Don't Blame the Powder
Posted on Saturday, February 1, 2014
I spend part of my life correcting other people’s mistakes. All
the jumping to conclusions provides plenty o’ exercise for many
a powder coating enthusiast. Here are some of the maladies
attributed to our fair technology.
Countless coaters have beseeched me
to help them remediate a catastrophe of
coating failure apparently due to poor corrosion
resistance. Their first inkling is that
they must have used a substandard powder
coating. Almost never is this the case. The
most common cause of corrosion failure is
due to poor metal preparation prior to the
powder coating being applied to the part. Often
a coater feels a solvent wipe will suffice
for an article to be coated that will eventually
be exposed to an extreme environment. It is
rare for a powder to provide any modicum
of corrosion protection with simple surface
cleaning. Epoxy-based powder coatings are
the only products capable of good corrosion
resistance on clean bare metal. Virtually all
other chemistries will fail.
Another scenario where a powder will
fail in a corrosive environment is when the
wrong metal pretreatment is used for a given
substrate. Don’t be duped into believing
a high-quality cleaning/pretreatment system
will provide corrosion resistance if it is not
specifically designed for your substrate. I
have seen fabricators who have been told that
a well-maintained iron phosphate pretreatment
system will provide an excellent anchor
for an aluminum alloy substrate. Don’t be
fooled. Iron phosphate (and zinc phosphate
for that matter) are designed for ferrous substrates.
They will not work on aluminum, no
matter how good its quality control.
Another recipe for corrosion resistance
disaster is undercuring the powder coating.
The metal can be clean and properly pretreated,
however if the powder doesn’t see the
right time at the correct temperature then all
bets are off. I have tested exceptional powder
coating formulas for corrosion resistance and
found that a lack of complete cure will cause
serious corrosion failures. A recent study by
our lab demonstrated how an AAMA (American
Architectural Manufacturers Association)
2605 grade powder prematurely failed salt
fog testing (ASTM B-117) due to a less-than-complete
Powder chemistries most susceptible
to lack-of-cure are polyurethanes and non-
TGIC systems based on HAA (hydroxyl-alkyl
amide). These products require full bake at
the prescribed temperature (typically 375 to
400°F). TGIC polyesters, hybrids and epoxies
have better low cure capability. however
this doesn’t give you license to be cavalier in
control of your part temperature.
Please keep in mind that heavy, dense
parts require longer oven dwell times than
light gauge metal parts. Ditto for higher loading
of parts. And be careful when increasing
the line speed of your continuous conveyor
finishing system. Make sure your parts and
therefore the coating get ample thermal energy
to completely cure the coating. Temperature
recorders, infrared pyrometers and
simple solvent rub tests are instruments
to ensure you’re getting the coating quality
The lesson here is: Do all things right
if you want the ultimate performance from
your coating system. And by “system” I mean
everything from the type and condition of
the metal to the cleaning/pretreatment and
coating cure conditions—not just the powder
When a coater experiences film defects—
specifically, craters—some jump to the conclusion
that the powder coating must be the
culprit. Very seldom is the powder the problem.
Craters are circular interruptions in the
coating that look like—you guessed it—lunar
impressions. These defects are caused by
a significant differential in the surface tension
of the molten coating and a foreign material.
The coating pulls away from the contaminant
leaving a void that can persist all the way
down to the substrate.
Craters almost always emanate from an
environmental contaminant. The worst actors
are lubricants. Silicones and penetrating
oils (e.g., WD-40™) are on the top of the list.
The intrepid maintenance guy who loves to
spray everything in his sight to keep things
lubricated is the first suspect to interrogate.
Materials like these can persevere in the air
for hours and sometimes days. And it doesn’t
take a high concentration to spell disaster. Indeed,
I have seen contaminants cause cratering
at parts per million concentration levels.
Not only can environmental contaminants
wreak cratering havoc, other sources
can be the cause. Poorly filtered compressed
air will guarantee the generation of craters in
your powder finish. It is essential to install
and maintain a high quality filter system on
your compressed air supply. Integral to this
system is a moisture separator installed just
after your air compressor. Aftercoolers that
refrigerate the air and thereby condense
moisture work best. Very close to the point of
your spray gun compressed air supply should
be a filter series comprised of a particulate filter
followed by a coalescing filter to trap any
oil particles before they can contaminate your
Just as important to installing a high quality
compressed air filtering system, is maintaining
it. Keep replacement filter elements
on hand and establish a regular inspection
routine to catch problems before they occur.
Another unsuspected source of cratering
is contaminated blast media. Blast media can
work wonders in preparing a metal surface
for adhesion of a powder coating. It can
erode the oxides that form on a metal surface
and provide a textured surface for the
powder to “bite” into to enhance adhesion.
It also can become contaminated if the metal
surface is oily or greasy. Contaminated media
can transfer foreign materials such as oil
and lubricants to subsequent metal surfaces.
Compounding this problem is the potential
of embedding contaminants into the surface.
These are some of the worst contaminants to
remove from a substrate.
Oils, lubes, silicone sealers and other
powders (especially acrylics and silicone-
based ones) can cause catastrophic
fisheye cratering. Something as seemingly
innocuous as WD-40™ can wreak havoc on
a shop. Contaminated media in a blasting
cabinet can also be a source of fisheyes. The
solution for this lies in isolating the cause
and eliminating it. The source can be the
coating environment or the compressed air.
Overloaded air filters and traps can harbor
compressor lubricants that can be fatal to a
powder coating operation.
In summary, when coating defects arise
it is wise to conduct a thorough analysis of
the root cause of the problem. Remember
that the powder quality is but a minor factor
in causing coating defects. More often the
defect is attributed to an environmental or
process shortcoming. Judicious investigation
of your application and oven environment
and review of your processes will usually
uncover the culprit and mitigate further
Kevin Biller is technical editor of
Powder Coated Tough and the president of
The Powder Coating Research Group. He can
be reached at firstname.lastname@example.org.