Write down the electrode reactions occurring during the corrosion of structural steel in seawater?

Write down the cathodic reactions for the corrosion processes of iron in de-aerated acidic water. Explain why corrosion rates are usually greater in acidic water than in neutral or alkaline water?

The rate of corrosion in acidic water is much faster than in neutral or alkaline water because the hydrogen production reaction, the cathodic reaction, is much faster than oxygen degeneration reaction because hydrogen is easier to diffuse in water rather than oxygen.

Write an account of corrosion issues relating to cargo tanks in ships?The SRB (Sulphate reducing bacteria) is mainly involved in the corrosion in cargo tanks. The bacterial oxidized sulphate to be the hydrogen sulphide which is very corrosive substance and attack locally in the cargo tank creating the pitting and crevices which introduced the macro cells to accelerate the corrosion. The cargo tank which is near the engine room will more suffer from the corrosion. The oil cargo tanks also will suffer more due to plenty of sulphur, and sulphate ion.In relation to marine, impressed current, cathodic protection equipment:- Summarise the factors that determine the choice of auxiliary anode.

1. Good electrical conduction, 2. Low rate of corrosion, 3. Low cost, 4. Able to withstand high current densities at their surfaces without forming resistive barrier oxide layers, etc. Explain why it is important to include electrode potential monitoring in the system.

The CP system needs the electrode potential monitoring becauseTo avoid forcing the potential to levels well below Eo) which:- Is wasteful in energy Can cause hydrogen embrittlement in susceptible materials (e.g. some high strength steel, titanium). Summarize the reasons why cathodic protection is almost always employed in conjunction with a paint coating for the corrosion protection of large steel structures in seawater.

To minimize the current demand and hence the cost because, thereby, the protective current only serves to protect any exposed parts of the metal surface.. Use Table 2c (below), to discuss the issues associated with the coupling of titanium or titanium alloys to other engineering materials in seawater. Briefly summarise the importance of the passive film on titanium in this situation?

Titanium occupies a position at the noble end of the seawater galvanic series in contrast with its location at the active end of Table 1. This is due to the influence of the passive oxide film which is present on the surface of titanium and its alloys in seawater and many other environments and which effectively transforms this metal from an active to a noble material. However, in some conditions (e.g. non-oxidising acids), the passive film on titanium is not stable and the metal behaves as an extremely active metal with an electronegative electrode potential (as indicated in Table 1). Other materials, such as stainless steels and aluminium alloys, exhibit similar characteristics, i.e. being passive (noble) in some environments but active in others.

A thermal sprayed coating of aluminium is often employed to protect offshore steel equipment from corrosion. Explain the mechanisms of protection for this strategy.?

The coatings function at least partly by exclusion of the environment from the substrate but some of them are not completely impervious to corrosive agents, such as moisture and oxygen, even when properly applied. Also sacrificial coatings (cathodic behaviour relatively to ions,): the thicker they will be the longer will be the protection.

Comment, with explanation, on the tactic of painting the cathode to minimise problems of bi-metallic corrosion. Make sure your answer covers the issue of area ratio effects on the intensity of bi-metallic corrosion.

The rate of bi-metallic corrosion can reduce by decreasing the cathodic reaction rate because the surface metal will try to restore its neutrality. Painting on the structure will reduce the anodic reaction area and therefore, it suppresses the corrosion rate. If painting is degraded or leaked since, the relative area of cathodic side over anodic side is still small. It means the corrosion rate due to bi-metallic galvanic is still relatively smaller than that is no painting on the cathodic. The corrosion at the anodic side will attack in general surface rather than locally attack which is more severe.

Write an account of corrosion problems and their control in ballast tanks on ships.

Corrosion problems in ballast tanks arise from high temperatures and warm, salty airless effective removal of water when emptying, possibly greater amounts of mud/silt left after emptying and consequent SRB, beneath such deposits which are ideal situations for MIC

Corrosion control methods include hugely increased steel area to be painted, inspected and maintained. Paint coatings have to withstand higher temperatures.

