(Corrosion Division Morris Cohen Graduate Student Award) Respirometric Monitoring of Corrosion
Authors: Strebl Michael G. , Bruns Mark P. , Virtanen Sannakaisa 
Published: December 2024

Accurate measurement of the corrosion rate of metals and alloys under realistic exposure conditions is crucial for the elucidation of corrosion mechanisms, the development of new corrosion protection measures, and for reliable corrosion prediction. A respirometric approach to monitor corrosion rates based on quantifying the rates of the main cathodic reactions during corrosion is presented. With the proposed gravimetric, manometric, or flow cell experimental setups it is possible to follow the rates of the H2 evolution reaction (HER), the O2 reduction reaction (ORR), or both cathodic reactions simultaneously.
The respirometric method can universally provide real-time corrosion rate monitoring for atmospheric corrosion conditions under thin electrolyte films, where suitable corrosion monitoring capabilities have been lacking so far. Furthermore, respirometric methods applicable to corrosion under immersion conditions are presented and coupled with electrochemical measurements. It was demonstrated on various relevant engineering metals and alloys that reliable and sensitive real-time monitoring is possible without influencing the corrosion process by the measurement itself. The accuracy and validity of the developed methodology was confirmed by independent parallel measurement methods such as mass loss, solution analysis by ion chromatography, profilometry, and electrochemical measurements. Parallel imaging of the corroding surface was carried out to visually link changes of the respirometric rates and of the exposure conditions to observations of the surface morphology, gas evolution and corrosion propagation.
Besides measuring corrosion rates, the respirometric method can be used to elucidate corrosion mechanisms. The effect of changes in different exposure conditions on the corrosion rate and on the rate of the cathodic reactions could be followed in situ. This includes changes in the temperature, the composition of electrolyte, or wet–dry cycling. The role of the ORR for Mg alloy corrosion was investigated and it was found that besides HER, the ORR contributes substantially to the cathodic reactions both for atmospheric corrosion and immersion. A change in the mechanism from ORR to HER was observed with the onset of localized corrosion of Al alloys under atmospheric conditions. In the context of biodegradable metallic implants, the time-dependent corrosion behavior of Zn and Mg alloys was investigated and compared in simulated body fluids of increasing complexity. The contribution of the two-electron ORR pathway was confirmed for Zn under immersion conditions and quantified for the different simulated body fluids.
By coupling the respirometric method with electrochemical measurements, it was shown that the contribution of the involved reactions and pathways to the net electric current can be extracted at different applied currents or potentials. As a result, it was possible to deconvolute dynamic polarization curves into the contributing partial reactions. Unexpected cathodic reactions (negative difference effect) during localized corrosion at anodic polarization as well as unexpected anodic dissolution (cathodic corrosion) under cathodic polarization could be revealed and quantified with the respirometric method. Moreover, the fraction of anodic current related to the O2 evolution reaction could be differentiated from the fraction of current that caused transpassive corrosion.
A more sensitive respirometric method was developed that is suitable to measure corrosion rates of more corrosion resistant materials or materials in weakly corrosive environments. Corrosion scenarios that were investigated with this high resolution respirometric method include breakdown of passivity of Al and stainless steel, H2 evolution of Zn anodes in Zn ion batteries and corrosion of metals in ionic liquids. While this approach is certainly promising for laboratory applications, also the applicability to respirometric field measurements will be demonstrated. A respirometric probe was developed that can be attached to corroding structures in the field.
Overall, the respirometric method provides a monitoring tool to establish the relationship between relevant exposure parameters and accurate corrosion rates. The advantages and possibilities of the developed respirometric methods open up new doors to in situ investigation of corrosion and other electrode processes

Figure 1