Corrosion of carbon steel in sodium methanoate solutions

The behaviour of steel electrodes in sodium methanoate solutions was studied by coupling electrochemical techniques (voltammetry, OCP vs. time) with in situ micro-Raman spectroscopy analyses of the corrosion products. The polarisation curves depended strongly on the methanoate concentration. For the smallest concentration (10−3 mol L−1), the current density increased regularly with the applied potential. So the behaviour of the electrode was typical of an active material. In contrast, for the largest concentration (10−1 mol L−1), the curves obtained were typical of a passive material. Methanoate ions favoured growth and stability of a passive oxide film more likely by adsorbing on its surface. The polarisation curve obtained for the intermediate concentration (10−2 mol L−1) was unusual and testified of an imperfect passivation of the steel surface. Finally, steel electrodes were left at the open circuit potential in the methanoate solutions. In any case, the passivity was rapidly lost and a general corrosion of the surface took place. In situ Raman spectroscopy analyses at the early stage of the corrosion process demonstrated that the first product to form was a green rust, GR(HCOO−). It was oxidised later into γ-FeOOH (lepidocrocite) by dissolved O2. The process is then typical of what is usually observed in neutral or alkaline media, whatever the anions present and responsible of the GR formation. A new and detailed characterisation of GR(HCOO−) by X-ray diffraction was performed and a crystal structure is proposed.