W H I T E PA P E R :
THE FEASIBILITY OF A
CORROSION RESISTANT SHIP
The effects of corrosion on naval vessels have become more
prominent as the acquisition of new equipment has slowed and
more reliance is placed on the service of aging equipment. Recent
studies in the US indicate corrosion is having an enormous impact
on military costs, representing one of the largest through life cost
components of military systems. These costs include the direct
costs such as the manpower and material that are used to repair
the damage resulting from corrosion and the indirect costs that,
were they to be quantified, would significantly increase the total
reported costs, such as the vessel or systems degraded availability.
Corrosion also poses numerous safety risks and is currently a
source of major concern to platform managers.
WHITE PAPER:
THE FEASIBILITY OF A CORROSION RESISTANT SHIP
Contents
Page
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
The cost of marine corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Corrosion in the marine environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Concept and design considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Specifying for corrosion prevention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
Manufacture and construction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
What tangible actions can be taken by project teams?. . . . . . . . . . . . . . . . . . . . . . . . . . 28
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
INTRODUCTION
This white paper seeks to draw the attention of potential ship owners to design considerations that will
mitigate the risk of unexpected corrosion of vessels and significantly reduce their through life costs.
These include:
• Stress and strain
• Geometry and crevices.
• Substrate surface preparation
• Influence of environmental factors.
• Material suitability, alone
and application.
and combined.
• Awareness and training.
• Corrosion management strategies.
Ship owners and operators recognise intuitively that combating corrosion impacts significantly upon
vessels’ reliability, availability, through life costs and budget availability for replacement projects.
However, until recently, the budgetary stovepiping often demonstrated by defence procurement
organisations in the UK and elsewhere precluded the adoption of a range of spend to save
measures including those related to corrosion avoidance at the design stage of a project.
Additionally, in the absence of a mandated corrosion prevention programme that would guarantee
continuity of initiatives through the procurement cycle, decision makers have often been forced to
trade off corrosion resistance as a cost saving measure when under budgetary pressure. Other
factors such as the short tenure in post of project personnel in comparison to vessel life-times and
the uncertainty in, or indeed lack of, estimates of costs and savings have conspired to drive early
consideration of corrosion prevention off the procurement decision makers’ radar screen.
In addition to the common corrosion prevention and control techniques such as coatings and
cathodic protection we will identify other areas for your consideration that can design-in improved
corrosion resistance. Correcting unanticipated corrosion when the vessel is operational may be very
time consuming and costly.
three
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
THE COST OF MARINE CORROSION
Mitigating unexpected corrosion can be very expensive in terms of direct cost. It also impacts heavily
on platform availability. If a ship and its systems were designed with corrosion resistance built-in, this
would result in less planned and unplanned maintenance and a substantial saving in through life
costs would accrue.
“At US$1.8 trillion, the annual cost of corrosion worldwide
is over 3% of the world’s GDP. Yet, governments and industries
pay little attention to corrosion except in high-risk areas
like aircraft and pipelines.”
George F Hays : Director World Corrosion Organization
This unpredictability of the extent and cost of corrosion can be mitigated by a realisation that
decisions made during ship design establish in-service corrosion properties and consequent through
life corrosion costs. For example, by considering the appropriate choice of materials, fabrication and
assembly processes, coatings and coating application, etc, through life costs can be reduced.
The cost of corrosion is poorly documented. Some operators in sectors such as highways and
pipelines with an acute awareness of public safety have often conducted corrosion cost studies but
the results have little relevance to the design and procurement of ships. There is little evidence that
the cost of corrosion in the marine environment has been the subject of study.
Some rough estimates have been made of the cost of corrosion and are rather intangible, but
they do provide an indication of the magnitude of the costs. The World Corrosion Forum recently
estimated the world wide cost of corrosion to be between 1.3 and 1.4 trillion Euros or almost 2% of
world GDP in 2007 (IMF figures). These figures reflect only the direct cost of corrosion – essentially
materials, equipment, and services involved with repair, maintenance, and replacement.
Improving acquisition practices to ensure that corrosion resistance is designed in ‘up front’ is the
only way to guarantee that a system will have the readiness, mission availability rates and ownership
costs that sustain themselves at predictable values. This is especially important as the design life of
weapon systems continues to climb.
