Crevice Corrosion of Grade-2 Titanium

Titanium alloys are highly corrosion resistant under oxidizing conditions, but can suffer crevice corrosion at elevated temperatures (>70°C) if the surface is occluded by another material, such as a deposit, a poor weld or a crack. The conditions which develop within a crevice are such that, in the event of the breakdown of the protective TiO2 film, Ti metal can dissolve and hydrolyzed, creating acidic conditions that induce corrosion. The key parameters influencing the crevice corrosion of Ti are oxygen concentration, temperature and material microstructure. In this study, the effects of these parameters are investigated.
Electrochemical measurements of the crevice current, Ic, and the crevice potential, Ec, (Figure 1) show the progression of the crevice corrosion. The planar potential, Ep, is from the planar electrode, which does not have a crevice, and is used as a comparison to Ec. The crevice current is a measure of the consumption of dissolved oxygen, the supporting cathodic reaction driving crevice corrosion. The relationship between Ic and Ec is clear. The variation of Ic with time suggests a number of periodic, short crevice corrosion events.

Dynamic SIMS images of a cross section of a corroded grade-2 Ti sample (Figure 2) show the redistribution of impurity elements at the corroded interface. The O and Ti images identify the interface between the alloy and the TiO2 corrosion product. The accumulation of Mo in the oxide at the corroding interface may assist the stifling of crevice corrosion (i.e. Ic decreasing to 0 in Figure 1). The accumulation of Ni on the Ti surface may also have assisted stifling by catalyzing proton reduction within the creviced area.
By Thalia Wells, Department of Chemistry, Western University