Corrosion Is The Destructive Attack Engineering Essay

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In the marine industry, metals can corrode in a variety of distinct and recognizably different ways caused by different corrosives in different situations. The effect of one metal on another when immersed in seawater is a particular aspect that is very important. Corrosion is the destructive attack of a material by reaction depending on its environment. The serious consequences due to corrosion have seen serious impacts globally. In addition to these encounters in the marine industry with this form of degradation, corrosion causes waste of valuable resources, loss or contamination or product, reduction in efficiency and high maintenance costs.

Actual corrosion behaviors are variable because corrosion depends on many factors in order to occur. As corrosion is a natural process, most structural materials used today are basically engineered in an unstable state. Iron, chromium, titanium, aluminum and most other metals naturally exist in the crust of the earth as oxides or sulfides. In the presence of a corrosive environment, these metals may rapidly revert to the initial state in which they were found in the earth. Corrosion is also a risk to safety, economy and the normal operation of systems and equipment.

An important category of damages found in ships and offshore structures usually come from the application of loads that do not necessarily approach the critical loads of the structure, but brings about strength loss due to repetitive actions under certain conditions. A normal operating vessel could very well operate for about 25 to 30 years, in which during this period of time, it will be subjected to various repetitive types of loadings. The continued action of loading and unloading, the effect of sea waves, the environment contributes to the occurrence of damages which leads to fatigue. Fatigue is the failure of a material that is caused by repeated application of external loads over a relatively long period of time.

The performance of a structural material in a marine environment will depend on the service operational parameters, the choice of materials and/or corrosion control methods, the type of environment, and design configurations. The proper material chosen are based on extensive knowledge of material interactions with the marine environment, corrosion control measures, and design to avoid corrosion. These effective measures are taken so as to reduce the total costs of a given structure from the time of its construction to the end of its planned life. The optimum choice of materials, corrosion control measures, and design will prolong structural life.

Although corrosion can be prevented at times, the aim is usually to control it within economic limits. There are situations where no corrosion is acceptable at all, but these cases are rare. The choice of materials is generally based on regular maintenance during the life of the construction. This is a sensible approach provided that it is part of the design philosophy.

Project objectives

This project aims to study how corrosion affects a vessel's operation or its period of life at sea. In this report, there will be :

Case study of corrosion observation at sea

Types of corrosion at sea

How the surrounding environment conditions onboard leads to higher corrosion rates

FPSO modules that are vulnerable to corrosion attack

Calculation of structural members that corrodes the surface which in turns reduces the thickness and how it affects the overall strength

Possibilities of a corrosion attack on areas of a vessel

Analyze a case study and causes of breakdown/abruption

Suggestion of ideas and solutions to eliminate corrosion issues

This project acquires to achieve results of a how strength of a metal or plate is affected due to corrosion. There will be a series of calculations involving a plate that has reduced thickness due to corrosion and how its overall strength is affected.

What is corrosion?

Corrosion is used to denote the physical and chemical changes in a structure with environment interaction. It changes a material's properties and degrades its performance. Corrosion is the most important cause of wear in metal structures of vessels and this cause reduction in thickness. Thus, the stresses in structure will gradually increase and the strength or load-carrying capacity is reduced. Corrosion also increases a material's surface roughness. This in turn increases a vessel's frictional resistance and thus the required power to propel the vessel through water will also increase.

Depending on the manner corrosion occurs, it can be classified into two types.

Chemical corrosion

It denotes the metal degradation that results from the reaction with substances in the absence of a liquid electrolyte. High temperature of dry corrosion is the reaction between metals and gases at elevated temperatures. These high temperature corrosion reactions are usually electrolytic with molten fused salts as the electrolyte.

Electro-chemical corrosion

Also known as wet corrosion, metals react with the environment usually at ambient temperatures. It is the degradation of a metal due to chemical processes in the presence of an electric current. External factors such as temperature, plays an influencing part and shows a more persistent phenomenon. Electro-chemical corrosion involves large quantities of energy and also to the extent of chemical degradation. This is the commonly encountered type of corrosion in the marine industry. Modern corrosion issues are classified as wet corrosion.

A typical example of rust formation occurring in shipbuilding is the annual thickness reduction of steel that can reach a 0.3mm in thickness. Porous corrosion occurs at 0.2mm with inadequate surface protection.

3.1 Corrosion mechanism in marine structures

A metal that corrodes rapidly is of little practical value as a structural material unless preventive measures are taken to reduce the rate of corrosion. Corroded mild steel is of no value but fortunately, there are preventive measures such as cathodic protection and coatings that are made available today.

