Relationship between microstructure and corrosion resistance in Zn-Al alloy coated galvanised steels

The effect of cooling rate on the coating microstructure and corrosion resistance of Zn - 4.5 wt-%Al alloy galvanising coatings applied to 0.7 mm gauge strip steel substrates is presented Increasing the cooling rate by the use of a high powered air cooler does not significantly alter the volume fraction of primary zinc in the alloy coating layer, which remains at similar to 14 vol.-%. There is, however, a dramatic change in the number and size of dendrites with far greater numbers (changing from 270 to 1500 mm(2)) of smaller dendrites as the cooling rate is increased using cooler outputs of 55 and 100%, respectively. This corresponds to a change in the surface appearance of the material with the surface spangle (i.e. eutectic cell) size being reduced from 7.5 to 2.1 mm as the cooling rate is increased Since the volume fraction of the primary zinc remains roughly constant and the dramatic change is in the size and numbers of dendrites, it appears that this phenomenon is related to increased nucleation of primary zinc. The scanning vibrating electrode technique (SVET) has been used to investigate the effects of these microstructural changes on the corrosion mechanism and kinetics. The S VET isocurrent contour maps show! a dramatic reduction in corrosion intensity with increasing cooling rate. This is borne out by metal loss calculations, which show, that the zinc losses from the coating are 2500, 1200 and 200 mug at cooler outputs 55, 80 and 100%, respectively. More significantly, the anodes display shorter lifetimes and lower intensities for the materials produced at the higher cooling rate. One quarter of the anodes in the specimen cooled at 55% remained active throughout the 24 h immersion period compared with 4% of the anodes in the specimen cooled at 100%. The spangle size is directly related to the cooling rate and the microstructure and can be related directly to the cut edge corrosion resistance as determined by SVET.