[TheForge] Pickling problem

[Bruce Freeman]

Recently I made a test piece - a candle "socket" and
drip cup for a chandelier.  To keep it "simple", I
forge-brazed the socket (a piece of pipe) to the cup
(a saucer-shape) with a copper-based flux-coated
brazing rod with supplemental borax flux.

The pipe and, especially, the flat were rusty, and I
had ground down to base metal only where the braze was
to be - the rest remaining rusty.  The rust wouldn't
hurt the final piece after normal cleanup.

Well, something happened unexpected.  The "rust"
turned into a thick, black layer, quite unsightly. 
"No problem", I thought - I'll just pickle it off. 
Well, a day or more in sodium bisulfate pickle (albeit
of uncertain strength) hardly touched it.  

So I tried electrolyzing it off in a bicarbonate bath
@ 12V (excessive, but my charger does only 12V).  That
obviously worked somewhat, but slowly.

Next I tried muriatic acid - straight.  It's been in
that for a day now.  It's at about freezing, but the
reaction rate would only be about twice as fast at
room temperature.  I see small progress.

So I'm thinking some really strange chemistry must
have happened in that flux.  Rust + borax + rod flux
(which is what?) seems to have given a black
glass-like compound not very amenable to  pickling
off.

In retrospect, I'd have cleaned the piece first.  (No
loss - it IS a test piece.)  I've never had this kind
of trouble using clean metal.

Anyone have any similar experiences or know what's
going on here?

Bruce
NJ




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