Lecture 8: Dissimilar Metal Corrosion

• EMF Series
• Galvanic Series
• Temperature Effect
• Area Effect
• Summary
• Reading Assignments

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Dissimilar metal corrosion results from the coupling of two different metals. It is also called bimetallic corrosion or galvanic corrosion.

The Electrochemical Motive Force (EMF) series

EMF series is an orderly listing of standard half-cell electrode potentials.

Galvanic Series

Galvanic series is an orderly list of corrosion potentials for various pure metals and alloys.

Comparison of EMF series and galvanic series

Point to note:

Sacrificial anode cathodic protection is the constructive use of the galvanic corrosion.

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Temperature Effect

The order of galvanic series may reverse under certain circumstances.

e.g. Zn and Fe at elevated temperatures in some potable water systems

The environment plays an important role in a corrosion process.

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Area Effect

Effects of coupling two dissimilar metals:

The more active metal will become the anode of the couple in a wet corrosion cell.

The more noble metal will become the cathode of the couple in a wet corrosion cell.

The corrosion rate of the more active metal will be accelerated, while that of the more noble metal is retarded (the cathode may
still corrode, depending upon the magnitude of the cathodic polarization induced by coupling of the more reactive metal).

Cathode to anode area ratio:

Greater area ratio will result in a greater accelerating factor in the rate of galvanic corrosion

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Summary

Both EMF and Galvanic series are based on thermodynamic analysis of potentials. The potential difference between two dissimilar metals is the driving force for galvanic corrosion. Despite of being the first recorded form of corrosion some 200 years ago, Galvanic corrosion continues to be one of the most commonly encountered engineering problems. The coupling of dissimilar metals will accelerate the failure of the anodic member. The acceleration factor is affected by the cathode to anode area ratio. Changes in temperature may cause potential reversal. 

Reading Assignments

To reinforce learning in this lecture read pages 130-143 (textbook)
To prepare yourself for the next lecture read pages 408-435 (Denny Jones's book)

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