JANUARY 31 2003
lowest energy level, or its natural state. After high levels of energy are added to the
metal, when it is exposed to the environment (an electrolyte), it will tend to revert to its
natural state. This process is normally extremely slow, and is dependent on the ion
concentration of the electrolyte that it is exposed to. Only under very extreme
conditions (acidic electrolyte) can this form of corrosion be significant. The corrosion
rate for steel climbs drastically at a pH below 4, and at a pH of about 3, the steel will
General corrosion tends to slow down over time because the potential
gradually becomes lower. Failures of pipelines or tanks would not quickly occur from
this type of corrosion since no pitting or penetration of the structure occurs, just a
general corrosion over the entire surface (except under very extreme circumstances
where the metal could dissolve in an acid electrolyte). However, in nature, the metal is
not completely uniform and the electrolyte is not completely homogeneous, resulting in
electrochemical corrosion cells that greatly overshadow this mild form of corrosion.
Concentration Cell Corrosion. This type of corrosion is caused by an
electrochemical corrosion cell. The potential difference (electromotive force) is caused
by a difference in concentration of some component in the electrolyte. Any difference in
the electrolyte contacting the metal forms discrete anode and cathode regions in the
metal. Any metal exposed to an electrolyte exhibits a measurable potential or voltage.
The same metal has a different electrical potential in different electrolytes, or
electrolytes with different concentrations of any component. This potential difference
forces the metal to develop anodic and cathodic regions. When there is also an
electrolyte and a metallic path, the circuit is complete, current flows, and
electrochemical corrosion will occur.
Soil is a combination of many different materials. There are also many
different types of soil, and even the same type of soil varies greatly in the concentration
of its constituents. Therefore, there is no such thing as truly homogeneous soil.
These soil variations cause potential differences (electromotive force) on the
metal surface resulting in electrochemical corrosion cells. Liquids tend to be more
uniform, but can vary in the concentration of some components such as oxygen varies
by depth and flow rates. Biological organisms are present in virtually all-natural
aqueous environments, these organisms tend to attach to and grow on the surface of
structural materials, resulting in the formation of a biological film, or biofilm. These films
are different from the surrounding electrolyte and have many adverse effects. Following
are examples of common forms of concentration cell corrosion.
Dissimilar Environment. Pipelines tend to pass through many different
types of soils. The metal exhibits different electrical potentials in different soils. The
electrical potential in those soils determines which areas become anodic and which
areas become cathodic. Since both the anode and cathode are electrically continuous