There are two types of paste flux that are commonly available for brazing, differentiated by their color: white flux and black flux. Both of these paste fluxes are water-based and contain fluoride and potassium compounds (making them somewhat acidic). They are both quite suitable for use on a wide variety of base metals when using a silver-based brazing filler metal (BFM). There are a number of manufacturers of these brazing fluxes, and each of their formulas is proprietary as well as being somewhat different from the others. There are certain additives, though, that each has in common, namely fluorides and some form of potassium borates. The presence of fluorides and borates in the flux enables the flux to be very effective at absorbing oxygen and thus preventing the formation of oxides on the metal surfaces onto which they are painted.
White Flux offers protection over a temperature range of approximately 320-870°C (600-1600°F). Its general characteristics are controlled by a number of industry specifications, including Aerospace Material Specification (AMS) 3410. White fluxes are suitable for use when brazing many ferrous and nonferrous metals such as carbon steels, copper and copper alloys.
Black Flux is similar to white flux except that it has additional powdered boron added to its chemistry. Powdered boron is a very black talcum-like powder, which, when added to the white flux, not only turns the flux to a black color but also increases the flux's ability to absorb more oxygen than white flux. It can also provide good surface protection to a slightly higher temperature (up to about 925°C/1700°F) and for a longer time than the white flux. This added boron improves the flux’s ability to enhance the surface wetability of certain tough-to-wet base metals such as austenitic stainless steel (because of the tenacious chromium oxides on the stainless) and certain refractory metals (such as tungsten and its alloys). Whenever these base metals are being brazed, black flux is by far the preferred flux to use. Black flux is controlled by a number of industry specifications, including AMS 3411.
NOTE: Boron is a biologically safe product, as evidenced by its use in boric-oxide eye washes that are readily available in all drugstores. Thus, the "safety" of black flux is identical to that of white flux in all respects.
Next time, we will finish the discussion of brazing fluxes by talking about cleaning.