01/01/2018
View of a 4 mm thick corrosion layer rendered from the computed tomography data. The segmented pore structure (yellow) showing the presence of elongated pores that are primarily oriented parallel to the sample surface.

View of a 4 mm thick corrosion layer rendered from the computed tomography data. The segmented pore structure (yellow) showing the presence of elongated pores that are primarily oriented parallel to the sample surface.

Source: BAM, Division Micro Non-Destructive Testing

Corrosion, that is the degradation of one material through chemical reaction with other materials in its environment, is the main cause of damage to steel-reinforced concrete structures. Because of unavoidable cracks in concrete, air and water can penetrate to the surface of the steel bar and initiate the corrosion process on the metal surface. The resulting corrosion products, which have a larger volume than their original component materials, generate internal tensile stress and this leads to further cracks that accelerate the process of degradation. In order to develop the best possible corrosion protection and to be able to make accurate damage predictions for existing structures, it is necessary to obtain a precise understanding of this damage process. Because the damage progress on the embedded steel bar cannot be directly observed, the amount of material reacted during the corrosion process must usually be determined by means of electrochemical measurements based on Faraday's law regarding electrical charge.

The use of micro-computed tomography (μCT) has proved its worth as a way of non-destructively visualizing individual stages of the corrosion process inside a component sample. The experience gained at BAM supports the collaborative work with French colleagues from CEA, Université Paris-Saclay. The selected work deals with the influence of the corrosion layer on the permeability of atmospheric oxygen to the steel surface. The specimens used were rust samples from the 18th-century Saint-Sulpice church in Paris. In addition to experiments on gas diffusion at CEA, these samples were also examined at BAM with a μCT system to show their internal structure in three dimensions. It was found that the cavities occurring in the corrosion products have an orientation parallel to the former steel surface. With a simulated flooding from one side of the corrosion layer, it was also demonstrated that there is no continuous connection over the pore space between the top and bottom of the approx. 4 mm thick layer. The cavities found using μCT, down to a minimum size of 20 microns (the resolution of the μCT images), do not appear to play a significant role in the diffusion process.

This insight along with the other investigation results could then be utilized for the calculation of a corrosion rate. The results this calculation clearly defined the dependence of the gas permeability of a specific corrosion layer on the layer thickness. The diffusion coefficient that was calculated for the corrosion layer from the Saint-Sulpice church is a key parameter that can now be used to assess the durability of other aging reinforced concrete structures.

Microstructure and diffusion coefficient of an old corrosion product layer and impact on steel rebar corrosion in carbonated concrete
S. Poyet, W. Dridi, V. L'Hostis, Dietmar Meinel
Corrosion Science, Volume 125, 15 August 2017, Pages 48-58
BAM Department Non-destructive testing, Division Micro Non-Destructive Testing