Where electrodes of two different metals are immersed in an electrolyte and externally connected by a conductive media, a 'battery' is effectively created whereby electrons will flow from one electrode to the other. According to the potential difference between the two metals, one electrode will become the donor electrode and will gradually be eroded.

The potential difference, or PD, of various metals commonly used in enclosures and shielding materials, and expressed in volts, is indicated in the table which follows based on the potentials against saturated calomel electrode in sea water:

The criteria most commonly employed to define a 'threat' is the likelihood of the equipment to be subjected to exposure to conditions of dampness or condensation through to wetting with salt water, salt mist/spray or the weather.Where exposure is likely, the general rule of thumb regarding PD is as follows:
¡ñ Outdoor exposure or saltwater/spray contamination 0.3 volts
¡ñ Dampness or condensation without salt presence 0.5 volts
If the equipment is hermetically sealed and any exposed gasket edges are 'elastomer protected', i.e. with dual or twin gasket configuration there would then be no restriction. Some combinations of metals, such as Silver (PD 0) and Aluminium (PD -0.75), have inherent problems. In these circumstances, gaskets should be totally enclosed or of 'twinstrip' construction.