Powder coating is not impervious to moisture so it alone will not stop metal from corroding. Without pre-treatment corrosion will quickly attack the metal, this can rapidly eat away under the surface of the powder coating, causing it to peel and flake off.
A quality pre-treatment provides a protective barrier to resist this corrosive attack and can add years to the life a product. So the ultimate durability of any powder coating is based on the quality of the pre-treatment applied to the metal product prior to powder coating.
The pre-treatment process carries out two functions; cleaning and conversion coating.
On our conveyorised systems the parts travel through various stages of the pre-treatment process. The first stages clean the parts by in line high-pressure sprays of heated water and detergent. This removes oil and dirt from the surface of the item to be coated.
The next stages apply a ‘conversion coating’, this converts the surface of the metal by depositing is a microscopic film of corrosion resistant crystals on the surface of the metal substrate, this also aids in adhesion of the powder coating. The corrosion resistant conversion coatings come in many forms, some specific to particular metals such as Chromate Conversion for aluminium. Ferric metal like steel require iron or zinc phosphate or the newly developed silane or zirconium pre-treatments which work effectively across a range of metals.
Our newest powder coating line boasts an eight stage silane pre-treatment system as used by major car manufacturers. Silane is the ultimate in corrosion protection on metal products. We currently coat products for Ford and Holden on this line to comply with their stringent corrosion resistance specifications.
Magnified 1000 time this photograph shows the silane crystals deposited on the surface of a product.
Corrosion is measured by a variety of testing methods, these are designed to accelerate the process of corrosion by exposure to heat, moisture, light and salt. One of the most common testing methods is by salt spray testing. This is carried out by cutting thought (scribing) the coated layer back to the substrate metal then placing the test sample in a ‘Salt Spray Tank’. Here the sample is constantly sprayed with heated salt water 24 hours a day. Every 100 hours the sample is removed and the distance that the corrosion has travelled under the coating from the scribe line is measured. When the corrosion has travelled 2mm under the coating from the scribe it fails and is removed from the tank.
The results are rated from category 1 to 5. Category 5 passes after 1000 hours exposure in the salt spray tank!! It is suitable for use in harsh marine environments.
There are many factors that can affect the efficacy of the pre-treatment process. The quality of the metal substrate is the most important factor. Clean lightly oiled bright steel is a good substrate that will clean easily and react well in a pre-treatment process. Rusty, black metal covered in mill scale and heavy deposits of grease and dirt will be difficult (or impossible) to clean and this will block the chemical reactions required for effective pre-treatment.
One aluminium panel coated on our Silane system achieved category 5 with no corrosion apparent after 1000 hours exposure in a salt spray tank. We have also achieved category 5 results on steel substrates.