Depending on the environment exposure, causes of marine corrosion can be classified as :

Immersion

Splash or tidal zone

Alternate wetting and drying that occurs on metal surfaces with deposition of salts from the sea.

Atmospheric

Semi-enclosed space

System failures and subsequent failure investigations has been increasingly important in current times. Conducting a failure investigation is important for identifying the mechanisms and causes of a problem in order to prevent its reoccurrence. In most cases of a standard operating vessel, immersion corrosion of mild and low alloy steels is a typical agent for corrosion.

3.2 Sources of corrosion

Corrosion occurring in marine structures is usually associated with three main sources. They are mainly -bimetallic contact, crevice corrosion and breakdown of surface films.

Bimetallic contact

It is not necessarily a measure of the severity of corrosion but is indicative of which metal in any combination will corrode. Compositional and environmental factors such as relative areas that determine the total current flow in a galvanic cell, determines whether corrosion will extent further or not.

Therefore, the connection of two dissimilar metals in a marine environment should be avoided. This is to ensure that alloy groups do not have varying differences in potential. Copper alloys such as cupro-nickel, bronze and gunmetals are classified as equipotential.

Breakdown of surface films

The presence of surface films on materials in a corrosive environment plays an important role, depending on whether they are protective or not. Certain materials derive its corrosion resistance from the films, namely oxides, as they are impenetrable and self-healing. These oxides must also be adherent, continuous and reasonably elastic to provide maximum protection.

The ultimate value of a protective film is determined by its ability to repair the metal when damaged. Metals like aluminum, titanium, chromium and stainless steels are only used in the marine sector as these films can repair damaged metal tremendously in the presence of oxygen.

Iron, for example, forms a loosely adherent layer of film that is not repairable. When the film is damaged, a corrosion cell is established between the exposed metal and forms an anode. The cathode is formed by the scale which conducts electrons.

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Fig. 3.2.1 Corrosion at a break in a surface film

Crevice corrosion

Corrosion problems in structural members are closely linked with the protective properties of surface films and scales. Crevice corrosion is the resultant of two areas of a metal that is exposed to an electrolyte under different oxygen concentration conditions. The area which has access to a reduced level of oxygen becomes an active anode. This may result in severe local corrosion areas that are not easily detected for a period of time. Any crevice on the metal where salt water and dirt may collect has a potential of experiencing corrosion.

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Fig. 3.2.2 A typical site for crevice corrosion

3.3 Types of corrosion

The rusting of ordinary carbon steel is the most common type of corrosion. This form of attack attributes to the expenses to corrosion control. The different form of corrosions represents different phenomena that can be categorized according to its appearance. These corrosions can be classified based on their ease of identification (C.P. Dillon (Ed.), 1982) and the three categories are :

Group 1 : Readily identifiable by ordinary visual examination

Group 2 : May require supplementary means of examination

Group 3 : Verification is usually required by microscopy (optical, electron microscopy, etc.)

Fig. 3.3.1 Main forms of corrosion attack grouped by ease of recognition

In a situation where crevice exists, it will create an environment favorable for pitting, inter-granular attack and also cracking. The actual importance of each corrosion type will differs depending on environment situations and systems.

The few common types of corrosion found in the marine industry are as follows :

Uniform or general corrosion-nature

The nature of corrosion layers dictates the corrosion rates occurring on a body. If a layer is continuous and impenetrable, it does not separate from the metal if an external load that causes development of stress fields in the plating is applied. The corrosion layer re-forms if plating is exposed to scratching.

In this case, the re-formed layer of the metal protects the remainder layers from further degradation. The duration of the protective layer depends on the amount of air exposure to the surface of the metal or plate. Lack of oxygen will give rise to pitting corrosion.

The corrosive action of seawater on marine steels increases with increase in temperature, oxygen content, water flow speed, content of corrosive elements and conductivity. This type of corrosion is the least harmful as it can be easily identified and the corroded metal can be easily replaced.

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Fig. 3.3.2 General corrosion, where x indicates the average corrosion loss

Pitting corrosion

Pitting corrosion is the local formation of corrosion products or the selective local solution of the surface to a certain depth. Electrolytic cells in the metal can cause accelerated galvanic corrosion. Factors such as difference in oxygen concentration, temperature and flow speed contributes to pitting corrosion. Corrosion also accelerates in the presence of still water with low oxygen count. As pitting corrosion is harder to be detected or predicted, it is considered to be more dangerous than uniform corrosion. In a corrosive microenvironment, which is not similar to a bulk environment, often plays a role in the initiation and propagation of corrosion pits. Apart from the localized loss of thickness, corrosion pits can be quite harmful as it acts as a stress riser. Fatigue and stress corrosion cracking may initiate at the base of corrosion pits, as shown in Fig. 3.3.3.

