Investigating Mechanical Fasteners Coatings for Corrosion Protection of Dissimilar Materials Joints

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Nowadays, several engineering fields focus their attention on the use of hybrid structures. In particular, the combination of traditional materials like steel or metal alloys together with Fiber Reinforced Polymer (FRP) materials have received growing attention.1 The demand of strong light weight structures is growing stronger, especially in the transportation industry due to the need for a high strength to weight ratio as required for better performance, safety, and environmental concern. This has become an important challenge with the increase in demand due to the requirement for high strength and lightweight materials.

Product Number: 51323-18998-SG

Author: Muhammad Haris, Sullivan Smith, Timothy Minton, Lorna Anguilano, Ben Robinson, Briony Holmes

Publication Date: 2023

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In a range of industrial sectors, multi-material constructions are used to optimise the performance of structures. In the transport sector, metal-to-carbon fibre reinforced polymers (CFRP) and steel-to-aluminium joints are seeing a dramatic growth in both R&D activity and commercial applications. As these materials are not weldable, directly to one another, current industrial joining solutions involve the use of mechanical fastening and adhesives. These dissimilar material combinations are highly susceptible to corrosion, because of the galvanic coupling between parent materials with greatly differing electrode potentials. In the automotive sector, carbon steel fasteners with zinc based coatings are widely employed for dissimilar metal and metal-to-composite joints. Nevertheless, there are widespread concerns regarding the long term durability of dissimilar joints. In production cars, dissimilar joints are used almost entirely in ‘dry’ areas of vehicles, where exposure to water, salt and stone chipping from road surfaces does not occur, theoretically. To further reduce the risk of corrosion-induced failure, dissimilar mechanical joints in cars are coated with sealants, zinc-phosphate crystals and multiple layers of paint and lacquer. These measures are taken because present data show that, if paint layers are penetrated and dissimilar joints are exposed to electrolytes, rapid deterioration of the fastener and joint occurs. Therefore, it is extremely important to find a solution that provides corrosion protection to the system by means of anti-corrosive coatings on the fasteners. This can be achieved by developing a better understanding of the underlying mechanisms of corrosion between interacting dissimilar materials and the development of specific coatings, which inhibit the corrosion at the interacting surfaces.
Therefore, this work investigated existing coatings technologies for mechanical fasteners in dissimilar material joining applications. The work focused on understanding the interactions of the coatings on CFRP/Al and CFRP/rivet interfaces when joined and enhancement of the corrosion resistance properties.