Corrosion inhibition by phosphate
Phosphates, together with hardness and corrosion products, form compact, firmly-adhering coatings on metallic surfaces, which dramatically slows down the corrosion process. It does not matter whether it is a new or damaged surface. The creation of this coating is an ongoing process which does not affect the quality of the water.
Even a small dosage of less than 5 ppm of phosphate (as P205) results in the rapid disappearance of brown water and the formation of a corrosion protection layer. In order for this layer to form, oxygen is needed to oxidize the iron, generate side-alkalinization and a minimum carbonate hardness in the water of approximately 15 mg/l of calcium carbonate (0.8 °dH).
Under these conditions, a phosphate dosing at the pipe walls separates out a mixture of iron and calcium phosphates and carbonates which, unlike normal iron corrosion products, are largely diffusion-tight and thus combat further metal dissolution. These protective layers have the advantage of not growing ever thicker and thicker. Furthermore, phosphate encourages the formation of the goethite modification of the iron oxide hydrate, whereby the layers are protected against transient corrosion.