Protecting the underwater hull
A ship's underwater hull is under constant attack from seawater. Hull anodes are fixed to the hull plating and corrode in place of the steel, protecting the hull, stern area and appendages between dry-dockings.
We supply hull anodes in zinc and aluminium alloys. Most are welded directly to the hull through steel inserts; bolted versions are available where the vessel design or maintenance plan calls for them.
Anode dimensions, insert design and mounting arrangement are produced to your drawings and CP requirements — for new builds and like-for-like replacement alike.
Mounting options
The mounting method is chosen to suit the vessel, the anode location and how the anodes will be maintained.
Weld-on anodes
Steel insert straps are welded directly to the hull, giving a secure mechanical and electrical connection. The most common arrangement.
Bolt-on anodes
Fixed to studs or bolts where welding is not preferred, or where quick replacement is a priority.
Custom inserts and shapes
Special sizes, insert designs and profiles for particular hull locations or customer requirements.
Key features
Designed for hulls
For submerged steel hull surfaces in marine service.
Zinc and aluminium
Alloy selected to suit the vessel and approved requirements.
Welded installation
Direct welding for a secure electrical and mechanical connection.
Custom configuration
Dimensions, inserts and mounting made to your requirements.
Technical data
Typical supply parameters. Final values are confirmed against your approved drawing and CP design.
- Product
- Sacrificial hull anode
- Typical applications
- Ship hulls and marine vessels
- Protection method
- Galvanic (sacrificial) cathodic protection
- Materials
- Aluminium alloy or zinc alloy
- Installation
- Welded or bolted, as required
- Configuration
- Hull-specific / customer-specific
- Dimensions
- To customer drawing and CP design
- Insert design
- To customer requirement
- Service environment
- Marine and seawater
- Electrical connection
- Through the approved hull mounting arrangement
- Design life
- As defined by the CP design
- Specification
- Applicable alloy specification, customer specification or approved drawing
Materials and standards
Choose the alloy to suit the environment and CP design, then the chemistry to suit your specification.
Aluminium (Al-Zn-In)
The usual first choice offshore: 2,550–2,750 Ah/kg and low weight for long design lives.
- Galvalum III equivalent
- DNV / ISO
- Alanode
Zinc
Compact, self-regulating and well proven — often chosen for hulls and confined spaces.
- MIL-DTL-18001
- ASTM B418 Type II
Available grades
Galvalum III equivalent
A well-established Al-Zn-In chemistry for seawater service, cast to equivalent composition limits.
DNV / ISO
Al-Zn-In composition in line with the limits given in DNV-RP-B401 and ISO 15589-2.
Alanode
Aluminium alloy chemistry supplied to the applicable Alanode composition limits.
US Military spec. (MIL-DTL-18001)
Zinc alloy with controlled aluminium and cadmium additions, widely specified for seawater service.
ASTM B418 Type II
High-purity zinc with very tight iron limits, typically specified for soil and fresh-water service.
Client specification
Your own chemistry, testing and documentation requirements, reviewed and confirmed with you before production starts.
Chemical composition
Composition limits in % by weight for the chemistries we supply.
Aluminium alloys
| Element | Galvalum III (equiv.) | DNV / ISO | Alanode |
|---|---|---|---|
| Zn | 2.0 – 6.5 | 2.5 – 5.75 | 0.50 – 5.00 |
| In | 0.01 – 0.02 | 0.016 – 0.04 | 0.005 – 0.05 |
| Si | 0.08 – 0.20 | 0.12 max. | 0.10 max. |
| Fe | 0.12 max. | 0.09 max. | 0.13 max. |
| Cu | 0.006 max. | 0.003 max. | 0.01 max. |
| Cd | 0.002 max. | — | — |
| Others (each) | 0.02 max. | 0.02 max. | 0.02 max. |
| Others (total) | 0.06 max. | — | — |
| Al | Remainder | Remainder | Remainder |
Zinc alloys
| Element | US Military spec. (MIL-DTL-18001) | ASTM B418 Type II |
|---|---|---|
| Al | 0.10 – 0.50 | 0.005 max. |
| Si | 0.125 max. | — |
| Cd | 0.025 – 0.15 | 0.03 max. |
| Fe | 0.005 max. | 0.0014 max. |
| Pb | 0.006 max. | 0.003 max. |
| Cu | 0.005 max. | 0.002 max. |
| Zn | Remainder | Remainder |
Specification note
Final chemistry should always be confirmed against the applicable material specification and approved project documents before production.
Electrochemical performance
Reference characteristics for our aluminium alloy anodes.
- Test environment
- Seawater at 23 °C
- Reference electrode
- Silver / silver chloride (Ag/AgCl)
- Open-circuit potential
- More negative than −1.10 V vs Ag/AgCl
- Closed-circuit potential
- More negative than −1.05 V vs Ag/AgCl
- Electrochemical capacity
- 2,550 – 2,750 Ah/kg
Performance note
Values apply to aluminium alloy anodes in seawater at 23 °C. In-service performance also depends on alloy chemistry, temperature, current demand and anode geometry, so final design values should always come from the approved cathodic protection (CP) design.
Applications
Commercial ships
Underwater hull protection for merchant vessels.
Marine vessels
Workboats, tugs and other vessels needing galvanic protection.
Shipbuilding
Anodes for new construction, to approved drawings.
Dry-dock replacement
Replacements matched to existing mounting arrangements.
Saltwater service
Zinc or aluminium selected for the vessel's operating waters.
Custom marine structures
Special sizes, inserts and mountings on request.
Manufacturing and quality
A controlled route from your drawing to a documented, ready-to-install anode.
-
01
Engineering review
We review your drawings, CP design and specification, and confirm alloy, net weight, insert design and tolerances before production.
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02
Insert preparation
Steel inserts are cut, formed and surface-prepared to the approved design for a secure bond with the anode.
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03
Melting and chemistry control
Each melt is prepared to the specified alloy, and its chemistry is checked against the required limits before casting.
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04
Casting
Anodes are cast to the required shape and net weight, with the insert soundly embedded.
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05
Inspection and testing
Visual, dimensional and weight checks, plus chemical analysis and electrochemical testing where the specification requires them.
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06
Documentation and dispatch
Anodes are identified, packed for safe transport and supplied with the agreed test records.
Inspection and documentation follow the project's inspection and test plan (ITP). Read about our approach to quality.
Frequently asked questions
What are hull anodes?
Sacrificial anodes fitted to ship hulls. They provide galvanic cathodic protection that reduces corrosion of the submerged steel.
Which materials are available?
Zinc alloy and aluminium alloy, selected to suit the vessel, CP design and approved specification.
How are hull anodes installed?
Usually by welding the steel inserts directly to the hull. Bolted versions are available where the vessel design or maintenance plan requires them.
Can you make custom sizes?
Yes. Dimensions, configuration and insert design are produced to your requirements and approved drawings.
Which specifications can you supply?
Galvalum III equivalent, DNV / ISO and Alanode for aluminium; MIL-DTL-18001 and ASTM B418 Type II for zinc; or your own client specification.
How is the right hull anode selected?
By the CP design, which considers vessel size, wetted surface area, coating, operating waters, current demand and the interval between dry-dockings.
Need hull anodes for your vessel?
Send us your vessel details, preferred anode material, dimensions or drawing, and we'll respond with a proposal.
Helpful to include
- Vessel type and hull drawings
- Anode material and specification
- Anode dimensions or drawing
- Insert and mounting arrangement
- Quantity per vessel
- Required delivery date / dry-dock schedule
