The morphology of atmospheric pitting corrosion in 304L stainless steel plate was analysed using MgCl(2) droplets in relation to changes in relative humidity (RH) and chloride deposition density (CDD). It was found that highly reproducible morphologies occur that are distinct at different RH. Pitting at higher concentrations, i.e. lower RH, resulted in satellite pits forming around the perimeter of wide shallow dish regions. At higher RH, these satellite pits did not form and instead spiral attack into the shallow region was observed. Increasing CDD at saturation resulted in a very broad-mouthed pitting attack within the shallow dish region. Large data sets were used to find trends in pit size and morphology in what is essentially a heterogeneous alloy. Electrochemical experiments on 304 stainless steel wires in highly saturated solutions showed that the passive current density increased significantly above 3 M MgCl(2) and the breakdown pitting potential dropped as the concentration increased. It is proposed that the shallow dish regions grow via enhanced dissolution of the passive film, whereas satellite pits and a spiral attack take place with active dissolution of bare metal surfaces.

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

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The effects of nitrate and sulfate salts on the chloride-induced atmospheric pitting corrosion of 304L and 316L stainless steel was investigated through automated deposition of droplets of magnesium and calcium salts. Nitrate was found to inhibit pitting under magnesium salt droplets when the ratio between the deposition density of nitrate anions and chloride anions was above a critical value, which was the same for both 304L and 316L. This critical ratio was found to decrease with increasing humidity. Sulfate was also observed to inhibit pitting for MgCl 2 + MgSO 4 mixtures, but only at higher humidities. Sulfate did not show any inhibition for CaCl 2 + CaSO 4 mixtures, an effect attributed to the low solubility of CaSO 4 . At low relative humidities, precipitation of the inhibiting salt was observed, leading in some cases to crevice-like corrosion under salt crystals. The pitting behavior was explained in terms of the thermodynamic behavior of concentrated solutions. In the UK, stainless steel is used to package intermediate level radioactive waste, ILW, which is characterized by relatively large volumes and variable levels of radioactivity.1 Whatever the strategic approach to its management c , most ILW has been packaged in thinwalled containers (2.3-6 mm thick, typically grades 304L or 316L, UNS S304003 and UNS S316003, respectively) and will undergo long periods of exposure to atmospheric conditions, either in surface or underground facilities prior to permanent disposal.During these periods, waste containers will be exposed to regimes of varying temperature and relative humidity (RH), as well as to chloride-containing salts arising from aerosol deposition. As a result, it is important to identify suitable storage conditions to ensure durability of waste containers, in particular to avoid conditions associated with the development of pitting and, even more importantly, atmospherically-induced stress corrosion cracking (AISCC). 2,3Monitoring of ILW storage facilities and other indoor locations considered broadly representative of ILW stores suggests that temperature and relative humidity, which are key parameters in the development of atmospheric corrosion, are expected to vary between ∼0-30• C and ∼30-100% RH, respectively. 3 Ionic chemical species deposited on surfaces after relatively long periods of indoor storage found in swab tests in a variety of real storage facilities include calcium, magnesium, sodium, potassium, chloride, nitrate and sulfate ions. Inside the storage buildings surveyed, chloride deposition densities were found to be below ∼20 μg/cm 2 , with deposition rates of the order of 1 μg/cm 2 per year estimated. 3 With such a deposition rate, the chloride deposition density could increase to ∼100 μg/cm 2 over the next century.In atmospheric conditions relevant to this work, a number of tests have been carried out to evaluate the atmospheric pitting corrosion of stainless steel in the presence of chloride deposits (e.g., Refs. 2, 4-6) but none of these have been carried out in the presence of...