Outgassing in Powder Coatings
Posted on Tuesday, April 7, 2015
Outgassing describes the phenomena of entrapped
gasses being released through a powder coating
during the cure 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
Causes of Outgassing
Outgassing can be attributed to the substrate material, a
surface contaminant, or the powder itself. Following are the
1. Castings: Outgassing can occur when a powder
coating is applied over a cast metal surface (iron, steel,
aluminum, 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
galvannealed steels are used, since the surface has been
annealed to relieve the stresses and release the entrapped
3. Surface Contamination: Outgassing can occur when
a powder coating is applied over a substrate that has a
contaminant on the surface. This contaminant can be oil,
grease, mold release, or any other material that can vaporize
through the powder coating during the curing process.
Although this situation is normally considered improper
cleaning of the substrate and not an outgassing problem, the
defect on the coated surface is the same.
4. Coating Thickness: Outgassing can occur when some
powder coatings are applied in a heavy film thickness in
a single coat. In this case, gasses given off during the cure
process are released through the outer surface of this coating
causing pinholes in the surface. Some coating are more
susceptible to this problems than others, like Primids and
How to Eliminate the Effects of Outgassing
Following are the different methods that have been
proven to eliminate this problem:
1. Preheating the Part: This method is the most popular
to eliminate the problem of outgassing. The part to be coated
is preheated above the cure temperature for at least the same
amount of time to cure the powder to allow the entrapped
gas to be released prior to applying the powder coating. This
solution may not eliminate all the outgassing if the part has
an enormous amount of entrapped gasses, where the gas just
keeps being released, no matter how much or how often you
preheat the part.
2. Seal the Part Surface: This method requires the
application of a material under pressure that is used to
seal the entrapped gasses within the substrate, therefore,
eliminating the outgassing from occurring entirely. Search
for companies that specialize in impregnating/sealing casting
technologies to obtain more information.
3. Change the Curing Technology: A change in curing
technology to IR or IR/UV can eliminate the outgassing
problem since the only the part surface is heated to cure the
powder coating. In this case, the part substrate is not entirely
heated, a necessity to release of the entrapped gasses.
4. Powder Formulation: The coating powder used for
your specific application can be changed by your powder
supplier to have enhanced flow characteristics. This means
that the powder will stay in a liquid form for a prolonged
period of time during the cure process. This allows the
entrapped gasses in the substrate to escape when the coating
is still liquid and flow over the pinhole, creating a smooth
and hole-free surface.
5. Change or Improve the Substrate: Substituting the
casting material with one that has less gassing issues can
be a graceful solution. Working with your casting supplier
to add vents or chills in particularly problematic areas is
another area that can improve the substrate or eliminate the
6. Eliminate the Contamination: Parts that have surface
contamination are best corrected by eliminating of the
contaminant. Identify the contaminant and remove it prior
to powder coating and this problem will go away.
7. Control the Coating Film Thickness: If the
outgassing problem is caused by excessive film build on
the part, then the easiest way of correcting the problem is
to reduce the film thickness. If a heavy film thickness is
required for the application, then select a different coating
material or apply the coating using two thinner coats.
Now that you know what causes outgassing and some
methods to eliminate it, you are better equipped to prevent
coating failure and poor coating appearance.
Nick Liberto, P.E., is president of Powder Coating Consultants,
division of Ninan Inc., an independent technical consulting
firm in Bridgeport, Conn. He can be reached at email@example.com