2. 2
INTRODUCTION TO CORROSION
Corrosion can be defined as the deterioration of metal due to
its interaction with the environment. Corrosion is a natural
phenomenon, which should not surprise one, but rather
should be expected to occur.
Metals are high energy materials, which exist because heat
energy was added to natural iron ores during the smelting
process.
Nature, by environmental contact, constantly attacks these
high energy materials and breaks them down to the natural
elements from which they were derived.
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3. 3
Conditions Required for Corrosion
Corrosion is an electrochemical process
occurring at the interface between metal and
environment. Three conditions must be
present for this to occur.
1. Two areas on a structure or two structures
must differ in electrical potential.
2. Those areas, called anodes and cathodes,
must be electrically interconnected.
3. Those areas must be exposed to a common
electrolyte (soil or water).
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4. 4
Corrosion Rate
As a result of this process, electric current flows
through the interconnection between cathode and
anode.
The cathodic area is protected from corrosion
damage at the expense of the metal, which is
consumed at the anode.
The amount of metal lost is directly proportional to
the current flow. Mild steel is lost at approximately
20 pounds for each ampere flowing for a year.
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Factors Affecting Corrosion Rate
Potential Difference Between Anode and
Cathode (Galvanic Series)
Circuit resistance – Resistivity of the
Electrolyte
Chemical Activity
Stray Currents
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6. 6
Potential Difference
Interconnecting two dissimilar metals
in an electrolyte will create a corrosion
cell. The strength of this cell increases
as the distance within the galvanic
series increases.
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7. 7
TYPES OF CORROSION
Uniform or near uniform - Corrosion attacks all
areas of the metal at the same or similar rate.
Localized - Some areas of the metal corrode at
different rates due to heterogeneities in the
metal or environment. This type of attack can
approach pitting.
Pitting - Very highly localized attack resulting
in small pits that may penetrate to
perforation.
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