Hot-dip Galvanized Coating Electrochemical Corrosion Behavior Research

Due to the fairly good corrosion resistance, Al-Zn alloy coatings by hot-dip process have been widely used as the protective material on steel products. The commercial Q235 steel coated with hot-dip 55 wt% Al-Zn-Si alloy has been applied for its excellent corrosion resistance in large market, especially for the one coated with 55%Al-43.4%Zn-1.6%Si alloy produced in Bethlehem steel company. Based on the relative literatures and preliminary studies on the commercial products coated with 55 wt% Al-Zn-Si alloy, the relationship among morphology, composition and corrosion behavior of the hot-dip coatings along with the formation mechanism was investigated, especially focusing on the influence of the pre-treating process of steel substrate, hot-dip plating technology (preheating temperature, dipping temperature and cooling methods, etc), and the addition of rare earth elements on the morphology, composition and corrosion behavior of hot-dip 55 wt% Al-Zn-Si coatings on Q235 steel. Main conclusions are drawn finally as follows:1. The fabrication process developed in our present study can produce the qualified products which has the quite similar chemical composition as the commercial abroad one.2. The morphology, microstructure and chemical composition significantly affects the corrosion behavior of the hot-dip aluminizing coatings on Q235 steel.3. Through electrochemical method, the corrosion test in the media of 3.5 wt% NaCl aqueous solution was carried out to indicate that the hot-dip 55%Al-Zn-Si coatings exhibited quite good anti-corrosion performance, which can be mainly ascribed to the formation of compact aluminum oxide layer and intrinsic corrosion resistance of Al metal.4. Different pre-treating processes of Q235 steel would lead to the significant effect on the morphology and corrosion resistance properties of the hot-dip coatings. It is shown in long-tern immersion tests that the hot-dip aluminizing coatings, prepared using steel substrate rinsed by cooling distilled water, exhibited better anti-corrosion performance than by hot one.5. It was showed that the hot-dip 55 wt% Al-Zn-Si coatings, prepared under the dipping temperature at 640°C and the preheating temperature for 640-650°C, exhibits excellent corrosion resistance and long cyclic life during long-term immersion tests. Too high or low preheated temperature has adverse effects on the corrosion resistance for the hot-dip coating. The hot-dip coating prepared by cooling water showed higher corrosion resistance than that of the hot-dip coating cooled in air.6. It was verified that the addition of a certain amount of rare earth elements could improve the morphology, composition and corrosion behavior of hot-dip 55 wt% Al-Zn-Si coatings on Q235 steel. The results showed that the pinhole on the surface of hot-dip coating was obviously reduced by adding rare earth elements, making the coating more uniform and smooth. The hot-dip 55 wt% Al-Zn-Si-RE coatings, prepared under the dipping temperature at 630°C and the preheated temperature for 640°C, exhibited excellent corrosion resistance and long cyclic life during long-tern immersion tests. Too high or low preheating temperature has adverse effects on the corrosion resistance for the hot-dip coating.