Figure Q3 shows an anodic polarisation result for type 316L stainless steel in seawater.in which the electrode potential of the steel specimen is driven in the positive direction from the free corrosion potential, Ecorr, whilst the current in the circuit is monitored.

(i) Explain what is happening to the stainless steel as the electrode potential shifts, in the positive direction, from the free corrosion potential, Ecorr.

The potential becomes more noble (more positive) by the formation of a thin, impervious oxide layer.

(ii) Explain what is happening to the stainless steel when the electrode potentialreaches the value, EB..

Specimen will reveal arrays of pits all over the free surface

(iii) Consider that, prior to undertaking an anodic polarisation exercise, the steel is immersed for a short time in nitric acid. Compared with the plot in Figure Q3, how would such a pre-treatment affect the results of the subsequent anodic polarization plot?

(iv) Type 316L stainless steel contains about 18% Cr and 2.5%Mo. What advantages are obtained by using a stainless steel with higher chromium and molybdenum contents for service in seawater? How would an anodic polarisation plot for a higher (Cr + Mo) steel appear, compared to the one for 316L, in Figure Q3?

Superaustenitic, 20Cr / 18Ni / 6Mo and the 25Cr / 7Ni / 3.5Mo (superduplex)stainless steels possess much enhanced performance in ambient-temperature seawater. However, higher alloyed superduplex and superaustenitic stainless steels possess significantly more positive breakdown potentials (and hence much superior corrosion resistance) than the conventional (say 316) grades.

The pipelines that transmit hydrocarbons from offshore fields are constructed of carbon steel or low-alloy steel and the internal fluid is a mixture of saline water (with all the normal constituents of seawater) and oil/gas. From many fields, this fluid will also contain high concentrations of carbon dioxide and have negligible dissolved oxygen.(4a) the internal surfaces of unprotected steel pipes typically suffer high corrosion rates in such fluids described above. Discuss how the carbon dioxide influences the mechanism of corrosion and identify the main strategy to control the corrosion attack inside the pipework.

The presence of appreciable amounts of CO2 can support corrosion, even when there is no oxygen present (as often in offshore pipes), by virtue of the products of its hydrolysis/dissociation reactions in water. Recalling that dissolved CO2 combines with water to produce carbonic acid, this is of major importance in calcium carbonate deposition phenomena, and also can influence corrosion processes.

CO2 + H2O -> H2CO3 This then partially dissociates as follows: H2CO3 -> H+ + (HCO3)-, Possible alternative cathodic reactions are:-

2H+ + 2e- -> H2

(but this is most important at pH less than about 4 not common in offshore pipelines). and (more likely at the normal pipework pHs of 5 8).

2H2CO3 + 2e- -> H2 + 2(HCO3)-2(HCO3)- + 2e- -> H2 + 2(CO3)2-

(4b) an additional mode of severe corrosion, and also stress cracking, inside the pipes is associated with the presence of sulphate reducing bacteria (SRB). Explain how the environmental conditions, within the pipes, stimulates the production of SRB and summarise the mechanism of the deterioration processes.

Deterioration promoted by sulphate-reducing bacteria is a potential issue in any environment (e.g. seawater) that contains sulphate, (SO4)2-, which fuels SRB metabolic activity. In order to thrive, SRB require anaerobic conditions and their metabolic activity results in the reduction of sulphate, (SO4)2-, ions to sulphides and especially hydrogen sulphide which are extremely corrosive to many materials.

Aspects of deterioration associated with SRB are:- This type of corrosion can often be recognised by the rotten-egg odour of hydrogensulphide exuding from black corrosion product which may be "capped" by a layer oforange / red rust The detailed corrosion mechanisms involved is likely that H2S gas or solid sulphides can stimulate the cathodic reaction and, thereby, increase the corrosion rate of steels. A particularly unfortunate characteristic of SRB is that they promote corrosion incircumstances (neutral pH / low oxygen) in which it would not usually be a problem. SRB can also promote hydrogen ingress into materials and this can be implicated in stress corrosion cracking and hydrogen embrittlement damage. In some circumstances, SRB and sulphur oxidising bacteria can become involved in cyclic deterioration processes in situations where the environment is alternatelyoxygen rich and oxygen denuded. Thus, in periods when oxygen is excluded, SRBthrive and cause corrosive attack to be followed, if the conditions become aerobic, by further extensive attack involving sulphur oxidising bacteria utilising the sulphide by products of the previous SRB-active period. Alternating activity can involve aerobic bacteria stripping out oxygen under a biofilm and also providing nutrients to be utilised by SRB.

