Bimodal trending in corrosion loss of magnesium alloys

Data for mass loss of a variety of magnesium alloys as a function of exposure period show that corrosion loss follows bimodal trending with time for different exposure environments, both laboratory and field support these findings. For data sets sufficient to discriminate bimodal behaviour, the instantaneous rate of corrosion at the commencement of the second mode is (close to) 4 times the instantaneous rate of corrosion at the end of the first mode (i.e. through the transition period). This observation is consistent with the theoretical relative diffusivities of oxygen and hydrogen through the corrosion product layer as it exists during the transition period. These findings support the notion that the bimodal model has corrosion in Mode 1 rate-controlled by the cathodic oxygen reduction reaction and the inward diffusion of oxygen while in Mode 2 corrosion is rate-controlled by the cathodic hydrogen evolution reaction and the outward diffusion of hydrogen. Similar findings have been made previously for various ferrous and other alloys and thus throws new light on the development of corrosion of magnesium alloys. It also provides reasons for measurements of hydrogen evolution and for electrochemical techniques underestimating magnesium corrosion rates. A new procedure for combining these is proposed.