Cathodic reactions of electrolytic A1_2O_3 deposition on MAR-M247 superalloy

摘要

Combining X-ray diffraction with the analysis of cathodic polarization tests in H_2O, NaNO_3, HC1 and the mixture of HC1 aqueous solutions, respectively, the cathodic reactions of electrolytic A1_2O_3 on MAR-M247 superalloy in A1(NO_3)_3 aqueous solution were investigated. It was verified that the cathodic polarization curve in A1(NO0_3)_3 can be comminuted into four regions: (1) major: 2H~+ + 2e~- →H_2 and minor: O_2 + 2H_2O + 4e~- → 4OH~- (―0.1 to ―0.5V); (2) the reduction of (A1~(3+))_2(H_2O)_7 complex ion: (A1~(3+))_2(H_2O)_7 + 6e~- → 2A1(OH)_3 ·H_2O + 3H_2 (―0.5 to ―0.9V); (3) the diffusion limit of (A1~(3+))_2(H_2O)_7 complex ion (―0.9 to ―1.0 V); (4) the reduction of H_2O: 2H_2O + 2e → H_2 + 2OH~- (―1.0 to ―3 V). The best deposition condition of A1_2O_3 was found at region 3. The deposited film of 2A1(OH)_3·H_2O hydroxide hydrate was further transformed into γ-Al_2O_3 after being annealed at 450 ℃ for 1 h.