Correlation of Stainless Steel Pit Morphology to Humidity-Specific Sea Salt Brine Constituents

Abstract

Immersion exposures of 304 stainless steel ground to a #4 finish were conducted in brines representative of the chemistry of sea salt aerosols at low (40%) and high (76%) relative humidity (RH). Low-RH-equivalent brines resulted in cross-hatched pits, whereas high-RH-equivalent brines produced ellipsoidal, faceted pits. Distinct surface microcracking was observed to be associated only with cross-hatched pits and appeared to correlate with a high concentration of dissolved carbonate species in low-RH-equivalent solutions while being absent in the high-RH-equivalent brines. Correlating these results to brine composition suggested that the concentrations of MgCl2 and dissolved carbonate species in the brines could, in the presence of machining-induced surface microstructure and residual stress, determine pit morphology in marine atmospheres, thereby potentially impacting stress corrosion cracking susceptibility and lifetime prediction.