In situ investigations of the interaction of small inorganic acidifying molecules in humidified air with polycrystalline metal surfaces by means of TM‐AFM, IRRAS, and QCM

Abstract

AbstractInitial atmospheric corrosion of copper, silver, and iron induced by humidity and oxidizing agents was studied in situ by three highly surface‐sensitive and complementary techniques: infrared reflection‐absorption spectroscopy (IRRAS), quartz crystal microbalance (QCM), and tapping‐mode atomic force microscopy (TM‐AFM). These techniques deliver information about the change of the topography of the sample surfaces with emphasis on the shape and lateral distribution of the corrosion products grown within the first 1300–2800 min of weathering (TM‐AFM), as well as chemical (IRRAS) and kinetic (QCM) data. A completely different mechanism of the initial stages of atmospheric corrosion of the three investigated metals could be observed. A uniform growth of corrosion products was seen on the copper surface (identified by IRRAS and XPS to be cuprite‐like) during exposure to synthetic air with 80% relative humidity (RH), whereas the iron surface remained unattacked. The investigations of the silver surface exposed to humidity revealed that silver is attacked by humidity and tends to form oxide and hydroxide surface species. While an increased humidity content of the surrounding atmosphere causes higher corrosion rates on copper, on the exposed silver sample only a change in the degradation mechanism could be observed.The addition of SO2 to the humidified air causes the growth of so‐called ‘second‐order’ features on copper, identified to be CuSO3 · xH2O‐like, which reveals the formation of a new chemical species on the investigated surface. These features are placed on top of the homogeneous formed oxide layer and tend to form well‐defined islands. In contrast to copper, on a silver surface exposed to humidity and SO2 no new chemical species are formed; nevertheless an increased corrosion rate could be observed owing to a change of the chemistry in the physisorbed water layer. Iron exposed to humidity and SO2 still remains unattacked. An iron surface is attacked only if exposed to humidity and SO2 and NO2, which show a synergistic effect by the oxidation of four‐valent sulfur‐oxygen species by NO2. Such an attack leads to the formation of pitting corrosion, which was observed in situ and time‐resolved. The pits mainly occur on predamaged surface structures, such as scratches caused from the polishing process of the samples, and therefore promote the initiation of the corrosion. The results obtained demonstrate the high potential of the surface‐sensitive methods applied for investigating the early stages of corrosion of different metals and for obtaining a better understanding of the molecular mechanisms during degradation. Copyright © 2007 John Wiley & Sons, Ltd.