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Pre-treatment before powder coating

Pre-treatment of the substrate before powder coating

The main purpose of pre-treating the metal surface prior to powder coating is to:
Thoroughly remove all foreign matter such as iron filings, grease, cutting fluid, dirt, welding slag, etc.
The surface is treated to make it suitable for painting.
Through the pre-treatment process, the entire surface of the treated workpiece is made uniform regardless of the source of the metal and the contaminants attached.
As with other methods using organic surface treatments, in order to fully exploit the potential of powder coatings, special attention must be paid to the pretreatment process.

The surface pretreatment process may vary with the specific end use requirements of the workpiece, including: from simple cleaning operations to multi-stage pretreatment of the conversion coating deposited on the metal surface.

To apply a static particle coating to a grounded metal surface, the metal surface must be free of any high resistance components. Any insulating film on the surface of the workpiece to be sprayed will limit or even prevent the deposition of powder.

Steel, aluminum, copper, zinc alloys and galvanized steel are common metal substrates for coating powder coatings. In many cases, a thoroughly cleaned metal can achieve a satisfactory coating effect if the coating conditions are normal.

For iron/steel surfaces, optimum corrosion and salt spray resistance is achieved by zinc phosphate conversion coatings.

For aluminum and its alloys, although the clean surface is easy to apply and the adhesion of the coating is excellent, a proprietary chromate conversion coating can still be used to improve performance.

Zinc alloy
For all zinc-containing substrates, such as Zintec, Mazac and galvanized steel, a suitable phosphate coating is recommended.

Porous castings and "sandblasting" surfaces
Due to entrained air, these surfaces may have serious "stomach" problems with powder coatings. Therefore, the shape of the metal and the thickness of the coating must be strictly controlled. In some cases, warming up for a few minutes can overcome this problem.

Remove oxides and scales
This can be achieved by mechanical friction, wire brush friction, or for larger surfaces, sandblasting can be used. In the UK and many European countries, the use of sand as an abrasive has been banned.

A disposable coarse abrasive or recyclable metal abrasive that replaces sand, now adds a range of ultra-fine abrasives, from 600 mesh fused alumina (as fine as talc), soft shell abrasives such as walnut shells and peach kernels. A tiny glass sphere of less than 25 μm in diameter. With these very fine abrasives, a very uniform surface can be obtained. Obviously, when using ultrafine abrasive grains, the iron filings are removed slowly, but the use of coarse abrasive grains causes the surface to be rough, so that during the drying stage, the powder fluidity is limited, resulting in loss of gloss and surface contour. Extremely rough.

To know the relative surface roughness of the blasted steel surface, the "peak-to-valley" measurement of the blasted steel surface should be about 100 μm. If fused alumina (180/220 grade) is used, it is 3-5 μm; if glass beads are used, it is 1-1.5 μm.

Remove grease
This is usually the first step in pre-painting of metal surfaces. There are a variety of methods of operation, which are described in more detail below.

Solvent wipe
The grease can be removed by wiping the workpiece with a cloth soaked in a suitable solvent. This method effectively removes grease and solid matter before the cloth and solvent dries. After that, this method only spreads the grease. To be effectively cleaned, the wipes and solvent must be replaced frequently. If the contaminant is just dust, a sticky wipe is often used.

Although this method is fast and convenient for small-scale production, labor and material costs are high, and some solvents present fire and health hazards.

Solvent impregnation
When using this method, the workpiece is immersed in a solvent bath and then removed, and after the solvent evaporates, all grease is removed.

As with solvent wiping, this method effectively removes impurities as the contaminants in the solvent pool increase and the grease is removed and re-deposited to equilibrium. The only difference between the two methods is that the grease is distributed throughout the assembly.

Better results can be obtained by arranging several numbers of solvent pools in tandem, but this will take up a lot of space and cost will increase due to the higher evaporation losses of the solvent.

Also, certain solvents present a fire and health hazard. Both solvent wiping and solvent impregnation methods are not recommended.

Solvent vapor degreasing
This technique is to suspend the workpiece in a chlorinated solvent vapor in a special device, such as trichloroethylene, etc., and the vapor condenses on the low temperature metal surface of the workpiece, so that the grease dissolves and leaves the workpiece together with the liquid into the solvent pool.

This process is much more efficient because the solvent is in a continuous boiling state to supplement the condensed steam.

This method can effectively degrease, but after the grease is removed, the solid particles may remain on the surface of the workpiece.

Equipping a boiling section or using ultrasonic agitation will improve. In addition, special additives can be added to the chlorinated solvent to increase efficiency.

The workpiece can be immersed in a hot water formulated detergent solution, preferably sprayed, then rinsed and dried. This effectively removes milder contamination, but does not remove aged grease or severe stains.

Milky detergent
Typically, the creamy detergent is a pre-emulsified kerosene/water emulsion, or the kerosene-based concentrate is emulsified with water. Like alkaline detergents, emulsions are most effective when used in spray equipment, but in many cases can also be used very effectively as a dip.

