Electrochemical behavior of ZrB2 in aqueous solutions

The corrosion behavior of a dense ZrB(2) material (hot pressed with 2.5 wt % Si(3)N(4) as a sintering aid) in aqueous acid solutions containing different aggressive anions such as chlorides, perchlorates, sulfates, oxalates, and fluorides was investigated by means of several experimental techniques. The methodology adopted consists in polarization curve recording, gravimetric corrosion rate determination, and potentiostatic tests coupled to solution analysis, scanning electron microscope observations and X-ray photoelectron spectroscopy surface chemical characterization. During the corrosion process, ZrB(2) is converted into both soluble [boric acid and complexes of the different anions with Zr(IV)] and insoluble (mainly ZrO(2)) corrosion products. The experimental results, compared to literature data, show that the higher the stability of the Zr(IV) soluble complex formed by a particular anion, the higher the corresponding solution aggressiveness and consequently the lower the protectivity of the surface oxide film. The solution aggressiveness was found to increase in the following order: perchlorates congruent to chlorides << sulfates congruent to oxalates, fluorides.