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Anticipated Severe Corrosion
Problems Associated with the Change from Single Hulled Bulk Carriers to Double
Hulled Bulk Carrier Designs
Amtec Consultants Ltd
December
2003
1. Background
The grounding and subsequent hull
rupture of the single hulled tanker “Exxon Valdez” resulted in massive
pollution of the Alaskan coastline by crude oil. Congress in the USA legislated
the use of double hulled tankers to help contain the cargo in the event of grounding
or other hull rupture. Recent tanker disasters with older single hulled tankers
have re-enforced this anti-pollution viewpoint.
A spin-off effect has led to the
proposed introduction of double hulled bulk carrier designs. There is however,
a very considerable difference between the operating conditions found on double
hulled tankers and bulk carriers.
In general, bulk carriers transport
cargo that is non-polluting and many have a tendency to sink rapidly and without
trace. When these unpredicted hull breaches occur with single hulled bulk carriers,
they can often be the result of the extremely high corrosion rates associated
with certain cargoes. Cycling between iron ore and coal has been found to be
a contributing factor to these unexpected sinkings.
This document seeks to explore how
these existing corrosion problems may very well be made significantly worse
by the prospective change from single hulled to double hulled bulk carrier designs.
2. Thermos Effect
After the current designs of double
hulled tankers were introduced, it was found that when hot cargoes were loaded
at temperatures in the order of 45oC, there was only a small drop in the temperature
of the cargo after a long voyage to North latitude ports.
Previous experience with single
hulled tanker designs had showed that the cargo quickly cooled and the cargo
temperature was approaching that of the sea temperature after half of the voyage.
The reason for this large difference in temperature behaviour was that for the
double hulled designs, the empty ballast space completely surrounded the cargo
with an insulating jacket of air, preventing direct heat transfer through the
hull metal between the cargo and the sea. This is the principle used for the
thermos flask or vacuum bottle, hence the “thermos effect”.
The humid, high temperature air
in the double hulled vessels ballast tanks has also resulted in more rapid corrosion
and coating failures in these areas. This is because, in adverse circumstances,
the rate of the corrosion reaction doubles with every 10oC rise in temperature.
In the worst cases, this could cause a four times increase in the rate of corrosion.
However, the corrosion rate is normally under the control of a diffusion reaction,
once some build up of rust has occurred.
The coated area of the ballast tanks
is very much greater in the double hulled design vessels and economic refurbishment
of the early double hulled tankers at ages of greater than ten years is proving
to be a great problem.
The corrosion within the cargo tanks
of double hulled tankers has been similarly affected. Previously, with single
hulled designs, the tank top and under deck areas could be left uncoated, but
with the double hull designs, the tank top shows pitting and grooving corrosion
much more rapidly (photograph
1) and severe under deck corrosion also occurs (photograph
2), necessitating coating.
The “Thermos” effect
will undoubtedly occur with double hulled bulk carriers. Where damp and humid
cargoes, such as grain and ore are loaded, corrosion rates will be significantly
higher than normal. In the case of damp, “active” cargoes such as
coal or petcoke, the anticipated corrosion rates will be extremely high at sites
of coating damage or removal.
In the case of double hulled tankers,
the thermos effect tends to be evenly spread across the entire cargo tank, for
double hulled bulk carriers this will not be the case, due to cargo compaction
effects and space that remains empty above the cargo. This will result in thermal
gradients within the bulkheads of the cargo holds. Any “hot spots”
will become strong anodic sites and suffer from both enhanced corrosion and
coating breakdown. Both processes will occur in the cargo holds and in the ballast
tanks at the same location.
3. Temperature effect on
Corrosion reactions
3.1 Corrosion Processes
Corrosion reactions take place in
two stages: a charge transfer step and then a diffusion step. The metal ions
have to diffuse away from the reaction site and oxygen or other cathodic reactants
have to diffuse to the metal surface.
The initial corrosion reaction is
the dissolution of iron from the surface:
Fe -> Fe2+ + 2e-
Within the rust layer, a second
reaction occurs that oxidises the ferrous ion to the ferric ion:
3 Fe2+ + 6 OH- -> 3 Fe(OH)2
The electrons produced by the anodic
reactions and their intermediate steps are consumed by a cathodic reaction.
