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Problems in Powder Coating of "Black Annealed" CRS

Q. Hi,

We make furniture using tubes (around 1 mm thickness) using cold rolled tubes.

We used to buy tubes with a shiny color. Lately we changed our tubes supplier. The tubes we received from our new supplier are black annealed CRS. When we sent our products to the powder coater, we got feedback that some sort of grains appeared on the surface of the tubes after coating.

powder coating is grainy / gritty

I have a few questions:
1. Is black annealed CRS not suitable for powder coating?
2. Besides the bad aesthetic look of our products because of these small grains, is there any functional drawback which might occur? (For example will the adhesion of the painting be less?)
3. If the adhesion will be lower and we might face the risk of painting falling off the product, is there any change we can instruct the coater to do to alleviate this problem (like exposing the product to a longer period in the pickling tank)?

Thank you

Nader FAM
January 3, 2024

A. Hello Nader.
In general, 'black annealed' seems to refer to annealed material which has received an oil treatment which gives it a black color. I suspect that it is theoretically possible to remove that oil coating, but that your powder coater uses a mild pretreatment sequence which does not remove it, and the result is an outgassing upon heating which creates those tiny volcanoes we see on the surface.

I would suggest asking your powder coater to scrub a few parts with a scrub brush [on eBay or Amazon (adv.)], wetted powder pumice [on eBay or Amazon (adv.)], and a little detergent as a pre-cleaning before the parts are sent through his pretreatment and powder coating system. If the sample parts still exhibit the defect, the answer is that you cannot use this material. If the sample parts come out okay, you will be able to look at your overall logistics and decide whether your best move is finding a different tubing supplier, finding a different powder coater, or requiring one or the other to rectify this oil problem.
Luck & Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

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