Summary for passivation test method Discussion
Passivation test method needed: the thread asks how to evaluate passivation layer thickness by electrochemical DC corrosion testing and, more practically, how to judge the effectiveness of stainless steel passivation. Arik Lin says thickness calculation by electrochemical means is “neither simple nor exact” and points to Vetter’s *Electrochemical Kinetics* for theory and graphical methods. For the stainless passivation question, James Watts recommends the ferroxyl test as a simple, very sensitive check, noting it may be too sensitive or not sensitive enough depending on the application. Lauren Keane adds that the ferroxyl test is very sensitive and says she likes the Koslow Passivation Tester because it avoids chemical mixing, suggesting it for fast testing of many parts. Overall, the discussion favors practical sensitivity tests over trying to directly measure passivation thickness.
Q. How can we evaluate passivation layer thickness by electrochemical method (DC corrosion test)?
Tapan K. Rout
– Orissa, India
A. The answer for that is neither simple nor exact. Calculation of thickness, some graphical representations, discussion, etc., is in Electrochemical Kinetics by Vetter K.J. Academic Press N.Y.(1967) pp 759-773.
Enjoy.
Arik Lin, Electrochemist
– Israel
Q. I am looking for a product or an instrumental technique that is able to determine the effectiveness of a stainless steel passivation process. There are many methods to determine whether stainless steel is passivated, but few that can determine the quality of passivation. I am looking for a method that is at least semi-quantitative so that I will be able to distinguish the quality of passivation between two different passivation methods (ie: nitric acid vs. citric acid).
Heather Johnson
– St. Paul, Minnesota
2000
 | A. Heather: The only accurate quantitative measure of the passivation layer is to run ESCA & AES testing on the surface. This measures the actual chemical composition of the surface, and calculates the ratio of chromium to iron on the surface, and how thick the chrome oxide layer is. There are a few electrical instruments around, but we have not found them to be accurate. We have test data that we can share with you comparing the nitric acid and citric acid systems using these tests. There are a number of test labs around the country (one in Minneapolis) that runs the tests. We can share these with you also, if you contact us. Lee kremer
A. Heather, This is a huge issue. Experience is that Electro Polishing provides the most passive film structure but even that varies from EP process to process. It is also dependent on post polishing processes being correct and not contaminating what was a clean surface. I have used every conceivable technique and am still amused by the degree of variability for every test other than the Scanning Electron Microscope. Mind you, once the EB passes over the piece it is no longer passive! SEM is just not feasible in the work shop! However, to quantify between citric and nitric is pretty easy. Simple EMF / Free Energy calcs will show you the differential in chemical activity. Remember Nitric is an oxidiser and so must be used in conjunction with a reducer such as HF. citric acid and its various formulations has its uses but does not have the solvation characteristics to deal with anything other than poorly bonded surface scales. The SURFIN 2000 proceedings contain a paper on the chemical models for this chemistry if you want further detail I would be happy to assist. Hamish Hunt 2000 A. The typical passivation tests, such as per ASTM A380 The second alternative is a corrosion test. Some sort of electrochemical test to measure the active passive behavior or pitting potential would be best. You may also be able to do some type of chemical exposure, where the time to visible corrosion signs would be your quantitative indicator. I hope this helps.
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A. The ferroxyl test is a rather simple test that is quite sensitive. Depending on your needs, it may be great or too sensitive or not sensitive enough. Try it, you may like it.
James Watts
– Navarre, Florida
2000
A. The Ferroxyl Test is very sensitive. I like the Koslow Passivation Tester because it doesn’t require mixing chemicals. Go for the fastest method for many pieces.
Lauren Keane
Koslow Scientific Company
Englewood, New Jersey
April 22, 2010
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