(4c) Corrosion control of the external surfaces of such underwater pipes involves the use of cathodic protection (CP). Without going into much detail (but using schematic diagrams), describe the two ways of application of CP.

Schematic representation of the two methods of cathodicProtection

The application of CP in seawater often involves the formation of calcareous deposits on the steel surfaces. Briefly explain:-(i) Why calcareous deposits are producedIn seawater, CP often results in the deposition of a calcareous scale on the component being cathodically protected. The scale is either calcium carbonate or magnesium hydroxide the deposition of both being promoted by high-pH conditions. Such conditions prevail on a surface subject to CP because of the stimulation of the oxygen-reduction cathodic reaction:-

O2 + 2H2O + 4e- -> 4(OH)-

causing relatively-high pH on the metal surface.

(ii) How the performance of CP systems benefits from such deposition?

This calcareous scale is beneficial because it reduces the magnitude of the required CP substantially hence lowering both the capital cost and energy costs of the CP equipment.

(ii) How the deposition of calcareous deposits, on the external; pipe surfaces, is stimulated if the internal fluid is at a relatively high temperature?

Iron carbonate often forms on the surface of carbon steel or low-alloy steel pipelines in saline water containing carbon dioxide (e.g. during transmission of hydrocarbons from offshore fields).(i) Very briefly, state what conditions of pH and temperature promote iron carbonate precipitation.

The most common type of deposit is calcium carbonate, CaCO3, which deposits when the calcium ion, Ca2, and carbonate-ion concentrations become high enough to exceed the solubility of CaCO3; in which circumstances the following reaction occurs spontaneously:-

Ca2+(aq) + (CO3)2-(aq) ---> CaCO3(s)

Two factors that can lead to deposition of calcium carbonate are rises in temperature (partly on account of reduced solubility of calcium carbonate with increasing temperature) and increases in pH. . Increases in pH, even at ambient temperature, tend to lead to precipitation of calcium carbonate on account of the preponderance of (CO3)2- ions at pH above about 9.

(ii) Discuss the benefits and dangers associated with such iron carbonate surface depositsA danger with the precipitation of iron carbonate or iron sulphide is that if the scale layer is incomplete (say by local spalling or erosion), extremely severe pitting attack can occur at uncovered site. The remaining deposit (possibly covering a large area) acts as cathode and small exposed pipe surfaces act as anodes with severe pipe wall thinning.

In offshore hydrocarbon extraction, seawater is often utilized for injection into the reservoir in order to maintain production.(i) Assuming that no pretreatment of the seawater is undertaken, summarise the potential corrosion problems within the water injection pipelines.

If raw, untreated seawater is used, two corrosion problems are:-1. Corrosion associated with the oxygenated water; i.e. with the oxygen reduction reaction as cathodic reaction; this will be a factor for such lengths of pipeline until the oxygen content is substantially reduced.2. At longer lengths, the oxygen-depleted conditions will support the proliferation of sulphate reducing bacteria (SRB) that will reduce the sulphate constituent of the seawater to sulphide and hydrogen sulphide.

Identify two treatments of the seawater that can be used to counteract the corrosion problems referred to above.The possible corrosion control measures are:1. deaeration of the raw seawater 2. Biocide dosing and/or removal of the sulphate constitutes of the seawater.

In recent decades, there has been a tendency towards the use of steel of higher tensile strength in shipbuilding. Discuss the impact of this on corrosion, hydrogen embrittlement and fatigue behavior.

