Evaluation of automotive coatings is often completed using laboratory cyclic tests. These tests use onset of red rusting and scribe creep to give an estimation of field performance of the materials. The most common automotive laboratory cyclic tests take 2-4 months to complete. These tests are often the rate limiting step in making progress toward product related decisions. This paper reports the use of EIS in combination with cyclic testing to estimate scribe creep performance. Materials were exposed in the GM 9540P-Cycle B laboratory test for 1 day or 8 days, and tested following removal from exposure. Material performance was compared to scribe creep performance after 40 days in the same test. Results of these studies will be discussed in this paper.
Evaluation of the effects of pretreatment type on painted corrosion performance typically involves use of an accelerated laboratory test taking ~2-4 months. Performance is usually determined by comparing average or maximum scribe creep. Scribe creep is a commonly used method in the automotive industry to estimate cosmetic corrosion susceptibility. An intentional defect, “scribe”, is introduced into a sample prior to the test. The scribe is made using a carbide tip or other sharp object and penetrates all layers of paint and the zinc or zinc alloy coating. Scribe creep refers to the delamination or creep-back due to corrosion around this defect. It is measured at regular intervals along the defect length at the conclusion of the cyclic corrosion test.
It would be desirable to have an alternate method to estimate corrosion performance in a shorter period of time. Previous studies have shown that electrochemical impedance spectroscopy (EIS) maybe an appropriate technique for this1. The purpose of this study was to confirm/reject earlier results that identified EIS as a promising technique for early estimation of scribe creep. The earlier study used a limited number of samples and conditions. This study used five zinc and zinc alloy coated sheet materials. Each material type received three phosphating conditions prior to painting and was tested in triplicate. EIS data was collected on these samples after 1 and 8 days of exposure in the GM 9540P-Cycle B test b. Nominally identical materials were also exposed in this test for the full 40-cycle test and scribe creep was measured after the test to compare to the EIS data.
In order to thoroughly evaluate the EIS method several other items were incorporated into the study. Materials were selected to represent performance extremes. This resulted in samples with a wide range of corrosion performance. Sample exposures were started on different days for both conventional samples and for the EIS samples to establish cabinet and test reproducibility. Phosphate uniformity, crystal size and coating weight were also determined.
In addition to testing the applicability of the EIS method for scribe creep estimation the study was also used to verify earlier results that painted corrosion performance of galvanneal is dependent on the pretreatment type and that painted corrosion performance of electroplated zinc is not as dependent on pretreatment typel-3.
