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Cloth pouch corrodes zn plate (yellow passivation)

 

Plated CRC sheet metal component (vertical support angle) with zinc followed by yellow passivation. This was later assembled on a networking rack. A red, linen or cotton, starched, pouch, containing a press fit plastic bag with Zn plated hardware, was then tied to this vertical support and the rack was packed for dispatch. However OSAMA happened and the dispatch was held up. It remained so for the next 3 months. The packing was then opened for a re-inspection prior to dispatch. Everything was fine until we picked up the red pouch. The entire small portion where the pouch was in contact with the plated component had severely discoloured and white corrosion had set in. The pouch was slightly stuck to the job. Am unable to explain whether it was the dye on the bag or was it the starch. The problem had never been faced before but that was probably due to no rack lying packed for such a long time. Any Explanations.

Regards,

Karan Gehani
Modern Engineers - Pune, Mah, India

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I don't think it's the dye or the starch, I think it's water. Perhaps the parts were incompletely dried, and the bag and it's starch retained dampness in this area. From the description it sounds like plain old white rust.

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

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Hello Ted,

Thanks for your reply. The jobs were dried and then after 24 hrs age-hardening were sent to the customer. Care for another explanation.

Regards,

Karan Gehani
- Pune, Mah, India

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Well you can change the dye that is used on this cloth, or you can change the sizing material, but I don't think either measure will preclude a recurrence. I think the presence of a hydroscopic material like a starched bag lying on the plated surface is what caused it. Whether the water got there by absorption of humidity from the air, or it retained moisture from a tiny roof leak you didn't know about, or a mouse made this cloth bag his home, white rust is usually moisture damage. That's only my opinion.

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