Why metal corrodes in sea water
Corrosion in water is an electrochemical process. When two different metals are electrically connected and sit in an electrolyte — sea water or brackish water — they form a galvanic couple, essentially a battery. The more active metal gives up electrons and dissolves (the anode); the more noble one receives them and stays intact (the cathode). Even a single steel plate has areas with different potentials, which is why steel rusts in water with no other metal nearby.
A vessel is full of such couples. Think of a bronze propeller and a steel hull, a stainless shaft and an aluminium sterndrive, brass fittings and steel pipework. Left alone, the most active metal in each couple is destroyed — usually the hull, the drive leg or the welds.
How the anode takes the corrosion
A sacrificial anode is a piece of metal that is more active than anything it protects. It is electrically connected to the structure — bolted or welded to the hull, shaft, drive leg or quay pile. It now becomes the anode of the galvanic couple and slowly dissolves. The steel, bronze and aluminium of the structure become the cathode and barely corrode. This is called galvanic (sacrificial anode) cathodic protection.
Hence the key rule: an anode must be consumed. If it looks new year after year, it is not working — it may be painted over, passivated or no longer in contact with the hull. See our anode inspection checklist.
The galvanic series in sea water
| Metal | Potential, V (approx., vs Ag/AgCl) | Role |
|---|---|---|
| Magnesium | −1.6 | Anode for fresh water |
| Aluminium alloy Al-Zn-In | −1.05 to −1.10 | Anode for sea and brackish water |
| Zinc | −1.03 | Anode for sea water |
| Carbon steel | −0.60 to −0.65 | Protected structure |
| Brass, bronze, copper | −0.25 to −0.40 | Protected parts: propellers, fittings |
| Stainless steel (passive) | −0.05 to −0.10 | The most noble metal on board |
The lower a metal sits in the table, the more noble it is. An anode must be more active — more negative — than everything it protects. Steel in sea water is considered protected once its potential is shifted to about −0.80 V or more negative.
What determines anode life
- Alloy. An aluminium alloy delivers roughly 2.5–3 times more ampere-hours per kilogram than zinc, so an anode of the same weight lasts longer.
- Salinity. In the brackish Baltic, zinc can passivate and stop working — see zinc vs aluminium anodes.
- Coating condition. The more bare metal on the hull, the higher the protection current and the faster the anode is consumed.
- Stray currents. Faulty wiring on board or shore power connections can multiply anode consumption.
- Contact. Rust, paint or a loose bolt between anode and structure break the circuit.
- Temperature and flow. Warm water and fast flow increase current demand and consumption.
Common mistakes
- The anode was painted along with the hull. Paint insulates the metal — the anode stops dissolving and stops protecting.
- No electrical contact. The anode sits on a painted surface or the fasteners have corroded. Resistance between anode and structure, measured with a multimeter, should be close to zero.
- Wrong alloy for the water. Zinc loses activity in brackish or fresh water, while magnesium in sea water over-protects: paint blisters and peels, and aluminium hulls suffer alkaline corrosion.
- Zinc and aluminium anodes mixed on one structure. They are consumed unevenly — choose one alloy.
Sacrificial anodes or impressed current
Large ships and port structures may also use impressed current cathodic protection (ICCP): an external power supply with inert anodes. It is controllable but needs power, adjustment and maintenance. Sacrificial anodes need no power supply at all. That is why they are fitted on smaller vessels, propellers and shafts, in ballast tanks and sea chests, and on quays. On large ships they also back up ICCP.
Anodes for your structure
We make zinc and aluminium anodes for hulls, tanks, shafts and propellers, quays and sheet piling — standard and to your drawing or sample. Send a drawing, a photo of the old anode with dimensions or its part number. We will recommend the alloy and confirm price and lead time.
Questions and answers
01How many anodes does a vessel need?
The number and weight of anodes come from the cathodic protection design: wetted area, coating condition, salinity and time to the next dry-docking. When replacing, go by the anodes currently fitted and how much they were consumed over the last period.
02Does an anode work when the boat is out of the water?
No. An anode only works in an electrolyte — in water. While the vessel is ashore there is no protection, but the anode is not consumed either.
03Can I paint an anode?
No. Remove or mask the anodes before painting the hull. A layer of paint switches the protection off.
04Why was my anode gone in half a season?
Typical causes are stray currents from wiring or shore power, a large area of bare metal, or an anode that is too light. Check the wiring and the contact, and fit a heavier anode if needed.