Which Pretreatment to Use?
The debate goes on about which pretreatment is best to use for powder coated aluminium for outdoor construction and infrastructure use. Due to 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 perform as well as their chrome counterpart.
It is relatively straightforward to move from chrome based pretreatment to a chrome free chemical pretreatment system. It usually requires a complete pretreatment plant deep clean with some part replacements, usually taking just a few days to complete. More recently specifiers and facade engineers have begun to specify anodising as a pretreatment for powder coating, sometimes referred to as PREOX or Flash Anodising. This process requires a completely new plant to be installed at considerable expense.
Currently the anodising pretreatment process consists of providing a thin anodised film of just 5 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 trains of thought on which standard is applicable for specifying architectural powder coated aluminium, either BS EN 12206-1 (2004) or QUALICOAT now in its 15th Edition (2017) of the standard. Each of these standards allow chrome, chrome-free and anodised pretreatment systems to be used. Whilst the BS EN Standard is European, the QUALICOAT Standard has worldwide recognition, the QUALICOAT Standard is also kept up to date continuously with a new edition being issued every two years with regular amendments added between editions.
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 the 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. They have recently set up a working group to research the application and current knowledge on such systems. Research is being undertaken by QUALICOAT members in collaboration with independent testing laboratories. What will result is a robust anodising pretreatment methodology which is independently monitored and specified in the QUALICOAT Standard.
Whilst both standards are similar, in fact, QUALICOAT was heavily involved in the writing of the 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 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 licenced 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 licenced applicator simply must provide data that their selected alternative conversion coating processes has been tested.
The next QUALICOAT UK & Ireland members meeting takes place in Norwich in November, please contact the General Secretary, Jan, via the QUALICOAT UK & Ireland website should you wish to attend. For details on the availability of various colours and finishes contact any QUALICOAT UK & Ireland member for more information. For updated information about the use and specification of QUALICOAT in the UK and Ireland, please visit the UK & Ireland associations' website at www.qualicoatuki.org
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