The disparity of corrosion resistance between Ni/Au and Ni–P/Au electrical contacts in mixed flowing and salt spray tests

Single-layer barrier systems such as Ni/Au and Ni–P/Au (the function of the ultra-thin Au top layer is not considered as a barrier), as opposed to bilayer stacks like Ni/Ni–P/Au, have been reported to display enhanced corrosion resistance. Within the single-layer barrier systems, Ni/Au appeared more corrosion resistant to neutral salt spray (NSS) test, while Ni–P/Au appeared more corrosion resistant to acidic mixed flowing gas (MFG) test. To explain such disparity factors potentially influencing corrosion resistance, such as internal stress, surface wettability and sulfur co-deposition have been examined. Interestingly, none of these factors could account for the disparity in the corrosion performance. Moving from a perspective of overall corrosion to one that focuses on the location of corrosion products was imperative to explain the disparity in corrosion resistance. Detailed pits analysis revealed that the disparity was caused by the edge porosity and corrosion product migration. These new concepts were further supported by a purposely designed experiment. Corrosion product analysis was also performed to explore the corrosion mechanisms in the NSS and MFG tests. The findings bring new insights into the design of corrosion-resistant metallic coatings in the future.