A sacrificial anode is a highly reactive metal connected to a steel structure so that it corrodes instead of the steel, continuously supplying protective current that prevents steel degradation. It is the simplest form of cathodic protection, requiring no external power, and it is used to protect buried pipelines, storage tanks, ship hulls, ballast tanks and submerged structures worldwide.
The performance of a sacrificial, or galvanic, cathodic protection system depends on choosing the right anode material for the environment. The three metals used are zinc, aluminium and magnesium, and each suits different conditions. This guide explains how they differ and how the correct anode is selected, sized and supplied.
How a sacrificial anode works
When two different metals are electrically connected in an electrolyte such as soil or seawater, the more reactive metal corrodes and protects the less reactive one. In a sacrificial cathodic protection system, anodes of a reactive metal are deliberately connected to the steel structure. The anode is the component that corrodes, providing protective current to the steel and keeping the steel at a stable, non‑corroding potential. The anode is consumed over time and is replaced at the end of its design life.
Because the current is generated by the electrochemical potential difference between the two metals, no external power supply is needed. This makes sacrificial systems simple, reliable and well suited to smaller structures, to locations without a power source, and to environments with low electrical resistance where the required current can be delivered without a driving voltage.
The three sacrificial anode metals
The electrochemical properties of each anode alloy—such as its open‑circuit potential, polarisation behaviour, and tendency to passivate—directly influence where it performs best.
Aluminium anodes
Aluminium alloy anodes offer high current capacity and long life for a given weight, and they perform reliably in seawater. This makes them the standard choice for marine applications such as ship hulls, ballast tanks and subsea structures, and for offshore foundations. Their favourable capacity-to-weight ratio means fewer or lighter anodes are needed to deliver a given protection period, which matters where weight and drag are considerations. They are also widely used for internal protection of crude oil storage tanks, where the aggressive water phase at the bottom requires a stable, high‑efficiency anode material that resists passivation and delivers consistent current output over long service intervals.
Zinc anodes
Zinc anodes are a robust and long-proven choice for seawater and brackish water. They deliver a stable, predictable output and are widely used on hulls, tanks, fittings and offshore structures where established marine practice applies. Zinc is well suited to lower-resistivity environments and is a dependable option where its performance history and reliability are valued. In soil applications, zinc can be used where the ground resistivity is relatively low and stable, providing consistent current output without the risk of passivation.
Magnesium anodes
Magnesium anodes have the highest driving voltage of the three, which makes them effective in higher-resistivity environments where zinc and aluminium cannot deliver sufficient current. This suits buried applications in soil and freshwater or brackish conditions, such as onshore pipelines, storage tank bases and water heaters. The higher driving voltage comes with faster consumption, so magnesium is chosen where its ability to drive current through resistive ground is the deciding factor. Magnesium is typically not used in seawater because its very negative potential leads to excessive self‑corrosion and over‑polarisation of steel, resulting in rapid anode wastage and unstable performance.
Choosing the right anode for the environment
Anode selection is driven primarily by the electrolyte and its resistivity, alongside the required design life and the structure being protected. As a general guide:
| Environment | Typical anode choice | Common applications |
| Seawater | Aluminium or zinc | Ship hulls, ballast tanks, subsea and offshore structures |
| Brackish water | Aluminium or zinc | Estuarine structures, harbour and port assets |
| Soil (buried) | Magnesium | Onshore pipelines, tank bases, buried steel |
| Fresh water | Magnesium | Water tanks, heaters, freshwater immersed steel |
Soil and water resistivity is the single most influential factor, so a soil resistivity survey is usually the starting point for a buried application. The final selection is then confirmed by calculating the current required to protect the structure, checking the anode resistance to ensure the anode can deliver that current and sizing the anode mass to sustain full design life.
Sizing and design life
A sacrificial anode system is designed, not simply fitted. The current requirement is calculated from the structure’s surface area, its coating condition and the environment. The anode’s ability to deliver that current is then verified by calculating its resistance in the electrolyte, ensuring the driving voltage is sufficient to produce the required output. The total anode mass is sized from the expected consumption rate and efficiency so that the anodes can supply the required current for the required period, since the metal is consumed as it works. Anode number, dimensions and spacing are set to distribute current evenly across the structure, avoiding areas that are under-protected or, at the other extreme, receiving more current than they need.
Getting this right at the design stage is what determines whether a system protects the asset for its full intended life. An undersized anode array leaves the structure unprotected before the next planned replacement; an oversized one adds unnecessary cost and can create additional risks. Backfill, appropriate to the environment, is used with buried anodes to lower resistance and maintain consistent output.
UK-manufactured anodes from Corrpro Europe
Corrpro Europe designs, manufactures and supplies a full range of galvanic anodes in zinc, aluminium and magnesium, cast at our foundry in Stockton-on-Tees. Our casting facilities include dedicated aluminium and zinc furnaces and a magnesium anode assembly plant, allowing us to produce anodes to the weight, dimensions and chemical composition a project requires, including bespoke sizes, cable lengths and backfill options. Aloline aluminium and Zincoline zinc hull and tank anodes are supplied worldwide, and our anodes are produced under an ISO 9001 registered quality management system.
Manufacturing in-house means anodes are made to the cathodic protection design rather than the design being constrained to fit a catalogue, and it supports short lead times and reliable quality. We supply anodes for industrial, marine and offshore applications, backed by the design capability to specify them correctly.
Frequently asked questions
What is a sacrificial anode?
A sacrificial anode is a block of a more reactive metal, such as zinc, aluminium or magnesium, connected to a steel structure so that it corrodes in preference to the steel. It supplies a protective current that keeps the structure at a potential where corrosion is controlled, and it is consumed and replaced over time. It needs no external power.
Which is better, a zinc or an aluminium anode?
Both protect steel in seawater. Aluminium offers higher current capacity and longer life for a given weight, so fewer or lighter anodes are needed, while zinc is a robust, long-proven option with predictable output. The best choice depends on the application, the water chemistry and the design life.
When are magnesium anodes used?
Magnesium anodes are used in higher-resistivity environments such as soil and fresh water, where their high driving voltage can push sufficient current through resistive ground. They are common on buried onshore pipelines, tank bases and water heaters. They consume faster than zinc or aluminium, so they are chosen where driving voltage is the priority.
How long do sacrificial anodes last?
Anode life is set at the design stage. The total anode mass is calculated to supply the required protective current for a defined design period, after which the anodes are replaced. On vessels this is typically aligned with the dry-dock cycle; on buried structures it is planned around the design life and monitoring.
To specify or order sacrificial anodes for your application, contact our team or explore our Cathodic Protection Materials range.