The cost estimates do not include the environmental damage, waste of resources, loss of
production, or personal injury resulting from corrosion and in 2001 a US Department of Defence
study estimated that corrosion cost the department at least $20 billion a year. Empirical evidence
gathered by BMT Defence Services when involved recently in the upkeep of a MoD owned support
vessel showed that coatings alone accounted for 20% of the total upkeep package costs.
four
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Some advocate that corrosion should be viewed as an acquisition risk and as such should
be managed like any other risk by inviting procurers to consider at an early stage a number of
corrosion prevention or reducing measures to mitigate the effects of corrosion and attendant
through life costs.
While it is difficult to project definitively the return on investment resulting from increased attention
to corrosion prevention and control during system design, one can appreciate the range of
potential benefits that will result including improved reliability, reduced maintenance, increased
availability, improved performance and efficiency, improved safety, increased service life,
and reduced life-cycle cost.
five
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
CORROSION IN THE MARINE ENVIRONMENT
It is generally accepted that the marine environment that combines the effects of saline seawater,
salt laden air, rain, dew, condensation, localised high temperature and the corrosive effects of
combustion gases is THE most corrosive of naturally occurring environments.
Corrosion rates
Metals can have very different corrosion rates in different circumstances and combinations.
Some metals only corrode via a pitting mechanism rather than by general corrosion and so it
is not possible to state a typical rate for corrosion of pitting-sensitive metals
• Steels containing less than 8% alloying elements tend to exhibit general corrosion
rates of about 10 microns per year.
• Stainless steels tend to corrode at less than 1 micron per year as a general rate,
however, like aluminium, they tend to pit.
• Copper and its alloys can corrode extremely slowly (less than 0.01 microns per year)
as a general rate.
• Lead can often show a better corrosion resistance than zinc and it is not as sensitive
to local environment changes, however, it is not commonly used on ships as
an anti-corrosion material.
Marine environments
Ocean going ships, including naval warships, travel globally and as such they experience the
extremes of marine environments that have often been noted to accelerate the decline in the
material state of a ship operating, for example, in the Gulf theatre of operations.
six
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Tropical marine environments are far more corrosive than cold European climates because the
temperature has a significant impact on the rate of corrosion. The rate of corrosion of structural
components or weather deck fittings will also relate directly to whether the material is completely
immersed, in what is termed the splash zone or in areas that are not normally immersed.
The external surfaces of a vessel are invariably coated with salt deposits but in other superstructure
locations the severity of the corrosion environment is intensified by high temperatures experienced,
for example, in the vicinity of the up-takes and down-takes associated with the propulsion system.
Equally significant is the corrosion experienced by internal pipe systems, valves and connected
machinery that, when it precipitates component failure, often requires costly restorative work. One
should also not ignore the fact that the salt laden air permeates some of the environments internal
to the ship that have direct access to the weather deck leading to corrosion in these zones as well.
Generic corrosion susceptible areas
Outer hull
Ballast tanks
Fuel tanks
Fresh, grey, black water tanks
Bilges
Pipe work and cooling systems
Holds and storage tanks
Boilers and engines
Rudders
Propellers
Bearings
Flanges
Valves
Pumps
Void spaces
Sea chests
Stabilizers
Corrosion in an inaccessible area
Impact of the corrosive environment
Having a firm understanding of the operational environment is crucial to designing a corrosion
resistant ship or weapon system. It is insufficient to simply have an understanding of the types
of corrosion that may beset a marine structure because, for example, solutions derived solely to
mitigate the effects of the galvanic interaction between different materials may actually exacerbate
corrosion by introducing other more corrosive effects.
seven
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Warships are particularly susceptible to stray current corrosion that originates from speed
controlled motors and weapons systems.
The word environment as used here describes the conditions to which a system may be exposed
while in service. For example, a ship afloat in the ocean is considered to be in a marine
environment, while turbine blades experience a high temperature environment inside a jet
engine during operation. Corrosion reactions can be significantly influenced by temperature.
Low temperatures can reduce corrosion rates and higher temperatures can increase corrosion
rates. Up to 40°C, aqueous corrosion reaction rates can double with every 10°C to 20°C increase
in temperature, depending upon local conditions. However, this generally only occurs during the
initial stages of corrosion. The later stages are usually less sensitive to temperature.
