All metals want to react with oxygen as the metals are heated. The higher the temperature, the greater the thermodynamic driving force to have those oxides form. 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. By Dan Kay
In June’s article, we’ll look at how creating “partial-pressures” in vacuum furnaces by back-filling vacuum furnaces with an inert atmosphere is sometimes necessary to achieve successful brazements!