I have been asked to help a business in town resolve what I thought would be a relatively simple question. However, a potential technicality has sidetracked my efforts. Specifically, this business sells and installs powder coated banisters in outdoor applications. They are currently subcontracting the powder coating of the steel pipe to a local business, but try as they might, the internal surfaces of the pipe rusts and discolors the carpentry it is installed with. I suggested that they convert to a chromated aluminum pipe. They can afford the additional cost and were considering it until one of their executives posed concerns about the changes in the aluminum structure (and its impact on strength) that accompanies the heat process of curing the powder. This executive worked in the scuba industry and is adamant about the strength change. Can you advise on this issue or on any other way to resolve the steel pipe rust problem? J.C.
People think consulting is easy. I’m just glad I’m not in the web site design business where any 10 year old with a computer can compete with someone designing web pages. My competition in the powder coating industry is limited to all the powder suppliers, all the equipment suppliers, all the pretreatment chemical suppliers and anyone who has used the powder coating process before—a much smaller group than all the 10 year olds in North America (just kidding). You see, a consultant must not only come up with an answer, they must convince the client that it’s the right answer. This is sometimes more difficult than determining what the answer was in the first place.
In your case, chromate conversion coated aluminum pipe was not the right answer. True, it is much better than mild steel, but the heat in the cure oven will degrade the chromate conversion coating in the uncoated pipe internal surfaces. Any chromated surface that is uncoated and subjected to more than 180F will degrade and not provide adequate corrosion protection. The only difference to what they have now is the rust would be white instead of red. If white rust is acceptable, then they should consider galvanized steel pipe, a much cheaper option to mild steel than aluminum and the zinc in the galvanizing won’t be affected by the cure temperature (usually less than 400F). Both internal and external surfaces of the pipe would need to be galvanized.
If they don’t like the white rust, then they should consider using steel pipe that has been electrocoat primed before powder coating. The electrocoat primer will provide adequate coverage and rust protection on both the inside and outside of the pipe, while the powder coating will provide a very durable and weatherable outside surface. Using a good outdoor formulated powder coating (for example polyester, TGIC polyester, acrylic, etc.) will ensure their satisfaction with the coated handrail.
As far as the “aluminum structure being affected by the curing temperature” question, your client need not worry. The cure temperature is well below any temperature that will anneal (make it soft) the aluminum. In fact, many aluminum parts are powder coated every day without problems, even in the scuba industry. So this shouldn’t be a concern.