THE EFFECT OF LOW LEVEL MAGNESIUM ADDITIONS ON THE MICROSTRUCTURE AND CORROSION RESISTANCE OF ZINC ALUMINUM ALLOY GALVANIZED STEELS

摘要

The magnesium levels were varied from 0.0 to 0.05% in 20μm thick 4.5%A1/Zn galvanized coatings applied to 0.7mm gauge steel under continuous galvanizing conditions. These additions increased the total volume fraction of primary zinc phase (from 6 to 22%) and the number of primary zinc dendrites (from 150 to 325 mm~(-2)) as a result of the depression of the eutectic temperature. These changes result in increasing cut edge corrosion on the exposed edges of materials embedded in resin to simulate organically coated steels. There is a systematic increase in zinc loss measured using a scanning vibrating electrode technique from 80 to 185 μg in 24 hours exposure to 5% NaCl. This increase in zinc loss results from an increase in active anode numbers (from 600 to 1700 m~(-1) cut edge) and from an increase in the number of long lived anodes resulting from the corrosion of large numbers of closely spaced dendrites.