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Comparing results obtained by various corrosion test methods?


Q. Due to the inability of "specialists" in the field being able to give me any data on the performance between various corrosion tests on powdercoated aluminium when the results obtained are compared with each other (e.g., 1000 hour NSS, Acetic Acid Salt Spray, Filiform corrosion test (DIN),etc.) is anyone able to comment? Is it that the pass/fail criteria (e.g., 2mm ASS) directly comparable to the pass/fail criteria of other tests or is it a case that one method may pass whilst the other may fail due to the test actually testing a different aspect of the system (e.g. the chrome itself, the efficiency the surface oxide has been removed, the paints adhesive properties to the chromate, etc.)? This is of specific interest when one sample is cut into pieces to be tested in different tests. If these tests are actually testing different aspects of the system, is this implying that using one test may not be analysing the finishing system totally (resulting in less confidence in the results)?

With particular relevance is that is anyone able to give a technical comment in comparison.

Alf Clarke
- Mt Gravatt, Qld, Australia


A. Hi Alf,

I don't think that there's much value in comparing results of different corrosion tests because the conditions of a particular test will give you a certain corrosion mechanism. Different corrosion conditions used in another test may yield different corrosion behavior. It's kind of like comparing the results of a coating weight determination to the results of an EDS analysis of the surface. The two tests are inevitably going to provide different results.

Rather than compare the results of different corrosion tests, I think it is more useful to look at the tests as being complementary to one another. Taken together, the results of various tests can provide a lot more information than any one of the individual ones can.

George Gorecki
- Naperville, Illinois


A. Hi Alf. I think George is on the money.

But actually, the problem is even deeper than that: salt spray test life is not an indicator of real life, and isn't intended to be because the corrosion mechanism is fundamentally different. It's only intended to try to determine whether your processing has gone off the rails. So multiple tests will always be better than any single one. Good luck.

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Salt spray testing immediately vs. waiting a month?

April 13, 2015

Q. Dear sir,

If I have plated parts and tested for neutral salt spray immediately.

And if I carry out the salt spray test after a month for the same parts, will the life vary?


Manjunath Shanmuga
- Bangalore, India

Salt Spray test and salt immersion test

April 13, 2015

Q. I am performing salt immersion test for 24 hrs on Zn plating we are observing black spots. Same parts if subjected to salt spray test we do not observe any black spots.
Please explain are these parts acceptable. What are these black spots?

Ravindra Naidu

April 2015

Hi Ravindra. What is the spec number for this "salt immersion test" -- sorry, I'm not familiar with it. According to Subramanian R in letter 35573, the black spots are only an intermediate stage on the way to corrosion and may be ignored.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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