Chloride Induced Corrosion of Reinforcement Steel in Concrete - Threshold Values and Ion Distributions at the Concrete-Steel Interface

Abstract: The chloride threshold value (Cth), or critical chloride content, is defined as the chloride concentration at the depth of the reinforcement, which initiates the depassivation of steel in concrete. However, very limited information is available regarding the chloride distributions at the interface with the steel. The main objective of this work is to improve the knowledge and understanding about the mechanisms leading to depassivation of steel in concrete, by studying the influence of the steel surface condition and the concrete-steel interface on the corrosion initiation and the chloride distributions along the concrete-steel interface at the time of depassivation. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used for spatial resolved chloride profiling in cementitious materials. A range of materials with increasing degree of heterogeneity (i.e. cement paste, mortar and concrete) and exposed to chlorides under different conditions (i.e. mixed-in, diffused and migrated) was studied. The system was optimized for maximum chloride sensitivity, while allowing for the detection of other elements such as calcium and iron. At a scan speed of 100 µm/s, a spatial resolution of 300-400 µm and limits of detection of 0.05 wt% of cement were determined. The chloride distributions along the concrete-steel interface and possible differences between passive and active regions were studied, for different steel surface conditions under free corrosion conditions as well as under potentiostatic control. The results have shown that along the interface, a range of chloride concentrations can be expected, with higher values around the corroding active sites. It was suggested that chlorides preferentially accumulate at the anodic regions even prior to depassivation, leading to pitting corrosion. A local migration mechanism was proposed to account for the chloride build-up around the anode regions, due to the formation of local potential gradients on the passive layer of the steel as a result of differences in the moisture content and oxygen availability, concentration of aggressive species and metallurgical properties, such as inclusions or mill-scale along the steel. In particular the steel surface condition and the presence of air voids at the concrete-steel interface were recognized as major factors influencing the development of potential gradients along the steel surface.