EFFECT OF HIGH TEMPERATURE OXIDATION ON STRUCTURE AND CORROSION RESISTANCE OF THE ZINC COATING DEPOSITED ON CAST IRON

摘要

The presented work describes results regarding influence of the high-temperature oxidation on anticorrosion properties of zinc coating created during hot-dip galvanizing at surface of cast iron contained different graphite precipitates: flake, vermicular, nodular. Test was made in Na_2SO_4 solution, using specific samples in form of cylinder, with groove lathed at the whole side wall circumference, to make the oxidation and galvanizing easier. Before corrosion test, samples were oxidized at temperature 850 °C, within 4h, sandblasted and etched chemically. For comparison, the corrosion resistance of cast iron samples without preliminary oxidation treatment and steel sample galvanized in analogical conditions were also measured. Research confirmed that to obtain subsurface composite cast iron layer two stage scale removal process is necessary: sandblasting combined with chemical treatment. When only sandblasting is applied the clean outside surface is achieved but zinc penetration depth inside after-graphite voids is slight. On the other hand, using one stage chemical treatment the completely removal of relatively thick outside scale layer was impossible. Research proved that cast iron oxidation process increases essentially the corrosion resistance of created zinc layer. This difference changes with dependence on graphite shape and is the smallest in case of nodular graphite and increases as graphite precipitates change to vermicular and flake. The achieved effect results from neutralization of negative influence of graphite precipitation on compactness and continuity of zinc coating and created subsurface composite layer.