Environmentally assisted cracking [EAC] caused by hydrogen assisted embrittlement ~7xxx-T7x Alloys?

[WKTaylor]

Please advise...

This report just came to my attention…

EASA SIB No 2018-04R1: Environmentally Assisted Cracking in certain Aluminum Alloys

https://ad.easa.europa.eu/ad/2018-04R1

This report states that EASA experience and testing of 7037, 7040, 7055, 7085, 7099, 7140 and 7449 have revealed 'SCC-like' cracking within these alloys caused by hydrogen embrittlement along grain boundaries. NOTE: I hate it when a technical document refers solely to an aluminum alloy designation number... and omits '-Txxx' temper and 'form' and specifications.

As a long-time user of 7XXX-T7XXX alloys where ST threshold of 25-KSIsqrt was considered 'SCC resistant'... along comes this related phenomena? Is 'EAC caused by hydrogen embrittlement' [EAC-HE] a fancy term for 'classic' SCC...? or is EAC-HE a subset of SCC...? or is EAC-HC an obscure similar [kissing-cousin?] phenomena that I have been unaware of... since it would normally merge with SCC during SCC testing?

PS: I have deliberately edited out the word alumininum… in favor of aluminum.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[blakmax]

Hey Will

PS: I have deliberately edited out the word alumininum… in favor of aluminum.

Are you going to to the same for titanum, radum, plutonum, palladim, ammonum, sodum, etc.?

You are right though. 7075-T6 SCC performance is vastly different to T76.

Regards

Blakmax

 

[Sparweb]

The only source in the SIB is a PhD thesis paper from one George Aloysius Young, Jr. in 1999:

http://adsabs.harvard.edu/abs/1999PhDT........72Y

No temper was reported and it does not seem to have been controlled in the tests. The alloys tested are 7050 and another "low copper variant (Al-6.87Zn-2.65Mg-0.06Cu)" which I can't identify (maybe 7076).

I don't know how EASA drew this out to a bulletin about all the other alloys.

I can't get the whole thesis paper from Dr. Young but if anyone can, it would be interesting to see if any standard corrosion tests were carried out as the basis of the thesis paper. It seems to reference the standard ASTM tests, but where they actually followed? I'd like to compare that to the claim in the EASA SIB that the ASTM test is inadequate. A lot of the SIB statements depend on the test procedure from the thesis paper.

They already seem to have had some pushback on this SIB, because this is a revision intended to show where this problem comes from or how it was found.

http://www.sparweb.ca

 

[Sparweb]

Lest we take SCC too lightly, though, there's an accident investigation report in my newsfeed today:

http://newsinflight.com/2019/09/29/...boeing-777-main-gear-axle-failure-on-landing/

Admittedly, that axle is made from steel and has probably been in service for a while.

Back to aluminum, I haven't found any more info on the subject of hydrogen embrittlement in aluminum. Searches for "hydrogen embrittlement" come up with steel and titanium, except for other instances of work from the same researcher, GA Young. Such as this article.

 

[verymadmac]

Found this paper for 2024-T3, the most interesting point being that blending out all the corrosion may not remove all of the region of hydrogen induced embrittlement.

http://www.mie.uth.gr/files/sdarticle.pdf

 

[rb1957]

the original post was talking about Al-Li alloys, no?
not so much our common 2024, 7050, and 7075.

another day in paradise, or is paradise one day closer ?

 

[LiftDivergence]

From NASA/TM-2016-218602:

