A NON-CHROMATE CONVERSION COATING FOR MAGNESIUM ALLOYS AND MAGNESIUM-BASED METAL MATRIX COMPOSITES

摘要

The development of a conversion coating in a stannate bath on magnesium-based ZC71 alloy and on a metal matrix composite (MMC), comprising a ZC71 alloy matrix and 12 vol% SiC particles, has been studied using weight gain behaviour, measurements of corrosion potential, X-ray diffraction and microscopic examination. The coating, typically 2-3 μm thick, is composed largely of crystalline MgSnO_3. The coating is formed by nucleation and growth of MgSnO_3 at cathodic sites on the metal surface, namely eutectic Mg(Zn,Cu)_2. As the coating develops, the unprotected regions of the alloy and MMC are corroded, in the presence of a film, resulting in scalloping of the metal surface by the treatment bath. The presence of silicon carbide particles in the MMC has no direct influence on the growth of the coating. However, the coating grows more rapidly on the MMC than on the ZC71 alloy because of the refined and more homogeneous distribution of cathodic sites in the former material.