Summary for microbubbles in epoxy powder coat Discussion
microbubbles in epoxy powder coat are reported only along silicon-bronze weld zones on a zinc-plated mild-steel enclosure, with some bubbles reaching the base metal and others staying near the surface. Murray Love notes that zinc plating thickness and current-density variation can create uneven results at inside corners and weld areas, so the welds may not be the only factor. Bill William Doherty adds that silicon-bronze welding often leads to heavy anti-spatter spray use, and warns that many anti-spatter products contain silicone, which is incompatible with powder coating. He suggests checking the anti-spatter used and trying a silicone-free alternative. The thread does not prove a single cause, but it points to the weld area, plating distribution, and possible silicone contamination as the main suspects.
Q. We are manufacturing a customer’s electrical enclosure design for the rail-transit industry. The enclosure is a folded sheet-metal box made of mild steel. The corners and flanges of the box are seam-welded with silicon bronze to create a one-piece sealed look. The box is currently zinc-plated to ASTM B633 Type V (RoHs-compliant) before powder coating using an Axalta epoxy powder.
Although the finish is generally very good, we are seeing lines of microbubbles (~0.020″ max. dia., though most are much smaller) forming in the powder coat on all the silicon-bronze welding locations, and only on those locations. Under magnification, some bubbles appear to go all the way to the base metal, others do not. Some welded areas have few bubbles; others have a line of bubbles all the way along the weld zone.
The powder coater is looking into it, and thinks the silicon bronze might be offgassing. In the past, I’ve designed parts with clear zinc over silicone bronze, and have never had a problem, but have never powder coated over it before. Any suggestions as to what might be happening here?
Murray Love
Manufacturing Engineer – Victoria, BC, Canada
March 29, 2016
A. Hi Murray.Remember that the zinc plating distribution varies according to the current density and might account for having problems at some welds (inside corners) and not at others. You have silicon bronze welding, zinc plating, chromate conversion coating, and powder coating — so, although the fact that the problem occurs at the weld areas makes us suspicious of the welds, other factors may be at work besides the welding. A picture might help.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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March 2016
Q. Thanks for your reply. I have attached a photo showing the bubbles in the powder coat.
Murray Love [returning]
Manufacturing Engineer, BC, Canada
March 29, 2016
March 30, 2016
A. Your situation is an unusual one.
Normally pre-galvanised sheet metal would be Si-Br welded and then pre-treated and powdercoated.
You appear to be using MS Sheet Si-Br welding and then electroplating the fabrication?
In either instance as Si-Br welds create so much weld spatter then there is a tendency at the fabrication stage to rather over do the anti-spatter spray.
Many of these contain Silicone, but there other effective anti-spatter sprays which are Silicone-free.
I suggest you look into the anti spatter used.
Silicone and Powder are totally incompatible.
Hope this helps.
Regards,
Bill
William Doherty
Trainer – Salamander Bay, Australia
Thanks, Bill. You’ve correctly described the fab process for this part.
This issue is related to another question I asked a couple of months back (letter 5163).
Our clients specified powder coat over mild steel with untreated threaded holes in the enclosure. We recommended zinc-plating to protect the threaded holes from corrosion, and to protect the substrate in the event of paint chipping. Pre-plated steel would protect the general substrate, but (as far as I know) not the threaded holes. Or am I mistaken?
Also, I hadn’t realized you could Si-Br weld pre-plated steel. Good to know.
This may have been a bad call, but we’re stuck with it now, and trying to make the best of it.
Murray Love [returning]
— – Victoria, BC
March 30, 2016
March 31, 2016
A. Hi Murray,
The use of pre-galvanised (electro-galvanised) steel and subsequent fabrication operations with Si-Br welding is very commonplace in Australia.
The idea, of course, is to present a galvanically protected substrate for further protection; first chemically with a suitable pre-treatment, in this case Zn phosphate, and then to further provide a barrier protection with, in your case, Epoxy Powder.
As for your earlier enquiry: Zn Phosphate as a pre-treatment provides a crystalline layer which is quite conductive.
Hope this helps,
Regards,
Bill
William Doherty
Trainer – Salamander Bay, Australia