Corrosion inhibitors are chemicals that can slow down or prevent corrosion of reinforcing steel in concrete. Corrosion inhibitors were first investigated in the 1960’s.
Some early inhibitors included sodium nitrite and the sodium and potassium salts of chromate and benzoate. Studies found that the sodium and potassium salts reduced the strength of the concrete and gave mixed results on corrosion inhibition. However, other inhibitors have shown promise as methods for protecting reinforced concrete from corrosion damage (Virmani and Clemena 1998).
A common inhibitor used today, calcium nitrite, was developed to be used in concrete as a noncorrosive set accelerator (Berke and Rosenberg 1989)
Corrosion inhibitors are typically divided into three categories: anodic inhibitors, cathodic inhibitors, and organic inhibitors. Anodic inhibitors, made up of chromates, nitrites, molybdates, alkali phosphates, silicates, and carbonates, act by minimizing the anodic part of the corrosion reaction.
These inhibitors form an insoluble protective film on anodic surfaces to passivate the steel. Some anodic inhibitors, such as nitrites, can cause accelerated corrosion and pitting if they are not used in large enough quantities. Cathodic inhibitors, consisting of zinc, salts of antimony, magnesium, manganese, and nickel, form an insoluble film on the cathodic surfaces of the steel. They are usually less effective than anodic inhibitors, but are also safer. Organic inhibitors, including amines, esters, and sulfonates, block both the anodic and cathodic reaction on the entire surface of the metal (Virmani and Clemena 1998).
the inhibitors not only participate in reducing the rate of corrosion but it also takes part in the properties like compressive strength of the structure.
The effectiveness of inhibitors depends mainly on the concentration of inhibitor, more the concentration more is the inhibitor effect on corrosion efficiency decreases with increase in time.. The application requires transport of inhibitor to the reinforcement where it has to reach sufficiently on the surface of reinforcement to protect the steel against corrosion or to reduce the rate of ongoing corrosion.
Following considerations should be taken care while evaluating use of inhibitors in concrete for corrosion protection of rebar:
* Long term stability & performance of inhibitors
* Inhibitors effect on corrosion propagation after corrosion initiation
* Inhibitors effect on concrete physical properties over the service life
* Inhibitors should remain chemically intact & physically present (not leaching or evaporating)
* Insufficient dosage of inhibitors may impact on corrosion progression
* Amount of cracking concrete resistivity decrease