Galvanic corrosion may occur in fresh or salt water. It generally happens whenever two/or more metallic elements having unlike galvanic-voltages connects to each other electrically in the presence of an electrolyte.
In other words, it can also be said that Galvanic corrosion is an outcome of the usual property of an active metal (greater in comparison to the other metal) to shred the electrons to the lesser active metallic counterpart. The corrosion increases speedily whenever unlike metallic elements connect to each other electrically. In more layman terms, we can say that a corrosion which happens due to the association of two/or more unlike metals is known as Galvanic Corrosion.
In the figure beside, The aluminium is having the loss of metal in terms of aluminium ion. The flow of electrons is from aluminium to the Steel via an electric medium.
The galvanic corrosion can only happen only in the presence of electric circuitry. With this statement, we can understand that for a galvanic corrosion to happen, it is mandatory to have two or more metals and an electrolytic medium for providing a path for the flow of electrons. In addition, the chemical reaction near the Cathodic material needs the availability of O2 in the electrolytic medium.
Lets understand What is ‘Anode’ and ‘Cathode’ now.
Anode :
The metallic element which loses (donate) the electrons when current flows is known as Anode. It has a ‘-ve’ galvanic voltage in comparison to another metal and so the rate of Corrosion is also more in that.
Cathode :
The another metallic element which accepts the electrons when current flows is known as Cathode. It has ‘+ve’ galvanic voltage in comparison to another metal and so the rate of Corrosion is less in that.
In order to stop the occurrence of Galvanic corrosion, we should at least maintain any one of the below mentioned three conditions :
- The galvanic voltage level of both the metals should be same (before coupling)
- The electric circuit should not be made.
- Presence of electrolytic medium to be removed.
By design a ship is made of steel and the natural tendency of the ships hull is to become anodic with reference to the nonferrous metals and the impurities present within steel. Therefore the ships hull gets corroded.
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