Effect of heat treatment on the properties of electroless Ni-P-carbon nanotube composite coatings

Electroless Ni-P-carbon nanotube composite coatings were deposited on API-5L X65 steel substrates. The coatings were vacuum heat treated at temperatures of 200-C for 2-h, 400-C for 1-h and 600-C for 15-min. Microhardness measurement, X-ray diffraction (XRD) technique, scanning electron microscopy, potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to study the effect of heat treatment on the properties of the coatings. XRD results indicated that the as-plated coating had either nanocrystalline or a mixture of amorphous and nanocrystalline structure. Heat treatment of the coatings above 400-C produced a mixture of polycrystalline phases. The highest microhardness was achieved for the samples heat treated at 600-C for 15-min because of the precipitation of NixPy phases and formation of an inter-diffusional layer at the substrate/coating interface. The lowest corrosion current density value was obtained for the coatings heat treated at 400/C for 1/h. EIS experiments confirmed the results.