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.