Products for drinking water treatment from the Neudos® product family
Neudos® TK and TF
Powdered special phosphates in food quality
Neudos® PS and TLS
Silicate and silicate/phosphate solutions
Dispersants based on organic polymers
How phosphates work
It well known for a long time that phosphates, even in very small quantities, form anti-corrosion layers on ferrous materials and at the same time stabilise the carbonate hardness of the water and suppress the formation of scale. At the same time, they are part of our natural nourishment and, thanks to their many years of use in the food sector, are among the best-studied chemical compounds of all.
Corrosion inhibition by phosphates
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.
Hardness stabilization with polyphosphates
The formation of insoluble calcium carbonate, which is responsible for the growth of pipelines, represents a crystallisation process. The crystal nuclei formed in the water grow on the pipe walls into ever thicker layers.
The Precondition for this is the formation of regular crystals with uniform surfaces, which can grow together to form hard crystal clusters. This is where the effect of so-called threshold inhibitors begins, which also include polyphosphates.
Polyphosphates prevent the further growth of calcite microcrystals by adsorption on the crystal surfaces, or promote the formation of distorted crystal structures, which are impossible to grow together into compact layers. For the stabilisation of high carbonate hardnesses, just a few mg/l of phosphate are sufficient.
Application of special phosphates
The practical experience of recent years has shown that mixtures of mono- and polyphosphates specially designed for corrosion inhibition are superior to pure phosphates in their effect. This is probably due to transport or diffusion phenomena in the surface layers, which have not yet been fully elucidated.
Where both corrosion and stone formation are present at the same time, phosphate mixtures are of further advantage, as they can effectively prevent stone formation in addition to corrosion. Furthermore, traces of iron hydroxides are masked, whereby brown water is combated with immediate effect.
Finally, phosphate mixtures are also capable of slowly degrading deposits already formed, provided that they also contain corrosion products and are not pure calcium carbonates, without impairing the water quality.