This is true for all metals, even though the oxides of some metals are not as stable as the oxides of other metals. Gold and nickel are examples of metals whose oxides are not stable at any temperature we would encounter in our daily activities, and thus, do not concern us at all. Copper oxides and iron oxides are examples of metals whose oxides are not stable at higher temperatures, in that those oxides are easily and quickly dissociated at elevated temperatures. Chromium-oxide, however, is an example of a fairly stable oxide (up to about 1850F/1000C before dissociating in a typical brazing atmosphere furnace), whereas aluminum-oxide will be extremely stable in a brazing furnace, and is beyond the capability of any standard brazing atmosphere to reduce that oxide. Titanium-oxides behave in a very similar fashion to aluminum oxides in typical brazing furnace atmospheres.
Brazing filler metals (BFMs) do not like to bond to, or flow over, dirt, oils, or surface oxides, and consequently, if any such surface contaminants are present on the faying surfaces inside a braze-joint, any externally applied BFM will find it hard, if not impossible, to flow by capillary action into that braze-joint. Instead, it will remain outside the joint. If the BFM is pre-placed inside the faying surfaces of a joint that is still contaminated with dirt, oxides, etc.,, the BFM may merely “ball-up” and not flow through the joint.
Cleanliness of both base metal and BFM is essential for a successful braze, since, as just mentioned, BFM will not “wet” surfaces that are dirty or contaminated with oxides. Yet, in spite of this, I often hear brazing-shop personnel say: “Don’t worry about that, the furnace will clean it up”, which is, unfortunately, a grave mistake, since I have rarely seen that happen! Yes, a typical furnace atmosphere (gaseous, or vacuum) may be able to “reduce”, i.e., eliminate, surface oxidation on the outside surfaces of components being brazed, but I defy anyone to truly be able to deep-clean surface contamination from the faying surfaces inside a joint once the parts have been assembled and the BFM applied to the joint. I have seen many such assemblies fail to braze properly, because the entrapped oxides, dirt, etc., inside the joint prevent the BFM from wetting those surfaces and properly flowing through a braze-joint.
Once assembled and placed in a brazing furnace, the furnace atmosphere will keep the parts clean, and allow brazing to take place.
Oxygen is present in ALL furnace atmospheres, including vacuum! Gaseous atmospheres, such as hydrogen, nitrogen, or argon, all have some moisture present in them as they are being delivered to the brazing furnaces from their storage tanks. The amount of this moisture content is measured by the dewpoint of the gas when it reaches the brazing furnace. A typical gaseous atmosphere might have a dewpoint of -60F/-50C when it reaches the furnace. At one standard atmosphere, the dewpoint of a gas is the temperature to which that gas must be cooled to get the first drop of moisture to condense (much like at night when the warmer ground of the earth causes “dew” to form when the cool night air touches the warmer ground).