Red waters result from the attack of aggressive waters on the iron pipes of the distribution system. The corrosion of iron is presented in short and the importance of oxygen in sustaining the cathodic reaction is revealed. Natural or desalinated waters contain the elements of self-inhibition in the form of Ca'+ and HCO; ions. These, when carefully adjusted at the proper pH value, produce a compact, pore-free inhibiting film of CaC03. The theoretical relations governing the precipitation of the film are outlined. The Langelier and similar indices, proposed and applied for the conditioning of potable waters, overlook the parts played by the metal and by oxygen in securing successful filming.
In the present paper emphasis is laid on the reduction of oxygen which produces the OH-ions that trigger the deposition of CaC03. The conditions allowing the successful formation of the film are defined as those where the fluxes of HCO; and Ca2+ are equal to or exceed that of the OH-ions. The new treatment is tested with the Umm Al Nar potable water.
The type and sequence of precipitates contributing to film formation are arrived at from a consideration of the thermodynamics of the probable reactions. It is concluded that the film is heterogeneous in nature, formed of CaC03, FeC03, Fe(OH)3 and Mg(OH)2 in varying proportions.
Factors interfering with film formation and/or stability are discussed briefly.