Proper Cleaning is a Two-Step Process
These two steps of proper cleaning are degreasing (removing oils, fingerprints, lubricants, etc. from each surface to be brazed) and removing surface oxides that are on the metal surfaces underneath those surfaces oils. Let’s look in more detail at each of these two factors.
As shown in Table 2, the removal of any oils and lubricants depends on the types of oils/lubricants you are dealing with. Namely, are they petroleum/mineral-oil based, or are they aqueous (water) based. Degreasing fluids are not necessarily going to be effective at removing all types of oils and lubricants. It is very likely that you will need to use a different type of degreasing solution to match the type of oil/lubricant that you are trying to remove. One-size-fits-all does not work when cleaning parts prior to brazing.
I have visited many brazing shops that have experienced greater-than-normal rejects from their brazing furnaces or their torch/induction brazing operations. In many cases, their cleaning processes prior to brazing were not good or were being improperly used.
For example, it was discovered that shop personnel had been using alcohols and acetones for many years to remove standard lubricants from the surfaces to be brazed (which worked fine). However, they did not realize that their pre-braze cleaning procedures were no longer effective when their supplier switched over to using only water-based (aqueous) lubricants. The alcohols and acetones used were no longer effective at removing oils/lubricants from the surfaces to be brazed (as shown in Table 2), resulting in increased braze failures in their shop. The same was true for other shops that had been using vapor-degreasing processes when the lubricants had been switched to water-based products.
Beware of synthetic, silicone-based lubricants. These lubricants, if allowed to dry on the surface of parts, can render them nonbrazeable. Silicones are not a good actor in brazing, and they must be kept away from your brazing areas. A number of shops have found that they need to either burn off or machine off any of the hardened silicone-based lubricants from the faying surfaces of parts they received from a supplier since attempts to braze those surfaces did not work even after normal de-greasing processes were tried. They did not realize the difficulty that silicone-based lubricants presented.
There is a big difference between silicon and silicone. Silicon is a naturally occurring chemical element (Si) in the Earth’s crust and is a widely used in many metals, including in the manufacture of many aluminum-based and nickel-based BFMs. Silicone, on the other hand, is a man-made synthetic polymer commonly found in many rubberized products used in our homes, as well as in many lubricants.
As was mentioned earlier, the removal of any oxides from surfaces that are to be brazed is essential since molten BFMs will not bond to oxidized surfaces. Surface oxides can come from many sources, and they must be completely removed from all brazing surfaces prior to being assembled together for brazing. Many metals easily react with oxygen at room temperature to form surface oxides. Iron, steel, aluminum and titanium alloys are common examples.
Perhaps the oxides have been added to the surface via surface prep, such as grinding, grit blasting, etc. Aluminum-oxide is a common material used for grinding wheels or grit blasting, and residues from those processes will remain on the surfaces, potentially ruining any hopes for effective brazing. Tumble-deburring, likewise, leaves a surface film from the tumbling media that is used.