Cathodic Protection

Corrosion is an electrochemical reaction between a material and its environment which results in the deterioration or destruction of the material. Corrosion is not necessarily restricted to metals.

In most circumstances metallic corrosion requires two factors in order for a reaction to occur:

1. At least one metal
2. An electrolyte such as water, soil or concrete or a moist atmosphere

WHY DOES CORROSION OCCUR?

When we refine metal ores there are varying degrees of mechanical, electrochemical and thermal energy used to reduce it from ore to its metallic state. A large portion of the energy remains stored within the metal. Metallic corrosion is the release of this stored energy as electric current.

Galvanic corrosion is corrosion that occurs when at least two metals are present and one metal corrodes(anode) in preference to another (cathode). The different metals must be electrically connected for this reaction to occur.

Corrosion can often be attributed to the poor compatibility of selected materials where one metal corrodes in preference to another, creating a anodic-cathodic cell. Correct selection of materials is one of the most crucial steps in the process of corrosion control.

THE GALVANIC CORROSION REACTION

This is best explained with a simple cell consisting of a zinc anode plate and a copper cathode plate immersed in an electrolyte solution. If the plates are not connected to each other, no galvanic corrosion takes place. As soon as they are electrically connected, the zinc plate dissolves (or corrodes) to form positively charged particles known as zinc ions in the solution.

With the dissolution of the zinc plate into the solution, electrons migrate from the anode to the cathode across the metallic bridge and are consumed at the copper cathode. Simultaneously, oxygen that is dissolved in the solution is also consumed at the cathode. Ions migrate through the solution and combine with other ions to complete the corrosion reaction.

If a voltmeter were placed in the circuit between the anode and the cathode, the difference in energy levels would be measured as a DC voltage. In the case of marine vessels, by employing Cathodic protection, the natural corrosion or deterioration mechanism of metals can be significantly reduced.

It is also important to keep in mind that a single piece of metal in an electrolyte, not electrically connected to any other metal will still self corrode. The energy used to create the metal from its ore state will still be released, but at its own rate of corrosion.

Cathodic protection is a technique used to reduce the rate of corrosion of a metallic surface by making the metal the cathode. This can be achieved by attaching galvanic (sacrificial) anodes or impressed current anodes.

Due to its extensive use in marine and other corrosive environments, steel is often the focus of Cathodic protection. Other metals widely used for mechanical and structural applications are aluminium alloys, stainless steels, brasses and bronzes, all of which require some level of protection against corrosion.

GALVANIC ANODES

Galvanic anodes are one of the most effective methods of protecting metallic structures or vessels against corrosion. Galvanic anodes, as distinct from impressed current anodes, do not require an external power source. They are a cheap and simple method of protecting against corrosion.

Galvanic anodes are cast from high purity primary magnesium, aluminium and zinc ingot, with additional “activating” elements added to the melt to ensure that the anode offers the most effective protection. The addition of the activating elements is strictly controlled according to national and international standards and verified by spectrographic and electrochemical analysis.

RATE OF CORROSION

When a metal is immersed in an electrolyte (water, soil, concrete) it generates an electrical current. The current is dependent on the type of electrolyte. Aluminium and zinc anode alloys are used in sea water which is a very conductive (low resistance) electrolyte. Magnesium alloy anodes are used in fresh water which is much less conductive (high resistivity).

Any metal will corrode much faster in sea water than in fresh water, typically ten times faster. The resistivity of salt water is 0.25 ohm metres, while fresh water (in most locations) is typically 50 ohm metres, a difference of 200 times.

It is important to remember that the aim of cathodic protection is to shift the natural voltage of a metal in a negative direction to a point at which corrosion is significantly reduced. For example steel has a natural potential of -500mV in sea water (with respect to a standard silver/silver chloride reference electrode, such as a Rust Seeker). The whole point of coating the steel with paint and fitting anodes is to shift the potential to around -800mV. At this point cathodic protection is achieved and corrosion of the steel is significantly reduced.

THE GALVANIC SERIES

All metals have an energy level which can be measured. This level is measured as the metal’s natural voltage within a particular electrolyte, e.g. sea water. Thus metals can be tabulated as a function of their natural voltage for a particular electrolyte. This table is referred to as the Galvanic Series.

