shallow dish regions. At higher RH, these satellite pits did not form and instead spiral attack into the shallow region was observed. Increasing CDD at saturation resulted in a very broad-mouthed pitting attack within the shallow dish region. Large data sets were used to find trends in pit size and morphology in what is essentially a heterogeneous alloy. Electrochemical experiments on 304 stainless steel wires in highly saturated solutions showed that the passive current density increased significantly above 3 M MgCl(2) and the breakdown pitting potential dropped as the concentration increased. It is proposed that the shallow dish regions grow via enhanced dissolution of the passive film, whereas satellite pits and a spiral attack take place with active dissolution of bare metal surfaces.
Atmospheric pitting corrosion of stainless steel was examined with synchrotron X-ray microtomography, laboratory based tests and electrochemical measurements in order to provide a mechanistic basis for the development of corrosion prediction models. It was found that the morphology of corrosion pits was affected by the presence of residual ferrite, and some pits showed dense covers that may affect pit stability. Fluctuations in relative humidity may lead to partial or complete repassivation of pits, sometimes leading to the formation of new pits. Electrochemical measurements in artificial pits containing concentrated salt solutions show a strong dependence of the diffusion limited current density on chloride concentration.
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