Local Electrochemistry and In Situ Microscopy of Pitting at Sensitized Grain Boundary of Type 304 Stainless Steel in NaCl Solution

Abstract

To elucidate the pit initiation behavior of sensitized stainless steels, the anodic polarization of a single grain boundary was examined in 0.1 M NaCl (pH 5.4) at 298 K using a micro-electrochemical system. For Type 304 heat-treated at 923 K for 2 h, no pitting was initiated on a small area (ca. 100 μm × 100 μm) with a sensitized grain boundary without MnS inclusions. However, stable pitting was observed on the electrode area that was larger than ca. 200 μm × 200 μm. In situ microscopy revealed that the first step in corrosion was a spherical pit generated at a MnS inclusion at a sensitized grain boundary, and that intergranular corrosion started at the pit. The local depassivation of the Cr-depleted zone along the sensitized grain boundary was thought to be introduced by the dissolution of the inclusion. The co-existence of the MnS inclusion and the Cr-depleted zone was considered to be the critical factor in the pit initiation of sensitized stainless steels in NaCl solutions. Stainless steels are widely used in chloride environments because of their high corrosion resistance, which is attributed to their high Cr content, at above 12 mass%.1 When stainless steels are heated to around 900 K (e.g., welding), sensitization occurs due to the formation of Cr-depleted zones at the grain boundaries. The susceptibility to intergranular corrosion and pitting at the grain boundaries increases as a result of sensitization-treatments. 2To assess the degree of sensitization and/or the mechanisms of intergranular corrosion, acidic solutions are usually used. ASTM A262 testing specification contains the following intergranular corrosion tests: 1) Huey test (boiling HNO 3 ), 2) Strauss test (boiling H 2 SO 4 + CuSO 4 + metallic Cu), 3) and Streicher test (boiling H 2 SO 4 + Fe 2 (SO 4 ) 3 ). After immersion, the specimens are visually examined and/or measured for weight loss. These methods have been widely and successfully used in corrosion engineering. ISO 12732 specifies a method that uses the double loop electrochemical potentiodynamic reactivation test (based onĈihal's method 3 ). In this method, the standard solution is 0.5 M H 2 SO 4 -0.01 M KSCN, and the degree of sensitization can be evaluated quantitatively from the comparison of the peak current densities between the anodic scan and the subsequent cathodic scan.Although acidic solutions are generally used to evaluate the degree of sensitization, in practical applications, pitting and stress corrosion cracking occur in stainless steels in near-neutral pH environments. NaCl solutions are used to evaluate the pitting corrosion resistance and the susceptibility of sensitized stainless steels to stress corrosion cracking (SCC). Cheng et al. studied the pitting corrosion of sensitized Type 304 stainless steel under wet-dry cycling and demonstrated that both the probability of pitting and the average size of the pit increased as a result of sensitization. 4 It was suggested that the observed change in the pit morphology from round to an irregular shape could be attributed t...

Influence of the welding current intensity and nitrogen content on the corrosion resistance of austenitic stainless steels

“…This can also be explained by depletion in chromium of the grain boundary areas at a higher heat input. In accordance with the observations presented, [57] it can be assumed that, with a heat input increase, the chromium-rich phases precipitate on the boundaries of MnS inclusions. As a result, there is a significant depletion in chromium in the regions close to MnS inclusions, which facilitates the formation of pits.…”

“…It has been shown that the Cr/Fe ratio is lower in this region than in the austenitic matrix; therefore, the formation of pits is easier. On the other hand, the co‐existence of MnS inclusions and chromium‐depleted zones at the grain boundaries is considered to be the critical factor in the pit initiation of sensitized AISI 304 stainless steels …”

“…On the other hand, the co-existence of MnS inclusions and chromiumdepleted zones at the grain boundaries is considered to be the critical factor in the pit initiation of sensitized AISI 304 stainless steels. [57] Table 3 presents the experimentally obtained values of the indicators of the pitting corrosion resistance for the welded joint (HAZ) formed both without nitrogen and in the presence of nitrogen in the shielding gas. During welding, nitrogen from the shielding gas was ionized at high temperatures of the electric arc, dissolved in the weld metal and diffused to the HAZ.…”

Corrosion behaviour of AA2024 aluminium alloy in different tempers in NaCl solution and with the CeCl3 corrosion inhibitor

“…The existence of these zones is responsible for susceptibility to intergranular corrosion of these steels . Pits are almost exclusively formed close to MnS inclusions, and in particular near MnS inclusions that intercept chromium‐depleted narrow zones near grain boundaries . It is possible that, in naturally aged AA2024 alloy, pit formation is facilitated near cathodic particles crossed by copper depleted zones near grain boundaries.…”

“…[55] Pits are almost exclusively formed close to MnS inclusions, and in particular near MnS inclusions that intercept chromium-depleted narrow zones near grain boundaries. [56] It is possible that, in naturally aged AA2024 alloy, pit formation is facilitated near cathodic particles crossed by copper depleted zones near grain boundaries. Pit formation in these zones can be the cause of insufficiently higher resistance to pit formation of this temper.…”