Galvanic Corrosion is a process of corrosion when two different metals are brought in electrical contact and the contact is bridged by a conductive fluid such as sea water, salted water etc. This is also known as Bimetallic Corrosion or dissimilar metal corrosion.

How Galvanic Corrosion take Place?

Every metal has acquire electrode potential when dipped into an electrolyte. A joint of two different metal are exposed to environmental conditions like dust, moisture etc. This moisture can ingress into the joint area which serves as the electrolyte. Thus the two different metal along with moisture make a cell. One metal acts as Anode and another as Cathode. Due to different electrode potential of metals, there is a flow of current through the joint and hence electrolytic reaction take place.Reduction reaction take place at Cathode and Oxidation reaction take place at Anode.
Let us take an example of Copper to Aluminium joint.
 
Galvanic Corrosion
 

For this joint, Aluminium acts as Anode and Copper as Cathode as decided from for electrode potential of copper (+0.34 Volt) and Aluminium (-1.66 Volt). The reactions at Cathode and Anode as follows:

3Cu+2 + 6e = 3Cu

2Al = 2Al+3 + 6e

Net reaction:

2Al + 3Cu+2 = 3Cu + 2Al+3

Due to the above reactions, current flows from anode to cathode as the flow of electron is from anode to cathode. From the net equation, it is clear that the product of reaction is Al+3 and solid copper. This means that during Galvanic Corrosion, cathode is protected from corrosion. But the corrosion of anode increases. As this corrosion of Anode is due to formation of Galvanic Cell between the Aluminium to Copper joint, this process of corrosion is known as Galvanic Corrosion.

Necessary Condition for Galvanic Corrosion:

Following are the necessary conditions:

1)    Two different metals are in electrical contact.

2)    The contact between the metals are bridged by conductive liquid.

3)    There should be a potential difference between the metals so that a current can flow.

The extent of Galvanic Corrosion depends on the position of metal in Galvanic Series. Galvanic Series is defined for different fluid but it is well defined for sea water. If the metals are separated farther in Galvanic Series, the extent of corrosion of Anode will be more. Let us understand this by example. A part of Galvanic Series for sea water is tabulated below just for the sake of simplicity in understanding. This table is not complete Galvanic Series for sea water.

Graphite
CATHODE














ANODE

Palladium
Platinum
Gold
Silver
Titanium
Tungsten
Tin
Lead
Copper
Steel
Indium
Aluminium
Cadmium
Beryllium
Magnesium

When a joint of Tin and Aluminium is made, the corrosion of Aluminium will more as compared to copper-aluminium joint since Tin & Aluminium are farther apart in Galvanic Series.

Apart from this, Galvanic Corrosion also depends on the surface current density. If the surface current density is more, corrosion of anode will be more. Thus anode surface are should be increased over cathode surface area. This reduces the current density at anode and hence the corrosion.

Prevention of Galvanic Corrosion:

Galvanic Corrosion can be prevented by the following ways:

1)  Jointing of dissimilar metals can be avoided. But it is not practical to avoid such joints. In industries we often need to make electrical joint of two different metals. Bimetallic strip can be used for such joints instead of direct joint. While using bimetallic, care should be taken that similar metals are facing each other in joint. For example a bimetallic of copper & aluminium, copper side of bimetallic should face copper side and aluminium side should face aluminium side of conductor. A typical bimetallic is shown below. Notice that inner surface is of this bimetallic is copper while the upper surface is of aluminium.

 
Copper-aluminium-joint-bimetallic

2)  Select metals as close to each other in Galvanic Series as possible. This reduces the corrosion of anode.

3)  Surface area of anodic metal shall be kept more than the surface area of cathodic metal.

4)  The joints should be protected from harsh environmental condition if bimetallic is not used.

5)    Before making a joint, contacts should be cleaned properly. 

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