Fig. 3.3.3 Small local anode and large cathode in the process of pitting

Fig. 3.3.4 Different types of pitting corrosion

Stress corrosion cracking (SCC)

Stress corrosion cracking is a process caused by the joint action of stress and a corrodent. It is known as one of the corrosion that brings about the most damage and will cause serious financial repercussions due to losses in materials. Stress corrosion only occurs with certain combinations of alloy and corrosive environments. Alloys which are almost inert to a particular environment are more prone to stress corrosion. It causes external loads to acts on surfaces with cavities due to pitting. These loads can cause brittle fracture of the whole cross-section.

At the site of initial crack, a stress concentration develops and the rate of corrosion near the crack becomes 10 times greater. The rapid propagation of cracks that occurs result in possible structure collapse and direction of the crack is perpendicular to the direction of the load. Stress required to produce stress corrosion cracking may be internal tensile stress with the alloy attributable to mechanical working processes applied to the alloy during manufacture or fabrication.

In the case of SCC, corrosion occurs due to alloys constituents, magnitude of applied load, type of corrosive environment, temperature and also time. Fig. 3.3.5 shows the failure stages due to stress corrosion from a localized breakdown of a corrosion product film. The formation of a corrosion pit acts as the initiation site for stress corrosion cracking and then, mechanical failure will occur due to loading above the mechanical strength of an alloy.

Fig. 3.3.5 Failure stages of components during stress corrosion

Cavitation erosion

Cavitation erosion occurs when local static pressure is smaller than saturation pressure of liquid at current temperature on a wetted body. It is also known as a continual exposure to collapsing cavities that results in metal removal. The vapour cavities grow rapidly using nuclei such as suspended solids within the fluid and minute gas bubbles trapped as imperfections in a solid surface. The bubbles will then liquefy on solid surfaces when cavitation reaches a location with higher pressure. When these bubbles collapse, large pressures acting on the metal can cause mechanical damage, wear of the surface and in turn opens small craters (pitting) and cavities.

A collapsing bubble does not occur adjacently to the metal surface. As the energy is partially absorbed with the surrounding fluid, the bubbles will still excite conditions of excessive fluid turbulence resulting in cavitation corrosion which is similar to an impingement attack. A common example would be the surface of rotating blades of pumps on propellers.

Bacterial corrosion

When environmental conditions are conducive for growth, it starts the spreading of bacterial activity. Bacterial corrosion is also known as microbiological or anaerobic corrosion.

The conditions required for bacterial corrosion are:

Stagnant waters

Bacteria that feeds on hydrocarbons eg. crude oil, protective coatings etc

Presence of sulphate in seawater

Optimum temperature of 20 - 40 degree Celsius

Occurs mainly on oil tanks/ballast tanks/oil cargo loading and discharge piping.

Symptoms are as such:

Smell of decomposing matter due to hydrogen sulphide (H2S)

Appearance of small pitting

Initial black colors which later on disappears following the air exposure due to oxidation of iron sulphide

Selective phase corrosion

This type of corrosion is confined to alloys having microstructures consisting of two or more phases which vary in chemical composition. Such phases may have different corrosion potentials leading to an internal bimetallic cell. The small electrical resistance between the anode and cathode, and the large current flowing through the two points contributes to rapid corrosion.

Impingement attack

Impingement attack is the accelerated corrosion due to removal of protective corrosion product films by turbulent water flow. Due to the rapid flow, an anode site is formed, continuously exposing a clean metal surface. This is showed in Fig. 3.3.6. The rate of attack is controlled by the hydrodynamics and geometry of the system which will determine the areas of anode and impingement attack and also the surrounding cathodes. This attack occurs at maximum speed when the anode is small and the cathode is large. Therefore, if a uniform high turbulent water flow can be achieved throughout the system, there would be no effective cathode and corrosion would be uniform throughout the surface. The rate of penetration would also decrease. The severity of the attack can be influenced by suspended solid matter and gas bubbles.

