Galvanic and Impressed Current Anodes

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.
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.
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Material
Voltage Range
Relative Position
Least Noble (More Anodic)
Magnesium
-1.60V to -1.67V
Zinc
-1.00V to -1.07V
Beryllium
-0.93V to -0.98V
Aluminum Alloys
-0.76V to -0.99V
Cadmium
-0.66V to -0.71V
Mild Steel
-0.58V to -0.71V
Cast Iron
-0.58V to -0.71
Low Alloy Steel
-0.56V to -0.64V
Austenitic Cast Iron
-0.41V to -0.54V
Aluminum Bronze
-0.31V to -0.42V
Brass (Naval, Yellow, Red)
-0.31V to -0.40V
Tin
-0.31V to -0.34V
Copper
-0.31V to -0.40V
50/50 Lead/Time Solder
-0.29V to -0.37V
Admiralty Brass
-0.24V to -0.37V
Aluminum Brass
-0.24V to -0.37V
Manganese Bronze
-0.24V to -0.34V
Silicon Bronze
-0.24V to -0.30V
Stainless Steel (410, 416)
-0.24V to -0.37V
(-0.45V to -0.57V)
Nickel Silver
-0.24V to -0.30V
90/10 Copper/Nickel
-0.19V to -0.27V
80/20 Copper/Nickel
-0.19V to -0.24V
Stainless Steel (430)
-0.20V to -0.30V
(-0.45V to -0.57V)
Lead
-0.17V to -0.27V
70/30 Copper Nickel
-0.14V to -0.25V
Nickel Aluminum Bronze
-0.12V to -0.25V
Nickel Chromium Alloy 600
-0.09V to -0.15V
(-0.35V to -0.48V)
Nickel 200
-0.09V to -0.20V
Silver
-0.09V to -0.15V
Stainless Steel (302, 304, 321, 347)
-0.05V to -0.13V
(-0.45V to -0.57V)
Nickel Copper Alloys (400, K500)
-0.02V to -0.13V
Stainless Steel (316, 317)
0.00V to -0.10V
(-0.35V to -0.45V)
Alloy 20 Stainless Steel
0.04V to -0.12V
Nickel Iron Chromium Alloy 825
0.04V to -0.10V
Titanium
0.04V to -0.12V
Gold
0.20V to 0.07V
Platinum
0.20V to 0.07V
Graphite
0.36V to 0.19V
Most Noble (More Cathodic)

Primary voltage range for material
Voltage range in crevices or stagnant and poorly aerated water

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