Corrosion and Cathodic Protection
Introduction
Cathodic Diecasting Queensland P/L has been operating since 1984.
An Australian owned company, Cathodic Diecasting specialises in the manufacture and supply of anodes to the corrosion protection and plating industries. Our anodes range from large anodes for oil rig platforms down to anodes for cray/crab pots. We supply anodes for ships, yachts, marine motors, tanks, hot water systems etc. and all are available in either Aluminium, Zinc or Magnesium.
Our Commitment to Quality
Cathodic Diecasting is committed to providing our customers with anodes of the highest quality. Our staff have a full understanding of the specialised nature of our products and the need for absolute attention to detail to ensure that our products meet these high standards.
Quality control
Standard operating procedures are employed in the production of our anodes and only the highest quality raw materials are purchased from ISO 9002 certified suppliers who can meet our demands for quality. Regular in-house laboratory checking with our Optical Emission Spectrometer ensures that all castings conform to the required analysis or chemical specification.
Quality assurance
Our practices and procedures follow the guidelines of the ISO 9002 quality system for manufacturing. All anodes produced comply with specifications detailed in Australian Standard 2239-2003, Galvanic (Sacrificial) Anodes for Cathodic Protection and N.A.C.E. standard RPO387-99. Metallurgical and Inspection Requirements for cast galvanic anodes for offshore applications.
Our Zinc anodes also conform to the U.S. Military Specification MIL-A-18001 K for chemical composition, recognised as the world standard for anode quality.
Where possible, every anode is stamped with a heat number identification, allowing complete traceability throughout the manufacturing process. Computer generated heat logs detail accurate alloy batches to ensure continuity in our process. Certificates of Analysis are available on request with any purchase order.
Assistance and after sales
Our staff includes an engineer, a metallurgist, and highly skilled trades people who collectively have over 50 years experience in the casting industry. Our subcontractors include corrosion (chemical) engineers, drafting, patternmaking, founding and general engineers.
Cathodic Diecasting Qld P/L is affiliated with the following organisations:
- Australasian Corrosion Association
- Standards Australia
- Australian Industry Group
- Australian Diecasting Association
SECTION 1: CORROSION AND CATHODIC PROTECTION
1.1 Theory of Corrosion
Corrosion of a material is an electro-chemical reaction between the material and its environment, which results in the destruction or deterioration of the material. The corrosion or degradation of materials is not necessarily restricted to metals.
Corrosion of a metal is known as an electrochemical or electrolytic reaction. This is associated with a flow of electrical energy, ie: corrosion current. In normal circumstances three factors are necessary to cause a reaction. At least one metal shall be present, an electrolyte such as water, soil or concrete shall be present and Oxygen must also be present.
It is also necessary in the case of galvanic corrosion between different metals, or metals with different energy levels, that they are electrically connected.
Corrosion occurs in a number of differing ways. It can usually be attributed to the poor compatibility in selection of materials. The selection and connection of various types of metals is one of the most crucial steps in the process of corrosion control.
The most widely used metal in industry that suffers from corrosion is steel. This is the metal that is also used extensively in the marine industry. It is therefore the metal most well known for marine corrosion activity. Other metals widely used in the marine industry for mechanical and structural applications are Aluminium Alloys and Stainless Steels. Brasses and Bronze Alloys are also widely used.
When Aluminium is used in the marine industry, the primary Aluminium metal is alloyed with other metals to provide it with varying degrees of mechanical properties and better corrosion resistance properties. For example, Marine grade Aluminium plate for the manufacture of Aluminium boats has considerable additions of other elements added to the alloy to render them less active and more corrosion resistant (or more passive). Aluminium anodes have active metals alloyed with them to render the Aluminium active.
Types of Corrosion
The main types of corrosion of most interest to marine applications are:
- Dissimilar Metals
- Pitting & Cavitation
- Stray Current
1.2 Why Does Corrosion Occur?
Or Why Metals Return To RockWhen man makes metals there is a varying degree of electro, mechanical and chemical energy used to refine the metal from the ore state to deliver the metal into a useable state. A large portion of the energy used in refining remains as stored or residual energy in metals.
Magnesium- the Hyperactive Alloy
Magnesium is a light and strong material. It is also one most prone to corrosion activity in its unalloyed state. This is due to the high levels of energy used in the manufacture of commercially pure Magnesium. Like all pure metals, Magnesium will have a tendency to return to its natural state as an ore or oxide of the metal. Part of this decay process also results in the release of the stored energy as DC Current (Milliamps) as seen in a typical galvanic corrosion cell.
