| Hot-dip galvanizing is one of the most commonly used methods in industry to providesteel articles with excellent performance against atmospheric corrosion. After they areexperienced a serious of pre-treatments of alkaline cleaning, acid cleaning and fluxing, steelworkpieces to be galvanized are dipped in molten zinc for times to gain a coating on theirsurface. As the operating temperature and corrosivity of molten zinc are high, the zinc cattleshould have a good performance. Some steels with low carbon and silicon, such as08F,05F,has a relatively low corrosion rate to molten zinc, so it is tradionally used to produce zinckettle. However, its high corrosion rate at higher temperature of over460℃and in moltenzinc with an amount of aluminum of over0.1percent limits its use under those circumstances.Ceramic is also an altanative material to produce zinc kettle due to its low corrosion rate. Butits critical shortcomings of bad heat conductivity and impact resistance greatly narrow it wideapplication in industry. It is important and urgent to persue a kettle material with excellentcomprehensive performance which can be used at higher temperature of over460℃or inmolten zinc with an amount of aluminum of over0.1percent for industry, expecially for batchhot-dip galvanizing.316L stainless steel has been successfully used in CGL production with an amount ofaluminum of0.15~0.18percent in bath as zinc kettle and sink roll for its good performance.Based on the results from some scholars and our team, typical zinc steel sheet XG08and316L stainless steel are chosen and their galvanized coatings immersed in Zn-0.2wt%Al andZn-5wt%Al molten alloy at450℃for a long time are studied by ways of scanning electronmicroscope(SEM), energy dispective spectrometer (EDS), X-ray diffraction (XRD) andelectrochemical peeling. The characters of the intermetallic layers of coatings in XG08and316L are kept an especial eye on.The main results are listed as follows:When XG08steels are dipped in molten Zn-0.2wt%Al at450℃for short time(3s), thelayers of Fe2Al5and FeZn7() intermetallic compounds are generated in the sequence fromsubstrate to coating on the surface of XG08. The Fe2Al5grows in the shape of needle and scale near the substrate, and the FeZn7with large grain crystals appears above the Fe2Al5.When XG08steels are immersed in molten Zn-5wt%Al at450℃for a long time, onlyone intermetallic layer-FeAl3(Fe4Al13) is generated. For a relatively short immersing time(<30min), the intermetallic layer can maintain a uniform barrier that can prevent the diffusionbetween matrix and liquid Zn-Al. With the increase of immersing time, the intermetallic layerbecomes loose and untethered，which can create a lot of diffusion paths. So the blockingeffect of intermetallic layer nearly disappears and Fe-Zn reaction increases.When316L stainless steel is immersed in molten Zn-0.2wt%Al at450℃for a long time,two homogeneous intermetallic compound layers are genertated. The outer layer that contactwith the liquid is FeZn7and the inside layer is Fe2Al5. The Fe2Al5grows prior to the FeZn7.316L stainless steel has a large amount of alloying elements like Cr, Ni which can fill thevacancy of Fe2Al5, prevent the diffusion between Fe, Al and Zinc. Therefore, the Fe2Al5layercan be stable for a long time under the environments. Although the FeZn7layer falls off alittle, it can still maintain a certain thickness. The Fe2Al5and FeZn7layers can preventdiffusion among the reactants, which minimize the corrosion rate in Zn-0.2wt%Al moltenalloys.When316L stainless steel is immersed in molten Zn-5wt%Al at450℃for a long time,only one compact intermetallic layer-FeAl3(Fe4Al13) is generated on the surface ofsubstrate. After a certain immersing time (>240h), this intermetallic layer can maintain acertain thickness. It is the reason that the Cr and Ni in316LSS influence the intermetalliclayer stability. Stable and dense FeAl3(Fe4Al13) layers can prevent mutual diffusion betweenthe Fe, Zn, Al, which minimize the corrosion rate in Zn-5wt%Al molten alloys. |
PDF Full Text Request