The objective of this research is to develop a method to prevent, so-called rinsing rust, in pickling and oiling line (POL) in a steel manufacturing company. In this company, a cascade type rinsing system has been adopted to remove residual acid remaining on the strip after pickling. Although it has shown effective rinsing capability under normal operating condition, rinsing rusts (spray burns) are evident on the strip as the line occur slow downed and stopped. Since the cost down due to rusting is significant(around $80.00/ton), it is necessary to develop a system which will prevent the rusting during rinsing.
First of all in this study, major operation conditions of the existing system were analyzed. The major problem in the existing rinsing system was identified as high acidity of the rinsing solution. Temperature of rinsing solution is 60~65℃, and its pH is 1.5~4. Because rusting is a corrosion process, corrosion behavior of hot rolled strips was investigated. Exposure tests were conducted under the same conditions as the rinsing system (60℃ HCl solution : pH 1.5~4) up to 200 seconds. As the acidity of solution increased, corrosion became severe. This phenomenon could be attributed to fast cathodic reaction of hydrogen evolution at low pH(high acidity). To resolve this low pH problem, the neutralization of rinsing solution by NaOH was considered. The results showed that when pH of solution is higher than 6, corrosion became moderate up to 100 seconds of exposure.