Corrosion Mechanism - Wet or Electro-Chemical Corrosion
Corrosion of steel concrete is an electro-chemical process. When there is a difference in electrical potential, along the reinforcement in concrete, an electro-chemical cell is set up. In the steel, one part becomes anode (an electrode with a +ve charge) and other part becomes cathode, (an electrode with a -ve charge) connected by electrolyte in the form of pore water, in the hardened cement paste. The +vely charged ferrous ions Fe+ at the anode pass into solution, while the -vely charged free electrons -pass through the steel into cathode, where they are absorbed by the constitutents of the electrolyte, and combine with water and oxygen to form hydroxyl ions (OH). These travel through the electrolyte and combine with the ferrous ions to form ferric hydroxide, which is converted by further oxidation to rust.
The reactions are described below:
Fe - - > Fe+++ 2e-
Fe+++ 2(OH)2 - - > Fe(OH)2
Fe(OH)2.2H2O +O2 -- > 4Fe(OH)3
4e- +O2+H2O -- - > 4(OH)
It can be noted that no corrosion takes place if the concrete is dry probably below relative humidity of 60%, because enough water is not there to promote corrosion. it can also be noted that corrosion does not take place, id concrete is fully immersed in water, because diffusion of oxygen does not take place into the concrete. probably the optimum relative humidity for corrosion is 70-80%
The products of corrosion occupy a volume as much as 6 times the original volume of steel, depending upon the oxidation state. Figure below shows the increase in volume of steel, depending upon the oxidation state.
It may be pointed out that though the 2 reactions Fe2 and OH originate iron the anode and cathode respectively, their combination occurs more commonly at the cathode, because the Sn. aller Fe2+ ions diffuse more rapidly than the larger OH ions. So, corrosion occurs at the anode, but rust is deposited at or near the cathode.
Increase the oxygen content has 2 effects :
(i) it forces the cathodic reaction to the right, producing more OH- ions and
(ii) it removes more electrons and therefore, accelerates the corrosion at the anode.
Each of these effects, in turn, supply more reactants for the forming reaction. Obviously, presence of oxygen greatly accelerates both corrosion and rust formation, with the corrosion occurring the entire anode, but the rust forming at the cathode.