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Basic Chemistry and Basic Corrosion Theory 2:22
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
In general, clays are deficient in oxygen and in mixed soils the areas of a
structure in contact with the clay become the anodes of an oxygen
concentration cell. In such cells it is the difference in available oxygen
between sand and clay areas in contact with a structure that produces the
electrical energy that drives the corrosion process.
Conductivity
The amount of current through an electrolyte is affected by the ion content.
The more ions, the greater the conductivity; the greater the conductivity, the
more current for a given cell voltage; and the greater the current, the higher
the rate of corrosion. Conductivity is equal to the reciprocal of resistivity. The
unit of measurement is the Siemen-cm (mho-cm).
Conductivity or its reciprocal (resistivity) is an important parameter in the
study of corrosion and its prevention. High conductivity itself does not
indicate a corrosive environment; it only indicates an ability to support
current flow.
Chemical Activity
It is the chemical activity of the electrolyte that provides the Redox
(Oxidation-Reduction) reactions necessary to drive a corrosion cell. Some of
the chemical species present in an electrolyte may assist in retarding or
slowing chemical action by aiding the production of protective films.
Carbonates, for example, may lead to the formation of a passive film on zinc;
in such environments galvanized structures may undergo virtually no
corrosion. If there are breaks in the galvanizing, however, the underlying steel
at such breaks may corrode rapidly, as it will be at a more active potential
than the zinc carbonate film. Passivation can also render zinc anodes
ineffective.
Of particular interest is the pH, which expresses the hydrogen ion
concentration in the electrolyte. The higher the concentration of hydrogen
ions, the lower the pH. Hydrogen ions readily accept electrons when in
contact with metals that are more electrochemically active than hydrogen. For
example, magnesium, aluminum, zinc, iron, and lead are all more active than
hydrogen. Other metals, such as copper, are less active (or more noble) than
hydrogen. Thus, in an acid environment, metals more active than hydrogen
will be corroded, and those more noble will not be corroded.
Highly alkaline environments, generally with a pH greater than 8, can cause
accelerated corrosion on amphoteric metals such as aluminum and lead.