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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.
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.
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.
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 [adv: item on
eBay &
Amazon]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.
Aerospace - Yeovil, Somerset, UK
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