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Novel Fastener Coatings for Corrosion in Dissimilar Joining

Fri, 13 December, 2019

TWI is developing novel coatings for corrosion protection of mechanical fasteners, specifically for joining carbon fibre reinforced polymer composites to metal, as well as other dissimilar materials combinations where corrosion is an issue.  

Multi-material constructions are used in a range of industrial sectors to optimise the performance of structures.

Focusing on the transport sector, the use of metal-to-carbon fibre reinforced polymer (CFRP) and steel-to-aluminium joints is seeing dramatic growth. As these materials cannot be welded together directly, many industrial joining solutions involve the use of mechanical fastening (riveting, flow forming screws, etc.) usually in combination with structural adhesives. Even with successful joining, however, these dissimilar material combinations remain highly susceptible to corrosion. This stems from the galvanic coupling between the materials, which have greatly differing electrode potentials.

Very little long-term corrosion data is available for mechanically fastened dissimilar joints between CFRP and metal, and reference to literature indicates that studies to optimise fastener materials and their coatings have not been performed for metal-to-composite joints. This is in spite of widespread concerns regarding durability. In production cars, for example, dissimilar joints of this type are used almost exclusively in ‘dry’ areas of vehicles, where exposure to water, salt and stone chipping from road surfaces does not occur. To further reduce the risk of corrosion-induced failure, dissimilar joints in cars are usually coated with sealants, electrolytically deposited zinc-phosphate crystals and multiple layers of paint and lacquer. These measures are taken because if paint layers are penetrated and the joints become exposed to electrolytes (moisture, salt water from the road, etc.), rapid deterioration of the fastener and thus the joint then occurs.

TWI is working on a coating based solution to be applied to the fastener itself. If successful, the issue of corrosion in fastened joints of dissimilar materials will be mitigated.

The main focus of this review is therefore to examine fastener coating developments that can improve the corrosion performance of dissimilar metal to composite joints.

Note that coating systems that act as a barrier preventing the electrical interaction between materials can potentially have applications across a range of dissimilar materials combinations, including not only metal to CFRP joints but also aluminium to steel.

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