Effect of pH of the galvanic bath on electrodeposition of Cu-Mn3O4 composite coatings

Cu-Mn3O4 composite coatings were prepared on the SUS 430 steel by means of the electrodeposition in an alkaline copper pyrophosphate bath containing Mn3O4 particles. The effect of bath pH on the content of Mn3O4 co-deposited in Cu matrix was investigated by electrochemical measurements including zeta potential, cathodic polarization and electrochemical impedance spectroscopy (EIS). The results showed that pH had a significant influence on co-deposition of Mn3O4 in Cu matrix. It was found that the absolute value of zeta potential of Mn3O4 particle, cathodic overpotential and Faraday resistance reached the maximum when the bath pH was adjusted to 9 or 9.5, which implied that mass fraction of co-deposited Mn3O4 particles in composite coating obtained at pH of 9 or 9.5 would be higher than those at other pH values. The compositions and microstructures of Cu-Mn3O4 composite coatings obtained at different pH values were consistent with the results of electrochemical analyses. The mass fraction of co-deposited Mn3O4 was highest (8.02 wt%) at pH of 9. It was proved that electrochemical tests were of great significance to ascertain suitable pH for Cu-Mn3O4 composite electrodeposition. Scratch test indicated the critical load (Lc) of the composite coating obtained at pH of 9 was 21.4 N.