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Galvanic Corrosion

When two different metals are immersed in a corrosive solution, each develop a corrosion potential. If the corrosion potential of the two metals is significantly different, and they are in direct contact immersed in an electrolyte, the more noble will become the cathode and the more active metal will become the anode. A measurable current may flow between the anode and the cathode. The corrosion rate of the anode will be increased and the cathode decreased. This increase in corrosion of the anode is called galvanic corrosion.

The greater the corrosion potential of each metal (the more active or more noble) the greater the likelihood for corrosion. The following list shows the various metals in order from most active to most noble: 

Magnesium Anodic
Galvanized Steel  
Aluminum  
Mild Steel  
Low Alloy Steel  
Cast Iron  
Lead  
Tin  
Muntz Metal  
Yellow Brass  
Red Brass  
Copper  
Aluminum Bronze  
Stainless Steel (430)  
Stainless Steel (304)  
Stainless Steel (316)  
Monel  
Silver  
Titanium  
Gold Cathodic

The greater the separation between two metals listed, the greater the corrosion potential. For example, if zinc, an active material, and stainless steel, a noble metal, were in direct contact in the presence of an electrolyte (water), galvanic corrosion would most likely occur.

In addition, the relative area of each of the exposed metal surfaces is also a consideration. If the area of the cathode (noble metal) is very large in relation to the anode (active metal), the current produced is likely to be high and the anode will corrode quickly. If the area of the cathode is small compared to the anode the anode will corrode very slowly, if at all.

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