Effect of Heat Treatment on Erosion-Corrosion Behavior of Electroless Ni-P Coatings in Saline Water

To prevent premature failure of heat exchanger tubes enduring rapid seawater flow, amorphous electroless Ni-P coatings were prepared on copper-nickel alloy (CuNi 90/10) substrates. The influence of post-deposition vacuum heat treatment at different annealing temperatures (190, 300, 400, 500, and 600 degrees C) on the erosion-corrosion resistance of the annealed coatings was examined by X-ray diffraction, scanning electron microscopy, potentiodynamic polarization, and electrochemical impedance spectroscopy. It was found that precipitation of a crystalline phase took place in the coating matrix at 400 degrees C. The highest microhardness was achieved for coatings undergoing heat treatment at 400 degrees C due to the formation of the stable Ni3P phase. Electrochemical results indicated that all the Ni-P coatings present passivity and better corrosion resistance than the uncoated CuNi 90/10 substrate. The Ni-P coating treated at 500 degrees C demonstrated the best erosion-corrosion resistance, which represented the optimization of the balance between erosion and corrosion resistance.