Making metal connections to an anode
There are two
connections to be considered regarding the anode. Firstly there is the
connection to allow the anode to be plated (relatively easy to get right) and
secondly there is the connection to the anode that is used when the anode goes
into service. This is the most difficult connection to get right. This
connection has to last for months and must pass substantial currents without
heating excessively. It must be protected from the salts, both from salts that
arrive externally to the connection (via spray from the cell) and also from
salts that are transported to the connection via the substrate by cappillary
action up the substrate (if the substrate is porous). This is the more difficult
problem to counteract. With Graphite or Ti substrate
anodes there is no connection problem as the supply is simply connected to the substrate.
Connections for long term service of the anode
The connection to carry
the heavy (10's of amps), long term (months) current for when the anode goes
into service in a (Per)Chlorate cell is the big problem. Most metals are not compatible with
Lead Dioxide and if you try to put a copper strap onto the anode the current
will stop flowing after a very short time. The problem is that non conductive
metal oxides always appear at the junction of the metal and the Lead Dioxide so
that current stops flowing. Silver forms a conductive Oxide when used to connect
to Lead Dioxide and is therefor the only suitable metal for the job. See JES
Vol. 105 page 101 (available on this web page) for a discussion.
If Graphite
is used to form a permanent connection it is difficult to get a low resistance
contact to the Lead Dioxide because the effective contact area between the rigid
Graphite and the rigid Lead Dioxide will be small. The Graphite will, at the
very least, have to be lapped so that it will have good contact.
It should be noted that silver is quite resistant to salts from the
cells and is MUCH more resistant than copper. The Graphite and Silver paint is
not a bad combination.
Silver paint can be purchased from car accessory stores. It is used for
mending rear windscreen heaters and is usually called 'Electroconnect' or
'Electrorepair' or some such like. It can vary in price from shop to shop so
shop around. It should cost about 8 US dollars for about 3ml. This sounds
expensive but 3ml will do quite a lot of connecting. Scan
of Silver Conductive Paint Box.Conductive silver paint can also be purchased
from electronic supply houses where it is sold for doing quick mending jobs to
printed circuit boards. It is important to realise that the paint must have
silver METAL in it, not just a silver colour. Aluminium paint is 'silver' paint
but it will be absolutely useless for the task here. Ebay may be a source. Use
'conductive silver paint' as a search string.
Silver can also be
electrodeposited on from a Silver Nitrate or Chloride tank, but it requires more
work.
The metal connection must be protected from the
corrosive effects of the (Per)Chlorate cell. This is important.
There should be no salts arriving to your connection via spray or such
like (externally) if you operate the cell with a proper lid and the anodes and
cathodes are installed in the lid properly.
Silver is quite resistant to the salts from the (Per)Chlorate cell and is
MUCH more resistant than Copper.
If your junction between the anode and the
conductor is not been overstressed (big current, relatively small area) then the
connection is not too critical and crocidile clips or clothes pegs can be used
to hold Copper braid in place against the Silvered areas. With Graphite or Ti substrate
anodes there is no connection problem as the supply is simply connected to the substrate.
See Silver and Copper electroplated connections.
It must be bourne in mind that the
connection will fail, no mater how good it is, if salts are getting to it.
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