Silicates

Sodium silicates have been used for many years as corrosion inhibitors in potable water systems. They hydrolyze in water to form negatively charged colloidal particles, which migrate to the anodic area and form a film. Solid corrosion products are needed for formation of the film, which eventually becomes a gel. This process is self-limiting because it stops when corrosion products are no longer available.

The film can form over rusted and clean surfaces eventually covering the entire metal, although its porosity does not completely retard oxygen diffusion. Its main strength, therefore, lies in its anodic inhibition.

In the treatment of potable water systems, 8-16 ppm silica (as Si02) is maintained above that silica level normally found in the water. The monomeric silica found in most water supplies does not provide protection. In water with a pH below 6.0, the general formula of the silicate used is Na2O“2SiO3. In waters with a pH greater than 6.0, a formula of Na20*3.3SiO3 is used. It is these glassy, amorphous polymeric silicates that provide corrosion inhibition. Optimum corrosion control, using silicates, is achieved in the pH range 8.0-9.5. 

The highly alkaline silicates used in domestic waters often cause the pH of the water to reach this level. They afford protection to a wide variety of metals and are particularly useful in preventing dezincification.

The use of silica is further improved by the use of polymeric silica and silicons. Silicons are creating an elastic protection film, which is not only creates an electrochemical barrier as to inhibiting corrosion, but also improves the mechanical strength of the metal surface.

So the metal presents better resistance to errosion, cavitation and attrition.