Hydrogen Embrittlement versus Stress Corrosion Cracking

When a metal is exposed to an aqueous environment that involves hydrogen, it is important to recognize the difference between HE and Stress Corrosion Cracking (SCC). In general, HE is considered as a cathodic mechanism and SCC as an anodic mechanism. The SCC is a dissolution mechanism of removing materials at the crack tip through a corrosion process, and the environments that cause SCC are usually aqueous in nature, such as by contact with condensed layers of moisture or by immersion in a bulk liquid solution. However, during a corrosion process, both anodic and cathodic reactions can occur near the crack tip for some materials, such that the cathodic mechanism is the hydrogen evolution at the surface, which would lead to the diffusion of hydrogen into the bulk of the material. For some materials, the synergistic mechanism of crack growth is thought to be a combination of local anodic dissolution and cathodic hydrogen embrittlement. Whether crack extension is by anodic dissolution or by HE remains an open question for some materials subjected to an aqueous environment.
In an aqueous medium, the distinction between HE and SCC sometimes can be made by noting the effect of small impressed currents on the time-to-failure in a constant load test. If cracking has occurred by an SCC mechanism, application of a small anodic current shortens the time to failure. When HE is the main mechanism, application of a cathodic current will accelerate the time to failure. The distinction between HE and SCC can also be made based on strain rates under HE testing. For SCC, if the strain rate is too high, ductile fracture will occur before the necessary corrosion reactions can take place; therefore, relatively slow strain rates must be used. However, if the strain rate is too low, corrosion may be prevented because of repassivation or film repair so that the necessary reactions of bare metal cannot be sustained and SCC reaction may not occur. Therefore, a unique range of strain rate must be determined in each SCC case [2]. For some materials, this mechanistic difference can be used to distinguish between anodic SCC and cathodic SCC mechanisms.

Also of interest:
NASA CR-134962, Hydrogen Embrittlement of Structural Alloys - A Technology Survey

Both break HE into several categories, Internal Reversible (IHE), Hydrogen Environment (HEE), and Hydrogen Reaction (HRE). SCC is a related phenomenon in that both effects can occur simultaneously and the presence of hydrogen may also alter the behavior of SCC which would otherwise be acting alone.

Keep em' Flying
//Fight Corrosion!

 

[LiftDivergence]

Also, blakmax,

Interesting historical tidbit here re: aluminum vs aluminium

https://www.thoughtco.com/aluminum-or-aluminium-3980635

Turns out "aluminum" actually was the original name given by Sir Humphry Davy (a Brit, of course). Later the name was changed to aluminium to fit better with existing elements.

The american chemical society (ACS) eventually reverted in 1925 since I guess it was deemed the original name should prevail. The rest of the world kept the non-Davy version.

Obviously the discoverer gets the official naming rights, so that would point to aluminum being "correct". However, the true discoverer is also a topic of debate, evidently.

Keep em' Flying
//Fight Corrosion!

 

[WKTaylor]

blakmax... quit messing with me... all those 'British words' spellings that you listed crossed to over to nonsense in Google search... 'A' for creativity, though!

Thanks to all !!!

The TWO 'scholarly articles' cited, suggesting hydrogen embrittlement in aluminum alloys is a 'real threat', is extremely unconvincing to me. NEVER have I ever heard discussion of 'hydrogen embrittlement' in aluminum... and almost all aluminum EXCO & SCC failures occurrs in 'electrolyte' [hydrogen-rich] environments with EXCO/SCC prone alloys/tempers... which sounds exactly like what is being described.

NOTE. The article suggested by VMM was a classic example of why I cannot accept this hypothesis. It is well established that 2024-T3xxx plate/extrusions/bar/etc [ >0.250"] is a known EXCO and SCC bad actor and is prohibited from MIL-Acft use for this reason [MIL-STD-1587, TABLE I. Commonly restricted structural materials].

NOTE.
Simply altering the 2024-temper to -T62 or -T8xxx significantly improves EXCO and SCC [and intergranular] thresholds. OK, 2024-T-anything still has to be carefully protected from corrosion since it is very prone to all other surface-centered and dissimilar-metals corrosion.

NOTE.
ALSO... ANY formally trained liaison engineer/mechanic knows that corrosion must always be completely removed for any in-service part to prevent continued degradation.... and that follow-up validation [FPI-NDI or EC-NDI] should attain "no detectable defects noted' in the final pass/fail to truly be serviceable.

NOTE.
When citing any NDI procedure, it is mandatory to have clear 'pass-fail requirements' [when NO further material removal is needed or allowed]... if NOT embedded within a spec or procedure, then the [liaison/service/design/etc] engineer must clearly define pass-pail for the final acceptance NDI.

I think at one time I recall that the presence of 'tramp metal Ions'... even in low percentages... could lead to accelerated EXCO and SCC failures... but the major aluminum OEMs 'know' how to consistently attain ideal chemistry to minimize/eliminate these tramps.




Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]