Got some expansion joints, stainless steed bellows with copper tails. What flux should I use to solder them? Manufacturer data sheet states: "When using soldered capillary fittings, do not over-heat. Shield the bellows from excessive heat. Most fluxes are chloride based and will attack stainless steel, best to use a phosphoric acid based flux. Clean off all residual flux." I currently use LACO regular soldering flux paste, which states "contains no zinc chloride to reduce corrosion concerns" - but doesn't list stainless steel specifically as suitable.