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Re: A490 bolts

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Charlie,

Re: > >Does anybody know why galvanizing A490 bolts is not allowed?
>

Assuming this is not a rhetorical question, (because I know how
knowledgeable you are,) here's some background and some information on the
'plated/coated A490' issue:

First, indeed ASTM's Fastener Committee (F-16) does strictly prohibit the
plating or coating of A490 bolts.  This prohibition may be traced to
failures attributed to 'hydrogen embrittlement'.  However, in the period
since these failures, much has been learned about additional risks related
to plating or coating of such fasteners.

History:
Originally, it was believed that the source of perhaps all such problems was
the acid pickling processes used to clean fasteners before plating.  These
processes introduce hydrogen which not only can embrittle the fasteners, but
may also resist extensive efforts to drive (bake) the hydrogen out.
Baking, even at 'high' temperatures for 24 hours or more is common remedy,
albeit only partially effective and expensive.   Unfortunately, moving away
from acid pickling to alkaline or mechanical cleaning methods does not
eliminate the risk of hydrogen-related failures.

There is also the potential for hydrogen introduction during the plating
processes themselves.  Certainly, in electro-plating this potential is well
documented.  For mechanical and hot-dip, it isn't quite as obvious.
Consider, however, that there is hydrogen potential involved in such things
as the copper 'flash-plating' operation which is the first step in a
mechanical plating process.

Behind the scenes:
The core of what is 'not stated' in ASTM's prohibition is this:  Other
factors come into play which can result in delayed brittle failure of coated
and/or plated A490s.  For example, stress.  Structural fasteners are
routinely tightened into yield and are supposed to be.  However, high stress
is correlated with environmentally induced hydrogen-related failures.

Another factor is galvanic action.  Zinc from a plated fastener will
'sacrifice' itself as it is designed to do to protect the base material of
the fastener.  Unfortunately, this process (aided by water) can introduce
hydrogen into fasteners in service by galvanic action.  Don't ask me why . .
. but the metallurgists at ASTM insist  . . . hydrogen introduced in this
fashion just loves to get cozy in places where it weakens the fastener's
intergranular structure.

Another set of factors is related to the optional fastener base materials
which may be used or to the manufacturing processes used to produce
fasteners.  One can imagine a rather complex set of factors which one would
need to consider before profound knowledge of hydrogen potential could be
established.  ASTM members have learned the hard way that the 'pass' vs.
'fail' tests that have historically been used to qualify fasteners as
'having' or 'not having' hydrogen embrittlement are far from foolproof.

Hardness is another factor, separate and distinct from stress.  High
hardnesses of some materials are problematic, while others not so.  One good
example for illustration would be case-hardened fasteners.  They are
particularly problematic.  (Note: All ASTM structural fasteners must be
'through-hardened'.)

Now and the Future:
Now, the good news:  ASTM has done a LOT of work in this area.  Under the
leadership of a gentleman named Salim Brahimi, much has been learned and put
into practice.  ASTM now has a Test Method which allows platers to establish
the hydrogen potential of their processes by use of test coupons which are
processed along with the fasteners and then submitted to what is called the
'Rising Step Load Test'.  This test method is an excellent tool for
providing quantitative data which can establish the risk associated with the
processing.

We are also in the last stages of refinements to a new test method (ASTM
F1624) for analysis of actual produced fasteners.  This test method is far
more informative than those used in the past.  It is not 'pass-fail' though.
It provides insight into the risks that may be associated with use of such
fasteners in certain environments.

My Prediction:
Honestly, I don't see this prohibition going away.  The field factors are
just too compelling.  Think about it.  Who would want to galvanize an A490
bolt?  Someone who intends to use it outside where it will be exposed to the
weather, right?  What about stress?  Would someone use A490s for connections
that don't need to be fully tightened?  No, so stress should be high.

If the A490s are tightened, that = high stress on a high hardness product
with galvanic potential in an environment where that potential will be put
to use.  Add time, (because installed bolts have nothing else but time,) and
the risk of hydrogen-assisted stress-corrosion cracking is substantial
enough to frighten off all but the least conservative engineer.

To Learn More:
I would encourage anyone interested in learning more about this topic to
contact Salim Brahimi of IBECA Technologies at (514) 944-3358 or
SalimBrahimi(--nospam--at)ibeca.ca   Tell him I sent you there and maybe he'll buy me a
beer at our next ASTM meeting in Kansas City.

David Sharp
TurnaSure LLC
57 E. 11th St. 8th Fl.
New York, NY 10003
(646) 602-1405 direct










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