The influences of monoethanolamine additive on the properties of nickel coating electroplated in post supercritical carbon dioxide mixed Watts bath

Post supercritical (PSC) carbon dioxide electroplating process uses the fully mixed electrolyte with CO2 in supercritical state and then performs conventional electroplating at atmospheric pressure. The oversaturated electrolyte provides the regenerated CO2 micro bubbles and similar improvement on the prepared coating as in the supercritical electroplating is obtained. However, after being exposed to atmosphere, the electrolyte diminishes gradually the influence of oversaturated CO2. In this study, monoethanolamine (MEA) was used as an additive for a Watts bath prepared for the PSC CO2 electroplating. MEA had been employed in CO2 capture for reducing carbon emission from power plants. The adoption of MEA in the electrolyte was intended to control the CO2 regeneration from the post supercritical mixing and, subsequently, to assist the electroplating. The experimental results showed that the nickel coating prepared by adding 5 mL/L MEA in the electrolyte increased its hardness and corrosion potential both for conventional and PSC electroplating. Cross-sectional TEM micrograph revealed the columnar grain growth in the prepared nickel coating. Adding MEA in the electrolyte provided the PSC electroplated nickel coating with the emergence of crystalline defects such as dislocation and twin along with nano-grains. Therefore, the coating's hardness increased accordingly. Moreover, the collateral increase in internal tensile stress due to these defects produced cracking in the coating. More cracking and higher surface roughness of the coating should be responsible for the larger corrosion current measured in the polarization curves.