The Effect Of Mg Addition On The Structure And Corrosion Resistance Of Hot Dip Aluminized Coatings

Hot-dip aluminized is widely used due to its good high-temperature oxidation resistance,but there are some problems such as the alloy layer of the coating is too thick,and and poor corrosion resistance in seawater environment.The addition of alloying elements can reduce surface quality defects caused by hot-dip aluminized progress,obtaining ideal layer organization and better corrosion resistance.Therefore,in this paper,the magnesium-containing coatings were obtained by hot dip aluminized method,the effects of different magnesium contents on the microstructure,high temperature oxidation and corrosion resistance of alloy coatings were characterized by scanning electron microscope,energy-dispersive X-ray spectroscopy and X-ray diffraction.The results are as follows:In this work,the experimental results of magnesium addition in pure aluminum bath showed that the addition of magnesium promoted the continuous and stable growth of FeAl₃ phase and inhibited the growth of Fe₂Al₅ phase.With the addition of magnesium content,the thickness of the alloy layer was gradually reduced.The experimental results of adding magnesium in the aluminum-silicon bath showed that with the addition of magnesium content,the thickness of the alloy layer increased first and then decreased.When the magnesium content in the molten pool was 2 wt.%,the phase composition of the alloy layer were Fe₂Al₅ phase,FeAl₃ phase and Al₇Fe₂Si phase.When the magnesium content was more then 2 wt.%,the phase composition of the alloy layer was converted from Fe-Al binary compounds to Fe-Al-Si ternary compounds.The growth of aluminum silicon coated alloy layer was transformed from interface reaction control to diffusion control after adding magnesium element.In order to better understand the influence mechanism of magnesium on the thickness of alloy coating,this work also used molecular dynamics simulation to study the effect of different magnesium content on atomic diffusion coefficient of aluminum melt.MSD curve slope results showed that the diffusion coefficient of the melt atoms increased with the increase of magnesium atoms,which was in agreement with the experimental results that the thickness of the sample alloy layer is reduced with the increasing magnesium content in the aluminum bath.In the high temperature oxidation performance tests,the results showed that the oxidation rate of the sample was the lowest when 0.5 wt.%magnesium was added to the aluminum bath.When the magnesium content in the molten pool was added to above 1 wt.%,the oxidation rate of the sample increases with the increasing of the magnesium content.The high temperature oxidation resistance of hot-dip aluminized silicon samples was not good as that of hot-dip aluminized.But after adding magnesium to the aluminum-silicon bath,the oxidation resistance of the samples was significantly improved.When the magnesium content in the molten pool was 1.5wt.%,the high-temperature oxidation resistance of the coating was optimal.Magnesium improved the high temperature oxidation resistance of the alloy coating,which was mainly due to the addition of magnesium promote the growth of the continuous dense FeAl₃ phase,while the dense FeAl₃ layer could hinder the diffusion of oxygen.The salt spray corrosion test results showed that addition of magnesium and silicon improved the corrosion resistance of the coating.The average corrosion resistance of the Al-Mg alloy coating samples was 3.6 times that of the pure aluminum coating,and the average corrosion resistance of the Al-Si-Mg alloy coating samples was 3.7 times that of the pure aluminum coating.The corrosion products of the plating layer were Al?OH?₃,Mg?OH?₂,Mg₂Si,FeAl,and so on.The corrosion product layer film could prevent chloride ions and other aggressive ions from penetrating into the substrate,and could effectively prevent the surface of the sample from being continuing corroded.