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In brazing, there is always the possibility of dangerous fumes and gases rising from base metal coatings, ink and cadmium-bearing filler metals, and from fluorides in fluxes. The following well-tested precautions should be followed to guard against any hazard from these fumes.
Ventilate confined areas. Use ventilating fans and exhaust hoods to carry all fumes and gases away from work, and air supplied respirators as required.
Clean base metals thoroughly, A surface contaminant of unknown composition on base metals may add to fume hazard and may cause a too rapid breakdown of flux, leading to over heating.
Use sufficient flux. Flux protects base metals and filler metal during heating cycle. Full flux coverage reduces fuming, Also, consult your MSDS regarding specific hazards associated with brazing flux.
Heat metals broadly. Heat the base metals broadly and uniformly. Intense localized heating uses up flux, increases danger of fuming.
Know you base metals. A cadmium coating on a base metal volatilize and produce toxic fume during heating. Zinc coatings (galvanized) will also fume when heated. Learn to recognize these coatings. It is recommended that they be removed before parts are heated for brazing.
Know you fill metals. Be especially careful not to overheat assembly when using filler metals that contain cadmium. Consult the Material Safety Data Sheet for maximum recommended brazing temperatures of a specific filler metal. The filler metal carries a warning label. Be sure to look for it and follow the instructions.
(For safety considerations, see the American National Standard Z49.1, "Safety in Welding and Cutting", published be the American Welding Society (AWS), 550 N.W. LeJeune Rd., Miami, Florida 33126.)
The pickling solutions recommended below may be used to remove oxides from areas that were not protected by flux during the brazing process. In general, they should be used after the flux residue has been removed from the brazed assembly.
Application |
Formulation |
Comments |
|---|---|---|
| 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. |
Note: The pickles recommended above will work with any of the standard silver filler metals, and no specific instructions are required for the individual filler metals. The phos-copper and silver-bearing phos-copper filler metals are different, and then only when used on copper without flux. In this case, a hard copper phosphate slag forms in small globules on the metal surface. Prolonged pickling in sulphuric acid will remove this slag, but a short pickle in 50% hydrochloric acid for a few minutes is more effective. When the brazed joint is to be plated or tinned, the removal of the slag is absolutely essential. A final mechanical cleaning, therefore, is advisable for work which is to be plated.
* Consult your supplier or local environmental restrictions regarding the proper product disposal information for your area.
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