The mechanism of electrolytic Al/sub 2/O/sub 3/ coating on MAR-M247 superalloy

Through an analysis of cathodic polarization experiments in H/sub 2/O, NaNO/sub 3/, HCl and a mixture of HCl and NaNO/sub 3/, solutions respectively, the mechanism of coating electrolytic Al/sub 2/O/sub 3/ on MAR-M247 superalloy was investigated. We suggest that the cathodic polarization curves in Al(NO/sub 3/)/sub 3/ can be comminuted into 4 steps: 1. H/sup +/+e/sup -//spl rarr/H/sub 2/ (-0.1 V/spl sim/-0.35 V), 2. reduction of Al/sup 3+/(H/sub 2/O)/sub 3/ complex ion: 2Al/sup 3+/(H/sub 2/O)3/spl middot/xH/sub 2/O+6e/sup -//spl rarr/2Al(OH)/sub 3//spl middot/xH/sub 2/O+3H/sub 2/ (-0.35 V/spl sim/-0.65 V), 3. diffusion of /sup +/(H/sub 2/O)/sub 3/ complex ion (-0.65 V/spl sim/-0.9 V), and 4. reduction of H/sub 2/O: 2H/sub 2/O+2e/sup -//spl rarr/H/sub 2/+2OH/sup -/ (-0.9 V/spl sim/-4 V). X-ray diffraction diagrams show the Al(OH/sub 3/) gel transforms into amorphous-Al/sub 2/O/sub 3//spl rarr//sup 623 K//spl gamma/-Al/sub 2/O/sub 3//spl rarr//sup 973 K//spl delta/-Al/sub 2/O/sub 3//spl rarr//sup 1123 K//spl theta/-Al/sub 2/O/sub 3/.