Higher tensile strength obviously yields weight savings on account of thinner steel sections but has the following disadvantages:-

1. reduced corrosion margins due to thinner steel because these higher-strength steels have essentially similar corrosion behaviour (rates) as conventional lower strength steels2. Greater possibility for hydrogen embrittlement and other cracking (e.g. fatigue) problems since higher-strength steels have inherently greater susceptibility to these mechanical deterioration processes.

Very briefly summarise the differences in corrosion behaviour in seawater of structural carbon steel and stainless steel.Structural CarbonStainless steel

1Possess extremely-poor corrosion resistance in virtually all aqueous environments essentially because they exhibit little tendency to form protective oxide films on their surfaces.

All types of stainless steels are characterised by having considerably superior corrosion resistance in most environments than has carbon steel. This is due to the presence, on the surface of a stainless steel component, of a very thin, adherent, protective layer of (chromium-rich) oxide.

2.The corrosion rate of these materials increases catastrophically with water flow rate

This excellent corrosion resistance in flowing liquids and results in their ability to be used without danger of erosion corrosion at much higher flow rates.

3.

Stainless steels are rather susceptible to some types of localised attack. Examples are pitting, crevice corrosion and stress corrosion cracking (the latter being more likely at higher temperatures - typically above about 80C - and at higher dissolved-oxygen concentrations).

A method of protecting carbon steel or low-alloy steel components from corrosion is to clad the component with a corrosion-resistant alloy. Summarise the methods of such cladding processes.This involves welding or hot rolling a relatively-thin sheet of a corrosion resistant material onto a component made from a material of insufficient corrosion resistance.Clad coatings that are applied by hot rolling the substrate and coating metal may yield superior adhesion of the coating.

2012 Including reference to the galvanic series at the front of this examination paper,compare and contrast the method of protection of a steel substrate with,(i) electrodeposited zinc coating and,

(ii) electrodeposited nickel coating.

Giving brief reasons, sketch a schematic diagram showing the electrode potential ranges in which corrosion occurs and in which corrosion can be prevented by the application of cathodic protection.

The following can be concluded from the foregoing sections.1. For corrosion of a particular metal, the aqueous environment must contain substances capable of sustaining a cathodic reaction which has an equilibrium electrode potential, Eo, more positive than Eo for the metal dissolution reaction,M = Mn+ + ne- (see Figure 8).2. Metals with relatively negative electrode potentials possess a greater tendency tocorrode (see Table 1). However, it is important to note that a greater tendency to corrode does not necessarily lead to a greater corrosion rate.3. If two regions of a component are at different electrode potentials, the region exhibitingthe more-negative electrode potential will act as anode (and corrode preferentially) withthe other region acting as cathode. Th

If, in an impressed current cathodic protection system, the applied current isinsufficient to achieve the target electrode potential for optimum protection,is there any benefit obtained?

Even if the electrode potential of a component, or part of it, is not lowered sufficiently to attain the target Eo value to stop corrosion completely, there will still be a benefit secured. This is because any reduction in the electrode potential of the component will result in lower corrosion cell currents and hence reduced corrosion rates. A negative shift in potential from Ecorr moves the current (and hence corrosion rate) down the anodic polarisation curve.

Outline the problems associated with an impressed current cathodic protection system, in which the applied current is such as to cause overprotection of a steel structure.

The disadvantages using impressed current are:- continuous DC power must be available care must be taken to ensure that the current is never connected in the wrong direction since this would cause acceleration of corrosion instead of protection more technical and trained personnel are required.

Describe how a paint coating system, on steel structures, can comprise of different coats employing different paint formulations and different protective mechanisms.

Paint can then be applied by dipping, brushing or spraying. Paint coatings may utilise one or more of the following mechanisms for protection of a structure:- By a barrier effect. Galvanic action. Iinhibitive action.

