Mechanisms of Adhesive Bond Endurance: Steel-Epoxy under Hydrothermal Stress

Abstract Stainless steel (AM 355) has been given a number of surface treatments in preparation for adhesive bonding with modified 121 C curing epoxy. The treatments divide into two groups, those that result in very durable joints under hydrothermal stress and those that do not. Those treatments that provide endurance have a chromium-oxide rich outer layer; those that do not have an iron-oxide outer layer. Strongly oxidizing solutions (acids with dichomate) and anodic treatments cause selective dissolution of Fe and reprecipitation of a dentritic layer (∼ 500Å) of an intermetallic compound of Cr. Ni and Mo. These dentrites (covered with a thin chromium oxide layer) form a capillary network that draws adhesive into the network, providing mechanical interlocking. However, the crucial criterion for endurance lies in the chemisorbed layer that results from the treatment. If reaction with water goes to completion in the oxidizing solution, the surface is covered with basic OH which can strongly bond with acid functional groups of the epoxy. Since the surface reaction has gone to completion with respect to water, there is no thermodynamic driving force for post-bond reactions in humid atmosphere. Those treatments that do not go to completion form strong bonds with the adhesive, but degrade under hydrothermal stress in post-bond conditions as the water reactions continue to completion.