Pulse electrodeposition of Ni–P alloy coatings from Watts baths: P content, current efficiency, and internal stress

Abstract

Ni–P alloy has been used as a protective coating for mechanical parts, a surface material for electronic parts, and an electrocatalyst for water electrolysis. Ni–P alloy electrodeposition was performed by adding H3PO3 to Watts baths under four different types of pulse current conditions and compared with direct current (DC) electrodeposition. When the peak current density was kept constant and the off time was extended, the P content increased, and the current efficiency decreased. When the off time was extended while the average current density remained constant, the P content peaked. This is thought to be due to a change in the phosphorus precipitation mechanism from indirect to direct. The internal stress trend of the Ni–P electrodeposition was measured in-situ using a spiral contractometer up to about 20 μm thickness. The results confirmed that it exhibited internal tensile stress due to hydrogen desorption. As the current efficiency decreased, the internal tensile stress tended to decrease because some excess hydrogen was not desorbed and remained inside. The internal tensile stress could be remarkably reduced to half that of DC electrodeposition while maintaining a high current efficiency by reducing the frequency of pulse electrodeposition to accelerate hydrogen desorption, or by applying a reverse current to consume the electrons that induce hydrogen production.