This invention relates to the stabiization of chlorinated solvents. More particularly, the present invention concerns the stabilization of 1,1,1-trichloroethane in the presence of metallic aluminum, copper, brass and zinc.
Chlorinated hydrocarbons such as trichloroethylene and perchloroethylene are commonly used as solvents in chemical processes, dry cleaning, and metal degreasing. Solvents of this type are subject to slow decomposition and oxidation reactions, particularly in the presence of impurities such as water, traces of acid or metal salts. Corrosion of metal surfaces in containers and process equipment and deterioration of solvent quality by formation of acidic and colored byproducts thereby become serious problems. Inhibitors such as acid scavengers and antioxidants are commonly added to these solvents in order to prevent such degradative reactions. Inhibitor concentrations are normally of the order of one percent by weight or less.
1,1,1-trichloroethane poses a particularly difficult stabilization problem because of its unusual reactivity with certain metals, notably aluminum. Traces of metal salts, moisture, or other impurities are not needed to initiate the 1,1,1-trichloroethane-aluminum reaction, for this reaction occurs spontaneously on a freshly exposed aluminum surface with spectacular results, converting the solvent and the metal surface in a few minutes to a blackened mass of acidic, carbonaceous material and aluminum salts. Conventional acid acceptor stabilizers cannot be depended upon to inhibit the aluminum-1,1,1-trichloroethane reaction and suitable inhibitors must be discovered by independent investigation. The search has yielded few effective compounds and these show little or no obvious pattern of structure. Typical 1,1,1-trichloroethane formulations contain about five percent by weight of inhibitor which is usually a combination of compounds to inhibit reaction of the solvent with a variety of metals.
Among numerous compounds mentioned as inhibitors for methylchloroform in the presence of aluminum is pyrazine. U.S. Pat. No. 3,798,170 discloses the use of pyrazine as a stabilizer of methylchloroform against aluminum, both in the anhydrous solvent and when water is present. While pyrazine apparently protects aluminum under such conditions, some indication is given that it is not as effective when other metals, e.g. steel and brass, are present.