A high anticorrosive chromium-free conversion coating prepared with an alkaline conversion bath on electroless Ni-P coating

A molybdate conversion coating (MCC) thin film was deposited on electroless Ni-P (ENP) coating by treating in an alkaline chemical conversion bath. A chromate conversion coating (CCC) was also prepared for comparison. Surface morphology of the conversion coatings exhibited no obvious difference with the as-plated ENP coating. Acid exposure test indicated that the oxidation resistance of the MCC was improved about 45 times higher than that of untreated ENP coating, respectively. The corrosion current density (icorr) values for both passivated surfaces were dramatically reduced by conversion treatment. However, MCC specimen showed the lowest icorr value of 0.19 μA cm−2 compared to 6.55 μA cm−2 for ENP and 0.38 μA cm−2 for CCC. EIS studies revealed that the MCC thin film possessed the highest charge transfer resistance (Rct) value, about 5 times higher than the CCC. XPS analysis indicated that the MCC thin film was made up of Ni, P, O, and Mo; the values of binding energy (BE) implied that both Ni and P participated in the formation of conversion coating thin film and the Mo was in the form of MO3.