Each metal has an inherent electrical potential. See Table 1. Un-passivated Nickel Alloys Brass Copper Bronze Silver Solder Passivated Nickel Alloys Passivated Stainless Steels Silver Titanium Graphite Gold Platinum Place any two metals in an electrolyte in contact with one another and a galvanic reaction takes place. Okay, enough chemistry. What this means to the brewer is that if he has mild steel in contact with copper, the steel will corrode.
Beer is an excellent electrolyte.
If the brewer has copper in contact with passivated stainless steel, the copper will corrode. Brass fittings and silver solder are right in the thick of things with regard to potential, but fortunately the difference is small and corrosion rates would be quite low. One rule of thumb is that if the cathode size is much smaller than the anode size, then the rate of corrosion will be very small. As a practical illustration, stainless steel rivets on a copper tank would cause minimal corrosion of the copper. Copper rivets on a stainless steel tank would soon be history.