Phosphorous alloying and annealing effects on the corrosion properties of nanocrystalline Co-P alloys in acidic solution

The corrosion properties of nanocrystalline Co-1.1 and 2.1 wt% P alloys and their annealed alloys as well as nanocrystalline Co were studied in a deaerated 0.1 M sulfuric acid (H₂SO₄) solution using electrochemical measurements and x-ray photoelectron spectroscopy (XPS) surface analysis. Potentiodynamic polarization testing and impedance measurements revealed that an addition of bulk P content to nanocrystalline Co enhanced the corrosion resistance; however, this enhancement was compromised by the annealing process at 350°C and 800°C. The enhanced corrosion resistance of nanocrystalline Co-P alloys at open-circuit potential was due to the increase of elemental P concentration on the surface that acted to hinder the anodic dissolution kinetics of surface Co following an initial selective dissolution of Co. However, the superior corrosion resistance of Co-P alloys did not last at the high anodic potential region, because of the formation of a nonprotective surface film consisting of mainly hypophosphite and phosphate as well as elemental P showing a pitting corrosion.