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A comparison of the cosmetic corrosion performance of zinc and zinc-iron alloy coated steels is made. Cosmetic corrosion can be defined in terms of paint creepage/blistering, red rust, and paint adhesion. Traditional scribe-type tests adequately address creepage and rusting, but do not adequately address paint adhesion. A new lab procedure called the Chipping Corrosion Test was used to assess paint adhesion of coated steels processed under various phosphate conditions. A review of the relative performance of zinc and zinc alloy coated steels in traditional scribed panel corrosion tests and the new chipping corrosion lab procedure is presented. Results of the Chipping Corrosion Tests showed significant differences in material performance when processed under plant conditions that were not evident under laboratory process conditions, or in traditional scribe-type tests.

A laboratory test procedure for evaluating the cosmetic corrosion resistance of auto body materials systems is presented. The test method combines periodic impact damage and scab corrosion cycles. Results are presented showing performance differences between coated sheet steels, phosphates, paint systems, and processing methods which were not discerned using conventional scribed panel scab corrosion methods. Four test methods are discussed which are capable of differentiating between alternate materials or processing methods.

A review is made of some of the existing methodologies for assessing phosphate characteristics on automotive sheet steels. Data obtained from on-line processing of zinc and zinc-iron substrates are presented comparing phosphate attributes to performance in the Chipping Corrosion Test. Phosphate characteristics measured were: crystal size; nickel content; coating weight; porosity; alkali solubility. A definitive relationship between these characteristics and chipping corrosion performance was not found.