Chemical passivation for stainless steel parts per ASTM A967

A. You are probably referring to ASTM A967, which spells out how to passivate Stainless Steel. There are 2 ways to do this, one is by a citric acid solution or by a Nitric Acid Solution. The Nitric Acid Solution has 5 different concentrations and temperatures that you can use. Get a copy of this Spec., it should answer all your questions.

A. ASTM A967 has officially replaced but not yet practically replaced QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil]C, both of which describe a method for either

1. removing surface iron with a citric acid solution or

2. removing surface iron & "CONTROLLED OXIDIZING THE NICKEL IN THE SURFACE" with nitric acid, and sometimes with the addition of potassium dichromate on certain alloys.

A. You can obtain a copy of ASTM A967 online. The objective of passivation is to remove iron from the surface so that it will not interfere with the passive layer which will be the chromium oxide (and nickel oxide in those cases where nickel is present). Passivation removes any free iron from the surface and then chromium (and nickel if present) enriches the surface by removing iron from the top 25-30 Angstroms of the surface. This layer then very quickly oxidizes in air to form the passive layer.

Although the nickel in austenitic and PH grades of stainless steel is very important to attain the best corrosion resistance, the semiconductor and orthopaedic industries measure the ratio of the chromium oxide to the iron oxide in the surface, and measure the thickness of the chrome oxide layer to determine the degree of passivity. This way of measuring (through ESCA & AES testing) has been proven to correlate to corrosion tests. The reason citric acid proprietary formulations attain so much better and thicker chromium oxide layers (and better corrosion resistance to salt spray, etc.) is because the chromium (and nickel when present) is left on the surface when using citric, while much of it is eaten away with nitric along with the iron. The steady state of the reaction leaves a much higher % of chromium on the surface with citric. I cannot write all of the test data here, but there are MANY reports and papers presented showing the increase in chrome oxide layer attained with the citric formulations. With the ferritic and martensitic grades of stainless steel (where little or NO nickel is present) it is the only way to attain good passive surfaces and corrosion resistance. Grades like 410, 440C, 420, 416 can be passivated very well with the proper procedure using citric proprietary formulations (not just citric acid ).

A. Xujin,

ASTM documents are protected by copyright, so I cannot send it to you. However, you can BUY it from ASTM at www.astm.org.

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Ed. note: One place to start would be this site's "Stainless Steel Passivation FAQs"

A. Johnny,
ASTM A967 as mentioned above gives a fairly good overview of passivation.

Is there an existing passivation line at your company or are you tasked with creating one?

adv.
If you have any specific questions, I would be happy to help.

A. Carlos,
The dessicated containers I've seen are essentially glass jars filled with a desiccant on the bottom with a rack over it, and a lid sufficient to keep an air-tight seal. You can probably buy one from a lab supply company.

Since the standard is vague on the matter, it's pretty much your own judgement call. As long as it looks dry to you, I'd go with it. Despite the standard, there's frankly not much need for the part to be dry if it's going STRAIGHT into the humidity chamber.

The proverbial little bird told me that the next revision of A967 may be reducing the burden here.

A. Good day Carlos.

I think this spec is a little vague also. How much moisture can a passivated part entrap?
I use the desiccator to dry my hard anodize panels for Taber abrasion, giving them 24 hrs.
I rely on weight loss to determine if samples are "dry".
Hope this helps.

Regards,