The need for a modified interface between the aluminum and stainless steel components was identified when a small percentage of the original fittings on the vacuum chambers failed. The new fittings were machined from roll bonded plates of 3003 aluminum and 304 stainless steel. The aluminum and stainless steel ends of the fittings were then welded to the aluminum chambers and the stainless steel piping, respectively.
The mechanism for the failure in the original fitting was identified as galvanic corrosion at the dissimilar metal interface, accelerated by poor DI water quality, as well as stresses from welding and piping connections. Steps have already been taken to deliver very high-quality DI water by continuous polishing, and to reduce the joint stresses by providing additional flexible connections.
A series of thermal cycling tests were conducted with the new water connections. The connections showed no leaks or galvanic corrosion even after an equivalent of 50 bakeout cycles. As of now, all the joints have been successfully replaced and leak tested.
-R. Dortwegt, J. Noonan, and S. Sharma
Accelerator Systems Division