In electrical power engineering and in electromobility, bimetallic joints can occur when e.g. the coated terminals of the lithium-ion battery are connected to the bare aluminum used as conductor inside the vehicle. The resistance of such detachable, current-carrying connections consisting of a bare aluminum contact member and one of bare copper or a coated contact member with a cover layer of tin or silver increases significantly over time. Previous research showed that the failure is due to the formation of Al₂O₃ in the contact interface. In this paper, the film growth is investigated more closely by supplementing the resistance measurements with metallographic investigations. Different mathematical models are developed that describe the film growth and aim to predict a connection’s resistance behavior over time. The growth is considered in lateral direction, decreasing the area of metallic contact, as well as in axial direction causing quasimetallic areas to rise in resistance and eventually become insulating. The entirety of the apparent contact area is considered in a macroscopic model as well as single a-spots in a microscopic model. The results yielded by the models were compared to the measured values of the long-term behavior of connections of different material combinations.