I'd appritiate if someone can help with understanding NACE TM0169.
There is a phrase i don't get about an inaccuracy of the analysis of test specimens:
At least duplicate test specimens should be exposed in each test. In laboratory immersion tests, corrosion rates of duplicate specimens are usually within +/-10% of each other when the attack is uniform. If the rates exceed this variance, retesting should be considered.
Can someone explane me the mentioned above, please? For example, if the results of the corrosion rates of duplicate samples are 0,35, 0,28 and 0,45 mm/yr?
Thank you!
Dear Dr. Ekaterina Mansurova ,
I personally understand the cursive statement as a cautionary tale as if exhibiting duplicate samples in a material that does not suffer from localized corrosion they remain within +/- 10% of each other
falls within an acceptable range. The case in which we have samples that suffer from localized corrosion (pitting, crevice...) and the attack is not uniform is different. In this case it is easier to overcome this variance and a retest need not be considered.
My best regards, Pierluigi Traverso.
Pierluigi Traverso, thank you!
I do understand the idea of 10% as a way to see if the corrosion is indeed general. But I don't quite get from this phrase what "within +/-10% of each other" means.
1) Calculate the average of 2-3 samples
2) Compere the difference beetween each sample from the average value in % ?
Thank you kindly.
Dear Dr. Ekaterina Mansurova ,
I think a method more similar to your second assessment is correct: the corrosion rate of duplicate specimens are usually within +/-10% (of one sample compared to the other); if you have more than two samples I would average and then comparison +/- 10% of the different corrosion rates with the mean itself.
My best regards, Pierluigi Traverso.
Dear Ekaterina Mansurova ,
On the surface, immersion testing appears to be a relatively simple test; however, the details are in the analysis/interpretation. It is best/helpful to have many repeats (e.g., if possible, at least 5) and to use some method of statistical analysis (e.g., ASTM G16) to assess the variance of the results to ensure you are capturing statistically repeatable data. If the material undergoes uniform corrosion (i.e., you verified this in a preliminary experiment or parallel experiment), then the standard is recommending a variance between results that is typically observed. However, as Pierluigi Traverso mentioned, the main purpose of the recommendation in the standard is to make the reader aware that, if the sample undergoes localized corrosion, then repeatability may not be as good and, more so, that another evaluation method may be required (e.g., cross-sections, specialized tests for localized corrosion). Think of the "10%" not as a hard-set rule (e.g., as stated in standard there are exceptions), but rather advice to investigate the corrosion mechanism as being uniform or localized.
I also agree with Pierluigi Traverso's interpretation that the standard implies that the samples should be within ~10% of the statistical mean of the sample set, in the example you provided; however, again it may be necessary to further investigate for possible localized corrosion and an explanation for the deviation in the results. For example, other factors could influence repeatability, including inconsistencies between samples (e.g., in their processing, their composition, their microstructure, impurities, crystallographic phases) and in the testing procedure (e.g., see ASTM G1 for discussion on repeatability).
You can also refer to these tests for additional reference:
R.A. Corbett, Ch.9 Immersion Testing in Robert Baboian (2005) Corrosion Tests and Standards: Application and Interpretation. https://www.astm.org/mnl20-2nd-eb.html
ASTM G31 Standard Guide for Laboratory Immersion Corrosion Testing of Metals
ASTM G1 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens
ASTM G16 Standard Guide for Applying Statistics to Analysis of Corrosion Data
All the best and happy testing!
Leo Monaco
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