Anti-corrosion Effect Of Magnesium Phosphate Cement Coating On Carbon Steel

Magnesium phosphate cement?MPC?is widely used in fast repair,immobilization of radioactive waste and mixed waste streams with the advantages of developing strength rapidly,strong environmental adaptability,high bonding strength and durability.Studies have been done on the hydration mechanism,retarding mechanism,microstructure,raw material ratio,doping modification mechanical properties and durability of MPC,primarily focusing on the block concrete samples.Earlier studies reported the use of MPC for metal substrate surface protection,and achieved good results.This thesis attempts to explore the corrosion resistance of magnesium phosphate cement coating on carbon steel surface by electrochemical measurement technique.In this thesis,MPC coatings were prepared on the surface of steel 45,and its corrosion resistance and corrosion resistance mechanism were evaluated.In the second chapter,three kinds of M/P magnesium phosphate cement coating with thickness of 500 ?m were prepared by brushing the surface of 45 steel with different manual brushing.The electrochemical characteristics of magnesium phosphate cement with M/P of 3/1,4/1,5/1 were compared by short-term electrochemical impedance spectroscopy and linear polarization method.The experimental results show that when M/P is 5/1,Magnesium phosphate cement coating has the greatest effect on impeding charge transfer.Then the effect of single brushing and multiple brushing on the coating properties was compared.The results showed that multiple brushing was more favorable for the protection of the metal substrate.In Chapter 3,MPC with M/P of 5/1 was used to brush the surface of 45 steel after phosphating,and the samples were characterized by SEM,XRD,long-term linear polarization and electrochemical impedance spectroscopy,long-term salt spray test,the bond test characterization of the corrosion resistance of the coating.The results show that the MPC coating is made up of dense prismatic crystals,with a small amount of microcracks inside,the main components are MgKPO4 · 6H2O,MgO and Al₂O₃.The R_p of the sample was maintained at 2.0×105?·cm² for 1 h to 12 d,and Rct remained at about 8×104?·cm²,Indicating that the coating may have long-term effective anti-corrosion effect.The results show that the bond strength is 4.8 ± 0.7 MPa,which proves that MPC can be used as the corrosion-resistant coating on the surface of 45 steel long-term stability.In Chapter 4,the corrosion resistance mechanism of MPC coating was analyzed by immersion test and potential dynamic polarization method.The results show that the corrosion rate of the samples immersed in 3.5%NaCl + MPC solution is the lowest,the open circuit potential is the most stable,and the polarization curve moves from the corrosion potential to the positive direction,and the anode branch is obviously decreased.Indicating that the incomplete reaction of MgO and slightly soluble phosphate in the MPC had an anodic corrosion inhibition effect on 45 steel.Therefore,MPC coating for the 45 steel provide physical barrier and chemical corrosion dual anti-corrosion effect.