First-principles modeling of corrosion potential/current and its application in AZ and AZ-RE magnesium alloys

Abstract

First-principles modeling for magnesium corrosion potential/current has been established in this work. The corrosion potential formulation should be UaSHE=ln10RTlgnFkTh+Gv+NAN×S×Esurf+(10ln10)RT+0.65λ2nF−(λ1+λ2)nF(the intersection point of anodic and cathodic branch). And the one for the corrosion current density should belgJtot=lgJm+km×∑(fm×∆Em)+ki×∑(fi×∆Ei)(including matrix corrosion lgJm, matrix interplane galvanic corrosion km×∑(fm×∆Em), second phase galvanic corrosionki×∑(fi×∆Ei) and ∆E is the intergranular potential difference between phase/matrix planes and Mg(0001)). Applied the modeling to simulate AZ and AZ-RE electrochemical corrosion, the predicted potential/current values are quite coherent with the experimental polarization ones. The role of alloying element on corrosion have also been discussed at atomic level. The main alloying element (such as Al or Zn in AZ, or AZ-RE) would affect work function Φ, surface energy density Esurf/ρ, and hydrogen atom adsorption free energy ∆GH*, so it influences anode/cathode reaction greatly and works a lot on Ua and lgJm. Furthermore, main alloy and minor alloy (such as RE in AZ-RE) would make their own contribution on galvanic corrosion by holding the potential difference ∆Em or ∆Ei and keeping mole fraction fm or fi. The work provide the modeling which could predict the corrosion potential/current values for magnesium alloy, which are directly correspond to the polarization curves. It also could give a theoretical understanding for alloying effect on corrosion. It suggests a more flexible way for seeking corrosion-resistant alloys.