In situ observation of electrochemical reaction of Zn primer steel plate in NaCl solution using confocal micro-x-ray fluorescence imaging

Steel plates have been widely used in bridges, vehicle bodies, guardrails, and so forth. Generally, to improve corrosion resistance, the surface of a steel plate is coated with a Zn layer. However, when the coated steel plate is scratched by external factors, the corrosion resistance decreases. Therefore, it is important to elucidate the elution processes of elements in coated layers and steel plates to develop new coated films and improve corrosion-resistant techniques. During corrosion, an elution reaction between the coated metal and Fe (anodic reaction) and a reduction reaction of oxygen (cathodic reaction) occur. Confocal micro-x-ray fluorescence imaging was employed to visualize the elution processes at the anodic and cathodic steel plates. A Zn primer steel plate was scratched and then immersed in sodium chloride solution. The Zn elution process at the anodic side was observed when a constant current was applied to the two steel plates by a galvanostat. The Fe elution process did not occur, and we believe that the sacrificial protection of Zn inhibited the process. The elutions of Zn and Fe were not observed on the cathodic side of the steel plate. Using the proposed method, we successfully visualized the elemental distributions at the anodic and cathodic sides of the steel plates.