Flux or No Flux? Welding and Soldering Platinum

Credit: http://www.jmny.com/

Apart from the high temperatures involved, platinum alloys weld readily without the need for fluxes. Because filler metal, if needed, is the same as the components to be joined, it is easily possible to close the seam without any visible colour change. This is a decided advantage in sizing rings, for instance. Even so, it is good practice to minimise the amount of filler needed (whether welding or soldering) by making joints as closely and accurately as possible; platinum alloys do not readily bridge large gaps with filler metal (which is not pasty at any stage).

The absence of fluxes, except for the lowest melting point solders (where the flux is to protect the solder, not the platinum – see below table), means that the usual adhesiveness of flux cannot be used to support the solder paillons in place. In practice, a small amount of non-borate flux may be used for this purpose, although it is not necessary metallurgically. Alternatively, solder may be clipped in the joint or supported by a thin extension of stock wire that is allowed to melt free at the last moment.

Platinum Solder Guide