Cathodic reactions of electrolytic Al2O3 deposition on MAR-M247 superalloy

Combining X-ray diffraction with the analysis of cathodic polarization tests in H2O, NaNO3, HCl and the mixture of HCl and NaNO3 aqueous solutions, respectively, the cathodic reactions of electrolytic Al2O3 on MAR-M247 superalloy in Al(NO3)3 aqueous solution were investigated. It was verified that the cathodic polarization curve in Al(NO3)3 can be comminuted into four regions: (1) major: 2H++2e−→H2 and minor: O2+2H2O+4e−→4OH− (−0.1 to −0.5 V); (2) the reduction of (Al3+)2(H2O)7 complex ion: (Al3+)2(H2O)7+6e−→2Al(OH)3·H2O+3H2 (−0.5 to −0.9 V); (3) the diffusion limit of (Al3+)2(H2O)7 complex ion (−0.9 to −1.0 V); (4) the reduction of H2O: 2H2O+2e→H2+2OH− (−1.0 to −3 V). The best deposition condition of Al2O3 was found at region 3. The deposited film of 2Al(OH)3·H2O hydroxide hydrate was further transformed into γ-Al2O3 after being annealed at 450 °C for 1 h.