Investigation Of Microstructure And Anticorrosion Mechanism Of New Types Of Zn Based Coatings Containing Mg

In order to further improve the corrosion resistance of continuously hot-dipgalvanizing Zn based coatings, the chemical compositions of new types of Zn basedcoatings were proposed in the present investigation based on thermodynamic analysisand calculated phase diagram together with measurement of fluidity of Zn liquid.Based on GI （Zn–0.2wt.%） processing parameters, three types of Zn based coatings,i.e., Zn–0.2wt.%Al–Mg, Zn–0.2wt.%Al–La and Zn–0.2wt.%Al–Mg–Lacoating, were prepared successfully by the continuously hot-dip galvanizing simulator.Furthermore, their microstructure, corrosion resistance and corrosion mechanism wereinvestigated deeply and systematically.According to isothermal sections of Zn–Al–Mg ternary calculated phasediagram near the conventional galvanizing temperature （4600C）, it was found thatthere is a liquid region near the Zn-rich corner for investigated temperatures, implyingthat the designed Zn based coatings can be prepared using the continuously hot-dipgalvanizing technique. In terms of the vertical section of Zn–0.2wt.%Al–Mgternary calculated phase diagram, the phase composition of Zn-Mg system containinga small quantity of Al consists of primary Zn plus （Zn+Mg₂Zn₁1） eutectics.Additions of Mg （1–3wt.%） or La （0–0.1wt.%） into the Zn bath can improvethe fluidity of Zn liquid, suggesting Mg or La added to Zn bath can improve thegalvanizability of the steel substrate.With the Mg content in the coating increasing from1wt.%to3wt.%, the surfacemorphology of Zn–0.2wt.%Al–（1-3） wt.%Mg （ZM） coatings is the same, i.e.,lamellar （Zn+MgZn₂） eutectics. The amount of （Zn+MgZn₂） eutectics in the coating increases till complete eutectics for ZM3coating.The Mg added to the Zn bath has a important effect on the growth of Zn andMgZn₂. With the Mg content in the Zn bath increasing, the morphology of Zn changesfrom bulk to strip and finally to mesh-like and morphology of MgZn₂changes frombar to mesh-like. Besides, Mg added into Zn bath makes the thickness of theinhibition layer thin and uneven.Mg can improve the corrosion resistance of Zn based coatings remarkably. Underthe condition of salt spray experiment, compared to GI coating, an improvement ofmore than2times in corrosion resistance was found. When the content of Mg in thecoating is less than2wt.%, the corrosion resistance of ZM coatings increases with theMg content increasing. When the Mg content is more than2wt.%, the corrosionresistance of ZM coatings reduces due to existence of Mg-rich regions in the coating.The corrosion resistance of ZM and GI coatings is as follows: ZM2（Zn–0.2wt.%Al–2wt.%Mg）> ZM1（Zn–0.2wt.%Al–2wt.%Mg）> ZM3（Zn–0.2wt.%Al–3wt.%Mg）> GI （Zn–0.2wt.%）.The enhanced corrosion resistance of ZM coatings can be attributed to inhibitionof grain boundaries corrosion due to Mg addition and formation of dense corrosionproducts. The results of electrochemical impedance spectroscopy （EIS） indicated thatduring the whole corrosion process, the GI coating was always subjected to grainboundaries corrosion. However, the grain boundaries corrosion was inhibited due tothe existence of Mg in the coating. With the Mg content in the coating increasing, thelocal corrosion regions for ZM coatings are different. For Zn–0.2wt.%Al–1wt.%Mg （ZM1） coating, the corrosion of Zn grains was always the main corrosionapproach. For ZM2coating, the localized corrosion was the main corrosion approachat the early stages of corrosion and after than, the general corrosion was the maincorrosion approach. On the contrary, for ZM3coating, the general corrosion occurredon surface of the whole coating at the early stages of corrosion and after that, Mg-richregions were subjected to localized corrosion. Mg in the coating can improve thedensity of corrosion products layer markedly. Under the condition of linear polarization experiment, Mg in the coating can improve the density of corrosionproducts layer. Under the condition of salt spray experiment, Mg in the coatingpromotes the formation of Zn5（OH）₈Cl₂H₂O. Especially for ZM2coating, thecorrosion products nearly consist of the plate-like corrosion product. When the Mgcontent in the coating is less than2wt.%, no exfoliation of corrosion products layerwas found. With the Mg content further increasing, the exfoliation of corrosionproducts is observed due to existence of Mg-rich regions in the coating, whichreduces the protective efficiency of corrosion products layer.Additions of a small quantity of La （0.04–0.1wt.%） into Zn bath can not varythe microstructure of the GI coating. Based on the results of salt spray experiment,additions of （0.04–0.07wt.%） La into the Zn bath should be recommended to obtainthe best corrosion resistance.Addition of a small amount of La into the Zn bath can alter the phasecomposition of ZM coatings. The phase composition of Zn–Al–Mg–La coatingsconsists of three phases, MgZn₂, Zn and La, with a addition content of1wt.%Mginto the Zn bath. The phase composition of Zn–Al–Mg–La coatings consists offour phases, MgZn₂, Zn, La and Mg2Al3with addition of more than1wt.%Mg intothe Zn bath. Segregation of La on the coating surface was observed. With the Mgcontent in the Zn bath increasing from1wt.%to3wt.%, the amount of （Zn+MgZn₂）eutectics on Zn–Al–Mg–La coatings increases till complete eutectics for aaddition content of3wt.%Mg. Based on the results of salt spray experiment,compared to GI coating, the corrosion resistance of Zn–Al–Mg–La coatingsincreases by more than2times in first red rust time and the best corrosion resistance（more than5times） is achieved with additions of2wt.%Mg and0.07wt.%La intothe Zn bath. With regard to GI coating, formability of Zn–Al–Mg–La coatings isbetter with a Mg content of1wt.%in the coating. With the Mg content furtherincreasing, the formability of Zn–Al–Mg–La coatings will decreases slightly.