This paper presents analytic expressions for calculating bounding conditions for pitting under atmospheric conditions. These expressions allow the prediction of the maximum pit size that can develop under known atmospheric conditions by considering the factors that can control the inherent galvanic coupling between a circular pit under a thin electrolyte layer surrounded by a concentric cathodic area. Expressions are developed for the maximum cathodic current and the minimum anodic current required for pit stability. An analytic expression for the maximum cathodic current that the surrounding area can supply to the pit is developed and validated by comparison to calculations using the finite element method. The effects of the controlling environmental parameters ͑deposition density and relative humidity͒ on the cathode bounding parameters are explored, as is the effect of the size of the pit. The analytical expression for the maximum cathodic current is then coupled to the Galvele pit stability product to estimate the maximum pit size that could develop for a given set of environmental conditions. Those results are then compared to data available in the literature from outdoor exposures of stainless steels for up to 26 years. Corrosion resistant alloys such as stainless steels rely on their passive films for the maintenance of their characteristically low dissolution rates. Under most conditions, this film provides outstanding protection against uniform corrosion. Corrosion resistant alloys do suffer from localized corrosion when discrete locations lose this protection. It is generally accepted that such local loss of protection occurs spontaneously in the presence of aggressive ions, although in most cases, the alloy surface is able to repassivate the failed oxide film, thereby limiting the damage. These metastable events have been the subject of many investigations 1-4 to understand the underlying processes that lead to the more damaging, stable pits. Models have been constructed that relate the frequency of metastable pitting to the likelihood of stable pitting. [1][2][3]5 Such models are of use for predicting the likelihood of corrosion damage occurring in service if the material and exposure conditions are known, although they generally do not predict the extent of damage, although there are exceptions.5 In many applications of corrosion resistant alloys, only stable pitting creates sufficient damage to be of concern. Others have fit exposure data to develop empirical power laws for pit propagationwhere d pit is the maximum pit depth measured, t is the exposure time, and A and n are constants that depend on the material and environment. While useful for the environments and materials used in the exposure testing, such approaches are difficult to extend to other material/environment systems as the underlying factors controlling pit size are usually not clearly delineated.In applications where the intended service life is very long, it may be considered unwise to extrapolate empirical power laws to times orde...

The corrosion of Ag contaminated with NaCl particles in gaseous environments containing humidity and ozone was investigated. In particular, the effects of relative humidity and UV light illumination were quantitatively analyzed using a coulometric reduction technique. The atmospheric corrosion of Ag was greatly accelerated in the presence of ozone and UV light. Unlike bare Ag ͑i.e., with no NaCl particles on the surface͒, Ag with NaCl exhibited fast corrosion even in the dark, with no UV in the presence of ozone. Samples exposed to different outdoor environments and samples exposed in a salt spray chamber were studied for comparison. Ag corroded at extremely low rates in a salt spray chamber partly because of the combined absence of light and oxidizing agents such as ozone. © 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3310812͔ All rights reserved. Despite their widespread use, the results of accelerated atmospheric corrosion tests such as ASTM B117 often do not correlate well with field exposures.1 As shown below, silver is an example of this phenomenon as silver oxidizes readily during outdoor field exposure but is barely attacked in a salt spray chamber. Clearly, some controlling factors of the chemical and electrochemical reactions in the field exposures are not accurately reproduced in the salt spray chamber environment.Atmospheric corrosion of silver is affected by factors such as relative humidity ͑RH͒, 2-4 airborne pollutants, 5,6 and temperature. Traditional atmospheric corrosion studies of Ag have focused mainly on sulfidation 3,7-9 as silver reacts strongly with sulfurcontaining species. The absence of such species in the ASTM B117 test is one reason why silver is relatively inert in the salt spray environment. However, there has been little focus on the effects of chloride and photoassisted corrosion.In a previous paper, 10 the effects of ozone, UV radiation, and RH on the atmospheric corrosion of bare silver were studied. Atomic oxygen generated by the photolysis of ozone by UV radiation reacted quickly with bare silver to form silver oxide. However, 254 nm UV radiation or ozone alone did not cause any corrosion of Ag within the exposure period. The corrosion rate of bare silver was relatively independent of RH, and it was suggested that the initial chemisorption of atomic oxygen or OH radical under wet conditions is relatively unaffected by RH. The effects of chloride on these reactions are not known. Furthermore, the atmospheric corrosion rate of metals in the field is typically dependent on RH as a result of the interactions of water with salts on the surface. 11Sea-salt aerosols can play a critical role in the atmospheric corrosion process. Cole et al. described marine aerosol formation, chemistry, reaction with atmospheric gases, transport, deposition onto surfaces, and reaction with surface oxides.12 The small size of sea-salt aerosols allows them to be carried away by the wind to places far away from the sea.13 These aerosols are created by physical processes such as the bursting of air...

Corrosion-resistant materials under atmospheric conditions can suffer from localized corrosion ͑e.g., pitting, crevice, stresscorrosion cracking͒. The stability of such a localized corrosion site requires that the site ͑anode͒ must dissolve at a sufficiently high rate to maintain the critical chemistry while a wetted surrounding area ͑cathode͒ provides matching cathodic current. The objective of this study was to computationally characterize the stability of such a local corrosion site and explore the effects of physiochemical parameters and electrochemical kinetics. The goal is to contribute to the establishment of a scientific basis for the prediction of the stabilization of localized attack. An analytical method for evaluating the stability of localized corrosion of corrosion-resistant alloys under thin-layer ͑or atmospheric͒ conditions is presented. The method uses input data that are either thermodynamic in nature or easily obtained experimentally. The maximum cathode current available depends on the cathode geometry, temperature, relative humidity, deposition density of salt ͑i.e., mass of salt per unit area of cathode͒, and the interfacial electrochemical kinetics. The anode demand depends on the crevice geometry, the position of attack within the crevice, and the localized corrosion stability product. By coupling these two approaches, the stability of a crevice can be determined for a given environmental scenario. The method has been applied to the atmospheric crevice corrosion of type 316L stainless steel.

The corrosion of Ag in an atmosphere of ozone and humidity with or without irradiation by ultraviolet (UV) light was investigated. A modified coulometric reduction technique was used, substituting sulphate solution for chloride solution, to prevent the spontaneous transformation of silver oxide corrosion product to chloride in the reduction solution. The presence of both ozone and UV radiation was required for fast corrosion of Ag to occur. The amount of corrosion product for a given exposure time increased with ozone concentration, whereas the relative humidity had little effect. An incubation time for the corrosion reaction was observed. The presence of both ozone and UV radiation were necessary for rapid corrosion because the photodissociation of ozone generates reactive atomic oxygen, which reacts with Ag rapidly to form Ag 2 O. The corrosion reaction on bare silver was minimally affected by the relative humidity in the environment, which is contrary to common atmospheric corrosion experience.