Study On Surface Pre-oxidation And Hot-dip Zinc Coating Structure Of Nodular Cast Iron

Under the dual protection of physical and electrochemical protection,hot-dip galvanization can protect steel material from corrosion for a long time,which is one of the most common surface protection methods.Nodular cast iron is a most commonly used material for water pipes and its surface protection measures have gradually become a hot topic.At present,the research on hot-dip galvanization is mainly based on steel,but there is no systematic research on hot-dip galvanization of nodular cast iron at home and abroad.Due to the high silicon content in the matrix of nodular cast iron,the Si reactivity similar to that of steel during hot-dip galvanization will appear,which has a negative effect on the growth of hot-dip galvanized alloy layer.Therefore,it is of great practical significance for the hot-dip galvanizing industry to study the surface pretreatment methods to improve the quality of the hot-dip galvanizing layer of nodular cast iron.In this paper,the pre-oxidation products,surface silicon content,microstructure and corrosion resistance of hot-dip galvanized layer of pre-oxidized nodular cast iron were systematically studied by means of SEM,EDS,XRD,XPS and electrochemical workstation.The results are as follows.Cr/Cr₂O₃ powder system was used for surface selective pre-oxidation of nodular cast iron.The results show that with the increase of pre-oxidation temperature,the thickness of Si O₂ layer formed on the surface of nodular cast iron increased.The silicon content on the surface of nodular cast iron after pre-oxidation shows a trend from sharp increase to slow increase.With the increase of pre-oxidation temperature,the silicon content on the surface of nodular cast iron decreases obviously after alkali washing and pickling,showing a trend of obvious decrease to slow increase.The silicon content on the surface of pre-oxidized nodular cast iron at800? decreases to 1.86 wt.%.The thickness of alloy layer in coating of pre-oxidized nodular cast iron is obviously reduced,and its variation law with pre-oxidation temperature is basically consistent with that of the silicon content on the surface of nodular cast iron after alkali washing and pickling with pre-oxidation temperature.The coating of pre-oxidized nodular cast iron pre-oxidized at 800? is the densest with a thickness of only 28.2?m.The effect of selective pre-oxidation time of Cr/Cr₂O₃ powder system on the growth and corrosion resistance of hot-dip galvanized alloy layer on nodular cast iron was studied.The results show that Fe₂O₃was the main oxide on the surface of nodular cast iron during pre-oxidation at 800? and1 atm.When pre-oxidized at 800? and low oxygen pressure,the surface of nodular cast iron undergoes selective oxidation,and the main oxidizing products are Si O₂.After alkali washing and pickling,the Si content on the surface of selectively pre-oxidized nodular iron is effectively reduced.The thickness of galvanized alloy layer of non-selectively pre-oxidized nodular cast iron is larger,and its growth is mainly controlled by interface reaction.The thickness of galvanized alloy layer of nodular cast iron after selective pre-oxidation for 5h is smaller,and its growth is mainly controlled by diffusion.Compared with non-selectively pre-oxidized nodular cast iron,the thickness of galvanized alloy layer of selectively pre-oxidized nodular cast iron for 5h decreases by 57.7%.After selective pre-oxidation,the corrosion current density of?phase layer of hot dip galvanized nodular cast iron decreases significantly and the impedance response increases significantly.With the increase of pre-oxidation time,the corrosion current density of galvanized layer decreases continuously and the impedance response increases continuously.It is found that when the nodular cast iron is pre-oxidized with sodium bicarbonate at 100?,the surface oxidized products are mainly Fe₂O₃.When pre-oxidized at 200?,the surface oxidized products are Fe₂O₃ and Si O₂.When pre-oxidized at 300?,the surface product is mainly Si O₂.The silicon content on the surface of the sample after surface pre-oxidation first decreases and then increases with the increase of pre-oxidation temperature and that of the sample after alkali washing and acid washing shows a decreasing trend with the increase of pre-oxidation temperature.With the increasing of pre-oxidation temperature,the thickness of alloy layer decreases.The thickness of galvanized alloy layer of nodular cast iron pre-oxidized at 300? is only 37.3?m,which is 43.9%lower than that of non pre-oxidized nodular cast iron.The experimental results show that the Si content on the surface of nodular cast iron can be significantly reduced by surface pre-oxidation method.The Fe-Zn reaction in the hot-dip galvanizing process can be inhibited,and a more compact alloy layer structure can be obtained.So the quality of hot-dip galvanizing layer can be significantly improved.