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Coatings on aluminum and his alloy which STN (Surface Treatment Netherlands) produces: |
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CHROMATE CONVERSION COATINGS |
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By means of chromating a thin chemical conversion coating (transformation of basic material to other chemical matrix) will be applied on the product. These coatings give a temporary or final preservation of the aluminum surface. Chromate coatings are intensively used to guarantee a good adhesion for paint applications.
Other often-used descriptions are chemical film, chem film or alodine.
The last name has been subtracted from the chemical trade name "Alodine", a product that is often used to apply these coatings. Chromate conversion coatings have a generally uniform, iridescent light- to dark-gold appearance.
The process provides corrosion prevention coatings that are far beyond the characteristics of bare aluminum. |
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Approvals |
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Allied Signal |
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Boeing (BAC 5719) |
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British Aerospace (ABP 1-5057) |
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DASA |
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Fokker |
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Lockheed Martin |
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Mcdonnell Douglas (HP 4-57) |
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CHROMIC ACID ANODIZING |
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Chromic acid anodizing is an electrochemical-applied coating with the following specific characteristics; |
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Fatigue-strength of the base material will not be affected (in contradiction to sulphuric acid anodized coatings) |
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Resident rests of electrolyte will have no harmful effect concerning corrosion of aluminum. This should be involved as a great advantage for assembled parts where a 100 % removal of electrolyte cannot be guaranteed. |
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Best pre-treatment procedure for parts that have to be assembled by means of adhesives. |
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The coating performs better corrosion protection than chromate films.
General thought as the best pretreatment for paint adhesion (anodic coating can best be not sealed).
Chromic acid anodizing can best be applied where optimal corrosion resistance is required and sulfuric acid anodizing is technical not responsible. |
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Approvals |
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Boeing (BAC 5019) |
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British Aerospace (ABP 1-1023) |
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DASA |
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Fokker |
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Lockheed Martin |
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McDonnell Douglas (HP 4-2) |
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TECHNICAL SULFURIC ACID ANODIZING |
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By anodic oxidation the aluminum surface is transformed to an aluminum oxide coating.
The anodic film has excellent adhesion to the substrate, as it is an integral part of the structure in contrast to, for example, a painted or electro-plated part.
Coatings provide an enhancement in properties over the base material particularly in respect to wear and corrosion resistance and for electrical insulation.
Normally sulfuric acid coatings are sealed to improve the corrosion resistance of the film.
Generally speaking, the surface finish of the original part is reproduced with slight roughening. Any defects such as corrosion or polishing "burns" will be emphasized rather than hidden. |
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Approvals |
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DASA |
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Fokker |
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HARD-COATINGS |
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Coating shall be used when optimal characteristics concerning the following properties are required: |
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Hardness (up to 750 Micro Vickers Hardness). |
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Abrasion resistance. |
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Corrosion resistance; hard coatings have smaller pores in relation to sulphuric acid anodized layers.
When corrosion resistance is about the primary function coatings must be sealed (shall be indicated on drawing or purchase order). |
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Good dielectric properties (breakdown voltage till 1200 V). |
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Heat resistant coating (shortly 2000 °C) |
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Hardcoatings are primary used for industrial applications.
Hard anodizing is mostly applied in a sulfuric acid based electrolyte with additives to minimize porosity thus achieving a harder finish coating.
Hard anodizing can be formed on all aluminum alloys but the purer the alloy the harder the coating. Since the coating builds at different rates on each alloy, it is important for the anodizer to know the specific alloy. Some alloys require different procedures and if the alloy is improperly identified, there is a possibility of damage to the part.
Coating globally penetrates base metal as much as builds up on the surface. The term thickness includes both the buildup and penetration.
The color of the coatings is darker as with sulfuric acid anodizing and will vary from light tan to black depending on alloy and thickness.
Therefore hard anodizing with a uniform color is extremely difficult.
Seen the relative low thickness of the coating the only good hardness indication will be performed by Micro Vickers measurement. Here for a square sleeve sample will be prepared on which microscopic hardness measurement is executed. |
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Approvals |
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Allied Signal |
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Boeing (BAC 5821) |
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British Aerospace (ABP 1-1022) |
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DASA |
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Fokker |
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McDonnell Douglas (HP 4-79) |
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SURLON-COATINGS (INTEGRATED COATINGS) |
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SURLON-coatings are integrated coatings of which both name and procedure are patented.
The Surlon processes provide a permanent dry lubricating surface combined with hardness. By leveling off the surface asperity with Surlon, surface energy is drastically reduced and lubricity greatly increased.
Surlon-coatings can be applied on basis of sulfuric acid anodized layers or hardcoatings.
Furthermore it's possible to dye the coatings although this is not recommended.
In the latter case the porous structure of the anodize-layer is both used to impregnate the dyestuff as well as the Surlon. |
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Sulfuric acid anodizing in combination with Surlon is suggested/applied when: |
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Very good lubricating properties are required. |
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Excellent non-stick surface is needed. |
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Hard-coatings based on Surlon are applied when: |
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Good lubricating properties are required. |
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Good non-stick surface is needed. |
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For excellent abrasion resistance. |
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Approvals |
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McDonnell Douglas (HP 4-68) |
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APPLICATION OF LACQUER SYSTEMS |
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From the start the painting facility has been designed to coat aerospace parts.
In combination with an optimal pretreatment Surface Treatment is capable to apply almost any liquid lacquer-system.
Of course we have facilities to force drying or bake the applied films. |
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At this moment the following coatings are applied: |
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Wash-, epoxy and polyurethane primers. |
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Diverse epoxy, polyurethane en alkyd topcoats. |
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Adhesive primers. |
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Teflon coatings |
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DAG coatings (used for EMC applications). |
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Approvals |
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Boeing |
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British Aerospace |
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DASA |
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Fokker |
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