In reality though, things are not quite that simple, because systems experience a variety of
simultaneous environmental conditions. Systems often contain many fluids and chemicals that are
necessary for their components to operate, but some of these can be very corrosive and cause
a material to degrade. For instance, designers must consider cleaning chemicals and hydraulic
fluids as sources of contamination that may cause a material to corrode.
Warship specific
corrosion susceptible areas
Flush deck fittings
Guardrail stanchions
Ladders
Boat davits
Fire main risers and hose connections
Fire hose baskets
Lights
Cable ways
Flight deck safety net fittings
RAS stations
Pipe hangers
Flight deck aircraft tie down points
Machinery bed plates
Screen doors
Lockers
Machinery space bilges
Galley steel decks
HVAC
Bathrooms / showers
eight
Corroded flange and pipe internal
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
There are many other materials and contaminants that exist within the operational environment
that may influence the rate at which a structure or component corrodes.
To mitigate the effects of the environment it is recommended that designers should initially gain
a firm understanding of all the environmental factors that will influence corrosion of the system or
ship before determining the corrosion prevention strategy.
It is important to note that an environment isn’t a single condition, but rather is a combination of
factors that work in concert, such as operating temperature and humidity, salinity, and mechanical
loading. Other contributing influences include chemicals, fuel, pollutants, solar radiation and
biological organisms and even the galvanic signature of the vessel’s berth during fitting out and
subsequent berthing conditions through life.
Types of corrosion prevalent in the marine environment
Corrosion is prevalent throughout a ship and although it tends to manifest itself in a commonly
recognised degradation of the material and attendant staining, often the causal factors differ and
initiate a different type of corrosion. For example, where the structure of the vessel is joined with
fasteners these are often susceptible to galvanic corrosion, pitting, and stress corrosion cracking.
This applies equally to electrical connectors. Without choice of appropriate materials propellers are
also susceptible to corrosion, notably erosion corrosion and galvanic corrosion.
Types of corrosion prevalent
in the marine environment
Crevice corrosion
Uniform corrosion
Microbiological corrosion
Hyrdogen embrittlement
Pitting corrosion
Erosion corrosion
Galvanic corrosion
High temperature corrosion
Crevice corrosion
Stress corrosion cracking
Stress assisted corrosion
Stray current corrosion
Waterline corrosion
Weld corrosion
Coating related corrosion
Corrosion under lagging
Intercooler and heat exchanger corrosion
nine
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
The hull, being constantly exposed to the corrosive seawater environment, experiences uniform
corrosion but it is also likely to experience pitting, galvanic corrosion and other forms. Pitting occurs
when the hull is exposed to stagnant or slow moving water like that found in dockyard basins.
The hull of a vessel may also experience stray current corrosion, which occurs when welding
equipment is incorrectly earthed. Galvanic corrosion may exist between the hull and a
more noble material.
What follows is a brief description of the common forms of corrosion likely to be generated in the
marine environment on a conventionally constructed ship.
Crevice corrosion
Crevice corrosion is a localised form of corrosive attack. Crevice corrosion occurs at narrow openings
or spaces between two metal surfaces or between metals and non metal surfaces. A concentration
cell forms with the crevice being depleted of oxygen. This differential aeration between the crevice
(micro environment) and the external surface (bulk environment) gives the crevice an anodic character.
This can contribute to a highly corrosive condition in the crevice. This type of rapid failure is dangerous
since it may jeopardize the integrity of the ship structure. For obvious reasons, crevice corrosion has a
tendency to occur in components where gaskets, washers, o-rings, fasteners and lap joints are used.
Uniform corrosion
Uniform or general corrosion is typified by the rusting of steel. Other examples of uniform corrosion
are the tarnishing of silver or the green patina associated with the corrosion of copper. The life of
components can be estimated based on relatively simple immersion test results. Allowance for general
corrosion is relatively simple and commonly employed when designing a component for a known
environment. Marine environments cause an amount of corrosion on metal surfaces exposed for
extended periods of time. Uniform or general corrosion usually occurs in stagnant or low flow seawater
at a rate of approximately 10 microns per year on mild and low-alloy steels. Uniform corrosion on
these types of steels is the most common form of corrosive attack on ships.
