Corrosion Behavior of SPHC Steel Plates by Cl‒ Concentration in Laminar Cooling Turbid Ring Water

The surface quality of steel products has always been a major concern. In the steel rolling process, in order to reduce production costs, laminar flow cooling of steel plates is often performed using recyclable water treated by turbid ring water systems. Due to the rapid evaporation of water during the cooling process, there will be some ions left on the surface of the steel plate, which will make the steel plate corrosive in the later storage process. In order to clarify the effect of the concentration of Cl‒ in laminar cooling turbid ring water on the corrosion of SPHC steel plates, so as to improve the surface quality of SPHC steel plates after corrosion occurs when they are stored in a humid environment, simulations were performed by full immersion corrosion tests. It was shown that the concentration of Cl‒ in the laminar cooling turbid ring water showed a linear and positive relationship with the residual amount of Cl‒ on the surface of steel plates, so the corrosion behavior of SPHC steel plates was studied by setting different concentrations of Cl‒. The macroscopic corrosion morphology of the specimens was analyzed by digital camera. The surface morphology and depth of corrosion pits on the specimens were analyzed by scanning electron microscopy and laser confocal microscopy, respectively. The loss-in-weight method was used to measure the uniform corrosion rate for different concentrations of Cl‒. Electrochemical tests were performed to analyze the electrochemical characteristics of specimens covered with corrosion product films at different concentrations of Cl‒. The results show that whether the concentration of Cl‒ is 60 mg/L, 300 mg/L or 1 000 mg/L, the uniform corrosion rate shows a trend of increasing first and then decreasing, which is caused by the formation and expansion of pitting corrosion in the early and middle stages of corrosion and the corrosion retardation of corrosion products in the late stages of corrosion. The increase in the concentration of Cl‒ increases the number of ions crossing the surface of the substrate, shortening the time for pitting to form and expand, and increasing the uniform corrosion rate. Although the accelerated corrosion rate will also accelerate the formation of corrosion products, the high concentration of Cl‒ will destroy the protective effect of corrosion products on the substrate. The analysis of the polarization curve shows that the increase of the concentration of Cl‒ makes the passivation film on the surface of the substrate be accelerated and destroyed, and the corrosion current density increases. In addition, the impedance value fluctuates at the high Cl‒ concentration, indicating that the rust layer formed is easily dissolved, and this rust layer does not have a good protective effect on the substrate. Therefore, the concentration of Cl‒ in the turbid ring water needs to be reduced in order to reduce the residual amount of Cl‒ on the surface of the steel plate. In summary, reducing the concentration of Cl‒ in laminar cooling turbid ring water is an effective way to improve the surface quality of SPHC steel plates after corrosion in a humid environment. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.