Electrochemical process of corrosion in RCC

Electrochemical process of corrosion

Steel in sound concrete is protected by a thin passive oxide film, formed in the highly alkaline pore solution (pH 12.5 to 13.5). If the film is destroyed (by carbonation reducing pH below about 9, or by chlorides above a threshold of about 0.4% by weight of cement), the steel can corrode when oxygen and moisture are available. The bar surface behaves like a small battery:

   concrete (electrolyte, pore water)
     anode               cathode
   Fe -> Fe2+ + 2e-    O2 + 2H2O + 4e- -> 4OH-
 ===[ steel bar ]==============[ steel bar ]===
         \-- e- flow in the steel -->--/
        Fe2+ ------- ion flow ------- OH-

1. Anode (corroding area): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$
2. Cathode (oxygen-rich area): $\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$
3. Ions react: $\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$, then it oxidises to $\text{Fe(OH)}_3$ and finally hydrated oxide $\text{Fe}_2\text{O}_3\cdot n\text{H}_2\text{O}$ (red rust).

The rust has 2 to 6 times the volume of the steel it replaces. Moisture, oxygen and a conductive path are needed; hence corrosion is high in wetting-drying and coastal zones.

Effect of corrosion on the concrete element

1. Expansive rust creates tensile stress; cracks run along the bars, followed by spalling of the cover and delamination.
2. Loss of bar cross-section (pitting) reduces the tensile capacity, ductility and fatigue life of the member.
3. Loss of bond between steel and concrete, so anchorage fails.
4. Rust stains, exposure of bars, more ingress of water and chlorides (accelerating the process).
5. Loss of serviceability (deflection, cracks) and finally structural collapse in severe cases.