SINUSOIDAL POTENTIAL SCAN-BASED NICKEL EL ECTRODEPOSITION ON ANODIZEDALUMINUM .4. ELECTROLYSIS AND BLACK COLORATION DURING REPEATED POTENTIAL SCANS USING VARIOUS WAVE PATTERNS

In a conventional electrolytic bath (5% NiSO4-4% H3BO3), the potential of anodized films of aluminum was scanned repeatedly in various sinus oid-based wave patterns. Continuous cyclic voltammograms in each patte rn showed that the two current peaks (I1 and I2) of Ni deposition and H2 evolution clealy observed in the first cycle decayed and coalesced into a low broad one in the second cycle and later until anomaly appea red due to the spalling of the films. Although Ni deposition increased with increasing cycles until the spalling took place, the molar ratio (S) of its increment per cycle to that of H2 evolution decreased shar ply in the intial few cycles. Interestingly introduction of a brief an odic scan prior to each cathodic scan above was effective in keeping t he S values of the later cycles significantly higher (0.5 similar to 0 .85) than the cases of simple cathodic scans (0.22 similar to 0.4). Co nsistently the lowest value of brightness index (L) as low as 23 was achieved by repeating such a scan. Irrespective of the wave patterns u sed,however, the ac impedances of the films were found to show rather unanimous behavior on increasing the number of cycles, i,e.,a slight d ecrease for the intial 600s of electrolsis, fllowed by an abrupt incre ase and a final decay due to the spalling. It is considered that the b arrier layer of the films suffers from corrosion during the electrolys is and that, once etch pits reach the alminum substrate, the hydrogen evolved there penetrates and accumulates in the barrier layer-substrat e interface, giving rige to the abrupt increase of impedance prior to the spalling.