The effects of heat treatment on structure, microhardness and corrosion behavior of electroless Ni-P deposits on Al 5083 alloy

In this work, electroless Ni-P deposits, of approximately 20 μm in thickness and a medium phosphorous content, of approximately 6.73 wt.%, were obtained on Al5083 substrate and the effects of heat treatment on structure, microhardness and corrosion behavior of the deposits were investigated. Deposits were heated at temperatures ranging from 200 to 400 °C for 1 h. The structural characterization was done by means of X-ray diffraction studies. As-deposited coating exhibit broadened X-ray reflections indicative of a semi-amorphous structure. Heat treatment decreased the amorphous phase and after heat treatment at 400 °C for 1 h, the final equilibrium phases were Ni3P and Ni crystals with size of 32 and 18 nm, respectively. The results of Vickers microhardness testing showed that after heat treatment at 200 °C, the hardness decreases and the maximum hardness was achieved for the samples which have been heat treated at 400 °C. This is caused by the formation of intermetallic Ni3P stable phase at this temperature, acting as a function of precipitation hardening. Corrosion resistance of the coatings was evaluated in 3.5 wt.% NaCl solution by potentiodynamic polarization and electrochemical impedance (EIS) methods. The fitting results indicated that on heating to 240 °C, the corrosion resistance increased and then reduced at higher temperatures. Heat treatment at 400 °C leads to crystallization of the amorphous phase, making available more number of grain boundaries which are highly prone to corrosion attack. Therefore, the heat treatment of electroless specimens at 400 °C significantly decreases their corrosion resistance.