Avoiding the threat of corrosion through cathodic protection

Corrosion can occur in all types of water and the consequences for boats can be catastrophic. That’s why it’s important to protect your vessel using strips of metal called sacrificial anodes. These anodes are part of a process called cathodic protection.

Below, we’ve provided an explanation of how corrosion occurs and a guide to how it can be prevented through cathodic protection and the use of sacrificial anodes. 

About corrosion

All leisure craft have metal parts that are susceptible to corrosion. How does corrosion occur? In short, a metal will corrode as a result of a chemical reaction to its surroundings, such as water.

Corrosion takes place in every type of water, whether fresh, salt or brackish, and, thanks to rising numbers of enclosed marinas and tidal barriers, boats are now regularly moving between all three.

Hence, without protecting your boat, corrosion is unavoidable. To avoid costly repair bills, it is imperative that you protect your boat with quality sacrificial anodes and that you know which ones to use.

Galvanic corrosion

Boats are principally at risk from galvanic corrosion, which is a process caused by the proximity of dissimilar metals to each other while in the water.

If these metals are in direct electrical contact and are immersed in water, they form a simple battery. The battery produces a small voltage, which drives a current from one metal component to the other through their metallic connection, and ultimately causes one of the components to corrode.

The component that corrodes depends on the position of the two metals in the galvanic series. The metals at the top of the series act as an anode and are subject to corrosion, while those lower down this ranking system are protected from corrosion (and are said to be more noble metals).

Electrolytic corrosion

Electrolytic corrosion is another common form of corrosion. Electrolytic corrosion is caused by stray current leakage, which occurs when a fault on board a boat causes an electric current from an external power source, such as a battery or an onshore power supply, to pass out through the hull or a fitting in the hull and flow through the water.

How to spot the signs of corrosion

Corrosion to steel and aluminium boats occurs in two forms: localised pitting to areas such as the hull plate, rudder and the bilge keel, and general wastage of the hull plate, which often occurs below the paint coating.

The impact of this corrosion can be very damaging. Localised pitting can lead to the complete penetration of the hull below the waterline, while general wastage can critically weaken the hull. Both can lead to expensive re-plating being required.

Corrosion to wooden and GRP boats occurs principally to the stern gear, such as the propellers, shafts, shaft brackets, stern tubes and rudder, which are critical to the viability of the vessel and expensive to replace.

The effects of corrosion to wooden and GRP boats can vary from localised pitting to components such as the propeller and shafts to the decomposition of the propeller alloy. The failure of something as small and inexpensive as a split pin can lead to the loss of a propeller.

Basic steps to prevent corrosion

Painting your boat

Applying the correct painting system to a boat is essential to helping protect it from corrosion. Advice on the most appropriate coating system to use should be sought from your paint supplier.

We recommend the application of an anti-corrosive primer if an anti-fouling is to be used. If you’re using a copper anti-fouling, you must not apply any paint directly to bare metal surfaces.

In addition, vegetable oil-based paints should not be used with cathodic protection systems as the paint is likely to saponify. The application of zinc metal spray is not recommended below the water line.

Electrical installation

The proper installation of electrical components on your boat will reduce the chance of stray current leakage and electrolytic corrosion.

The easiest way to avoid electrical problems is to have electrical work carried out by a qualified marine electrician. However, if you are doing any work yourself, we recommend the following:

  • Use only high-grade insulated wiring of suitable capacity. Undersized wiring will cause resistance and voltage drop.
  • Clip or support all wires at suitable intervals to prevent fatigue and fracture.
  • Use only corrosion-resistant terminals and connectors, and ensure they are clean and tight.
  • Attach only the main battery leads to the battery terminals.
  • Fit an isolation switch in the battery circuit.
  • Ensure that all battery circuits are correctly fused.
  • Keep all wiring, connections and junction boxes above the bilge area and other areas likely to become wet.

Maintenance

Regular maintenance is a good way to help prevent corrosion. Metal work, paint work and electrical components should all be regularly inspected.

If you have a steel boat, it is important that you pay special attention to areas around the water line, which are particularly vulnerable but receive no protection from sacrificial anodes placed above the water line.

What is cathodic protection?

Cathodic protection is an electrochemical process that halts the natural corrosion of metals in a particular environment, in this case water.

In scientific terms, it works by superimposing an electrochemical cell more powerful than the corrosion cell. Put more simply, it diverts the corrosive process away from the metal components of your boat and on to stripes of metal called sacrificial anodes.

These sacrificial anodes are attached to the metal components of your boat and, because they are made of a metal higher placed in the galvanic series, they attract corrosion and protect against damage. The anodes corrode and not your boat.

The number and size of sacrificial anodes that you should use depends on what your boat is made from and the size of the surface area being protected. In addition, the size of your boat, the type of water that it is used in and the length of time it spends in the water have a bearing on how often the anodes should be replaced.

How to choose the correct type of sacrificial anode

As a rule of thumb, you should choose sacrificial anodes according to the water you most regularly use your boat in. The following is a useful guide:

Salt water Zinc or aluminium anodes
Brackish water Aluminium anodes
Fresh water Magnesium anodes

Moving between water types

Boats often move between different waters, either travelling between salt and fresh water, or being berthed in marinas or behind tidal barriers where the water is enclosed and brackish.

Importantly, not all anodes are suitable for every type of water. For example, zinc and aluminium anodes are not suited to fresh water. If they are exposed to fresh water for a prolonged period, they become covered in a white oxide crust that effectively seals the anode and stops it from working when the boat returns to salt waters. Zinc anodes suffer a similar problem in brackish water.

Magnesium anodes are not suited to salt water. In salt water, they become much more active than usual and this accelerated activity shortens the lifespan of the anodes and can leave a build-up of an off-white calcareous on your boat that is difficult to remove. In addition, magnesium anodes should never be fitted to wooden hulled vessels as they can damage the timber.

Anode Outlet recommends that if you are moving into fresh water from salt water or vice versa for more than two weeks, you should change the type of sacrificial anodes on your boat.

The bottom line is that if the anode isn’t corroding, your boat is. Hence, it is vital to check the condition of your sacrificial anodes as you move between types of water and, if necessary, clean or change them. 

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