| Corrosion is a common reason for serious damage in modern industry,and the amount of failed steel due to corrosion is about 25%30%of annual production.Among the coating protection technologies,hot dip galvanized coatings are widely used,due to the low cost,formability and high strength.Traditional hot dip galvanized coating faces some problems,such as poor structure uniformity,large coating thickness,and moderate corrosion resistance.The addition of rare earth?RE?elements in the galvanized coating could improve the microstructural homogeneity of coating,decrease the coating thickness,and enhance appearance quality as well as corrosion resistance.However,the effects of RE content on the microstructure and corrosion resistance of galvanized coating are unclear.In addition,there is still a lack of analysis on the corrosion resistance of the coating from the perspective of phase composition.The purpose of this paper is to investigate the effects of La and Ce on the phase composition,microstructure and corrosion resistance of galvanized coatings.The variation of corrosion resistance with the addition of RE is also studied to guide the development of Zn-La/Ce coatings with excellent corrosion resistance.In this paper,the solubilities of lanthanum and cerium in iron-saturated liquid zinc were measured,which provided data for the amounts of RE added into the galvanizing bath.The coatings on the mild steel surface with different contents of La or Ce were obtained by hot-dip process in a laboratory simulative equipment.The actual content of RE elements in bath alloy was determined by inductively coupled plasma spectroscopy?ICP?.Optical microscope?OM?and scanning electron microscope?SEM?were used to analyze the microstructure of the coatings.The phase compositions were identified by X-ray diffraction?XRD?.Moreover,the corrosion resistance of different coatings was systematically evaluated by neutral salt spray?NSS?test as well as electrochemical measurements,and the corrosion mechanism of the different coatings was investigated.Main conclusions are drawn as follows.The solubility of La in iron-saturated liquid zinc was 0.012 wt.%at 460°C and0.028 wt.%at 480°C.After La was added to the galvanizing bath,the formation of LaZn13 phase with square shape was observed on the surface of Zn-0.2La coating and this phase is beneficial to inhibit the cathodic reaction.Besides,La promoted the zinc dendritic growth of coating and decreased the coating thickness.The results show that the corrosion resistance of the coating increased with the increase of La content.Compared with pure Zn coating,the addition of La reduces the weight loss of the coating by 525%.The main reason for the improvement of corrosion resistance is the addition of La that inhibits the transformation of the dense corrosion product Zn5?OH?8Cl2·H2O to the porous Zn5?CO3?2?OH?6 and significantly inhibits the cathodic reaction.Moreover,the surface potential of La-containing coating tends to be uniform and the number of micro cells is decreased.Therefore,the stability of the coating is enhanced.The solubility of Ce in iron-saturated liquid zinc was 0.011 wt.%at 460°C and0.014 wt.%at 480°C.After Ce was added to the galvanizing bath,the formation of CeZn111 phase with equilateral triangular shape was observed on the surface of Zn-0.2Ce coating and this phase is beneficial to inhibit the cathodic reaction.Ce promoted the zinc dendritic growth of coating and decreased the coating thickness.The corrosion resistance of the galvanized coating increased with the increase of Ce content.Compared with pure Zn coating,the addition of Ce reduces the weight loss of the coating by 1527%.When the same amount of La or Ce was added,the improvement of the corrosion resistance of the coating was slightly higher for Ce addition than La addition.There are two main reasons for the improvement of corrosion resistance of the coating.The addition of Ce improves the stability of Zn5?OH?8Cl2·H2O,which displays strong and compact structure.The addition of Ce greatly reduces the concentration of the cathodic reactant?dissolved oxygen of the adsorption reaction?,and significantly suppresses the cathodic reaction during the corrosion process.After 0.1 wt.%La+0.1 wt.%Ce was added to the galvanizing bath,La and Ce existed in the form of?La,Ce?Zn13 in the interdendritic area of the coating surface.In the neutral salt spray test,the time to red rust formation of Zn-0.1La-0.1Ce coating was96 h.In addition,the corrosion currents of Zn-0.1La-0.1Ce,Zn-0.2La and Zn-0.2Ce coatings were decreased by 63%,70%and 72%,respectively,compared to that of pure Zn coating.The mischmetal addition with La:Ce=1:1 was slightly less effective in improving the corrosion resistance,compared with single rare earth addition with the same total content.The low-frequency impedance modulus of Zn-0.2La-0.2Ce coating was only 700?cm2,similar to that of pure Zn coating.Excessive RE leads to the decrease of corrosion resistance. |
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