The less corrosion resistant or the "active" member of the couple experiences accelerated corrosion while the more corrosion resistant or the "noble" member of the couple experiences reduced corrosion due to the "cathodic protection" effect.
The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi-metallic joint, the degree of accelerated attack is reduced, the dissimilar metals do not have to be widely different i.e. copper and steel, two different steel water pipes will produce the same effect.
What causes galvanic corrosion? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. When the corrosion potentials of various metals and alloys are measured in a common electrolyte (e.g. natural seawater) and are listed in an orderly manner (descending or ascending) in a tabular form, a Galvanic Series is created. It should be emphasized that the corrosion potentials must be measured for all metals and alloys in the same electrolyte under the same environmental conditions (temperature, pH, flow rate etc.), otherwise, the potentials are not comparable.
The potential difference (i.e., the voltage) between two dissimilar metals is the driving force for the destructive attack on the active metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte will also affect the degree of attack. It is generally accepted that the Mediterranean sea is the saltiest with mid Atlantic and the Caribbean being second saltiest (All the popular yacht cruising grounds) the Arctic is the least salty. The cathode to anode area ratio is directly proportional to the acceleration factor.
How to prevent galvanic corrosion? Galvanic corrosion is best prevented by not putting your vessel in the sea! However this not practicable so some of the following will help.
- Select metals/alloys as close together as possible in the galvanic series.
- Avoid unfavourable area effect of a small anode and large cathode, this effect is utilised for protection with the typical zinc anodes used on ships where the anode is encouraged to corrode rather than the hull.
- Insulate dissimilar metals wherever practical or bond them together to achieve close to zero potential difference a minimum of 16mm² copper cable is needed to achieve low Ω bonding.
- Apply coatings with caution. Paint the cathode and keep the coatings in good repair on the cathode and make sure the anode is clean.
- Avoid threaded joints for materials far apart in the galvanic series, better still avoid joints with greatly dis-similar metals.