Toward understanding the effects of strain and chloride deposition density on atmospheric chloride-induced stress corrosion cracking of type 304 austenitic stainless steel under MgCl ₂ and FeCl ₃ :MgCl ₂ droplets

Type 304 (UNS S30400) austenitic stainless steel was exposed for 6 months under elastic (0.1%) and elastic/plastic (0.2%) strain to MgCl ₂ and mixed MgCl ₂ :FeCl ₃ droplets with varying chloride deposition densities (1.5 μg/cm ² –1,500 μg/cm ² ) at 30% relative humidity (RH) and 50°C. The occurrence of pitting corrosion, crevice corrosion, atmospheric chloride-induced stress corrosion cracking (AISCC), and hydrogen embrittlement (HE) was observed, and the average crack growth rates estimated. Exposure to elastic/plastic strain resulted in longer and more severe cracks. AISCC was found at chloride deposition densities down to 14.5 μg/cm ² , whereas no cracks were seen at lower deposition densities, with cracks developing at pit or crevice corrosion sites. More severe cracks were seen under MgCl ₂ droplets as contrasted to mixed MgCl ₂ :FeCl ₃ salt droplets, which were seen to promote more localized corrosion sites with deeper penetration and in conjunction with shorter crack lengths. Differences in AISCC propagation rates and associated crack morphologies are discussed in relation to understanding long-term atmospheric corrosion exposures.