전기화학적 임피던스 분석을 통한 유기 도장된 자동차 부품의 부식 거동에 관한 연구

Abstract

Automotive chassis parts are susceptible to corrosion due to external physical impact and chemical effects from contaminants. Therefore, it is required to evaluate the degree of corrosion and corrosion resistance of the parts. In this study, corrosion characteristics of organic painted chassis components from a passenger car that has been driving in North America for five years were examined. The degree of corrosion of each components was evaluated by visual inspection and thickness measurement. The phase of corrosion products and the cross-section structure of corroded samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The electrochemical properties were measured by electrochemical impedance spectroscopy (EIS). It was found from visual inspection that the press, fuel/brake, brake disc, fuel tank/band and other parts except suspension parts were severely corroded and the organic coating on them are totally peeled off. Metal thickness measurement showed that the metal loss of the press parts was the largest, that of the suspension parts was the least among the samples used in this study. α-FeO(OH), β-FeO(OH), γ-FeO(OH), Fe3O4, and SiO2 compounds were detected from the XRD pattern of corrosion products. Most of the oxide and the oxyhydroxides of iron were mixed on the surface of corrosion samples. SiO2 was found in some corrosion products especially on the rear parts, possibly originated from the sand on the road. Based on the SEM cross section observation, the layered structure of the cross-section were classified into four groups; C/O/M group that has a continuous coating layer over corrosion products, O/M group with corrosion product layer only on the metal, C+O/M group that has a mixed layer of corrosion products and coating, C/cr/O/M group with clear delamination of coating over corrosion product layer. Many of suspension, disc cover parts showed the C/O/M structure, while subframe and disc parts mostly belong to the O/M group. The total resistance (Rp + Rct) was 3.28 ∼ 1521 Ω-cm2, meaning 9.92 10-6 ∼ 4.60 10-3 mm/year of corrosion rate. The O/M group showed the lowest resistance and highest corrosion rate, while the C/O/M group showed the highest corrosion resistance. Delamination area () of the O/M group was 0.0063 ∼ 0.8190, delamination area () of the C/O/M group was 0.0218 ∼ 0.5032. Therefore, the more corrosive parts appear the more severe the cracking of the corrosion products and peeled off the organic coating at corrosive cross-section. The lower the resistance (Rp) or the total resistance (Rp + Rct) of the coating obtained from the electrochemical impedance, the higher the corrosion grade and the lower the corrosion resistance. Also, the delamination area was high.