The Chloride Ion
Concrete contamination by chloride ions can occur through the use of inert material and/or water contaminated with chlorides or exposure to the marine environment.
For the corrosion process to start, the free chloride content in the steel-concrete interface must be above a certain value, known as the threshold limit. This value depends on the alkalinity of medium and is higher, the higher the pH. Generally, for concrete made using Portland cement, a Cl_/(OH_) ratio of <0.6 is acceptable. Standard BS 8110 limits the total chloride content to 0.4 % mass, relative to the mass of the cement. European standards recommend the same amount.
Below the critical chloride level, the corrosion process does not start, since chlorides have the ability to adsorb on the surface of the cement hydration products, establishing stable chemical bonds with cement, producing chloroaluminate. Later, these chlorides may be released into the solution, if for some reason there is deterioration of the aluminates, as happens, for example, during acid attack [10–12].
Figure 15.20 shows a diagram of the action of the chloride ion on the progression of corrosion in reinforcing steel, and the role played by the cover thickness (the thickness of the layer of concrete between the reinforcement and the exterior). This barrier has a protective role. However, it possesses small pores and defects that allow diffusion of water, chlorides, and oxygen to the steel-concrete interface.
The chloride ions penetrate the passive film through defects and/or damaged areas and lead to the formation of anodes, where the conversion of metallic iron into iron ions (Fe2+ and Fe3+) takes place. The electrons released in this process are used at the cathode for oxygen reduction and the consequent formation of OH~ ions. These ions, dissolved in the electrolyte (the aqueous phase that fills the pores in the concrete), combine with the iron ions and lead to the formation of several voluminous iron oxides and hydroxides, in accordance with the chemical reactions shown below:
Fig. 15.20 Diagram evidencing the process of reinforcing steel corrosion induced by the chloride ion
Fe! Fe2++ 2e~ (15.9)
Fe2++ 2OH~ ! Fe(OH)2 Iron (II) hydroxide (15.10)
Fe(OH)2 + OH-! Fe(OH)3 + e~ Iron (III) hydroxide (red rust) (15.11) Cathode:
4e~ + O2 + 2H2O! 4OH – (15.12)
The action of chlorides leads to the development of localized corrosion (pits) on the reinforcing steel. Consumption of the reinforcement, i. e., reduction in its cross section, reduces the steel’s tensile strength, and irreparably affects the strength of the entire structure. Conversely, the accumulation of expansive corrosion products (Fig. 15.21) at the interface steel-concrete generates internal stresses that lead to cracking and/or delamination of the concrete cover.