Process description: Conversion coatings (phosphate layer)

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Phosphating layer conversion coatings

Phosphate coatings are the most widely used conversion coatings and probably the most widely used surface treatment. They are used to treat steel, aluminium and zinc for:

• cold forming: this involves very high surface stresses and phosphating is used in all types of cold forming operations, i.e. drawing of wire, tube, or profile; deep drawing; cold heading, cold extrusion, cold forging. These applications are described in the reference.

• Coil coating: steel strip electroplated with zinc is phosphated in the process line to improve formability in subsequent drawing operations, such as steel can forming, as well as for corrosion resistance and subsequent paintability.

• Rust proofing: heavy zinc and manganese phosphate coatings retain a protective oil film and provide substantial corrosion prevention, e.g. for nuts, screws, bolts, and tubes

• Bearing surface lubrication: manganese phosphate improves the retention of lubricant and shortens running-in periods. It is used for pinions, camshafts, pistons, gears, and valves

• Paint base: phosphating enhances the adhesion and corrosion protection of paints.

• Electrical insulation: phosphate layers can be used to coat the silicon steel sheets forming the cores of electric motors, generators or transformers. A phosphate coating of 1 – 6 μm thickness is sufficient insulation to prevent eddy currents.

There is a wide variety of phosphating processes, but the most important are alkali (iron) and zinc phosphating. The surface weight of layers is 0.05 – 5 g/m2.

Methods of application

The phosphating solutions are generally applied by spraying or by immersion depending on the number, size, and shape of the parts to be treated. The type of application may lead to differences in the composition and morphology of phosphate coatings. Coiled strip is also coated by the roll-on, dry-in-place process, in which phosphating solution is applied to the strip and, without rinsing, is dried to form the phosphate coating.

Phosphating requires workpieces or substrates to be degreased and pickled. Activation prior to phosphating may be with hot water or with special proprietary titanium or manganese phosphate dispersions to induce the formation of a fine-grain phosphate coating in the subsequent step. Final rinsing may be without deionised water or passivating chemicals based such as Cr(VI) and Cr(III) compounds. Rinsing with water is needed between the processing stages.

Alkali phosphating

This is mainly when corrosion protection does not have to satisfy stringent requirements. For steel substrates, the solutions (pH 4-6) consist of acid alkali phosphates, free phosphoric acid, and small amounts of additives; oxidising agents (e.g. chlorates, chromates, or nitrites), condensed phosphates (e.g. pyrophosphate or tripolyphosphate), and special activators (e.g. fluorides or molybdates). The first reaction is the pickling reaction which produces Fe2- ions from the substrate (steel). These ions react with phosphate ions from the solution to form sparingly soluble iron phosphate that precipitates and adheres strongly to the metal surface. Iron phosphating processes may not require acceleration. The coating weight varies with the bath composition. Coatings formed on ferrous surfaces contain iron oxides and phosphates. Iron phosphating solutions normally contain surfactants for cleaning and oily surfaces may thus be treated in one step (so-called “cleaner-coater”).

On zinc surfaces, zinc phosphate layers are formed in an analogous reaction sequence. Aluminium is usually treated with solutions containing fluoride; thin, complex coatings are formed that contain aluminium, phosphate, and fluoride. The baths are adjusted to a concentration of 2-15 g/l. Treatment may be spraying, flooding, or dipping. The bath temperature is normally 40-70 °C, but can be lowered to 25-35°C with special bath compositions. Treatment times are 5-10 seconds (spraying of strip material) and 1-3 minutes (spraying or dipping of individual parts). Iron phosphating includes both thin-coating (0.2 – 0.4 g/m2) and thick-coating methods (0.6 – 1.0 g/m2). The colour of the layers is blue-green, but may be reddish iridescent. The surfaces become more matt and grey with increasing coating weight.

Zinc phosphating

Zinc phosphating is primarily used for the surface treatment of steel and zinc (or zinc coatings on steel) as well as composites of these metals with aluminium. Application may be spraying or immersion. Essential constituents are zinc, phosphate ions, and an oxidizing agent, often sodium nitrite. The pH value is between 2 and 3.5. Concentrations vary considerably; additives such as nitrate, fluoride, silicofluoride, nickel ions, or manganese ions are common.

