Effect of the Time-Temperature Conditions of Annealing on the Corrosion Resistance of Electroless Ni–P Coatings

We analyze the effect of different temperature-time conditions of the process of annealing applied to electroless Ni–P (12 wt.% P) coatings in order to improve their corrosion resistance in a 3.5% NaCl solution. A process of annealing at 400°C for 4 h applied to Ni–P electroless coatings (12 wt.% P) induces the crystallization of the coating accompanied by the formation of stable Ni₃P and Ni phases. This Ni–P crystalline coating presents better corrosion resistance in the 3.5 wt.% NaCl solution than the amorphous alloy and the coatings annealed at 400 and 500°C for 1 and 2 h. The corrosion resistance of the Ni–P coatings is evaluated by the method of electrochemical impedance spectroscopy for 4 h of immersion. The microstructure of the Ni–P coatings prior to and after annealing at 400 and 500°C for 1, 2, and 4 h is studied by X-ray diffraction (XRD) and scanning-electron microscopy. The procedure of annealing at 400°C for 4 h induces the preferential orientation of the plane (112) of the Ni₃P phase, as shown by the XRD analysis, which modifies the kinetics of the electrochemical reactions. The corrosion resistance of the Ni–P coatings annealed at 400°C for 4 h is related to the formation of the texturized Ni₃P phase, which is chemically stable and comprises 80% of the coating volume.