Paint coatings usually consist of more than one coat of different paint formulation and protective mechanism. As an example: a primer coat may possess inhibitive or galvanic properties top coat functions by the barrier principle. Many protective paint systems incorporate a primer, intermediate layer and topcoat.Thick-film paints, of the barrier type, are often employed on buried structures. Also,steel pilings exposed to the severe conditions (water plus oxygen-rich) represented by atmospheric, splash, tidal and shallow-immersed zones, require low-permeation coatings such as spray-applied or fusion-bonded epoxy or polyurethane paints.The most common type of galvanic-action paint, used for protection of steel structures, comprises inc-rich paints, containing a high loading of zinc particles or flakes dispersed in a binder, For the beneficial galvanic action, there must be electrical continuity with the substrate; hence they are used as primer coats with or without a top (barrier) coating.

Briefly comment on whether cathodic protection (CP) can be utilised to protect steel above the waterline on ships Only submerged equipment can be protected by CP; hence the superstructures/deck regions of ships cannot be so protected.

What is the function of the dielectric shield in CP systems on ships? The dielectric shields ensure that the current output from the anode does not short-circuit near the anode and thereby reaches furthermost parts of the hull of a ship.

Discuss why it is important to incorporate electrode potential monitoring in impressed-current CP systems. Monitoring of the electrode potential is carried out in order to avoid "overprotection" (i.e. forcing the potential to levels well below Eo) which:- is wasteful in energy can cause hydrogen embrittlement in susceptible materials (e.g. some high strength steels, titanium).

Compare and contrast the corrosion behaviour of stainless steels and copper-nickel alloys in seawater including identifying limitations on the use of these classes of materials. Stainless SteelCu-Ni

Considerably superior corrosion resistance due to the presence, on the surface of a, of a very thin, adherent, protective layer of (chromium-rich) oxide. This excellent corrosion resistance is a particular feature of all stainless steels in flowing liquids and results in their ability to be used without danger of erosion corrosion at much higher flow rates than is possible for the copper-nickel alloys.

The protective film on the surface of the copper based materials is thicker than that of stainless steel.

The corrsosion resistance of cu based alloys is flowrate dependent. The protective film on these materials are vulnerable to damage at much lower velocities

Describe the phenomenon of Accelerated Low Water Corrosion (ALWC).

It occurs on steel marine structures, especially in ports and harbours, such as piers, quays, flood defence structures and mooring chains. Occurs just above and below region of lowest astronomical tideRapid attack > 1mm per yearMay cause up to 2/3 reduction in design life if not protected againstFEATURES / IDENTIFICATIONOccurs in patches with loose orange-coloured corrosion product over a layer of black coloured iron sulphide with the subjacent steel appearing bright with extensive pittingDETERIORATION MECHANISMSNot completely understood but involves microbiological + corrosion processes (i.e. a form of MIC)It is considered that SRB are involved which thrive in the low-oxygen conditions and produce iron sulphide corrosion productPREVENTIONThe Use of conventional corrosion control tactics, protective paint coatings and cathodic protection.

The data below was obtained from an experiment in which a welded plate of carbon steel was cut up to yield small specimens representing the parent metal (i.e. remote from the weld) and the weld metal itself. These separate specimens (of equal area) were then exposed to seawater at ambient temperature and the values of electrode potential were recorded as indicated. (Note: SCE stands for the Standard Calomel Reference Electrode.)

(i)Use the data in the above table to predict, with explanation, what would be expected to happen if the parent metal and the weld metal specimens were in electrical contact (as in the original welded plate) during the experiment.Write down the electrode reactions occurring on the weld metal and parent metal

Describe what is meant by weld decay in stainless steels.This type of intergranular corrosion can occur in the heat-affected zone (HAZ) of welded stainless steel components due to precipitation, during cooling after the welding operation, of chromium carbides at the grain boundaries (and hence loss of chromium in the immediately-adjacent zone .The local loss in corrosion resistance arises because the chromium is crucial in promoting the formation of a Cr-rich passive film on the surface of stainless steels.

Write down the electrode reactions occurring during the corrosion of structural steel in aerated seawater.

Describe the phenomenon of bi-metallic (galvanic) corrosion. Use the data sheet to provide an example of this type of corrosive attack.

