Corrosion and metal release rates of Ni substrates under a porous gold coating

Noble metal coatings intended to protect devices from corrosion may have small porosity that allows corrosive fluid to attack the substrate alloy. A technique combining electrochemical tests and chemical analysis is described that measures low levels of porosity and low corrosion product release rates from the substrate. The technique has been applied to an electrodeposited gold coating on a nickel substrate. The results yielded a porosity of 1.8 × 10−3, defined as the area of substrate exposed to corrosion/area of the coating. Galvanic coupling between gold and nickel was found to increase the rate of nickel corrosion by about a factor of 10. A simplified model showed that if the substrate alloy does not passivate, the corrosion product release rate is expected to increase with exposure time.