Initiation of Localized Corrosion of Ferritic Stainless Steels by Using the Liquid-Phase Ion Gun Technique

Abstract

The initiation of localized corrosion of types 430 and 443J1 ferritic stainless steels was evaluated in 0.15 mol dm −3 Na 2 SO 4 solution. A liquid-phase ion gun (LPIG), a silver microelectrode covered with a silver chloride layer, was cathodically polarized to generate Cl − in the vicinity of the stainless steel polarized at 0.4 V SSE . Contact of the stainless steel surface with the Cl − -concentrated environment by the LPIG operation induced a rapid increase in anodic current flow through the stainless steel electrode after the induction period t d and consumption of the cathodic electric charge Q d by the LPIG microelectrode. Numerical modeling using the LPIG microelectrode current for t d gave a critical Cl − concentration [Cl − ] d needed for the initiation of localized corrosion. These parameters obtained by LPIG tests showed that type 443J1 stainless steel was superior to type 430 stainless steel in localized corrosion resistance. AES depth profiling of the stainless steel surfaces after the LPIG operation revealed enrichment of Cl in the outermost oxide film as well as decreased film thickness. The mechanism of degradation of the stainless steel surface due to contact with the Cl − -concentrated solution is discussed. The adsorption of Cl − on the oxide surface is thought to be a trigger of the oxide degradation. Stainless steel, an Fe-based material with a minimum of 11 wt% Cr content, has corrosion resistance, which is attributed to the formation of Cr-and/or Fe-oxide films, so-called passive films, on the surface. The resistance is related to the chemical composition of the film, which is strongly dependent on the chemical composition of the substrate. The pitting resistance equivalent (PRE) number (= wt% Cr + 3.3 (wt% Mo + 0.5 wt% W) + 16 wt% N) has been used to determine the localized corrosion resistance of stainless steel.1 The larger the PRE number of stainless steel is, the greater is the resistance of the stainless steel to localized corrosion. The PRE number is related to the contribution of elements to the resistance of a passive film on stainless steel. Alloying with Cr forms non-crystalline Cr 2 O 3 on stainless steel by a direct reaction of Cr with H 2 O and improves the resistance to localized corrosion.2 Alloying with Mo enhances the protectiveness of a passive film to Cl − by increasing oxygen affinity of the stainless steel.3 Alloying with W inhibits localized corrosion by dissolved WO 4 2− from the passive film to an aqueous electrolyte or by forming insoluble WO 3 , which enhances the stability of the passive film.4 Alloying with N decreases local pH by decreasing acidity in pits due to NH 4 + formation and promotes repassivation. 5 For ferritic stainless steel, however, the effect of N is not quite satisfied and usage of the simple PRE number (= wt% Cr + 3.3 wt% Mo) is proposed.

Effect of carbon addition on the passivity of hypoeutectic high chromium cast irons

“…Various researchers have approached to explore the growth kinetics and the electronic properties of the passive lm with a stable condition in the alkaline buffer solution. [12][13][14] All the electrochemical measurements were carried out in a boricborate buffer solution (0.15 mol dm −3 H 3 BO 3 and 0.15 mol dm −3 NaB 4 O 7 ), with pH 8.4, purged with 99.999% N 2 gas for 1 h at room temperature. In all electrochemical tests, the reproducibility was examined at least three times with different specimens.…”

Revealing How Alkali Cations Affect the Surface Reactivity of Stainless Steel in Alkaline Aqueous Environments

“…Most often, studies of alloy corrosion have focused on Cl – species due to their well-known role in accelerating the decomposition of the passivating oxide film, and thereby initiating corrosion. − Three models have been proposed for oxide layer breakdown in the presence of halides, namely, (1) the adsorption model, , in which adsorbed halides form a metal cation complex within the oxide layer, which accelerates film removal, (2) the penetration model, , in which the penetration of halides results in the formation of conductive pathways within the oxide film that facilitate the transfer of metal cations from the surface into the bulk, and (3) the film breakdown model, , which suggests that the presence of adsorbed halides decreases the surface tension of the oxide, resulting in its mechanical rupture. These mechanisms are, however, applicable for systems involving a halide ion and a well-oxidized substrate.…”

“…Of particular interest is KOH because the K + ion has a smaller charge density compared to Li + and therefore, it exhibits a lower affinity for the negatively charged – OH ion, , making water chemistry control more manageable. Moreover, KOH is substantially (at least 5 times) cheaper than LiOH and can be sourced much more readily than nuclear-grade Li-hydroxide. As such, the use of KOH as compared to LiOH would result in substantial reductions in plant operational costs.…”

Passivity of alloy 31 in green‐death solution

“…Many researchers have reported the presence of Fe and Cr in the passive film formed on FeCrNi alloy in acidic aqueous solutions, while a detailed content between Fe and Cr in the film is different. On one hand, it has been reported that the outermost layer of passive film becomes enriched with Fe cations, while Cr are concentrated as cations near the metal‐passive film interface . Conversely, some studies have shown that the Cr is more concentrated across the entire passive film than Fe .…”

“…Conversely, some studies have shown that the Cr is more concentrated across the entire passive film than Fe . The film composition varies with substrate alloy composition and passivation conditions such as solution temperature, pH, anions concentration, and passivation potential, which is related to the film's electrochemical behavior . It is important to investigate the composition of passive film formed on FeCrNi alloys under a particular corrosive condition.…”