Evaluation of porosity and discontinuities in zinc phosphate coating by means of voltametric anodic dissolution (VAD)

In this study, the porosity and the presence of discontinuities, such as cracks, in zinc phosphate coatings were evaluated by the voltametric anodic dissolution (VAD) method. Zinc phosphate (PZn), zinc phosphate with niobium (PZn + Nb), and zinc phosphate with ammonium niobium oxalate (Ox) and benzotriazole (PZn + Ox + BTAH) coatings deposited on SAE 1010 carbon steel were investigated. Coating porosity was evaluated by estimating the charge densities associated with the substrate passivation process for samples with a phosphate layer and comparing the results to the charge densities for passivation of the same substrate without a coating phosphate layer. Weight loss measurements, induced coupled plasma optical emission spectroscopy (ICP-OES), and scanning electron microscopy (SEM) were also used to investigate the solubility of the phosphate layers tested. The electrolytes used were four buffer solutions with pHs of 7.0, 8.0, 10, and 12. Scanning rates of 30, 50, 100, and 150 mV s− 1 were used in the VAD tests. The porosities of the PZn, PZn + Nb, and PZn + Ox + BTAH layers were estimated by VAD to be 4.35, 1.96, and 1.37%, respectively. The lower porosities of the PZn + Nb and PZn + Ox + BTAH layers are related to their morphologies, which promote better surface coverage compared to the PZn layer.