Uniform corrosion
ten
Pitting corrosion
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Pitting corrosion
Pitting corrosion is a form of extremely localised corrosion that leads to the creation of small holes in
the metal. The driving power for pitting corrosion is the lack of oxygen around a small area. This area
becomes anodic while the area with excess of oxygen becomes cathodic; leading to very localised
galvanic corrosion. The corrosion area tends to burrow into the mass of the metal, with limited diffusion
of ions, further pronouncing the localised lack of oxygen. This kind of corrosion is extremely insidious,
as it causes little loss of material with small effect on its surface, while it damages the deep structures
of the metal. The pits on the surface are often obscured by corrosion products. Pitting may be initiated
by a small surface defect, being a scratch or a local change in composition, or damage to protective
coating. Polished surfaces display higher resistance to pitting, providing the polishing is carried out
correctly. Poor quality polishing may accelerate corrosion. Alloys most susceptible to pitting corrosion
are usually the ones where corrosion resistance is caused by a fascination layer: stainless steels, nickel
alloys, aluminum alloys. Metals that are susceptible to uniform corrosion in turn do not tend to suffer
from pitting, e.g., regular carbon steel will corrode uniformly in sea water, while stainless steel will pit.
Addition of about 2% of molybdenum increases pitting resistance of stainless steels. The presence of
chlorides, e.g. in sea water, significantly aggravates the conditions for formation and growth of the pits
through an auto catalytic process. Stagnant water conditions favour pitting.
Hydrogen embrittlement
Welds are common in ship and submarine
structures but are especially susceptible to
hydrogen embrittlement. The high temperature
environment caused by welding may break down
molecules such as hydrocarbons and produce
hydrogen (atomic or molecular), which can then
diffuse into the metal and initiate embrittlement.
Thus proper cleaning of the metal surfaces
before welding to remove handprints grease;
paint or solvents will reduce the potential for
hydrogen contamination and ultimately
Galvanic corrosion between the copper deposits
and the hull
hydrogen embrittlement.
Galvanic corrosion
Galvanic corrosion is an electro-chemical process in which one metal corrodes preferentially when
it is in contact with a different type of metal and both metals are in an electrolyte. When two or more
different metals come into contact in the presence of an electrolyte a galvanic couple is set up as
different metals have different electrode potentials. The electrolyte provides a means for ion migration
whereby metallic ions can move from the anode to the cathode. This leads to the anodic metal
corroding more quickly than it otherwise would. The presence of electrolyte and a conducting
path between the metals may cause corrosion where otherwise neither metal alone would have
corroded. Even a single type of metal may corrode galvanically if the surface varies in composition,
forming a galvanic cell.
eleven
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Microbiological corrosion
Microbial corrosion, or bacterial corrosion, is corrosion caused or promoted by micro-organisms
and can apply to both metals and non-metallic materials. The phenomenon is often referred to as
Microbiologically Influenced Corrosion or MIC. A range of bacteria react uniquely in the presence of
materials producing corrosive chemicals and other reactions with adverse affects such as oxidisation
of the material, damage to the protective coatings, a reduction in the efficiency of the cathodic
protection system, production of harmful environments such as H2S, or increase drag and therefore
stress, thus increasing the propensity for stress corrosion cracking. MIC is also known to accelerate
corrosion of stainless steel (e.g. 304L, 316L, AL-6XN), nickel alloy (e.g. alloy 400) and copper alloy
(e.g. 90-10 cupro-nickel) weldments.
Erosion corrosion
Erosion corrosion is a degradation of material surface due to mechanical action, often by impinging
liquid; abrasion by particles suspended in fast flowing liquid or gas; bubbles or droplets; cavitation,
etc. Metal corrosion generally increases with increasing seawater (relative) velocity until it reaches a
critical velocity where the deterioration is much more rapid. Typically, erosion corrosion is greater with
metals that are exposed to seawater with higher salinity than to those that are in a brackish (lower
salinity) or fresh water environment; thus, erosion corrosion varies with salinity. A more specific form
of erosion corrosion that typically occurs on the propellers of ships and submarines is caused by
cavitation. The formation and immediate collapse of vapour bubbles (cavitation) repeatedly hitting a
particular location will often result in surface damage on the propeller. Cavitation may enhance the
erosive capability of the seawater that is moving, due to the extreme fluid phenomena that occurs at
and near the surface of the blade. A propeller’s rotational motion may result in a high relative velocity
of the seawater moving over the propeller blades, which causes cavitation to occur.
