Preparation of chemical conversion super-black films on ZK61M magnesium alloy

Abstract

Four different super-black conversion films on ZK61M magnesium alloys were investigated in this work. Macro- and micro-morphologies of the conversion films were observed using a digital camera and scanning electron microscope, respectively. Salt spray testing and electrochemical measurements were used to examine the corrosion resistance of the conversion films. The emissivities of the conversion films were measured using the Fourier transform infrared spectrometry. The results show that all conversion films reduced the corrosion rate of the magnesium alloy substrates effectively; in particular, the film prepared using 230 g L⁻¹ HF + 150 g L⁻¹ Na₂Cr₂O₇·2H₂O + 2.5 g L⁻¹ CaF₂ mixed solution had a higher corrosion resistance. Moreover, the emissivities of the conversion films prepared using 230 g L⁻¹ HF + 180 g L⁻¹ Na₂Cr₂O₇·2H₂O + 270 g L⁻¹ HNO3 and 230 g L⁻¹ HF + 150 g L⁻¹ Na₂Cr₂O₇·2H₂O + 2.5 g L⁻¹ CaF₂ mixed solutions were very high; the values exceeded 0.9 at room temperature.