Phosphating and Accelerating Corrosion Behavior on Al-Si Coating of Hot Stamping 22MnB5 Steel

Abstract

In order to study the relevant of corrosion resistance with hot treatment process condition, verify whether Al-Si alloy coating surface can be treated with zinc salt phosphating. It is prepared several different kinds of hot stamping Al-Si coating of 22MnB5 steel plates. The heating treatment temperature, holding time, thickness of coating and diffusion layer are discussed in this paper. Then phosphating to above plates which are obtained under several different hot treatment conditions. It is found that there exists some phosphating films on surface of several plates, such as No. 4, No. 5 and No. 9 sample, others have none. By scanning electron microscopy analysis method, morphology and microstructure of Al-Si coating and phosphating film are analyzed. By energy spectrum analysis method, all element contents are characterized. The content of Al element in the coating is decreased fom 87% to 8% through phosphating treatment. The content of Si element in the coating is decreased from 10% to below 1%. Because of corrosion of phosphating liquid, Al-Si alloy coating is broken at acid environment. Al is oxidated and become aluminum ion, Si is dissociated and deposited. With the decreasing of Al, anode region is oxidated. Because reduction in hydrogen concentration at cathode zone, pH become higher, when condition reachs to the solubility of zinc phosphate, the phosphating films appeared on surface metal material. The films element increasing, Zn is up to 20%, P is near to 9%, O is up to 34%, phosphating film crystal is main Zn3(PO4)2•4H2O and small amount of Zn2Fe(PO4)2•4H2O. In order to research corrosion resistance between phosphating film and Al-Si alloy coating, it is adopted CCT method to evaluate two passivation films. The result shows that phosphating film can not improve corrosion resistance of materials. On the other side, Al-Si alloy coating has better corrosion resistance, it can reduce corrosion rate of substrate. Through 50CCTs and 70CCTs, the sample NO. 8 shows excellent corrosion resistance. The result shows that the corrosion resistance of relatively good sample of heating treatment process condition is as follows: When heating treatment temperature is 930-950°C and holding time is 3-5min, hot dip plating and stamping of 22MnB5 steel with aluminum, silicon and other elements. It is obtained a continuous and complete coating, which toatal thickness is 42-52um.