Galvanic Anodes and Galvanic Corrosion Prevention – FAQ

Galvanic anodes are a class of anode that do not require an external power supply. Instead they rely on their natural potential being more electro-negative than the potential of the structure being protected. This creates the voltage differential necessary to generate cathodic protection current flow.

Galvanic anodes are masses of metal alloy available in a wide range of shapes, sizes, and configurations.

The other type of cathodic protection anode is the impressed current anode which requires an external power supply to provide current and generates the voltage differential necessary for cathodic protection.

Sacrificial anode is a term commonly applied to galvanic anodes. This is because galvanic anodes consume as part of the electro-chemical reaction required to create current for cathodic protection (thus they sacrifice themselves when used for cathodic protection).

The term sacrificial, while commonly applied as synonymous with galvanic, is not the appropriate technical term. Many impressed current anodes are also consumed in a similar “sacrificial” manner as galvanic anodes, but they require a power supply to create the requisite differential voltage to drive current in the cathodic protection circuit. In practice however, the term sacrificial is used interchangeably with galvanic.

There are three common galvanic anodes used in cathodic protection systems: Magnesium, Zinc and Aluminum. Each of these anode materials have different driving potentials and performance characteristics and are typically deployed in different applications. The right type for given application depends largely on the nature of the structure and the environment where the anode will be installed (the electrolyte).

Galvanic anodes have several advantages:
• They do not require an external power supply
• Low maintenance costs throughout the design life
• Low potential virtually eliminates stray current concerns with other structures

Galvanic anodes have several limitations that must be considered:
• The driving force available is very limited
• The service life and efficiency tend to be low requiring either large quantities of anode or more frequent anode replacement
• May not be suitable for some acidic environments
• Generally, not suitable for high resistance environments (typically anything greater than 10k ohm-cm)

Cathodic protection is an electro-chemical process. As an electrical circuit (the electro part of electro-chemical) the basic Ohms law applies, such that Voltage = Current x Resistance (V=IR) where voltage (V) is the difference between the voltage (potential) of the anode and that of the structure.

This difference is often referred to as the “driving potential”. When the structure and the anode are connected in a circuit, the amount of current (I) is determined by this driving potential (V) and the overall system resistance (R).

As noted earlier, cathodic protection is an electro-chemical process and as an electrical circuit, CP systems follow Ohms law (V=IR).

When we talk about the chemical part of electro-chemical, we have to consider exactly how electrons are transferred and what chemical reactions have to occur. Metals are tightly bound accumulations of atoms in a specific structure.

For most anode systems, the chemical reaction at the anode surface is an oxidation reaction where the metal anode at the surface reacts to release a positive metallic ion and an electron. The metallic ion reacts with water to create a metallic oxide. The metal is not actually consumed, but it is transformed–from its metallic structure into an oxide form that lacks the tightly bound structure of the metal and often goes into solution in the environment around the anode.

This oxidation reaction is stoichiometric, meaning there is a chemical equation that defines exactly how many electrons are liberated when a certain number of metal atoms are oxidized.

The amount of weight loss is a mathematical function tied to the number of electrons transferred per atom of metal over a given period of time – this is known as Faraday’s Law.

While this varies with each anode type, it is possible to calculate the mass of metal being consumed to create a given amount of current. This consumption rate is often expressed as kg/amp-yr (lbs/amp-yr).

Galvanic anodes are not perfectly efficient – in that 100% of the mass of the anode cannot be fully utilized in the cathodic protection circuit. Impurities in the metal alloy, self-corrosion reactions, and uneven consumption all combine to limit the complete utilization of the anode mass, some of that mass will simply not convert into electron flow. This utilization factor varies with the type of galvanic anode and must be considered when designing the galvanic anode system.

Magnesium anodes are available in two common alloys: high potential and standard.

The high potential magnesium anode has a natural potential of -1.75V/CSE (relative to a copper sulfate electrode CSE) while the standard potential formulation has a -1.55V/CSE. Magnesium anodes provide the highest available potential of all the common galvanic anodes and are often used in higher resistivity environments including fresh water and most soils. Magnesium anodes have a relatively low efficiency in the 50% range. In lower resistivity applications they can consume quickly, limiting their economic value. In soil applications, they are commonly installed with a special ion-rich backfill to improve performance.

Zinc, with a potential of -1.1V/CSE, has a lower natural potential than magnesium. Thus, the driving potential between a zinc anode and a steel structure is quite low.

This limits zinc anodes to applications where the resistance is low such as fresh, brackish and sea water applications. Zinc anodes are much more efficient than magnesium anodes with a utilization factor of 90-95%. Zinc is an amphoteric metal that can react both with an acid or a base.

Typically, all anodes operate in acidic environments as the outward flow of electrons leaves behind a concentration of H+ cations; however, certain conditions of temperature and environmental chemistry can cause zinc to change states and begin accepting electrons. This renders the zinc ineffective as an anode. Some conditions to be concerned with include elevated temperature applications and areas with high concentrations of bicarbonates, carbonates or nitrates.

As with magnesium anodes, zinc anodes for use in soil are often packaged in an ion-rich gypsum bentonite backfill.

In the image below, steel galvanic anodes are used to protect the steel pipe piles for a jetty under construction.

Galvalum III is an aluminum/indium/zinc alloy commonly used in seawater. This alloy formulation has a higher potential than other aluminum anodes and works well in a wide range of seawater environments.

Aluminum anode potentials, similar to zinc, are in the -1.1 to -1.15V|CSE. Most of the physical shapes for aluminum anodes are designed for marine applications and include bracelets for attachment to offshore pipeline, configurations with steel straps for attachment to ship hulls, large hooks and eyes for suspension from piers and jetties, and formed with cast-in standoff supports for use on offshore structures. Aluminum anodes are efficient with utilization factors between 85% and 95%.

Because these anodes operate on the natural potential difference between the anodes to drive cathodic protection current, there is no external power supply. This makes it difficult to monitor the anode system performance. If monitoring is required, then it is common to have the anode connected to the structure through a test station equipped with a shunt. This arrangement provides access to measure the anode system’s current output without having to dig up the anode to structure connection. For the most part, however, these galvanic systems are often installed with no significant monitoring provisions and are run until failure. This install-and-forget feature is simultaneously both an advantage and a disadvantage of galvanic anode systems.

Generally for magnesium anodes a 75/20/5 mix of gypsum, bentonite and sodium sulfate is used while for zinc anodes a 50/50 mix of gypsum and bentonite is typical.

Because there is no external power supply, the design of galvanic anode systems can be quite unforgiving. An external power supply in other types of CP systems allows current output to be adjusted up or down by varying the applied voltage to meet the desired current output or structure potential.

With a galvanic anode system, once installed there is no volume switch. It either works or it doesn’t. When these anodes operate too fast, they consume quickly (it is possible in some cases to use a resistor to alter the circuit resistance and throttle back the output) and when the resistance is too high they are unable to discharge sufficient current to meet the application’s requirements.

As with Goldilocks’ porridge, for galvanic anodes, the conditions have to be just right.