The “black pad” phenomenon, which rarely occurs in electrolessly plated nickel–phosphorous (Ni–P) films, was reproduced. This work reports that nodular variation of the P content induces potential differences large enough to drive galvanic corrosion when the film is exposed to an electrolyte containing gold cyanide. Subsequent corrosion couple experiments using Ni–P films with different P contents demonstrated that preferential corrosion occurred at the low P side, and the propensity of forming black pad increased with the difference in P content (ΔP).