Effects due to Corrosion
Corrosion is defined as the process of deterioration (or destruction) and consequent loss of a solid metallic material, through an unwanted (or unintentional) chemical or electro-chemical attack by its environment, starting at its surface, is called Corrosion. Thus corrosion is a process of 'reverse of extraction of metals'.
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 constituents 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:
Anodic Reactions:
Fe - - > Fe+++ 2e-
Fe+++
2(OH)2 - - > Fe(OH)2
Fe(OH)2.2H2O
+O2 -- > 4Fe(OH)3
Cathodic Reaction
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. alert
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 cathode 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,
supplies 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.
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.