WATER CRAFT MAINTENANCE

Anodes and Coating Systems Work Together

Anodes and a good quality coating work best when applied together. The protective coating reduces the amount of DC current discharging from the hull by reducing the exposed surface area. When the coating starts to deteriorate over time, and more of the steel becomes exposed, the anodes offer protection to those exposed areas.

The application of coatings and anodes should be seen as complimentary to each other.

Engines

Most marine engine manufacturers install small zinc or aluminium anodes inside the cooling jackets of salt water cooling systems. These anodes minimise corrosion damage to the metallic internal surfaces of these components. Due to the active degradation of these small anodes in high flow conditions, they should be checked regularly and replaced as necessary.

Bilges and Ballast Tanks

The internals of a vessel’s hull are also prone to corrosion. Bilges and ballast tanks are considered wet areas that can contain substantial quantities of water and other fluids and should be treated like any other structure requiring protection against corrosion. A bilge or ballast tank should be correctly coated and anodes employed.

Often long slender anodes are installed in bilges or ballast tanks. Another alternative is the “string type” anode(s) where a stainless steel wire core is used to connect the “string of anodes” to the hull.

Skin Fittings

If two dissimilar metallic skin fittings are bonded (electrically connected), galvanic corrosion activity on the immersed and embedded surfaces may occur. When skin fittings, cooling pipes, shafts or propellers show signs of corrosion, it is important that correct electrical bonding is undertaken and sufficient anodes are installed.

Bonding and Electrical Continuity

It is often assumed that the propeller shaft and hence the propeller are bonded through the gearbox to the engine and to other earthed structures or fittings on the vessel. This is usually true when the vessel is sitting idle, however once shafts and gears start moving, there can be sufficient resistance across the lubricated moving parts to cause a galvanic disconnection or isolation between the hull and the moving parts.

If electrical continuity between the hull and shafts and propellers is necessary, these fittings should be directly bonded to the hull or other conductors. In cases where the gearbox does not provide electrical continuity, bonding of the shaft to a metallic hull or engine can be achieved by utilising a slipring and soft copper/carbon contact brush kit, which is then connected from the hull to the shaft.

Similar problems have been observed with some rudder assemblies where the pintle can become isolated. A flexible bond cable from the rudder stock to the hull plating usually overcomes this problem.

Skin fittings can be effectively isolated from the engine by using non-conductive plastic hoses in lieu of metallic piping if necessary. Very little can be done to effectively isolate instrumentation. However, double insulated wire power systems are preferable on steel and aluminium vessels.

Should electrical earthing/bonding be necessary, then a continuous negative loop earth is recommended where all earthing is connected to the negative loop. The use of direct multiple earthing to many points on any hull wiring system is generally considered to be bad practice.

Stainless Steel

The corrosion resistance of stainless steel is primarily provided by the formation of a protective oxide layer on the surface of the stainless steel. If the oxide layer is depleted or removed and is unable to re-form, it becomes active, which can then lead to localised corrosion. Marine grade stainless steels can corrode if used or installed in the wrong environment.

There are some grades of stainless steel that have better corrosion resistance than others. For example, Type 304 exhibits good corrosion resistance in air, but not so good in sea water. Types 316, 316L and 2205 Duplex offer good corrosion resistance in marine environments.

Galvanised Steel

Galvanising is a metallising process that was developed to provide protection of steel from corrosion in the atmosphere (air). If galvanised coated fittings are immersed in sea water without the addition of an effective coating, the bare galvanising will become the anode and break down (similar to a zinc anode) exposing the steel to corrosion and potential failure.

Decks, Deck Fittings and Rigging

All non-immersed fixtures are subject to the marine environment and therefore should be selected for their corrosion resistance properties. Where mild steel is utilised it is necessary to apply suitable protective coatings in conjunction with a routine maintenance program.

Note: In order to offer protection, anodes require an electrolyte, such as sea water. Anodes will not work in atmosphere (air).