This specific form of corrosion may also occur in other components that are in contact with water that
cavitates. A key to preventing a significant amount of erosion corrosion is designing the component
or system to minimize turbulence and cavitation.
High temperature corrosion
High temperature or hot corrosion can occur in ships, primarily in the engine components, for
example, gas turbine engines. The turbine blades made of nickel and cobalt based super alloys have
been known to experience this accelerated form of corrosive attack and severe material deterioration.
Temperature is a significant environmental factor affecting cracking. For stress corrosion cracking to
occur three conditions must be met simultaneously. The component needs to be in a particular crack
promoting environment, the component must be made of a susceptible material, and there must
be tensile stresses above some minimum threshold value. An externally applied load is not required
as the tensile stresses may be due to residual stresses in the material. The threshold stresses are
commonly below the yield stress of the material.
twelve
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Stress corrosion cracking
Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material,
and tensile stress. Stress corrosion cracking is an insidious type of failure as it may occur without
an externally applied load or at loads significantly below yield stress. Thus, catastrophic failure may
occur without significant deformation or obvious deterioration of the component. Pitting is commonly
associated with stress corrosion cracking phenomena. Aluminium and stainless steel are well known
for stress corrosion cracking problems. However, all metals are susceptible to stress corrosion
cracking in the right environment.
Corrosion is a major through life cost that can be minimised; however, it does require a deep
specialist understanding to ensure that an accurate prediction is made of the full range of likely
processes and that the prevention techniques are both effective and complementary.
Corrosion mitigation measures conceived from a basic or naïve understanding of the
forces at work often lead to the acceleration of corrosion in other areas.
CONCEPT AND DESIGN CONSIDERATIONS
Corrosion engineering should not be seen simply as a reactive discipline, or one that is brought
to bear after the system shows the effects of corrosion; it should figure at the concept and design
stage of a project when decisions are made that will have a significant impact upon the structures’
ability to avoid corrosion and its attendant costs.
This confined space and shape provided inadequate access for surface preparation and painting
and would always be a location for corrosion and coating breakdown.
It is recommended that the following factors be considered at the concept, design, construction
and in-service stages of a project. All these factors can have an impact on the propensity of a
structure or system to corrode:
thirteen
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Design Considerations
Stress and strain
•Residual stress from cutting, welding and fit up.
•Strains induced during construction and service.
•Cyclic processes.
•Corrosion enhanced stress and strain.
•Fretting, wear, vibration and erosion.
Geometry and crevices
•All joints need care in design and maintenance.
•Flange crevices should be avoided.
•Design to avoid liquid under lagging.
•Awareness of possible construction and maintenance
issues.
Preparation and application
•Good surface preparation is essential.
•Contamination must be removed or managed.
•Anti-corrosion measures must be applied or installed
as specified, without unauthorised changes.
•Curing times must be observed.
Influence of environmental
factors
•Temperature, humidity and oxygen.
•Liquids, e.g. sea water, fuel, chemicals.
•Gases, e.g. H2S, CO2, NH4.
•Ionic contamination sources.
•Soot, oil, grease.
Material suitability
•Must be able to withstand the environment.
•Must be compatible with adjacent materials.
•If above are not possible, then management strategies
must be considered and implemented.
Awareness and training
•Corrosion awareness for designers and specifiers.
•Training for site teams during ship building.
•Training for Officers and crew for maintenance and
repair during service conditions.
repair
during service conditions.
•
•Repair of damage during construction.
•Cathodic protection via sacrificial anodes and
Corrosion management
strategies
fourteen
Impressed Current Cathodic Protection (ICCP) systems.
Coatings
– correct selection for construction and
•
maintenance phases of ship life.
•Inhibitors: Vapour Phase Inhibitors (VPI); boilers; in paint.
•Regular inspection and repair.
•Planned and emergency maintenance.
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Stress and Strain
There are many forces and circumstances that introduce stresses and strains into a warship.
