All adhesives tend to lose strength on immersion in water. Water acts in two ways on a bonded assembly: It can be absorbed into the adhesive and soften it, and it can degrade the interface between the adhesive and the bonded surfaces. You should ensure that the samples have dried out thoroughly when you test after the water immersion; you may find that much of the initial strength has been retained.

Epoxy adhesives have traditionally been the gold standard for water immersion. The organosilane primers that are commonly used for bonding metals to glass will often give improved water resistance with epoxies when used to bond metals. Epoxy coatings are routinely used for long-term protection of metal water tanks and pipes, where the contact area is much larger than the edge of an adhesive bond. If you think about it, a bonded coating is the same as an adhesive bond, except the coating is only bonded to one surface instead of two. It should be noted, however, that the tensile shear forces on a vertical coating are much lower than for a typical adhesive bond—it only has to support the weight of the coating!

An additional approach you should examine is to see whether it is possible to protect the bond line from water. If you can seal it with a moisture-resistant sealant such as a silicone or polysulfide, this should provide additional protection for the bond.
 

Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.