Together with hardeners and corrosion products, phosphates form compact, adhering outer layers on metallic surfaces that drastically slow down the corrosion process. It does not matter whether it is new or already damaged surfaces. The surface courses are built up during operation without the water suffering any loss of quality.
Even the dosage of less than 5 ppm phosphate (as P2O5) leads to a rapid disappearance of brown water, as well as to the formation of a corrosion protection layer. Precondition for the formation of a protective layer are the presence of oxygen for the oxidation of metallic iron and for the production of wall alkalinity, as well as a minimum carbonate hardness of the water of about 15 mg / l calcium carbonate (0.8 °dH).
Under these conditions, mixtures of iron and calcium phosphates and carbonates are deposited on the pipe walls during phosphate dosing, which, in contrast to normal iron corrosion products, are largely diffusion-tight and thus suppress further metal dissolution. These protective layers have the positive property of not growing into ever thicker layers. Furthermore, phosphate promotes the formation of the goethite modification of the iron oxide hydrate, whereby the outer layers are protected against transient corrosion.