In neutral solutions, in the absence of other ions or pollutants, this is usually
the reduction of oxygen to hydroxyl ions:
O2 + 2 H2O + 4 e- -> 4 OH-
In acidic or polluted conditions,
the cathodic reaction is the evolution of hydrogen via the reaction:
2 H+ + 2 e- -> H2
3.2 Corrosion Rates vs Temperature
If the rate of the reaction is determined
only by its initial oxidation step, then the corrosion rate increases exponentially
with temperature in a manner governed by the Arrhenius expression:
r = A exp (-E / RT)
Where:
r = corrosion rate
A = a constant
E = activation energy
R = gas constant
T = absolute temperature.
Diagram 1 shows a typical example of this type of behaviour for low alloy steel
immersed in dilute hydrochloric acid.
Diagram 1. The effect of temperature on corrosion rate under acid conditions.
The temperature of the steel and the temperature of the environment cannot be
viewed as independent. The specific conductance of seawater increases with temperature.
At 0oC seawater conductance is in the order of 0.023 reciprocal ohm cm, whilst
at 25oC it is 0.042 and continues to change at a similar rate over the region
of interest, 20oC to 50oC. The more conductive that the environment becomes,
the faster the corrosion rate.
3.3 Diffusion Rates vs Temperature
Dissolved oxygen in the seawater
also varies with temperature. At 5oC it is 0.65 mg/litre and at 30oC it is in
the order of 0.41 mg/litre. The diffusion of oxygen and iron ions through seawater
is faster with increasing temperature and is governed by the Nernst equation:
D = RT/F2 { [(V1 + V2) (L1 x L2)]
/ [V1 Z1 (L1 + L2)] }
Where:
D = diffusion co-efficient.
F = Faraday’s constant.
T = absolute temperature.
L1 and L2 = cation and anion
limiting equivalent conductances.
V1 and V2 = number of cations
and anions from one molecule of electrolyte (one for NaCl).
Z1 = cation valency (one
for NaCl).
From the above equation it can be
seen that for diffusion controlled reactions, the corrosion rate would be expected
to increase in a linear manner with temperature.
On balance therefore, the effects
of temperature on the charge transfer step, the conductivity and the diffusion
constant, far outweigh the effects of changes in oxygen solubility.
Water line corrosion is a particularly
strong effect of the diffusion of oxygen (photograph
3). Water lines are areas of oxygen super saturation and hence the corrosion
rate here is not under diffusion control but under activation (Arrhenius) control.
It would be anticipated that static water lines in ballast tanks in double hulled
vessels would be prone to the severe effects of this type of corrosion in a
similar manner to aft peak tanks located in close proximity to hot engine rooms.
3.4 Surface Area Effects
The reactions are extremely dependent
on surface area. The surface area of the cathode is of particular importance.
When the cathode is of high surface area, such as a cargo of petcoke, then the
rate of the anodic corrosion reaction can be extremely high.
This area effect is not only limited
to cargo holds but has caused high corrosion rates in repaired vessels such
as recent, near disastrous, structurally damaged vessels due to “accelerating
corrosion”, which had high corrosion rates on the repaired area because
the older coated areas acted as a large cathode.
Repairs carried out in the void
spaces of double hulled bulk carriers are likely to suffer from high corrosion
rates for similar reasons and exacerbated by the temperature effect.
3.5 Localised Corrosion
The above discussion has dealt with
general, uniform corrosion phenomena. Localised corrosion types are also extremely
susceptible to the temperature effect. Pitting corrosion and corrosion fatigue
are highly affected by the increased diffusion of oxygen as temperature increases.
3.6 Microbially Induced Corrosion
Localised microbial corrosion reaches
a peak around 37oC when the microbes are most active. Anaerobic bacteria, such
as sulphate reducing bacteria, follow the Arrhenius equation because the cathodic
step:
SO42- + 8 H -> S- + 4 H2O
And the anodic step:
Fe2+ + S2- -> FeS
Are under activation control
3.6 Sacrificial Anodes vs Temperature
The relative corrosion rates of
iron and zinc very with temperature in the manner shown in diagram 2.
Diagram 2. Corrosion Rates of Iron and Zinc vs Temperature.
The striking feature of this graph is that as the corrosion rate of iron rapidly
increases with increasing temperature, it is mirrored by a corresponding decrease
in the corrosion rate of zinc.
Zinc sacrificial anodes are commonly
used to protect ballast tanks and heavy weather ballast holds against corrosion,
by corroding preferentially. It can be seen that the implication of the curves
of diagram 2 is that over four times the amount of zinc anode surface will be
required at 40oC, compared with required at 20oC.