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Fig. 3.3.6 Impingement attack due to rapid water flow

Crevice corrosion

This form of attack occurs when a thin film of electrolyte is trapped in a crevice and becomes stagnant. Such corrosion forms between closely fitted components. Crevice corrosion occurs due to concentration cells created by limited oxygen availability or difference in metal ion concentration. With limited oxygen diffusion, available oxygen to repair any breakdown is the passive film is also limited, thus the material becomes active.

The sea water path between the active and passive sites determines the resultant current which will flow, with the area under the crevice acting as the anode and the uncreviced area as the cathode. Another alternative mechanism of crevice corrosion is the metal ion concentration cell. With a restricted electrolyte flow, the crevice rapidly becomes saturated with the corroding metal ions. Metal ions are free to diffuse or be removed in the non-crevice areas.

This action results in the area surrounding the crevice to become anode, with the area under the crevice being a cathode. Consequently, a localized area of corrosion may occur around the edge of the crevice.

Corrosion fatigue

Fatigue is known as the application of stress on a surface that may result in failure of the metal. Corrosion fatigue is the combined action of a corrosive environment and cyclic stressing. Fatigue failures occur mainly due to a corrosive element. Neither cyclic stressing nor environmental attack applied to a metal will produce the same type of deterioration. Any alloy that is vulnerable to corrosion in seawater is also vulnerable to corrosion fatigue in the same environment.

SCC (stress corrosion cracking) may not necessarily be obtained with the same alloy in this situation. Thus, corrosion fatigue crack propagation can occur under SCC conditions if negligible.

Fatigue properties of alloy in air are dependent of the surface conditions of the specimen used whereas in corrosion fatigue processes, it is the generation of notches which play an important role in the crack initiation stages. This factor influences the reduction in fatigue life. With a low frequency corrosion fatigue, the ultimate corrosion fatigue value is influenced by the alternating stress acting on the metal.

3.4 Conclusion

In the marine and offshore industry, a vessel is vulnerable to all sorts of corrosion, cracking or fouling. Factors such as sea environment, weather conditions and also material contact, plays an important part in order for corrosion to take place. The actual importance of each corrosion type will also differ between systems and other operational variables.

Factors influencing corrosion

4.1 Corrosion and the environment

With studies and microscopic inspections, corrosion is more intense at the interface of corroding metal, an increase in electrical conductivity of the corrosive environment, at irregular surfaces of corroding metals and alloys due to pollution or seawater. When the corrosive environment or its properties changes, corrosion gets intense. Other factors such as the presence of oxygen-filled seawater, high temperatures, acidic solutions that get into contact with the metal, contribute to the corrosion rate. The most intense corrosive environment is seawater, as it contains salt which increases conductivity. Micro-organisms cause pollution and this leads to an un-uniform material surface, thus increasing the chances of corrosion.

Certain operating conditions lead to the acceleration of corrosion. They are:

Corrosion with friction

Friction corrosion occurs on two different metal components. As the surface in not smooth, contact only occurs at the tips of anomalies. Therefore, the relative motion of the two surfaces causes wear of the tips and rupture of the protective oxide coating. Thus, this action increases the corrosion rate.

Temperature differences

Within a metal body, temperature differences give rise to a potential difference between the hot and cold region. The colder region, which is more electropositive than the hotter region, corrodes more rapidly.

4.2 Effects of corrosion on ships on a global scale

Corrosion has its significant consequences in relation to ship strength, operability and operating life. For cargoes that contain seawater, its structural members are vulnerable to corrosion induction.

Reports and articles show that it is evident that corrosion in the holds is more extensive than in ballast tanks. Protection against corrosion is made possible as the continued presence of water in the ballast tank has made it necessary. In cargo holds, no measures are taken since the effects of corrosion is expected to be low. Furthermore, cargo holds are often loaded with corrosive cargoes. In the case of a bulk carrier, the inner surfaces of the holds are damaged by cargo handling equipment.

4.3 Corrosion rates in seawater

As seawater is generally known to be a corrosive environment, the corrosion rates of a metal can increase significantly if not fully immersed. Corrosion rate will be naturally reduced if plating is fully immersed in mud. This is due to the lack of oxygen. The flow of water also contributes to corrosion action. This occurs as flowing water removes rust deposit on the surface of plating which results in continuous corrosive attack on the exposed surface. Steel occupies a relatively low position in the galvanic series. Other factors that contribute to corrosion are as follow:

Proportion of time spent in ballast condition

Type and condition of protective coatings as well as the type of surface preparation

Maintenance of corrosion protection system

Type of inert gas system used

Speed and route of vessel

Structural layout of vessel and the tanks

Design against corrosion

Corrosion in marine structures is commonly due to the ballast sea water and cargoes. In the case of an oil tanker, corrosion occurs due to acid water that contains sulphur compounds from oil which in turn, forms oil deposits on cargo tank, ballast tank bottom and cargo transfer piping. For ore carriers, the grabs of a moving crane and machinery moving cargo damages the corrosion protection system on both sides of exposed plating.

