re: chrome plating (way geeky)

["Hardwick, Robert C" <Robert.Hardwick@xxxxxxxxxxxxxx>]

Chrome plating has enough advantages that it's used in highly stressed parts
when really needed.

Hydrogen embrittlement is a concern for steels heat treated >180ksi.  The
typical procedure is a post plate bake for 8 hrs @ 350F.

The chrome plate also imparts a tension stress to the surface of the part.
To counteract this, the part is shot peened and also stress relieved prior
to plate.  The latter is especially important if there are welds.  The welds
are also subject to hydrogen embrittlement because the hydrogen concentrates
in areas of high residual stress.  Another reason to stress relieve.

Even aluminums don't escape the need for consumer heat treatments.
Frequently, welding material is used to fill surface defects, such as
porosity (weldment is then ground flush - you won't see it).  If the part
isn't reheat treated, the fatigue strength is reduced at the weld/base
material interface.  It seems likely that the same wheel supplier that has
high casting porosity would not be likely to redo the heat treatment after
cleaning up the defect via weld.  Frequently the part will warp during the
heat treatment, which requires more machining or straightening - expensive.

Chrome does have lots of cracks because its structure is like stacks of tiny
flakes.   But the chrome isn't bonded to the base material well enough for
these cracks to propagate.  The problem with the cracks is that they allow
moisture to penetrate, especially if the plating is thin, i.e. cheap.  

Chrome isn't a very good finish for corrosion protection (because of all
those little cracks), but it is good for wearing surfaces and for sealing,
the latter because of those little cracks' ability to hold oil.  It's this
combination that makes chrome useful for parts such as landing gear
cylinders.  A good powder coat should be durable enough to not be scraped
off by tire machines with some care, and would be better for corrosion
protection.  So it's hard to justify chrome wheels on a technical basis.

Warren Reid is on the money about dye penetrant of aluminum, and magnesium
for that matter.  Magnetic particle inspection is the ticket for steels and
is able to "see through" chrome to some extent.  Another method, Barkhausen,
uses fluctuations in a magnetic eddy field to inspect through really thick
chrome, but is well beyond the means of most folks.

Back to the real world, a lot can be gained through careful visual
inspection of a well cleaned part.  If the crack is critical when so small
that more sophisticated inspection techniques are required, then it might be
argued that insufficient safety margin was designed in to begin with.  This
might mean more load paths, as opposed to massive thickness of fewer load
paths.  Intuition should guide you to concentrate around holes, fillets and
weldments.  Good cleaning and dental mirrors to all!

robert hardwick, seattle usa
'79 spider
'91 164

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End of alfa-digest V7 #307
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