4. Active Cargoes
What is an “active”
cargo? For this document, it can be defined as one that physically affects the
cargo hold coating and directly influences the corrosion reaction on the underlying
steel. Examples of active cargoes in this article are coal, coke, bauxite, sulphur
and petcoke. They commonly produce the type of coating breakdown on the corrugated
bulkheads of cargo holds that is shown in photograph
4. Rapid corrosion occurs at these damage sites, as shown in photograph
5.
What makes these cargoes “active”
is that they are ionically conducting, possess a large surface area with a significantly
high moisture content or associated water and the cargoes act as good cathodes.
The corrosion reaction that is taking
place on the steel is the anodic half of the process and generates electrons.
The electrons are consumed by the reduction reaction of oxygen and water that
takes place at the cathodic site. Normally the cathodic reaction is very slow
and controls the rate of the anodic dissolution of iron, however, the extremely
large surface area of the cargo removes this limitation. The corrosion reaction
can occur at significantly increased rates when an active cargo is carried.
The corrosion products formed can lever the paint off the steel, causing further
fresh steel to be exposed to the aggressive environment. In severe cases, the
corrosive solutions that develop can also result in the formation of pits.
Pitting corrosion occurs when there
is a small anodic site (in this case the exposed steel) and a large cathodic
site. Cathodic sites generally form on the intact coating, but some cargoes
are capable of supporting the cathodic reaction. Pitting corrosion can be accelerated
with active cargoes, which drives the corrosion process at a greater rate. This
is frequently the situation that occurs with carbon based cargoes such as coal
and coke. Products such as coke and petcoke have very large surface areas and
can form excellent cathodic sites and pits may grow rapidly under such circumstances.
Active cargo corrosion occurs in
a sequence of events that results in coating damage, often with the characteristic
tree pattern, together with steel corrosion. The first stage of the damage to
the paint occurs when the sharp, hard and angular cargo scratches into the coating
due to settlement during both loading and the voyage. Eventually the cargo cuts
through sites of weak paint to the steel and exposes the metal to the cargo
environment.
It is first seen as a small puncture
in the coating that creates a pathway for water to reach the steel. Water associated
with the cargo can permeate along the interface between the paint and the steel
at the site where the coating has been damaged, resulting in the loss of adhesion
of the coating.
When examined visually, the damaged
area may appear to be small, but when it is investigated with a penknife, it
is often found that the paint is loose around the damage site. When this loose
coating is removed, the true extent of the corrosion and associated coating
delamination becomes apparent. Photograph
6 shows two damage sites that were initially very similar with only tiny
punctures visible in the paint. It can be seen that the corrosion under the
coating and the loose paint extend several centimetres from the visible damage
site.
As the cargo settles, soft or weak
paint can move with the cargo creating blisters and “sags” of the
type shown in photographs 7 and 8.
The loose coating sag is removed by further settlement of the cargo, which in
turn can cause the formation of another sag. Repeated “stick – slip”
cycles of cargo settlement result in the tree shapes of photograph
9.
5. The Domino Effect and
Coating Lifetime Issues in Double Hulled Vessels
Coatings tend to have a reduced
service lifetime expectancy at more elevated temperatures. One of the main reasons
for this is that they become less elastic more quickly in these circumstances,
due to a more rapid loss of residual solvents and an enhanced level of final
cure.
In the period immediately following
application, epoxy coatings show elongations in the order of 2%, due to the
incomplete cure and retained solvents. Over a period of about four years, this
becomes reduced to some 0.5% as the level of cure increases and the retained
solvents are lost. Coating shrinkage can accompany this effect and results in
the coating becoming more easily detached due to internal stress.
At the temperatures in excess of
30oC that are to be anticipated to be more common in double hulled designs,
the time to embrittlement of the coating will be approximately halved.
These factors will tend to exacerbate
the “Domino Effect” described by Peter Contraros of ABS. In the
Domino Effect, coatings breakdown more quickly at highly stressed areas as these
areas preferentially become the anodic sites of the corrosion reaction. The
rapid corrosion both levers the coating from the metal surface and locally thins
the structure. This in turn increases the stress levels.
Localisation of the anodic sites,
increased corrosion rates and coating breakdown are all significantly worse
at elevated temperatures. It is therefore to be anticipated that the Domino
Effect will be a more serious problem in double hulled bulk carriers than in
their single hulled equivalents. The relative novelty of the double hull bulk
carrier design will make prediction and possibly inspection of this effect problematical.
