A quartz crystal microbalance study of the corrosion of iron thin films in neutral aqueous solutions

The quartz crystal microbalance (QCM) technique is capable of detecting small mass changes in the region of nanograms per square centimetre from resonant frequency changes of the quartz crystal. In this study, the QCM technique, combined with electrochemical measurements, was applied to the minute corrosion of iron thin films in deaerated neutral solutions. An iron thin film with a thickness of 200 nm was electroplated on the gold electrode of a quartz crystal. The mass changes of the iron thin film during natural immersion or galvanostatic polarization in deaerated pH 6.48 borate solution, pH 6.42 borate solution with 10^-2 M chloride ions, pH 6.0 borate solution with 0.5 M chloride ions and pH 6.48 phosphate solution were measured as a function of time or potential to evaluate the iron dissolution rate or iron dissolution current. The corrosion rate of the iron thin film on natural immersion increased in the order pH 6.48 phosphate > pH 6.0 borate with 0.5 M chloride ions > pH 6.42 borate with 10^-2 M chloride ions > pH 6.48 borate solution. The net current flowing through the external circuit during galvanostatic polarization near the corrosion potential was successfully separated into the iron dissolution current and hydrogen evolution current. Tafel plots of the iron dissolution current and hydrogen evolution current were made to evaluate the corrosion mechanism of the iron thin film. The Tafel slopes of iron dissolution and hydrogen evolution thus obtained depend on the electrolyte solutions, from which conclusions can be drawn on the corrosion mechanism.