In situ studies of the corrosion during drying of confined zinc surfaces

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Abstract
The corrosion process during the drying out of zinc surfaces confined in crevices was studied using real time photograpy and in situ FTIR microspectroscopy. A pH‐indicator was used to visualise differences in the pH during the drying process. The distribution and the composition of the corrosion products after several wetting and drying cycles were studied with FTIR microspectroscopy and SEM‐EDS. An area with high pH formed during the drying process at the border of the electrolyte, with a zone of white corrosion products that contained zinc hydroxycarbonate in the electrolyte inside this area. A differential aeration cell is present at the border of the electrolyte, and the cathodic oxygen reduction reaction takes place close to the border of the electrolyte during the drying process. The corrosion attack and the distribution and composition of the corrosion products on the surface depend strongly on the drying process of the surface. The corrosion attack of confined surfaces was localised, with a significantly higher corrosion attack in some areas. Outside the drying front a thin layer of electrolyte formed as a result of surface tension driven flow of electrolyte from the electrolyte border. This effect was attributed to the alkaline pH of the electrolyte due to the oxygen reduction reaction at the border. A galvanic element was formed between the local cathodes in the area outside the drying front and the anode in the area with bulk electrolyte. The main corrosion products detected after several wet dry cycles were ZnO, Zn5(OH)6(CO3)2 and Zn5(OH)8Cl2 · H2O, but Na2CO3 · 10H2O was also detected. The corrosion products were non‐homogeneously distributed on the surface and the distribution was related to the anodic and cathodic processes that took place in different regions on the surface during the corrosion process.

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