Pit Initiation and Repassivation of Stainless Steels Exposed to Cyclic Relative Humidity Changes

Abstract

Pit initiation and repassivation of stainless steels (SS) SS430 and SS304 were investigated under a thin solution film of MgCl 2 . The potential difference between the SS plate and the thin silver wire set 100 μm above the SS surface was measured during the wet-dry cycles. The wet-dry cycles were carried out by changing the relative humidity (RH) between 95% and 45% at a constant rate of 5%/h. A rapid potential drop due to initiation of a growth-type pit was observed when the RH was decreased from 95%, while a rapid potential rise owing to repassivation was observed when the RH was increased from 45%. The RH pit and RH rep at the onset of pitting corrosion and repassivation were determined at various temperatures. At 298 K, the RH pit was obtained to be 68%-48% for SS430 and 58%-47% for SS304. The RH rep ranged from 67% to 73% for SS430 and 56%-70% for SS304. The RH pit and RH rep increased significantly as the temperature was increased from 283 K to 313 K.In marine atmospheric environments, airborne sea salts are deposited on metallic surfaces. The salts absorb moisture from the air and form a thin solution film, which leads to atmospheric corrosion of the surface. In daily wet-dry cycles, the relative humidity (RH) generally increases at night and decreases during the day. In general, atmospheric corrosion of non-passivated metals such as carbon steel and weathering steel will be enhanced at RH higher than 80%. 1,2 Conversely, passivated metals such as stainless steel (SS) may suffer from pitting corrosion in environments of relatively lower humidity as the chloride concentration of the surface water film is increased. 3-6 Pitting may cause rusting, which is a cosmetic problem of stainless steel structures. Furthermore, pits may cause mechanical deterioration by acting as crack initiation sites.Electrochemical techniques are often used to evaluate corrosion resistance of metallic materials, as well as for mechanistic studies. In most cases, these studies are carried out in bulk solutions; however, atmospheric corrosion progresses under a thin solution film. Consequently, the mechanism for atmospheric corrosion is significantly different from that in bulk solution. Over the last few decades, several electrochemical techniques have been employed to investigate atmospheric corrosion and its mechanism. Stratmann et al. utilized an oscillating Kelvin probe (KP) to measure the surface potential under a thin electrolyte film. 7-9 They measured the anodic and cathodic polarization curves of steel under electrolyte films using the KP as a reference electrode (RE). From the polarization curves obtained at different electrolyte film thicknesses, they were able to estimate the corrosion current density. The KP method is very useful for atmospheric corrosion studies because the RE does not interrupt the thin electrolyte film. Nishikata et al. employed electrochemical impedance spectroscopy (EIS) to evaluate the corrosion rate of metals covered with thin electrolyte films. 10-12 They reported that the metal/thin electrolyte ...

Observation of Pit Initiation and Growth of Stainless Steel under a Chloride Solution Droplet —Effect of S Content on Pit Initiation, Growth, and Repassivation—

“…Figure 3(a) shows typical changes in the RH and potential (E SS-Ag ) for 60S steel during the drying process. The E SS-Ag gradually increases with the decrease in the RH from the start of the test up to 8.2 h. However, as reported in a previous study, 3) the [Cl ¹ ] in the droplet increases with decreasing RH, which induces the shift of the RE potential in the positive direction. Thus, this potential change does not correspond to the pitting corrosion of 60S steel.…”

“…In addition, because aluminum (Al) and oxygen (O) were also detected, it is highly possible that MnS inclusions are present near the Al 2 O 3 inclusions. As described in a previous report, 3) the Ag wire used as RE partially dissolves as the Ag complex ions in the high [Cl ¹ ] droplet (³10 M in MgCl 2 saturation concentration). These ions react with the MnS inclusions to form AgS on the MnS surface.…”

“…Therefore, we improved this method to avoid crevice corrosions in the recent works. 3,4) In this study, we further improved the conventional evaluation system for pitting corrosion in an atmospheric environment. The conventional system was combined with a new system that can automatically stop the pit growth at any time during the pit growth stage.…”

“…However, we previously reported that the change of E corr(WE) due to the pit initiation and growth is more significant than that of E Ag/AgCl . 3) Thus, the pit initiation and growth can be determined by continuously monitoring E SS-Ag . In this study, the pit initiation was defined as the change in the potential of E SS-Ag by ¹0.01 V s ¹1 .…”

“…The detailed experimental procedure has been previously reported. 3) To evaluate stochastic phenomena, such as pitting corrosion, three droplets were dropped on each sample and the samples were subjected to five wetdry cycle tests twice. In total, 30 test cycles were performed.…”