Substrate pre-treatment prior to coatings
The main aims in the preparation of a metal surface prior to powder coating may be defined as follows:
- The complete removal of all foreign matter, eg. scale, grease, cutting oil, soil, welding splatter, etc.
- The conditioning of the surface so as to render it suitable for the coating that is to be applied.
- The pretreatment should impart uniformity throughout all treated workpiece surfaces, irrespective of the source of the metal or of the contaminants that might adhere.
As with other methods of organic finishing attention to the pretreatment stage is essential in order to achieve the full potential of the powder coating.
Surface pretreatment may vary depending upon the specific end-use requirements of the finished products – from a single-step cleansing operation to a multi-stage sophisticated pretreatment which deposits a conversion coating on the surface of the metal.
Application of a coating of electrostatically charged particles to an earthed metal surface can only be achieved if the surface is free of any composition which has a high electrical resistance. The presence of any insulating film on the surface of the workpiece to be coated will limit or in some cases prevent powder being deposited.
Steel, aluminium, copper, zinc alloys and galvanised steel are common metals on which powder is used. In a number of cases where normal service conditions apply, satisfactory properties can be obtained on thoroughly cleaned metal.
For iron/steel surfaces maximum corrosion and salt spray resistance are given by a zinc phosphate conversion coating.
For aluminium and its alloys, although the clean surfaces are easily coated and adhesion is excellent, performance can be upgraded using a proprietary chromate conversion coating.
With all zinc based substrates such as Zintec, Mazac and Galvanised Steel a suitable phosphate coating is recommended.
Porous castings and ‘blast cleaned’ surfaces
These surfaces can give considerable difficulty with ‘blowing’ of the powder coating due to entrapment of air. The profile of the metal and thickness of coating must therefore be strictly controlled. Preheating for a few minutes sometimes overcomes this defect.
Oxide and scale removal
This can be achieved by mechanical scuffing, wire brushing or for larger areas, by abrasive blasting. Sand as an abrasive material has been banned in the UK as well as in many European countries.
The coarse expendable types of abrasive or re-usable metallic abrasives which took over from sand are now augmented by a whole range of ultra-fine abrasives, ranging from 600 mesh fused aluminous oxide (which is as fine as talcum powder), soft vegetable abrasives such as walnut shell and peach stones, through to tiny glass spheres less than 25µ in diameter. With these extremely fine abrasives a complete surface uniformity can now be achieved. Obviously using a very fine grit the rate of scale removal is rather slow, whereas a too coarse grit will give such a rough surface that the flow of the powder during stoving will be inhibited with consequent loss of gloss accompanied by an extremely rough surface profile.
To provide some idea of the relative surface roughness on a steel surface which has been shot blasted the ‘peak to valley’ measurement would be about 100µ. With fused aluminous oxide (grade 180/220) it would be 3-5µ, while with glass beads it would be 1-1.5µ.
Oil and grease removal
This is usually the first step in the preparation of metallic surfaces for coating. There are numerous ways of carrying out this operation and some of the more common methods are briefly enumerated below.
Grease removal can be achieved by wiping the workpiece with a cloth soaked in a suitable solvent. This method will remove grease and solid matter fairly efficiently until first the cloth and then the solvent become dirty. Thereafter this method will only spread the grease and for really effective cleaning the rags and solvent will have to be replaced frequently. If only loose dust is the contaminant, tack-rags are often used.
Although this method is quick and convenient for small scale production, it suffers from high labour and material costs and, depending on the solvent selected, can be a fire or health hazard.
With this method the workpiece is immersed in a tank of solvent and after withdrawal, when the solvent has evaporated, all oil and grease should have been removed.
This method remains effective until, like the solvent wipe, contamination has built up in the solvent dip tank and an equilibrium is reached whereby as much oil or grease is redeposited on the work as it takes off. The only difference between the two methods is that this oil is spread over the whole component.
Better results can be obtained by having a number of tanks in line on a cascade principle, but this takes up considerable space and is expensive as solvent losses due to evaporation are high.
Again, depending on the solvent used it can be a health or fire hazard. Neither the solvent wipe or solvent dip methods are recommended.
Solvent vapour degreasing
Using this technique the workpiece is suspended in the vapour of a chlorinated solvent such as trichloroethylene in a specially designed plant and the metal is degreased by the condensation of the vapour on its cold metal surface, which solubilises the oils and grease which run off the parts with the liquid as it returns to the sump.
This is a much more efficient process because the solvent is continuously boiled up to replace the vapour that condenses.
On its own this method will degrease effectively but any solid particles left on the surface may remain there after all the oil and grease has been removed.
Improvements can be obtained by including a boiling liquor stage or by the use of ultrasonic agitation. In addition special additives can be put into the chlorinated solvent to improve efficiency.
The workpiece can be dipped into or preferably sprayed with a solution of a suitable detergent in hot water and then rinsed and dried. This will effectively remove light contamination but will not deal with aged oil, grease or heavy soils.
