Passivation Test Method Needed

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.

A. The typical passivation tests, such as per ASTM A380 , are generally 'go-no go' types of tests. I can think of two options for a quantitative/semiquantitative evaluation. The first is an analysis of the chemical composition and thickness of the passive layer on the passivated materials. This could be done by Auger Electron Spectroscopy or X-ray Photoelectron spectroscopy. The composition, thickness and chemical state of the metal surface are good semiquantitative indicators of the passivations state.

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.