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Stainless Steel is Suffering Black Patches after Passivation



November 28, 2008

I have passivating instruments in nitric acid, I am immersing them for 1 hour and some of the instruments get black patches on them after the passivation.

The steel is Grade 420.

Any ideas what could be causing this? I am carrying out no other processes.

John Watford
Finisher - Nottingham, UK



December 3, 2008

John,

You need to give us some more details such as which specification the parts were processed to, what method (e.g. nitric, nitric/dichromate, citric etc.), what your cleaning regime is, what the heat treatment state of the alloy is, whether there is any scale, your analysis of the solutions, etc.

Brian Terry
Aerospace - Yeovil, Somerset, UK



December 9, 2008

Hi,
I have used nitric acid in this instance, the items have come to us cleaned after polishing in an ultrasonic washer. The alloy has been tempered, I am not sure about scale or solutions?

What is the best way to test passivation and test resistance to corrosion by stainless steel, at the moment I am using a boil test of boiling RO water and immersing the items in there for 24h.

John Watford
Finisher - Nottingham, UK



December 15, 2008

According to AMS2700 [affil. link] and QQ-P-35C [link is to free spec at Defense Logistics Agency, dla.mil] you should really be using a nitric/dichromate solution as described in method II of QQ-P-35 and methods I, II & III in AMS2700 [affil. link]. This may help with the black smut problem.

Testing of the passivation can be done in various ways. The quickest is the acidified copper sulphate test. This is usually used as a rapid test for the presence of free iron. It has variable results on certain alloys and may give a false result (a failure when the passivation is fine).

Other methods include humidity, salt spray and water immersion. These methods can be found in the AMS or QQ spec.

Brian Terry
Aerospace - Yeovil, Somerset, UK


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