Re: Nickel plating of aluminum for good EMC still ok?

Dr. Barry L. Ornitz wrote:
"James Arthur" <bogusabdsqy@xxxxxxxxxxx> wrote in message news:V40_l.2919$u86.1099@xxxxxxxxxxxxxxxxxxxxxxx

As far as galvanic corrosion goes, aluminum and gold don't mix:

http://www.eaa1000.av.org/technicl/corrosion/galvanic.htm

Active SS 430 looks like a "maybe" compatibility-wise w/ aluminum 6061,
but I don't get why it appears twice, widely separated, in lines 28
_and_ 41...that's a bit mysterious.

You do not buy "active" stainless steels.

I imagined "active" just meant stainless steel that hadn't been
passivated, e.g.:

http://www.mmsonline.com/articles/how-to-passivate-stainless-steel-parts.aspx

Because they are alloys
with distinct crystal phases, stainless steels are far more complex
in their corrosion than are simple metals. Stainless steels can be
both passive and active with respect to itself. In other words, a
piece of stainless can selectively undergo galvanic corrosion in
certain areas while other areas are protected. The work history
(machining), the temperature history, and the strain the part is
subjected to determine whether stainless is active or passive and
whether stress corrosion cracking occurs.

People often buy stainless exhaust systems for their vehicles because
they are tired of the conventional components rusting away. But they
are usually wasting their money. Stainless steels have poor
resistance to chloride attack so if your exhaust system is rusting
away because your state salts the roads in winter, they will still
corrode away - just a little slower. The much higher price of
stainless in these areas is virtually never recovered by longer life.

For chloride and other halide exposure, the alloys of choice are the
Hastelloy alloys. Not only are they extremely expensive, they
require extreme care in machining as they work harden. When you
consider that stellite machine tools are made from similar alloys,
this makes more sense.

By the way, the galvanic series referenced in your post contains a
number of typographical errors, especially in regard to the stainless
alloys. 430 SST should only appear twice, once as active and once as
passive.


Nickel plating the Al would prevent oxidation (and it looks great) but it peels off, IM limited E. I don't know if that's inherent or just bad workmanship, but that's what I've seen.

Nickel plating copper is easy. Plating nickel over aluminum requires
that the aluminum surface must be free of oxidation. But as soon as
the piece is lifted from the etch tank, it oxidizes again. Generally
the aluminum is cleaned in a slightly acidic solution containing zinc
salts. This produces a very thin layer of zinc that protects the
aluminum during the transfer. The nickel is then plated over the
zinc to any desired thickness. I think that the peeling referred to
above is with electroless nickel. I have had aluminum waveguides
nickel plated for abrasion resistance and I never saw the plating
peel off, even when gouged with a screwdriver. Electroless nickel
can be applied over plastics (like buttons) but with no true metallic
bonding. I once visited a startup company that was doing nickel
coatings via the thermal decomposition of gaseous nickel
tetracarbonyl. Because of the extreme toxicity of Ni(CO)4, I would
avoid this process completely. The OSHA permissible exposure limit
for nickel tetracarbonyl is one part per billion. This is 50,000
times lower than carbon monoxide. [If you do get a stainless exhaust
system for your automobile, carbon monoxide in your exhaust will
slowly corrode the nickel out of the stainless forming nickel
tetracarbonyl. At International Nickel, I saw pieces of stainless
tubing and stainless fittings that had been eaten through by carbon
monoxide flowing through them.]

I have a number of files on galvanic corrosion including a few that
specifically discuss stainless steels and marine environments. Email
me if you would like copies of them.

Barry BLOrnitz48@xxxxxxxxxxx



Joerg's the guy with the requirement, and he's decided to
leave the final choices up to his customer. Nonetheless,
that was a very interesting treatment of several related
questions.

Thanks!

--
Cheers,
James Arthur
.

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