Corrosion Characteristics Of Carbon Steel With Ultrasonic Phosphating And Sealing Coatings In Simulated Hot-Dry-Rock Geothermal Water

The issue of corrosion in hot-dry-rock environment seriously restricts the effective utilization of energy in hot-dry-rock system.In this study,the environment-friendly ultrasonic phosphating and surface coating techniques were employed to improve the corrosion resistance property of carbon steel in the hot-dry-rock system based on the summary of progress on the methods of inhibition of corrosion.To determine the optimal preparation conditions of ultrasonic phosphating,effects of phosphating process parameters on corrosion properties of phosphating film were studied by SEM analysis and potentiodynamic polarization curves at ambient temperature.The results showed that the size of phosphate crystals was reduced and the corrosion resistance of phosphate coatings was improved by utilizing ultrasonic treatment.Phosphate films using the combination accelerators of hydroxylamine phosphate bath p H value and phosphating time are two key parameters affecting the anticorrosion performance of the phosphate films significantly,both of which should be kept in the optimal range.The optimal conditions for phosphating films with high corrosion resistance were obtained with ultrasonic treatment,p H value of approximate H₃PO₄?85 wt.%?,8-10 g·L^-1 of HAS,20-30 g·L^-1 of Na NO₃,2-4 g·L^-1 of Mn?NO₃?₂?50 wt.%?.The corrosion properties of the phosphating films in simulated hot-dry-rock geothermal water were investigated by immersion tests.SEM analysis of phosphate films after immersion indicated that ultrasonic phosphate coatings were scarcely damaged after exposing to geothermal water at 80^°C.The phosphate particles suffered a certain degree of damage after immersion at 150^°C,but no obvious corrosion products were generated on the sample surface.The surfaces of traditional phosphate coatings and industrial phosphate coatings were all full covered by corrosion products after immersion at 80^°C and 150^°C,which demonstrated inferior protection compared with UP coatings.The results of electrochemical impedance spectroscopy?EIS?measurement showed that UP coating encountered a drop in corrosion protection with the increase of temperature of geothermal water and the ability of corrosion protection of coatings decreased with the increase of concentration of Cl^-.It should be mentioned that the temperature and concentration of scaling ions have certain effect on the property of fouling that can reduce the corrosion reaction between the sample and solution.Although the resistance of phosphate coatings tended to decrease as immersion time went on,R_ct of UP coatings even after exposure to geothermal water for 336 h was 36.90 k?·cm² that was higher than that of carbon steel?15.54 k?·cm²?,suggesting that ultrasonic phosphate coatings could prevent the interface reactions as much as possible.Sol-gel SiO₂ was coated on the phosphate coating to seal the pore with the purpose of further enhancing the properties of corrosion protection.The performance of ultrasonic phosphate coating with sol-gel SiO₂ post-sealing was evaluated by surface analysis and immersion tests.The surface of UP-SiO₂ coatings was denser,the roughness was smaller,and the contact angle of water was larger.The immersion tests in geothermal water at 150^°C declared that the corrosion attacked at UP-SiO₂ coatings was much lighter than that at UP coatings.The EIS measurements of UP-SiO₂coatings after immersed in geothermal water with time showed that the penetration was slower than that of UP coatings.The value of R_f and R_ct of UP-SiO₂ coating were about 20 times and 2 times higher than that of UP coating after 336 h of immersion respectively,indicating that more effectively protective films were obtained by sol-gel SiO₂ post-sealing on phosphate coatings.