As explained previously, galvanic corrosion occurs when two or more different metals are coupled together and are immersed in a common electrolyte such as sea water. However, we have all seen heavily corroded steel structures with no other metals connected. This type of corrosion occurs due to variations in stress levels placed on the structure which leads to the creation of anodic-cathodic galvanic cells.

For example, when steel is welded or joined, the point of the weld or joint is subjected to huge stress concentrations. The welds become anodic to the surrounding steel and become far more prone to corrosion. The unstressed steel then becomes the cathode and places large demands on the dissolution of the welded anodic areas. This stress also occurs when steel is bent, bashed or mechanically worked.

Similarly, major variations in oxygen concentration will cause anodic and cathodic sites on a single surface of immersed steel. The area with the least oxygen becomes anodic and the area with more oxygen becomes cathodic.

This section provides a brief overview of galvanic anodes and how they apply to water craft. For further advice contact an accredited Marine Surveyor.

Aluminium Vessels

For aluminium used in hull construction, the primary aluminium is mixed with additional elements to create an alloy that has improved mechanical properties and greater corrosion resistance (e.g. Marine Grade Aluminium). This is in contrast to aluminium anode alloys, where additional elements are mixed with the primary ingot to make the anode actively corrode.

Aluminium grades used in hulls can be subject to corrosion from acid and alkali degradation of the natural protective oxide layer on the surface of the aluminium. Damage to this layer can render the hull susceptible to corrosion. Aluminium or zinc anodes and protective coatings help protect aluminium hulls against corrosion.

Alkali problems on aluminium hulled vessels can be the result of too many anodes being fitted. This is best controlled by adhering to the manufacturer’s recommendations on the installation of anodes, by assessing the hull using a reference electrode such as a Rust Seeker or by seeking advice from a corrosion engineer.

It is considered good practice to use a doubler plate for attaching anodes to the hull (see photo on right). This ensures that mechanical damage due to unexpected grounding does not tear studs out of the hull plating, jeopardising the integrity of the hull.

The action of cathodic protection can produce an alkaline solution in the immediate vicinity of the anode, so it is recommended that a sound hull coating be applied to doubler plates and surrounding areas.

Steel Vessels

Steel is a very robust material. If the hull is well maintained with a good marine coating and suitable galvanic anodes, the vessel should last for many years. A good understanding of basic corrosion principles and how they apply to a steel vessel and a suitable maintenance schedule are critical in ensuring that steel vessels are adequately protected against corrosion.

Fibreglass Vessels

Corrosion on fibreglass vessels is usually restricted to the shaft/propeller, rudders and skin fittings. Most corrosion can be controlled by installing shaft anodes or bonding to a hull anode.

To reduce drag on racing yachts, owners often suspend anodes over the side on cables when moored and withdraw them during races. Professional advice should be sought if owners wish to follow this practice, as the vessel has no cathodic protection during races.

Note: Osmosis on fibreglass hulls is the result of osmotic or ionic migration of moisture through poorly applied fibreglass which results in delamination of the fibreglass from the matting. Such defects do not usually occur on hulls manufactured under strict factory conditions. Such damage generally has no relationship to Cathodic protection.

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).

Electrolytic corrosion is corrosion caused by current from an external source, such as a vessel’s battery or the shore power supply in a marina. This current is known as stray current. Electrolytic corrosion can cause serious damage to a vessel’s hull, underwater metallic components, coating systems and anodes.

If an underwater inspection reveals unusually rapid depletion of anodes or serious localised damage to the coating, this may be an indication that the vessel has been or is currently being subjected to stray current.

In addition to a comprehensive inspection and possible repair, identification of the cause of the stray current is critical. This can be achieved using a reference electrode, such as a Rust Seeker to narrow down the source of the stray current.

Alternatively, a specialist, such as a corrosion engineer, may be required to assist with the investigation.

Welding in a Marine Environment

Sea water has very low electrical resistivity. If any electric welding is undertaken whilst the vessel is in sea water, it is essential that the welder’s earth be located immediately adjacent to the job. Any other location may cause stray current corrosion (at the rate of up to 10Kg /amp/year).