Humidity effects on pitting of ground stainless steel exposed to sea salt particles

The effect of relative humidity (RH) on the corrosion of coarse-ground 304 stainless steel exposed for one year under sea salt particles was investigated. Total corrosion damage accumulation was higher at 40% RH than at 76% RH. At 40% RH, pits were numerous and irregularly shaped with a rough, cross-hatched structure. At 76% RH, pits were much fewer in number and ellipsoidal with crystallographically faceted surfaces. Higher E_pit resulting from lower [Cl⁻] impeded initiation at 76% RH. Cathodic resource competition likely limited growth and resulted in lower total volume loss. At 40% RH, lower E_pit due to higher [Cl⁻] led to initiation of multiple pits supported by discrete cathodes under individual droplets. Despite more cathodic current available at 76% RH, higher damage accumulation at 40% RH was due to lower anodic stability requirements resulting from higher [Cl⁻]. At 76% RH, pitting proceeded with increasing active area at conditions above critical stability, leading to ellipsoidal pits with facets. The cross-hatched morphology at 40% RH was ascribed to growth at the critical stability conditions, driven by constant current through a fixed active area. Small cracks at the 40% RH pits might have been caused by hydrogen environment assisted cracking.