There is a second precaution necessary when brazing aluminum to stainless.
Nickel Plate to Cover Up Oxide Layer on Stainless Steel
Stainless steel stains less than regular steels because of a tenacious chromium-oxide layer that is built up on the surface of the stainless. During brazing it is necessary to eliminate that oxide layer so that the molten BFM can alloy with the chromium and other metal components of the stainless alloy since any oxides on the base-metal surfaces can prevent brazing from occurring.
When heating the stainless steels for brazing, the chromium-oxide layer builds up even more and gets thicker. It only begins to dissociate (chemically break up into pure chromium with the release of oxygen into the furnace atmosphere) at temperatures much higher than those used for brazing aluminum. When aluminum brazing, therefore, the stainless steel should be plated with a thin layer of pure, electrolytic nickel (which will not oxidize) in order to prevent any oxygen in the furnace atmosphere from oxidizing the chromium in the stainless any further. It also covers up the chromium-oxide layer that is already there so that when it melts the BFM will see a clean, non-oxidized layer of pure-nickel to which it can readily braze. On the aluminum side of the joint, we’ve previously discussed about how that oxide layer breaks up during brazing operations so as to be brazeable.
Whenever you are brazing metals with significantly different expansion rates, a ductile intermediate core of soft metal may be needed to absorb the expansion stresses that may occur during the heating/cooling of the brazing processes. It is also very important to remember that molten BFM does NOT like to bond to surface oxides. Therefore, surface oxidation must be carefully considered (and prevented) during the brazing process.
When these two issues are properly handled, brazing can then be performed reliably, with excellent joint integrity and strength, fit for the end-use service conditions to be encountered.