Discussion paper: “In situ SR-XRD study of FeCO3 precipitation kinetics onto carbon steel in CO2-containing environments: The influence of brine pH” D. Burkle et al.. Electrochimica Acta 255 (2017) 127–144

Burkle et al. [1] present an elegant study using a flow cell designed to allow synchrotron X-ray diffraction study of the growth of siderite layers on carbon steel at elevated temperature in the presence of CO2 under rigorously de-oxygenated conditions. However, they state that in earlier work, “the growth of the corrosion product was accelerated by applying excessive currents/voltages (unlike in this research) to the sample under study which were not realistic of the actual corrosion kinetics encountered in the field and consequently prevented the true processes of film formation from being established”. In the present comment, the hydrodynamics of the cell of Burkle et al. are analysed using an approximate model and it is shown that the results of the different studies are in fact completely consistent, further supporting the deductions and model of the earlier work for the effects of hydrodynamics on anodic crystal growth. Further it is shown that fitting of their results to the Avrami model used in the earlier studies illustrates the transition from anodic to cathodic reaction rate control with increase of solution pH. Comparison of the different studies indeed illustrates “the true processes of film formation”. The hydrodynamic model in conjunction with that developed in previous studies implies a significant variation over the surface of the anodic electrocrystallisation rate in the cell used by Burkle et al..