Hydrogen embrittlement for economists

Over the past decade there have been an overwhelming number of techno-economic studies of hydrogen deployment as a means to reduce national carbon emissions. To reduce cost, these have always considered the possibility (or necessity) of re-using existing natural gas pipe networks.

Far too many of these economist-written reports contain an (unreferenced) of-the-cuff warning about hydrogen embrittlement, and these well-meant (but ignorant) warnings are then picked up by campaigners and lobbyists with no respect for truth or any evidence whatsoever, only rhetorical scoring points.

Some years ago I was so appalled (I am a metallurgist and was a Fellow of the UK’s ‘learned body’ the Institute of Materials, Minerals & Mining and also a chartered materials engineer) that I tried to find out the source of all this misinformation. I discovered one them: the Wikipedia entry on hydrogen embrittlement had been so severely polluted by hearsay, modern myths and unsubstantiated gossip that it needed serious attention. I and several other Wikipedia editors worked on it for a while and today it is in much better shape (though the reading-age required might still be beyond many journalists I fear: more work is required). But it is interesting to note that every few months some poor well-meaning soul edits it to add more biased and un-evidenced garbage, so keeping this clean requires constant effort.

This is particularly galling since

• the basics of this embrittlement have been well understood since 1875
• ordinary steel is today, and always has been, the material almost universally used for compressed hydrogen cylinders (at 300 bar!)
• much of the existing UK iron pipe network has already carried coal gas (50% hydrogen) for many decades

This is not to say that embrittlement can be ignored. New networks proposed using higher-strength steel alloys certainly need to use fracture mechanics for correct design, and rotating machinery in hydrogen atmospheres needs to be specified while understanding the environmental effects on crack growth rates. The danger from hydrogen embrittlement in pipelines comes from welding the pipe with damp welding rods, from acid cleaning, or even from external rusting, but not from the hydrogen inside.

In the past 2 or 3 years the level of stupidity of this sort has seemed to reduce, so it looks as if correcting Wikipedia had a useful effect. There were still some articles published, but mostly from local newspaper journalists, or geographers or economists (who couldn’t be expected to know any better), and hardly any from science journalists (who certainly could be expected to know better, or at least to check with Wikipedia) and even a gratifying reduction in the number from outraged loons (who wouldn’t care about knowing better).

Until last October.

Physics World published an otherwise-excellent article on heat pumps which also discussed hydrogen for heating and unfortunately contained this phrase “… would require not only new boilers but also replacement pipes, since hydrogen in high concentrations causes steel to become brittle“. This is not just condescending side-insult, a wink and a nod, or a sly insinuation of trouble if you go in that direction, but an outright and complete untruth. What is more, neither the writer nor the journal editor thought it important enough to quote or to find any supporting evidence whatsoever. Such a level of ignorance and self-importance in thinking that this is unimportant is staggering. If this were the parish journal of the Outer Boondocks Chronicle and Advertiser, I would just sigh and move on, but this is the institution journal of one of the most respected learned societies in the UK. This article will no doubt be quoted as authoritative evidence by anti-technology campaigners for decades to come: the damage has already been done.

There are two authoritative sources I recommend, in addition to the Wikipedia article:

• Prevention of Hydrogen Embrittlement in Steels, by Harry Bhadhesia (2016). The first two pages of this long review article.
• Technical Reference for Hydrogen Compatibility of Materials, Sandia Report SAND2012-7321 (2012), which has all the numbers you could ever need. Though you may need a masters degree in linear elastic fracture mechanics to be able to use them properly, or at least understanding why the units for fracture toughness have square-root(metres) in them: MPa m^1/2

[ Addendum: 31 March 2024 ]
ASME standard B31.12-2014 unfortunately contains these words: “Hydrogen gas molecules adsorb onto metal surfaces, like many atmospheric gases, and dissociate to its atomic form.” This is entirely and completely incorrect in every particular. The dissociation energy of hydrogen is huge, 423 kJ/mol, so this Simply Does Not Happen (not unless the pipe is glowing red-hot, in which case other failure mechanisms may be more worrying). Check with Prof. Bhadhesia’s review article.
This ASME standard, quite rightly (from a position of ignorance) mandates a large safety factor to be used with things that the revision team did not understand. This does not mean that the “materials performance factors” they mandate have any connection to reality. I look forward to the next revision of this standard.

[ Addendum: 3 March 2024 ]

While a home electrician installing a solar hot water system in Outer Boondocks, Lower Nebraska, may honestly believe that the broken weld he is looking at is due to this mysterious ‘hydrogen embrittlement’ thing, and while his distant cousin in Blackstumpton, Eastern Australia (who is a bit hazy on the distinction between atoms and molecules) may encourage him in this belief¹, this is not objective evidence backed up by careful, long-term experiments, deep thought by a community of clever people, and a lot of electron microscopy.
That is the the level of the material which was excised from Wikipedia.

This is what Wikipedia says now: “Gaseous hydrogen is molecular hydrogen and does not cause embrittlement though it can cause hot hydrogen attack (…). It is the atomic hydrogen from chemical attack which causes embrittlement because the atomic hydrogen dissolves quickly into the metal at room temperature.”

It is worth noting that the fast fracture²of a specimen filled with atomic hydrogen is an effect with origins in the bulk of the specimen material, whereas the effect of gases on fatigue crack growth are (almost certainly) due to the adsorption of gas molecules on the freshly-formed metallic surface at the crack tip on each cycle of crack growth. The term “hydrogen embrittlement” refers specifically and only to the bulk effect.

1. I do not wish to cast aspersions on the many fine universities which no doubt exist in Eastern Australia, but Bruce (for that is surely his name) has never been to any of them. ↩︎
2. “Fast-fracture” is a technical term. It is quite different from fatigue crack growth. Look it up. ↩︎