Initial Atmospheric Corrosion of Zinc Exposed to Formic Acid, Investigated by in Situ Vibrational Sum Frequency Spectroscopy and Density Functional Theory Calculations † Jonas Hedberg,* ,‡ Jaime Henriquez, § Steven Baldelli,* ,| C. Magnus Johnson, ⊥ and Christofer Leygraf ‡ Department of Chemistry, DiVision of Corrosion Science, Royal Institute of Technology, Drottning Kristinas V. 51, SE-100 44 Stockholm, Sweden, Chemistry and Natural Resources Institute, Talca UniVersity, 2 Norte 685, Casilla 721, Talca, Chile, Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003, and Department of Chemistry, Surface Force Group, Royal Institute of Technology, Drottning Kristinas V. 51, SE-100 44 Stockholm, Sweden ReceiVed: June 24, 2008; ReVised Manuscript ReceiVed: August 13, 2008 Vibrational sum frequency spectroscopy (VSFS) and ab initio density functional theory (DFT) calculations of formic acid on ZnO/Zn have been performed in order to understand the first step of atmospheric corrosion on zinc initiated by formic acid. In addition, infrared reflection absorption spectroscopy (IRAS) has been employed to complement the surface sensitive VSFS results to identify the corrosion products. Oxidized polycrystalline zinc samples were exposed to 120 ppb formic acid in either humid or dry air where, the formic acid adsorption on ZnO/Zn is observed to have a low dependence on the humidity, as deduced by VSFS. Formate is formed on the surface in both dry and humid air and stabilized in configuration after about 90 min exposure in 120 ppb formic acid as seen in the VSFS results. This is evidenced by the occurrence of the CH and symmetric COO - vibrations of the formate ion. The DFT calculations support the VSFS results, showing a coordination of the formate to zinc ions without participation from water molecules. Introduction Atmospheric corrosion is a costly and complex form of corrosion that involves chemical, electrochemical, and physical processes in three phases (solid, liquid, and gas) and three interfaces (solid/liquid, liquid/gas and solid/gas). Since the development of interface sensitive spectroscopic techniques, it has become possible to perform molecular in situ analyses of the interfaces involved under ambient atmospheric pressure conditions. 1-3 From these studies a conceptual framework has evolved to describe the initial stages of atmospheric corrosion. A crucial step here is the coordination of a proton or a ligand to the corroding surface, as a precursor to metal ion dissolution from the oxide-covered metal surface. 3 Formic acid is regarded as one of the most important indoor corrosion accelerators. It is emitted from wooden objects, adhesives, and plastics. Field exposures have shown that formic acid plays an important role in indoor corrosion. 4 For example, the indoor corrosion of lead in organ pipes have been seen to be greatly affected by formic acid. 5,6 Indoor formic acid concentrations have been found to be around 20 ppb. 7 Aims of the Study. This study is part of a larger effort to describe the initial atmospheric corrosion of zinc and copper exposed to small organic acids by studying all the interfaces that take part in the process. Earlier studies include a vibrational sum frequency spectroscopy (VSFS) study of formic acid at the air/water interface. 8 It was observed that the formic acid is still in its protonated state at this interface and forms an ordered structure at the air/water interface, where the CH group is directed toward the vapor phase and the COOH into the bulk liquid. At high concentrations of formic acid, the air/water interface is dominated by formic acid and no water is observed. This can be explained by the fact that there are fewer water molecules in the solution at high formic acid concentrations. A similar study has also been performed on acetic acid, 9,10 where it was observed that acetic acid is more surface active at the air/water interface, while for both acids, no ionized species were observed at the interface. Infrared spectroscopy (IRAS), which probes thin films on metals, has been used to study zinc exposed to formic acid. 2 Here it was concluded that formate ions are present on the surface and that a high relative humidity increases the rate of formation of the zinc formate. Also, work on zinc and copper exposed to acetic acid have been performed 2,11,12 and displays slower corrosion kinetics than formic acid on both metals. For studying the interface between the substrate and the thin liquid film on the surface, in situ surface sensitive VSFS and grazing incidence infrared reflection absorption spectroscopy (IRAS) experiments, along with ab initio density functional theory (DFT) calculations have been employed. VSFS is used to elucidate in what form the formic acid adsorbs and also to gain information about the structuring and ordering of the adsorbed species. The role of the DFT calculations is to provide information on adsorption sites for the formate ion and its interaction with the ZnO substrate and water molecules. Growth of Corrosion Products and Substrate Heterogene- ity. The simplest model for the growth of thin films of corrosion products is as a uniform layer-by-layer type of growth, but in atmospheric corrosion this model is generally not valid. 13,14 There are many reasons for this, such as the use of polycrys- talline material, oxides, roughness of the substrate and rate of † Part of the special section “Physical Chemistry of Environmental Interfaces”. * To whom correspondence should be addressed. Fax: +46 8 208284 (S.B.). E-mail: jhed@kth.se (J.H.); sbaldelli@uh.edu (S.B.). ‡ Division of Corrosion Science, Royal Institute of Technology. § Talca University. | University of Houston. ⊥ Surface Force Group, Royal Institute of Technology. J. Phys. Chem. C 2008, 113, 2088–2095 2088 10.1021/jp805582h CCC: $40.75  2009 American Chemical Society Published on Web 10/01/2008