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In Situ Analyses of Corrosion Under Alternate Wet and Dry Conditions, 20-9995 Printer Friendly VersionPrincipal Investigators Inclusive Dates: 11/01/96 - Current Background - Corrosion under condensed water layers and cyclic wet and dry conditions poses significant problems for many engineering applications. Atmospheric corrosion of structures such as buildings and bridges is strongly dependent on the duration and conditions of daily dry and wet cycles. In addition, for high-level nuclear waste (HLW) containers, which must prevent release of radionuclides to the accessible environment for several thousand years, the corrosion rate may also be accelerated by exposure to cyclic wet and dry conditions. Approach - The objective of this project was to develop test technologies for the in-situ analyses of corrosion under wet and dry conditions, including the characterization of the electrolyte chemistry and the redox reactions occurring in the corrosion product layers. Raman spectroscopy, DC linear polarization, AC electrochemical impedance spectroscopy, and corrosion potential measurements were used to investigate the corrosion of high-purity iron and copper under cyclic wet and dry conditions. The evolution of the corrosion potential was followed during successive wet cycles to characterize the behavior of a metal specimen covered with corrosion products. Electrochemical impedance spectroscopy and DC linear polarization were used to measure the corrosion rate of the metals under the thin electrolyte layer. Measured weight losses were used to validate the corrosion rate measurements. Raman spectroscopy was used to identify the corrosion product layer and to determine the relative humidity at which a water film can be formed on a steel surface. Accomplishments - A corrosion cell was developed to monitor corrosion under thin electrolyte layers. Electrochemical impedance spectroscopy showed that the corrosion rate of high-purity iron increases with the number of wet cycles prior to reaching a maximum corrosion rate two times that observed under continuously immersed conditions. Initial high corrosion rates upon rewetting were shown to be the result of the reduction of Fe(III) to Fe(II) species, which increased the corrosion potential. Corrosion rates at the end of each wetting cycle were also accelerated by the enhanced rate of oxygen reduction and the increased salt concentration resulting from water evaporation. Although acceleration of the corrosion rate of copper was not observed under alternate wet and dry conditions, the corrosion rate increased as a result of the enhanced oxygen reduction rate that occurred when the thickness of aqueous film decreased due to water evaporation. The corrosion products of high-purity iron exhibited a layered structure. Investigation of corrosion product layers using Raman spectroscopy revealed the formation of oxides and oxyhydroxides, including g-FeOOH (lepidocrocite) at low and intermediate chloride concentrations and b-FeOOH (akageneite) at high chloride concentrations. Reduction of g-FeOOH to Fe3O4 (magnetite) was observed during wet cycles, followed by oxidation of Fe3O4 to g-FeOOH in the final drying stages and when the specimen was completely dry. Analyses of corrosion products on copper was performed, but many of the expected corrosion products were not Raman active. A sharp increase in the amount of adsorbed water on a steel surface was detected with Raman spectroscopy when the relative humidity was greater than the critical value required for the corrosion of steel. Results of this investigation showed that a combination of Raman spectroscopy and electrochemical impedance spectroscopy can be used to monitor atmospheric corrosion. Raman spectroscopy is an effective technique for in situ identification of the layered corrosion products and the detection of water films formed on metal surfaces. Both Fe(III) and Fe(II) species were identified in the corrosion product layer under cyclic wet and dry conditions. The rate of corrosion under such conditions is controlled by both the rate of the redox reactions in the corrosion product layer and the rate of transport of oxygen to the metal surface through the corrosion products and the aqueous film.
Raman spectra of high-purity iron corrosion products are shown during alternate wet and dry cycles. Initial spectra obtained after corrosion products were partially removed. Raman peak at 667 cm–1 due to the presence of Fe3O4 formed by the reduction of g-FeOOH. Oxidation of Fe3O4 to g-FeOOH observed during late stages of evaporation and dry periods. Materials Research and
Structural Mechanics Program |
