Electrochemical Dissolution Behavior and the Residue Formation Mechanism of Laboratory Made Carbonyl Nickel

• Why residue is formed during anodic dissolution of carbonyl nickel was explained.
• Spatiotemporal pattern of pitting in anodic Ni dissolution was described.
• The role of sulfur impurities on anodic Ni dissolution was explained.

The anodic dissolution of two laboratory-made Ni samples obtained using the carbonyl method was investigated to understand the origin of residue formation in the anode basket in an electroplating tank. The first sample was obtained with 3 ppm addition of carbonyl sulfide to introduce a small amount of sulfur (CN-S sample). The second was obtained without sulfur impurities (CN sample). Linear sweep voltammetry and chronopotentiometry were applied to characterize the dissolution of these samples. The dissolution of the CN-S sample took place in the active region at low overpotentials. This behavior is determined by the presence of sulfur impurities that break down the passive layer and facilitate Ni dissolution. The CN sample without sulfur was dissolved at high overpotentials. The overpotential-time plots displayed regular large amplitude oscillations in which the overvoltage periodically moved between the transpassive and passive regimes. The anodic dissolution of this sample was controlled by two competing processes: breakdown and formation of the passive layer. Scanning electron microscopy and white light interference microscopy were applied to monitor the morphological changes of the two samples as a function of the dissolution time. The results of these studies showed that the CN-S sample dissolved uniformly across the surface. However, the roughness and the aspect ratio of the protruding features on the surface increased with time. This sample produced a fine residue due to detachment of small protruding crystallites. In contrast, the dissolution of the CN sample involved pit formation and took place predominantly from the bulk of the pits. The dissolution of this sample left a porous skeleton of more passivated Ni. The residue in this case consisted of large, porous chunks of the skeleton.