Assessing the effects of shear, compression, and peel on the cathodic degradation of elastomer-to-metal adhesive bonds

Abstract

For cathodically polarized elastomer-to-metal adhesive bonds, two modes of bond degradation are known to occur: weakening and delamination. Weakening is the degradation caused solely by the high pH environment created by hydroxyl ion (OH−) generation at cathodic sites. Weakening manifests itself as an environmental crack at the primer/metal interface detectable only if the bond is pulled destructively apart. Delamination occurs if peel stress of sufficient magnitude acts on a weakened bond to cause the rubber to separate from the metal.To quantify the effects of compression and shear on the rate of bond weakening, the authors used custom-developed specimens called the compression specimen and the shear specimen, respectively. These were simple elastomer–metal specimens bonded with a commercially available, two-coat (primer and adhesive) system. A number of these specimens were conditioned under a variety of cathodic conditions, including some that were highly accelerated. Static compressive stress was found to decelerate bond weakening in specimens placed in highly alkaline (pH 14) environments. The experimental evidence revealed no significant effect (neither accelerating nor decelerating) of shear stress on bond weakening rate.Tests to evaluate the effect of peel stress on the delamination mode of bond degradation were also performed. To this end, a large number of custom, self-loading specimens called the strip blister specimen, were tested in cathodic environments according to a test matrix that included five levels of cathodic current density and four temperatures in two electrolytes. The rate of bond delamination was found to be sensitive to cathodic voltage, temperature, as well as the electrolyte. Blending the control primer with γ-aminopropyltriethoxysilane was found to significantly retard the rate of bond delamination. Similarly, passivation of the mild steel substrates by a zinc-phosphate coating treatment was found to considerably slow delamination.