Atmospheric corrosion is a highly costly and complex form of corrosion that involves chemical, electrochemical, and physical processes in three phases (solid, liquid, and gas) and two interfaces (solid/liquid and liquid/gas). With the access of interface sensitive analytical techniques, it is now possible to perform molecular in situ analyses of the whole interfacial region under ambient atmospheric pressure conditions. From these studies a conceptual framework has evolved to describe the initial stages of atmospheric corrosion. A crucial step is the coordination of a proton or a ligand to the corroding metal surface, a precursor to metal dissolution.
This study explores the possibility of combining two vibrational spectroscopy techniques for directly probing the coordination of the ligand towards the metal surface during exposure at atmospheric pressure conditions. Zinc is exposed to humidified air to which formic acid (HCOOH) has been added as corrosion stimulant. Sum frequency generation (SFG) is used herein for probing the interface between the oxide-covered metal and the spontaneously formed aqueous adlayer, whereas infrared reflection absorption spectroscopy (IRAS) acts as a complementary technique to detect the multilayers of corrosion products formed during extended exposure.
With SFG, the asymmetric stretch of the carboxylate group of the formate ion is observed within very short exposure time to 120 ppb formic acid and humidified air. The intensity of the peak does not increase with longer exposure time, indicating a saturation of the formate species at the interface. The results provide direct experimental in situ evidence of the formate ion coordinated to the zinc substrate under corroding exposure conditions. IRAS shows the intensity of the formate peak to grow continuously with longer exposure time, indicating a steady growth of multilayers of corrosion products. A closer inspection of the formate peaks obtained with SFG (probing the true interface) and IRAS (probing multilayers of corrosion products) reveals significant differences that reflect differences in average orientation of formate in different parts of the interfacial region between zinc and the corrosive atmosphere.
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