Phosphating principally involves the protection of ferrous metals (steel alloys and cast iron).
Treatment is applied for four main reasons:
To improve corrosion resistance
To offer improved anchorage for wax, oil, paint, laccar, rubber, Teflon, polymers, etc.
To facilitate cold extrusion or other mechanical working.
To provide the treated parts with wear resistance and anti-seizing characteristics.
Phosphating is a surface conversion process (1).
The use of products for the activation (2) and acceleration (3) of phosphating baths is essential to ensure the correct morphology (4) of the crystalline layers and fast reaction times.
The total thickness of the surfacing is measured in microns or in grams per square metre (g/m2).
Our main phosphating treatments are:
Zinc phosphating
Manganese phosphating
Phosphating for paint priming
Hystorical background
The history of phosphate surface treatments is relatively short.
In 1906 T.W. Coslett laid down the basis for modern phosphating.
In 1911 important improvements were made by R.G. Richards with the introduction of the diacidic manganese phosphatic treatment.
Phosphating methodology was further improved in 1916 by the American W.H. Allen with his systems adopted by the ‘Parker Rust Proof Comp.’
In 1929 ‘Digofat‘ was created in Russia, and Parker and Metallgesellschaft of Frankfurt introduced bonderizing.
In 1934 a phosphatic-electrolytic methodology was proposed, called ‘electrogranodizing‘ by the ‘Am. Chemical Paint Co.’, based on the work of J. H. Gravell.
The use of accelerating agents further contributed to the development of phosphating, making it practicable for industrial applications
Conversion(1)
This term has particular importance when talking about the chemistry of surface treatments.
Some modern industrial processes aimed at improving the corrosion resistance of highly oxidable
materials like raw steel, are based on the ‘surface conversion’ of the same.
The chemical process aims to ‘convert the surface’ making it chemically more inert (metastable)
and therefore more resistant to chemical-physical attack.
In the case of phosphating steel, the transformation comes about in a series of stages.
Initially there is the corrosion of the base material as a result of contact with solutions containing phosphoric acid.
This corrosion depends on the co-ordinates of the metal surface (topo-chemical).
Micro-cathodic and micro-anodic areas are created permitting the dissolution of the metal.
The overall reaction is:
Me° + 2H+ ——————-> Me++ + H2
In the subsequent stages and in suitable conditions there is the formation of a neutral insoluble phosphatic deposit in crystalline form on the surface of the metal, replacing the corroded layer and providing additional protection.
The number of chemical and crystallographic forms present in a phosphatic layer is extremely high.
The most important are:
hopeite : Zn3(PO4)2 .4 H2O
fosfofillite:Zn2(Fe,Mn)(PO4)2.4H2O
hureaulite: (Mn, Fe)5H2(PO4)4.4H2O
Accelerating agents
The accelerating agent in phosphating processes is generally a mineral based oxidising element which speeds up the conversion process. Chemically this element depolarises the metallic surface in the micro-cathodic zone and oxidises the metals dissolved in the micro-anodic area, permitting the precipitation of phosphatic slime.
The speed of a production process is undoubtedly of considerable importance
Activation(2)
The word ‘activation’ in its chemical sense means a process that acts on the molecules of a system to make them react differently and in a reduced time.
In phosphating ‘activate’ means conditioning the metal surface to be treated in order to increase the number of crystallisation centres (nuclei).
Increasing the crystallisation nuclei increases the number of crystals while reducing the dimensions and weight of the same in the phosphatic layer.
The result is the rapid creation of a thin microcrystalline layer that satisfies the requirements of the phosphating procedure.
Accelerating agents(3)
The accelerating agent in phosphating processes is generally a mineral based oxidizing element which speeds up the conversion process. Chemically this element depolarizes the metallic surface in the micro-cathodic zone and oxidizes the metals dissolved in the micro-anodic area, permitting the precipitation of phosphatic slime.
The speed of a production process is undoubtedly of considerable importance
Morphology(4)
This term is used in the phosphating sector to define the appearance, form, and orientation of the crystals that make up the conversion surface.
A well oriented and ordered microcrystalline ‘morphology ‘ is a good index of quality.
The morphology of the phosphatic layer is checked using a scanning electron microscope (S.E.M.).
If you have any questions about or would like a quote phosphatizing techniques
