Simultaneous In Situ Infrared Reflection Absorption Spectroscopy and Kelvin Probe Measurements during Atmospheric Corrosion D. Persson, a,z S. Axelsen, b F. Zou, a and D. Thierry, a, * a Swedish Corrosion Institute, SE-104 05 Stockholm, Sweden b SINTEF Materials Technology, Corrosion and Surface Technology, N-7465 Trondheim, Norway An experimental setup is described which allows simultaneous in situ measurements with the Kelvin probe and infrared reflection absorption spectroscopy IRAS. Variations in the Volta potential were measured during corrosion of zinc surfaces in humid air, and information about the composition and the growth kinetics of the surface film was obtained by IRAS. The results show the Volta potential drops to between -600 and -700 mV/SHE after introduction of humid air due to activation of the zinc surface. With IRAS the growth of a surface film containing ZnO and Zn 5 OH 8 Cl 2 •H 2 O and zinc hydroxy carbonate was followed. With increasing thickness of the surface film the zinc surface is passivated and the Volta potential increases to between -100 and -200 mV/SHE after prolonged exposure. The results show that the experimental setup for simultaneous Kelvin probe and in situ IRAS measurements is a powerful tool for obtaining information about the changes in corrosion potential associated with the formation and growth of different types of corrosion products on metal surfaces. © 2001 The Electrochemical Society. DOI: 10.1149/1.1342096 All rights reserved. Manuscript submitted September 15, 2000; revised manuscript received November 26, 2000. Available electronically January 8, 2001. Despite the scientific and technological importance of atmo- spheric corrosion there is still a lack of understanding of the funda- mental processes that occur on metal surfaces during exposure to humid environments. One reason for this is the difficulty of obtain- ing electrochemical information about metals corroding in the atmo- sphere when the metal surfaces are covered with adsorbed water layers, the thickness of which may range from a few monolayers up to bulk layers of water. However, with the development of the Kelvin probe for investigation of corrosion of metals in humid at- mospheres corrosion potential changes can be followed during cor- rosion in humid air. 1-3 The Kelvin probe has been successfully ap- plied to study corrosion processes of metals in humid conditions. The technique have been used to investigate changes in corrosion potential of metals during wet-dry cycles and to detect electrochemi- cal spatial variations in the corrosion potential on bare and coated metal surfaces. 1-5 The main drawback of the technique is that infor- mation is not obtained about the rates of the electrochemical reac- tions occurring on the metal surfaces. The potential changes mea- sured by the Kelvin probe are strongly dependent on the composition and thickness of surface films and adsorbed species on the surface. Therefore, it is important to have knowledge about the changes in composition that occur on a surface in connection with a potential change. It is also desirable to analyze the surface films under in situ conditions because changes in the surface film may occur during surface analysis performed in laboratory air or in vacuum. Infrared reflection absorption spectroscopy is a powerful method for in situ studies of the composition formed on metals during corrosion in humid air, and information about the initial pro- cesses that occur during the interaction of SO 2 6,7 and SO 2 and NO 2 , 8 with different metal surfaces have been obtained. In addition to information about the composition of the surface films, the growth kinetics of the film can be followed and the thickness of the surface film can be estimated. Thus, the combination of the two techniques in a single experimental setup could significantly in- crease the understanding of the potential changes measured with the Kelvin probe during corrosion processes in humid air, and would be a powerful tool for more fundamental studies of atmospheric corro- sion of metals. In this work, a new experimental setup for simulta- neous in situ infrared reflection absorption spectroscopy and Kelvin probe measurements is described, and results are shown of measure- ments performed during corrosion of chloride-contaminated zinc surfaces in humid air. Experimental Experimental setup for in situ IRAS and Kelvin probe measurements.—A schematic picture of the experimental setup for simultaneous in situ infrared reflection absorption spectroscopy and Kelvin probe measurements is shown in Fig. 1. The combined in situ infrared reflection absorption spectroscopy IRAS and Kelvin probe cell consists of a stainless steel body with KRS-5 thallium bromoiodide windows, an observation window in polymethyl meth- acrylate PMMA and inlets and outlets for humid air. An oscillator is enclosed in a stainless steel cylinder which is attached to the lid of the cell. The tip of the Kelvin probe consists of a 0.6 mm thick NiCr alloy attached to the oscillator. The polyvinylidene fluoride PVDF sample holder is fastened to the bottom flange of the cell and elec- trical contact with the sample is made by a brass rod pressed against the back side of the sample. The Kelvin probe signal is picked up by a custom-built preamplifier and demodulated by an EG&G lock-in amplifier, model 5302. The Kelvin probe tip was oscillated approxi- mately 50 m above the surface with a frequency of 777 Hz. The Volta potential was obtained by changing the bias voltage and tak- ing the intercept of the linear curve of current vs. voltage with the current axis. This procedure was implemented in the software which controls the Kelvin probe measurements. The measured Volta potential was converted to corrosion poten- tials by calibrating against the Cu/CuSO 4 electrode at the end of the experiments. All potentials are given against the standard hydrogen electrode. The combined in situ IRAS and Kelvin probe cell was mounted on an aluminium holder that fits in Harrick’s variable angle attach- ment, the Seagull, equipped with a KRS-5 wire grid polarizer. The angle of incidence of the p-polarized light was 78° with respect to the surface normal. The IRAS spectra were collected using a Biorad 175C spectrometer equipped with a DTGS detector and a cesium iodide CsI beamsplitter. Spectra were obtained by averaging 125 scans using a resolution of 16 cm -1 . The in situ IRAS spectra were obtained after different exposure times using a spectrum obtained 5 min after the introduction of humid air as background. The spectra are presented in absorbance units which in this work corre- sponds to -log(R/R 0 ), where R is the reflectance of the surface after a certain time t , and R 0 is the reflectance of the background. All spectra were obtained at different times after introduction of humid air. The background spectrum, R 0 was recorded 10 min after the introduction of humid air. Materials and exposure conditions.—Samples were made of zinc sheets 99.9 % with a size of 22  19 mm which were polished * Electrochemical Society Active Member. z E-mail: dan.persson@corr-institute.se Electrochemical and Solid-State Letters, 4 2 B7-B10 2001 1099-0062/2001/42/B7/4/$7.00 © The Electrochemical Society, Inc. B7