Studies On New Blackening Processes For Ni-W-P Alloy Deposits,Mechanism For Blackening And Properties Of Black Coatings

Black coatings have been widely used in industries due to their excellent decorative,physical and mechanical properties. Super black coatings are used for decoration, opticalequipment, high solar absorber and extinction. Up to now, different surface treatmenttechnologies are developed to produce ultra black coatings, including electroplating andelectroless black treatment methods. However, different black films produced could not meetthe demands of high hardness, good wear and corrosion resistance for equipment used inindustries. So new environmental friendly blackening processes which produce black filmwith high hardness, good wear and corrosion resistance is needed in industries.Ni-W-P alloy deposits have been widely used due to its shiny appearance, high hardness,good corrosion and wear resistance in machinery, printer, petroleum industry and handware.Moreover, Ni-W-P alloy is nominated as a promising environmental friendly alloy to replacehard chromium, due to satisfactory appearance of coating, mechanical and anti-corrosionproperties. However, little work has been seen about the blackening of electrodepositedNi-W-P alloys. In this thesis, two methods for the blackening of electrodeposited Ni-W-Palloys are proposed to fabricate uniform black Ni-W-P coatings. The detailed research workwas listed as follows.1. Blackening process for the Ni-W-P depositA new blackening process for the Ni-W-P deposit with W content at26wt.%is reported.The uniform black film is fabricated by simply immersing the Ni-W-P deposit inHCl-H₂O₂-HF solution. Process optimization was carried out by investigating the influence ofvarious conditions, viz, solution component, temperature and time. The optimized operatingconditions and bath compositions for blackening of Ni-W-P deposits were determined asfollows: HCl28.7mol/L, H₂O₂1.9mol/L, HF5.0mol/L, time20-100s, temperature19-25°C. Structure, morphology and growth mechanism for the black coating of the Ni-W-Pdeposit were studied by XRD, SEM and EDS/XPS, respectively. Before blackening, Ni-W-Pdeposit was dense and smooth, and micro-crack of meshwork appeared on the surface afterblackening. After blackening the content of Ni decreased, while W, O, P increased. Thisindicated that some Ni dissolved and Ni, P, W elements are oxidized in the blackening process.The black film was composed of NiO, Ni（H2PO4）2, Ni₂O₃, WO₃, metallic Ni and W.Properties study showed that the black Ni-W-P coating has good thermostability, solarabsorption, corrosion resistance, and adhesion to Ni-W-P plating.2. Formation mechanism of blackened film Tungsten content in Ni-W-P alloy deposits plays a significant role in achieving theuniform black coatings. Ni-W-P deposits with different W contents were used for blacktreatment, and mechanism for blackening is studied in detail. With W content less than29wt.%, a uniform black films of Ni-W-P deposits were obtained by proposed blackeningprocess. The black coatings consisting of oxides （NiO, Ni₂O₃and WO₃） and phosphateproducts cause intrinsic absorption, meanwhile, two-dimensional light-trapping meshworkstructures which formed on the surface of Ni-W-P plating after blackening enhanced theabsorption of visible light. The micro-crack of meshwork may resulted by the corrosion ofcrystal boundary. The mechanism study showed that the black color of Ni-W-P deposit wascaused by both its unique dense meshwork morphology and the formation of oxides （NiO,Ni₂O₃and WO₃） and nickel phosphate on the surface. As soon as W contents were higher than36wt.%, the Ni-W-P alloys are amorphous which are hard to be etched in the oxiding acidsolution. After blackening, meshwork cracks disappeared and micro-pitting corrosionproduced with the absoption of Cl-on the Ni-W-P deposit. The blackening process proposedin this paper did not affect the corrosion resistance of Ni-W-P deposits, which is related to theblackened passive film. The passive film can prevent or slow down the erosion of the Ni-W-Pplating.3. A new blackening process for Ni-W-P in20wt.%H₂SO₄solutionAs HCl and HF are easily volatile and HF is toxic, the environmental-friendly blackeningprocesses are needed. Based on the studies for the blackening mechanism of Ni-W-P withdifferent W contents, it can be seen that the Ni-W-P platings with W contens at22-29wt.%turn black in the oxiding acid solution.20wt.%H₂SO₄is used for the blackening of Ni-W-Pplatings. The effect of blackening in H₂SO₄solution is the same as HCl-H₂O₂-HF solution.Uniform black coatings can be produced by dipping the Ni-W-P platings in20wt.%H₂SO₄for15min at70°C-80°C. The electroless black treatment for Ni-W-P alloys is easily operatedand costed effectively. The black coatings have good corrosion resistance, solar absorptionand extinction properties.