The initial stresses can be introduced at the time that the plates from which the ship is
constructed are rolled. Each successive step of cutting, forming and welding can introduce
further stresses and strains.
In some areas these are cumulative and both good design and good construction practice and
inspection can help to minimise the effects. Once the ship has been constructed, the initial
“shake down” period will allow some stress relief to occur.
Stress corrosion and stress corrosion cracking will accelerate the rate of metal loss, particularly
at sensitive and often at critical areas. Highly stressed areas tend to corrode more readily than
non-stressed areas, so the corrosion is focussed on the areas of stress. As the metal becomes
thinner due to corrosion, the local stresses and strains increase and accelerate the process.
Cyclic processes, and those such as vibration, fretting and wear, continually expose a fresh
metal surface to the environment, preventing the formation of a passive film on the surface and
allowing corrosion to occur. These processes can also prevent organic coatings from performing
satisfactorily by causing paints to crack.
Geometry and Crevices
Geometry is important because it can contribute to increased or decreased stresses and strains.
Crevices that are due to inadequate design will be extremely difficult to prepare and paint.
Crevices that are formed between mating surfaces, such as flanges, must be eliminated whenever
possible, as crevice corrosion can cause localised pitting and failure of the component.
Again, good design is important and good maintenance is necessary to ensure that crevices
are eliminated.
Surface preparation and application Issues
Good surface preparation is essential for the long lifetime of any system. It is essential that this
is understood by all people involved in the design, construction, operation and maintenance
of a warship.
Edges and welds should be carefully prepared and inspected before painting to ensure that all
contamination is removed and that sharp edges or rough surfaces are rounded. Additional coats
of paint on the edges and welds (stripe coats) are best applied onto a dry surface and allowed
to dry themselves, before the next coat is applied.
fifteen
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
The application of paint systems must take place within the boundaries specified by the paint
manufacturers, or the performance and lifetime of the paint could be substantially reduced.
The conditions under which paint is applied and cured can be crucial, particularly with regard
to temperature, relative humidity and dew point.
The time between coats is also important for some types of paint and, again, these times should
be carefully followed to ensure optimum performance. While it may seem to be expedient to
change these times to comply with an overall timetable, it should be remembered that this could
lead to significantly increased maintenance time and costs in the future.
Influence of environmental factors
There are a wide range of possible factors that a warship can encounter during the construction
process and during its lifetime. These include:
• Sea states, grounding, wind effects and ultra-violet radiation.
• Temperature has a major influence in many areas including: cutting and welding;
• Impacts with jetties, tugs and other objects can distort steel and remove paint.
• Storage and handling of aqueous liquids in ships storage tanks such as: ballast,
prevent explosions, can sometimes accelerate corrosion.
• Individual areas of the warship will be exposed to local corrosive conditions, such as
crevices can cause localised corrosion.
• The presence of contaminants in inert gas, which is introduced into fuel tanks to
fresh, grey and black waters.
• Storage and handling of fuels, chemicals, armaments, etc.
• The presence of contaminants (such as salts, oils, soot, etc) under paint or in
daily heat/cool cycling; proximity to heated pipes or engine rooms.
the water tanks, chimneys, bathrooms and heads, bilges, etc.
• Flow rates through pipes and valves can induce erosion or cavitation effects which
may can be accelerated by corrosion processes.
All environmental factors, both alone and in combination will affect the rate of corrosion and the
type of corrosion products formed. They affect the integrity and performance of the coatings and
the effectiveness of other anti-corrosion strategies.
Material suitability
It is important to consider that metals are often used in combination with other materials such as
plastic, rubber and wood. Different materials my also be used in combination, such as pipe
work and valves.
sixteen
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Compatibility of metals, to ensure that galvanic corrosion does not cause one metal to dissolve,
is important, particularly at the design and operating stages of the ship’s life.
The correct choice and use of welding consumable is essential to ensure that the weld does not
become the focus of localised galvanic corrosion due to differences between the compositions
of the parent and weld metals.
The addition of coatings, seals and insulators should be carefully considered, as these may
introduce possible additional corrosion problems such as crevices. In some instances, corrosion
may only be managed to an acceptable rate, rather than prevented.
