The Contrast between the Pitting Corrosion of 316 SS in NaCl and NaBr Solutions: Part II. Morphology, Chemistry, and Stabilization of the Pits

Abstract

This paper intends to evaluate the pit chemistry characteristics of 316 austenitic stainless steel in NaCl and NaBr solutions while discussing the pitting transition from metastability to stability. Results reveal that increasing the relative concentration of bromide increases the critical and saturation concentrations of cations, which are, respectively, requisites for pit survival and salt precipitation. Besides, bromide causes the ohmic solution resistance of the pit solution to increase. Consequently, it intensifies the pit transition from metastability to stability and retards the occurrence of the stable pitting. The morphology of the pits and the influence of the pit lacy cover on the pit stabilization in NaCl and NaBr solution are also discussed. It was also observed that contrary to metastable pits, salt-covered stable ones have higher growth kinetics in the presence of bromide.

Localized corrosion and stress corrosion cracking of stainless steels in halides other than chlorides solutions: a review

“…The actual potential at the pit bottom might be lower due to an ohmic drop in potential, and it can be estimated by different procedures detailed in the literature (Gaudet et al, 1986;Laycock & Newman 1997;Li et al, 2019). An increase in E T measured with pencil electrodes correlated with an increase in E pit measured on flat electrodes, as the Br − /Cl − concentration ratio increased in solutions with a total halide concentration of 0.2 m (Pahlavan et al 2019b). Kaneko and Isaacs (2000) reported similar results for 1 m Br − and 1 m Cl − solutions.…”

“…For example, according to Srinivasan and Kelly (2017), stable pitting of a 316L stainless steel at room temperature requires a concentration around 50% of the saturated solution at the pit bottom (Srinivasan & Kelly, 2017), otherwise the pit repassivates. A deeper understanding of how measurements with the artificial pit electrode can explain E pit differences in Br − vs. Cl − solutions can be achieved with a suitable model for pit growth (Li et al, 2018;Pahlavan et al, 2019b). During pitting corrosion, metallic cations are produced by anodic dissolution at the pit bottom and diffuse out of the pit down a concentration gradient.…”

Investigation of the Microstructure Dependence of Critical Pitting Temperature and Pitting Potential in a 2205 Duplex Stainless Steel

“…The stability product is a criterion introduced by Galvele, 33,42,52,59 a critical value of which must be attained for pit stabilization. The critical value of the stability product depends on the saturation concentration of the cations dissolved during the process of pitting corrosion, 60 which should be similar for all studied microstructures. Therefore, the higher values of the stability product for the Widmanstätten specimen implies a closer situation to transition from metastability to stability.…”

“…Nonetheless, as there are no secondary precipitates in the equiaxed and hot-rolled microstructures, it can be said that the distribution of MnS inclusions might have been different in these two microstructures, resulting in their difference in terms of λ; the MnS inclusions are known as preferential sites for the pit nucleation. 60 On the other hand, the metastable pits developed in the equiaxed specimen at 55 °C have higher values of the stability product compared to the hot-rolled microstructure. As presented in Table II, the austenite is the weaker phase in both equiaxed and hot-rolled specimens, due to its lower PREN, and the pit propagates within its grains.…”

Unraveling the effects of surface preparation on the pitting corrosion resistance of austenitic stainless steel

“…The resistance of the passive film against pitting attack was correlated to E c -E corr [24]. The magnitudes of E c -E rp [25] and the hysteresis loop area [26] are indicative of the severity of pitting. The absence of the loop (absence of E rp ) was considered as an indication of the excellent resistance to pitting corrosion [27].…”