(5) The application of anodic coating to magnesium and zinc protects them against corrosion.
The protective oxide film that is provided by an anodic treatment is of the same general type as that
afforded by a natural oxide film. The natural film is very thin; thus, anodic coating, because of its
greater thickness, uniformity, and abrasion-resistance, offers better protection against corrosion. Look
again at Table 5-3, page 5-15, and study the passivation types that are applicable to different metals as
well as the chemicals used for passivation.
(6) Passivation by the electron theory deals specifically with metals and alloys that become
more noble because of their electrochemical behavior. In application, advantage is taken of the
corrosion-inhibiting action of ions in protective coatings that passivate by both electrochemical and
mechanical means. Coatings and claddings are also used and frequently provide the most economical
solution to corrosion problems.
b. Cathodic Protection. Cathodic protection is a method used to protect metal structures from
corrosive action. As explained before, galvanic cell corrosion is the major contributing factor to the
deterioration of metal by an electrochemical reaction. The area of a structure that corrodes is the anode
or positive electrical current that leaves the metal and enters the electrolyte. Galvanic cathodic
protection is designed to stop this positive current flow. When the current is stopped, the corrosive
action stops and the anodes disappear. This type of protection depends upon the neutralization of the
corroding current and the polarization of the cathodic metal areas. Galvanic cathodic protection is a
procedure for reducing or preventing metal surface corrosion by using sacrificial anodes or impressed
current methods. The sacrificial anode method is known as the galvanic anode method. The impressed
current method is the galvanic cathodic method; however, it is referred to as the impressed current
method. Depending on the corrosive characteristics of the electrolyte surrounding the structure,
galvanic anode and impressed current methods are used separately or in conjunction with each other.
(1) Galvanic anode method. This cathodic protection method uses an electrode that is
referred to as a sacrificial anode which corrodes to protect a structure. This sacrificial anode is
electrically connected to, and placed in, the same electrolytic area of the structure. To protect iron or
steel structures, use a sacrificial anode made of magnesium or zinc so that it will produce a sufficient
potential difference to cause the structure to become a cathode. The action of this galvanic protection
causes the electrical current to flow from the sacrificial anode through the electrolyte to the structure to
be protected. The electrical connection between the two metals completes the circuit and allows current
to return to the corroding metal. The sacrificial anode becomes the anode of the established, dissimilar
metal galvanic cell; the structure to be protected becomes the cathode. The current from a sacrificial
anode is intense enough to oppose or prevent all positive current flows from leaving the anodes in the
structure to be protected. The prevention of the positive current flow from the anodic areas in the
structure reduces the corrosion rate to almost zero. Galvanic cathodic protection is used in areas where
the corrosion rate is low and electric power is not readily available. The gas-fired, hot-water tank and
the buried oil tank, shown in Figure 5-10, are typical examples of galvanic cathodic protection.