Long-Term Effects of Humidity on Stainless Steel Pitting in Sea Salt Exposures

Abstract

Ground 304 stainless steel (SS) samples were exposed to sea salt particles at 35 °C and two relative humidity (RH) levels for durations ranging from 1 week to 2 years. For all exposure times, pit number density and total pit volume at 40% RH were observed to be considerably greater than those at 76% RH. Statistical analysis of distributions of pit populations for both RH conditions showed that pit number density and total pit volume increased rapidly at first but slowed as exposure time increased. Cross-hatched features were observed in the 40% RH pits while ellipsoidal, faceted pits were observed at 76% RH. Optical profilometry indicated that most pits were not hemispherical. X-ray tomography provided evidence of undercutting and fissures. Piecewise curve fitting modeled the 40% RH data closely, predicting that corrosion damage would eventually plateau. However, a similar treatment of the 76% RH data suggested that corrosion damage would continuously increase, which implied that the piecewise power-law fit was limited in its ability to model atmospheric corrosion generally. Based on these observations, the operative mechanisms determining long-term corrosion behavior were hypothesized to be different depending on the RH of exposure.

The evolution of pit morphology and growth kinetics in aluminum during atmospheric corrosion

“…When all individual pit volume growth is summed to show cumulative volume loss (directly proportional to mass loss) in Fig. 2b, the 4N-Al material had similar trends to other materials under atmospheric corrosion scenarios and constant contaminant environments where the corrosion rate slows over time 41,[60][61][62] . However, in the current study, the early stages of corrosion damage did not show the commonly described power law volume/mass loss relationship with time.…”

Influence of Realistic, Cyclic Atmospheric Cycles on the Pitting Corrosion of Austenitic Stainless Steels

“…At lower RHs, below NaCl deliquescence, crevice corrosion was observed to occur under precipitated NaCl crystals with MgCl 2 deliquesced in solution. Srinivasan et al and Weirich et al, 2,3 observed similar correlations with RH through atmospheric exposure of artificial sea salt droplets. At 76% RH, with NaCl dominated brines, the resulting pits were hemispherical with crystallographic surfaces, whereas at lower RH (∼40% RH), with MgCl 2 dominated brines, irregularly shaped pits, with cross-hatching, and micro-cracking were observed.…”

“…Specifically, relative humidity (RH) and temperature (T) can control the surface brine composition, chemistry, conductivity, electrolyte distribution and volume, and the subsequent pitting damage. [2][3][4][5][6][7] Recent corrosion studies have attempted to replicate aspects of atmospheric exposures in laboratory settings through droplet exposures under controlled atmospheric conditions. Specifically, Street et al, 5 studied the influence of exposure RH on atmospheric corrosion with MgCl 2 droplets exposed under static RH and T conditions and short time intervals (⩽ 24 hours (h)).…”

“…Pit quantification/analysis requires the use of different imaging techniques, analysis software, and user input to be able to accurately determine pitting characteristics like pit depth, volume, density, etc. For example, Srinivasan et al and Weirich et al, 2,3 utilized optical and electron microscopy, profilometry, focused ion beam (FIB), and X-ray tomography (XCT) to image and quantify pits. However, due to limitations with line-of-sight nature of some instruments, determining a threshold procedure for profilometry, and post processing data quantification of pitting damage was still a challenge.…”

Effect of Stress Magnitude on Pit Growth Rate of 304 Austenitic Stainless Steel in Chloride Environments

“…The surface roughness also significantly affected the number of corrosion pits. The effect of humidity on the generation of corrosion pit for 304 stainless steel was investigated under sea-salt environments by Srinivasan et al [13]. It was found that, as the RH decreased from 76% to 40%, the corrosion susceptibility increased gradually.…”