The role of pyrophosphate in copper deposition from cupric pyrophosphate solutions

Cathodic polarization characteristics of copper was examined in cupric pyrophosphate solutions of different concentrations and temperatures. The structure and composition of the deposited copper was examined by scanning electron microscopy, X-ray diffraction, electron probe microanalysis and Auger electron spectroscopy. The polarization characteristics implied that the coverage of the electrode by adsorbed pyrophosphate ions decreases with increasing cathodic polarization and it becomes almost zero past a critical potential E_b of about ?0.75 V vs sce. E_b becomes more negative with increasing concentration of free pyrophosphate ions. As expected, pyrophosphate was incorporated at relatively high concentrations (0.05–0.07 wt% as P) in the deposits in the potential region I (E ? E_b) and at lower concentrations (ca. 0.02 wt%) in region II (E < E_b). Smooth deposits with random crystalline orientations were obtained in a part of region I for cd ? ca. 5 mA/cm². The rough deposits obtained outside this showed preferred (220) orientations as could be expected from the orientation theory. The smooth deposition in region I is due to the pyroposphate adsorption which inhibits the ordered deposition of copper.