16.2 METAL BONDING

Metals, having a relatively high surface energy, are generally considered easy to bond. However, several problems could occur when one is working with metallic substrates. These are related to the following characteristics:

1. Durability related to environmental effects on the substrate surface and the interface of the adhesive joint

2. Variation in the surface chemistry depending on alloy, processing, preconditioning, etc.

3. Relatively low thermal expansion coefficient and high thermal conductivity compared to most epoxy adhesives

One difficulty in bonding metals is the durability of the joint. It is not so much a problem of making a strong joint as one of keeping it that way throughout its expected service life. A weld may have strength of only 600 lb, but it is likely to remain that strong for 5 to 10 years afterward. An epoxy adhesive, on the other hand, may have 3 to 4 times the initial strength of a weld, but it could weaken when exposed to high humidities, cycled between hot and cold temperatures, or immersed in salt water and then dried. By definition, a structural adhesive must be able to withstand these conditions without significant deterioration.

A second difficulty in bonding metals lies in understanding the surface. One of the important points to consider in bonding metals is that only the surfaces are involved. Adhesives and sealants are active only on the molecular surface layer that forms the joint interface and on any surfaces contained in the porosity of the metal itself. Thus, if unprepared...