The first person who fully understands hydrogen embrittlement (HE) will
win a prize of some kind - or at least write a book a few of us
metal-geeks will buy.
A Google search on the two words will yield up all sorts of background
information.
I've never seen the topic discussed in the context of ham radio however,
so here are a few thoughts:
- HE is a form of stress corrosion, both can lead to stress corrosion
cracking. For either to occur, the metal in question must be under
tensile stress and must have been, or be in, the process of being
corroded. Most of us encounter corrosion due to oxidation - typically
producing rust - but hydrogen is as bad or worse a corroder than oxygen
and does not produce rust (iron oxide) hence making it impossible to
see. My first guess for the failed tower legs in question was simple
stress corrosion cracking, but the complete absence of rust in the
photos makes me think it was HE.
- Various metals are more and less susceptible to HE. Medium strength
steels and stainless steels are susceptible to HE failures. This would
include tubular tower legs. This would also include the "mystery water
pipe" that is often used for masts, but masts are seldom subjected to
the main culprit for causing HE - welding. If you weld to a mystery
water pipe the point on either side of the weld will not carry the same
tensile stress as the rest of the pipe and your mast may fail from a
clean annular crack rather than bending. This is a catastrophic failure
- just like a cleanly cracked tower leg will typically be catastrophic.
Cr-Mo masts "the good stuff" are virtually immune to HE.
- The result of HE is a loss of ductility - i.e., embrittlement. A lot
of the performance (strength) of a metal in tension results from the
ductility of the metal - loss of ductility equates to loss of strength.
- The strength of a metal undergoing HE goes down with time. Sometimes
lots of time - like years. Both elements are at work - corrosion and
applied tensile stress. The exact role applied tensile stress plays is
not understood.
- For HR, we first need to avoid plain old stress corrosion at high
tensile stress points in our metal systems - like at the bottom of tower
legs. Tower leg bottoms often get their galvanizing knocked off or sit
in water - obvious problems. More sinister - the welds at the bottom of
tower legs may have pin holes in the galvanizing permitting corrosion
that is hard to see. Tower leg bottoms should be painted with cold
galvanizing paint (or some other oxidation inhibitor) now and then and
inspected regularly.
- The next thing we need to do is avoid bad designs. Welding at the
bottom of tower legs is asking for trouble. Rohn 25 and all its bigger
cousins are good designs because of three things: 1. The stress at the
bottom of the legs is well below the tensile limit of the unembrittled
metal (unless you make the tower higher than Rohn recommends), 2.
embrittlement of the entire leg cross-section is unlikely given the tack
welds of the cross members are very small, and 3. as guyed designs, the
stress at the bottom of the legs does not rise appreciable with wind
load. I have personally failed two Rohn towers, unfortunately, and in
both cases the legs failed by exceeding the modulus of the steel
producing bending, not cracking. Good job Rohn. Bad job N4GG - we
sometimes learn the hard way. The failed tower we are discussing has
significant welds at the highest stress points in the legs, and is self
supporting with reasonably significant wind load. The wind loading at
the top is applied to the weld-embrittled location on the legs
multiplied by the moment that is proportional to the height of the
tower. If the modulus of the legs had been exceeded they would have
crumpled, if the welds had cracked it would tell us we had bad welds -
the nice clean leg cracks tell us stress corrosion (embrittlement) was
the culprit.
- After a little reading, you can get convinced everything causes HE.
HE is most often caused, in my experience, by welding or plating. Most
ham stuff is not plated (galvanizing within this context is not
plating). Welding plasma contains hydrogen as do some welding rods.
There are special hydrogen-free welding techniques - these are typically
not available to hams or ham-related manufacturers, i.e., they are very,
very expensive.
- Poor welding leads to cracks at the weldment. Even perfect welding -
if there is such a thing - can lead to crack failure in the base
material (not the weld) immediately adjacent to the weld due to HE.
I hope this is helpful...
N4GG
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See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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