Areas with constricted accessibility, such as ballast tanks, are prone to extensive corrosion especially unprotected surfaces. The rate of corrosion can be slowed down or prevented thus, particular attention should be paid when handling cargo and ballasting water. Empty spaces are to be made accessible to facilitate surveys and repairs, allowing accurate assessment of condition. Special attention also has to be paid during design and construction stage and during hull surveys. After surveying on the corrosion condition, an adequate corrosion protection system can be applied.

5.1 Initial features of corrosion

A number of design features promote corrosion because they influence the immediate local environment and also because they affect some other part of the structure or system. Some features are mainly applicable to structures, tanks, holds etc, and they are due to the following factors.

Entrapment of moisture and salts

Moisture and salts are usually collected in many parts of a structure where there are open spaces or channels that can collect water. As it evaporates, concentrated salt solutions may get in contact with the protective coatings. We can expect a local environment to be more corrosive thus the probability of premature coating failure occurring is high.

Ground level corrosion

At ground level, there are frequent problems where steel columns enter or rest on the ground. This is due to the collection of water that runs from a structure and the build-up of debris which acts as a poultice at the foot of the column. The element of liquid splashing contributes to the initiation of corrosion.

Shape

The shape of a structural member has a bearing on coating performance. Rounded sections are easier to coat and are less vulnerable to damage as compared to edges on rectangular sections.

5.2 Design considerations

Corrosion control can be done during the design stage of a structural work by observing a few basic rules.

Direct connection of dissimilar metals

Areas on deck where it is necessary to join two dissimilar metals should be able to incorporate components that insulate one from the other effectively. Such areas are the joints between a steel decking and an aluminum alloy super-structure. Adding of corrosion inhibitors increases the effectiveness of the metal compounds. Effective insulation of bolted joints can be achieved by non-metallic washers and bolt sleeves. As direct contact is unavoidable in some fastening cases, the component with the smallest exposed area should be of the more noble material.

Protective layers

The type of protective coatings is to be considered as it plays an important role. Paint systems used may reduce effectiveness in areas where surface preparation is inadequate.

Type of weld metal

Corrosion occurring during fabrication and repair during welding can be hazardous. In order not to allow bimetallic corrosion to occur, there must be a difference in composition between the weld deposited metal and parent material. If incompatibility is unavoidable, the weld metal should is to be more noble. Other factors such as minor structural changes in the weld and parent metals may affect corrosion susceptibility in the welded area during cooling. Heat affected zones that are adjacent to the weld is susceptible to corrosion as well depending on the alloy or weld metal combination and welding conditions.

Different corrosion types require different design requirements.

Design to avoid cavitation erosion

Major design considerations to be taken in the step of avoiding cavitation erosion to prevent the pressure in any part of a fluid system from falling to its saturated vapour pressure.

Design to avoid impingement corrosion

Corrosion attack is promoted by excessive local turbulence in a sea water system and is best avoided by designing and constructing a system such that damaging levels of turbulence are kept at minimum.

The initial design planning determines the outcome of corrosion attacks in the long run. For example, if a hold is made quite inaccessible, corrosion can easily take place and this will cause corrosion control to be harder. Thus, the design stage of a structure member plays an important role to prevent or minimize corrosion attacks.

5.3 Conclusion

Even though design is only a compromise between many competing factors, problems that are more or less due to nature are bound to happen. That is if only corrosion is not fully taken into account at the beginning of a project. An overall design may consist of different types of materials and parts from different manufacturers. Thus, serious problems could occur if the complete design, materials and coating requirements are not being considered in the early design stage.

Another important consideration in design is the necessity for maintenance. In the case of structures that requires re-painting or part replacements such as valves, will foresee a high maintenance cost. In order to keep repair costs at its lowest, proper access to areas that require regular maintenance is essential.

