Heavy/Thick Zinc Phosphate Coating For Corrosion Protection
On account of their porosity and micro-roughness, phosphate coatings have high oil holding properties. This can be demonstrated by partly immersing blank and zinc phosphate coated steel sheet in commercially available corrosion protective oils.
The use of such heavy phosphate coatings for improved corrosion protection can be found in nearly all branches of metal working industry. Typical examples include screws, nuts and bolts, motor vehicle components in brake and clutch assemblies, engine components, leaf or coil springs, drill bits, washers, anti-vibration washers, tools, magnet cores, casing interiors and other small items.
To ensure good corrosion resistance, phosphate coatings should not be less, than 10 g/m² except for undemanding environments where values of 5-10 g/m² are acceptable. The preferred phosphating processes are those based zinc and manganese phosphates. Zinc phosphate coatings are, in general, more corrosion resistant than those based on manganese phosphate but many users prefer manganese based ones because of darker, attractive appearance.
The type of post treatment oil or wax also affects corrosion resistance. Increase of viscosity, and thus the film thickness and its resilience, usually leads to improved resistance while equally significant are the effects of corrosion inhibitors included in the oil formulations. Surfactants, on the other hand, which are called for in the water emulsifiable oils, increase the danger of the protective oils being washed out by rain or formation of condensate.
Oils and waxes used for post treatment of phosphate coatings can be classified according to the mode of use, as follows:
- Oils and waxes dissolved in organic solvents
- Aqueous solutions of oils and waxes
- Undiluted oils
- Dewatering fluids or water-repellent oils
The main components of these oils are generally mineral oils, solvents, corrosion inhibitors and surface active agents as well as special agents to inhibit oxidation, polymerisation or bacterial action.
Corrosion protective oils, used diluted either solvents or water, are relatively viscous. Organic dilution solvents are usually low-boiling point hydrocarbons or chlorohydrocarbons. The concentrations of such oils lie in the region 5-25%. The phosphated components, after drying, are immersed in the oil baths for 0.5-2.0 minutes, allowed to drain and excess solvent evaporate. The thickness of resulting oil film depends of the oil used, its concentration and the nature of the solvent. Small parts are often barrelled or centrifuged to remove excess oil and solvent.
More important technologically, are the corrosion protective oils in aqueous emulsion. Since the parts to be treated do not first have to be dried, this approach integrates readily with the main wet processing sequence. Oil concentrations are usually 10-30% and the water should be softened since some corrosion protective oils form undesirable reaction products with salts in hard water. Drag-in electrolyte from preceding stages is also undesirable since this can affect the emulsion stability. These emulsion based systems are normally used at 60-80 ºC, sometimes above 90 ºC. Such relatively high temperatures facilitate subsequent drying of the work-pieces. Immersion times range from 0.5-2.0 minutes. Subsequent drying in a circulating air owen is only called for if the bath temperature is less than 80 ºC and there is an excess of fresh air movement. Drying at higher temperatures reduces the thickness of the oil film and thus the corrosion resistance.