Evaluation of electrochemical impedance spectroscopy for the detection of oil films on steel surfaces

As a prerequisite for successful thermochemical heat treatments such as gas and plasma nitriding, a clean and active steel surface is needed. After multiple machining steps, conservation, transport and storage, passive layers may be present on the surface to be treated. The presence of such layers may inhibit diffusion of molecular nitrogen into the border zone of the steel part, thus leading to problems such as soft-spotting and a lack of compound layer. We developed a method for detection of oil films on steel surfaces using electrochemical impedance spectroscopy. A selection of electrolytes was evaluated regarding corrosiveness to the steel surface, conductivity, and reproducibility to show the suitability of EIS as a method for assessing the surface cleanliness state of steel parts. A certain increase in the charge transfer resistance R ct could be linked to defined amounts of Castrol Rustilo DWX 30 or 7165 Emulsin 5382. Surface coverages as low as 1.7 × 10 –2 mg cm −2 of oil are detectable via electrochemical impedance spectroscopy. Graphical abstract