INTRODUCTION TO MARINE CORROSIONS

Effective corrosion control structure of the hull is one of the most important in the shipping industry to ensure reliability and minimize risk and a significant suppression characteristic of the hull structure. The problem of corrosion of the hull has become a major concern for owners, as they have recognized the need for more and more to protect your investment and to demonstrate a commitment to maintaining the structural integrity of the ship.

Corrosion can be minimized by mainly two methods. One method is the technical approach, which involves coating, cathode protection and inhibitors, etc. The other method is a method of management (CM), which is less technical and more managerial, and was not a major concern for owners for many years. To ensure the continued integrity with minimal costs, erosion must actively manage the planning phase until the end of life ships. Erosion Management System is an approach that includes all aspects of the human factor in reducing the risk of corrosion failure. Management of erosion associated with the development, implementation, revision and maintenance of corrosions policy. Policy of corrosion provides a structural framework for identifying risk for corrosion and the development and operation of appropriate measures to control the risks. A policy is a policy that determines how long operational issues should be addressed in the long term. Policy is to minimize the risk of corrosion failure and major renovation of the hull structure and prolong the life of the vessel by the proper management of corrosion. It provides a basis for further detail below with respect to strategies, organizational structure, performance standards, procedures and other managerial processes.

Year after year, the cost of marine corrosion has increased so far is estimated at 4 % of GDP. An informed choice of materials, approach control and corrosion protection is necessary to reduce the burden of construction waste, waste of energy and money lost. These notes were compiled by members of the forum on marine corrosion to help designers of marine engineers and equipment users, to understand the causes of marine corrosion and how they used protective devices and most durable materials to reduce or elimination of seawater corrosion problems.

Many different types of destructive attacks can occur in the structures, containers and other materials used in the service seawater The term "aqueous corrosion" describes most of the more annoying problems which are in contact with seawater, but atmospheric corrosion of metals exposed at or near the coast, and hot salt corrosion in engines operating at sea or taking salt laden air is also problematic water corrosion and require a systematic approach to eliminate or manage them

(www.ipedr.com ) .

The ballast tanks could then be designed that are more accessible and easier to spray-paint, repair and maintain, with flat surface without cambers affording better drainage and with better placement of outfit items that do not result in corrosion trap (Shipping world and shipbuilder, April 2010).

Corrosion by seawater:

Corrosion by seawater, aqueous corrosion is an electrochemical process, and all metals and alloys in contact with sea water has a certain electric potential (or corrosion potential) at a certain level of acidity or alkalinity of the sea water – pH,

Figure 1: Ship visit (Irish vessel)

Effect of marine corrosion

Corrosion affects bulk flexural capacity of the ship hull to reduce the thickness of the plates. Corrosion may also reduce the ability of the local structure, which can direct lead to failure. In the land of the ship or collapse, large plastic deformation out of the plane happens. When erosion occurs in these elements plates highly distorted vessels, high strain localization can occur in the corroded plate strain at break directly resulting in the fracture. The study of the reliability of the local faults are given effects of corrosion is important but has not been discussed extensively in power

Types of marine corrosion

Uniform corrosion Pitting corrosion Crevice corrosion Galvanic corrosion Stress cracking corrosion Erosion corrosion High temperature corrosion Fretting corrosion

Uniform Corrosion

Uniform or general corrosion is characterized by the formation of rusting of iron steel. Other example of the uniform corrosion is tarnish silver or green patina related to corrosion of copper. General corrosion is quite predictable. The life of the components can be estimated from the results of relatively simple tests immersion. Provision for general corrosion is relatively simple and commonly used in the design of a component for a known environment. Uniform corrosion is characterized and is expressed as the weight loss per unit area and unit time, e.g. mm / year. If the corrosion rate is less than 0, 1 mm / year, from stainless steel, which is usually considered to be durable (www.smt.sandvik.com).

The problem of uniform corrosion in stainless steel is more likely to occur with exposure to acidic and alkaline environment which is very hot. In general, inorganic acids are more aggressive than organic; hydrochloric acid is one of the worst.

Figure 2: uniform corrosion (www.smt.sandvik.com).

