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Powder Coat

Powder Coat

Powder Coat


I am a mechanical engineer working at a small company. We produce hydraulic cylinders and we powder coat all of our cylinders. When we powder coat it cant be fully assembled as the heat would melt the internal seals so it is just the welded tube assemblies. We use plugs to prevent any powder coat getting inside as it is honed tube and has to hold spec for the internal components to work properly. However, when we use plugs it seems to always cause flash rusting on the inside of the honed tube. My best guess is that as it heats up and pressure builds the water is pulled from the trapped air and causes the tube to rust.

Can anyone with more knowledge help me understand exactly why this happens and if there are any solutions?

This seems like the best forum for this question, sorry if it really belongs somewhere else.

Thank you,
Garrett Green

RE: Powder Coat

It is not a controlled environment (though I have asked for that in the past). That is why I expect that when they insert the plugs in a humid environment it traps that humid air in then it goes through the wash cycle (cools it down), drying (heats it up), then baking the powder coat (450°F) and during that process the water trapped in the air on the inside causes the rusting.

I guess my question is, has anyone found a good solution to this that doesn't require climate control? my company has refused that option unfortunately.

Thank you.

RE: Powder Coat

You could fill the tube with argon before putting the last plug in. Maybe CO2 would work as well?

Is the furnace sealed? Can you purge it with nitrogen?

RE: Powder Coat

The furnace is not sealed and I am not sure it would help to do that regardless because we plug the tube first. Purging the air inside the cylinder would be possible but we are building 3500-4500 cylinders a month and purging would add a decent amount of time to the process and the cost of the gases. We would also probably have to plug one port with a quick connect fitting that would be destroyed after a few uses and those are expensive as well. I have tried to look into liquids you can swap but at that temp everything (that I have found) would either evaporate off or double the cost of our cylinder.

Thank you both for your input. Both are fixes that would work! I expect that the response from those above me would be "that will be too expensive" for both solutions. That doesn't mean I can't push for one of them but I am hoping someone knows of a relatively quick and cheaper option to solve this.

Again thank you both for the solutions you have already presented!

RE: Powder Coat

Have you considered adding a pickling step after curing the coating? Phosphoric acid is very effective for removing corrosion, test to make sure it doesn't etch your coating.

RE: Powder Coat

Moisture from the air trapped inside being an issue is silly. What are your plugs made from? Water and other corrosives will off-gas from rubber when heated. Pre-baking the plugs may help. Silicone plugs may work better but, again pre-baking may help. Full cure and devolatilization of rubber and silicone take time, and thus many manufacturers don't do it. Measure the weight loss of the plugs upon baking.

RE: Powder Coat

TugboatEng, I will absolutely look into this!

Compositepro, our plugs are silicone https://www.caplugs.com/p/tapered-silicone-caps-pl.... I find the humidity in the air to be more likely though. We have some plugs that we have been running for up to a year and the inside of the tube still rusts when these are used. However, I will still take a new plug and pre-bake it to measure weight loss. If I find a noticable difference I will look into how to Fully Cure and devolatize the plugs.

Thank you both!

RE: Powder Coat

You mention a wash step that happens after installing plugs? Is there any check that plugs haven't leaked (letting water pass to the inside)?

Regardless of the mode of moisture entry, I'd give the i.d. of the tube a quick spritz of VCO (Voltatile Corrosion Inhibitor) spray, or a wipe with a sponge wet with same, before installing the final plug and then sending it.

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