6. Enhanced Corrosion in
Cargo Holds
The combination of the “thermos
effect” with “active” cargoes in double hulled bulk carriers
is anticipated to lead to extremely high corrosion rates. When this occurs,
it will place severe limitations on the operations of such vessels and will
limit the vessel’s ability to trade all cargoes world wide.
The carriage of products such as
sulphur and sulphurous materials, for example, could become almost impossible,
unless the cargo is completely dry. Some active cargoes undergo an exothermic
reaction with water, causing their temperature to rise during the course of
a voyage. With the single hulled design, the danger of fires in the holds was
reduced, as the vertical stiffeners protruding into the cargo tended to act
as “heat sinks” and drain the heat away into the surrounding sea
water. In the double hulled designs, this will not occur and the danger of very
severe corrosion as a result of the significantly higher temperatures, or in
the worst case cargo fires, is therefore increased.
Some idea as to the level of accelerated
corrosion attack to be expected in the double hulled design, can be obtained
from comparing different areas of existing holds that exhibit this problem.
When warm cargoes are carried, the corrugated bulkheads between holds experience
the same temperature on both sides, often about 40oC. The shell plating has
the sea at a much lower temperature on the outside, typically 5-10oC. The face
plates of the vertical stiffeners on the shell plating are at an intermediate
temperature because they project into the cargo but have a heat sink to the
shell plating. In a double hull design, the shell plating and stiffeners would
remain at a similarly high temperature as the corrugated bulkheads, allowing
an increased rate of corrosion.
Photographs 10 and 11 were
taken in the same hold of a bulk carrier that had only carried one cargo. Photograph
10 shows a section of corrugated bulkhead between two holds, with over 200
defects per square metre that are associated with rapid corrosion. Photograph
11 is of a typical area of shell plating at the same height. It can be seen
that there is an order of magnitude fewer corrosion sites and that the degree
of corrosion is also very much less.
The stiffeners projecting into the
cargo are condition that is intermediate between that of the shell plating and
the corrugated bulkhead. In the double hulled design, the side plating would
be expected to behave in a similar manner to the corrugated bulkheads.
7. Enhanced Corrosion in
Ballast Tanks
The ballast tanks of bulk carriers
are subjected to significantly higher levels of mechanical damage than other
vessels, due to the effects of reverse impact damages. A typical example is
shown in photograph
12. This type of damage is caused both by grabs, bobcats, etc operating
in the holds and by external impacts from tugs and floating debris.
The reduced scantlings on the holds
and the outer hull will have the effect of increasing the severity of this type
of damage in double hulled design vessels. The more severe impact damage in
combination with the “thermos effect” where the cargo remains at
higher temperatures for the duration of the voyage, will cause considerable
problems with maintenance in the ballast tanks of double hulled vessels.
The new designs of double hulled
bulk carriers should enable full access, inspection and maintenance of the ballast
tanks, to a greater extent than is currently available on single hulled designs.
If this does not happen, there is a significant danger that rapid failures of
the coating and steel may be undetected between, or even at, major surveys.
8. Maintenance of Coatings in Double Hull Areas.
The double hulled bulk carrier designs
differ from their tanker counterparts in that there will be a smaller distance
between the outer shell and the inner skin. This distance is expected to be
in the order of 1000mm to 1400mm.
The internal stiffeners on both
the shell and inner bulkhead will take up some 400mm of this small space leaving
approximately 600mm of access space; just enough for a person to squeeze through.
Inspection and repair of the coating in these areas will be extremely difficult
and full refurbishment (using methods such as standard abrasive blasting) will
be virtually impossible.
Poor accessibility, in combination
with difficulties in the initial application and inspection is likely to lead
to a situation where poorly applied coatings are poorly inspected and repaired,
in an unusually corrosive environment.
9. Conclusions.
From the viewpoint of corrosion
and coatings, any proposed legislation mandating double hulled designs for bulk
carriers could be regarded as a retrograde step. Any safety or pollution advantages
to these designs should be judged very carefully against the potential economic
and operational problems likely to be encountered as the result of the more
rapid degradation of the fabric of the hull.
Free initial telephone guidance is available on all the topics covered on this web site.
e-mail enquiries sent to the address below will be promptly answered.
Contact Amtec:- Tel +44 (0)1928 734996 Fax +44 (0)1928 734998
e-mail enquiries@amteccorrosion.co.uk
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