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Letter 29035
Iron release from aging passivated 304
steel
++++
We passivate stainless steel type 304 with 37 % nitric acid
40°C if we take 2 items and test 1 after passivation it shows
nearly no release of iron, if we after some try the other item there
is suddenly iron release. Does anybody know why ?
Bo Kønig
Food industry - Odense, Fyn, Denmark
++++
You should get excellent passivation with this. What test method
are you using to determine free iron on the surface? What is the
second part subjected to between the time of passivationa and the
time of testing? Even your first sample should show NO free iron on
the surface after this passivation in nitric.
Any carbon steel being worked on in the same shop anywhere? If so,
it is likely that you are getting iron dust from the air.
++++
The item is a tube with a valve in one end, so my test method is
to fill the tube with 1 % phosphoric acid, leave it for 3 minutes and
then measure the rise in iron content in the acid. First test shows
very little iron release, like 1/10 of my costumers demand. I believe
you are right to think that item get polluted with iron during the
storage. I have started test, where I wrap the item in plastic after
passivation and then leave them in storage for 2 month; only problem
is, my costumer wants answer now.
Bo Kønig
Food industry - Odense, Denmark
++++
Phosphoric acid is not a viable test for free iron on the surface,
as it tends to etch into the steel. I realize that your customer may
be demanding this test. It might be better to ask him to use one of
the tests in ASTM
A967 [link is to spec at TechStreet]. You may want to
consider a better passivation method, also.
++++
You are right Lee, costumer demands this test, because the liquid
is acidic (pH between 4 and 5) and would get taste of iron if posible
to dissolve from tube. So in costumers mind this kind of test is
closest to real world. What better passivation method would you
suggest ?
Bo Kønig
Food industry - Odense, Denmark
++++
There is a couple of excellent German reports that shows how much
better citric acid formulations are for removing iron than nitric is.
One of these is an old study by Adolph Coors. The other one is by NMI
Research:
NMI
Naturwissenschaftliches
und Medizinisches Institut
an der Universität Tübingen
Markwiesenstraße 55
D-72770 Reutlingen
Attn: Dr. Reichl
If you need copies of these contact us. Is the acidic nature of
the product in the tube in end use an organic or mineral acid?
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