Merging Electrochemistry and Water Capillary Condensation to Understand the Corrosion Mechanism of Steel in Carbonated Concrete
Authors: Stefanoni Matteo , Angst Ueli , Elsener Bernhard 
Published: February 2020

Reinforced concrete is one of the most used man-made materials worldwide, one of its main causes of degradation worldwide is carbonation induced corrosion, accounting every year for billions of dollars for main­tenance, repair and rebuilding. The alkali produced by the Portland cement hydration (mainly Ca(OH)2) allows steel passivation. CO2 from the atmos­phere can enter the concrete pores and react with the Ca(OH)2, leading to a decrease of pH that can propagate to the reinforcement level. In presence of oxygen the steel starts to corrode, the corrosion products precipitate in the surrounding of the reinforcement, generating stresses that can lead to cracking and spalling of the concrete cover. The topic of carbonation induced corrosion is becoming of key importance in the reduction of environmental footprint of Portland cement production, responsible for about 8% of the man-made CO2 and thus for global warming and climatic change. The replacement of Portland cement by using supplementary cementitious materials (SCM) is one of the most viable, short-term solutions. This, however, might impair the durability of reinforced concrete; reducing the Portland cement content leads to a reduced production of calcium hydroxide and therefore to a much faster carbonation process. When the carbonation front reaches the reinforcement before the end of service life, the corrosion propagation becomes an important part of the life of the structure.
To be able to collect data of corrosion propagation rates in a reasonable time a new experimental set up has been designed, which consists of small (8x8 cm) and thin (0.6 cm) mortar samples instrumented with a reference electrode (Ag/AgCl), 5 steel wire electrodes and a stainless steel grid counter electrode. The thin sample allows rapid carbonation and equilibration of environmental humidity. Electrical resistivity, oxygen cathodic current, open circuit potential and corrosion rate of the steel can be measured. The results have shown that the corrosion rate increases markedly with increasingly higher relative humidity (up to 99% RH), maximum values around 2 µA/cm2 have been found in wet/dry cycles. Increasing corrosion rate was associated with a decrease of the corrosion potential, the ohmic resistance of the electrochemical cell was found to correlate with the corrosion rate. The oxygen reduction limiting current was always about five to ten times higher than the corrosion rate. The influence of cement type and water/binder ratio was of minor importance.
The mechanism of corrosion of steel in carbonated concrete is under debate since 1980 - and after almost 40 years there is still no agreement. The traditionally proposed controlling mechanisms such as resistive, cathodic or anodic control have been found not suitable for corrosion of steel in carbonated mortar, which has always been considered as a uniform process. Instead, being the cement paste a porous system, the capillary water condensation had to be taken into account. With the Kelvin equation and the porosity curves, the volumetric water content could be calculated for every different sample and relative humidity condition. A correlation between the water content and the corrosion rate was found.
Merging electrochemistry and water capillary condensation theory allowed explaining the mechanism of corrosion of carbon steel embedded in a porous system. The porosity and degree of pore saturation define the electrochemically active area with respect to the total steel surface. This active area varies by orders of magnitude depending on the exposure condition: from immersed conditions to 99% RH the active area goes down to ca. 40 %, at 80% RH only 1 %. The volumetric water content also determines the available diffusion volume of produced Fe2+ ions, which modifies the anodic reaction reversible potential. A larger diffusion volume leads to a lower Fe2+ concentration and therefore lower reversible potential and higher corrosion current according to the Evans diagram. The two contributions can be recognized and the huge variation of the corrosion rate with the exposure condition can thus be explained. In conclusion the corrosion mechanism of carbon steel in carbonated concrete could be determined and the corrosion rate can be uniformly expressed, for every different humidity state, water to binder ratio and binder type, as a function of pore solution pH, water content and porosity.

Figure 1