Prevention of uniform corrosion:

Uniform corrosion can be limited or prevented by proper selection of the material, including the use of metal or organic coatings, the modification of the medium (pH, temperature, dissolved oxygen concentration and electro active impurities, addition of inhibitors, etc.), and cathodic protection (www.materialsengineer.com) .

Pitting corrosion

Pitting corrosion often takes place at the points where the passive layer is weaker, for example, slag inclusions or defects in a damaged passive layer. Once the attack began, the material can be penetrated quite soon. Problem with pitting attacks mainly depends on the chloride content, pH (acidity), and temperature. If pitting has occurred and if the environment is not very corrosive to stainless steel, the spontaneous repair of the passive layer occur in the presence of oxygen. Natural sea water is more corrosive than expected from the content of chloride. The reason is that biological activity occurs. Thus, chlorination is often used as a biocide to prevent the growth of microorganisms in sea water pipe mounting systems.

Figure 3: Showing pitting corrosion (www.cdcorrosion.com).

Prevention of pitting corrosion

How to reduce or prevent corrosion pitting: select the most suitable for the conditions of service equipment, avoid stagnant zones and deposits, reducing the aggressiveness of the environment, using a cathodic protection (www.smt.sandvik.com).

Galvanic corrosion

Galvanic corrosion is an electrochemical process in which a metal is preferably corrodes when in contact with a different type of metal, both metals in an electrolyte. When two or more different kinds of metals are in contact, in the presence of an electrolyte of a galvanic couple is established as different metals have different voltages electrodes. The electrolyte provides a means for ion migration, so that the metal ions can move from the anode to the cathode. This leads to corrosion of metal rising faster than they would otherwise. The presence of electrolyte and a conductive path between the metals can cause corrosion of metal or otherwise have only eroded. Even a single type of metal can corrode galvanically if the surface varies in the composition, the formation of a galvanic cell.

Figure 4: galvanic corrosion (www.smt.sandvik.com).

Prevention of galvanic corrosion

Some ways to prevent galvanic corrosion: choose combinations of metals in which voters are as close as possible in the galvanic series corresponds avoid an unfavourable surface. Whenever possible, use a waterproofing, insulation, carpets, etc., to avoid direct contact between the two dissimilar metals, prevent the screw connections between widely separated materials in the galvanic series (www.cdcorrosion.com).

Erosion corrosion

Erosion corrosion is the degradation of the material surface by mechanical action, often by spraying the liquid, abrasive particles suspended in liquids and gases, fast, bubbles or droplets, cavitation, etc. Corrosion of metal generally increases with increasing level of seawater (relative) until it reaches a critical level

which is much faster decay. Typically, the corrosion resistance is greater than the

corrosion of metals exposed to seawater higher than those in the salinity water (lower salinity) or freshwater therefore environmentally salinity, erosion corrosion varies depending on the salt content. A more specific form of erosion corrosion

usually occurs in the propellers of ships and submarines caused by cavitation. Cavitation erosion can improve the ability of sea water to move due to the extreme of the liquid (www.amteccorrosion.co.uk).

Prevention of erosion

Ways to reduce corrosion, erosion and abrasion erosion: select a stronger material, improving plant design, regulation of media (check the oxygen content, the lower the temperature, using inhibitors, filter the solids particles, etc. (www.cdcorrosion.com).

Crevice corrosion

Crevice corrosion is a localized form of corrosion attack. Crevice corrosion occurs in narrow gaps or openings between two metal surfaces or between metals and non-metallic surfaces. This form of cell concentration to break the oxygen is depleted. This difference between aeration crevice (micro environment) and outer surface (high temperature) gives the crevice an anodic character upward. This can contribute to highly corrosive conditions in the crevice. This type of rapid failure is dangerous because it can compromise the structural integrity of the ship. For obvious reasons, crevice corrosion tends to occur in the components used gaskets, washers, O-rings, fasteners and joints.

Figure 5: crevice corrosion (www.smt.sandvik.com).

Prevention of crevice corrosion

Ways to prevent or reduce corrosion crack: Use welding instead of bolting or riveting, design feature to allow full discharge (without corners or stagnant zones), empty water repellent that cannot be eliminated, particularly accessories grease all gaskets and designs using a non-porous solid planks, etc. ( www.cdcorrosion.com).

