Effect of nickel immersion pretreatment on the corrosion performance of electroless deposited Ni-P alloys on aluminum

Ni-P alloys with 7 wt% to 10 wt% phosphorus were deposited by sodium hypophosphite onto industrially pure aluminum substrates after different pretreatments. Two nickel immersion pretreatments (NIPs) were tested prior to electroless Ni-P plating, and their effects on the corrosion properties of the Ni-P alloy layers were evaluated using polarization curves and electrochemical impedance spectroscopy. The surface morphology and chemical composition of the Ni-P coatings were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis. Both proposed NIPs were found to improve the corrosion resistance. Thus, a double NIP method similar to the double zincate treatment method was developed. The alloy layer pretreated by this developed method showed the highest corrosion resistance, which can be mainly attributed to the high phosphorus content and low microporosity.