Why Magnesium Anodes are used as sacrificial Cathodic Protection

Introduction

Magnesium sacrificial anodes are widely used in cathodic protection systems to prevent corrosion of metallic structures, such as pipelines, underground tanks, and steel foundations. By acting as a more active metal, magnesium corrodes preferentially, protecting the steel or iron structure.

Key Concept:

Magnesium has a higher (more negative) electrochemical potential than steel, making it an ideal anode in low-resistivity environments like soil or freshwater.

Principles of Cathodic Protection

Sacrificial Anode Method (Galvanic Protection):

A more “active” metal (anode) is electrically connected to the metal to be protected (cathode).

The anode corrodes instead of the cathode.

This works because magnesium has a more negative electrode potential than iron:

Magnesium -2.37 V
Zinc -0.76 V
Aluminum -1.66 V
Iron/Steel -0.44 V

Reaction:
Mg → Mg²⁺ + 2e⁻
Steel (cathode) receives electrons → corrosion is prevented.

Properties of Magnesium Anodes

Chemical Composition:

High-purity magnesium or magnesium alloys (often Mg-Al-Zn).

Key Properties:

Driving Voltage: ~1.6 – 1.7 V in soil

Theoretical Capacity: 2200 – 2300 Ah/kg

Typical Utilization Efficiency: 70–80%

High Current Output: Suitable for low-resistivity soils

Environmental Suitability:

Excellent for soil and freshwater.

Not recommended for seawater (zinc/aluminum preferred)

Installation Considerations

Anodes buried vertically or horizontally near the structure.

Use backfill (gypsum, bentonite, sodium sulfate) to reduce soil resistivity.

Maintain good electrical connection to structure (cable, clamp, welding).

Typical spacing: 10–15 m for pipelines, depending on soil resistivity.

Limitations and Challenges

Poor performance in high-resistivity soil (>200 Ω·m)

Risk of overprotection, leading to coating disbondment or hydrogen embrittlement.

Passivation in certain environments reduces anode effectiveness.

Magnesium can’t be used in seawater for long-term protection (aluminum/zinc preferred).

Maintenance & Replacement

Typical lifespan: 5–10 years (depends on environment and load)

Inspect at intervals: every 6–12 months for soil systems.

Replace anodes when more than 70–80% of material is consumed.

Ensure connections remain intact to maintain electrical continuity

Conclusion

Magnesium sacrificial anodes are a cost-effective and reliable solution for cathodic protection of steel structures in soil and freshwater. Understanding their principle, design, installation, and maintenance ensures long-term protection against corrosion.