Q: What is the corrosion mechanism for steel reinforcement in concrete?
A: In fresh concrete steel is protected by the alkali pore solution. Chloride or loss of pH breaks down this protection causing surface atoms of iron to lose electrons 2Fe(s) ==> 2Fe2+ + 4e- (oxidation half equation). The released negatively charged electrons participate in the process of formation of negatively charged hydroxyl ions ( OH- ) which are the product of water and oxygen interaction on the surface/interface of steel and electrolyte (concrete) - 4e- + 2H2O(l) + O2(g) ==> 4OH-. As the corrosion process is oxygen limited and oxygen is consumed during this interaction, the process is also described as oxygen reduction or cathodic oxygen reduction. And the above formula for the hydroxyl ion formation is described as the reduction half equation.
The overall or net equation is
2Fe(s) + 2H2O(l) + O2(g) ==> 2Fe2+ + 4OH-
The negatively charged hydroxyl ions (anions) now interact with the out of equilibrium, following the loss of electrons, positively charged iron atoms/ions to form iron hydroxide.
- Fe2+ + 2OH- ==> Fe(OH)2(s)
- Fe(OH)2(s) =O2=> Fe(OH)3(s)
- Fe(OH)3(s) =dehydrates=> Fe2O3.nH2O(s) or rust
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Max Merzlikin, Surtreat Holding, LLC 2015