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Problem with passivating SS410 screw with 25%-45% Nitric Acid

2005

Dear all,
I worked for a plating company in Singapore. As I just started working for a few months, I encountered rolls of problems. Here is one of my problems.
Our company is doing passivation on mostly SS302 material(also some other SS3** series materials), which had no problem. Recently we tried to passivate and SS410 material, but it become black after the process.
Our process is normal alkaline degreasing--> Nitric Acid passivation(25%-45% Nitric Acid, 50-60 Deg.C, 30 mins)--> neutralization(dilute NaOH)--> DI water rinse.
Anybody know what's the cause for this kind of problem, and can it be solved?
Thanks for your support.

Best regards,

Bob Wang
Plating shop - Singapore

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2005

For the 400 series Stainless Steel you should try the following formulation
20 % Nitric Acid
6 % Hydrofluoric Acid ( added as Ammonium Bifluoride )
1% Hydrochloric Acid
This will etch the parts but will give a goos oxide coating
Regards,

John Tenison - Woods
- Victoria Australia

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2005

Dear Sir,
Thank you for your reply. I will do some testing on your recipe and keep you updated of my progress.

Best regards,

Bob,Wang
plating house - Singapore