Inhibition of Localized Corrosion of Ni-Ti Superelastic Alloy in Sodium Hypochlorite Solution by Sodium Carbonate Addition

Inhibition of localized corrosion of Ni-Ti superelastic alloy in 0.1 M NaOCl solution has been attempted by adding Na2CO3. The addition of Na2CO3 scarcely affects the pitting potential. Nevertheless, at a more noble than pitting potential, the increase in current density is inhibited by increasing the amount of Na2CO3 added. In the immersion test for 24 h, with the addition of 0.01 M or 0.05 M Na2CO3, the corrosion potential often shifts rapidly in the less noble direction similarly to that observed without Na2CO3 addition, but it tends to shift back in the noble direction in a short time. By increasing the amount of Na2CO3 added to 0.1 M, the corrosion potential is stabilized at the noble potential. The mass losses due to localized corrosion are significantly inhibited by the addition even of 0.01 M Na2CO3. With the addition of Na2CO3 above 0.05 M, only very small corrosion pits are observed on the surface of the specimen. The relative XPS peak intensity of OH- increases with the amount of Na2CO3 added. The present results indicate that the addition of Na2CO3 is effective in the inhibition of the localized corrosion in NaOCl solution.