Pitting Corrosion of Bare Stainless Steel 304 under Chloride Solution Droplets

Abstract

Pitting corrosion behavior of stainless steel 304 ͑SS304͒ under droplets of chloride solution was investigated using a Kelvin probe ͑KP͒. Droplets of different volumes of MgCl 2 solution were placed on the steel surface and exposed to a constant low relative humidity ͑RH͒. As the concentration increased during the exposure of the drop to low RH, the open-circuit potential ͑OCP͒ and the shape change of the drop were monitored by the KP. Pit initiation was detected by a sudden decrease in the OCP. Pits initiated earlier under small droplets than under large drops. The chloride concentration at initiation was between 3.0 and 8.4 M for droplets with a starting concentration of 0.88 M Cl − . The initiation concentration increased when the initial concentration of the droplet was higher. The anodic current demand of pits growing at the OCP decreased with time as did the available cathodic current. When the current demand exceeded the available cathodic current, the active pit area decreased. A mechanism for pit formation and growth under droplets of MgCl 2 solution was proposed. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3467850͔ All rights reserved. Stainless steels and other corrosion resistant alloys exhibit strong passivity but are susceptible to localized corrosion in the presence of chloride or other aggressive ions.1,2 These alloys are often immune to attack in dilute solutions, but breakdown can occur during atmospheric or alternate immersion conditions by the following scenario. A droplet of salt solution on the surface loses water due to a temperature increase or a decrease in relative humidity ͑RH͒. The chloride concentration thereby increases until pitting corrosion initiates, which is accompanied by a sharp drop in the open-circuit potential ͑OCP͒. 3,4Tsutsumi et al. exposed stainless steel 304 ͑SS304͒ samples to seashore and rural atmospheres.5 No pits were found on samples in the rural area, while pitting initiated between 35 and 75% RH in the marine environment. A humidity of 35% corresponds to a saturated magnesium chloride solution, and 75% RH relates to 3 M MgCl 2 ͑6 M Cl − ͒. According to these results, the chloride concentration for pit initiation under a thin electrolyte layer is around 6 M. This finding agrees with the results of other papers by the same authors.6,7 Tsutsumi et al. determined the onset of pitting, which is associated with a critical chloride concentration, from the sudden drop in the OCP of the stainless steel electrode measured vs an Ag/AgCl reference electrode ͑RE͒.7 The Cl − concentration of the thin electrolyte layer was controlled by varying the RH. The initiation chloride concentration increased with increasing drying rate and with decreasing amount of salt on the surface. The same researchers also studied the influence of electrolyte drop size on the occurrence of the pitting and concluded that pitting is less likely under drops with a small diameter ͑Ͻ5 mm͒ due to a small cathode area. 6,8 The placement of the RE relative to the sample is complicated for a thin-...

Effect of Nitrate and Sulfate on Atmospheric Corrosion of 304L and 316L Stainless Steels

“…While this was not the main focus of the experiment, it may support the idea that corrosion is cathodically limited since increasing the droplet size increases the coverage of the electrolyte over the metal surface, thus increasing the available area for a cathodic reaction to take place. 21,29,36 As such, the propagation of initiation sites to stable and visible pits may be limited or even impeded under small droplets. Another factor may be that smaller areas of electrolyte are less likely to cover a suitable initiation site from which stable pitting can propagate.…”

“…It is worth noting that even the work undertaken at a chloride concentration of 3.12 M by Uhlig and Gilman is still not sufficient a concentration to cause pitting corrosion of 304L under atmospheric conditions using magnesium chloride. 20,29 For droplets below the inhibition threshold, the amount of rust observed appeared to be independent of the amount of nitrate within the droplet (e.g. Figure 5).…”

“…2,20,[27][28][29] The current work uses automated deposition of combinatorial arrays of droplets to identify inhibiting nitrate:chloride deposition density ratios (NDD:CDD) and sulfate:chloride deposition density ratios (SDD:CDD) for both 304L and 316L stainless steels, at ∼31 • C and several fixed exposure humidities.…”

“…20,21 Methods employed to simulate atmospheric corrosion conditions used in previous studies include the formation of extended thinfilms of electrolyte, [22][23][24] ink-jet printing of salt layers that deliquesced to form droplets, 21,25,26 and direct droplet deposition. 2,20,[27][28][29] The current work uses automated deposition of combinatorial arrays of droplets to identify inhibiting nitrate:chloride deposition density ratios (NDD:CDD) and sulfate:chloride deposition density ratios (SDD:CDD) for both 304L and 316L stainless steels, at ∼31 • C and several fixed exposure humidities.…”

Atmospheric Pitting Corrosion Studies of AA7075-T6 under Electrolyte Droplets: Part I. Effects of Droplet Size, Concentration, Composition, and Sample Aging

“…Utilizing the SKP, Maier and Frankel were able to study the pit initiation behavior on SS304 under droplets of varying initial volume and electrolyte concentration by monitoring the shape of and potential beneath the center of the droplets. 12 Morton and Frankel used the same approach to study the pitting behavior of AA7075-T6 under NaCl droplets containing inhibitors. 13 They found the formation of secondary droplets at the periphery of the initial droplets, under which accelerated corrosion attack took place by the large-scale separation of the anode beneath the secondary droplet and the cathode beneath the primary droplet.…”