Corrosion is a leading cause of premature failure in metallic structures. Corrosion is a spontaneous electrochemical process in which a metal reacts with its environment to form Oxides (Ferrous Oxide, Ferroso Ferric Oxide) or other compound (Hydroxides), the commonest example being the pitting and rusting of steel.
Principle of Cathodic Protection
Cathodic Protection (CP) is an electrochemical process where DC current is applied to a metal to slow or stop the process of corrosion. Thus, after application, it stops the corrosion reaction from occurring. Cathodic protection works by placing an anode in an electrolyte to create a circuit. As in case of an electrolytic cell, current flows from one electrode through the electrolyte to another electrode. Thus, in case of a ship, if the whole metal surface to be protected is made sufficiently cathodic, corrosion will not occur. This is the basic principle of Cathodic Protection.
Areas more exposed to wear & tear
Some areas of the ship are more prone to the regular or random wear and tear. The wetted surface area of the hull and the appendages (like bilge keel, rudder, and propeller) require protection. In a new hull, typically the propellers and small regions where the protection provided by coatings may not be adequate (including small breaks in the paint) surely need extra protection. New coatings are usually of good dielectric materials. These coatings provide good protection themselves. During the life-cycle of the ship, the coatings degrade, additional metal surface is exposed to moisture. Hull, therefore requires increasing levels of protection.
Similar & Dissimilar Metals
The usual electrolysis process is possible between different metals. On ship, such corrosion cells may result from the use of dissimilar metals for different purposes. The propeller, hull plating and the rudder are likely to be of different metals. Thus, solid cast steel for stern frame, mild steel on the hull plating, etc have different electrolytic protection.
On the other hand often, localised anodic and cathodic areas (local cells) arise on the surface of the same metal due to various reasons such as:
- Differences in aeration and exposure.
- Variations in protective films.
- Changes in the electrolyte at different locations due to: temperature, salinity, etc.
Any of the reasons stated above can cause the difference required for the electrolysis. Corrosion may be prevented by removing one or more of these corrosive elements. In case of marine structures, the most practicable method is to apply a protective coating, thus, introducing an electrical resistance between the metal and the electrolyte. Paint in various forms normally, provides the first level of protection. The paint, according to the type and basic property it possesses, has certain limitations offered due defects during application or service. The corrosion occurring on the other hand on the exposed area is inevitable & positive.
Thus, it can be said that the two systems nicely complement each other. The cathodic protection, along with a high performance paint system is used for a valuable protective maintenance system in life span of ship. For the continuous care it would be very effective and economic tool against corrosion.
Types of Cathodic Protection
The essential factor in cathodic protection is to ensure that the unwanted anodic reactions are suppressed by the application of an opposing current. This opposing current can be achieved by either of the two ways:
- Sacrificial Anodes
- Impressed Current
The total current required to protect the vessel is calculated as follows:
Total weight of sacrificial anode:
Where 8760 = Number of hours in a year.
Galvanic or Sacrificial Anode System
Galvanic corrosion is an electrical-chemical process where one metal is more susceptible to corrosion than another when both metals are linked. Galvanic (also called sacrificial) anode utilized to protect steel structures is an example of galvanic corrosion, where the galvanic anode corrodes or gets sacrificed to protect the structure. Anodes based on alloys of Zinc, Aluminum and occasionally Magnesium, are attached to the hull structure, which as stated above, corrode in preference to the protected metal. Consequently, these anodes require renewal at routine intervals. During dry docking the replacement of anode plates is a compulsory activity. Galvanic or sacrificial anodes are made in various shapes and sizes using alloys of Zinc, Magnesium and Aluminum. In order that the galvanic cathodic protection works, the anode must possess a lower (that is, more negative) potential than that of the cathode.
Hydrogen embrittlement, as the name suggests involves the production of hydrogen ions and is the outcome of improperly applied cathodic protection. It is caused due to the production of hydrogen ions, leading to its absorption in the protected metal. The welds, joints and other materials tend to get very hard. Under normal conditions, the ionic hydrogen will combine at the metal surface to create hydrogen gas, which cannot penetrate the metal. These ions, however, are very small. If they pass through the crystalline steel structure, can cause, hydrogen embrittlement, affecting the strength of steel.
