High-strength steels are increasingly being used in applications such as ship hulls and oil and gas pipelines which are subjected to corrosive environments. These steel grades exhibit more than an order of magnitude higher crack growth rates in corrosive environments when compared to the crack growth rates in air. Mechanistic understanding of the crack growth in the presence of corrosive environment has pointed out that the synergistic action between crack tip electrochemical and chemical conditions along with the mechanical crack driving force has been the primary factor in the enhancement of crack growth rates. The crack tip conditions are determined by a complex interplay between the high metal ion concentrations resulting from crack tip anodic reactions, mass transport (including ionic migration, diffusion, and advection), and electrochemical polarization of the metal surfaces which determines the extent of anodic and cathodic reactions occurring in the crack environment.
In this study, a mathematical model for a corroding crack is developed based on the multi-species transport model. The species distribution inside the crack is evaluated to infer the pH, potential drop and the corrosion rate inside the crack. Additionally, it is known that the stress and strain fields near the crack tip influence the potential and pH drop in the crack. The model takes into account the effect of plastic strain on electrode potential. It was found that the pH at the crack tip for a corrosion fatigue crack is lower than that for a static crack indicating pH accelerated corrosion at the crack tip. It was also found that the loading frequency has a more significant effect on the corrosion rate and the potential drop when compared to the load ratio. Thus, a simplistic model that can predict the crack tip's corrosion rate for various loading conditions in a particular environment is developed