The Electrochemical or Galvanic Series

The electrochemical tables or galvanic series scales, often quoted in technical literature as a measure of corrosion, show the electro-potential between pure metals, not between their oxides, carbonates or chlorides.

Although theoretically correct, these tables can give a misleading indication of the performance of different materials in contact.

This series only applies to pure metal. Under certain conditions, some metals react with the environment or chemicals to form a passive surface, which renders them less active, so that any ranking can be misleading. "Passivity" becomes an important phenomenon in controlling corrosion rates.

However, the Electrochemical series table is still a useful indicator of electrode potential. The further apart two metals are in the electrochemical series, the greater the potential difference between them.

Metals termed anodic, active, negative, or less noble corrode in preference to metals that are deemed more cathodic, noble, positive or less active. The less noble metal becomes the anode and is subject to corrosion. The greater the potential difference, the more corrosion there will be of the less noble metal; i.e., on the anode.

The difference in nobility is why zinc can protect a steel substrate.

Different electrolytes can lead to different rankings, and metal alloys may display more than one potential than that which applies to their "active" state.

The exposed surface ratio of anode and cathode determines the rate of dissimilar metal corrosion. For instance, if a fastener which has a small surface compared to the cladding becomes the anode, its current density will be high, and the fastener will corrode quickly; e.g., an aluminium rivet in a copper sheet. When the opposite is the case, the effect is not so great.

The Electrochemical table shows zinc is more active than steel. Contact between steel and zinc, in the presence of moisture, will cause the zinc to corrode or sacrifice itself, to protect the steel.



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