[Antennas] RE: Soldering Stainless Steel (and aluminum)

[Barry L. Ornitz]

Lots of messages in the last digest were on soldering stainless steel.
I'd like to add a little.

Stainless steel can be soldered with ordinary tin/lead solder but the
results will be better if a special solder alloy is used.  These
alloys generally contain cadmium to provide better wetting of the
nickel in the stainless.  While lead is toxic enough, cadmium is also
very toxic and it tends to leach from the solder far faster than lead
can.  Thus you do not want to use stainless solder to repair kitchen
utensils or anything used to prepare food.  Many automotive parts
stores carry small rolls of stainless solder and the necessary acid
flux.  They take a somewhat higher temperature soldering iron than do
conventional tin/lead solders.

The reason stainless steel is so difficult to solder is the same
reason aluminum is difficult to solder.  The surfaces of both are
quickly oxidized upon exposure to air.  This oxide layer is
tenaciously bonded to the surface of the base metal and it prevents
the solder from sticking.  [It also prevents further oxidation.]  In
the case of stainless steel, a good acid flux (stronger than the zinc
clhoride flux often seen with plumbing solder) can remove it allowing
the solder to bond to the metal.  With aluminum, the oxidation occurs
so quickly that it takes both a very aggressive flux and one that can
immediately cover the bare aluminum preventing new oxidation.

Those aluminum "welding rods" seen at hamfests are really a form of
brazing rod.  [And technically brazing is just a higher temperature
form of ordinary soldering.  The higher temperatures allow more
alloying of the base metal with the bonding metal.]  They work
extremely well if used within their limitations.  Most welding
suppliers carry these rods at far lower prices than seen at hamfests.
Chemically they are mainly zinc with a few percent of aluminum (I had
an analysis done once of a hamfest rod).  Here you heat the aluminum
to the proper temperature and scratch the surface with the rough end
of the rod.  This displaces the oxide and the rod melts.  If the piece
is hot enough, the zinc bonds to the aluminum and lifts the oxide away
as the zinc flows under it.  It works great for filling holes, but to
join two pieces it takes additional work.  It will not flow into a
joint by capillary action like we normally see in conventional
soldering.  To join two pieces, you have to thoroughly cover both
pieces with the zinc, clamp them together, reheat and melt the zinc
between them.

The failures normally seen when soldering aluminum or stainless are
generally the result of dissimilar metal corrosion when the joint is
later wet.  I have had aluminum to aluminum soldered joints hold up
for years outdoors, but an aluminum to copper soldered joint always
would fail within a day or two after getting wet..  If you have to do
an aluminum to copper connection, use the aluminum solder to join
aluminum to steel.  Then use conventional solder to join the steel to
the copper.  Stainless to copper joints are generally not too much of
a problem.

High temperature silver solder is also an option.  Again it is a form
of brazing if you go by temperature.  [Note that welders usually only
refer to brazing when brass rods are used.]
These joints are extremely stong.  The flux is normally a borax
related compound that melts and forms a glass over the surfaces
protecting them from oxidation.  The metal oxides dissolve in the
glass.  After the joint is made, you need to clean off the residual
flux to prevent future corrosion.  The high temperature "silver
solder" rods contain varying amounts of silver; the ones with the
highest silver content are the strongest (and most expensive).

With the proper equipment and rods, welding stainless is a breeze.  I
have never understood why stainess has the reputation as being
difficult to weld.  When I was in the research labs at Eastman
Chemical Company, our welders in the research shops preferred to work
with stainless over virtually anything else - and being a chemical
company, we had stainless everywhere!

Someone else mentioned MAPP gas.  This is a mixture of
methyl-acetylene and propadiene.  With oxygen, it reaches temperatures
almost as high as oxy-acetylene welding.  But the mixture is far safer
than acetylene to work with.  You can also get MAPP-air torches.  They
are similar to, but not interchangeable with, propane-air torches.
They do go to much higher temperatures than do propane torches.  These
torches are good for hard silver soldering.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net