Following is worth noting in respect of anodes:
- Anodes normally are in the form of plates (stripper bars) so that they can be welded directly onto the hull.
- The ends of the plates should not be galvanized. This will prevent the toxic gases being produced when they are mounted.
- The distribution of the anodes are according to the underwater hull geometry and stress areas.
- Anodes must not be placed on the bottom as this will cause problems in entering and leaving the dock.
- The anodes must never be painted as this will defeat the very purpose of protection.
There are many ways to calculate the area of wetted surface or the hull in general. It may be available as the data provided by shipyard. A data about the half girths of hull may be available to find the hull area at different drafts. An approximation of the wetted hull surface area may be calculated using the following formula:
Wetted Surface Area = (1.8 x LBP x D) + (LBP x C b x B) Where:
LBP = Length between perpendiculars
D = Draft
Cb = Block coefficient
B = Breadth
Anode plates welded in area of increased electrolytic activity.
Following factors are generally considered in calculating the number of anode plates that will be mounted on the hull:
- Length between perpendiculars.
- Moulded breadth and maximum draft.
- Block coefficient.
- Type of coatings.
- Desired system life.
- Nature of service.
- Whether the propeller is bonded with a slip ring.
- Number of propellers and rudders.
- Thrusters, sea-chests.
Installation of anodes within the tanks:
Magnesium, used to be the favorite anode for the protection of ballast tanks and was extraordinarily well suited, having an electromotive force of 700mV over that of polarised steel. It has rapid power of polarisation. Difficulty however, being excessive emission of hydrogen and its electrochemical power being approximately 55%. Today this metal is rarely used due to the restrictions imposed on it by the Classification Societies. However, it is still used in tanks with platforms. Zinc and Aluminum show a relatively low potential. The condition voltage being lower (as compared to Magnesium), over Polarized Steel; however, they have an efficiency of 80%.
The result of the Aluminum Anode depends largely on the additives (Indium and Zinc), which immunise the tendency of the steel to form a rust film with passive effect.
Both, Aluminum, and Zinc have been favourite as cathodic protection materials. One of the advantages of Aluminum is that, in its installation, only one third of the weight is used compared with Zinc, which is important when considering the ship’s deadweight. One disadvantage, in agreement with the Classification Societies, is the possibility of sparks, which gives rise to the situation whereby the aforementioned Societies impose certain restrictions on the use of Aluminum Anodes. The distribution of these Aluminum Anodes, due to the possibility of sparks, must be studied so that they are placed in low areas of the tanks. In order to do precise calculation regarding the number of anodes for the protection of tanks, the following data is normally considered:
- General layout, the principal dimensions.
- Capacity plan.
- Type of tanks; ballast, cargo segregated ballast, etc.
- The density of the current required.
- The life expectancy of the anodes.
- If the tanks are painted.
- The type of paint used.
- The type of expected ballast.
- Any heating coils, their material.
- Whether the coils are insulated from the structure.
- Recommendations of classification society.
Impressed Current Cathodic Protection (ICCP)
The cell formation is caused by impressing the current artificially. Thus, as the name suggests, a protective current is impressed on the structure through semi-inert anodes.
For larger structures, galvanic anodes cannot deliver economically enough current to provide complete protection. Impressed current cathodic protection (ICCP) systems use anodes connected to a DC power source. Usually this will be a cathodic protection rectifier, which converts an AC power supply to a DC output. A typical ICCP system consists of:
1. Non-sacrificial noble anodes connected to power supplies;
2. Reference cells to monitor hull potential state; and
3. A controller to adjust the current output of the anodes.
AC supply is converted into a controlled low voltage DC output, which is then delivered onto the metal surface by attached anodes. The anodes are insulated from the hull structure. To ensure the correct level of protection it is necessary to measure the potential of the steel against a known and reliable reference cell. This potential is monitored by reference electrodes mounted on the underwater hull surface. The distribution of electrodes is carefully selected in conjunction with the anode configuration matching the hull geometry. Solid state circuitry within an automatic control unit compares the reference potential against a desired and pre-set optimum. An error signal, depending on the difference, regulates the DC power supply to the anodes.
Thus, cathodic protection, though quite effective has the drawback of being expensive, needing regular monitoring. The sacrificial anodes get consumed quite fast. In case of impressed current system, the need of electricity and related components with regular maintenance would be necessary.
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