By Angus Mackie – QUALICOAT Chair
Chrome-based surface treatments for metals have been around for some time providing excellent corrosion resistance by offering an impervious coating to a metal substrate that eliminates surface oxidisation. These systems have been used in the pretreatment of aluminium profiles, sheet and castings for some time and have demonstrated their excellent resistance to extreme weather across the British Isles. So, why are chrome-based pretreatment systems being withdrawn in the UK later this year?
After forming by either extrusion, rolling or casting, on contact with air, an aluminium surface begins to oxidise. Not seen by the naked eye initially, the aluminium oxide quickly builds up over time and is seen as a light grey surface discolouration. Without any pretreatment a subsequent surface coating will have reduced adhesion on the aluminium. If the applied finish is porous, as is the case for most wet applied and powder coated finishes. then exposed to weathering, the aluminium substrate will slowly continue to oxidise under the applied coating, eventually forcing the applied finish off the aluminium over a period of time, this is seen as surface finish peeling.
The importance of a quality pretreatment process cannot be overstated, it forms the foundation for a robust powder coating finish by cleaning the aluminium surface of all contaminants and oxidisation. The water based immersion or spray pretreatment applied to aluminium by the powder coating applicator includes an initial degrease process followed by an acid or alkaline etch. These processes clean off any surface contaminants and oxidisation. The aluminium pretreatment, sometimes referred to as a ‘conversion coating’ or ‘surface passivation’, essentially seals the aluminium surface prior to applying a wide choice of powder coated finishes.
Chromates or phosphor-chromates, often referred to as just ‘Chromates’, have been the traditional choice of pre-treatment for architectural aluminium since the introduction of powder coating in the 1970’s. Over the last few decades alternative, chrome-free systems have become popular and have gained a reputation for being as robust as their chrome counterpart. Many powder coaters now use chrome-free pre-treatment systems with some UK and Ireland members of QUALICOAT successfully using chrome-free pretreatment systems now for over 20 years.
Prior to Brexit the European Court placed a sunset date on the use of Chromates until 21st September 2024. Following Brexit, the UK set up UK REACH for which the HSE has responsibility for regulatory functions. As the UK grandfathered over the original termination date for the use of Chromates the termination date was subsequently incorporated into UK legislation.
Last year, following submissions by a consortium of seven Authorisation Holders, the European Court annulled their termination of the use of Chromates. Annulment means that effectively the law never existed, so Chromates can continue to be used as a pretreatment in the Republic of Ireland and Europe, pending any further European legislation being put forward.
Since Brexit, the UK no longer recognises European Law, therefore the termination date for the cessation of use of Chromate remains in the UK as 21st September 2024 irrespective of any developments within the EU after 31st January 2020. This position has subsequently been confirmed by the UK’s HSE.
QUALICOAT continue to undertake extensive research in the pretreatment of aluminium developing further changes in the process that can offer extensive levels of protection for various exposed locations, more information is available on the Association website.
The up-to-date QUALICOAT Specification is freely available for download via the UK & Ireland Association website, qualicoatuki.org and the website offers an up-to-date list of licensed UK and Ireland Powder Suppliers, Pretreatment Suppliers and Applicators including the applicators who carry the Seaside class accreditation. Telephone support is available from QUALICOAT UK & Ireland Head Office in the Midlands on 0330 240 9735, the Association can also be followed on Twitter @Qualicoatuki or on LinkedIn www.linkedin.com/company/qualicoat-uk-&-ireland.
