Field Emission AES Characterization of Corrosion Products Formed on Copper in Chloride Containing Solutions

The first 24 hours it is immersed in a 0.5 M NaCl solution, a copper surface undergoes a series of transformations. A corrosion product layer is formed that acts as a barrier layer between the aggressive solution and the substrate, offering some protection to the material. This barrier layer itself is subsequently attacked and localized corrosion is observed. The evolution can be divided into three stages. In the first hour, a homogeneous corrosion layer is formed over the entire surface. In the next hours, the thickness of this layer keeps on increasing. After two hours local corrosion starts to appear, piercing the formerly grown oxide layer in certain places. This attack is localized mostly at the grain boundaries. After 20 hours of immersion almost all these local attack sites gradually get covered anew by the constantly growing corrosion product layer. Electrochemical Impedance Spectroscopy (EIS) studies conducted on these surfaces generate a response that is a combination of the response of both reactions, making it impossible to discriminate between global and local reaction properties [1,2]. Knowing the reaction products present on different parts of the surface would however represent a great deal of information if one needs to identify the reactions taking place on this surface. Extensive studies have already been carried out on these surfaces, but still some lack of clarity remains, illustrated by some yet unidentified features in electrochemical impedance spectra (EIS) recorded on these surfaces[1,2,3]. From the various works it becomes clear that multiple species have been identified to form on the surface. Among them Cu2O [4-8] CuCl [5,6,8,9-13] or CuO [14-19]. Auger spectroscopy was already used in various works dedicated to the study of copper corrosion. A number of papers has been published on the identification of copper oxides, formed on various substrates [20-23]. Also a limited amount of literature exists on the identification of copper chlorides [24]. Lineshape analysis was used in a number of works as a tool to discriminate between different copper compounds, concentrating however on thermally formed oxides and chloride compounds grown in modified vacuum conditions. The study presented in this work uses Field Emission AES (FE-AES) to identify different copper compounds on the same surface. All of these were formed during the corrosion process of copper in aqueous chloride containing solutions. Lateral distributions of the composition were visualized by Auger imaging. Identification of the compounds was done by comparing the recorded high resolution FEAES spectra on corroded surfaces with spectra obtained from reference materials. With the acquired reference spectra Target Factor Analysis (TFA) was carried out to analyze the chemical composition of the corrosion products. Both the in-depth product distribution in the uniform corrosion layer and the distribution of the products in and around a local corrosion site were studied.