Letter 30658

How does copper sulfate test for passivation work? [Indiana] 

(2004)

Dear Readers,

I have heard that a way to check for iron on the surface of a passivated stainless steel part is with the copper sulfate test. From what I can gather the copper replaces the iron due to the fact that the iron is more easily oxidized. Chromium is more easily oxidized than iron, so why doesn't the copper also replace the chromium?

Thanks for your input.

Shaun C [name deleted for privacy]
manufacturing w/ in-house plating - Sheridan, IN, USA

Chromium and nickel will not be replaced with the Copper sulfate test solution (which also contains sulfuric acid), only the Fe or other less noble metals. If there is free iron on the surface it will be replaced by the copper, forming the distinctive copper color on the surface in this 6 minute test.

Of course, this test should only be used on austenitic stainless steel. Some people use it on other grades, but you can get false failures.

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA

Why is it that Fe is less noble than Chromium? From their locations it appears that Iron is more noble.

Chris L [name deleted for privacy]
co-op/manufacturing engineering - Sharon, MA, USA

I need to determine effectiveness (pass/fail) of an up-coming passivation procedure. I have read that using copper sulfate is a proper method, however, I have not found a test kit, only the copper sulfate itself. How do I go about constructing a procedure or test method?

John Stiglbauer
- Columbia, South Carolina, USA

^-- I want to contact this person and my company is a supporting advertiser of finishing.com

To answer John's question, I'd look up a standard test and use that, rather than make my own. Primarily because I'm lazy, but you can also argue that something published by a known organization probably has had some thought behind it, maybe it's been vetted as well.

So I'd go to Assist Quicksearch by typing that into Google and hitting "I'm feeling lucky". Then I'd type "753" into the number box, and click on the icon that will display Mil-Std-753 revision C. Then I'd read that one, and if it met my needs, I'd use it.

Lee Gearhart
- East Aurora, NY

I want to answer or follow-up on this subject publicly (in non-commercial fashion).

 

I want to post a new question or inquiry of my own on a different subject.