Recently, in a work on ZK60 magnesium alloy and its corrosion behavior change due to different mechanical treatments (mainly ECAP), we observed a sort of filiform corrosion (the electrolyte was phosphate buffer solution, PBS) (the paper is in the attached ink, see pages 318-319).
This type of corrosion basically occurs under a protective film (either an externally applied protective film such as lacquer or an intrinsically formed film such as passive oxide films). So, attribution of such phenomenon to magnesium is quite plausible (see this paper: http://pubs.acs.org/doi/abs/10.1021/ie50533a053 ). However, there are very few reports about such observation for Mg alloys (I personally did not find any explicit case).
I would like to know if anybody had such experience with Mg alloys and whether has tried for a detailed explanation and characterization.
Article Microstructure, texture evolution, mechanical properties and...
Filiform corrosion is generally observed on magnesium alloys. A detailed study on filiform corrosion can be seen in the reference as follows:
Yingwei Song et. al, Corrosion characterization of Mg–8Li alloy in NaCl solution, Corrosion Science 51 (2009) 1087–1094.
Hi Masdak,
The following paper is an essential read on the filiform corrosion mechanism on both coated and uncoated magnesium:
http://dx.doi.org/10.1016/j.electacta.2010.09.005
The important highlight is that the filiform corrosion mechanism on magnesium and its alloys cannot be explained with the same "differential aeration" mechanism that is generally accepted to occur on steel and aluminum alloys. This is because of the following:
- The main cathodic reaction on Mg is hydrogen evolution rather than oxygen reduction
- Mg2+ ions tend to increase the local solution pH, while Al3+ or Fe3+ ions tend to decrease the local solution pH
Hi Zach,
Thanks for the suggested paper and for your comments. This point is well explained in there:
“FFC of Mg arises not from differential aeration, but rather via a differential electrocatalytic mechanism. Thus the principal driving force for filament advance on both coated and uncoated Mg is a consequence of the activation of the corroded surface towards cathodic hydrogen evolution, compared to the intact (uncorroded) surface."
In our study entitled:
Combined effect of composition and surface condition on corrosion behaviour of magnesium alloys AZ31 and AZ61
Corrosion Science, Volume 68, March 2013, Pages 66-71
Alejandro Samaniego, Irene Llorente, Sebastián Feliu Jr.
We observed filiform corrosion on AZ31 and AZ61 alloys :
According to the literature, filiform corrosion is assumed to proceed on magnesium alloys under a relatively resistant oxide film from points where this film is no longer continuous
I hope it helps.
Kind Regards.
S.
Dear Sebastián,
Thanks for letting me know of your work and your helpful comment.
Regards,
Mazdak
Dear Mazdak,
Thank you for your answer. I have found quite interesting the manuscript attached to the question.
Kind Regards.
S,
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