Improvement of Pitting Corrosion Resistance of Spring Steels for Automobile by Alloying and Chemical Passivation Treatment

In order to reduce an exhaust gas, it has been necessary to reduce a weight of automobile. One of the most effective method for the weight reduction of cars is the strengthening of parts. Therefore, in order to reduce the weight of suspension coil springs, the strengthening of spring steels has been carried out in recent years. However, the strengthening of steels brings increase in sensitivity for corrosion fatigue caused by pitting corrosion. Improvement of the pitting corrosion resistance by alloying and chemical passivation treatment was investigated in this research. Measurements of anodic polarization curves were carried out for four commercial production low alloy spring steels and 22 laboratory steels modified on JIS SUP 10 in 5 mass% NaCl solution at 308±0.5K. Furthermore, the effect of chemical passivation treatment on the pitting potential was evaluated for the specimens treated in 63 mass% conc. HNO3 solution at 308K. The quasi-pitting potential of V'cq100 corresponding to V'c100 of stainless steels, has been adopted to evaluate the pitting corrosion resistance of low Cr-bearing spring steels. The major results of this research are as follows. (1) The effective alloying elements to improve the pitting corrosion resistance of the low alloy spring steels are Si, Ni, Cr, Cu, Co, Sb, and B. (2) The chemical passivation treatment for 3.6 ks on the practical spring steels containing less above-mentioned alloying elements, is also effective to improve the pitting corrosion resistance. (3) The quantitative alloying design guide based on regression analysis between V'cq100 and content of alloying elements was proposed.