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Corrosion Theory

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CORROSION THEORY

The search for a better understanding of the actual mechanism of corrosion has been actively pursued for over a hundred years. We understand what occurs during many corrosion processes, but the search is by no means complete; many areas remain unexplored or little understood.

However, our current knowledge of what takes place during the corrosion process has been sufficient to create effective tools to combat corrosion in most situations.

Corrosion is an electrochemical process in which a difference in electrical potential develops between two metals or between different parts of a single metal. This voltage can be measured when a metal is electrically connected to a standard electrode.

The electrical potential of a metal may be more or less than the standard, in which case the voltage is expressed as either "positive" or "negative". This difference in potential allows current to pass through the metal causing reactions at anodic and cathodic sites.

These sites constitute the corrosion cell, as shown in the figure below.


 The anode is the region of lower potential. Conversely, the cathode is the region of higher potential. At the anode, metal Ions go into solution. In general, the lower the potential of the anode, the greater the amount of metal dissolution and the more serious the corrosion problem.

The extent of the corrosion is also a function of the capability of ions and electrons to travel through the water phase and participate in chemical reactions. Waters high in dissolved solids are more conductive and cause more severe corrosion problems. Thus, seawaters are generally more corrosive than surface supplies.

Any metal immersed in water will soon develop a measurable potential. Those of lower potential can be expected to corrode more easily and extensively than those of higher potential. Theoretically, we can, therefore, assume that if two metals are coupled, the one of lower potential would become the anode and actively corrode. The figure below shows the galvanic series developed for metals immersed in seawater. When two metals, as shown in the figure, are coupled, we would expect that the metal lower in the series would corrode.

It should be noted that the measured potentials shown in the illustration were developed under specific conditions of composition, temperature and velocity; and that in practical situations, the actual conditions may differ.

 

Anodic

Zinc Galvanized Aluminum Steel Brasses Copper Cu/Ni Alloys Nickel

Corrodes First 

Cathodic

Stainless Steel

Protected

 

Practical galvanic series of metals and alloys

In an actual situation, if a pair of metals of different potential were coupled, problems within the system would probably exist. The result of coupling two metals of closer potential is more difficult to predict. Many other factors must be considered before predictions can be safely made.

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