Application of Raman spectroscopy as a complementary techniquefor studying localised corrosion

Damien Larroumet, Dr Robert Akid, Professor Jack Yarwood

Electrochemical cell and localised corrosion

Corrosion science involves a study of electrochemical processes that happen at metal-liquid interfaces.

An electrochemical reaction is characterised by the fact that it takes place by either donating or receiving electrons. This phenomenon can occur at the surface of the same material (figure 1) and anodic (A) and cathodic (C) sites, where localised reactions develop, can be identified.

Figure 1 the four requirements for an electrochemical cell

Project aim

The principal aim of the project is to develop the analysis of corrosion by Raman spectroscopy in order to be able to predict and monitor corrosion processes. The project has been started using a basic model system exposed to simple environment. Once these chemical processes and the corrosion kinetics are well understood and controllable, more complicated system (e.g. alloys, coated substrates) will be used and analysed.

Corrosion analysis by Raman spectroscopy

The electrons and ions produced from the metal during the corrosion process react with the environment and yield chemical species that can be studied by Raman spectroscopy.

In-situ Raman spectroscopy permits localised corrosion analysis by focusing through the solution onto an anodic site. The spectra obtained during the corrosion experiment can give different types of information.

First, qualitative information can be obtained by comparing peak positions and band shapes with those from reference spectra (fig.2), hence corrosion products present at a sample surface can be identified.

Figure 2 Example Raman spectra of different iron oxides: Goethite (y-FeOOH), Magnetite (Fe₃O₄), Hematite (Fe₂O₃) and Lepidocrocite (y-FeOOH)

Secondly, by following the evolution with time of the recorded spectra, it is possible to monitor surface changes. Figure 3 presents Raman spectra of an Electro chromium coated steel (ECCS) substrate exposed to 3.5 per cent NaCl solution for 12 hours. The pink spectra has been recorded just after the start of the experiment, the green one that presents a band a 377 cm-¹ and a smaller one at 250 cm-¹ characterising lepidocrocite (y-FeOOH) has been recorded seven hours later and finally the red one (top spectrum) was acquired at the end of the experiment.

Figure 3 Development of corrosion product at the surface of an Electro Chromium Coated Steel during it exposition to sodium chloride solution

Eventually, by mapping a specific area, as illustrated by the line map presented in Figure 4, the spatial distribution and relative quantity of oxides can be monitored.

The example on the left identifies lepidocrocite as the main compound in the middle of the scanned area which is surrounded by a mixture of magnetite and goethite ( -FeOOH) on the edges of the analysed area.

Figure 4 Distribution of corrosion product across a corroded area of steel exposed to sodium chloride solution