| Hot-dip aluminium is a simple and efficient surface coating technology, hot-dip aluminium of steel is used to improve its corrosion resistance, weatherability and resistance to high temperature and oxidation. Hot-dip aluminium can reduce the severe economic losses caused by corrosion of steel, prolong the life of ferrous products greatly and bring good benefits. Hot-dip aluminium is oxidized rapidly by oxygen to form a passivation layer on the surface, it usually works as cathodic deposit, so it can not provide effective protection to the substrate when the coating is defective, except in the solution containing high chloride ion concentration. Although hot-dip 55% Al-43.4%Zn-1.6%Si alloy sacrificial anode has been developed, its corrosion resistance is lower than the hot-dip aluminium significantly. In this paper, some improvements have been made according to the principle of aluminum alloy sacrificial anodes, in order to develop a new kind of hot-dip aluminium layer, which has a full media sacrificial anode performance and better corrosion resistance than 55% Al-43.4%Zn-1.6% Si alloy coating.In this paper, smooth hot-dip Al-Zn-Si, Al-Zn-In-Si, Al-Zn-Sn-Si coatings are prepared by flux method by adding elements such as zinc, indium, tin in the molten aluminum. First, the effect of adding elements such as zinc, indium, tin in the hot-dip coating is researched in immersion corrosion test and polarization curves. Second, the composition and micro structure of hot-dip aluminium coatings are analyzed. Finally, the polarity reversal of hot-dip Al-Zn-In-Si, Al-Zn-Sn-Si is investigated when the chloride ion concentration is dropped. The following results are obtained.(1) In hot-dip Al-Zn-Si coating, the integrated results show hot-dip aluminium coatings with 4%Zn,5%Zn,6%Zn have better corrosion resistance; In hot-dip Al-Zn-In-Si, the integrated results show hot-dip Al-5%Zn-0.02%In-0.1%Si has the best coiTosion resistance among these hot-dip coatings; In hot-dip Al-Zn-Sn-Si, the integrated results show hot-dip Al-5%Zn-0.06%Sn-0.1%Si has the best corrosion resistance among these hot-dip coatings, followed by Al-5%Zn-0.06%Sn-0. l%Si coating.(4) The hot-dip Al-Zn-Si, Al-Zn-In-Si, Al-Zn-Sn-Si coatings are consisted of two layers, an external layer (defined as Al-riched layer) consisting of Al and FeAl3, and an internal layer (defined as Al-Fe diffusion layer) consisting of Fe2Al5 and FeAl3. Metallurgical bond is observed between coating and substrate, just as the hot-dip aluminium coating.(3) In 0.12%NaCl solution, when the potential is stable, the open circuit potential of hot-dip aluminium coating and steel appear stable at around -0.66V(vs. SCE). Hot-dip aluminium coating turns to cathodic deposit and steel becomes the anode, polarity reversal occurred.(4) Although the chloride ion concentration is dropped to zero, the open circuit potential(in descending order) of Al-5%Zn-0.02%In-0.1%Si, Al-4%Zn-0.06% Sn-0.1%Si and Al-5%Zn-0.06%Sn-0.1%Si coatings are still lower than that of steel, they are always anodic coatings and do not occur polarity reversal.(5) The corrosion rate of Al-5%Zn-0.02% In-0.1% Si, Al-4%Zn-0.06%Sn-0.1%Si and Al-5%Zn-0.06%Sn-0.1%Si coatings are lower than that of 55%Al-43.4% Zn-1.6%Si coating and hot-dip galvanizing coating in immersion corrosion test of 3.5%NaCl solution. The corrosion potential of Al-4%Zn-0.06%Sn-0.1%Si and Al-5%Zn-0.06%Sn-0.1%Si coatings are lower than that of 55%Al-43.4%Zn-1.6%Si coating and hot-dip galvanizing coating in 3.5%NaCl solution, so they can provide better protection for the steel. The hot-dip Al-5%Zn-0.02%In-0.1%Si, Al-4%Zn-0.06%Sn-0.l%Si and Al-5%Zn-0.06%Sn-0.1%Si coatings not only have better corrosion resistance but also can be used as sacrificial anodic coatings, so the purpose of this article has been achieved. |
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