The primary objective of the pretreatment process is to remove grease and impurities from the surface of the steel strip, thereby forming a chemical conversion coating on the steel substrate to enhance both the adhesion between the coating and the substrate, and the overall corrosion resistance. A typical pretreatment sequence comprises steps such as degreasing, brushing, hot water rinsing, surface conditioning, a second hot water rinsing, passivation, and drying. Surface conditioning is an additional surface treatment step introduced prior to phosphate or chromate treatment; it involves spraying the steel strip surface with a weakly acidic solution containing phosphate, nickel, and fluoride ions. This process aims to improve coating performance and the adhesion of the passivation agent, while simultaneously removing precipitates-such as aluminum and lead-that have accumulated at the grain boundaries of galvanized substrates. The treatment solution for passivation can be applied via several methods, primarily spraying, dipping, and roll coating. Among these, roll coating involves applying the solution to the steel strip surface to create an insulating barrier between the metal substrate and the subsequent coating layer; this method offers the advantages of easy process control, uniform film thickness, and minimal waste liquid generation, thereby contributing to environmental protection.
There are two primary design configurations for the pretreatment process layout. The first involves positioning a pre-cleaning section upstream of the looper (accumulator) and the main pretreatment section downstream of the looper; this design is widely adopted by various European suppliers of color-coating lines. Its key advantage is the ability to pre-treat and remove large particulate impurities when the surface quality of the raw material is poor. The second configuration involves situating the pretreatment section exclusively downstream of the looper, with no pre-cleaning section located upstream; this design is more commonly observed among Asian suppliers of color-coating lines.