Galvanized iron, whether in pipe, sheet or other form, is iron or steel coated with zinc to protect it from corrosion. After cleaning, the workpiece is dipped in a galvanizing tank containing molten zinc.

Gas Welding. Zinc has a melting point of about 420°C (786″F), and will volatilize when heated to about 930°C (1700°F). If galvanized iron parts are welded with steel welding rods or electrodes, the zinc coating is heated to the melting temperature of steel, about 1510°C (2750″F), and is burned away in the path of the weld, forming clouds of white zinc oxide. The zinc oxide smoke produced when zinc volatilizes is harmful to the welder. Alternatively, bronze melts at about 885°C (1625″F), below the volatilization point of the zinc; and when a bronze rod is carefully used with the oxyacetylene process, no fumes result.

Galvanized sheets or pipes should generally be bronze welded with Tobin bronze, or manganese bronze, or a high-strength bronze rod if an oxyacetylene torch is used. The zinc coating alloys with the bronze rod and spreads on each side of the joined edges, serving as a protective agent and leaving no bare steel for exposure to corrosion.

A good way to protect the galvanizing on the underside of the sheets is to make a paste of flux and water and paint it on the underside of the sheets with a paint brush. This flux coating prevents the hot sheet from exposure to the oxygen in the surrounding air, and avoids oxidation. The flux paste may also be painted on the upper side from which the seam is welded.  Another way to apply flux is to heat the welding rod and dip it into the flux box, and the flux will cling to it.

The seam should be a butt seam, and the sheets should be held in a jig during welding. When correctly done, the bronze will appear on the lower side of the seam.

Galvanized pipe is easily brazed, using bronze rods and a brazing flux. The pipe is beveled to an angle of at least 30″, and if cut with a cutting torch, all oxide should be removed.

If Tobin or manganese bronze rods are used, the torch flame may be adjusted for a slight excess of acetylene, or it may be neutral. However, with some bronze rods, manufacturers recommend a slightly oxidizing flame. Specific rods will often require varying flame adjustments and techniques.

 

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