Post Braze Flux Residue

It is generally accepted that the presence of flux residues on a heat exchanger enhances its corrosion resistance [80,81,82]. However, it has always been difficult to quantify the level of corrosion resistance enhancement. In corrosion testing of flat panels or coupons coated with flux residue, there is no doubt that there is a beneficial effect. With heat exchangers on the other hand, the general trend shows a longer corrosion life but factors such as uniformity of flux residue coverage and variations in flux load sometimes confuse the corrosion test data. 

There is no indication of interactions between flux residue and coolants, refrigerants [83], turbine oils, and polyalcylene glycol lubricants. Recent experimental work has revealed the correlation of flux residue solubility and fluoride release [84]. 

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P. Garcia and H.-W. Swidersky, Study on the hydrolysis of fluoroaluminates in post-braze flux residues, A.F.C. Holcroft International Invitational Aluminum Brazing Seminar, Novi, October 2008, conference proceedings. 

The appearance of the post-braze surfaces predominantly depends on initial flux loading and uniformity of coating. Once the flux melts, it is completely liquid. In its molten state, the flux has no particles. Once the flux is liquid, it immediately spreads out and wets the surfaces. Upon cooling and solidification, the amount of flux residue and its distribution on the surface of the work-piece is related entirely to the initial flux loading, and not particle size. 

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