Perforation Corrosion on Automotive Steel - Comparison of Accelerated Indoor Corrosion Test and Field Performance 971001

The use of cyclic corrosion tests which consist of salt spraying, humidifying, and drying phases is desirable in the evaluation of the corrosion performance of automotive body panels. In this work, coupons of zinc-coated steel and of cold rolled steel were exposed to a defined cyclic corrosion test (CCT) which has shown to be suitable for simulating diverse outdoor conditions for evaluating the cosmetic corrosion of precoated steel. Micro-environment data in the gap of lapped panel specimens, such as wetness, corrosion rate of test materials, pH value, chloride content, have been obtained during the CCT test. The galvanic current flowing between a gold electrode and the test materials was measured to monitor the degree of wetness inside the crevice. The results show that the surface of the test materials never became really dry even during the dry period of the CCT cycle. Furthermore, the wetness during wet and dry periods in the crevice increased with increasing number of CCT cycle. Electrochemical Impedance Spectroscopy (EIS) has been used to determine in-situ the corrosion rate of test materials in the crevice during the CCT. EIS measurements suggested that the corrosion performance of the test materials changed greatly with the number of exposure cycles during the wet and dry phases of the CCT. This is probably due to the increase in the degree of wetness and in the chloride content in the crevice geometry. At the end of the cyclic test, corrosion evaluation by visual inspection and microscopic examination has shown good agreement with the EIS and galvanic current measurements. A comparison of the perforation corrosion attack on both zinc coated steel and cold rolled steel during the field exposure and that under CCT exposure shows clearly that this type of accelerated indoor test cannot appropriately simulate conditions for perforation corrosion in actual automotive bodies.

The corrosion products after the CCT test under both crevice and cosmetic conditions were analysed by X-ray diffraction (XRD). In crevice conditions, zincite and hydroxycarbonate were the main corrosion products, whereas hydroxychloride was detected under cosmetic condition for zinc coated steel sheets. These difference are attributed to the different geometries and the different micro-climate conditions.