Corrosion Behavior Of Supersonic Arc Spraying Al Coating In Sewage Storage Tanks

In the petroleum and petrochemical industry chain,sewage tanks are widely used as important facilities for sewage storage,transportation and post-treatment.However corrosion and perforation failure seriously affect the long-term and safe operation of storage tanks.Aiming at the long-term anti-corrosion problem of storage tanks,this paper introduces Al coated Q235 steel with good seawater service performance into sewage storage tanks on the basis of clarifying the failure mechanism of perforation,and proposes a hole sealing method for the pore penetration problem of Al coated Q235 steel,so as to realize the preparation of Al anti-corrosion coating for sewage storage tanks.To this end,this paper using scanning electron microscopy(SEM),energy spectrum analysis(EDS)and X-ray diffractometer(XRD)study the corrosion mechanism of air/water interface,water layer,water/mud interface and mud layer.The corrosion failure form and mechanism of Al coated Q235 steel were clarified by the bolt and electrochemical experiments,and the mechanism of the resistance of Al coat Q235steel after the sealing treatment is clarified,providing a basis for the application of Al coat Q235 steel in sewage storage tanks.The results show that the corrosive medium of the sewage storage tank infiltrates into the film layer and the inner wall of the tank to form sub-scale corrosion.The oxygen concentration difference corrosion and SRB corrosion jointly act on the inner wall of the tank to form local corrosion pits,resulting in the most serious corrosion in the mud layer.When the depth of the corrosion pit reaches 65%of the initial wall thickness of the tank,the stress level at the bottom of the corrosion pit exceeds the internal pressure strength of the tank,and the tank pierces to form corrosion perforation.The as-sprayed Al coating is a typical layered structure with no obvious oxidation.The porosity is about 5.4%and the thickness is about 60μm.After60 days of immersion,the self-corrosion current density of the aluminum coating was 3.54μA/cm~2,which was always lower than that of Q235 at 4.85μA/cm~2.The thinning rate of each position of the coating before and after the protection of the aluminum coating was significantly reduced,and the aluminum coating played a good protective effect on the substrate.However,due to the factors such as oxygen concentration corrosion and porosity,the corrosion rate of the coating serving in the mud layer was higher than that of the oxide film passivation rate,and the corrosion of the coating was serious.Local corrosion pits and cracks occurred,and the delamination failure occurred.After sealing treatment with sodium silicate and silicone resin,the porosity of Al coating decreased to 1.2%and 0.7%,and the residual pores were mainly closed pores.The sealing agent has a blocking effect on the immersed corrosive medium after penetrating into the open pores,so the corrosion resistance is improved after sealing treatment.No significant corrosion occurred in the coating after 60days.Silicone resin not only improves the compactness of the coating,but also forms a layer of resin film with physical shielding effect on the surface of Al coating.Therefore,the corrosion resistance of Al coating after sealing treatment of silicon resin is improved.