Challenges in the Measurement of Passive Dissolution of Steel in Concrete
Authors: Vazquez Damián , Sagüés Alberto A. 
Published: February 2020

The corrosion rate of the steel reinforcement of concrete is measured and monitored in laboratory and civil structures by various techniques that include anodic or cathodic galvanostatic pulse, linear polarization resistance (using a given polarization rate), electrochemical impedance spectroscopy, etc. Considerable attention has been given to the suitability of those methods to measure the corrosion rate of steel in the active state, after passivity breakdown due to aggressive species contamination. However, fewer studies have evaluated the case of service in structures with required durability much in excess of one hundred years (as for example containment of radioactive waste) where the corrosion process is expected to remain in passive state, with anodic current densities typically lower than 10-8 A/cm2. The electrochemical measurement of such low corrosion rates is a challenging goal as it is difficult to separate polarization currents due to interfacial charge storage processes (which may be of actual or apparent non-Faradaic origin) to those related to the anodic, metal dissolution process itself. That separation often leads to the need for long duration of the measurements, which would make it impractical to monitor a large number of reinforcement locations in an actual structure. Understanding of the relevant measurement factors is needed to eventually develop more time efficient assessment methods for those systems.
As part of developing that understanding, the objective of this work is the experimental and modeling analysis of steel in concrete in the passive state, in response to the application of cathodic cyclic polarization with polarization rates in the order of 0.001 mV/sec and polarization down to -300 mV with respect to the (passive) corrosion potential. This technique, implemented with higher polarization rates after appropriate adjustment as a refined form of Tafel extrapolation, is a possible candidate to monitor structures of interest. In order to interpret the passive corrosion process response to polarization excitation, a modified Randles interphase model analog was used. A novel feature of this study is the introduction of large amplitude time domain behavior of a constant phase angle element, which is then combined with the large amplitude response of the Faradaic current treated with a Butler-Volmer approach. The system steel/interface/concrete is described by a fractional order differential equation that is solved by the implementation of a finite differences aproach. Comparisons between the experimental results from multiple specimens at various scan rates and the response calculated by solution of the equations of the system were made, showing good agreement between both. Corrosion current densities obtained by numerically fitting the model to experimental results yielded values typically lower than 10-8 A/cm2 in agreement with other values reported in the literature for long term exposures of passive steel in concrete. Open issues on the applicability of these results for mechanistic interpretation of the processes at work on the rate of passive dissolution of steel in concrete are discussed.