Let me make two important statements right at the start: 1. Surface-oxidation of metals will prevent effective brazing. 2. Brazing filler metals (BFMs) do not like to bond to or flow over, oils, dirt, greases, or oxides on metal surfaces. Thus, if any of the surface contaminants just mentioned are present on the metal surfaces to be brazed, effective brazing will not occur. Effective brazing requires the BFM to be able to alloy with (i.e., diffuse into) the base-metal being joined in order to form a strong, leak-tight metallurgical bond. The amount of alloying required is not large, e.g., copper BFM on steel actually alloys/diffuses much less than 5% and yet forms very strong, leak-tight brazed joints on steel.
Surface-oxidation is a common source of problems in commercial brazing, especially in those shops where production personnel say: “Don’t worry about that oxidation; the furnace will take care of that!” Wishful thinking, and highly impractical, since furnace atmospheres may be able to “clean up” the outside surface of the assembly, but will NOT be able to effectively clean deep down inside a braze-joint if any of those inside surfaces (faying surfaces) were oxidized or contaminated prior to assembly. Parts to be brazed must be cleaned BEFORE assembling the parts for brazing, and then must be kept clean during the brazing process.