All powder coaters strive for a consistent paint finish, however upon application powder does not evenly coat all areas of a workpiece, especially in recessed or restricted areas. Intricate shapes can have an effect on the electrostatic properties of the workpiece, such that powder repulsion can occur. This effect, known as a ‘Faraday Cage’ actually prevents a standard coating thickness from being achieved and results in the workpiece having only a thin powder film in these areas. Thin film powders are porous and susceptible to moisture ingress. As a result, the coated workpiece is likely to have a poor corrosion resistance.
Many Iron based materials are susceptible to rapid oxidation and / or moisture attack. This is particularly apparent in our global location, where the coastal areas of South East Queensland are prone to high humidity for much of the year and worse still this moist air allows for a high dissolved salt content, especially within a kilometre of the sea. This highly salt and moisture laden air is extremely corrosive to Iron based materials.
A recent study by the International Molybdenum Association of Australia found that the heavy surf and strong winds on the Sunshine Coast create salt (chloride) particulate levels in the air, and from this salt is being deposited on surfaces at a daily rate of 3 gms/ sq.mtr. This salt laden air, together with the high average temperatures significantly accelerates corrosion, even on Stainless Steel. http://www.imoa.info/_files/pdf/casestudy9_06.pdf
At Ridgewood Powder Coating, we are fully aware of the climatic conditions in South East Queensland. Based at Mudjimba, we are only 1500 metres from the Coral Sea. In our location we see the first signs of corrosion starting on blasted Iron within approximately 4hrs on humid days.
To combat the rapid onset of corrosion, unless we are specifically instructed otherwise, we use Zinc priming on all Iron related products, such as Mild Steel, Stainless Steel, Galvanised Steel and other ferrous alloys immediately after abrasive blasting.
Our reasoning may initially appear as overkill on some components, but in the long-term our view is that the additional cost of this process far outweighs the cost of component removal, refurbishment and re-installation due to the rapid onset of corrosion associated with an inferior coating system.
The Zinc priming process involves the application of a specially formulated powder coat [containing no less than 70% Zinc], to your pretreated or abrasive blasted components. This application is usually applied within 2 hours of the blasting process so as to avoid any re-oxidation of your components surface. This timeframe ensures that an effect bond is created between the coating and the component surface, thereby providing enhanced adhesion of the Zinc coating to the substrate surface.
Once applied, unless specifically requested as a ‘primer coat only’ finish, the Zinc primer is ‘green cured’ prior to the immediate application of your specified top coat colour. This green-cure stage is highly important to the integrity of the total powder coating process. Essentially, the primer curing process is stopped, such that only a partial crosslink of the coating occurs. This enables initial adhesion to the blasted substrate surface profile to occur, but prevents the complete reaction of the powder coat.
After the application of the top coat, the curing process is reactivated and continues until a full powder coating is achieved. During this second baking stage, the primer and top coats not only fully cure to form coated layers, but also fuse together at the interface of the coating layers to create an overall uniform layered coating. It is this layered coating which provides the powder coating with its exceptional corrosion resistant performance.
By using this special primer we achieve not only good corrosion resistance, but also for many items, good chip resistance. This is especially useful for items such as Bullbars, Shock Absorbers, Road Wheels and Suspension Springs.