Phenomenal Pitting Corrosion on Coating Damaged Surface of Automotive Suspension Coil Springs

摘要

This study focuses on a better understanding of significant pitting corrosion on coating surface damaged carbon steels, or low alloy steels, during salt spray testing for automotive applications. Anodic cyclic polarization test was used to evaluate the severity of pitting corrosion, and to estimate the corrosion rate of raw materials. Corrosion potential E_(corr), pitting potential E_(pit), and pitting protection potential E_(pp) were measured by conducting anodic cyclic potentiodynamic polarization (CPP) on 3 carbon steels. The scattering of the three potentials were observed within 35 mV on a 1 cm~2 exposure surface of the 3 steels. To simulate the surface damage on automobile suspension coil springs and stabilizer bars, gravel shocking and a hardness indenter were used to generate damage on ZnP pretreated and coated shot peening surfaces. The corrosion potential evolution of steel substrate, ZnP pretreated surface, and damaged coating surface were monitored in 5% NaCl solution for 72 hours. The timing of pretreatment exposure and metal substrate exposure to the NaCl solution were noticeable. A pitting model and a correlation between pitting occurrence and fatigue cracking are further discussed.