I guess it is very highly situational. I think we will continue our practice of cleaning thoroughly and using flux to keep it clean. Operating on the principle that anything is cheaper than rework. (A bit broad, perhaps, but a pretty good rule of thumb.)
Tom
From the AWS Handbook
Fluxes are not designed or intended for the primary removal of oxides, coatings, oil, grease, dirt, or other foreign materials from the parts to be brazed.
And 2 paragraphs later
Oxides are the principal source of surface contamination. The dissolution and removal of oxides during brazing are the most common functions of a flux.
AWS Brazing Handbook 4th edition
FLUXES AND ATMOSPHERES
The purpose of a brazing flux' is to promote the formation of a brazed joint by protecting the base metal and filler metal from oxidation. The brazing flux may also serve to remove surface oxides and therefore reduce surface tension to promote freer flow of filler metal.
When metals are exposed to air, chemical reactions occur. The rate of these reactions generally is accelerated as the temperature increases. The most prevalent reaction leads to formation of oxides, though nitrides and even carbides are formed in some instances. The rate of oxide formation varies with each metal composition and the nature of the oxide. Oxide tenacity, structure, thickness, and resistance to removal or further oxidation are all factors that need consideration. Oxide formation in air on some metals such as aluminum occurs instantaneously, even at, or below, room temperature. In almost all conditions, such surface oxides or other compounds create barriers to the formation of brazed joints.
FLUXES
Fluxes are not designed or intended for the primary removal of oxides, coatings, oil, grease, dirt, or other foreign materials from the parts to be brazed. All parts prior to brazing must be subjected to appropriate cleaning operations as dictated by the particular metals (Chapter 7).
In making a braze, the flux is often needed to combine with, remove, or dissolve those unwanted residual compounds or products of the brazing operation which would otherwise impair filler metal flow.
Oxides are the principal source of surface contamination. The dissolution and removal of oxides during brazing are the most common functions of a flux. So as not to impair metal flow, the flux must also be fluid and displaced readily by the molten brazing filler metal. Wetting ability and viscosity of a flux at brazing temperature, therefore, are important properties.
Under some circumstances, flux also may suppress volatilization of high vapor pressure constituents and the formation of oxide fume condensate in a filler metal.
Brazing by Mel Schwartz ASM International
One of the functions of a flux is to remove the oxide layer on the joint area and expose clean base metal.
ASM Vol. 6 Welding. Brazing and Soldering 1993
Definitions
Flux - Material used to prevent, dissolve or facilitate removal of oxides and other undesirable surface substances.
ASM Vol. 6 Welding. Brazing and Soldering 1993
Selection Criteria for Brazing and Soldering Consumables by A. Rabinkin, AlIiedSignal Amorphous Metals
Cleaning the base-material components being joined and protecting them from oxidation during heating are essential in both brazing and soldering processes. Chemically active substances, called fluxes, are commonly used to provide such cleaning and shielding. The use of nonmetallic agents in fluxes is generally undesirable, because of the increased propensity for their entrapment and the resultant formation of voids in the finished joint. However, the use of certain active elements that can be added to filler-metal alloys to act as a flux can be advantageous. However, this concept usually only works with brazing filler metals. For solders, working temperatures are generally too low for the reduction reactions to take place with adequate speed, although in some cases these reactions do indeed occur.
Thomas J. Walz
Carbide Processors, Inc.
