Design of Corrosion Resistant Alloys Using Density Functional Theory: Part II. Chloride Susceptibility Index (CSI) as a Scientific Descriptor for Corrosion Resistance

Abstract

Evaluating corrosion resistance of alloys is vital to the design and application of the next generation of superior corrosion resistant alloys (CRAs). Here, Chloride Susceptibility Index (CSI), a scientifically based quantitative indicator for CRAs, is proposed as a metric constructed through combining atomistically resolved information regarding adsorption and alloy surface composition with environmental conditions such as temperature, pH, and Cl- concentration. Using this method, CRAs can be evaluated for any given application environment(s). The pathway to determining the CSI is as follows: (1) Thermoequilibrium surface coverages of O and Cl are calculated with a Langmuir isotherm model using adsorption energies of different species (O, Cl, OH, H₂O) obtained from DFT. (2) CSI is then calculated by integrating Cl surface coverage over a realistic window of applied potentials, which incorporates the effect of Cl surface coverage and electrochemical environment simultaneously. A series of Ni-Cr-X alloys was used as an example to develop and validate the method through studying the effect of different alloy solutes X on chloride resistance. It is found that the trends and relations predicted by CSI are in qualitative agreements with experimental observations. Moreover, a quantitative relationship is found between CSI and the repassivation potential.

Keywords: Corrosion, Nickel alloys, Repassivation, Surface