Area Effects 
The corrosion is caused by a current flow from the anode (active) to the cathode (noble). A large cathode and a small anode means high current densities at the anode, hence, high corrosion rates - anode corrosion may be 1000 times more than if the two areas were equal.

Protection Possibilities  
When dissimilar metals are used in contact with each other and when they are exposed to an electrically conducting solution, then combinations of metals should be chosen that are as close as possible in the galvanic series. Coupling two metals which are widely separated in this series will normally lead to accelerated attack of the more active metal. Often, however, protective oxide films and a number of other effects tend to reduce bimetallic corrosion. Bimetallic corrosion can further be prevented by the application of dielectric insulating materials between dissimilar metals. For example, if dissimilar plates are bolted together, plastic or rubber washers may be used to insulate the metals. Dielectric unions separate and insulate dissimilar plumbing components.

Checklist

• Select combinations of metals close together in galvanic series.
• Provide for complete dielectric insulation of dissimilar metals.
• Avoid small anodic and large cathodic areas.
• Extend distance between dissimilar metals in a conductive medium.
• Corrosion inhibitors help by reducing the aggressiveness of the environment.

Examples

• unprotected carbon steels in steam, condensate, cooling water, gas systems
• galvanized steels in heating systems, cold and hot tap water systems
• bare carbon steels in cooling water systems
• aluminum, magnesium and alloys in heating loops or cold or hot tap water