Metal finishing Q&As since 1989
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Is black film on stainless steel drinking cups hazardous?
Q. I have purchased stainless steel cups (assume food grade 403) for use by school children. I have discovered a black film, not visible to the eye but comes off on my skin or a cloth when rubbed. The film is very difficult to remove. I have been told it's a finish put on the cup to prevent corrosion during shipping.
I am concerned about the children ingesting this film. Regular washing does not remove it. Vinegar
[in bulk on
eBay or
Amazon] seems to assist its removal and Vim cleanser [on
eBay or
Amazon]
seems to do the best job.
Do you think the film is a finish on the Stainless Steel? In your opinion is this film toxic to humans? Can you suggest another solution that might assist with this film's removal?
Thank you.
- Richmond, BC Canada
February 10, 2011
A. A lot of times a black "residue" that shows up on a white cloth wipe is just some residual carbon, freed from the confines of the metal during the passivation acid dip but still clinging to the surface. It is essentially harmless, though certain industries that need a "clean" surface as determined by a "wipe test" get thrown into a panic.
The idea of a finish to prevent corrosion on stainless steel is an odd one given that the entire point of stainless is to be corrosion resistant without an applied coating, but people do funny things. Though I would hope that a coating or finish used on drinking cups would be a non-toxic one, again people do funny things but toxic material on a drinking cup is the kind of behavior that will get a manufacturer shut down by the government so it's doubtful that they would intentionally be that foolish.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
March 1, 2011
Q. Hello,
I recently acquired some stainless steel canteen cups. They are slightly magnetic as a rare earth magnet lightly adheres to their surface. Shaking them forces the magnet to drop. They have a dull gray matte finish on them. Scratching the surface removes the coating. Handling it leaves my fingerprints in the coating. I have sanded this coating off of one of the cups with some 220 grit sandpaper. I plan on using lemon juice to further clean them.
Does anyone know what this coating is? It has the dull grey look of a glass beaded surface. Is it heat related oxidization? Is it simply unpolished stainless steel? Is it a releasing agent from the press mold? Is it safe to eat/drink out of?
Thank You for any help.
D.
- Belleville Illinois, USA
September 19, 2012
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