Extending the Passive Region of CrFeNi-Based High Entropy Alloys

This study provides principles for designing new corrosion resistant high entropy alloys. The theoretical framework is a percolation model developed by Newman and Sieradzki that predicts the ability of an alloy to passivate, i.e., to form a protective surface oxide, based on its composition. Here, their model is applied to more complex materials than previously, namely amorphous CrFeNiTa and CrFeNiW alloys. Furthermore, the model describes a more complex passivation process: reforming the oxide layer above the transpassive potential of Cr. The model is used to predict the lowest concentration of Ta or W required to extend the passive region, yielding 11-14 at% Ta and 14-17 at% W. For CrFeNiTa, experiments reveal a threshold value of 13-15 at% Ta, which agrees with the prediction. For CrFeNiW, the experimentally determined threshold value is 37-45 at% W, far above the predicted value. Further investigations explore why the percolation model fails to describe the CrFeNiW system; key factors are the higher nobility and the pH sensitivity of W. These results demonstrate some limitations of the percolation model and offer complementary passivation criteria, while providing a design route for combining the properties of the 3d transition metal and refractory metal groups. A percolation model is used to predict corrosion resistant high entropy alloy compositions. A single refractory metal (Ta,W) is added to CrFeNi-based alloys to extend the passive region to higher potentials. The minimum required concentration of Ta agrees with the model, while that of W is underestimated. Additional chemical criteria are then formulated for the corrosion resistance of complex alloys.image