Research On In-Situ Growth Of Mn-P Corrosion-Resistant Coating On Q235 Carbon Steel And Its Application

Due to the high hardness and relatively simple smelting of ironware,and the widespread existence of iron ore in nature,it is becoming more and more popular in promoting industrial and agricultural production.Unfortunately,humid air can easily lead to corrosion of iron utensils,resulting in lower product quality and increased economic losses.At the same time,it may increase the probability of accidents and environmental pollution.In order to meet the requirements for the protection of ironware in the new era,this paper has developed a chromium-free Mn-P coating protection technology with low pollution and excellent corrosion resistance.In-depth research and discussion are carried out around its performance,structure,formation mechanism and corrosion mechanism.This article uses a combination of manganese sulfate（MnSO4）,potassium dihydrogen phosphate（KH2PO4）,ferrous sulfate（Fe SO4）,and cetyltrimethylammonium bromide（CTAB）as a template to obtain a phosphating solution,Mn-P corrosion-resistant coating is obtained by further phosphating treatment on the surface of Q235 carbon steel.Taking the ratio of MnSO4 to KH2PO4in the phosphating solution,the concentration of each substance,the phosphating temperature,the phosphating time,and the p H value of the phosphating solution as the optimization conditions,the corrosion resistance of the Mn-P coating was optimized and inspected mainly through Tafel-type polarization curve and salt spray accelerated corrosion test.Finally,within the scope of research,the optimal process conditions for in-situ coating growth were determined:m（MnSO4）:m（KH2PO4）=2:10,c（MnSO4）=9g/L,c（KH2PO4）=45g/L,c（CTAB）=4g/L,c（Fe SO4）=1g/L,T=45℃,t=12h,p H=5.At this time,the corrosion current（Icorr）in the Tafel-type polarization curve is6.56×10^-11 A/cm²,which can be reduced by 7 orders of magnitude compared to the blank sheet.At the same time,Mn-P coated carbon steel sheets grown under optimal conditions can withstand more than 30h in the salt spray accelerated corrosion test（5%Na Cl）,there is no obvious rust change on the surface,and the longest salt spray resistance time can reach 120h,and then there will be slight corrosion,which has significant corrosion resistance compared with the blank sheet.In addition,it was found that changes in the ratio of phosphorus to manganese have a significant impact on coating performance during the optimization process,a scanning electron microscope（SEM）equipped with an energy spectrometer（EDS）was used to determine the element content of the Mn-P coating formed with different ratios of phosphorus to manganese,and in-depth study of the crystal micro morphology.The X-ray diffractometer（XRD）compares the composition changes of the coating crystals under different phosphorus-manganese ratios,and the X-ray photoelectron spectrometer（XPS）analyzes the compound valence state of the coating crystal components under the optimal conditions.The micro-scratch test is used to test the adhesion performance of the Mn-P coating under the optimal conditions,and through electrochemical impedance（EIS）and acid copper sulfate spot test to further judge the difference in corrosion resistance caused by different phosphorus-manganese ratios.The results show that a thin and dense coating composed of MnHPO4*2.25H2O and Mn5²⁺（PO4）2（PO3（OH））2*4H2O was grown in situ on Q235 carbon steel,the elements of the coating are evenly distributed,and the microscopic morphology gradually changes from a loose spherical structure to a dense needle-like structure.Under the optimal conditions,the thickness of the coating is about 9.28μm.The micro-scratch test show that the bearing capacity of the coating is about 5.65N.The electrochemical Impedance（EIS）shows that Rct=4.878×10⁸Ω·cm² under relatively optimal conditions,and Q235 carbon steel coated with Mn-P coating under optimal conditions,the dripping time of acid-resistant copper sulfate can last up to 30 minutes without discoloration.SEM and EDS tests were used to analyze the growth process morphology of the coating and the changes in element content before and after corrosion,and deeply analyzed the growth process mechanism and corrosion process mechanism of the Mn-P coating.At the end of this article,an ancient iron lamp is selected for protection treatment,and it is found that the corrosion resistance of the protected iron lamp has been significantly improved,which proves that the phosphate coating can prolong the service life of iron utensils.