Let’s conclude our discussion of magnesium and brazing. Part 1 can be found here.
When does this Mg reaction with oxygen begin? All metals, when heated, strive to react with oxygen to form a metal-oxide on its surface. Some oxides are not stable at room temperature, such as gold or nickel, but for highly reactive metals such as magnesium, aluminum, titanium, chromium, etc., readily observable oxides will form whenever the metals are heated up a few hundred degrees (°F or °C). Thus, equation #1 (2Mg + O2 → 2MgO) will occur whenever Mg is placed in open air. Like aluminum, Mg will maintain a tenacious, adherent oxide layer at all times in open air. When heated, this oxide layer can get thicker.
When Mg chips (Fig. 2) are heated in a vacuum furnace, the oxide layer will not grow very rapidly due to the tiny amount of oxygen available in a clean, tight vacuum furnace designed for aluminum brazing. Then, when the Mg is heated in a strong vacuum, it will reach a temperature at which the metal will begin to change from solid to a gaseous state (i.e., it will begin to volatilize – evaporate/sublimate/outgas). Magnesium, in its gaseous state, is highly reactive with any form of oxygen and becomes an excellent “getter” in a vacuum chamber used for aluminum brazing.
But at what temperature can Mg actually begin to volatilize? Surprisingly, it can begin as low as about 225°C (440°F) in a hard vacuum (Fig. 3). This chart is based on research work published in March 1965 by Ames Research Center (one of the U.S. national research laboratories) in California using a specially designed vacuum chamber (Fig. 4).
Therefore, it is necessary to be sure there is enough Mg available for the brazing process. Since the vacuum pumping system is steadily pulling out any gases in the vacuum chamber, it is important that a sufficient amount of Mg be present. This can be in the aluminum base metal, in the aluminum brazing filler metal (BFM) or in the form of chips in a small container inside the chamber (or a combination). Doing this will supply enough Mg in the furnace to produce an effective Mg cloud to “getter” any oxygen in the furnace at the time of actual brazing.
1. Gilbreath, William P., The Vapor Pressure of Magnesium between 223° and 385°C, Ames Research Center, Moffett Field, CA (March, 1965). NASA Tech Note D-2723