Deposition of electroless Ni–Cu–P coatings on L80 steel substrates and the effects of coatings thickness and heat treatment on the corrosion resistance

In this study, the effect of adding colloidal copper nanoparticles to the electroless Ni–P bath on the corrosion behavior and microstructure of electroless coatings was investigated. Hence, the effects of coating thickness and heat treatment on the corrosion behavior of Ni–P and Ni–Cu–P coatings were studied. For this purpose, Ni–P and Ni-Cum-P electroless coatings with different thicknesses (0.1–75 μm) were deposited on L80 steel substrates. Moreover, the resulted coatings were heat treated at 200 and 400 °C. Microstructural examinations and X-ray diffraction patterns confirmed the alteration of cauliflower to orange peel morphology at high thicknesses by the addition of Cu nanoparticles to the electroless Ni–P bath. The results of Tafel polarization and electrochemical impedance spectroscopy tests in 3.5% NaCl solution indicated that for non-treated samples treatment, the corrosion resistance was improved by increasing the thickness of the coating due to the filling of the micropores. Furthermore, the corrosion resistance was highly increased for the sample treated at 400 °C for 1 h because of the increase in the Ni3.8Cu phase in the electroless Ni-Cum-P coating. In addition, it was observed that at thicknesses above 50 μm, the corrosion resistance of the samples treated at 400 °C was reduced due to the formation of microcracks. Nevertheless, the highest corrosion resistance was attributed to the electroless Ni-Cum-P coating with a thickness of 25 μm and the Ni–P coating with a thickness of 50 μm.