Visualization of pH and pCl Distributions: Initiation and Propagation Criteria for Crevice Corrosion of Stainless Steel

Abstract

Both pH and pCl sensing plates were fabricated, and the H + and Cl − distributions in the crevice solution for 18Cr-10Ni-5.5Mn stainless steel were visualized at 0.3 V (vs. Ag/AgCl, 3.33 M KCl) in 0.01 M NaCl (pH 3.0, adjusted with H 2 SO 4 ) at 298 K. While the pH inside the crevice gradually decreased with time, no accumulation of Cl − ions occurred. The SO 4 2− ions that were added to adjust the pH probably accumulated to maintain electroneutrality. When the pH reached 1.7, a metastable pit was generated inside the crevice, providing a sharp decrease in pH to 0.4 and an increase in Cl − concentration to above 4 M. After that, the corroded area grew steadily with time, and the pH and the Cl − concentration remained constant at pH 0.4 and above 4 M Cl − . Acidification from pH 1.7 to 0.4 was induced by the hydrolysis of the dissolving metal ions from the pit and resulted in the depassivation of the surrounding steel matrix. The metastable pitting also caused the Cl − accumulation inside the crevice by electromigration. The crevice corrosion was found to be initiated when the pH and Cl − concentration inside the crevice reached the conditions for metastable pit formation.Crevice corrosion of stainless steels is a problem in chemical industries and marine structures, and it has been the subject of many investigations largely because crevice corrosion sometimes happens in steels that generally exhibit excellent corrosion resistance. 1 The prevention of crevice corrosion requires an in-depth understanding of its initiation process. In crevice corrosion, geometric restrictions cause deoxygenation inside the crevice, resulting in the separation of anodic and cathodic sites. On the anodic site (inside the crevice), metal ions accumulate by passive dissolution, which cause acidification of the crevice solution by hydrolysis. 2 The electromigration and accumulation of Cl − ions also occur inside the crevice. In the incubation time to crevice corrosion, the pH and the Cl − concentration of the crevice solution gradually reach the critical values for depassivation, and then the transition from passive to active dissolution occurs. This is the classical initiation mechanism (passive dissolution model) of crevice corrosion. 2 However, this mechanism appears not to be consistent with experimental observations. 2,3 The IR drop and metastable pitting models have been proposed for the initiation of crevice corrosion.In the IR drop model, 4-13 crevice corrosion starts when the potential difference between the outside and the inside of the crevice becomes large enough to cause the transition from passive to active inside the crevice. The IR (ohmic potential) drop arises from the anodic current (I) from the inside to the outside of the crevice and the electric resistance (R) of the crevice solution and corrosion products. In this model, acidification and Cl − accumulation bring about an active loop in the polarization curve in the crevice solutions, and both factors also accelerate the active dissolution rate. 13 Metastable (micro)...

Electrochemical investigations on crevice corrosion of a martensitic stainless steel in a thin-layer cell

“…3c) for which the depassivation of the martensitic SS surface occurs for a pH = 3.2 ± 0.2. This result confirms that both the Cl À concentration and the pH influence the depassivation process [5,9,14,15].…”

“…The two stainless steel electrodes were unconnected and a cathodic polarisation (À1 V/SCE for 1 h) was imposed on the smaller electrode (working electrode), in the thin-layer cell, to remove the passive film [24]. The dissolution of the passive film induced a modification of the electrolyte composition in the confined medium due to the release of metal ions which was progressively accompanied by both acidification of the solution through hydrolysis and by an accumulation of chloride ions in the thin electrolyte layer [3][4][5]. Then, the polarisation was stopped and impedance measurements were performed, at the corrosion potential, on the smaller electrode during immersion time to evaluate the ability of the martensitic SS to repassivate in the confined medium.…”

“…Outside the confined area, the cathodic reaction of oxygen reduction still occurs on the metal surface, whereas, in the confined area, the metal surface becomes the anode. The corrosion processes lead to the modification of the chemical composition of the electrolyte in the crevice with simultaneous acidification and an increase of chloride ion concentration [3][4][5]. When the pH and the Cl À concentration in the crevice solution reach critical values, depassivation of the stainless steel with active dissolution occurs.…”

Proton exchange membrane (PEM) fuel cell bipolar plates prepared from a physical vapor deposition (PVD) titanium nitride (TiN) coated AISI416 stainless-steel

“…Typical PVD coating defects are of a micron-scale sizes, and thus the substrate corrosion at coating pits has all the features of confined (pitting, crevice, gap) corrosion. The specific attribute of such corrosion is that the concentrations of the corroding agents (H + , halogen ions) inside such confined areas are usually substantially higher than in the ambient milieu [21], therefore most of the metallic substrates are prone to such pitting corrosion; e.g., if the ambient sulfuric acid solution holds a pH value of 3, the solution inside the thin gap may possess a pH value of 0.4 in the course of crevice corrosion of stainless steel [22]. This feature makes it important to focus on preparation of defect-free TiN films, without relying on self-healing features of the substrate material (being stainless steels and titanium).…”

Crevice Corrosion of Stainless Steels 904L, 2205, and 2507 in High-Temperature Sulfuric Acid Solution Containing Chlorides: Influence of Metal Cations

“…In contrast, in less corrosive environments [10] or for more corrosion-resistant alloys [11,12], an incubation period is required for the active peak to appear. This can occur when i) the passive current increases [13] or metastable pitting gives raise to current (I increases), ii) the potential range of the active peak increases (EA/P increases) due to an increase in temperature or a change in the electrolyte composition (pH [14], chlorides), iii) R increases due to the formation of constriction by gas bubbles or corrosion products [15], or iv) the corrosion potential of applied potential ESURF shifts in the negative direction. Then the crevice corrosion initiates at some distance xA/P from the crevice mouth.…”