Study on Corrosion of Automotive Coil Spring Steel by Electrochemical Impedance Spectroscopy, 전기화학 임피던스 분석을 통한 자동차용 코일스프링 강의 부식 평가

Coil spring steels from the automobile suspension part after field exposure for 10 years and those after anti-corrosion validation test in proving ground of 5,000 ~ 10,000 km were examined for corrosion damages. Partial loss of paint, accumulation of corrosion product, and cracking of paint and superficial material were observed. The surface and subsurface region of spring steels had compressive residual stress and high hardness by shot peening. The surface hardness values of the specimens were 620 ~ 670 Hv. They were 60 ~ 80 Hv higher than those of the samples taken from the middle part of the spring. The maximum compressive stress was -916 ~ -1208 MPa measured at depth of about 100 μm. Electrochemical impedance spectroscopy showed that the resistances of charge transfer and the paint layer of the spring steels ranged from several tens to millions Ωㆍcm 2 . The resistance of the field samples was much higher than that of the proving ground samples used in this study, implying that the proving ground test condition would be more corrosive than the field environment.