Q. We apply powder coating over steel parts that have been treated with a zinc phosphate. Some parts show bubbling after running through the powder line. The parts were clean and rust-free before running. What could cause a bubble/blister on steel parts? I have seen it on zinc die-cast parts where cold flow happens and we have tumbled them too long, but not on steel parts. K.O.
A. Bubbles in a coating applied over a casting are caused by trapped porosity. Heat causes air or moisture trapped in porous pockets in the substrate to rupture through the coating as the air escapes. Bubbles in coating over a steel part also are most likely caused by trapped air or moisture. If you have bends in the steel, you can create micro-fissures that will hold air. If you have contamination on the surface (oil, unreacted chemicals, salts, etc.) you can trap moisture that will boil out during cure.
Another possible source of bubbles is back ionization. If the micro-amps are high or the film is too thick, you will create too much negative energy at the surface and generate positive ions that will blow holes or texture in the coating.
Do you use a polyester urethane powder? If so, check the film build. A thicker film may cause some pin-holing as the missive component in the powder evolves from the film during cure.
Have some test parts cleaned at a different facility and see if you still have the problem. This will indicate if the problem is the pretreatment or some other cause. Preheat the parts to relieve the trapped material. Test a powder formulated for extended flow with an agent that helps to avoid the bubbles. Try using a longer cycle for cure with lower temperature. Make sure the powder gun is set up for low amperage (10 microamps or lower).
A lot of things could cause your problems. I hope that this advice points you in the right direction.