Structural Bonding of Single-Layer E-Coated Steel Structures in the Agricultural Sector

Mobile working machines usually have a welded frame structure which, due to its size, is commonly painted by spray application, which has a considerable influence on corrosion protection. An optimized corrosion protection could be achieved by electro-dip painting, but this is not economically legitimate due to the small lot sizes. This problem could be solved by shifting the joining process to the end of the process chain and using adhesive bonding technology. This means that smaller parts are dip-coated before the joining process. In order to achieve maximum corrosion protection, it is necessary not to damage the painted surface, so that only an adhesive joining process is possible. For successful implementation, it is therefore necessary to match both the adhesive system and the paint system to each other and additionally to the (steel) materials used and to qualify them with regard to the requirements. In the presented study, the main parameters of both coating properties and adhesive properties are investigated. The acrylate (AY) e-coats, which are scientifically analysed for the first time with regard to adhesiveness, showed good weather resistance, but inferior corrosion protection compared to epoxy-based (EP) e-coats. The adhesive strength of the joints has been investigated by lap shear tests. After stressing the bonds by corrosion tests (neutral salt spray, cyclic corrosion tests) and temperature changes, the failure changed in some cases from cohesive fracture of the adhesive to adhesive fracture in the coating–adhesive interface. Another critical point was detected: if the coating is injured near the bonded area, the corrosive under creepage can penetrate under the joint.