Metal finishing Q&As since 1989

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Clear zinc plating develops brown stains with time!

Q. Acid Zinc barrel plating with trivalent Blue passivation on mild steel CRCA pressed component, discolouration into light brown on the LCD / step area after 2 months. 200 pcs out of the total assembled 5000 components in one particular lot supplied. End customer has inspected and confirmed that it is definitely not red rust.

We are suspecting plating solution being entrapped in the porous area and slowly leaking out causing this discolouration. Please share your thoughts on this. And how to eliminate this problem.

Yukash Ganesan
- Chennai, TamilNadu, India
January 20, 2022

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⇩ Closely related postings, oldest first ⇩

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June 9, 2008

Q. We have some chassis that have developed brown stains over time (approx. 1.5 years).
They were built and plated in Minnesota, spent some time in New Jersey, and are languishing in Austin, Texas.
The plater says they can't figure it out.
They were packaged in a standard type corrugated box- not sealed up in a bad with dessicant.
Some of the badly stained areas have been in direct contact with the corrugated.
Other areas were not. It does not appear to be a plating solution entrapment issue.
I am thinking it is packaging related, with humidity contributing during storage.
I've heard the acid in corrugated or brown kraft paper can attack zinc plating with a chromate conversion coating.

        

(click pictures to see 4x as large)

We need to re-make the chassis's, but I am reticent to use the same suppliers when they have not determined a root cause and cannot guarantee this won't happen again.
Experts- please help!
Thanks,
Jon

Jon Perdue
product designer - San Jose, California

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June 12, 2008 Expert help on line is easier said than done. Remember, the definition of an expert is someone who comes from over 100 miles away! Seriously, it is difficult or not impossible to determine the problem from the pictures. Perhaps your vendor can get a failure analysis from the technical service department of one of his vendors. Otherwise, I feel that if you are determined to find the cause of the failure it would be necessary to hire a consultant. Any yes it is possible that the box contributed to the problem. It is also possible that the environment in which the parts were stored contributed (not the geographical location but the specific location). And for further confusion, yes it is possible that a deficiency in the plating was highlighted by the first two. Gene Packman - Great Neck, New York Brown corrugated boxes and any brown paper is a source of sulfur [affil links] which reacts with humidity and corrodes zinc and steel. This corrosion is even moire likely with clear chromate than heavy yellow chromate. And it is less corrosion when the chromate has been topped off with one of the newer silicate type top coats. But how about thickness, was his zinc so thin that the chromate ate most of it off, leaving you closer to the iron which goes into "brown" oxide. Take the parts to a local plater who has a magnetic type thickness gauge and check the thickness. You should have at least 0.0002 or 0.0003 inch after chromate. Tell your next plater the thickness you want after chromate, and to wrap in clean sulfur free paper. Robert H Probert Robert H Probert Technical Services Garner, North Carolina June 12, 2008

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Brown stain on zinc plating

Q. HI EVERYBODY, WE HAVE THE PROCESS ZINC ELECTROPLATING , AT THE LAST DAYS I HAVE SOME BROWN STAIN ON THE PIECES , SOME OF THEM ARE IN CIRCLE BUT OTHER ARE ASYMETRIC, DO YOU HAVE ANY IDEA WHAT'S HAPPENING? I MADE IRON TREATMENT, FILTER SOLUTION BUT THEY DON'T DISAPPEAR.

MARY CARMEN RETANA
RESPONSIBLE FOR GALVANIZE - MÉXICO
September 17, 2019

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Ed. note: Here's Mary's pic which I apparently misfiled: