Interfacial Chemistry of an Aluminum-to-EPDM Rubber Bonding System

During recent assessment of aging in aluminum-to-rubber bonds on stored solid rocket motors, corrosion and minor insulator debonds were observed. A test was conducted to study the progressive effect of exposure to high humidity on the bondline; elevated temperature was used to accelerate the aging. In a parallel test, samples were held at elevated temperature in a dry atmosphere. The test results were compared with the analyses of corroded and noncorroded hardware samples. The predominant corrosion product detected at the bondlines was aluminum oxide/hydroxide. In general, there was a very good correlation between the Cl:Al atomic percent ratio calculated from X-ray photoelectron spectroscopy analysis of the ruptured bondline surfaces and the visual characterization of the extent of corrosion. The Cl:Al ratio, which represented the ratio of primer to corrosion product at the locus of failure, varied from 0.4 to 47. The implications for metal-to-rubber bond fabrication and storage are discussed.