Localised corrosion: general discussion

Abstract

Philippe Marcus opened a general discussion of the paper by Roger Newman: Should the passive lm not be signicantly involved in pit initiation (as you suggest), how would you explain that the time to initiation is very much dependent on the nature of the passive lm?Roger Newman responded: I never said that the passive lm is not involved in pit initiation. I understand (of course) that longer passivation gives a longer induction time for pitting. What I say is that the effects of parameters like alloy composition, environment composition, potential, and temperature, are not easily accommodated (at least not predictively) within a passive lm breakdown model, but fall out naturally from a modied Galvele type of model that uses pitting potential data (or, if one has the time, lower extremes of pitting potential distributions). Now I don't know whether or not the nest details of lm breakdown, detectable at the pA level or lower in electrochemical experiments (not nA to mA -those are already pits), and/or on very pure, at alloy surfaces, follow the same rules that we nd using pitting potentials on industrial or semi-industrial alloys. Those measurements have not been done. Actually I don't think stainless steel is necessarily the best model system for such studies. Under certain conditions, as shown by Bardwell many years ago, iron shows blizzards of pits that are clearly not impurity-particle-related; probably aluminum too. In stainless steel we really don't know whether pit initiation ever occurs without a microcrevice and/or an impurity particle.From the viewpoint of practical utility, the Galvele type of approach clearly has the advantage. The Critical Pitting Temperature (CPT) is a propagation-related transition below which metastable pits never become stable at any potential. My group has published extensively on that. One can make a foolproof, if expensive, † Electronic supplementary information (ESI) available: SVET scans movie. See

Integrated computational materials engineering of corrosion resistant alloys

“…64,84 Given the high rates of ionic phase diffusion away from small localized corrosion sites, propagation (i.e., pit or crevice growth) may also be stifled by the inability of the coupled cathode to polarize the local corrosion site to high enough potentials to achieve the dissolution rate required to maintain the aggressive local environment. 85,86 Therefore, coupled oxygen reduction reaction (ORR) kinetics are important.…”

The Effect of Deposition Conditions on Atmospheric Pitting Corrosion Location Under Evans Droplets on Type 304L Stainless Steel

“…It has been suggested [28] that the lack of trend in position of pitting site on highly passive alloys may be due to a reduced rate of oxygen consumption on the altered passive layer, preventing differential aeration from developing in the droplet. However, WBE arrays constructed with 304 stainless steel [29] show that large droplets cause a trend in corrosion towards the droplet edge rather than the center, suggesting oxygen access does play a role.…”

Low-Temperature Environmentally Assisted Cracking of Grade 2205 Duplex Stainless Steel Beneath a MgCl2:FeCl3Salt Droplet

“…The formation of oxygen concentration cells between the actively dissolving central area (anode) and the peripheral droplet region (cathode) formed an active-passive coupling system. [53][54][55][56][57] The occurrence of corrosion on passive stainless steel surfaces under a droplet deposit, however, consists typically of one or more local anodically confined sites (e.g., pits/crevices), with cathodic regions distributed around these sites over the area covered by the deposit. 24,[57][58][59][60] As shown in Figure 1(c), the anodic areas were not entirely at the center of the droplet, which is in line with previous observations.…”