A corrosion-inhibiting mixture of compounds has been developed for addition to the water used to rinse metal parts that have been cleaned with aqueous solutions in preparation for adhesive bonding of the metals to rubber and rubberlike materials. Prior to the development of this corrosion inhibitor, the parts (made, variously, of D6AC steel and 7075-T73 aluminum) were rinsed by deionized water, which caused corrosion in some places on the steel parts — especially in such occluded places as sealing surfaces and threaded blind holes.
An integral part of the particular cleaning process is the deposition of a thin layer of silicates and silane primers that increase the strength of the adhesive bond. The corrosion inhibitor is formulated, not only to inhibit corrosion of both D6AC steel and 7075-T73 aluminum, but also to either increase or at least not reduce the strength of the adhesive bond to be formed subsequently. The corrosion inhibitor is a mixture of sodium silicate and sodium tetraborate. The sodium silicate functions as both a corrosion inhibitor and a bond-strength promoter in association with the silane primers. The sodium tetraborate buffers the rinse solution at the optimum pH and functions as a secondary corrosion inhibitor for the steel.
For a given application, the concentrations of sodium silicate and sodium tetraborate must be chosen in a compromise among the needs to inhibit corrosion of steel, inhibit corrosion of aluminum, and minimize cosmetic staining of both steel and aluminum. Concentrations of sodium silicate in excess of 150 parts of silicon per million parts of solution (ppm Si) have been determined to enhance inhibition of corrosion; unfortunately, because of the alkalinity of sodium silicate, even a small concentration can raise the pH of the rinse solution to such a level that aluminum becomes corroded despite the inhibiting effect. The pH of a solution that contains a high concentration of sodium silicate can be decreased by adding sodium tetraborate. On the other hand, the addition of sodium tetraborate increases the concentration of dissolved solids to such a high level that cosmetic staining becomes an issue.
One suitable compromise calls for establishing the pH of the rinse solution at 9.5 and including sodium silicate at a concentration of 50 ppm Si. This choice is justified by the observation that at a pH of 9.5, the rates of corrosion of both steel and aluminum are relatively low, and, therefore, only a minimal concentration of corrosion inhibitor is needed. By reducing the concentration of silicate, one reduces the amount of sodium tetraborate needed to buffer the rinse solution, thereby also reducing the level of cosmetic staining.
The figure depicts the effectiveness of the corrosion inhibitor in blind holes in D6AC steel specimens. In other tests on D6AC steel specimens, rates of corrosion in water containing this corrosion inhibitor were found to be approximately 10–5× those in deionized water. In similar tests on 7075-T73 aluminum specimens, rates of corrosion in water containing this corrosion inhibitor were found to be only slightly greater than those in deionized water. In still other tests, the strengths of adhesive bonds on steel and aluminum specimens rinsed with corrosion- inhibited solutions were found to be approximately equal to those on specimens rinsed with deionized water.