Gases in Metals

Gases enter solid or liquid metals during smelting or electrolysis or upon the interaction of metallic articles with the atmosphere. For example, when steel is produced from pig iron in open-hearth furnaces or in converters, oxygen and nitrogen enter the molten metal from the furnace air; when nickel is produced by electrolysis of its aqueous solutions, the solid metal becomes saturated with hydrogen, which separates out onto the cathode. There are three types of interaction between gases and metals: adsorption, solution, and the formation of chemical compounds.

In adsorption, the gases react only with the surface of the metal, forming a film on it that is one or more molecules thick. Adsorption decreases as the temperature is raised and as the pressure of the gas on the metal is lowered. Gases adsorbed on the metal parts of electrovacuum devices (in measuring apparatus), radio broadcasting equipment, and electrical transformers are desorbed when the apparatus is in use, disrupting the normal operation of the apparatus—for example, they alter the electrical conductivity. The elimination of adsorbed gases during the manufacture of such apparatus is achieved by deep scavenging with gas absorbents (getters). This is one of the most important goals of vacuum technology.

The majority of gases, other than inert gases, form true solutions with solid or liquid metals. Gases with molecules consisting of several atoms (for example, sulfur dioxide, carbon dioxide, hydrogen, and nitrogen) break up into atoms upon dissolution in metals. This facilitates the penetration of the gases into the metal, since it decreases the energy necessary to move aside the strongly interacting metal atoms. In addition, part of the energy expended is made up by the energy gain upon chemical interaction of the atoms of the gas and the metal. Therefore, the solution of multiatomic gases is accompanied by their dissociation. For example, the diatomic gases hydrogen and nitrogen are dissolved in iron according to the reactions

H₂ = 2H_{in iron} N₂ = 2N_{in iron}

The solubility of gases in molten metals is considerably higher than in solid metals. This often leads to the spoiling of metal ingots by the formation of gas bubbles, internal cavities, and porosity. These flaws arise because during the gradual hardening of the ingot in the casting mold the concentration of gases in the still uncrystallized liquid rises to such an extent that the gases are liberated in the mold as crystallization proceeds, forming bubbles in the ingot that cannot float to the surface and be eliminated before the hardening process is complete.

Gases often form chemical compounds (oxides, sulfides, and nitrides) with metals. These compounds are not soluble in the metals and are formed as independent phases—the so-called nonmetallic inclusions—whose presence greatly decreases the mechanical and anticorrosion qualities of metals and alloys. Therefore, various methods are used in industry to eliminate gases from metals. One of the most effective methods is vacuum treatment. When the pressure is reduced, the gases are released from the metals; this occurs with particular intensity when the metal is molten.

The smelting of metals and alloys (especially steel) in vacuum furnaces and the vacuum treatment of liquid metal during its casting and while it is in the ladle are widely practiced. Another method of attaining the same result is to send a blast of an inert gas such as argon through the liquid metal. In a number of instances the metal is smelted or heated in a protective gas atmosphere that does not contain components harmful to the metal.