Environmental and technical considerations of preflux bath composition on structure of hot dip zinc galvanised coatings

摘要

The influence of the preflux bath composition upon the structure of zinc coatings deposited with hot dip galvanising has been examined. Various compositions were studied containing chloride salts of Na, K and Sr. The as formed coatings were examined by optical microscopy, scanning electron microscopy and X-ray diffraction. Depending on each specific compound and its concentration in the preflux bath, different crystallisation procedures are induced, which finally results in the formation of Fe-Zn intermetallic phases with different morphologies, thicknesses and qualities. Fluxing in an aqueous solution of NaCl or KCl introduced several cracks in and between the phases while fluxing in an aqueous solution of SrCl₂ caused total failure of the coating because it had no cohesion to the substrate. However, when the same salts were dissolved in an aqueous solution containing 50 wt-%ZnCl₂·2NH₄Cl instead of water, the coatings were improved.