Cleaning the Brazed joint
After you've brazed the assembly,
you have to clean it. And cleaning is usually a two-step
operation. First - removal of the flux residues. Second -
pickling to remove any oxide scale formed during the brazing
process. Flux removal is a simple, but essential operation.
(Flux residues are chemically corrosive and, if not removed,
could weaken certain joints.) Since most brazing fluxes are
water soluble, the easiest way to remove them is to quench the
assembly in hot water (120°F/50°C or hotter). Best bet is to
immerse them while they're still hot, just making sure that the
filler metal has solidified completely before quenching. The
glass-like flux residues will usually crack and flake off. If
they're a little stubborn, brush them lightly with a wire brush
while the assembly is still in the hot water. You can use more
elaborate methods of removing flux as well - an ultrasonic
cleaning tank to speed the action of the hot water, or live
steam.
The only time you run into trouble
removing flux is when you haven't used enough of it to begin
with, or you've overheated the parts during the brazing process.
Then the flux becomes totally saturated with oxides, usually
turning green or black. In this case, the flux has to be removed
by a mild acid solution. A 25% hydrochloric acid bath (heated to
140- 160°F/60-70°C) will usually dissolve the most stubborn flux
residues. Simply agitate the brazed assembly in this solution
for 30 seconds to 2 minutes. No need to brush. A word of
caution, however - acid solutions are potent, so when quenching
hot brazed assemblies in an acid bath, be sure to wear a face
shield and gloves. After you've gotten rid of the flux, use a
pickling solution to remove any oxides that remain on areas that
were unprotected by flux during the brazing process. The best
pickle to use is generally the one recommended by the
manufacturer of the brazing materials you're using. (See the
Handy & Harman
recommendations for pickling solutions.) Highly oxidizing
pickling solutions, such as bright dips containing nitric acid,
should be avoided if possible, as they attack the silver filler
metal. If you do find it necessary to use them, keep the
pickling time very short. Once the flux and oxides are removed
from the brazed assembly, further finishing operations are
seldom needed. The assembly is ready for use, or for the
application of an electroplated finish. In the few instances
where you need an ultra-clean finish, you can get it by
polishing the assembly with a fine emery cloth. If the
assemblies are going to be stored for use at a later time, give
them a light rust-resistant protective coating by adding a water
soluble oil to the final rinse water.
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