"Ferroxyl" Test for Passivated SS-304
Can some one furnish me the details of "Ferroxyl" Test for passivated SS-304. I am using SS-304 tank for 93.5% concentrated sulfuric acid service and have specified passivation of the tank. Vendor has proposed two types of tests (i) Ferroxyl, and (ii) Koslow-2026 Passivation Tester.
Any recommendations on which test to follow?
Thank you.Gopal Murti
The ferroxyl test is described in detail in the ASTM specification ASTM A380 [link is to the practice at TechStreet] . You can obtain the standard on-line (for a fee). It is a relatively simple test, but uses a cyanide compound which should only be used by personnel qualified to handle hazardous chemicals.
Check out ASTM A-380 for the recipe for the ferroxyl text. However, it is just an indicator for free iron. The metal turns blue in the presence of any iron. People I have spoken with say it is easily contaminated and gives false readings. My only time using the ferroxyl had the solution contaminated before use. Granted, it was probably my poor lab technique but I wasn't impressed.
The A-380 standard also talks of the copper sulfate. Most of the "old hands" I have spoken with like this more. It seems to be more forgiving.
The Koslow has some merits. An associate of mine described its application as an indicator to the passivation trend of the metal. As stainless steel passivation is not an either/or condition, the Koslow will show whether the metal is tending toward being more passive or less passive.
To be quite frank, at this point he started using more and more multi-syllable words and my eyes glazed over :-) I got the basic idea but can't fully explain it in a scientific way.
Good luck on your tests.
By the way, have you checked out ASTM A967 [affil. link to spec at Techstreet] rather than A-380? I am now using it and really prefer citric over nitric from a safety and environmental perspective. Some recent observations on an industrial project are implying it does a better job of free iron removal.
Stay safe.Todd Turner
- Monroe, Louisiana
Just a note on the dangerous cyanide in the test solution. It is also used as an anti-caking agent in table salt.
surface treatment shop - Stroud, Glos, England
The Ferroxyl Gel used in testing porosity is at least as old as 1941, from where I found one formula in a Formulary:
potassium ferricyanide...5 grams
potassium ferrocyanide...0.5 grams
sodium chloride...........10 grams
distilled water to make 1 liter
Falls Township, Pennsylvania
It is much easier to use the oxiliser test; it is more accurate than the ferroxyl test and not harmful.Frédièn Vermeire
- St-Niklaas, Belgium
April 4, 2008
We are trying to determine porosity in Ni coatings over steel. During ferroxyl test, the entire surface turns blue immediately as it is dipped in the ferroxyl test solution. Will ferroxyl test turn blue even in the absence of iron? what can be the reasons for 'false positive' in a ferroxyl test?Prat George
metallurgist - Baton Rouge, Louisiana
April 15, 2008
"We are trying to determine porosity in Ni coatings over steel. During ferroxyl test, the entire surface turns blue immediately as it is dipped in the ferroxyl test solution. will ferroxyl test turn blue even in the absence of iron? what can be the reasons for 'false positive' in a ferroxyl test?"
Ni coatings are iron pigmented, the ferroxyl test will be always positive.
To determinate porosity, you must use the spark test. The voltage to use depends on the dry film thickness.Frédièn Vermeire
- St-Niklaas - Belgium