Study On Processing And Properties Of Phosphate Conversion Film On The Surface Of Magnesium Alloys

Phosphate conversion on the surface of magnesium alloys was studied in this dissertation in order to obtain the phosphate conversion films with well corrosion resistance. Orthogonal design experiment was used to acquire the best formula and processing parameters. The surface morphology, chemical composition, phase status, corrosion resistance and binding strength of phosphate conversion films were analyzed by scanning electron microscopy(SEM), X-ray diffraction(XRD), energy dispersive spectrometer(EDS), electrochemical impedance spectroscopy(EIS), polarization curves, corrosion weight-loss and tensile test. The formation mechanism of phosphate conversion film was also studied. The phosphate conversion film with well corrosion resistance was acquired by optimized formula(NH4H2PO4:80 g/L,KMnO4:20 g/L,NaF:0.3 g/L)and processing parameters(temperature 30℃,time 15 min,pH value 3.5). Further study results showed that sodium molybdate would improve the passivation of magnesium alloys.The results of SEM observation showed that optimized formula would effectively reduce the number of cracks and holes after sealing, and improve corrosion resistance of phosphate conversion films. The reduction of the numbers of cracks and holes might be related to matching of phosphate conversion films and solidification of sealing agent. EDS results showed that the phosphate conversion films mainly consisted of elements of phosphor, magnesium and manganese. There was a little F element in optimized formula, but Mo was not found in compounded formula. It indicated that mainly elements participated in formation, but all additive elements do not. The results of XRD showed that the film treated by basic formula was mainly amorphous mixed with a little crystal of phosphate magnesium, while those treated by optimized formula and compounded formula were both amorphous. The experiments of corrosion weight-loss, polarization curves and EIS showed that the phosphate conversion films in optimized and compounded formula have well corrosion resistance. The results of tensile test showed that the best binding strength of the phosphate conversion film and the coating was from the compounded formula. Sealing treatment would reduce the binding strength of the phosphate conversion film and the coating, although it would improve the corrosion resistance of phosphate...