Effect of tungsten doping on the performance of MAO coatings on a Ti6Al4V drill pipe

A tungsten (W)-doped microarc oxidation (MAO) coating on a TC4 titanium (Ti) alloy drill pipe was prepared by adding sodium tungstate (Na₂WO₄) into electrolytes. The effects of adding different concentrations on the electrical conductivity of the electrolyte and the performance and structure of MAO coatings on the titanium alloy drill pipe were studied. The electrical conductivity of the solution was tested using Ohm’s law. The surface morphologies of the coatings were observed by scanning electron microscopy. The element contents of the MAO coatings were analyzed by energy-dispersive spectroscopy. The hardness of the coatings was measured by using a microhardness tester. The phase composition was analyzed by X-ray diffraction, and the corrosion resistance was evaluated by polarization curves. The results show that orthotungstate (WO₄ ²⁻) begins to form tungsten trioxide (WO₃) when the temperature of the microarc discharge area is above 1314 K. With increasing sodium tungstate doping concentration, the conductivity of the solution increases first and then decreases, and the hardness and corrosion resistance of the coatings also show an identical trend. Tungsten doping slightly affects the phase composition of the MAO coating, and it is helpful in improving the hardness and corrosion resistance of the coatings. The optimum doping concentration is 3 g/l in this work.