Study On Structure And Corrosion Resistance Of Zinc-Aluminium Based Alloy Coatings

The most common element added in pure zinc bath is aluminum. The traditionalflux，zinc-chloride and ammonium-chloride, will become invalid and result in defectssuch as pretermission of plating. So the plating-supporting process had been aproblem of desiderate to solve. In this work，a new flux without ammonium wasdeveloped by a series of orthogonal experimental design and also the correspondingtechnological parameters was ensured. Heating the flux to75degrees and keeping for3min, by which the coating with smooth surface would be obtained. The experimentalresult revealed that the method is suitable for coatings varied from Al-Zn alloy withhigh aluminum to pure Zinc. Moreover, to the zinc coatings，the addition of NiCl2inthe flux could inhibit the growth of ζ Phase，refine the grain size and enhance thecoating’s corrosion resistance. This could be an feasible way to solve silicon reactivityin galvanizing.In this paper, SEM and EDS was applied to investigate the effect of Silicon andMagnesium on the structure of45%Al-Zn alloy coating. It was found that Q235steelwill react violently with the45%Al-Zn bath, which resulted in excessive growth ofcoating. The structure of FeAl3will change with time increased. There is anincubation time for the formation of Fe2Al5, but Fe2Al5could exist on the interfacemore stably than FeAl3to prevent the formation of Fe-Zn phase. At the later stage, theamount of aluminum reduced obviously and zinc riched in the area near the interfaceand a portion of zinc atom diffused into the substrate through the grain boundary. Thesilicon added in the45%Al-Zn bath could inhibit the extreme reaction between thesteel and bath. The composition of the intermetallic compound varied with the contentof silicon in the bath. The silicon will be precipitated in the form of Si-riched phase inthe coating when the addition of silicon reached to2.0%. The more silicon containedin the bath, the longer incubation time had of Fe2Al5phase. Magnesium could enhancethe nucleation of Fe-Al compounds on the interface between steel and bath, improvethe intensity of FeAl3and shorten the incubation time of growth of Fe2Al5phase onthe interface, but could not inhibit the extreme reaction between the steel and bath.Meanwhile a mixed structure of periodic lamellar FeAl3phase and liquid phase was observed in the present study when Mg was added in the bath. To explain theformation mechanism of the periodic lamellar FeAl3phase, a model was also set up.The effect of Silicon and Magnesium on the corrosion resistance of45%Al-Znalloy coatings was discussed by salt spray test chamber and XRD. According to theresult of5%NaCl neutral salt spray test, it is found that the coatings’ corrosionresistance increased at beginning and then decreased with the addition of siliconincreased in the Al-Zn alloy bath. On the contrary, the coatings’ corrosion resistanceincreased with the addition of Magnesium increased. But the excessive Magnesiumwould lead the surface coatings’ quality decreased. By the observation and analysisresult of corrosion products, it is found that Magnesium could prevent loosen productsZnCO₃（OH）₆·H₂0to generate and promote good adhersive productsZn₅（OH）₈Cl₂·H₂O to form to keep the coatings’ long-term and stable corrosionresistance.