Galvanic corrosion may occur when two different metals are in contact, say by welding or bolting, or connected by an electrical conductor, and are exposed to an aqueous environment. This arrangement constitutes an electrochemical cell in which one of the metals becomes the anode and thus corrodes.AnodeMagnesium alloys

CathodeCarbon Steel

The rule is that the more electronegative of the two metals (i.e the metal which has the more negative electrode potential if measured as in becomes the anode and the other metal becomes the cathode.

The result of this is that the more-electronegative metal(less noble metal) now suffers more corrosion and the less electronegative (more noble) metal component (providing sites for a cathodic reaction such as oxygen reduction) suffers less corrosion than if they were placed unconnected in the same environment. The driving force for galvanic corrosion, that is for the intensification of attack on one of the two connected metals, is the difference in electrode potentials of the two components when exposed separately in the same environment.

Discuss how the phenomenon of bi-metallic (galvanic) corrosion can be taken advantage of in one important method of corrosion control. Use the control of corrosion on carbon steel as an illustration of your answer.

By connecting the Carbon steel component to a material which is more reactive (i.e. has a more-negative electrode potential).(e.g Zinc) This forces the electrode potential of a Carbon steel component from its naturally-occurring values (more positive than Eo) to a value equal to or more-negative than Eo. This Known as the sacrificial anode method of cathodic protection, because it relies on the sacrificial corrosion of the reactive material in order to protect the component.

With the aid of a sketch, summarise the principles of the impressed current method of cathodic protection (ICP). Comment on the importance of using the correct polarity in the ICP set-up. Describe what is meant by overprotection when using ICP and identify the problems associated with the occurrence of overprotection.

With the appropriate polarity connection to the DC current source (most important), electrons flow to the component to be protected thereby forcing its potential in the required negative direction. This stimulates cathodic reactions on the component with anodic reactions being stifled.

The decrease in electrode potential of the component will be in direct proportion to the magnitude of the current supplied from the DC source and the larger the surface area to be protected, the greater is the required current.

Thus the method should include monitoring of the electrode potential in order to avoid "overprotection" (i.e. forcing the potential to levels well below EO) which:- is wasteful in energy can cause hydrogen embrittlement in susceptible materials (e.g. some high strength steels, titanium).

Summarize the environmental conditions in which stainless steels possess excellent corrosion resistance in water. at much higher flow rates at lower temperatures In deaerated water

Summarise the environmental conditions in which stainless steels are vulnerable to corrosive attack in water. Stainless steels are susceptible to some types of localised at higher temperatures - typically above about 80C Higher dissolved-oxygen concentrations. Chlorinated water.

In relation to the types of corrosion involved in, give a brief indication of the key alloying elements that confer increased corrosion resistance (actual alloy compositions not required). Chromium and molybdenum.In order to provide an indication of the influence of alloy chemistry upon the pitting or crevice-corrosion resistance, the idea of the "pitting resistance equivalent" (PRE) is often used. This parameter is defined by empirical formulae like the following:-PRE = % Cr + x % Mo + y % N.

Giving brief reasons, comment on whether you would recommend Copper-10%Ni alloy or a stainless steel alloy for service in rapidly flowing seawater.

I would recommend Stainless steel because of its considerably superior corrosion resistance due to the presence, on the surface of a very thin, adherent, protective layer of (chromium-rich) oxide, without danger of erosion corrosion at much higher flow rates. This is because Corrosion of stainless steel in seawater is controlled by the availability of oxygen to the metal surface. Thus, under static conditions, Stainless steel corrodes faster

The protective film on the surface of the copper based materials is thicker than that of stainless steel. But are susceptible to impingement attack occurs when the hydrodynamic effect caused by seawater flowing across the surface in which protective films are removed and erosion-corrosion occursStainless steels are not subject to impingement attack.

There are a number of ways in which a coating of zinc can be applied to a steel substrate for corrosion protection. Without providing much detail of the processes, identify three such zinc-coating processes.

