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How to protect internal surfaces from plating while outside is zinc plated

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We are having some difficulty rack zinc plating steel tubes that require an external thickness of 8 microns but must not have any internal plating (they are power steering cylinders). The tubes are from 45 - 50mm in diameter and 250mm in length and have two holes that are 20mm in from each end. We have concluded that the tubes must be held vertically in the rack to allow for drainage of the solution. The tubes are held via two nylon plugs backed with steel contacts. The original idea was that the nylon plugs would insulate the inside of the tube from the current. That didn't work so we've resorted to placing rolled up A4 overhead transparency sheets inside, which is not very desirable. The tubes can't be sealed because of the plating temperature.

Would a more cathodic steel placed inside the tube be worthwhile?
Is there another method to protect the internal surface?
Is there a way to stop the current flowing internally?

Thanks in advance.

Andre Lees
TRW Australia

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I would think that thieves would successfully draw the plating current away. I would picture a screen, at cathode potential, capping these tubes or placed with a very small gap between them and the tube. A piece of wire as a thief should handle the two holes in the side of the tube.

Ted Mooney, P.E. finishing.com Brick, New Jersey