Corrosion and protection of the automotive structure

This chapter discusses the corrosion and protection of the automotive structure. As most of the experience in volume car manufacturing and service is related to steel structures, the knowledge gained with this material is presented. Although varied in terms of nature and geometry, the types of corrosion experienced in automotive situations usually have electrochemical origins. Because of the basic differences in chemistry between aluminum and its alloys and steel, aluminum exhibits specific features, which tend to characterize its general behavior. Effective design principles are divided into influences arising from styling features, subassembly, panel and design, and associated production process practices. To confirm and monitor an effective materials policy, most automotive manufacturers now carry out two types of evaluation, the first is a running vehicle assessment involving 12-week exposure to regular, alternating, salt spray, and humidity cycles. The empirical tests described above allow general performance and material comparisons to be carried out, and enable weaknesses to be identified in bodies or test pieces but do not allow detailed scientific analysis to be carried out on the nature of reactions taking place at a local level. More recently, it has been shown using a scanning vibrating electrode test (SVRET) technique that different situations can develop on opposite sides of a formed coated steel profile in IZ steel. The concave side of a semi-spherical cup appears to behave anodically whereas cracks opened on the convex side become cathodic.