Aluminium & Zinc - Energetic Anodes
Considerable energy is also used in the manufacture of Aluminium and Zinc. Whilst not as extensive as Magnesium, these metals in the unalloyed state have a strong tendency to corrode or oxidise and return to their 'ore' (the metals oxidised state). This reaction is most commonly referred to as corrosion.
The energy used to manufacture one kilogram of high-grade Aluminium is approximately double the energy that is required to produce a similar amount of steel.
1.3 The Corrosion Reaction
This is best explained by considering how a simple battery cell operates. A battery consists of a combination of a Zinc anode plate and a copper cathode plate immersed in a solution of salt water. If the plates are not connected to each other, no galvanic reaction takes place. Immediately they are electrically connected, the Zinc plate dissolves (or corrodes) to form positively charged particles known as Zinc ions in the salt water solution. Simultaneously, oxygen that is dissolved in the salt water solution is consumed at the copper cathode plate. With the dissolution of the Zinc plate into the solution, electrons are released. These flow through the circuit and are consumed at the copper 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 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 arrested. It is IMPORTANT to remember that if the metals are not connected, no galvanic corrosion cells will operate.
1.4 Rate of Corrosion
When a metal is immersed in an electrolyte it generates an electrical current. The current generated is dependent on the type of electrolyte. Any metal will corrode at a much faster rate in sea water than in fresh water. The corrosion rate is typically ten times faster. Sea water is a very conductive electrolyte (high corrosion rate or very low in resistivity). Aluminium alloy or Zinc alloy anodes perform best in seawater conditions. Freshwater is much less conductive (high resistivity) and pure water is a very poor conductor (higher in resistivity).
This theory can be applied to the following statement:
'The resistivity of salt water is 0.25 ohm metres. Freshwater used in most city water supplies is typically 50 ohm metres. This results in a difference of 200 times in resistivity and correspondingly reduces an anodes capacity to deliver protective current.'
Refer Ohms Law: I = V/R or V=IR
Where: I=(amps), V=(volts), R=(resistance)
The type of electrolyte affects the performance of Aluminium and Zinc alloy anodes. The salinity determines the best performance of aluminium anodes. Zinc however is more commonly used for both high and low salinity (brackish water).
It is important to remember that the aim of cathodic protection is to shift the natural corrosion energy or voltage in a negative direction to a point at which corrosion does not occur. With steel it is normal to shift the natural corroding potential (voltage) from -500mV to -800mV (with respect to a standard silver / silver chloride half cell). At this point cathodic protection is generally achieved and corrosion of the steel should cease.
(Note: -800mV is the minimum protection criteria).
1.5 Galvanic Energy in Metals
All metals have an energy or stress level, which can be measured. This level, is measured as the metal's natural voltage. This is the same in all electrolytes at any location worldwide (using like circumstances of temperature and velocity). Thus metals can be tabulated as a function of their natural voltages or potentials.
This tabulation of energy levels for alloys of metals is referred to as the Galvanic Series. It refers to the potential voltage exhibited by all metals in their most widely used form - as an alloy with other metals.
The Galvanic Series
The table is intended to give a very general picture of the order of the series in respect to the materials used in the Marine Environment. More detailed data (complete breakdown of the copper alloys, stainless steels, etc) can be located in published data on the Galvanic Series.
Metal or Alloy Potential in Volts
| Anodic or Active Corrodings(Base) End | |
|---|---|
| Magnesium Anode (High Potential Alloy) | -1.64 |
| Magnesium Anode (Low Potential Alloy) | -1.53 |
| Aluminium Anode | -1.10 to -1.08 |
| Zinc Anode (AS2239-Z1) | -1.03 |
| Aluminium Alloys | -0.96 to -0.68 |
| Cast Iron | -0.61 |
| Carbon Steel (Mild Steel) | -0.50 to -0.55 |
| Copper Alloys (Brass/Bronze) | -0.36 to -0.29 |
| Cupronickels | -0.29 to -0.22 |
| Copper | -0.20 |
| Silver | -0.13 |
| 316 Stainless (Active-Passive) | -0.18 to -0.06 |
| Protected(Noble) End | |
| Cathodic Carbon (Graphite) | +0.25 |
| Platinum | +0.26 |
The Galvanic Series
The Galvanic Series is a table of metals that ranges from the most reactive (anodic metals) to the least or non-reactive (Cathodic or Noble) metals. Metals shown low in the series are said to be more Noble (more resistant to corrosion), whilst metals at the top of the series are called Active or Anodic (active - prone to corrosion).
