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.