Kinetics of the development of a nonchromate conversion coating for magnesium alloys and magnesium-based metal matrix composites

Kinetics of the development of a conversion coating from a stannate bath on commercial purity magnesium (Mg-comm), magnesium-based alloys ZC71 and WE43, and a metal matrix composite (MMC), comprising a ZC71 alloy matrix and 12 vol% silicon carbide (SiC) particles were studied using linear polarization resistance, potential-time, potentiodynamic polarization, x-ray diffraction, Rutherford backscattering spectroscopy, and microscopic examination, The coating, typically similar to 3 mu m to 5 mu m thick, was composed largely of crystalline magnesium tin oxide (MgSnO3 . 3H(2)O), and developed by a nucleation and growth process through an initial corrosion film on the substrate. Nucleation probably occurred on regions where a critical concentration of magnesium ions was reached for coating crystals to form. Specific sites of nucleation, such as particles of eutectic phase and of reinforcement, were revealed in some cases, but frequently the precise sites of nucleation were not disclosed. A longer treatment time (at least 35 min) was suggested by polarization resistance data for improved coverage of the substrate than the previously recommended time of 20 min The coating continuity on the substrates, after a particular time of treatment, depends upon alloy composition increasing in order: Mg-comm, 12% (SiC)p/ZC71 alloy MMC. ZC71 alloy,and WE43 alloy. Polarization resistance (R-p) changed systematically with coating development showing a decrease in R-p in the early stages of the coating process, related to the initial corrosion of the substrate surface for the nucleation of the the coating material:subsequent increase in R-IP related to the nucleation and growth of the coating material: and the approach to a relatively constant, high R-p value as the coating neared completion. The main cathodic and anodic reactions during coating development were hydrogen evolution from the decomposition of water and oxidation of magnesium from the dissolution of the substrate respectively.