This is the reason for the use of fluxes and atmospheres to reduce the oxide film on the metal surface. If the surface cannot be wetted, the filler-metal flow on the metal surface will not occur. This affects the joint metallurgy of what should be a successful joint. The result of the attempted braze will be voids within the filler-metal flow area, thus reducing the tensile strength of the so-called brazed joint.
Hydrogen is a reducing atmosphere. However, care must be taken when selecting hydrogen as an atmosphere for reducing the metal surface of oxides because of the potential for hydrogen embrittlement. The hydrogen gas must be of a very low dewpoint to prevent it being contaminated with moisture. The moisture will have the potential to accelerate the formation of metallic surface oxides that would be contaminated with moisture/oxygen.
Because of its small size, hydrogen is the most diffusible gas, and it will readily diffuse into the surface grain boundaries of the metal being brazed. If oxygen is present at the surface of the metal (and if the temperature is high enough), the hydrogen can react with the oxygen to form water vapor molecules at the grain boundaries. The depth of diffusion is dependent on how much hydrogen and oxygen is present and how long the metal takes to get to the braze temperature and its complete cycle (heat-up and cooldown).
In steel brazing, the oxygen will not diffuse but only react with the surface. The hydrogen, however, will diffuse and will tend to form hydrogen molecules around the surface grain boundaries.
Careful selection of the flux is critical. Some fluxes, such as the fluoride fluxes, can reduce some oxides and, in conjunction with hydrogen, reduce the oxides further (dependent on the braze temperature selection). The chemistry formation of the oxides and the rate at which an oxide reaction forms will be dependent on the base metal itself and the oxidizing agent.
The metallurgy of the brazed joint can only be seen with X-ray or perhaps with ultrasonic methods. This means that the joint integrity can only be seen at the surface of the braze filler metal. One would then have to rely on mechanical testing or destructive metallurgical examinations such as tensile testing. Therefore, joint preparation, as far as oxide contamination is concerned, is of a critical nature. This leads the way to vacuum, or low-pressure, brazing techniques, which will keep the metal surface free and clear of oxide formation.