Kinetics of electrochemical dissolution of metals in porous media

Abstract

Metals embedded in porous media interact electrochemically with the liquid phase contained in the pores. A widespread form of this, adversely affecting the integrity of engineered structures, is corrosion of steel in porous media or in natural environments. While it is well-documented that the rate of this electrochemical dissolution process can vary over several orders of magnitude, understanding the underlying mechanisms remains a critical challenge hampering the development of reliable predictive models. Here we study the electrochemical dissolution kinetics of steel in meso-to-macro-porous media, using cement-based materials, wood, and artificial soil as model systems. Our results reveal the dual role of the pore structure, that is, the influence on the electrochemical behavior through transport limitations and an area-effect, which is ultimately due to microscopic inhomogeneity of the metal-porous material interface. We rationalize the observations with the theory of capillary condensation and propose a material-independent model to predict the corrosion rate.

Laboratory tests simulating corrosion in geothermal power plants: influence of service conditions

“…Concerning this capacitance (C dl ), Table 3 also shows an increase between the beginning of the tests (phase 1) and the last phases (4 and 5) by a factor of 3, which may again be related to a higher effective steel surface area due to surface roughening. This hypothesis is in agreement with earlier studies showing that corrosion of metals embedded in porous media (here, a porous corrosion product layer) is at the microscopic scale non-uniform and thus leads to surface roughening (Romanoff 1957;Stefanoni et al 2019). Thus, in summary, the increase in corrosion rate by a factor of 3 between phase 1 (approx.…”

Corrosion of steel in carbonated concrete: mechanisms, practical experience, and research priorities – a critical review by RILEM TC 281-CCC

“…Another effect promoting depassivation at pH values around 8-9 may be the formation of small galvanic cells. This is supported by the view that corrosion of metals in meso-tomacro-porous media is a non-uniform process at the microscopic scale [37]. The complexity of the steel-concrete interface [38] gives rise to local heterogeneities in water presence at the steel surface, and may thus promote local differences in pH and oxygenation (similar to the "Evans drop" [39]).…”

“…A number of studies showed that the instantaneous corrosion rate in carbonated concrete increases exponentially with the relative humidity in the concrete [42][43][44][45][46]. This relationship was recently explained by the dual role of the pore structure of the cementitious medium surrounding the embedded steel [37]. First, the pore structure governs the amount of water that is held in the pore system when in equilibrium with a given exposure moisture condition.…”

“…This concept essentially means that in dry concrete, only a minor fraction of the steel area is in contact with liquid water and thus most of the steel cannot corrode at significant rate, while in wet concrete, this area fraction increases and leads to higher corrosion currents [46]. Second, the pore structure controls the rate at which ferrous ions, released at the rebar surface in the anodic iron dissolution reaction in the corrosion process, can be transported away [37]. This transport process influences the ferrous ion concentration at the electrode and thus has a direct impact on the electrochemistry of the system.…”

The influence of defects at the steel/concrete interface for chloride-induced pitting corrosion of naturally-deteriorated 20-years-old specimens studied through X-ray Computed Tomography

“…A lower average volume loss of 2.5% was found also for 2550-S3, apart from the bottom part of the bar where a localized volume loss up to 9% was observed. Recently, Stefanoni et al [23] observed that the more porous the medium around the steel reinforcement, the more aggressive corrosion occurs due to the dual role of the pore structure (in terms of transport limitations and inhomogeneity of the interfacial zone). Since blended cements lead to a denser microstructure than that of OPC, it is not surprising that, on average, rebars embedded in OPC-based cores had the highest volume loss.…”