Method of corrosion
A metal in contact with an aqueous solution functions as an electrode. The potential of this electrode is a measure of the tendency of the metal to act as an anode and become oxidized when coupled with some other material or with localized portions of the same metal which can act as cathodes. The anodic or corrosive half reaction can be represented as
Where, M1 is metal corroded. The e.m.f. of the galvanic corrosion cell formed is given by
Ecell = -Eanode + Ecathode
This gives a driving force behind the process under equilibrium conditions. The values of e in the above expression for both anode and cathode are considered as reduction potentials which explain the negative sign for anode.
Cooling water systems provide an ideal environment for corrosion of the material of supply pipe and equipment, by which the metal returns to its natural state. Water acts as a galvanic corrosion cell unit in which one part of the system acts as an anode and becomes corroded, while another part of the system functions as a cathode. The rate of corrosion is proportional to the current density and this in turn is proportional to the cell potential. Metal ions dissolve into water (electrolyte) at anode and electrically charged particles are left behind. These electrons flow through the metal to other points (cathode) where electron-consuming reactions occur. As a result of this, metal losses and often deposit forms.
Corrosion is an electrochemical process, in which, iron ions tend to go into the water and replace hydrogen ions, and thus the metal disintegrates. The half reactions of the unit are partially shown below:
Cathodic reaction
The factors that contribute to pipe corrosion are:
(i) pH of flowing water,
(ii) presence of water in surrounding soil and its pH,
(iii) composition of pipe material,
(iv) temperature,
(v) oxygen.
Action of corrosion inhibitors
Chemical inhibitors reduce or stop corrosion by interfering with the corrosion mechanism. Usually it affects either anode or cathode such that it creates a protective film in between the metal and water.
Problem if inhibitors dosed less than required
In this case, the entire corrosion potential occurs at the unprotected sites. This causes severe localized (pitting) attack. Pitting is the most serious form of corrosion because the action is concentrated in a small area; it may perforate the metal in a short time.
Problem if inhibitors dosed more than required
In this case the film produced will be more and it may cause to reduce the heat exchange, or the cooling efficiency will be affected.
Action of corrosion inhibitors
Chemical inhibitors reduce or stop corrosion by interfering with the corrosion mechanism. Usually it affects either anode or cathode such that it creates a protective film in between the metal and water.
Problem if inhibitors dosed less than required
In this case, the entire corrosion potential occurs at the unprotected sites. This causes severe localized (pitting) attack. Pitting is the most serious form of corrosion because the action is concentrated in a small area; it may perforate the metal in a short time.
Problem if inhibitors dosed more than required
In this case the film produced will be more and it may cause to reduce the heat exchange, or the cooling efficiency will be affected.