| Various hot-dip Zn-Al and Zn-Al-Ce alloy coatings were produced on Q235 steel by adding a certain amount of aluminum and cerium into zinc bath in this study. The structure of the coatings and alloys were investigated. The surface feature of these coatings were characterized with structure of the bath alloy. In the next, a kind of passivation treatment which consists of Mn(H2PO4)2·2H2O and Na2 Si F6 as film-forming agent was prepared on the hot-dip Zn-5Al-0.08 Ce alloy coatings. Then ammonium citrate tribasic was added into the basic passivation solution to improve the basic conversion coating. The surface microstructure, chemical composition, growth of the basic manganese salt conversion coating and improved conversion coating were analyzed by SEM, EDS, XPS. And corrosion resistance of the coatings were studied by neutral salt spray(NSS)test, electrochemical polarization method, and electrochemical impedance spectroscopy. The mechanism of film-forming was also discussed in this paper.Being very different from hot-dip zinc coating, hot-dip Zn-Al-Ce alloy coating only contains tow layers.The inner is intermetallic compound of Fe-Al-Zn and the outer is nearly the same with alloy bath. The rearch reveals that the bath alloy microstructure consists of first eutectic phase and Zn-Al eutectic phase. The Zn-Al eutectic phase became the most smallish and pyknotic when 5wt%Al and 0.08Ce% were added into the bath at the same time. What shoud be noted is that eutectic cells are different in orientation compactness from each other.The corrosion resistance of hot-dip coatings increased when Al and Ce were added,which was proved bay the results of NSS test and electrochemical test. Hot-dip Zn-5Al-0.08 Ce coating has the smallest area of white rust in NSS test, highest electrochemical impedance and lowest corrosion current density in 5% Na Cl solution, which indicates the best corrosion resistance.The optimal process parameters of the basic manganese salt conversion coating are as follows: Mn(H2PO4)2·2H2O 30g/L, Na2 Si F6 0.3g/L, p H 3.0,treating time 2 min. The optimal concentration of ammonium citrate tribasic is 1g/L to improve the basic passivation solution.The results of SEM show that the thickness of both the basic and the improved conversion coatings increase with the increase of treatment time. Compared to the basic conversion coatings, the improved conversion coatings grow more rapidly and has fewer defects. While the processing time exceeds 2 min, the films appear cracks and exfoliation which leads to poor corrosion resistance.The results of NSS test reveal that corrosion resistance of hot-dip Zn-5Al-0.08 Ce coating was greatly improved with treatment of basic passivation solution. The sample treated by the optimal process parameters appeared 32% the corroded area after three cycles of NSS tset. When the ammonium citrate tribasic was added, the area of white rust decreased further, which indicated better corrosion resistance.Compared to untreated hot-dip Zn-5Al-0.08 Ce coating, the polarization resistance and the electrochemical impedance increased significantly while the corrosion current density of the basic and improved conversion coatings decreased. The results of electrochemical test show that the corrosion resistance was improved by forming conversion coatings. The improved conversion coating which was treated with 2min gained the highest electrochemical impedance, and the corrosion current density decreased to the minimum.The analysis of the results of EDS, XPS reveals that the basic manganese salt conversion coating contains Zn, Al, O, P, Mn, F and Si elements, and the surface of the conversion coating consists of Zn O, Zn3(PO4)2, Zn F2, Al2O3/Al(OH)3, Si O2 and the oxides of Mn. While certain carbon was found in the improved conversion coating except the elements existing in the basic conversion coating.The improved conversion coating is composed of Zn3(PO4)2 and the the oxides of Zn, Al, Si and Mn. Citrate was also found in surface of the film, which means ammonium citrate tribasic join in the forming process of the conversion coating. |
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