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Powder coating on alloy items and outgassing, Please read the article written by the Powder coating institute. 

Whilst we endeavour to offer the best results possible there are certain factors you need to know and be aware of.

Powder coating is cured at a temperature of at least 180dc, at this temperature the chemistry starts to happen in some metals and castings. Castings as Ali alloys, magnesium and some cast irons tend to be porous (see below)  and may outgas during the curing process. Known as porosity as the castings heat up any gas, air or any other contaminants trapped inside these microbubbles expand and push their way out of any small holes/micro cracks under the curing powder coat in the wetting out stage. This can leave small but very annoying bubbles under the cured powder. These are often confused with poor prep work or work contamination, I can assure you this is not the case.  If you were to look at these with a powerful magnifying glass they appear to resemble tiny volcanoes. There are some methods that  negate this problem by reworking the parts or using an anti gassing product but by using these products does not always work however in the case of poor and cheap castings (see the article below) there is nothing that can be done to stop this other than offering wet paints & good  fillers. 

Corrosion that sets into castings from standard alloys/ cast irons is also a problem, steel to magnesium products can also have the same effect on the finished items whereas the corrosion eats its way into to the material and gives the same effect. Despite of careful prep work and making sure all the corrosion has been removed by media blasting and yet will leave behind micro-holes that again can give rise to outgassing. 

You may ask why fillers can not be used. Standard fillers are not suitable for powder coating as they burn and/or can expand and fall away from the work, there are other high-temperature fillers available but these are not used on unseen micro holes although they can be used in some corrosion cases. 

When outgassing occurs we are unable to offer any guarantees. 

The article below written by The Powder Coating Institute explains this problem.  

Outgassing describes the phenomena of entrapped gasses being released through a powder coating during the curing process. When this gas passes through the coating, it creates pinholes. These pinholes can provide a path for moisture or corrosive materials through the coating and to the substrate, causing coating failure. Furthermore, these holes can be unsightly on high-quality appearance parts.

Causes of Outgassing Outgassing can be attributed to the substrate material, a surface contaminant, or the powder itself. Following are the specifics:

1. Castings: Outgassing can occur when a powder coating is applied over a cast metal surface (iron, steel, aluminium, brass, etc.). Gasses are entrapped into the casting material during the pouring process of both sand and die type castings. These gasses can be anything from entrapped air to gas formed during the cooling of the molten metal. The quality of the metal used and the care taken in the pouring process will directly affect the amount of gas that is entrapped. For instance, higher quality castings using higher quality metals have lower amounts of entrapped gas.

2. Galvanized (Zinc Coated) Substrates: Outgassing can occur when a powder coating is applied to a substrate that has a zinc-coated surface (i.e. galvanized steel). As with castings, the process of applying the zinc to the steel substrate produces gasses that can be entrapped within this surface coating. This is especially true when “hot-dip” galvanizing is used. However, this does not occur when galvanized steels are used since the surface has been annealed to relieve the stresses and release the entrapped gasses.

3. Coating Thickness: Outgassing can occur when some powder coatings are applied in heavy film thicknesses in a single coat. In this case, gasses given off during the curing process are released through the outer surface of this coating causing pinholes in the surface. Some coating is more susceptible to these problems than others, like Pyramids and emissive Polyurethanes.

Trevor Lodge.

M.D.

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