|
Fluxes & Fluxing
Brazing fluxes are
proprietary formulations, some standard specifications
which do exist are very general regarding compositions.
Of the constituents in AGR Matthey brazing fluxes,
boron and fluorine compounds are the active deoxidizing
constituents.
The selection of a suitable flux has to take into
consideration the active range of the flux relative
to the melting range of the brazing alloy to be used.
The flux must be active at least 50°C below the
solidus and must remain active at least 50°C above
the solidus of the brazing alloy. In addition to this,
special fluxes are recommended for certain applications
such as stainless steel and tungsten carbide brazing.
Flux Removal
5 reasons for removing residual flux after brazing:
1. Enable proper inspection of brazed joints
2. May be contributing to bonding of joint giving
false impression of integrity.
3. May give incorrect conclusions on pressure/leak
testing and after leaching out in service, cause service
leaks.
4. Absorbs moisture promoting oxidation and corrosion.
5. Hinders painting, electroplating etc.
Flux removal should not be delayed beyond 24 hrs to
avoid corrosion. Hot water rinse removes most residues.
Tenacious flux is often an indication of overheating
during brazing or using too little flux.
10% caustic soda immersion is necessary for residues
of fluxes used in stainless steel and carbide brazing.
Acid pickling is not very effective in removing flux
residues unless the residues are burnt or blackened.
If pickling is necessary it should be carried out
after flux residue removal.
Cold 5% sulphuric acid can be used for more tenacious
flux residues. Small addition of sodium dichromate
accelerates action but care with metal etching.
Residues from
SilBRAZE P alloys:
The greyish coloured residue of the slag left on the
work-piece and the brazing alloy fillet does not subsequently
cause corrosion and need not be removed.
When a tenacious green residue is present after brazing
this is a sign of excessively long brazing cycles,
which have led to flux exhaustion
When SilBRAZE P alloys are used to braze brass or
bronze, conventional borate fluxes have to be used
as phosphorous does not dissolve zinc or tin oxide.
Testing for Residual Flux
- Halide Tests: To ensure that residual halide-containing
flux has been removed. If the flux is known not to
contain fluorides, tests for chloride need not be
made; if the flux is known not to contain fluorides,
the test for fluorides need not be made. If tests
indicate the presence of halides, parts must be subjected
to additional cleaning and testing procedures until
removal is complete.
1. Chlorides: Rinse
the test area with 40 to 50 cc of hot (80°C approx)
ASTM D 1193 Type IV water. Collect rinse water in
a 100cc beaker and add 3 to 5 drops of concentrated
nitric acid (sg 1.42) and 2 to 3 cc of 10% silver
nitrate solution. Stir the contents and allow to stand
for 5 to 10 minutes. A solution as clear as a blank
of ASTM D1193 water treated in the same manner as
the rinsings indicates the absence of chlorides. A
white-to-grey precipitate or turbidity indicates the
presence of residual chloride flux
2. Fluorides: Rinse
the test area with 200 cc of hot (80°C approx)
ASTM D 1193 Type IV water. Collect rinse water in
a 250cc beaker. Use approx 200cc of ASTM D1193 water
as a comparison sample. Test both samples in accordance
with ASTM D 1179. A higher concentration of fluoride
in the rinse water than the comparison sample indicates
the presence of fluoride containing residual flux.
Shelf Life of Fluxes
Under cool, dry conditions and with unbroken seals,
flux powders should be satisfactory for a minimum
of three years from date of supply and flux pastes
for one year. After these times powders contain lumps
due to settlement and paste may show signs of drying
our but both should still braze satisfactorily. Once
seals are broken it is recommended that powders be
used within one year and pastes within 6 months of
opening.
|
