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Flattening the end of mild steel tubes leads to corrosion?

Q. We are working with some tubular structure designs. I have been told that flattening ends of MS tubes which are then powder coated tends to have corrosion due to moisture having entry points at the ends of tube. We then get corrosion leeching out from the ends in (outdoor product) the rains making things ugly apart from the obvious corrosion. This is a consumer product so aesthetics are key factor. Our powder coats have a CED base.

Given that this is such a commonly used, cheap and effective way to make tubular products I was wondering what the general consensus in the industry is with this issue?

Roy Allan
- UK
September 11, 2019

A. Hi Roy. Moisture accumulation & entrapment are certainly a corrosion issue, and this is somewhat amplified by the fact that the stressed metal is anodic to the rest of the tube, tending to sacrificially corrode. I think the best answer (in addition to designs which discourage water from getting in and encourage it to get out) is proper zinc phosphatization, plus your e-coating before powder coating.

I bought a backyard gazebo/screen tent constructed with such flattened end tubing and it was rusting in under a week, so I appreciate the issue.

opinion! My strong suspicion is that no attention had been given to proper phosphate pretreatment -- especially because there is zero environmental pressure towards sustainability and making metal fabrications durable, but enormous pennywise/pound foolish environmental pressure towards harassing finishing shops to reduce pretreatment chemical usage, sabotaging any sustainability efforts :-(

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2019

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