Abstract
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 Ecorr, pitting potential Epit, and pitting protection potential Epp 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 cm2 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.
Introduction
The corrosion of carbon steel has been studied for decades. The corrosion behavior and the factors accelerating or inhibiting the atmospheric corrosion of carbon steel have been systematically summarized in well-known corrosion textbooks.1-3 The exposure of carbon steel to a humid environment quickly generates severe corrosion on the entire exposed area. Corrosion caused by road salts and emissions was a major problem in carbon steels during the early years of the automotive industry. However, the corrosion problem in the automotive industry was significantly improved by the development of coating technologies. 4 Currently, carbon steels with pretreatment and powder coating are being used for many automotive applications, taking into account the balance between mechanical properties and the cost of materials and manufacturing. To understand the corrosion mechanism and to evaluate the corrosion rate of these carbon steels, electrochemical testing on metal samples under imperfect coating conditions (such as flawed, cracked, or partially damaged coating surfaces) becomes critical.
