Effects of temperature and pH value on the morphology and corrosion resistance of titanium-containing conversion coating

In this paper, the effects of temperature and pH value of the conversion solution on the morphology and corrosion resistance of the titanium-containing conversion coating prepared on AZ91 magnesium alloy were studied. The morphology and composition of the coatings were studied by a scanning electron microscope, an energy dispersive spectrometer and an X-ray photoelectron spectroscopy. The potentiodynamic polarization tests and electrochemical impedance spectroscopy tests were performed by an electrochemical workstation. The results showed that both of the temperature and pH value of solution had an obvious effect on the micro-cracks and thickness of the coatings. With the increase of solution temperature, the micro-cracks became wide and deep. And the corrosion resistance of the coatings increased firstly and then decreased with the increase of temperature. The coating thickness and number of the micro-cracks decreased with the increase of pH value. Increasing the pH value of solution was favorable to the corrosion resistance improvement. The coating prepared in the solution at 40°C with the pH value in the range from 3.5 to 4.5 possessed good corrosion resistance.