I am not corrosion expert, however, my answer is as follows. You must remove the oxide layer from the specimen surface before carrying out corrosion tests on the Al alloys. You can remove it by normal polishing using different grades of emery papers (say up to 2500 grade). The actual corrosion behavior will not be reflected unless you remove the already existing oxide layer, which acts as passive layer. Hope this will help.

If you perform electrochemical testing for egsamample according to ASTM G-5 standard it is recommended to remove the oxide layer. The existing oxide layer on your sample surface will lead to strong deviations between your measurements.

I recommend to choose a procedure as follows:

1. grinding up to P1200 meshsize

1a. rinsing and further cleaning using an ultrasonic bath

2. polishing using polycrystalline diamond suspension, here the final step should be 1 µm

2a rinsing and further cleaning using an ultrasonic bath

3. after setting up your cell (is it a three electrode setup?) you should apply a cathodic voltage to your sample. Here a triangle function, strating with a potential of -1970 mV and decreasing the voltage with a rate of 1 mV/s to -2000 mV and than increasing it back to -1970 mV could be used. This is what I always use in my lab for investigation of stainless steel in bodyfluids. Other labs perform this kind of electrochemical cleaning in different ways. A friend of mine for example uses a cathodic potential at -600 mV for 10 min. We never compared for the influence of the procedure (which definitley should be done (-: ).

Try to keep the time span between step 2a and setup of your experiments as constant as possible.

Fact is, if no electrochemical cleaning is done, you never know the exact surface condition at the beginning of your experiment.

But with respect to your alloy and electrolyte initial testing of the applicability of your procedure should be done!

Michael... thank you very much for your useful informtion. I am using Gamry instrument. To remove the oxide layer on the surface before running actual scan they have provided one option "conditioning" . I have applied that before scanning. There is a significant difference between the results with and with out conditioning. Thank you once again.

I don't have any experience with gamry instruments. Sorry! But anyway good luck!

:-)

My short answer is no, but the panel must be cleaned carefully to remove surface contaminants, and then further "electro-chemically cleaned" by putting the panel under a cathodic potential for a brief time to remove electrochemical contaminants. See "Using DC Electrochemical Techniques to Assess the Relative corrosiveness of Water-Based Coatings and their Ingredients," by F. Louis Floyd, Sumeet Tatti, and Theodore Provder, J. Coatings Technology: Research, 4(2), 111-129, 2007. This paper focuses on steel, but we subsequently found that the technique was also suitable for several metals of commerce, including zinc (as galvanized coatings on steel), aluminum, magnesium and various magnesium-aluminum alloys.

No metal of industrial significance exists without its oxide layer. Removing it creates an artificial condition that is not at all representative of how the material exists or behaves in actual use. The technique described in the above paper seeks to preserve that layer, and characterize it, since that is what will be used in actual practice.

By carefully measuring the difference between the open circuit potential (rest potential) and the breakdown potential (scanning in the anodic direction), one obtains what we called the passivation index. The larger the index, the more protective the substrate behaved once coated and exposed to corrosion tests. We used the Gamry ZRA for our more recent studies.

Regards,

Lou Floyd

Thnak you Floyd ,,It is very useful for me.