Powder Coat Failures and How to Avoid Them

Powder coating is applied in various plants across the UK using the well known methodology of spraying electrostatically charged fine polyester based powders onto a product rather than using a wet solvent or painting method. Products powder coated are then baked at an optimum temperature to melt the powder which allows it to flow, the product is then allowed to cool and the coating sets. The problem is, like most opaque coatings, whilst the finish looks good, any coating can hide a multitude of sins on the surface of the metal, the substrate, which will become evident over time.

A traditional wet applied, or powder coat finish, is generally porous which allows some air and moisture to reach the substrate.  When the metal substrate begins to oxidise, any applied coating will lose adherence, no matter the quality of the coating itself.  This is particularly true of architectural powder coating which is applied to aluminium products such as, extrusion, sheet or castings.

Newly formed aluminium, as soon as it is exposed to the oxygen in the air,  begins to oxidise, this normally forms quickly into a thin layer and then slows down as the layer becomes thicker. Invisible to the naked eye at first, the process continues until the oxygen in the air is effectively stopped from gaining access to the exposed aluminium surface.  This naturally occurring oxidisation builds up over time to completely seal the surface and form a protective surface, this is why aluminium is often chosen unfinished for external metalwork. Whilst a natural finish, this oxidisation is often not seen as an attractive finish and therefore aluminium is either coated or anodised prior to being used externally. Anodising is simply a rapid and controlled oxidisation of the surface of the aluminium. Typically just 25 microns in thickness the finish is very hard and wear resistant, but colours are limited generally to bronze or grey finishes.

Prior to coating architectural aluminium, it is crucial that the substrate is subject to a ‘pretreatment’ which includes a product to ‘seal’ the surface of the aluminium and provide a ‘key’ for the adherence of the coating. This is formed by an applied pretreatment system or by producing a very thin anodised finish on the substrate.

A multistage process, the pretreatment includes the cleaning of the aluminium and etching to remove contaminants prior to the pretreatment being applied or created. For an architectural powder coating operation a pretreatment plant is a significant investment often costing as much as the powder coating line itself. Few powder coating lines in the UK have this facility and those that do will powder coat to BS EN 12206 or become licensed to powder coat to the more substantial QUALICOAT standard.

So where do things go wrong? Often in an attempt to reduce costs through the supply chain, architectural aluminium products will be coated by powder coaters who do not possess the correct pretreatment lines. There is also the possibility that some coaters who do have the correct pretreatment systems do not have the correct controls in place and the quality of the coatings can vary, even though they can claim to coat to BS EN 12206.

The QUALICOAT licences applicator, on the other hand, has to use a pretreatment system that is approved and powders that are also approved by QUALICOAT which ensures quality is maintained through the supply chain. Furthermore, QUALICOAT licence holders are randomly inspected twice a year to ensure the QUALICOAT specification is being met. Licensed coaters who fail these inspections, will lose their licences.

So the best written specification for any powder coating should simply be ‘coated by a QUALICOAT licensed applicator’. Known across the globe, the standard can be achieved on any continent.

Failures in powder coating falls into two broad categories, that of a complete failure of the coating where it simply peels off and something called ‘filigree corrosion’ where oxidisation can form at a cut or punched hole in the aluminium and looks like strands of corrosion under the surface of the coating. Often this failure is seen several years after installation and clearly shows a poor pretreatment during the powder coating process.

Other problems with powder coating can include fading of colour, loss of gloss and ‘chalking' of the surface finish. The first two are generally a normal ageing process and the powder coating should still adhere fully to the aluminium and continue to protect the aluminium. Often happening over 20 years or so, this ageing can be reduced by specifying a more robust class of powder such as a QUALICOAT Class 2. The latter, ‘chalking’, is also part of the ageing process and looks like there is a dusking of chalk on the coated surface, this only forms when the finish is neglected and not cleaned down to an agreed cleaning regime. Often chalking can be removed by simply cleaning the finish over a period of time.

How to avoid these failures? Ensure that the powder coater has the necessary pretreatment systems in place and that they can demonstrate that the coat and test to BS EN 12206. Alternatively, insist on a specification that coating must be applied by a QUALICOAT licensed applicator. In the UK, the majority of architectural powder coaters are QUALICOAT licence holders. If you are proposing to approve an architectural coater who powder coats to BS EN 12206, be more vigilant to ensure the process is being maintained.

Should the finish require a longer life expectancy, possibly where access could be difficult, then a Class 2, more robust powder can be specified. The most important aspect to specification is the involvement of a specialist coater at the early design stages of any project. All QUALICOAT members in the UK are available to advise and can be found on the associations website.

Copies of the sixteenth edition of the QUALICOAT Standard, complete with the amended updates, 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 enquiries@qualicoatuki.org

The Association can also be followed on Twitter @Qualicoatuki