We powder coat welded steel tubing (ANSI 1008-1010). We have a five-stage washer followed by a dry-off oven before we apply powder coating. On parts that are 48 inches and longer, we often get rust from the ID of the tubing within a few days after powder coating. Do you have any suggestions on how we might eliminate this problem? M.P.
It may seem obvious, but the rust from inside the tubing is caused by subjecting an unprotected steel surface to moisture and air. The solution lies with protecting this inside surface. Since you are using a conveyorized system, the easiest way to add protection is by immersion cleaning and iron phosphating this surface using an off-line process. This can be done in as little as three stages (wash/iron phosphate, rinse, rinse/seal). However, a five stage process, as you perform for the outside of the part, will provide more protection against rust. The greater the protection, the longer the time before rusting will become evident.
If you cannot obtain the desired protection using iron phosphate alone, you may have to apply an organic coating as well. Applying an electrocoat primer after completely cleaning and phosphatizing the part will provide an excellent barrier for corrosion protection. You can then process the part on your powder coating line as before to obtain the exterior appearance and performance properties you desire. The degree of coverage by the electrocoat process on the interior of the tubing is directly related to the ID of the tubing. The larger the ID, the more electrocoating will be deposited within the tubing. Good luck.