3.4 Sacrificial Anode Materials: Mg, Zn, & Al Alloys

Sacrificial Anode Materials: Mg, Zn, & Al Alloys

The success of a galvanic cathodic protection system hinges entirely on selecting the correct anode material for the specific environment. Not all metals make good sacrificial anodes. A suitable anode material must have a sufficiently negative open-circuit potential to provide a driving voltage, a high electrical capacity (amp-hours per pound), a high and predictable efficiency, and a tendency to corrode uniformly without passivating.

The corrosion industry relies almost exclusively on three metals and their highly specific alloys: Magnesium, Zinc, and Aluminum.

Magnesium (Mg) Anodes

Magnesium is the workhorse for underground (soil) galvanic CP systems. It is highly active in the galvanic series, providing the highest driving voltage of the common anode materials. This makes it the only practical choice for soils with moderate to high resistivity.

Types of Magnesium Anodes

There are two primary alloys of magnesium used in CP, distinguished by their purity and resulting potential:

1. Standard Potential (H-1 Alloy or AZ63):

   • Composition: Contains specific amounts of Aluminum (approx. 6%) and Zinc (approx. 3%).
   • Potential: Typically -1.45 V to -1.55 V (vs. Copper-Copper Sulfate Electrode, CSE).
   • Driving Voltage (to a -0.85V polarized structure): ~0.65 V.
   • Application: Used in low to moderate resistivity soils (typically less than 2,000 ohm-cm). They offer slightly higher efficiency than high-potential anodes but less driving voltage.

2. High Potential (High Purity) Alloy:

   • Composition: Extremely pure magnesium with trace amounts of manganese to improve efficiency.
   • Potential: Typically -1.70 V to -1.75 V (vs. CSE).
   • Driving Voltage (to a -0.85V polarized structure): ~0.90 V.
   • Application: The go-to choice for higher resistivity soils (up to roughly 10,000 ohm-cm). The extra driving voltage is necessary to push current through the resistive earth. They are the most common anode used on pipelines today.

Properties of Magnesium

• Theoretical Capacity: Very high, approximately 1000 Amp-hours per pound (Ah/lb).
• Efficiency: Relatively low. Magnesium is so reactive that it undergoes significant "self-corrosion" (local cell action on its own surface that doesn't contribute protective current to the pipeline). Typical efficiency in soil is around 50%. This means only about 500 Ah/lb of useful current is actually realized.
• Environment: Used almost exclusively in soil and sometimes fresh water. Seldom used in seawater because they consume far too rapidly and can over-protect structures, leading to coating damage.

Zinc (Zn) Anodes

Zinc has been used for cathodic protection longer than any other metal. It is less active than magnesium, providing a lower driving voltage, but it is highly efficient and reliable in the right environments.

Properties of Zinc

• Potential: Extremely stable at approximately -1.10 V (vs. CSE).
• Driving Voltage (to a -0.85V polarized structure): ~0.25 V.
• Theoretical Capacity: Lower than magnesium, approximately 372 Ah/lb.
• Efficiency: Very high, typically 90% to 95%. Zinc suffers from very little self-corrosion. Therefore, its practical capacity is around 335 Ah/lb.

Applications of Zinc

Because of its low driving voltage (only 0.25 V), zinc can only push current through very low-resistance circuits.

• Low Resistivity Soils: Zinc is excellent in soils with resistivity below 1,000 ohm-cm (like coastal marshes or saline soils).
• Marine Environments: Zinc was historically the primary anode for seawater applications (hulls of ships, offshore structures) because seawater has very low resistivity (often < 30 ohm-cm). However, aluminum has largely replaced zinc in seawater due to weight and capacity advantages.
• AC Mitigation and Grounding: Zinc is uniquely suited for grounding structures to mitigate induced AC voltage or fault currents. It provides a safe grounding path while simultaneously offering some cathodic protection, unlike copper grounds which would cause severe galvanic corrosion to a steel pipeline.
• Temperature Limitation: Zinc anodes should not be used in environments exceeding 140°F (60°C). Above this temperature, a phenomenon called "polarity reversal" can occur, where the zinc actually becomes cathodic to the steel, causing the steel to corrode rapidly to protect the zinc.

Aluminum (Al) Anodes

Pure aluminum passivates easily, forming a tight oxide film that stops current output. However, by alloying aluminum with small amounts of metals like indium, zinc, or mercury, it remains active. Aluminum alloys are now the dominant sacrificial anode for marine environments.

Properties of Aluminum Alloys

• Potential: Similar to zinc, typically -1.10 V to -1.15 V (vs. Silver-Silver Chloride electrode, Ag/AgCl, which is standard for seawater).
• Driving Voltage: Similar to zinc, relatively low.
• Theoretical Capacity: Extremely high, approximately 1350 Ah/lb.
• Efficiency: High, typically around 85% to 90% in seawater. This yields a massive practical capacity of over 1150 Ah/lb.

Applications of Aluminum

• Seawater Exclusivity: Aluminum anodes are used almost exclusively in seawater and brackish water (offshore platforms, ship hulls, submerged pipelines).
• Why not soil? Aluminum anodes rely on the abundant chloride ions present in seawater to prevent that tight oxide film from forming. If placed in typical soils, they will quickly passivate, shut down, and stop providing current.
• Weight Advantage: Aluminum is significantly less dense than zinc. An aluminum anode of the same physical size as a zinc anode will weigh much less but provide far more total amp-hours of protection. This makes aluminum highly cost-effective for large offshore structures where the weight of the CP system is a structural consideration.

Material Selection Summary

Understanding these material properties allows the CP tester to identify when the wrong material might have been installed, explaining system failures.

EPG Practical Galvanic Series (vs CSE)

The AMPP CP1 Exam Preparation Guide lists these approximate open-circuit potentials versus saturated Cu/CuSO₄ for exam reference:

Use −1.60 V CSE for standard magnesium alloy anodes and −1.75 V CSE for high-potential magnesium—not a −1.45 to −1.55 V band.