Galvanic anodes, (often called sacrificial anodes), protect metallic structures from corrosion without requiring an external current source. The difference in electrical potential between the anode and the metallic structure cause electrons to flow from the anode to the structure. The metallic structure is known as a cathode in this cathodic protection system. As the anode loses electrons, it becomes corroded, and the cathode remains free from corrosion. In time, the anode will become so corroded that it can no longer protect the metallic structure and will need to be replaced.
Cathodic protection engineers take advantage of this process, and design systems where anodes are linked to steel structures to extend the lifetime of the structures. Galvanic cathodic protection is often used for storage tanks, smaller ships and bridges. It is a more simple system than an impressed current cathodic protection system. Installation, inspection and monitoring are simple for trained staff, and stray current and over-protection are unlikely.
A sacrificial anode must have a electrical potential at least 0.2V more negative than the metallic structure in order to be effective. Typical materials for galvanic anodes are zinc, aluminum and magnesium. These 3 metals are among the most anodic metals in the galvanic table, and all are more anodic that any type of steel.
Impressed Current Anodes
The primary difference between galvanic and impressed current cathodic protection systems is an external power source. The external power source forces current in the desired direction, regardless of anode material. Materials used in impressed current anodes include high silicon cast iron, graphite, mixed metal oxide, platinum, titanium, niobium and more.
Impressed current cathodic protection (ICCP) systems, though more complex than galvanic systems, provide several advantages. Because of the increased flexibility in anode materials, the ICCP designer can select anodes that will last longer. Also, the external power source provides greater control and monitoring over the entire system. Insufficient or too much current can be detected and changed after installation to ensure the appropriate amount of protection is being delivered.
Because of the greater flexibility and control, ICCP systems are often used on larger projects. Gas and oil pipelines, large ship hulls, reinforced concrete and seawalls are some of the many structures that are protected with ICCP systems.