Ask Joe Powder

Posted on Wednesday, February 4, 2015

Phospate Debate

Dear Joe,

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.

Dear Jeff,

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.

-Joe Powder

A Galvanizing Response

Dear Joe

What is the most difficult material to powder coat?

Frans P., Aruba

Dear Frans

Ice cubes. These are tough to coat because they are very difficult to dry and their dimensions are constantly changing.

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 minimal spangle.

-Joe Powder

Faraday Cage Rage

Dear Joe,

How do you prevent “Faraday”?

Brenda W., Athens, Ala.

Hi Brenda

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 tight corner.

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.

-Joe Powder

Orange You Glad You Asked?

Hi Joe,

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?

Thank you,

Hey Ken,

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 done cleaning.

-Joe Powder

Joe Powder is our technical editor, Kevin Biller. Please send your questions and comments to Joe Powder at askjoepowder@yahoo.com.

Editor’s Note: Letters to and responses from Joe Powder have been edited for space and style.