Research Of Preparation And Modification Of Anticorrosive Magnesium Phosphate Cement Coating On Q235 Steel

Magnesium phosphate cement（MPC）is a novel inorganic material with structures and properties partially analogous to ceramics.With rapid hardening,high early strength and environmental benefits,MPC has been used in fast repair of road surfaces,hazardous waste encapsulation and other applications.As an inorganic material,MPC overcome the advantages of organic and metallic coatings such as pollution and poor resistance to high temperatures and weathering.This thesis explored the anticorrosion performance and mechanism of magnesium phosphate cement coating on Q235 steel by electrochemical methods.In this thesis,magnesium phosphate cement paste was prepared by mxing KH₂PO₄（P）and MgO（P）at different P/M ratios,and applied to the surface of Q235 steel.The physicochemical characteristics and corrosion resistance of the resultant MPC coatings were studied by XRD,SEM,linear polarization,electrochemical impedance,and salt spray tests.It was found that the charge transfer resistance of MPC coating with P/M of 1:5 was the largest and the corrosion resistance of this P/M ratio was the highest.After neutral salt spray for 1440h,no corrosion was visible on the surface of MPC coated Q235 steel,indicating that MPC coating offered a long-term anti-corrosion protection for the steel.MPC coating was mainly composed of unreacted MgO and Al₂O₃ particles,and relatively few hydration products MgKPO₄·6H₂O.Meanwhile there were a few microcracks on the coating surface,and surface structure of coating with P/M of 1:5 was denser.The MPC coating had a bonding strength 4.6±0.7 MPa with the metal substrate.Immersion experiment and dynamic potential polarization method showed that,the corrosion of Q235 steel in 3.5%NaCl solution was fast.In NaCl solution containing MgO and MPC particles,the corrosion rate of Q235 steel was significantly reduced,indicating that both MgO and MPC had an corrosion inhibition effect on Q235 steel.It is thus concluded that the anti-corrosion mechanism of MPC is not only due to its physical barrier effect,but also the phosphate ions and OH^-liberated from MPC,which forms an inert layer on the metal surface to protect the metal from corrosion.Additionally,MPC coatings were prepared by substituting the same volume of Al₂O₃ in the raw material with fly ash.The experimental results of XRD,SEM and electrochemical tests show that the charge transfer resistance in MPC coating with fly ash slightly decrease compared with the MPC coating with Al₂O₃.It indicated that the mixing fly ash did not significantly improve the corrosion resistance of the MPC.The MPC coating with fly ash was mainly composed of MgKPO₄·6H₂O,unreacted MgO,Al₂O₃ and SiO₂.And the coating surface is smoother,but with many cracks.As an industrial waste,the use of fly ash may offer significant economic and environmental benefits.Nevertheless,the influence rule of fly ash to the long-term corrosion resistance of MPC still needs to be further investigated.MPC coatings were also prepared by replacing the same volume part of Al₂O₃ in the raw material with silica fume.The experimental results of XRD,SEM and electrochemical tests show that the charge transfer resistance in MPC coating with silica fume and Al₂O₃increased obviously,indicating that the introduction of silica fume and Al₂O₃ improved the corrosion resistance of the MPC.The coating is mainly composed of MgKPO₄·6H₂O,unreacted MgO,Al₂O₃,SiO₂ particles,and MgSiO₃ gel.The particles of different sizes fill the holes and gaps in the coating,increasing the density of the coating.Chemical modification of magnesium phosphate cement coating was carried out by surface treatment with sodium methyl silicate（SMS）,a waterproof agent.The results of electrochemical test showed that the corrosion resistance of the MPC coating was improved with doping a certain amount of PMS,and the greater the doping amount,the more obvious the effect of corrosion resistance.However,the dosage of PMS should not be too large,or it will leave a lot of pores that have not been discharged in time in the MPC,resulting in a decline in corrosion resistance of the coating.In summary,this thesis studied the corrosion resistance of MPC coating on Q235 steel,and modification of the MPC coating by several approaches.Results of this thesis may provide additional insights and new perspectives for improving the anti-corrosion properties of magnesium phosphate cement.