Abstract:

The influences of the mass concentration of zinc oxide(the main film-forming substance) in the phosphating bath on the microstructure and properties of the phosphate coating on AZ61 magnesium alloy were studied. The structure, surface morphologies and compositions of the phosphate coatings were observed and analyzed by using a metallographical microscope,SEM and EDS. Anodic polarization measurement was used to assess the corrosion resistance of the phosphate coating on the magnesium alloy. The results indicated that the phosphate coating obtained from a phosphating bath with a lower concentration of ZnO was a loose and defected structure, without exhibiting enough corrosion resistance. The phosphate coating obtained from a phosphating bath with a higher mass concentration of ZnO is composed of coarse crystals and the thickness of the coating was  uneven,  showing poor corrosion resistance. The uniform and intact phosphate coating which was obtained from a phosphating bath with a medium mass concentration of 2.0g/L ZnO showed high corrosion resistance in a 3.5％ NaCl solution.

Key words: magnesium alloy; phosphate coating; phosphating; corrosion resistance