Aluminium coating with powder paint is described within the British Standard EN 12206-1:2021 published on the 19th May 2021. (Paints and varnishes. Coating of aluminium and aluminium alloys for architectural purposes. Coatings prepared from thermosetting coating powder)
Specifiers should be aware that EN12206-1:2021 replaces EN12206-1:2004 (Withdrawn May 2021) and BS 6496 (Withdrawn 2017).
The scope of EN 12206-1 covers:
- the pre-treatment of the substrate prior to the coating process;
- the coating powder;
- the coating process;
- the final product.
This briefing paper has been prepared by ALFED to outline the pre-treatment processes permitted under EN 12206-1 and is not intended to provide a comparison between their relative merits.
The Requirement for Pre-treatment
The pre-treatment of an aluminium substrate prior to powder coating provides a conversion coating with two primary objectives:
- to provide continuing corrosion protection given the known porosity of the cured external powder layer thereby avoiding premature failure of the coating;
- to promote adhesion of the powder to the aluminium by forming a “key”.
Mill finish aluminium oxidises readily to form a substrate which is not suitable for the application of a durable powder coating. In all pre-treatments recognised under EN 12206-1, the natural oxide layer of aluminium is removed by acid and/or alkaline etching and then a conversion of the surface is performed.
There are three stated recognised conversion methods of pre-treatment in EN 12206-1.
- chemical treatment with aqueous solutions containing either chromate ions or chromate and phosphate ions
- chromium-free systems;
- electrolytic conversion (pre-anodising).
To ensure compliance with EN12206-1:2021 specifiers should ascertain that powder coaters are part of the relevant approved applicator networks administered and audited by both chemical and powder suppliers. A third-party accreditation scheme such as QUALICOAT or GSB could provide additional comfort. Project specifications should clearly identify the standard being worked to.
Chemical treatment with chromate or chromate and phosphate ions
Chromates or phosphochromates (“Chromates”) have been the traditional pre-treatment for architectural aluminium since the introduction of powder coating. They are relatively simple to operate and provide proven corrosion protection in service and good powder adhesion.
Hexavalent Chromium (Cr VI) poses a health risk to workers due to potential occupational exposure and an environmental risk on any accidental release from the processing plant, as chromates are both carcinogenic and mutagenic. For this reason, the use of chromates was scheduled to be prohibited under ECHA REACH from 21 September 2017. This was appealed by a consortium of users and the revised sunset date is now 21 September 2024.
It should be noted that where phosphochromates are used there will be no residual Hexavalent Chromium (Cr VI) on the surface of the pre-treated aluminium.
Chromates can continue to be used under UK REACH until 21 September 2024 provided authorisation holders and downstream users comply with more stringent regulations which include mandatory workplace exposure measurements.
In our opinion, specifiers need to be mindful that it appears unlikely that there will be any extension to the revised sunset date for architectural coating.
There are some Trivalent Chromium (Cr III) based pre-treatments using chromates in a safer form, but these are not typically used for external applications. Where Hexavalent Chromium (Cr VI) is used in the production of such pre-treatment chemicals, the use of those Trivalent Chromium (Cr III) products may ultimately be proscribed in any case.
Chemical Treatment with Non-Chromate Systems
Chromate free pre-treatments (“Chrome-free”) generally use chemistry based on Zirconium, Titanium or a combination of both.
Chrome-free technology has been used in Europe since 1994 and is proven to the performance criteria of EN 12206-1 having undergone the natural weathering testing required over a number of years for full approval under EN 12206-1 along with many thousands of real-life exposures.
Chrome-free systems need greater process controls to operate and monitor compared to Chromate based technology, with a significantly reduced margin for error in forming the conversion coating.
For specifiers and contractors, it is important to assess that powder coaters have management and audit systems in place to demonstrate that they can maintain appropriate controls over the pre-treatment process. External accreditations such as ISO 9001, approved applicator status, QUALICOAT or GSB could help evidence this.
Experience in use of Chrome-free technologies is developing in the finishing industry and there is a growing consensus within the industry that properly applied their performance is comparable to Chromates.
Chrome-free pre-treatments are the most widely used alternative to Chromate. They are cost effective, have equivalent throughput and are well established.
Electrolytic Conversion (Pre-anodising or Flash Anodising)
Pre-anodising is a method using electrolysis to build a natural oxide layer on aluminium to a thickness within specified limits which introduces strong corrosion protection.
In contrast to full anodising this layer is left unsealed creating a surface that allows good adhesion to the paint.
The process is the most complex to control, has reduced throughput capacity compared to the dipping, deluge or spray techniques used in Chromate and Chrome-free applications and is consequently more expensive.
Pre-anodising is not always suitable for pre-treating certain parts or components as the handling post pre-treatment and electrical contact points should be considered.
In extreme environments pre-anodising can be more effective than Chromate or Non-Chromate systems.