Laboratory studies of perforation corrosion on automotive materials are decribed, in which a perforation coupon simulated a crevice on a car body. Electrochemical impedance spectroscopy (EIS) was used to determine in situ the corrosion rate of precoated steel sheet in a particular crevice geometry. Measurements were taken during an accelerated cyclic laboratory test, while visual inspections and microscopic examinations of corrosion attack were performed at the end of the exposure. The results show that the crevice width (the distance between the two sides of the crevice) has a significant influence on the corrosion performance of coil coated steel. High corrosion rates were observed during the first stage of the drying period. The degree of wetness inside the crevice was monitored during the wet-dry cycle by measurements of the galvanic current flowing between a gold electrode and the coil coated steel. Results of galvanic current measurements in crevices with different widths may provide an explanation for the corrosion performance of coil coated steel as a function of crevice width. It has been shown that EIS measurements with a flat three electrode configuration is a suitable technique to study the performance of automotive materials against perforation corrosion. During a wet-dry cycle, the corrosion rate increased with the crevice width in the range 0.5-0.25 mm. When the crevice width increased to 0.5 mm the corrosion rate decreased again. A maximum corrosion rate was thus obtained when the crevice width of the coupon was 0.25 mm. The maximum rate of corrosion occurred for coupons with a crevice width of 0.25 mm. It has been deduced that this is a result of a combination of the high rate of corrosion during the wet period and a long period of wetness during the drying time. The corrosion rate of galvanised steel during the CCT (cyclic corrosion test) is highest during the first stage of the drying period owing to the combined effect of a reduction in thickness of the moisture film and an increase in oxygen reduction at the metal surface.
Application of electrochemical impedance spectroscopy to study perforation corrosion of automotive materials
Anwendung der elektrochemischen Impedanzspektroskopie zur Untersuchung der Durchrostung von Automobilwerkstoffen
British Corrosion Journal ; 31 , 2 ; 113-118
1996
6 Seiten, 7 Bilder, 1 Tabelle, 19 Quellen
Article (Journal)
English
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