Under normal conditions, emulsion detergents operate at a lower temperature than alkaline detergents and, in some cases, can be used at ambient temperatures.

Alkaline Detergent - Again, the workpiece can be impregnated or sprayed with a high temperature aqueous solution of a suitable alkaline mixture, then rinsed twice and dried. Spraying is more efficient and less expensive than impregnation because of the higher operating temperatures (40-90 ° C) and the use of concentrates for impregnation. The spray time is between 5 and 60 seconds, while the impregnation takes 1-5 minutes. The immersion agent emulsifies the fat into the solution to disperse it. There are also some detergents that separate the oil into layers that float on the surface of the detergent.

Alkaline detergents are effective at removing grease and dirt and can easily remove the most severe contaminants.

Some alkaline detergents can be tailored to effectively remove the corresponding contaminants. In many cases, these detergents also contain a crystal refiner to ensure that the phosphate coating applied to the steel surface later has a fine crystalline structure.

In addition to the base, the mixture contains a cleaning agent, an emulsifier, a sequestering agent, and occasionally a soft water additive.

Please note that alkaline detergents can only be used in light alloys, zinc, electroplated metals or aluminum under controlled conditions, otherwise they will be corroded by alkali.

Pickling can use inhibitory sulfuric acid or hydrochloric acid to completely remove rust and iron filings, as well as finishing the surface. This method is usually limited to iron or steel surfaces.

When using an aqueous washing process, special care must be taken to ensure high quality of subsequent rinsing to ensure that the components that have been dried and cleaned are not contaminated with acids, bases or emulsions. In addition, if the conversion coating system is not immediately performed, the workpiece must be dried quickly and efficiently to prevent surface rust.

Phosphate conversion coating
Prior to powder coating, the pre-approval of steel substrate pretreatment is phosphating, and the weight of the conversion film layer may vary.

The heavier the weight of the conversion film layer, the higher the corrosion resistance; the lighter the weight of the conversion film layer, the better the mechanical properties. Therefore, it is necessary to strike a balance between mechanical properties and corrosion resistance. A higher phosphate coating weight may adversely affect the powder coating because crystal damage may occur when the coating is subjected to local mechanical forces such as bending or impact.

Because of the excellent adhesion of the powder coating to the phosphate coating, the problem of interlayer adhesion typically occurs at the interface of the phosphate and metal substrate rather than at the interface of the phosphate coating and the powder coating.

The phosphate coating is specified in BS 3189/1959, with zinc phosphate being grade C and iron phosphate being grade D.

If the zinc phosphate conversion film layer weight is 1-2 g/m2, if iron phosphate is used, the conversion film layer weight is 0.3-1 g/m2, and spraying or dipping may be employed. Chromate passivation is usually not required.

Under normal circumstances, the iron phosphate coating is sprayed in three or four processes. Usually, the workpiece needs to pass through two washing sections and then dried.

Zinc phosphate can be sprayed or impregnated in five steps, namely alkaline degreasing, water washing, zinc phosphate, and two water washing.

It is essential that the workpiece be dried as soon as possible after the phosphate treatment.

Zinc surface pretreatment
A lightweight zinc phosphate coating is recommended. In general, there is no pretreatment problem with electrostatically deposited zinc coatings, but hot dip galvanized coatings may affect adhesion. The higher the zinc flower, the worse the adhesion performance.

Chromate conversion coating
The primary conversion coating of aluminum and its alloys is a chromate coating which may be colorless or chrome oxide yellow or chromium phosphate green. The recommended coating weight is 0.1-0.5 g/m2.

Under normal circumstances, five processes are required: alkaline degreasing, water washing, chromate conversion coating, and then two water washes.

Also, the film weight of the chromate coating should be reduced to achieve optimum adhesion.

For high quality coatings, it is usually necessary to use a demineralized water for the final water wash. Then, the conductivity of the final washing tank is monitored to ensure its cleanliness.

Waterless washing system
One way to avoid final water washing is to use an in situ drying or waterless washing process. Mainly in the form of chromate. Whether it is a true conversion coating or a dry film that reacts only with the substrate remains to be explored, but the advantages of the water-washing process are obvious.

No heavy metal pretreatment
As environmental standards in developed countries become higher and higher, the use of heavy metal pretreatment begins to decrease, especially chromate. Early chromate-free pretreatment was less effective, but recent standards have improved, and the Qualicoat organization first licensed it in the aluminum coating of buildings in 1996.

Wastewater treatment
Wastewater discharge standards developed by local authorities vary. However, their regulations are becoming more stringent and cautious for wastewater that is allowed to be discharged.

In general, the iron phosphate solution can be directly discharged without treatment, and the zinc phosphate solution usually must be lower than the specified concentration, which can be achieved by dilution with water.

Some final water wash solutions contain chromate, which requires special handling due to the toxic effects of chromate on marine life.
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