- Understanding Corrosion
“Rusting” or corrosion, is the natural tendency of a refined manmade
(tank or pipe) wanting to revert to its original and more stable
the form of a salt and iron oxide (rust). The pathway of corrosion
a cell: a cathode, an anode, an electrolyte (like water) and a
path. This “corrosion cell” can occur on the same manmade metal
localized areas in the form of “pitting” or crevice corrosion,
where the coating may be defective. Pitting can be very deceptive;
deterioration of a relatively small area can ultimately cause
failure of a tank or pipe. Galvanized corrosion occurs when two
metals (like a copper pipe attached to a steel tank) couple together
electrolytic environment, causing one metal to become the anode and
and the other metal to become the protected cathode.
All metals have stored energy in the form of electrons, distinct to
nature- magnesium with the most, followed by zinc and aluminum, and
steel and cast iron. Lead, brass and copper are low energy thus only
affected by corrosion with carbon metal and gold finishing last.
stored electrons travel along the pathway from higher energy to
energy- much like water flow, creating an electric current.
high energy metal magnesium bar (anode) forces the lower energy
tank/pipeline (cathode) to accept electrons, thus preventing
(corrosion.) This installation of an anode is called cathodic
The chemical component of corrosion involves the combination of
and other negatively charged ions with positively charged iron to
which we call rust. Dissolved salt and other minerals in water
When a tank corrodes, the iron atoms break away from the steel
this leaves pits in the steel which eventually become holes.
anode. Therefore, pits occur at anodic spots on the surface.
cathode. Therefore, the unpitted areas are cathodic spots.
The most commonly installed anode consists of a 17lb. bar of
packed inside a cloth bag containing 28 lbs. of tightly packed
conductive earth backfill with an insulated copper wire that is
the magnesium bar and the tank/pipe. The magnesium bar is a
energy (voltage) electrons with an open circuit potential of
steel structure to be protected is around -.50 volts. When the
connected via the copper wire, this creates a direct current (DC
can be measured above ground and recorded. A good voltmeter
equalized average of the two at about -.85 volts or a greater
confirming cathodic protection.