| Hot-dip galvanizing is one of the major methods used zinc-based coatings on steel in order to provide sacrificial protection against corrosion. As a de-oxidant agent or strengthen element, alloying element Si is introduced into steels, When galvanizing Si-containing steels, Sandlin effect arise and reduces the quality of the coatings. The addition of alloying elements is one of the most used methods to restrain the Sandlin effect. One of the most [common] important alloy elements added into zinc-bath is Al. As Al added into zinc-bath, the formation of Fe2Al5 layer can prohibit the appearing of Fe-Zn compound. Meanwhile the Al containing in the coatings affect the process of Galvaneal largely. Another method for restraining the Sandlin effect is arising the temperature of hot-dip galvanizing. By galvanizing at 540°C instead of the typical temperature at 450°C, the reactivity of steel is reduced and thin, uniform coatings with acceptable appearance are produced.The 450°C isothermal section of Zn-Fe-Al-Si quaternary system with Zn being fixed at 93 at. % has been studied experimentally by means of optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). There were 10 four-phase fields in this section. Three Fe-Al-Si ternary phases,τ1,τ23 andτ, have been found in this section and the Zn solubility in those three phases were rather low. No quaternary phase occurred in this section. FeSi was found to be in equilibrium with FeSi2, Si,η-Zn,τ1,ζandτ. The Fe-Zn-Al ternary phase T(Fe8Al6Zn86)was found to be in equilibrium withη-Zn, Fe2Al5 andτ1 phase. The Al solubility in FeSi phase was 17.04 at%, and that in FeSi2 phase was rather limited. Experimental findings indicated the possibility of three phase equilibrium between FeSi, FeSi2 and Si.The microstructure of high temperature hot-dip galvanized coatings has also been studied experimentally .The coatings of high temperature hot-dip galvanizing of 10# steel are consist ofδlayer,δ+η-Zn layer and free ofζphase. Due to the stress produced by the phase transformation, theδphase enriched with zinc disintegrated into a phase mixture referred as diffusedδ. However, theδphase enriched with iron is continuous and the microstructure is compact. As the Silicon content in the steel increase, theδphase in the diffusedδlayer breaks into parts and theδphase enriched with iron are becoming thinner. There are micro-cracks in all of the coatings. The formation of the micro-cracks can be inferred from the observation of the [micro-cracks] microstructure. Micro-cracks initiated at the interspaces of theδ+η-Zn layer, as coupling of the interspaces, the micro-cracks extended to theδphase layer. Then the micro-cracks extended along the grain boundary ofδphase. Finally a micro-cracks initiated at... |
