Powder Coating & Substrate Pretreatment
The quality of any powder coating relies on its ability to adhere to the substrate. If there is no ‘key’ to adhere to, or the substrate could oxidise, then no matter the quality of the powder coating that is applied, it simply will not adhere over time.
The debate goes on about which pretreatment is best to use for powder coated aluminium outdoor construction and infrastructure use. Due to ongoing concerns raised by REACH regarding the use of chromium trioxide, many architectural coaters have now moved to chrome free alternatives. Chrome free systems have been around for many years and, whilst more difficult to apply consistently, they are reported to perform as well as their chrome counterparts.
Increasingly, specifiers and facade engineers are turning to anodising as a pretreatment for powder coating on aluminium, this is sometimes referred to as PREOX or Flash Anodising. This process requires a completely new plant to be installed at considerable expense hence there are few powder coaters in the UK that can offer this service.
The anodising pretreatment process consists of providing a thin anodised film of just five microns created on the surface of the aluminium and then left unsealed. The process of powder coating then needs to be undertaken quickly in order to avoid potential contamination of the surface of the anodising or further oxidisation as the anodising is unsealed. This being the case, it is only powder coat plants with an anodising facility that can offer this form of pretreatment.
In the UK there are two main standards for architectural powder coated aluminium, either BS EN 12206-1 (2004) or QUALICOAT now in its 16th Edition (2020) of the standard. Each of these standards allow chrome, chrome-free and anodised pretreatment systems to be used. BS 6496:1984 - ‘Specification for powder organic coatings for application and stoving to aluminium alloy extrusions…’ was withdraw in February 2017 and should not be used.
Whilst theBS EN Standard is a European national standard, the QUALICOAT Standard has worldwide recognition. The QUALICOAT Standard is also kept up to date continuously with new editions being issued every two years with regular amendments added between editions.
The 'cross hatch adhesion test' to check powder coating adherence to the aluminium substrate.
Technically, the QUALICOAT standard is more robust as it specifies minimum etch requirements within the pretreatment process and a finished powder coated surface test for filiform corrosion resistance, which BS EN 12206-1 does not require. Crucial to removing contaminants from the extrusion process is an acid or alkaline etch of 1g/sm, without this potential corrosion could form on the aluminium substrate once coating has been completed. Furthermore, QUALICOAT offer a ‘Seaside’ class of powder coating where the minimum etch is increased to 2g/sm.
QUALICOAT, with their global headquarters in Zurich, recognise the increase in specification of anodising as a pretreatment system and have a working group dedicated to developing the technology further. Research is constantly undertaken by QUALICOAT members in collaboration with independent testing laboratories which results a robust anodising pretreatment methodology based on extensive testing which is then independently monitored and specified in the QUALICOAT Standard.
Whilst both BS EN 12206 and QUALICOAT are similar, in fact, QUALICOAT was heavily involved in the writing of BS EN 12206-1 in 2004, the BS EN 12206 requires any alternative conversion coating processes to be tested to ISO 8565 (2011) for a duration up to five years ‘Industrial and Coastal Outdoor Weathering’. However, QUALICOAT uses both accelerated laboratory testing together with Florida and Genoa outdoor weather testing for higher correlation. The matrix of tests laid down in the QUALICOAT standard requires location tests for up to two years duration repeated every three years for all alternative conversion coating processes.
So which one to use? Like many processes it is down to the quality of the application, each pretreatment process, chrome based, chrome free and anodising are all approved and offer guarantees. Whilst some companies can claim to powder coat to BS EN 12206-1 (2004) it is only QUALICOAT that monitor QUALICOAT licensed applicator plants twice yearly for the correct application and ongoing testing of pretreatment and coatings. Specifiers are recognising the importance of this independent assessment of compliance and as a result the number of QUALICOAT specifications being received by applicators is on the rise, which in turn is increasing membership in QUALICOAT UK & Ireland association.
For QUALICOAT applicators to comply with a BS EN 12206-1 (2004) specification the licensed applicator simply must provide data that their selected alternative conversion coating processes has been tested.
Copies of the sixteenth edition of the QUALICOAT Standard, complete with the amended updates mentioned, are available through the UK Association website, www.qualicoatuki.org together with an up-to-date list of licensed UK and Ireland Powder Suppliers, Pretreatment Suppliers and Applicators. Printed literature and telephone support is available from QUALICOAT UK & Ireland Head Office in Birmingham on 0330 236 2800 or email email@example.com. The Association can also be followed on Twitter @Qualicoatuki
QUALICOAT UK & Ireland with offices based in the Midlands is the National Association for QUALICOAT approved companies. QUALICOAT is a global quality label organisation committed to maintaining and promoting the quality of lacquering, painting and coating on aluminium and its alloys for architectural applications.
Please browse our website and should you have any questions about architectural powder coatings and the correct specification, please contact one of our local members or phone our office on the number below.
QUALICOAT UK & Ireland Chairman