Quantifying the Role of Transition Metal Electrodeposition in the Cathodic Activation of Corroding Magnesium

E Michailidou,G Williams,H N Mcmurray

doi:10.1149/2.0251805jes

Abstract

High purity (80 ppm iron) magnesium immersed in aqueous sodium chloride solution exhibits a filiform pattern of localized corrosion in which hydrogen is evolved at local (filament head) and remote (filament tail and uncorroded surface) cathode sites. Transition metal cations in solution are shown to significantly accelerate rates of corrosion, principally by activating (catalyzing) the remote cathode sites. The degree of activation is cation concentration dependent and efficiency increases in the order Mn2+ < Fe2+ < Zn2+ < Cu2+. It is proposed that activation occurs as a result of transition metal electrodeposition through a displacement reaction. It is also shown that precipitation of insoluble transition metal (hydr)oxides through time-dependent cation hydrolysis competes with, and reduces the efficiency of, electrodeposition-induced cathodic activation.

Highlights

Materials Research Centre, College of Engineering, Swansea University, Bay Campus, Crymlyn Burrows, Swansea SA1 8EN, United Kingdom
These include: i) transition metal particles in electrical contact with the Mg matrix accumulating at the Mg/solution interface as the Mg matrix corrodes, ii) transition metal particles becoming electrically separated from the Mg matrix, corroding with the release of the ions and these ions electrodepositing onto the Mg surface in an electrochemical displacement reaction and iii) transition metal ions released as in ii) becoming incorporated in the Mgoxide film at the Mg/solution interface and rendering it more conductive or electrocatalytically active.[1,5,7,14,15]
The present paper aims to quantify the effect that transition metal ions have on the cathodic activation of corroding magnesium when added to solution in a known concentration

Summary

Results and Discussion

In order to estimate the influence of competing precipitation reactions a second set of volumetric H2 evolution measurements were performed in which the solid, crystalline metal salts were deposited directly onto the exposed area of the magnesium sample. This approach was intended to minimize the effect of hydrolysis by reducing the ion mass transport (diffusion and/or migration) path length and the time during which hydrolysis could occur.

Metal Ion

RT P

Conclusions

Amplification Factor