Magnesium, Zinc and Aluminium alloy (anodes) have a large voltage difference when connected to metals such as Copper, Platinum, Gold, and Titanium (cathodes) and therefore the rate of corrosion / oxidation and loss of Magnesium, Zinc or Aluminium would, on comparative surface areas, be quite rapid.
Connection of a more negative metal such as a steel hull (Pa) (-500mV) to a bronze propeller (Pc) (-300mV) will generate a driving voltage of 200mV (Pa - Pc). In this circumstance the steel would become anodic and the bronze cathodic and the driving voltage will cause a current to flow.
As an example, if steel is connected to a piece of copper, brass or bronze, the steel will become an anode and the copper, brass or bronze will become a cathode. The steel will be consumed at the rate of 10 kg of steel / amp / year. This means that if one amp of current were discharged from a steel hull of a vessel for one year, there would be ten kilograms of steel lost to corrosion. A steel plate 14mm x 300mm x 300mm will weigh approximately ten kilograms.
1.6 Size of Anode & Cathode Sites.
A small anode site connected with a large cathode site will result in rapid consumption of the anode site.
A large anode site connected with small cathodes will generally result in slow consumption of the anode.
It is the surface area of the anodic and cathodic metals and the resistivity (or resistance properties) of the water (electrolyte) that determines the amount of DC current (milliamps) that flows from an anode. It is the flow of electrons (mA) driven by the galvanic energy (voltage) of the metal/s that results in the amount of corrosion, oxidation or degradation of any metal.
Using Ohms Law (I=V/R) and the surface area of an anode (Resistance), it is possible to calculate the amount of protective current (milliamps) able to be delivered when a sacrificial anode is connected to a Steel or Aluminium cathode.
1.7 Corrosion Of Steel - Where No Dissimilar Metals Are Present
As explained, galvanic corrosion occurs when two or more differing metals are coupled together and are immersed in a common electrolyte such as sea water. However, we have all seen steel piles and other steel structures, or simple floating barges, with no other galvanic metals connected which are corroded. This occurs from a number of the basic mechanisms already discussed. If one focuses on STRESS levels or energy risers created in the various metals, it is also possible to create stress risers in steel structures.
For example, when steel is welded or joined, the point of the weld or joint is subjected to huge stress concentrations during the welding and solidification process. Thus welds will become anode sites and thus are more prone to corrosion as they try to release this energy in order to become very active anodic sites. The unstressed parent plate then becomes the cathodic site and places large demands on the dissolution of the welded anodic areas. This stress also occurs when steel is bent, bashed or some how 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 the Anodic site and the area with more oxygen becomes cathodic. Thus it is also possible to achieve a form of galvanic corrosion on steel structures that have been welded, mechanically worked or have differing oxygen levels in the immersed condition. In fact it is possible to cause this anode cathode reaction on almost any metals immersed in water under similar conditions to those above.
Back to topSECTION 2: MATERIAL SELECTION & GALVANIC CORROSION
2.1 Steel / Aluminium Vessels
Generally metallic hulls tend to have more problems due to the various metals used for their specific purposes. Bonding should be attended to and cathodic protection should be provided from sacrificial anodes for immersed surfaces. Steel is a very robust material and if painted with a good marine coating system, it can generally be easily protected and will not subsequently suffer many adverse effects. (Refer Sections 3.4 & 3.5, stray current activity for steel damage effects).
On metallic vessels it is generally found that fifty percent of the anodes are required to be installed in the stern frame region. (Aft end of vessel).
Aluminium is an amphoteric material and can be subject to a phenomenon where it can suffer corrosion from both acid and alkali degradation of the protective oxide layer on the Aluminium. Damage to this natural protective layer on the Aluminium can then render the Aluminium susceptible to corrosion activity or degradation.
For any acidic or anodic reactions, the solution is usually to install a system of sacrificial Aluminium Alloy or Zinc Alloy anodes. These anodes are usually supplied with stainless steel insert straps for attachment to the studs on the hull. Alkali (overprotection) problems, are usually the result of excessive anodes fitted to the hull. This is best controlled by adhering to the manufacturers recommendations on the installation of anodes or alternatively by seeking the advice of a corrosion engineer.
Note: For Aluminium vessels, it is considered good practice to use a doubler plate for connection of the anodes to the hull. The action of cathodic protection can produce an alkaline solution in the immediate vicinity of the anode. It is also good practice to ensure the application of a very sound hull coating system on the doubler plate and immediate surrounding areas. The attachment of stainless steel studs to the doubler plate also ensures that any mechanical damage due to unexpected grounding does not tear studs and anodes out of the hull plating and thereby jeopardise the integrity of the hull.