Stress corrosion cracking (scc)

Stress corrosion cracking (SCC) is a process that involves cracking and proliferation, perhaps even a complete failure of a component due to the combined action of mechanical action of tensile loading and a corrosive environment. In fact, the presence of tensile stress is dangerous, compression exerting a protective effect.

Stress corrosion cracking occurs frequently in media with little or no harmful to the metal or alloy of these, in the absence of a tensile load (for example, austenitic stainless steel in high temperature water and steam). Weight loss is associated generally very small, even insignificant compared to the measure of all losses.

Figure 7: stress corrosion cracking (www.cdcorrosion.com).

Prevention of stress corrosion cracking

How to reduce or prevent stress corrosion cracking are: elimination of residual stresses by the stress relief heat treatment , cleaning the environment , choosing the most suitable material , better surface finish , avoid the limitations of the surface treatment , blasting treatments perform welds cause surface compressive stresses , the methods of external protection ( cathodic protection , inhibitors , and organic or inorganic protective coatings ).

Fretting corrosion

Corrosion due to friction is a combined mechanism involving corrosion damage in areas where metal surfaces rubbing motion. This happen mostly when the interface is subjected to vibration (repeated relative movement of the two contact surfaces) and compressive loads. The amplitude of the relative motion is very small, typically on the order of a few microns. When the motion of friction in a corrosive environment is constant, the resulting process is called fretting corrosion (www.cdcorrosion.com).

Figure8: Friction wears of cylinder piston

Prevention of fretting corrosion

Lubricating oil or grease to reduce friction and exclude oxygen from the interface.

Increasing the stiffness of one or both of the materials in contact. Certain combinations of materials exhibit better friction behaviour of others. The surface hardening treatment may be beneficial.

Using seals to absorb shocks and to exclude oxygen and / or moisture. Reduced friction loads in some cases, or, conversely, increasing loads friction

reducing vibration. Modification of the amplitude of the relative movement between the two

surfaces in contact (www.cdcorrosion.com).

Way of preventing corrosion

Protection by painting

Ship Painting isolates the steel from the corrosive medium. The painting should also be resistant to the marine environment and restricted application to ensure the full and effective coverage of steel. Regular inspection and repair of coating may be necessary to ensure a reliable and durable protection (www.marinecorrosionforum.org).

Cathodic protection

Anodes allow the potential of the system to change and provide temporary steel exposed by wear or damage to the protective layer of protection. Systematic position of the anodes is essential to the overall effectiveness. It should also be maintained and replaced when it happened regularly (www.marinecorrosionforum.org).

Inhibitors protection

Through inhibitors vessels alter the corrosion process can effectively prevent the attack in the cellars and other areas where sea water to accumulate and stagnate. Reliable systems for monitoring and maintaining the correct concentration of the inhibitor is an important aspect of this prevention strategy (www.marinecorrosionforum.org).

Conclusion

Conclusions, Reliability assessment by considering corrosion and fatigue is very important for the optimal scheme of maintenance and repair. Different corrosion models would have significant influence on the assessed reliability. By studying some of the currently available corrosion models, it is argued in this paper that these models may not fully reflect the reality. A new corrosion model is proposed to improve this situation. This model is compared with two representative existing corrosion models and the effect of corrosion models on the time-dependent reliability is also studied using a steel plated element. Through this study, the following conclusions can be drawn:

In the new model, the whole corrosion process is divided into three stages: no corrosion, corrosion accelerating, corrosion decelerating. There does not exist any sudden jump in the corrosion rate for metals with CPS which might be better to reflect the real corrosion process.

The new corrosion model is more flexible and can accommodate other existing corrosion models as its special cases. Thus, the new model would have a better curve-fitting capability.

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www.ipedr.com/vol12/1-C001.pdf Shipping world and shipbuilder, April 2010, volume 210

no:4260/IMaREST/Accessed: 5th February, 2014 Marine Engineer Review, (May 1979) Marine Management

(holdings), ISSN 0047-5955 Leicester, England. Accessed: 2nd February 2014.

Marine Engineer Review, (July 1981) Marine Management (holdings), ISSN 0047-5955, Cradley. Accessed: 5th February 2014.

Shipping World and Shipbuilder, (March 1992), Volume 193 No. 4081 ISSN 0037-3931. IMarEST, Hampshire. Accessed: 5th February 2014.

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