RWTH Aachen University, Chair of Corrosion and Corrosion Protection
Michael Strebl

The degradation of metals by corrosion is an important issue with consequences related to sustainability, safety, health and economic losses. To monitor the corrosion rate and to investigate the corrosion mechanisms under different exposure conditions it is crucial to have suitable in situ real-time monitoring methods. With approaches similar to respirometry it is possible to monitor corrosion processes indirectly via the most important cathodic reactions that accompany the metal oxidation which are the Oxygen Reduction Reaction (ORR) and the Hydrogen Evolution Reaction (HER).

Methods

The corroding metallic object is placed in a hermetically closed chamber with a constant volume. The object can be surrounded by humid air (atmospheric corrosion) or by a bulk electrolyte (immersion) with a gas headspace. A PICO-O2 meter with a PIC-FIB2 optical fibre is used to read out the partial pressure or concentration of O2 in the chamber with contactless O2 sensor spots (OXSP5) (Fig. 1). A second sensor for absolute pressure is used to obtain the H2 partial pressure simultaneously. The number of moles of evolved H2 and consumed O2 can be obtained with ideal gas law from the partial pressures and the temperature of the experiment, and are converted to a charge with Faraday’s law.

FIG. 1 Measurement of O2 consumption of a corroding screw in a glass chamber with rubber stopper

Conclusion from data and application

The data in Fig. 2 show that the kinetics of the HER and ORR of a corroding nut and bolt can be monitored simultaneously. The sum of HER and ORR rates gives the total cathodic reaction rate which is equal to the corrosion rate. The increase in the rates coincides with the onset of the HER which might be due to localized corrosion in the crevice accelerated by the galvanic coupling of the different materials.

The technique can be used to assess the corrosion properties of metals with different alloy compositions, heat treatments and microstructures. It is suitable to investigate the efficacy of corrosion protective coatings or inhibitors. In addition to static exposure, different (intermittent-) flow-cell set-ups were realized to change the exposure conditions (e.g. wet-dry cycles) and follow the effects on the corrosion rate and the ratio of the two cathodic reactions in situ.

FIG. 2 Respirometric monitoring of atmospheric corrosion of a combination of nut and bolt contaminated with de-icing salt and exposed in humid air