Corrosion behaviors of hybrid ultrasonic phosphating coatings on carbon steel in simulated 150 °C hot-dry-rock fluids

To improve the corrosion resistance of carbon steel in hot-dry-rock geothermal environment, Hybrid zinc phosphate coatings on carbon steel with and without sol-gel SiO2 post-sealing were prepared under ultrasonic conditions. The surface morphology, composition and crystal phases of the coatings were studied by SEM, EDS and XRD. The corrosion characteristics of the phosphate coatings in four types of simulated hot-dry-rock geothermal water were investigated by the immersion test and corrosion electrochemical method after immersion for 72 similar to 336 h at 80 similar to 150 degrees C. The results revealed that the ultrasonic phosphate (UP) coating suffered from homogeneous corrosion and the corrosion increased with increasing temperature. When the temperature increased from 80 degrees C to 110 degrees C, the influence of temperature on scaling was more significant than on corrosion. When the temperature further increased, corrosion became more severe. The UP coating and UP coating with SiO2 post-sealing (UP-SiO2) had the lowest corrosion current densities and the highest charge transfer resistance (R-ct) due to better coverage, finer crystals and the seal of pores among crystal particles. The corrosion protection performance was improved because the ultrasonic phosphate coating prepared with SiO2 provided an effective barrier in simulated hot-dry-rock fluids.