A prerequisite for successful powder coating

Preparatory treatment is very often a theme of discussion between the powder supplier and the powder user. How necessary is pre-treatment and what type of pre-treatment should be adopted?

On the following pages we will attempt to provide a guide to assist the selection of pre-treatment systems, based on the stresses anticipated for the finished products

An absolute prerequisite for successful powder coating is that the surface to be coated must be clean and uncontaminated.

Contamination of the surface may give rise to aesthetic defects in the coatings, (bumps, craters, etc) and may cause poor adhesion between the coating, and the base. The most typical contaminants are oil and grease, surface corrosion, mill scale and what can collectively be called particles - both loose and fixed.

A clean surface will in many cases be insufficient to achieve the necessary or desirable corrosion protection. In such cases - where corrosion protection is a primary requirement - chemical pre-treatment should be chosen.

Pre-treatment to suit the definitive use of the product

"Solvent degreasing" is very often employed as the sole pre-treatment prior to powder coating. To illustrate what can happen when such a system is exposed to a corrosive environment, salt spray chamber tests are suitable. In comparison, a system recommended for pre-treatment of higher corrosion classes can be used.

How do you select the proper pre-treatment method?

A series of factors will consequently affect the choice of pre-treatment method, some of which will be specific to the individual user.

The following factors should be identified:

Cleaning/Degreasing

Whether you consider using cleaning/degreasing as the only pre-treatment or as an integral part of a more comprehensive treatment, the method employed should be based on the types of contamination to be removed, as well as the type of substrate (base) in question.

Solvents remove only "greasy" contaminants such as oil, fats, lubricants etc. The most common being tri and perchloroehtylene. Subsequent rinsing is not necessary after degreasing with solvent.

Degreasing with acidic, neutral or alkaline chemicals can also be employed. Such agents may also remove corrosion, mill scale and other oxides.

Mechanical cleaning

Mechanical methods are used both to remove stubborn contaminants such as welding flash, mill scale etc., and to provide better adhesion for the subsequent surface coating.

If blast cleaning is used (the abrasive agent to be chosen be determined by the base in question and the desired roughness), it is important to remember that grwasy contaminants should be removed in advance.

Phosphate treatment

Iron phosphate
Treatment with iron phosphate (often called thin layer phosphating) provides very good adhesion properties and has no adverse effects in the mechanical properties of the powder coating. Iron phosphate provides good corrosion protection for exposure in the low and middle corrosion classes, though it cannot compete with zinc phosphate in this respect. Iron phosphate can be used in either spray or dip facilities. The number of steps in the process may vary from 2-7, depending on the basemetal and the requirement for protection. In relation to zinc phosphate treatment, the iron phosphate process is generally cheaper and simpler to accomplish The phosphate layer normally weighs between 0.3-1.0g/m2.

Zinc phosphate
The zinc phosphate process deposits a thicker layer than iron phosphating, and is securely anchored to the base material. Zinc phosphate also has very favourable adhesion properties, though in some cases it may reduce the mechanical integrity (flexibility of the system. Zinc phosphate provides excellent corrosion protection and is recommended for pre-treatment of steel and galvanized steel for exposure in high corrosion classes. Zinc phosphate can be used in either spray or dip facilities. The number of steps in the process varies between 4-8.

Zinc phosphating is normally more expensive than iron phosphating, due to both higher plant costs and more expensive operation.

Chromate
A series of different systems are available within the chromate group of treatments. The system selected depends on the type of metal or alloy, the type of object (method of manufacture: casr, extruded etc.) and of course, quality requirements.

Chromate treatment may be sub-divided into:

The latter is the most common method for pre-treatment prior to powder coating. The number of steps in the process may vary, depending on how extensively the goods have to be prepared for chromating, for example by pickling, neutralization etc. and consequent rinsing steps.

Pre-treatment and environmental concerns

Use of traditional chemical pre-treatment may involve chemicals subjected to strict regulations, resulting in high expenses on water/effluent treatment and waste disposal.

More environmentally friendly pre-treatment systems are continually being developed, examples being non-rinse processes and chrome-free systems. Some of these more novel pre-treatment processes also offer good prospects for use with powder coatings.

Reputed suppliers of pre-treatment chemicals will be able to give background, experience and documentation for their systems with powder coatings.

Filiform corrosion

This is special kind of corrosion appearing mostly on aluminum. The phenomenon resembles a worm creeping under the coating, always starting from a cut edge or a damage in the layer.

Filiform corrosion develops easily when the coated object is exposed to salt in combination with temperatures 30/40°C and relative humidity 60-90%. This problem is therefore limited to coastal areas and linked with unfortunate combination of aluminum alloys and pre-treatment.

To minimize filiform corrosion it is advised to ensure a proper alkaline etching followed by an acidic wash prior to the chrome conversion coating. An aluminum surface removal of 2g/m2 (minimum 1.5g/m2) is recommended.

Anodizing as pre-treatment for aluminum is a technology specially developed to prevent filiform corrosion. A special anodization process is required when the thickness and porosity of the anodization layer is of vital importance.

Corrosion Classification

As an aid in finding what requirements should be made for pre-treatment, we can define different classes of corrosion:

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