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.