Silver solder for SS wire?

[TomP]

Douglas,

First off, you are correct...GTAW is gas tungsten arc welding...better known as TIG.
You are also correct that temperatures as low as 700F can, and do, have an effect on the corrosion resistance capability.
Insofaras using clay dams to limit the heat affected area...no. Clay is a natural insulator, and therefor would not proived a "limiter" to the heat affected zone (HAZ). TIG, due to its very nature, is a precise weld. A skilled TIG welder should produce no more than a 1/16 to an 1/8 of an inch on either side of the weldment itself. Thus limiting the HAZ. There are commercially available "heat pastes" for delicate welding. For the most part, they are an amalgam of tin and aluminum powder suspended in a base...designed to act as a heat sink. I do not have first hand experience with these products, but some welders I know use them. Though I personally feel it is to hide their own short comings as a welder and their failure to control the HAZ.

I co-authored a paper on this...it is quite long, so I will just elaborate on the specifics to this conversation:
True, heat does play into the corrosion resistance of materials. However, it is not necessarily the heat that is the culprit in reduced corrosion resistance, but the process in which it was heated. In this case, let us focus on fabrication, i.e soldering and welding. During fabrication, metal products often come in contact with steel components and tools. Transportation, handling, forming, grinding and welding can all result in physical contact with iron-based structures. During such contact, iron may become embedded into the surface of an alloy component. When stainless steels (e.g. austenitic grades such as AISI 316, 317 and 904L) and superaustenitic grades (e.g. INCOLOY alloy 25-6MO) are
exposed to aggressive acid and acid-halide environments, the embedded iron can accelerate localized attack such as crevice and/or pitting corrosion. Embedded iron has not been shown to affect the corrosion resistance of higher alloyed materials, such
as INCONEL alloys 622, 625, C-276 and 686.

So what can be done? One word: Passivation. Passivation is the formation of a thin, protective film on a metal which makes the metal passive (corrosion-resistant).
While stainless steels require an acid treatment to passivate their surface, nickel and the high-nickel alloys become passive upon exposure to air. The film itself is a chromium-nickel-iron-molybdenum oxide for the chromium-bearing nickel alloys. Chromium is the key constituent giving the film its outstanding corrosion resistance. The film formed in air at the mill is stable. However, during fabrication, the film may be damaged locally when
iron, weld spatter, arc strikes and heat tint scale create local defect sites (i.e. local imperfections in an otherwise passive film). The undamaged surface remains passive
throughout fabrication. Removal of defects, by pickling, electropolishing, or mechanical means, immediately restores the film and passivity so passivating by acid washing is not required. Nitric acid cleaning/passivation treatments for stainless steels are described in ASTM A380. It should be noted that these treatments are not suitable for cleaning structures after fabrication. They are applicable only to passivating chromium-nickel stainless steels and are not applicable for cleaning nickel alloys after fabrication. As already stated, such treatments are not required to optimize the corrosion resistance of the nickel-based alloys.

Whichard made a product called Whichinox (sp) and marketed it as a passivation paste. It essentially was a mild phosphoric/nitric acid pasted with some mild abrasives added to the emulsion. It worked quite well, and from a mettalurgical standpoint, it did, to a degree, passivate the metal.

I hope this was not too much of an overload.

Tom