12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Zinc coated steel sheets are used in order to protect corrosion of automobile bodies. It is well known that the type of automotive corrosion can be divided into perforation corrosion and cosmetic corrosion. Although the mechanism of the perforation corrosion has been studied well, the mechanism of the cosmetic corrosion has not been clarified yet. In this study, an in-situ observation technique was developed and an analysis of cosmetic corrosion was performed. Cold rolled steel sheets without galvanizing were used as test samples. After painting with ED, the surface of the samples was scratched with a cutter. The corrosion resistance was evaluated under the test condition which is in accordance with SAE J2334 (1 cycle: Salt dip, 15 min => Dry process, 17 h, 45 min => Wet process, 6 h) . The cosmetic corrosion behavior of the samples in initial 2 cycles was observed in-situ by the developed in-situ system. The samples were taken out at regular time intervals and analyzed. As results, it was found that the progress of under-film corrosion consisted of 3 steps. The 1st step is in the initial stage in the dry process of the 1st cycle. In this step, red rust gradually changed to black rust composed of magnetite at the scribed part, implying that the rust changed from iron hydroxide to magnetite by a cathodic reaction. The 2nd step is in the latter stage in the dry process, and in this step, under-film corrosion progressed from the scribed part. Cl was enriched at the scribed part when under-film corrosion started, and then it moved to the tip of under-film corrosion. The progress of under-film corrosion in the 2nd step is based on the Funke mechanism. The 3rd step is in the wet process, and in this step, the tip of the under-film corrosion grew so as to swell. It is assumed that water penetrated at the Cl-enriched part of the tip of the under-film corrosion by osmotic pressure. In 2nd cycle, the 2nd step and 3rd step in 1st cycle was repeated. The under-film corrosion progressed at almost the same rate as 1st cycle, and the under-film corrosion of the 2nd step was dominant as to the total corrosion rate. This in-situ observation technique can be also applicable to zinc coated steel sheets and will lead to a new coating design.