Automotive Corrosion and Accelerated Corrosion Tests for Zinc Coated Steels

Numerous studies on zinc and zinc alloy coated steel sheets have appeared since corrosion problems were widely recognized in the automotive industry in the 1970s. In general, accelerated corrosion tests including the Salt Spray Test and Cyclic Corrosion Tests have been applied to evaluate the corrosion resistance of coated steels in the laboratory in order to shorten the product development period. However, the reproducibility of corrosion in actual use conditions by corrosion tests is always a controversial issue. In this paper, author's previous studies regarding corrosion of coated steel sheets in automobiles and corrosion tests are reviewed. The perforation corrosion behaviors of coated steels at lapped parts in snowy and tropical regions were clarified on the basis of an analysis of corroded vehicles. Several types of accelerated corrosion tests used in the automotive industry were conducted for comparison with the corrosion observed in the actual vehicles. The Perforation Corrosion Index (PCI), which is the corrosion rate ratio of steel and zinc coating, was introduced as an index for comparing the correlation of corrosion behaviors. Coating weight is the major factor determining the perforation corrosion resistance of existing coated materials in automobile use environments. Corrosion tests that have a PCI value close to that in the use environment can simulate similar corrosion tendencies. The influence of the specimen configuration is also a key factor for adequate evaluation of corrosion resistance. Specimens without an intentional large clearance and bare metal surface in the lapped portion are recommended for perforation corrosion tests.