Once you have properly brazed the assembly, you will need to clean the brazed joint. Cleaning a braze joint is a two step process. The first step is to remove all flux residues. Second, pickle the joint to remove any oxide scale formed during the brazing process.
Step 1: Flux Removal
Flux removal is a simple, but essential operation. Flux residues are chemically corrosive and, if not removed, could weaken certain joints. Since most brazing fluxes are water soluble, the easiest way to remove them is to quench the assembly in hot water (120°F/50°C or hotter). Best bet is to immerse them while they're still hot, just making sure that the filler metal has solidified completely before quenching.
The glass-like flux residues will usually crack and flake off. If they're a little stubborn, brush them lightly with a wire brush while the assembly is still in the hot water. You can use more elaborate methods of removing flux as well like an ultrasonic cleaning tank to speed the action of the hot water or live steam.
Troubleshooting When Removing Flux
The only time you run into trouble removing flux is when you haven't used enough of it to begin with or you've overheated the parts during the brazing process. Then the flux becomes totally saturated with oxides, usually turning green or black. In this case, the flux has to be removed by a mild acid solution. A 25% hydrochloric acid bath (heated to 140- 160°F/60-70°C) will usually dissolve the most stubborn flux residues.
Simply agitate the brazed assembly in this solution for 30 seconds to 2 minutes. No need to brush. A word of caution: acid solutions are potent, so when quenching hot brazed assemblies in an acid bath, be sure to wear a face shield and gloves. After you've gotten rid of the flux, use a pickling solution to remove any oxides that remain on areas that were unprotected by flux during the brazing process.
Step 2: Pickling Oxide Scale
The best pickle to use is generally the one recommended by the manufacturer of the brazing materials you're using.
Highly oxidizing pickling solutions, such as bright dips containing nitric acid, should be avoided if possible, as they attack the silver filler metal. If you do find it necessary to use them, keep the pickling time very short.
The pickling solutions recommended below may be used to remove oxides from areas that were not protected by flux during the brazing process and are typically used after the flux residue has been removed from the brazed assembly.
|Oxide removal from copper, brass, bronze, nickel silver and other copper alloys containing high percentages of copper.||10 to 25% hot sulphuric acid with 5 to 10% potassium dichromate added.||Pickling can be done at same time flux is removed. Will work on carbon steels, but if pickle is contaminated with copper, the copper will plate out on the steel and will have to be removed mechanically. This sulphuric pickle will remove copper or cuprous oxide stains from copper alloys. It is an oxidizing pickle, and will discolor the silver filler metal, leaving it a dull gray.|
|Oxide removal from irons and steels.||A 50% hydrochloric acid solution, used cold or warm, More diluted acid can be used (10 to 25%) at higher temperatures (140-160°F/60-70°C.)||A mixture of 1 part hydrochloric acid to 2 parts water can be used for Monel and other high nickel alloys. Pickling solution should be heated to about 180°F/80°C. Mechanical finishing is necessary for bright finishes. This HCI pickle is not like bright dips on nonferrous metals.|
|Oxide removal stainless steels and alloys containing chromium.||20% sulphuric acid, 20% hydrochloric acid, 60% water, used at a temperature of 170°-180°F(75-80°C.)||This pickle is followed directly by a 10% nitric dip, and then a clean water rinse.|
|20% hydrochloric acid, 10% nitric acid, 70% water, used at about 150°F(65°C.)||This pickle is more aggressive than the sulphuric-hydrochloric mixture listed above, and will etch both the steel and the filler metal.|
Once the flux and oxides are removed from the brazed assembly, further finishing operations are seldom needed. The assembly is ready for use, or for the application of an electroplated finish. In the few instances where you need an ultra-clean finish, you can get it by polishing the assembly with a fine emery cloth. If the assemblies are going to be stored for use at a later time, give them a light rust-resistant protective coating by adding a water soluble oil to the final rinse water.