Practically all metals and alloys can be brazed or hard soldered satisfactorily with the exception of alloys having zinc, tin, lead or magnesium as base and aluminium alloys containing more than 2% magnesium.
Cleaning of Components before Brazing
Mechanical cleaning methods are filing, scraping, grinding, polishing and sandblasting. The cleaned parts should not be touched with bare hands. Alloys, on which oxide films grow rapidly, should be brazed soon after cleaning. For chemical cleaning, components should be degreased as suggested below:
(a) Steel and Iron. Oxide or scale can be removed by dipping the parts in a hot solution of 5 to 10% sulphuric acid (H2SO4) in water.
(b) Brass. It requires pickling in a solution of 10% sulphuric acid in water for approximately 10 minutes. After the parts have been pickled, they should be washed with clean water. The components should not be over-pickled with this solution, particularly if they have fine screw threads.
(c) Copper. Only degreasing is carried out before brazing copper parts.
(d) Stainless Steel. Parts should be immersed in a mixture of 3 parts of sulphuric acid, 1 part of nitric acid and 10 parts of water until all visible oxides are removed. After pickling, a water wash is required.
Cleaning after Brazing
All traces of flux must be removed. Otherwise flux which has acid constituents causes corrosion. In oxygen and other gaseous equipment, explosion may occur from the action of the gas on the flux residue. Fluoride flux residues are removed by scrubbing parts in water with a wire brush or leaving the parts immersed in hot water. Borax type flux is removed by diluted sulphuric acid pickle, followed by a clean water rinse. Non-ferrous metals in most cases may be quenched in water without harmful results as soon as the brazing alloys have set. If a borax type flux has been used, the certain contractions caused by this quench assist in cracking off some of the glass like residue of flux.