Galvanizing---This involves immersion of the component in a vat of molten zinc and this type of coating has the ability to provide a degree of protection to coated steel components for a time even at breaks in the coating on account of localized sacrificial protection provided by the coating. Hot-dipped coatings---This is one of the oldest, simplest and generally cheapest coating methods. It involves applying a low-melting-point metal to a high-melting-point substrate. The coating metals are: zinc, aluminium, tin, lead and the mo

Spray coatings---These are applied to equipment by a variety of thermal-spray techniques - one of the most effective being the high velocity oxy fuel (HVOF) process in which the coating material is fired onto the component at supersonic velocities and high temperatures to produce a coating usually of excellent adhesion to the substrate and very-low porosity. thermally-sprayed coatings is not as good as, say, electrodeposited hard chromium

Explain how a zinc coating continues to provide protection to a steel substrate even at sites of local damage to the coating. By localised sacrificial protection provided by the coating to substrate at breaks in a galvanized coating .When base steel is exposed, such as at a cut edge or scratch, the steel is cathodically protected by the sacrificial corrosion of the zinc coating adjacent to the steel

Without presenting any great detail, compare and contrast the deterioration processes of stress corrosion cracking and corrosion fatigue.

Both these phenomena involve eventual failure of a component by the initiation and growth of cracks during service.

Stress-corrosion cracking (SCC) occurs in the presence of a steady applied or residual tensile stress and is a very specific type of phenomenon in that a particular material will only be susceptible to SCC in certain environments and another material is likely to be immune to SCC in a similar environment but itself prone to SCC in a different set of conditions. The environmental requirements for SCC may be extremely specific. e.g. a narrow pH range or a certain temperature range. The progress of SCC comprises an initial phase in which one or more cracks initiate on the surface of the component; this is followed by a period of slow crack growth and finally by component failure by fast fracture or leak. Corrosion-fatigue is the result of the combined action of an alternating or cycling stresses and a corrosive environment. The fatigue process is thought to cause rupture of the protective passive film, upon which corrosion is accelerated. If the metal is simultaneously exposed to a corrosive environment, the failure can take place at even lower loads and after shorter time.

Compare and contrast the corrosion control methods of cathodic protection and anodic protection. Ensure that you identify their potential, or otherwise, for corrosion protection of carbon steel used in ship construction and marine equipment.

CATHODIC PROTECTIONANODIC PROTECTION

The basis of cathodic protection is to force the electrode potential of a corroding metallic component from its naturally occurring value(more +ve than E0) to a value more ve than E0The basis of Anodiv Protection is to passivate a metal by shifting its electrode potential to a more +ve value.

Cathodic Protection can be applied to metals in any mediumThe Carbon steel is protected by shifting its electrode potential from Ecorr to a value of Ep (or more positive) where the establishment of a passive film will confer corrosion protection in the same manner as naturally-occurring films. The required positive shift in electrode potential can be achieved by using a set-up rather like the cathodic protection apparatus but with the opposite polarity and this procedure is known as anodic protection.

Anodic protection has only restricted application and is especially unsuitable when chlorides are present (e.g. seawater).(ii) Anodic protection is inherently risky if proper electrochemical control is not maintained. This is because shifting the electrode potential of a component in the positive direction from its natural value, Ecorr, results in an acceleration of corrosion rate.

Use Table A (see next page), to discuss the issues associated with the couplingof titanium or titanium alloys to other engineering materials in seawater.Briefly summarise the importance of the passive film on titanium in this situation.

Titanium and its alloys possess excellent corrosion resistance in many environments including seawater.Also care is necessary in bimetallic contacts involving titanium and its alloys. The reasons for this are twofold:-1. Titanium is noble to many other materials in seawater2. Titanium is susceptible to hydrogen embrittlement. One possible source of hydrogen uptake in metals is via the cathodic reaction during corrosion. Thus, if titanium is in contact with another metal and significant bimetallic corrosion is occurring on the less-noble component, this may lead to hydrogen ingress into the titanium. For this reason, special care is needed in coupling titanium to the most electronegative metals, aluminium, zinc, magnesium. Another possible source of hydrogen is if a titanium component is attached to a structure which is receiving cathodic protection, since a poorly-controlled CP system say pushing potentials down to about -0.9 V (silver/silver chloride) or more negative - will increase the dangers of damage to the titanium, especially if this component is also under stress.

