Ask Joe Powder
Posted on Wednesday, February 4, 2015
Can a standard iron phosphate pretreatment
be used as a corrosion inhibitor for
steel? That is, can I process my parts with
iron phosphate at one location and then
ship them to another location for powder
coating? The parts are made in Asia and the
coating process is in the United States.
Jeff S. in Conn.
I would strongly advise against this strategy.
Iron phosphate pretreatment is used to provide a
chemical anchor between the ferrous substrate and
the organic coating, in this case powder coatings.
Iron phosphate pretreatment was never intended
as a protective coating for steel. The amorphous
structure formed on the metal surface is relatively
porous and will allow corrosive oxidation
especially if transported in a saline atmospheric
environment. I would recommend either a rust
preventative compound specifically designed for
the protection of steel or an electrocoat primer.
A Galvanizing Response
What is the most difficult material to powder coat?
Frans P., Aruba
Ice cubes. These are tough to coat because they are
very difficult to dry and their dimensions are constantly
If you’re referring to materials commonly encountered
in a factory, then I would say highly spangled hot
dipped galvanized steel. Hot dipped galvanized steel is
difficult to coat because the zinc alloy produces hydrogen
gas in the oven which can cause pinholes with most
powders. In addition highly spangled (“spangle” refers
to glittery star-like formations) galvanized surfaces are
a bear to adhere to. So even if you overcome the pinholing
issue you may run into problems with acceptable
coating adhesion. Hence I recommend customers use
electro-galvanized steel or hot-dipped with
Faraday Cage Rage
How do you prevent “Faraday”?
Brenda W., Athens, Ala.
By “Faraday” you undoubtedly are referring to
the “Faraday Cage” effect. This is the issue of penetrating
a tight inside corner with electrostatically
charged powder coating. The electric field produced
by the corona charge delivered by the gun
creates a dead space of charge within the corner.
This dead space repels the deposition of powder.
In addition the air volume exiting the gun creates
blow-back the further impedes powder deposition.
So not only do you get electrostatic repulsion but
pneumatic repulsion as well.
So, how do we mitigate this common problem?
A few actions will eliminate this issue. First, use a
powder spray gun that has a charge limiter device.
Most of the recent models have this feature. It
essentially detects a build-up of excess charge in
the Faraday Cage and automatically reduces the
current of the corona charging tip. This diminishes
electrostatic repulsion. If you do not have a gun
with this feature simply reduce the current of the
gun by about 30 percent. For example, if you are
spraying at 18 microamps, cut back to 12 microamps.
This can usually be accomplished by simply
reducing the gun voltage.
Second, use a high volume/low air velocity
powder pump system. The major powder gun suppliers
have this option which conveys a very high
concentration of powder with significantly less air
velocity. This helps with Faraday penetration by
delivering more powder and less air to the
Third, use a slotted gun tip instead of a fan
spray tip. This will concentrate the powder stream
to the corner.
Fourth, ensure that you have a good connection
from your parts to ground. You can check
this visually and also with a megohm meter. You
should measure less than 1.0 megohms. It is important
to use a megohm meter and not a simple
volt-ohm meter. Megohm meters actually put out
voltage that more precisely ascertains if an acceptable
ground is present.
Good luck, Brenda.
Orange You Glad You Asked?
We bought a powder coat system and love it. I have one question, there are black scuff marks on the floor. In the past, the guys have used plastic pipe cleaner to get them off. This sounds scary to me. What would you use to clean the marks off, that does not have the volatile vapors?
I would stay away from using the plastic pipe cleaner.
The solvents contained in it are flammable and present a
serious hazard in your shop. I recommend that you use
a citrus-based cleaner. I learned this trick when my son
worked in the detail department at a car dealership. The
guys would swear by this stuff. It’s safe, bio-based and
leaves a pleasant orangey smell when you’re
Joe Powder is our technical editor, Kevin Biller. Please
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