Characteristic V-Mn conversion coating in reinforcing corrosion resistance of electroless Ni-P coated LZ91 Mg alloy

A V-Mn conversion coating was studied and furthermore employed to improve the corrosion resistance of electroless Ni-P coated LZ91 Mg alloy. A thin, continuous and compact V-Mn conversion coating was formed on LZ91 after the immersion of 60 s in the conversion coating solution. The results of electrochemical impedance spectroscopy (EIS) and salt spray test (SST) showed the enhanced corrosion resistance of LZ91. The impedance in EIS increased from 573 Omega.cm(2) to 11,319 Omega.cm(2), and moreover the deteriorated area ratio of the V-Mn conversion coated LZ91 decreased to < 5% after 120 h of SST exposures. Further study of a V-Mn conversion coating/electroless Ni-P composite coated LZ91 was used to investigate the improvement of the corrosion resistance in the electroless Ni-P coated LZ91. The composite coated LZ91 demonstrated better corrosion resistance of mere electroless Ni-P coated LZ91. Furthermore, the corrosion resistance of the composite coated LZ91 was greatly enhanced. The impedance of the composite coated LZ91 was 500 Omega.cm(2), larger than that of the electroless Ni-P coating of 5 Omega.cm(2). The composite coated LZ91 showed better corrosion resistance in the SST exposures in comparison to mere electroless Ni-P coated LZ91. The composite coating indicated that the positive characteristics of a V-Mn conversion coating usefully enhanced the corrosion resistance electroless Ni-P coated LZ91. Moreover, based on the above results, a mechanism for the conversion coating and corrosion resistance was proposed.