Austenitic stainless steels develop a chrome oxide layer that makes soldering with Cu/Ag solder at temperatures of 1000°C or lower impossible because the layer doesn’t wet.
Only higher temperatures tear up the chrome oxide layer and wetting becomes possible in those cracks.

To achieve wetting at around 800°C with Cu/Ag solder the area partially gets plated with a special galvanic copper or nickel layer that measures between 5 and 20 µm. The pretreatment rids the stainless steel of its oxide layer, which also won’t return below the galvanic layer. However, if soldering parameters are off, a galvanic copper layer might dissipate within the Cu/Ag solder. Since this doesn’t happen with nickel, an adhesive coating of that kind is preferable for a stainless steel/copper soldering with Ag/Cu solder.

Similarly, materials like Glidcop as sinter material, copper-chrome-zirconium (ELBRODUR), copper-beryllium or tungsten are easier to electroplate adhesively than to solder them.