Enhanced Corrosion Resistance and Microhardness of Titanium with Electroless Deposition Ni-W-Cr-P Coating

In the present study, Ni-W-Cr-P quaternary alloy coatings were prepared on titanium-based alloys by electroless deposition. The morphology, crystal structure, microhardness, and corrosion resistance of samples were studied by scanning electron microscope (SEM), X-ray diffractometer (XRD), microhardness tester, and electrochemical analyzer. The results show that the microstructure of these alloy films changes with the increase of annealing temperature. As-deposited alloy layers belong to nickel-based mischcrystal structure, and its crystallinity was enhanced to 200 degrees C and 300 degrees C, gradually; the new phase Ni3P began to appear at 400 degrees C, completely crystallized at 600 degrees C; Cr1.12Ni2.88 and Ni3Ti phases were present at 700 degrees C; Ni and Ni3P phases were totally decomposed at 800 degrees C, and Cr4Ni15W, Cr2Ni3, Ni17W3, Cr, Cr3P, and Ni2P generated. Because of the different structures after annealing, microhardness and corrosion resistance of the alloy coatings possess a relevant change. The Ni-W-Cr-P alloy coatings heated at 600 degrees C exhibit better microhardness and corrosion resistance than those of samples at other annealing temperature. The related mechanisms of those results were also discussed, briefly.