- Pre-Treatment of the metal surfaces
Correct Preparation – A Guide
An absolute prerequisite for successful powder coating is that the surface to be coated must undergo pre-treatment to clean and decontaminate.
Contamination of the surface may give rise to aesthetic defects in the coating (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:
A) Metal type and quality
B) Surface condition, i.e. degree of contamination and what contaminants to be removed
C) The finished products and areas of application and its protective quality requirements
D) Economic and environmental considerations (most often a question of various alternatives to similar or equivalent pre-treatment systems)
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 perchloroethylene. 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 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 should be determined by the base in question and the desired roughness), it is important to remember that greasy contaminants should be removed in advance.Zincshield0b
Zincshield® is a zinc rich epoxy based thermosetting powder coating designed to inhibit rust and adhesion loss on ferrous metals. Zincshield® has been designed as a powder coating undercoat. Suitable top coats include Fluoroset®, Duratec®, Electro™, Duralloy®, Duralloy® FG and Alphatec® ranges.
Zincshield® may be used with confidence for improved corrosion resistance and durability and can also be used as a functional topcoat where appropriate. Suggestions for use include ironwork, street and garden furniture, gas cylinders and tanks, agricultural machinery, transport (trailers), valves, and transformers. This method has excellent corrosion and chemical resistance, film integrity, very good surface hardness, no solvents or emissions and sacrificial layer increasing service life and displays excellent corrosion protection.
- Pre-Treatment 2 Chromate
A series of different systems are available within the chromate group of treatments. The system selected depends on the type of metal of alloy, the type of object (method of manufacture: cast, extruded etc.) and of course quality requirements.
A) Chromate treatment may be sub-divided into:
B) Thin layer chromate treatment
C) Green chromate treatment
D) Yellow chromate treatment
The yellow chromate treatment is the most common method for pre-treatment prior to powder coating and it is what we use here at Powdercoating Services Beenleigh Pty Ltd. 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 is a special kind of corrosion appearing mostly on aluminium. 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 of 30-40ºC and relative humidity 60-90%. This problem is therefore limited to coastal areas and linked with unfortunate combination of aluminium alloys and pre-treatment.
To minimise filiform corrosion it is advised to assure a proper alkaline etching followed by an acidic wash prior to the chrome conversion coating. An aluminium surface removal of 2 g/m2 (minimum 1.5 g/m2) is recommended.
Anodizing as pre-treatment for aluminium is a technology specially developed to prevent filiform corrosion. A special anodisation process is required when the thickness and porosity of the anodisation layer is of vital importance. If in doubt, ask Powdercoating Services Beenleigh.
These pages do not give you answers to all conceivable queries relating to pre-treatment, but we trust you have gained an idea of the criteria applying to pre-treatment prior to powder coating.