JANUARY 31 2003
Different metals have different potentials in the same electrolyte. This potential
difference is the driving force, or the voltage, of the cell. As with any electrochemical
corrosion cell, if the electrolyte is continuous from the anode to the cathode and there is
a metallic path present for the electron, the circuit is completed and current will flow and
electrochemical corrosion will occur.
Dissimilar Metals. The most obvious form of this type of corrosion is when
two different kinds of metal are in the electrolyte and metallically bonded or shorted in
some manner. All metals exhibit an electrical potential; each metal has its distinctive
potential or voltage (paragraph 2-4). When two different metals are connected, the
metal with the most negative potential is the anode; the less negative metal is the
cathode. An "active" metal is a metal with a high negative potential, which also means it
is anodic when compared to most other metals. A "noble" metal is a metal with a low
negative potential, which also means it is cathodic when compared to most other
metals. Dissimilar metal corrosion is most severe when the potential difference
between the two metals, or "driving voltage," is the greatest.
Examples of active metals are new steel, aluminum, stainless steel (in the
active state), zinc, and magnesium. Examples of noble metals are corroded steel,
stainless steel (in the passivated state), copper, bronze, carbon, gold, and platinum.
One example of this type of corrosion occurs when coated steel pipelines are
metallically connected to bare copper grounding systems or other copper pipelines
(usually water lines).
Figure 2-8. Galvanic Corrosion Cell Caused by Different Metals
Copper Water Pipeline
Steel Gas Pipeline
Current Flow in the Earth