Through a combination of mechanical stresses and corrosive environments, a material’s performance may be hindered by the complex evolution of damage due to stress corrosion cracking (SCC) or corrosion fatigue (CF). Understanding the contribution of the localized corrosion features, loading state, crack-formation features, local microstructure, and environment remains a critical issue when predicting crack initiation and propagation leading to potential metal failure. As such, the lifetimes of many exposed alloys are greatly reduced by the presence of corrosion damage and the prediction of this deleterious influence via standard fracture mechanics methods is nontrivial. Current knowledge is insufficient to fully address governing features and mechanism of the pit-to-crack transition, a common initiation mode of SCC and CF. This review examines current research of pit-to-crack transitions for various alloys and loading conditions and highlights critical areas of research necessary for informing the mechanism related to a material’s lifetime in a stressed corrosive environment.

Thermodynamic modeling has been used to predict chemical compositions of brines formed by the deliquescence of sea salt aerosols. Representative brines have been mixed, and physical and chemical properties have been measured over a range of temperatures. Brine properties are discussed in terms of atmospheric corrosion of austenitic stainless steel, using spent nuclear fuel dry storage canisters as an example. After initial loading with spent fuel, during dry storage, the canisters cool over time, leading to increased surface relative humidities and evolving brine chemistries and properties. These parameters affect corrosion kinetics and damage distributions, and may offer important constraints on the expected timing, rate, and long-term impacts of canister corrosion.

Calculated maximum pit sizes for stainless steel 304L (SS304L) are presented for atmospheric conditions with elevated chloride concentrations and temperatures. These calculations were carried out by experimentally determining the pit stability product ((i · x)_sf), repassivation potential (E_rp), and cathodic kinetics. These results were aided by determination of a solubility curve as a function of temperature for a stoichiometric metal salt mixture of iron chloride (FeCl₂), chromium chloride (CrCl₃), and nickel chloride (NiCl₂). Based on these newly measured saturation concentrations, metal ion diffusivities were calculated. Overall, it was found that (i · x)_sf decreases with increasing chloride concentration and increases with increasing temperature. E_rp was determined to decrease with increasing concentration and increase with increasing temperature. When incorporated into the maximum pit size model, these experimental results yielded a small increase in maximum pit size with increasing bulk chloride concentrations and/or increasing temperature.

The natural convection boundary layer ( δ n c ) and its influence on cathodic current in a galvanic couple under varying electrolytes as a function of concentration (1 − 5.3 M NaCl) and temperature (25 °C−45 °C) were understood. Polarization scans were obtained under quiescent conditions and at defined boundary layer thicknesses using a rotating disk electrode on platinum and stainless steel 304L (SS304L); these were combined to determine δ n c . With increasing chloride concentration and temperature, δ n c decreased. Increased mass transport (Sherwood number) results in a decrease in δ n c , providing a means to predict this important boundary. Using Finite Element Modeling, the cathodic current was calculated for an aluminum alloy/SS304L galvanic couple as a function of water layer (WL) thickness and cathode length. Electrolyte domains were delineated, describing (i) dominance of ohmic resistance over mass transport under thin WL, (ii) the transition from thin film to bulk conditions at δ n c , and (iii) dominance of mass transport under thick WL. With increasing chloride concentration, cathodic current decreased due to decreases in mass transport. With increasing temperature, increased cathodic current was related to increases in mass transport and solution conductivity. This study has implications for sample sizing and corrosion prediction under changing environments.

Assessing the lifetimes of alloys in humid, corrosive environments requires growth kinetic information regarding individual instances of damage, e.g. pit growth rates. Corrosion rates measured at the continuum scale using mass change convolute the rate of pit nucleation and growth, providing limited information on local kinetics. The current study used in-situ X-ray computed tomography to measure growth rates of individual pits in aluminum over 100 h of exposure in a humid, chloride environment. While pits grew at relatively constant rates over the first hours after nucleation, significant growth-rate nonlinearities subsequently occurred. These were linked to both droplet spreading, which altered the cathode size, and changes in the mode of pit growth. Pit morphology appeared to influence the dominant growth mode and the duration of pit growth. Post-mortem serial sectioning revealed pits preferentially attacked grain-boundary triple junctions and dislocation boundaries.