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From: (Carl Ijames)
Newsgroups: rec.crafts.metalworking
Subject: Re: Brazing SS?
Date: 22 May 1999 18:53:34 GMT

>It is most surely not something one should do at home. The major
>component of the solution is sulfuric acid, but commercial products that
>I have seen are propriatory mixes. The label on the drums did not

From what I remember the most common recipe is 50% concentrated sulfuric
acid (about 3 times more concentrated than battery acid) and 50%
concentrated phosphoric acid plus some water, but I've also tried a citric
acid solution that wasn't bad and was much safer (sorry, don't have the
recipe any more :-().  I haven't looked, but I'd bet that the metal
finishing handbook has some recipes.  The stainless is the anode, and the
cathode can be lead, stainless, or copper since it gets plated onto
instead of dissolving (unless you get the leads reversed, as one poster
mentioned :-)). Current density is on the order of 1-10 amps per square
inch and about 3-8 volts (?), and polishing times are 1-10 minutes.
Warming the solution and stirring it both give better results.  The idea
is that the current density is greatest at sharp protrusions so they erode
faster, which tends to level or polish the surface.  It also tends to
erode corners and the edges of threads, too, so protect what you don't
want polished (wax, tape, paint, anything to cover the surface).  The
solution needs to be a good electrolyte to carry the current, and it also
needs to contain a good sequestering agent to keep the metal ions that are
produced at the surface in solution and to promote even polishing.  That's
where the proprietary mixes of surfactants and such come in, and what the
citric acid and phosphoric acids are for - phosphate and citrate are good
chelating agents.  Sorry I can't be more specific about recipes, and if
you do want to try a concentrated acid bath I pray you are careful.

Carl Ijames

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