Emulsion cleaners are usually pre-emulsified kerosene/water emulsions, or kerosene-based concentrates which emulsify when added to water. Like the alkali cleaners, emulsions are most efficient when used in spray equipment but can be quite effective as immersion cleaners in many instances.
Emulsion cleaners normally operate at lower temperatures than the alkali type and in some cases can be used at ambient temperatures.
Alkali cleaners – Again the workpiece can either be dipped or sprayed with a hot aqueous solution of a suitable alkali mixture and then rinsed twice and dried. Spray application is more effective than dipping and is cheaper as higher operating temperatures (70-90oC) and concentrations have to be used with the latter. Spray application varies in time from 5-60 seconds whereas dip takes from 1-5 minutes. Immersion cleaners can disperse the grease and oil by emulsifying them into the solution. Alternatively cleaners are available which separate the oil into a layer so that it can be floated off the cleaner surface over a suitable weir.
Alkali cleaners can effectively remove oil, grease and soils and will cope readily with the heaviest contaminants.
There is a wide variety of alkali cleaners whose properties can be adjusted to give effective cleaning from any set of contaminants. These cleaners often include grain refining agents to ensure that phosphate coatings subsequently applied to steel surfaces have a fine grained crystal structure.
In addition to the alkali the mixtures contain detergents, emulsifiers, sequestering and chelating agents and occasionally water-softening additives.
It should be noted that only under controlled conditions are alkali cleaners suitable for light alloys, zinc, galvanised metal or aluminium which are all attacked by alkali.
Acid pickling using either inhibited sulphuric or hydrochloric acid can completely remove rust and scale and can also condition the surface. This method is usually confined to iron or steel surfaces.
It is of paramount importance that when aqueous cleaning methods are employed great care be taken to ensure that subsequent water rinsing is of high standard to ensure that the dried and cleaned components are not contaminated with acid, alkali or emulsion. Also if a conversion coating system does not follow on in sequence the work must be dried rapidly and effectively to prevent rusting of the surface.
Phosphating conversion coatings
The recognised pre-treatment for steel substrates just prior to application of powder is phosphating which can vary in coating weight.
The greater the conversion coating weight the greater the degree of corrosion resistance achieved; the lower the coating weight the better the mechanical properties. It is therefore necessary to select a compromise between mechanical properties and corrosion resistance. High phosphate coating weights can give trouble with powder coatings in that crystal fracture can occur when the coating is subjected to locally applied mechanical forces, eg. bending or impact.
Due to the excellent adhesion of the powder coating to the phosphate coating, disbondment will usually occur at the phosphate/metal substrate interface rather than at the phosphate/powder coating interface.
Phosphate coatings are covered by BS3189/1959, Class C for zinc phosphate and Class D for iron phosphate.
A fine grain crystalline zinc phosphate is recommended at coating weights of 1-2g/m2 and for iron phosphate at 0.3-1g/m2. Application can be made by spray or dip. Chromate passivation is not usually necessary.
Iron phosphate coatings are normally spray applied in a three or four stage operation. The work usually passes through two water rinse sections before drying.
Zinc phosphate can be either spray or dip applied in a five stage operation, ie. alkali degrease, rinse, zinc phosphate, two water rinses.
It is essential that the workpiece after phosphating is powder coated as soon as possible after drying.
Pre-treatment for zinc surfaces
A lightweight zinc phosphate coating is recommended. Generally electro-deposited zinc coatings present no pre-treatment problems but hot dipped galvanised coating can affect adhesion. Increasing degree of spangle decreases adhesion characteristics.
Chromate conversion coatings
The main conversion coating for aluminium and its alloys is a chromate coating which can be colourless or of the yellow chromium oxide or green chromic phosphate type. The coating weight recommended is 0.1-0.5g/m2.
The five stage process normally consists of an alkali degrease, rinse, chromate conversion, followed by two rinses.
Again the chromate coating should be of low film weight for maximum adhesion.
For high quality applications it is usually necessary to employ a final rinse with de-mineralised water. The conductivity of the final rinse bath is then monitored to ensure its cleanliness.
One way of avoiding the need for this is to use a dry-in-place or no-rinse process. These are predominantly a form of chromate. It is arguable whether they are true conversion coatings or merely dried-on films with some reaction with the substrate but the advantages of needing no rinse are obvious.
Heavy-metal free pretreatments
The increasingly strict environmental standards in the developed world mean that there is a move away from heavy-metal containing pre-treatment, particularly chromate. Early chromate-free pretreatments had poor performance but more recently standards have improved with the first approvals for use on architectural aluminium applications being awarded by the Qualicoat organisation in 1996.
Local authorities work to different standards in dealing with effluent discharge. However they are all becoming more stringent and cautious as to what effluent they will accept.
Generally iron phosphate solutions can be passed to drain without treatment, zinc phosphate solutions usually have to be below a specified concentration level which can normally be achieved by diluting in ordinary water.
Some final rinse solutions contain chromate, which requires special treatment because of its toxic effects on marine life.