A physics-based model has been developed to improve the fundamental understanding of the active chemistry of metal and alloy surfaces that are exposed during aggressive pitting or the growth of an environmentally assisted crack in media containing chlorides and sulfides. In such cases, the perpetuation of corrosion or cracking depends on the interactions between the chemical species in the environment, the composition of the freshly exposed material surfaces and the underlying microstructure. A method for directly calculating the dominant chemical reactions occurring at such freshly exposed alloy surfaces has been developed, using interaction energies calculated by quantum chemics as inputs for the surface reactivity coefficients. The resistance of a material to a particular environmental condition (specified as pH, chloride concentration, temperature and/or pH2S) can then be quantified using a ‘chloride susceptibility index’. In this paper the scientific approach is presented and applied to provide insights regarding the role of composition and environment on the localized corrosion of steel and nickel-based alloys.
During crack growth and localized corrosion, fresh metal surfaces become exposed to the chemical and electrochemical environment. Subsequent alteration of the material in the nascent pit or at the crack tip zone will be initiated by the surface chemical processes that template the subsequent reactivity. Quantum chemical calculations can provide a physics-based method to simulate the adsorption phenomena that occur in systems possessing multiple chemical species (such as water, chloride, hydrogen-sulfide, inhibitors, etc.), various pH levels, temperatures and electrochemical conditions. Recent innovations have also allowed assessment of adsorption phenomena on to metal alloy surfaces having complex composition (such as high-entropy alloys).[1] In this paper, the mathematical framework for simulating these adsorption processes with the help of adsorption energies calculated using quantum chemistry is presented. The significance of phase transitions predicted by the model with respect to the dominant surface chemisorption states are then quantified by introducing "susceptibility indices". These indices are calculated by integrating the fractional surface coverage of important chemical species (such as chloride or, possibly in future applications, hydrogen) as a function of the electrochemical potential.