WARNING: Magnesium alloy anodes should NEVER be used on any vessel in salt-water service without the advice of a corrosion engineer. The use of magnesium alloy anodes will usually generate excess alkali on Aluminium hulls and they can also cause major damage to the coating systems on steel and Aluminium hulls.
2.2 Coating Systems
The benefits of a system of good coatings and a cathodic protection system
In order to provide cathodic protection from a number of sacrificial anodes attached to a hull, it is necessary for the anodes to deliver an amount of DC Current (milliamps) to the immersed surfaces of a hull. However in order to reduce the amount of DC Current required to achieve protection for the desired life of the hull,it is necessary to reduce the demand for cathodic protection current (milliamps).
This is best achieved in the marine industry by the addition of a dielectric (high resistance) barrier on the surface of the metal. This is normally achieved by the application of a resistive layer of coating (or paint). It is important to understand in this application of paint, that the primary objective of corrosion protection is to achieve elevated dielectric strength (resistance) of the hull plating to the sea water.
The application of paint coatings that have poor dielectric properties will not reduce the corrosion protection load on the anodes. Anti fouling paints and some metallic based paints applied directly to the metallic hull have poor dialectric properties.
The application of coatings and anodes (CP) should be seen as complimentary to each other.
2.3 Internal Corrosion - Bilges
The internal side of a vessel's hull is also prone to corrosion. A bilge is considered a wet area that can contain substantial quantities of water and other fluids. A bilge should therefore be treated like any other structure; regardless of whether it has any galvanic connection. A bilge should be correctly coated and sacrificial anodes can be employed. Often long slender anodes are installed in a bilge or ballast tank to protect the internals of the area. 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.
2.4 Timber Hull Vessels & Skin Fittings
If two dissimilar metallic skin fittings are bonded (or interconnected), galvanic corrosion activity on the immersed and embedded surfaces may occur.
When skin fittings, cooling pipes, shaft or a propeller show signs of corrosion, it is important that correct electrical bonding be undertaken and sufficient anode material be provided. (Further information can be found in section 3.7 Bonding & Electrical Continuity). It is also important that the use of suitable corrosion prevention coatings to the hull and fittings are utilised to reduce the amount of cathodic protection current required to be delivered by the sacrificial anodes.
2.5 Galvanised Steel
Galvanising is a metallising process that was developed to provide protection of metals from corrosion in atmospheric (above the water) situations. If galvanised coated fittings are immersed in the ocean without the addition of a dielectric coating, the bare galvanising will be subjected to the same corrosion mechanisms as a Zinc anode. This will result in dissolution of the Zinc coating and later potential corrosion failure of the metal.
2.6 Fibreglass Vessels
Generally corrosion on fibreglass vessels is restricted to the shaft/propeller, rudders and skin fittings. The problem can normally be rectified by a shaft anode or bonding to a hull anode. To reduce drag on very competitive yachts, owners will often suspend anodes over the side on conductors when moored and withdraw them during races so as to reduce drag. Professional advice should be sought if owners wish to follow this practice, as it is not the most ideal means of achieving cathodic protection.
2.7 Osmosis
NOTE: Osmosis on fibreglass hulls is the result of osmotic or ionic migration of moisture through poorly applied fibreglass to areas of lamination of the fibreglass and the matting. These are referred to as low-density defects and normally do not occur on hulls manufactured under strict factory conditions. Such damage generally has no relationship to any sacrificial anode cathodic protection systems installed on fibreglass hulls.
Back to topSECTION 3: MAINTENANCE
3.1 Decks, Deck Fittings, and Rigging
All non-immersed fixtures are subject to the marine environment and therefore should be selected for their corrosion resistant properties.
Where mild steel is utilised it is necessary to apply suitable protective coatings in conjunction with a routine maintenance program.
3.2 Water Tanks
The fresh water used for potable drinking water on boats/ships is generally more than one hundred times higher in resistance than salt water. Therefore the driving voltage (energy) available from typical Zinc or Aluminium anodes is too low to provide sufficient cathodic protection or DC current in this environment. Where corrosion protection is required in fresh water tanks and systems, it is normal to use Magnesium anodes.
Warning: Magnesium anodes should NEVER BE USED in salt-water environments without seeking professional advice.
3.3 Engines
Most marine engine manufacturers install small Zinc alloy anodes inside the cooling jackets of salt water cooling systems. These are present to minimise any 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.