Corrosion effects on structural members

Ships and offshore structures are often subjected to mechanisms that degrade their mechanical properties. It will be difficult to establish a reduction in material strength when corrosion has occurred due to the complicated effects of uneven surfaces and material properties on the stress field and failure modes of the structure. Defects of material are known to affect a ship's hull buckling strength, shear strength and collapse strength. A mismatch of thermal expansion coefficients between the bulk material and passive film can induce stresses within susceptible surface films. This will then lead to film fracture and loss of passivity. Hydrogen that enters several structural alloys leads to a loss of ductility. In order for loading controls and failure prediction to be applicable, the understanding of corrosion effects on structural failure is essential.

When uniform corrosion occurs, the structural strength calculated by deriving the plate thickness or weight loss per unit area from the original values. The decrease in strength resulting from localized defects such as pitting is more complicated as compared to when general corrosion occurs. Nakai and Yamamoto (Marine Structures, 2004), pointed out that "overall tensile strength decreases gradually while total elongation decreases drastically with increase in pit depth" and "tensile strengths of specimens with pit corrosion" were reduced more than specimens which are subjected to average thickness reduction.

For stiffened plates, its ultimate collapse load has little effect when the corrosion loss is lower and a higher loss will result in local buckling. For the same volume of material loss, the global collapse and ultimate strengths is influenced by different corrosion locations. Initial residual stress in a corroded area will affect the local buckling and plate post-ultimate response.

6.1 Reliability of corroded steel

As we all know it, when corrosion occurs, a plate experiences uneven surface effect. For structural members with large uneven pitting corrosion on its surface, its ultimate strength decreases with the increase of number of pits. For support structures like beams and stiffeners, the major effect of corrosion is the loss of metal section due to a reduction of structural carrying capacity. As metal has a varying deterioration process, its structural performance will vary as well.

Accumulation of corrosion damage reduces a metals load effect and resistance with time. The rate of deterioration can often be non-uniform and difficult to predict and this affects various structural parts differently. When a superstructure is heavily exposed to a corrosive environment, its dynamic loads may increase and its resistance will decrease. Corrosion does not only cause fracture but also the yielding and buckling of members. In the case of a steel girder, there may be an increase in stress, a change in geometric properties and a build-up of corrosion products.

A loss of material is usually associated with a change in member stresses and geometric properties. Surface corrosion is one common problem that the marine industry faces. It will cause a reduction in member cross section properties, such as section modulus or slenderness ratio. Such properties are crucial in a member's ability to resist bending moments or axial forces. Corrosion build up also affects structural performance as accumulated rust retains moisture that further promotes corrosion.

6.2 Structural corrosion and its effects

The corrosion rate in the marine environment affects economic interest since the loss of steel in marine structures has an impact on structural safety and performance. There has been an increasing interest in predicting corrosion rates at a given location with a given period of exposure in order to maintain existing structures in the service. Corrosion allowances are prescribed for structural members by using standards based on the corrosion protection provided, expected rate of corrosion and the service life of the structure.

Fig. 6.2.1 Typical location of corrosion on a steel girder bridge

Two main corrosion mechanisms are generally present in steel plates-general waste that is reflected in a generalized reduction of plate thickness and also pitting which consists of much localized corrosion with deep holes on the steel. Fig. 6.2.1 is an example of surface corrosion that occurs typically on structural members and how its decreases its thickness due to surface loss. Loss of material may affect bending, bearing and sheer resistance in a supporting structure. Loss of flange material causes a reduction in the net area that is available to resist bending while a reduction in inertia will cause an increase in deflection. A reduction in a structures ultimate bending strength will also cause a reduction in maximum carrying capacity. Loss of web material may influence the resistance modes of shear and bearing.

6.3 Conclusion

Corroded structural members will face a variety of differences in strength as it experiences surface or material loss. The type of surface present in a corroded plate is far from uniform. A plate that is subjected to general corrosion has a random distribution of thickness over its area. We can evaluate the overall compressive strength of the steel by using a model of the spatial variation of the plate's thickness. A decrease in ultimate strength can be said due to plate thickness while a reduction in effective thickness near a plate boundary causes a decrease in the rotational restraint at the plate edges. This not only implies a decrease in compressive strength but also cause the deflections to be more out of plane. These conditions are ideal for the occurrence of stress corrosion for a structural plate as the plate thickness decreases due to the increment of stress level in the plate.

It is the combination of factors mentioned earlier that causes the corrosion rates in ship structures to be higher as compared to the corrosion rates on steel specimens. Other factors such as material properties, location of components and environmental conditions on board play an essential part in determining the results. It can be seen that corrosion rates in marine structures are significant and their effect on the structural strength that will affect ships and its operation in the long run.

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