Subject: hydrogen ebrittlement

From: [email-masked] (Chris Demke)
Date: 11 Jan 1994 22:06:47 GMT

What is hydrogen embrittlement?  The rocker arms on the motor
	of my dad's dragster are prone to break.  They are aluminum
	with a hard anodize coating.  My dad claims that they may have     
	weakened by hydrogen embrittlement during the anodize pro-
	cess, but he couldn't give me a good answer as to what hydro-
	gen embrittlement is and what causes it.  Any takers???

			Chris Demke
			Sun Microsystems
			[email-masked]

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From: [email-masked] (michael jensen)
Date: 12 Jan 1994 15:27:00 GMT

>			[email-masked]

This doesn't sound like hydrogen embrittlement to me.  It more then 
likely is a case of overstressing the rocker arms with a little creep 
and metal fatique throw in.  From your post I can't give you a 
exact answer but there are things that you can look for in the broken 
parts that will help you find out what is causing the failures of the 
rocker arms.  The first question would be are they breaking at a 
area of high stress consentration.  This would be near a hole, a sharp
bend or corner.  How many cycles are they going through before they 
break. Aluminum that is subjected to stress cycles will eventually 
fail.  It might take 6 million cycles in very low loadings situations, 
but in a much higher loading of the rocker arms this could be 
decreased greatly.  And finally how hot do the rocker arms get.  Maybe
at the elevated temperatures in a racing engine the aluminum weakens, 
the ultimate strength is not as high.  

To answer your question about hyrogen embrittlement from the plating 
process. From Guidebook and Directory of Metal Finishing " All ferrous
materials with a hardness greater than Rockwell C40 are subject to 
becoming brittle as a result of their absorption of hydrogen during 
the plating process."  Aluminum is not a ferrous material.  Anyway 
hydrogen is  a impurity which causes internal cracks and weakens the 
metal.  

By the way, hydrogen embrittlement is a real problem in nuclear 
power plants.  When the uranium has it's reaction it gives off hydrogen
which is absorded into the stainless steel containment vessel walls. 

Mike J.
--
Mike Jensen ([email-masked]) 
Chemistry Instrument Shop              
University of Nebraska - Lincoln
(402) 472-3542

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From: [email-masked] (bui tho xuan)
Date: 12 Jan 1994 15:45:03 GMT

[email-masked] (michael jensen) writes:
>This doesn't sound like hydrogen embrittlement to me.

Neither to me.  But I can't say for sure unless I have the part in
my hand to analize. 

>hydrogen is  a impurity which causes internal cracks and weakens the 

Hydrogen can indeed causes embrittlement in aluminum (and most metals,
for that matter).  At least I hope so, 'cause this is a part of my
research topic :-).  The main problem is that the oxide layer in aluminum
is impermeable to hydrogen.  If there is a disruption to the oxide layer
due to, say, corrosion or an ultrasonic field, you can put enough hydrogen
into aluminum to create havoc.  It is possible that hydrogen is introduced
during the anodizing process (an oxidation procedure, not like plating), 
but as I said, to positively place the blame you'll need more detective
work.

tho  
p.s.:  for a rather spectacular demonstration, place a piece of kitchen
       foil in an ultrasonic cleaner with water.  Turn it on, and come
       back 15-30 minutes later.  The foil should be broken into pieces
       (disappear in some cases).  This is one of the common technique
       for charging hydrogen into aluminum.  The other processes are
       plasma charging, or corrosion charging (with NaOH--lye).

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From: [email-masked] (Harry Phinney)
Date: Wed, 12 Jan 1994 20:45:39 GMT

Chris Demke ([email-masked]) wrote:
: 	What is hydrogen embrittlement?

As pointed out by the previous posters, it's possible but unlikely that
the rocker arm problem is caused by hydrogen embrittlement of the Al
alloy.

:	The rocker arms on the motor
: 	of my dad's dragster are prone to break.  They are aluminum
: 	with a hard anodize coating.

A layer of anodization can provide convenient nucleation sites for
cracks, however my first suspicion would be the rest of the valve train.
How close are the springs to binding?  Is there enough spring travel
left to allow a bit of float in the case of over-reving?  Is he running
solid or hydraulic lifters, and if hydraulic, is he sure they're not
pumping up to the point of binding the coils?  I assume these are roller
rockers.  Are the pivots staying free, or could they be siezing?

Harry Phinney   [email-masked]

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From: [email-masked] (Clive Washington)
Date: Thu, 13 Jan 1994 10:09:53 +0000

In article <[email-masked]>, [email-masked] (bui tho
xuan) wrote:

  for a rather spectacular demonstration, place a piece of kitchen
>        plasma charging, or corrosion charging (with NaOH--lye).
I'm not sure this is hydrogen embrittlement.  Where does the hydrogen
come from?  I think that what happens here is that ultrasonic cavitation
(which is how ultrasonic cleaners work) lifts off bits of the passivating
oxide layer.  This reveals the very reactive aluminium metal underneath.
This then disintegrates in contact with water.

For a demo. of the importance of this oxide layer, try the following.
Put a drop of mercuric chloride solution on a piece of Al. foil and 
nothing much will happen.  Take a needle and gently scratch the foil
surface under the liquid drop, and the foil will rapidly corrode on that
scratch.  You can write your name if you like, and it will etch through the
foil.  The reason is that the bare aluminium causes galvanic deposition
of mercury, which amalgamates with the aluminium.  Its a striking reminder
of how reactive this useful metal is.  If it weren't for that thin oxide
film, aluminium (and magnesium) would be about as useful to metalworkers
as sodium and potassium.

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From: [email-masked] (bui tho xuan)
Date: 14 Jan 1994 17:22:44 GMT

[email-masked] (Clive Washington) writes:
>, [email-masked] (bui tho xuan) wrote:
>

The hydrogen comes from water.  What you said is correct, but did not
account for how the aluminum disintergrates when it contacts water.
The aluminum reacts with water to form aluminum oxide.  The left over
hydrogen is absorbed into the metal.

The equilibrium concentration of hydrogen in aluminum at room temperature
is normally very small, and has to be extrapolated from high temperature
data--in the order of 10e-7 atomic ppm.  When charged using the techniques
I mentioned, the hydrogen concentration can go to 1800 appm (measured by
gas chromatograph).  Interestingly, the oxide can reform and seals in the
hydrogen.  We found that there is no drop in H concentration several months
after charging.

This and the mechanical effects of hydrogen in aluminum is well documented.
I don't really work directly with this topic (my specialty is radiation
damage), and from the top of my head,  I can suggest the work of F. Zeides
(an ex-grad student), and one of my bosses (HKBirnbaum).  FZ's work is
on aluminum only (about 5-6 years ago), and HKB's work is a bit too volumous
to search through.  A quick computer search will provide more details.

tho

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From: [email-masked] ([email-masked])
Date: 28 Jan 1994 00:34:30 -0500

>>	gen embrittlement is and what causes it.  Any takers???

I agree with [email-masked] sounds more like fatigue, possibly caused
by cracks in the anodized coating (The coating, by the way, is a hard
layer of aluminum oxide.) Hydrogen embrittlement as far as I know does
not occur in aluminum. Also, it usually happens after a time at constant
stress, and almost always in steel which is harder than Rockwell C 40. It is
caused by migration of dissolved hydrogen to the tips of
tiny cracks, which allows the cracks to grow further until the part
finally breaks. It can be cured as long as the part has not already
cracked by heating the part to about 400 degrees F for three to four
hours.    [email-masked]