On Sat, 7 Mar 2009 13:17:17 -0500, "Ed Huntress"
wrote:


"Tim Wescott" wrote in message
...
On Fri, 06 Mar 2009 22:49:51 -0500, Wild_Bill wrote:

I'm wondering what plating materials can be applied to polished cast
aluminum parts, and how difficult the process would be, such as
unobtainable chemicals or other problems.

It dawned on me that I've seen chrome plated alternators and other
aluminum dress-up parts for cars, but never really paid that much
attention to them. I've seen many applications where a flash-thin chrome
plating finish peels off, and thicker, apparently poorly done or poorly
prepared plating that peels, forming razor-sharp cutting edges (on hand
tools, BTW).

I have an old Dumore 1/3 HP universal electric motor (approx. 3.5" dia.)
that has a lustrous-silver appearance, and I've been wondering what the
plating might be.
The plating is old, I suspect 1950s or maybe as old as the 1940s, and
doesn't look like new or old chrome generally does, instead, it's
slightly dull (like myself) with a greyish and yellowish cast to it.
This finish hasn't been cleaned any time in recent years, which looks
better, to me anyway.

There is an area where some of the plating has been ground away
(probably to remove an engraving) where a sub-layer looks like copper.

The bare base metal looks like oxidized aluminum, and it's non magnetic,
so I assume it's cast aluminum, based on the unusual shape (motor end
bells with various features such as bearing bosses and small oil ports
with reserviors).
Unlike most other electrical gear, I didn't disassemble this to see what
it's like inside (only beause it didn't appear to have been tampered
with).

I don't have any desire to restore the plated part, I just thought it to
be a bit unusual for aluminum to be plated on a utility tool-type part,
when just polishing the aluminum would have provided a good appearance,
at least while the motor was new.
It appears to have been polished prior to plating, but plating generally
outlasts paint or other coatings.

The plating may have been a "pride in our name" and/or a better quality
than our competitors' products appearance-comparison issue, in that
bright plating would appear to be a much better grade of finish than
wrinkle paint, I suppose.

My understanding, from one of the mechanical engineers at one of my
clients, is that copper is sort of the universal primer of metal plating
-- it'll stick to most anything, and most anything will stick to it.

I'd never heard of a flash of nickel under the copper to plate steel,
though.

"It is in most cases still essential to deposit a thin initial layer of
copper from a cyanide colution onto both steel and zinc alloy diecastings
for reasons already discussed, prior to their being acid copper plated.
Alternatively, a thin initial nickel layer deposited from any standard bath
is also quite satisfactory on ferrous substrates. In both cases, the thin
initial layer is often called a 'strike plate'. (_Nickel and Chromium
Plating_)

Punctuation is per the original -- in other words, this is a British book.
g In American parlance, this initial layer is called a "strike layer." If
you Google [nickel "strike layer"] you'll see that nickel is used for the
strike on many other metals as well, often under a copper leveling layer,
which is then topped with nickel, and then chromium. But cyanide-based
copper can also used for strike layers. The leveling layer is sometimes
electroless, but more commonly it's copper plated from an acid bath, rather
than a cyanide bath.

Top-quality chrome plating, therefore, actually is a four-layer process,
even if the first two layers are different methods of plating copper. My
limited understanding is that nickel is used for the flash more often than
copper in high-volume operations. It's done that way where durability is
important and they don't want to use excessively thick layers of plating.

Caswell offers an alkaline copper (no cyanide) that works well as a
flash or strike layer.