The passive oxide film which is present on the surface of titanium and its alloys in seawater and many other environments and which effectively transforms this metal from an active to a noble material. However, in some conditions (e.g. non-oxidising acids), the passive film on titanium is not stable and the metal behaves as an extremely active metal with an electronegative electrode potential (as. Other materials, such as stainless steels and aluminium alloys, exhibit similar characteristics, i.e. being passive (noble) in some environments but active in others.

A thermal sprayed coating of aluminium is often employed to protect offshore steel equipment from corrosion. Explain the mechanisms of protection for this strategy.The high velocity oxy fuel (HVOF) process, in which the coating material is fired onto the component at supersonic velocities and high temperatures, to produce a coating usually of excellent adhesion to the substrate and very-low porosity.

The compositions of three stainless steel alloys are given in Table A below. Discuss the corrosion issues associated with the use of each of the alloys in seawater and provide a little detail on the influence of the more relevant alloy constituents on the types of corrosion that you refer to.

Different grades of stainless steel exhibit significant differences in resistance to these localised forms of corrosion with better resistance being conferred by increases in chromium and molybdenum contents of the stainless steel

Type S31600 stainless is unsatisfactory for seawater applications due to its susceptibility to localised corrosion.Type 316L is also rather vulnerable in most marine conditions.

Type 31254 stainless steels possess much enhanced performance in ambient-temperature seawater.

The following statement has been made by one of the major classification societies: Corrosion of ballast tanks is often the reason for scrapping of ships. Discuss the factors that account for the above statement.These are especially prone to serious corrosion problems. They are periodically filledwith seawater and emptied. These tanks are probably more vulnerable after discharge of the ballasting seawater on account of:- High corrosion rates during drying out in contact with air (ready supply of oxygen to fuel the cathodic reaction) the usual failure to completely drain the tank thus leaving a shallow pool of corrosive oxygenated water at the base of the tank. Moreover, there is often a residue of silt left in the tank under which there may prevail local anaerobic conditions that can stimulate additional mechanisms of corrosion associated with microbial activity with associated extremely-rapid corrosion Air spaces above the ballast water surface are vulnerable again on account of the ready availability of oxygen. Care must be taken to avoid seepage of corrosive cargo into ballast tanks. A recent issue relates to the development of regulations governing the treatment of ballast water to kill biological organisms. This has become important in order to avoid transfer of biological species from one part of the world to another region where the local ecosystems may become overwhelmed by the nonindigenous species with serious environmental and economic consequences. A wide range of anti- biological treatments have been mooted and there is a need to ensure that specific treatments do not promote corrosion problems.

Briefly state the electrochemical basis of the corrosion control method known as cathodic protection (CP).

Fe(s) Fe2+(aq) + 2 e

and the cathodic steps can be any of

O2 + 2 H2O + 4e 4 OH

H+ + e H2(g)

M2+ + 2 e M(s) Summarise the two ways of applying CP.Impressed current methodWith the appropriate polarity connection to the DC current source (most important), electrons flow to the component to be protected thereby forcing its potential in the required negative direction. This stimulates cathodic reactions on the component with anodic reactions being stifled.Sacrificial anode methodBy connecting the component to the more active "sacrificial anodes", a bi-metallic cell is set up in which the sacrificial metal acts as anode, corrodes and thereby supplies the required electron flow to the component.

Discuss the use of CP for the protection of ballast tanks in ships.

As mentioned above, only submerged equipment can be protected by CP; hence the superstructures/deck regions of ships cannot be so protected.CP is often employed, in the form of sacrificial anodes, in ballast tanks but, again note that such a strategy will only be useful when the ship is carrying ballast water and, even then, only for the part of the tank up to the ballast water level.