Phosphating (or phosphatization) mainly concerns the protection of ferrous metals (alloy steels and cast irons).
Phosphating is a surface conversion process (1) where, unlike painting or galvanic plating, the base metal participates in the chemical reaction and is a constituent of the surface layer.
The first stage of the phosphating reaction involves microcorrosion of the base metal by the phosphating bath, which is acidic due to phosphoric acid.
The second stage involves the formation of a crystalline phosphate layer, which restores the corroded layer and creates a further deposit.
This treatment is performed for four main reasons:
The use of products for activation (2), acceleration (3), and phosphating baths are essential to ensure adequate morphology (4), crystalline layers, and rapid reaction times.
The total thickness of the coating is measured in grams per square meter (g/m2), or in microns.
Historical Notes
The history of phosphate coatings is relatively recent.
1906
In 1906 T.W. Coslett laid the foundations of today's phosphating.
1911
In 1911, important changes were made by R.G. Richards with the introduction of manganese diphosphate.
1916
The phosphating method was then perfected in 1916 by the American W.H. Allen and his systems were taken over by the ‘Parker Rust Proof Comp.’
1929
In 1929, ‘Digofat‘ salts were created in Russia and Parker and Metallgesellschaft of Frankfurt launched bonderization.
1934
In 1934, a phosphate-electrolytic method called ‘electrogranodization’ was proposed by the ‘Am. Chemical Paint Co.’ by J. H. Gravell.
The use of accelerators contributed to the development of phosphating, making it advantageously applicable to the industrial sector.
The main phosphating treatments we carry out are:
Zinc Phosphating
Manganese Phosphating
Conversion(1)
This term is of particular importance with regard to the chemistry of surface treatments. In fact, some of the modern industrial processes used to increase the corrosion resistance of materials such as crude steels, which are easily oxidized, are based on the principle of their ‘surface conversion’.
The chemical process is aimed at ‘converting the surface’ by transforming it into a more chemically inert (metastable) one and therefore more resistant to chemical-physical attacks.
Activation(2)
In chemistry, the word “activation” has been used to describe the process that affects the molecules of a system to enable them to react differently and more quickly.
In phosphating, “activating” means conditioning the surface of the metal being treated to increase the number of crystallization centers (seeds).
By increasing the crystallization seeds, the number of crystals increases, while their size and weight in the phosphate layer decreases. The result is therefore to obtain, in a short time, a thin microcrystalline layer that satisfies the phosphating requirements.
Acceleration(3)
In phosphating processes, the accelerator is generally a mineral oxidizing element, which has the purpose of speeding up the conversion process.
Chemically, this element depolarizes the metal surface in the microcathodic zone and oxidizes the dissolved metals in the microanodic area, allowing the precipitation of phosphate sludge. Speed in a production process is undoubtedly of considerable interest.
Morphology(4)
In the phosphating sector, this term is used to define the appearance, shape and orientation of the crystals that make up the conversion layer.
A well-oriented and ordered microcrystalline morphology is a high quality indicator.
The morphology of the phosphate layer is checked using a scanning electron microscope (SEM).
