Protective layers on the hard aluminium substrate are formed by Cr ion beam once mixing

twice mixing and oxidizing respectively. The anodic polarization curves and annular polarization curves of every sample are measured with three electrodes potentiodynamic scanning method in 3% NaCl solution. The electrochemical parameters of samples with ion beam twice mixing treatment are superior to those of samples with once mixing or oxidizing treatments. RBS analysis has given the profile of Cr concentration in the samples and that Cr is well mixed with the matrix on the surface during ion beam twice mixing.