Mitigating the Potential Effects of Corrosion

Budget for the correct process as well as the colour

Corrosion is a subject that may appear quite simple but in reality, is complicated. Guarantees often mention corrosion alongside colour and gloss, but it is a more nuanced element and requires serious consideration. As I often say to customers: “It’s no use asking for a great colour if it’s all peeled off and laying on the ground in five years.” This is my first point when specifying metal finishing – the relative cost between one colour and another may not be a major cost driver whereas, in the long term, using the correct process is.

A simple questionnaire will confirm your project corrosion category

Approved architectural powder coaters will have access to powder manufacturers’ databases of locations and related corrosion data. For example, you will be able to get information on corrosion levels affecting a hotel in Brighton or a car park in Bradford from 40 years of project experience and environmental monitoring. A simple questionnaire confirming the location, elevation, distance from the seashore, prevailing winds and other factors will enable a powder coater to confirm the availability of a corrosion guarantee and the process required to achieve it.

Corrosion categories are becoming more frequently used. Look for notations C1 to C5 which define the corrosion features of that particular environment. The relevant standard is BS EN ISO 9223: 2012, annex C; of which describes categories C1 to C5. The categories are defined by the first-year corrosion rates of standard specimens of exposed copper, steel, zinc and aluminium. In practice, the corrosion rates can be estimated using the description of the environment.

Category C3 Outdoor is described as: “A temperate zone with medium pollution (SO2) or some effect of chlorides (Cl), e.g. urban areas, coastal areas with low deposition of chlorides…”.

From this, we can see the key determinants of the corrosive categories are pollution, measured by the atmospheric concentration of sulphur dioxide (SO2) and salt, measured by the atmospheric concentration of chlorides (Cl). When combined with elevation, wind direction etc., the specifier builds up a picture of the atmosphere.

Coastal locations are those closer than 2000m to the shore

Land-locked Bradford and coastal Brighton may have the same ‘C’ category, but in Brighton it will depend upon the actual distance from the shore to the building in question. The cut-off point is usually 2000m, i.e. a building that is more than 2000m from the shoreline is not generally considered to be coastal in the UK.

Knowing the corrosion category gives us a start point for the specification of the powder coat finish.

Qualicoat class 1 and class 2 powder products have no difference in corrosion protection properties

In most UK locations, a single coat of polyester powder with an average thickness of 60 microns will give excellent longevity. An increase in SO2 or Cl (higher pollution or shorter distance to the sea) will require a thicker single-coat film or perhaps a primer coat beneath to further protect the metalwork. Discussing your needs with your powder applicator and powder manufacturer (preferably an approved applicator/manufacturer partnership) will result in the best practice. It is worth noting that use of either Qualicoat class 1 or Qualicoat class 2 products has absolutely no influence on the corrosion resistance of the system. The performance properties of a more expensive Qualicoat Class 2 powder relate to the retention of colour and gloss – not corrosion.

Chrome-free and chrome-based pre-treatments are equal and equivalent

Between the powder coat and the metal is perhaps the most important process of powder coating – the bit that you don’t see – pre-treatment. For best results in final coating, it is worth budgeting for chemical pre-treatments that are approved to powder coating standard BS EN 12206 or Qualicoat specifications. The treatments will change dependent on the corrosion category. You may hear of two types of pre-treatment, Chrome VI and Non-Chrome. Performance-wise there is no difference between the two; however, Chrome VI may be withdrawn from use.

Design detail influences the performance of metal finishing in architectural applications

An attractive, functional design is important but must address the corrosion category and powder coating process so that its lifespan is not compromised. Specific design details are key; namely the bends, folds, welds, gaps, edges, material gauge, recesses and fixing points of an architectural fabrication. For instance, sharp edges are corrosion origination points, so need eliminating. Similarly, if the material needs to be coated all over to achieve corrosion resistance, then the design must allow for this to happen. Your powder coater can provide helpful advice on all of these issues.

Low maintenance is not ‘no’ maintenance

Finally, it makes sense to specify and design a system that supports a low-maintenance solution. This can be achieved by spending time (and money) at the start of the process so that the cost can be quantified. For example, ensuring no water traps where chlorides (salt) can build up and initiate corrosion will add a known cost, whereas the future cost of repair and replacement is unknown. Design with access for cleaning is also important. Low maintenance is not ‘no’ maintenance, and your maintenance plan will generally be a simple regular washing routine.

I hope that I have persuaded you that knowing and understanding the corrosion ‘rating’ of your environment, discussing pre-treatments and paying attention to design elements are essential stages in your powder coating selection. Once this framework is set, you can select colour, texture and sheen with confidence that will look good for its guaranteed period and often longer.