Galvanic corrosion also is also known as bimetallic corrosion or dissimilar corrosion. In the process of galvanic corrosion, metals are placed within an electrolyte with another metal. Due to the involvement of two metals, it is called as bimetallic corrosion. Corrosion usually refers to a metal degrading in the presence of oxygen and moisture whereas galvanic corrosion involves degradation of one metal and protection of another metal placed with the precious metal. Here there is one thing that is to be noted. The two metals are needed to be electrochemically different from each other that is why it is denoted as dissimilar corrosion.
Every galvanic cell functions due to potential difference. This potential difference causes the flow of electrons within the cell. The ability of the metal electrode to lose electrons and to become oxidized is known as oxidation potential. The more is the oxidation potential, then more easily metal will give up its electrons. Opposite to oxidation potential is the reduction potential. It is the ability of a metal ion in solution to pick up electrons and become reduced. On the basis of the oxidation potential of metals, an electrochemical series is arranged.
The elements at the top of the series are more easily oxidized thus there are major chances of losing the electrons. The entire mechanism depends upon the oxidation potentials of the two metals in the galvanic corrosion. The difference in the oxidation potential of the two metals is the driving force of galvanic corrosion.
When two metal electrodes are placed in an electrolyte and the circuit is completed by joining the metal through a wire then this is the point where the process of corrosion takes place. Metals with the higher oxidation potential will start acting as anode and will keep losing electrons whereas the metals with lower oxidation potential will start acting as cathode respectively. The activity of the metals is based on the electrochemical series. Thus the flow of current is from more active metal (anode) to less active metal (cathode).
Few metal pairs that undergo galvanic corrosion are given below-
1. ZINC AS ANODE AND COPPER AS CATHODE
When zinc and copper metal are placed in an electrolyte which can be of water/aqueous solution containing any salt like copper sulfate it then the circuit gets completed. Now Zinc starts to oxidize due to its high oxidation potential compared to the cathode. Thus zinc slowly starts to corrode and form zinc ions. Copper ions, on the other hand, start to gain electrons and are reduced so it is protected. The process continues until the zinc electrode entirely gets dissolved. The hydrogen ions present in the electrolyte also gets reduced to hydrogen gas and is seen as bubbles at the cathode.
Reaction at Anode
Zn-------> Zn+2 + 2e-
Reaction at cathode
2. STEEL AND ALUMINUM
When steel and aluminum are placed in the electrolyte then aluminum will act as anode and will start to corrode whereas steel will be protected.
Galvanic corrosion is not required in many places thus has to be prevented. There are various methods of preventing galvanic corrosion.
Some of them are given below.
1. AVOID GALVANIC COUPLING
This means that two metals that can undergo galvanic corrosion must not be placed together otherwise one out of the two metals will corrode. We have already seen a few examples of such metals. Avoid using metals that are electrochemically different from each other.
2. USE OF INSULATING MATERIAL
If two different metals are kept together then insulating materials must be used between them which will prevent the circuit formation and flow of electrons.
Paint creates a barrier by covering the metal so it is also a way to prevent galvanic corrosion.
4. AVOID LARGE CATHODE AND SMALL ANODE.
This also reduces dissimilar corrosion.
Galvanic corrosion starts when metals are dissimilar, if there is an electrolyte present or if the circuit gets completed. All these conditions must be avoided to prevent galvanic corrosion.
1.How larger anode and smaller cathode can prevent galvanic corrosion?
The surface area ratio of anode and cathode determines the rate of galvanic corrosion. If a large anode is taken with a smaller cathode then the rate of corrosion of metal is slowed down as anode has a large surface area. On the other hand when we take the inverse-large cathode and small anode then the corrosion will be accelerated and the anode will dissolve very quickly. That is why it is advised to avoid large cathodes and smaller anodes for the prevention of galvanic corrosion.
2. Mention some real-life incidences when galvanic corrosion was proved to be a problem.
Galvanic corrosion could be problematic as seen in the following incidences. In America, it was proposed that the drinking water was getting contaminated with lead due to galvanic corrosion taking place between copper and lead in the pipes. To prevent this authorities decided to use some insulating materials which can prevent galvanic corrosion.
Another example is the damage of weld metals of ships due to the coupling of steel with the weld metal. Even though steel is painted in ships but due to collisions with icebergs, the paint gets removed leading to galvanic corrosion.