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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 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 Index (source: Corrosion Doctors.org)

Metallurgy	Index (V)
Gold, solid and plated, Gold-platinum alloy	0.00
Rhodium plated on silver-plated copper	0.05
Silver, solid or plated; monel metal. High nickel-copper alloys	0.15
Nickel, solid or plated, titanium an s alloys, Monel	0.30
Copper, solid or plated; low brasses or bronzes; silver solder; German silvery high copper-nickel alloys; nickel-chromium alloys	0.35
Brass and bronzes	0.40
High brasses and bronzes	0.45
18% chromium type corrosion-resistant steels	0.50
Chromium plated; tin plated; 12% chromium type corrosion-resistant steels	0.60
Tin-plate; tin-lead solder	0.65
Lead, solid or plated; high lead alloys	0.70
Aluminum, wrought alloys of the 2000 Series	0.75
Iron, wrought, gray or malleable, plain carbon and low alloy steels	0.85
Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type	0.90
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate	0.95
Hot-dip-zinc plate; galvanized steel	1.20
Zinc, wrought; zinc-base die-casting alloys; zinc plated	1.25
Magnesium & magnesium-base alloys, cast or wrought	1.75
Beryllium	1.85

For harsh environments, such as outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the "Anodic Index". For example; gold - silver would have a difference of 0.15V being acceptable.

For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the "Anodic Index".

For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions

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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	CORROSION REACTIONS
	CATHODIC POLARIZATION

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