The following is an example of process suitable for the phosphating of steel sheets as a pretreatment prior to painting:

Zn2+ 1.2 g/l, Ni2+ 1.0 g/l H3PO4 + H2PO4- 15 g/l, and NO2- 0.1 g/l

A pH of 3.4 is achieved with sodium hydroxide. The process is usually carried out at up to 95°C. For cold forming applications total concentrations may well be ten times higher.

Typically, the phosphating reaction may be broken down into five steps:

1.) Pickling reaction

Part of the metal surface is dissolved by the acid:

Fe + 2H -> Fe2+ + H2 for Fe or Zn + 2H+ -> Zn2+ + H2 for Zn

The phosphating of aluminium requires fluoride ions to attack surface oxides which only slowly dissolve in phosphoric or nitric acid:

AlOOH + 3HF -> Al3+ + 3F- + 2H2O; Al + 3H+ -> Al3+ + 3/2 H2

2.) Acceleration

The pickling reaction is accelerated by oxidising agents called accelerators, which also prevent the evolution of excessive amounts of molecular hydrogen thus minimising hydrogen embrittlement:

H2 + 2Ox -> 2HOx; Fe2+ + H+ + Ox -> Fe3+ + HOx for Fe

3.) Complexation

When coating aluminium, sufficient fluoride ions must be available for complexation of surplus Al3+ ions as, in concentrations as low as 3 mg/l, they prevent the formation of zinc phosphate coatings. If different metals including aluminium are to be treated, the use of fluoride-free phosphating solutions may allow phosphating of steel or zinc without coating the aluminium:

Al3+ + 6F- -> AlF63- for Al

4.) Coating formation

Metal dissolution in the pickling reaction results in a significant increase of the pH value close to the metal surface. Consequently, the equilibrium constant for the precipitation reaction is exceeded and zinc phosphate is precipitated as the metal surface offers favourable nucleation sites. Once the whole metal surface is covered, the reaction ceases:

3Zn2+ + 2H2PO4- + 4H2O -> Zn3(PO4)2*4H2O + 4H+ 2Zn2+ + Fe2+ + 2H2PO4- + 4H2O -> Zn2Fe(PO4)3*4H2O + 4H+

5.) Sludge formation

Dissolved iron and AlF63- ions from the pickling reaction do not accumulate in the phosphating solution but are precipitated as iron(III) phosphate or triosodium hexafluoroaluminate, respectively. When treating galvanised surfaces and in spraying processes, relatively small amounts of tertiary zinc sludge are formed as well. Nitrate-accelerated processes often do not involve sludge formation. Iron(II) nitrate accumulates until equilibrium is reached between iron dissolution and drag-out.

Fe3+ + H2PO4- -> FePO4 + 2H+ for Fe; or AlF63- + 3Na+ -> Na3AlF6 for Al

The sludges formed must be removed either periodically or continuously.

Pre-paint processesZinc phosphating processes carried out prior to painting can be classified as high zinc or low zinc processes. High zinc processes operate at 3-4 g/l Zn2+ and low zinc processes at 0.7 – 1.5 g/l Zn2+, the upper limit for dip applications. Low zinc concentration, i.e. high phosphate to zinc ratio, improves corrosion protection. Coatings on steel produced by low zinc processes consist mainly of phosphosphyllite and show superior paint-base performance to the high zinc processes with their hopeite coatings, such as stone chipping resistance and wet adhesion on galvanised substrtes. Performance has further increased with the introduction of trication processes, which contain zinc, nickel, and manganese both in solution and in the coating. These processes have become commonplace in, for example, the automotive industries.

For powder coatings, iron phosphate often gives optimum results.

Low temperature processes for cold forming

These are accelerated by nitrate and the iron(III) concentration in the bath is limited to 5-8 g/l by oxidation with air, preferably continuously in a separate reaction tank. The operating temperature has thus been lowered by about 30 °C to 50-60°C, and the phosphating tank remains virtually free of sludge.

Manganese phosphating

This is performed by dipping only, and iron(II) nitrate is often used for acceleration.

Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.

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