Why the Baltic is a special case
Ocean salinity is about 35 ‰. In the central Baltic it is roughly 6–8 ‰, and lower still in the Gulf of Finland and near river mouths. That is brackish water: its conductivity is much lower than sea water, which matters for cathodic protection.
In such water zinc gradually builds up a dense layer of oxides and salts — it passivates. The anode looks almost intact but has nearly stopped working: protection current drops and corrosion moves to the propeller, shaft and hull. An aluminium alloy with indium stays active in brackish water.
Zinc vs aluminium: comparison
| Zinc Zn-Al-Cd (MIL-DTL-18001) | Aluminium (Al-Zn-In) | |
|---|---|---|
| Sea water | Yes | Yes |
| Brackish water (Baltic) | Limited — risk of passivation | Yes |
| Fresh water | No | No — use magnesium |
| Capacity, Ah/kg (order of magnitude) | ≈ 780 | ≈ 2000–2600 |
| Density | 7.1 g/cm³ | 2.7 g/cm³ |
| Weight for the same protection | Higher | Roughly 2–3× lower |
Because of its higher capacity, an aluminium anode of the same weight lasts longer — or a lighter anode gives the same service life. On larger vessels and in ballast tanks this is a noticeable weight saving.
When to choose zinc
- The vessel operates mainly in full-strength sea water (North Sea, ocean).
- Zinc anodes are already fitted and you are replacing like for like — do not mix types on one structure.
- The vessel, class society or equipment maker specifies zinc.
When to choose aluminium
- Ships, boats and workboats sailing the Baltic, the Gulf of Finland and the Gulf of Riga.
- Vessels moving between the Baltic and the North Sea: aluminium works in both.
- Quays, sheet piling and port steelwork in brackish water.
- Ballast tanks and places where anode weight matters.
For fresh water (lakes, rivers) neither zinc nor aluminium is suitable — you need magnesium. We do not make magnesium anodes.
When to replace an anode
The general rule is to replace an anode when it is about half consumed. If an anode has barely changed over a season, that is not good news: it may have passivated (typical of zinc in brackish water) or lost contact with the hull. Check the mounting and electrical contact, and for the Baltic consider switching to aluminium. More in our anode inspection checklist.
Non-standard shape or size
We cast zinc and aluminium anodes of any shape and weight to your drawing, sample or part number: hull, tank, shaft and propeller, quay anodes. Send a drawing or a photo of the old anode and we will select the alloy and confirm price and lead time.
Questions and answers
01Can zinc and aluminium anodes be fitted on the same vessel?
Better not to mix them: the alloys have different potentials and one type will be consumed faster than the other. Choose one type for the whole structure.
02Does an aluminium anode work in sea water?
Yes. Al-Zn-In works in both sea and brackish water, so it suits vessels sailing between the Baltic and the North Sea.
03Why is my zinc anode barely wearing?
In brackish water zinc can passivate — a surface layer stops the protection. Check the contact with the hull; for the Baltic, aluminium is usually the better choice.
04Can you make an anode from an old sample?
Yes. Send a photo with dimensions or the sample itself and we will make an anode of the same shape in the right alloy.