3.4 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 by the addition of alloying elements such as nickel and chromium. If the oxide layer is depleted or removed, the stainless steel can then be subject to localised corrosion. Marine grade stainless steels can corrode if used or installed in the wrong environment.
There are some grades of alloy in stainless steel that have better corrosion resistance than others. For example, Type 304 stainless exhibits good corrosion resistance in many atmospheric situations, however types 316 and 316L stainless have much greater corrosion resistance and are therefore more suitable for corrosion resistance service in marine environments.
3.5 Welding Procedure in a Marine Environment
Seawater is extremely low in electrical resistivity. If any AC or DC Arc, Mig or Tig welding is undertaken whilst the vessel is in the 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 10Kg /amp/year)
3.6 Marinas - Shore Power
Marinas normally provide 240VAC as a common power supply and Multiple Earth Neutral (MEN) system. If this MEN shore power is connected to a number of vessels, then effectively all vessels connected to that power system are interconnected to the main MEN power earth grid on shore.
Therefore it is possible for a vessel to be adversely effected whilst another vessel will benefit by the connection. Alternatively, all vessels connected to the MEN power earthing system might become more anodic relative to the copper earth on shore. This may result in early deterioration of anodes or accelerated corrosion or loss of cathodic protection from the sacrificial anodes fitted to the individual hulls.
No common rule applies to this type of problem and each would require individual consideration. The problem with moored metallic hulls can be reduced or eliminated by the installation of a galvanic isolation device or the provision of isolating transformers between the ship and shore power supply. Often the suspension of additional anodes (attached and earthed to the hull) at the mooring point can provide an economical solution.
3.7 Bonding And Electrical Continuity - Bolt On Anodes
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 often true when the vessel is sitting idle at a mooring. However experience has shown that once shafts and gears start moving, there is sufficient resistance across the lubricated moving parts to cause a galvanic disconnection or isolation between the hull and the moving parts.
If electrical continuity is deemed necessary between the hull and shafts and propellers, it is better to make allowance to directly bond such fittings to the hull or other conductors. In cases where the gearbox does not provide electrical continuity, then 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 isolate instrumentation effectively. However, double insulated wire power systems are preferable on steel or Aluminium vessels.
Should electrical earthing / bonding be considered 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 very bad practice.
Back to topSECTION 4: ANODE ALLOY GROUPS AND APPLICATIONS
4.1 Magnesium Alloy Anodes
Magnesium Anodes have a high negative driving potential which makes them suitable for the protection of steel structures where the environment has a high resistivity, such as in soil and fresh water.
Magnesium Anodes are used extensively for the protection of buried pipelines and also in hot and cold potable water applications. Their corrosion by-products are generally considered non-toxic. Magnesium anodes are generally never used in a salt water environment as they can cause disbanding or removal of paint on wetted surface areas.
4.2 Zinc Alloy Anodes
Zinc alloy anodes have been used for many years as a most reliable and economic means of providing cathodic protection to the hulls of the world's steel hulled boats and ships. With the changes in technology in recent years, Aluminium alloys have shown distinct improvements in anode applications for marine use in sea water. However because of the universal benefits of being able to operate in sea water and fresh waters, Zinc anodes remain very popular in the marine and boating industry.
Large marine structures which require considerable anode mass for cathodic protection, will economically justify the use of Aluminium alloys over Zinc alloy anodes. For most small vessels, the benefits between Zinc and Aluminium alloys may be marginal.
Zinc anodes are widely used in marine environments. However in the right geometry, Zinc anodes, in conjunction with a very good hull coating system, can provide sufficient output to protect steel in higher resistivity environments such as some tidal fresh water estuaries.
It is important to note that at temperatures in excess of 50°C, hard non-saline waters (such as some fresh water cooling systems) may cause the polarity of a Zinc anode/steel couple to reverse. That is, the steel may become anodic to Zinc and corrode at a rate more rapidly than existed prior to the installation of the anode.
Seek the advice of a corrosion engineer if you expect to experience these conditions.
4.3 Aluminium alloy anodes
The use of Aluminium anodes is generally limited to marine applications.
The driving potential of Aluminium anodes is slightly higher than zinc anodes. Aluminium alloy anodes are more efficient than the zinc anode alloy, which can provide performance and lifetime benefits of the anode protection system. This results in more protection in the same anode sizes due to the corrosion efficiency of the Aluminium alloys.
Typical uses include applications such as ship hulls, salt water ballast tanks, offshore structures, steel wharf piling and submerged (offshore) pipelines.
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