Identification of corrosion products on metallic coated steel with raman spectroscopy

Abstract

Consulta en la Biblioteca ETSI Industriales (8913)

[EN] Steel is a very versatile product and it is one of the most widely used alloys throughout the world. It is applied to many end uses including steel buildings, automotive panels, signs, and appliances. Unfortunately it is prone to corrosion, a phenomenon that causes the surface to become unsightly and, over time, may contribute to product failure. For this reason, steel is protected by a variety of methods. One of them is the protection by metallic coatings. These coatings of metallic materials can provide a satisfactory barrier between the metal surface and its environment. However, the protection offered by the coating is not perfect and eventually the phenomenon of corrosion can damage the material, resulting in the loss of its mechanical properties. For this reason, it is of fundamental importance to study the corrosion process of metallic coated steel. The identification of corrosion products can contribute to the understanding of the mechanisms of corrosion of metallic coated steel. Knowing the products formed during the corrosion process, it is possible to know the reactions that take place and therefore necessary measures to prevent damages can be taken. Many different analysis techniques are used for the investigation of the composition of the corrosion products on metallic coated steel. However, few of them provide the required information in a non-destructive way. Raman spectroscopy (RS) has been used successfully in this field. Therefore, the aim of this study was to identify the different corrosion products on metallic coated steel by means of Raman Spectroscopy. The samples studied were organic coated steel sheets with different metallic coatings which had been subjected to the effects of corrosion by Salt Spray Testing (SST) for different time spans. The composition of the metallic coating was zinc (galvanized steel) or a zinc/aluminium alloy (Al rich coated steel). It is known that there are some differences in corrosion behaviour between both types of steels and this difference is especially seen at the cut-edge. In a first step, a database of reference products was set up. This database is necessary for the identification of compounds, as this is principally done by comparing and attributing an unknown, measured spectrum, to a corresponding reference spectrum. The reference compounds consisted mainly of minerals, which were expected to form during corrosion of galvanized steel and zinc/aluminium coated steel. In a second step, corroded samples were analyzed by RS, so that different spectra were obtained for each of them. Measurements were performed with different laser powers and different types of laser (excitation wavelength) to carry out a complete analysis and avoiding possible erroneous results. The spectra obtained for each of the samples were compared to references and the corrosion products formed on the sample were identified when it was possible. To support the study of Raman spectroscopy another additional method was used on a limited set of samples. This method was X-ray photoelectron spectroscopy (XPS). It can be used to detect the elements present on a surface. For zinc coated samples, it was possible to establish a relative evolution of the corrosion products over time. However, in the case of Al/Zn coated samples or in the case of cut-edge corrosion it was not always possible to identify the corrosion products on all samples. The main problems were the fluorescence and noise present in most of the measurements, the difficulty in attributing the spectrum to specific compounds and probably the lack of adequate databases of reference spectra (mainly for aluminium compounds). The XPS results confirmed the results obtained by Raman spectroscopy. The methodology developed during this study indicates that RS can be an efficient tool for the non destructive investigation of corrosion on metallic coated steel. Despite the problems encountered, this method allowed to identify in many cases the corrosion products. However, it would be advisable to extend the database with other reference compounds and to analyze the samples with additional analysis techniques for comparison with the results obtained with the Raman analysis and for getting complementary information.