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Black "rub" on stainless food equipment

January 5, 2009

Q. Technically, this may be an "un-finishing" situation

We have a problem with black rubbing off a type 304 stainless steel conveyor belt and getting onto the food product. The belt is used in a steam blancher running at not much above boiling.

We've installed hundreds of these and have never had a problem in the past. Generally, any "black rub" (as it's referred to in the food business) calms down in short order and goes away. However, in this particular unit it seems to keep coming back a day after cleaning. We've tried passivation with citric acid as well as various cleaning methods to no avail. The cleaners the customer uses are generally mildly chlorinated and tend toward basic but have not (according to them) changed from the past and don't seem to be different than we usually see

This is a replacement belt in an existing piece of equipment which the customer says never showed the problem before.

I also can't find any mechanical rubbing that could be inducing the problem.

Any ideas on ANY path to try and alleviate this problem? I'm simply looking for any direction to go as we've exhausted all the simple and usual fixes.

Krey Miller
Fabricator - Kent, Washington, USA

January 8, 2009

A. Hi, Krey. Often such problems resolve to some employee doing something that the engineers and management are not aware of. I would think that someone is wiping this equipment with bleach or using significant amounts of bleach or muriatic acid [affil link] in close proximity. Has the installation recently had floor tiling done?


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

January 9, 2009

A. Are you sure that the stainless steel is the same grade that you have used successfully in the past? I don't mean whether you specified it the same - I mean is it actually the same?

Bill Reynolds
Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.

January 12, 2009

Q. Thanks for the responses.

I'm checking to see if there is any evidence of chemical use that is different than stated or if anything odd has been used recently in the area.

We have the wire mill's spec sheet for the batch of material and it matches T304 but have not done our own testing on it. We may have to do that but I am still holding out hope of a "magical" solution being found. 99.9% of the time, the answer is something simple that's been overlooked.

Thanks again for the help

Krey Miller [returning]
- Kent, Washington, USA

Can dry citric acid powder passivate stainless steel?

November 8, 2019

Q. Can dry powdered citric acid be used for passivation. I work in powdered metals and we are having issues with our stainless steel belts not rebuilding the chromium oxide layer which is causing the belt to stick to the iron parts and causing slag. I know that this is a bit different from finishing metals but I'm out of ideas.

Amber Lawson
- Brockway

December 13, 2019

A. Amber,

Can dry powdered citric acid be used for passivation.

Well... if you add it to water, yes.

We are having issues with our stainless steel belts not rebuilding the chromium oxide layer which is causing the belt to stick to the iron parts and causing slag.

The chromium oxide layer protects against corrosion. I wouldn't expect passivation or lack thereof to have any effect on "sticking" to iron. Perhaps this would work better for you if the surface of your belt were smoother?

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

December 16, 2019

A. Amber, you can purchase spray-on citric acid passivating products from The Rust Store (which I have no affiliation with whatsoever):

Brian Johnson
Large Synoptic Survey Telescope - Tucson, Arizona, United States

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