The rust is generated by the corrosion of the metal. When the metal is soaked in the water, minute local anodes and local cathodes are
formed on the surface of the metal due to impurities in the metal, non-uniform concentration of dissolved oxygen or uneven temperatures.
On the anode part, the iron becomes a ferrous ion of the divalence and dissolves into water. And on the cathode part, hydroxide ion (OH-)
is formed by electrons discharged during the anodic reaction, with oxygen and water. As a whole, iron corrodes by the reaction of Fe＋H2O＋
The iron hydroxide (Fe(OH)2) is further oxidized to become red rust (Fe2O3∙nH2O), black rust (Fe3O4∙nH2O), and blue rust (FeO∙nH2O) and accumulates on the metal surface (【Fig-01】)
The dissolved oxygen is indispensable for the corrosion to progress. The oxygen is easily dissolved into the water when water comes into contact with air.
In cooling towers, the oxygen dissolves into water from air rapidly, because the contact surface area of water and air is large in order to promote the evaporation. Corrosion easily progresses in cooling-tower water systems since they contain sufficiently dissolved oxygen. Therefore corrosion occurs frequently in pipes and devices in which cooling-tower water is supplied.
Dissolved oxygen is the major cause of the corrosion in the cooling water system. Corrosion does not progress unless oxygen reaches metal surfaces. 【Fig-02】 shows the relationship between the concentration of dissolved oxygen and the corrosion rate. From this figure, you can see that the corrosion rate is in proportion to the dissolved oxygen concentration.
|2010 Functional Fluids Ltd. All rights reserved.|