Awareness and Training
In many cases, corrosion occurs because the people involved are not sufficiently aware of the
importance and causes of corrosion and cannot therefore take the relevant factors into
consideration at the design stage. Similarly, the crew are often unaware the lifetime of the ship
can be enhanced and repairs made more effective by good practices with regard to corrosion.
Courses and training to upgrade the knowledge of the personnel involved in the design,
construction and operation of a warship can have many benefits and prevent unnecessary
work and re-work being performed.
Corrosion management strategies
In some instances, it is not possible to prevent corrosion either by design or material selection and
so management of the corrosion rate and its process must be considered. Depending upon the
structure to be protected and its operating environment, there is a wide range of anti-corrosion
strategies that may be used. It is important to ensure that the selected option is feasible and is
available at the construction location. Corrosion prevention and control methods can include
adding additional layers of protection such as paint or galvanising, the use of cathodic
protection as sacrificial anodes or as an Impressed Current Cathodic Protection (ICCP) system.
The effectiveness of each method will depend upon the local conditions and on the method itself.
Cathodic protection systems that use sacrificial anodes to prevent the corrosion of the outer hull
or salt water tanks will be effective only when the sacrificial anodes and the structure are both
under water. Once the sacrificial anodes are consumed, the protection will cease until the
sacrificial anodes are replaced.
seventeen
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Impressed current, i.e. ICCP, systems are used on the outer hull only, as they can generate
hydrogen if they malfunction. Regular checks on ICCP performance are essential to ensure that
they are providing the required level of protection for the steel. Too great a protection level will
protect the steel but can also cause paint to be removed.
Strategies for the inspection, maintenance and repair of the parts that can corrode and their
protections systems, should be planned and implemented. Permanent means of access,
such as ladders, should also be carefully maintained as they are crucial to the protection regime.
The protection of spaces that will be sealed for long periods of time, can be achieved using
vapour phase inhibitors. Larger spaces may be protected by dehumidification. This will allow
emergency access by people, if necessary. Many other possible protection methods
(such as inert gas) may require long purging periods before they are safe to enter.
The choice of suitable corrosion resistant materials is often influenced by both cost and workability.
For example, higher grades of stainless steel may perform ideally under marine conditions;
but they can be very expensive and in many cases, extremely difficult to machine or weld.
This does not mean that it is not possible to select the best materials for the environment.
Careful choices made from from a basis of knowledge can allow the right materials, or at least
the best compromises, to be selected.
specifying for corrosion prevention
Organic coatings
In determining which materials to specify, it is
important to obtain as much relevant data as
possible from other vessels and structures,
particularly with regard to failures and whether
any successful replacements have been
established. Several sources of information are
available, although many companies prefer not
to publish detailed data on corrosion failures,
for commercial reasons.
If time and budgets allow, the most favoured option would be to assess the potential corrosive
environment and then screen available materials via a controlled test programme designed
by a corrosion engineer. Ideally, this would include laboratory pre-selection followed by
service environment trials.
eighteen
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
The influence of other components in the environment would need to be assessed by the corrosion
engineer before final material selection. When selecting coatings, for example, data will be available
from paint manufacturers on laboratory testing and from vessels and structures in service. Inspections
and non-destructive tests can also be carried out on the paint in service, to investigate its performance
and assess its long term suitability. A structured inspection of existing vessels can provide valuable
information on coating performance and aid material selection for repair and new construction.
Organic paint coatings are generally the most common form of protection applied to marine
structures. Organic coatings are applied to ships and marine structures to protect against corrosion
in terms of metal loss or component failure due to corrosion. They are also applied to improve the
cosmetic appearance in a positive manner in terms of adding colour, camouflage, gloss and other
such desirable effects as radar absorption. Organic coatings also guard against undesirable
cosmetic effects such as rust staining, mechanical damage and deterioration due to weathering.
Most of the costs associated with the application of coatings and their repair are driven by the
requirement that warships present a smart, well turned out appearance. Coatings are available in
many specialist types for particular situations, such as anti-fouling paint for underwater hulls,
anti-corrosive paint for tanks and pipes, decorative paint for accommodation areas, etc. It should
be remembered that all paint systems will suffer from degradation with time and ultimately
will need replacing.
The major factors to consider with organic coatings are:
• Select the correct surface preparation and coating specification.
• Confirm the products have a track record of application under the expected service
conditions (making allowance for more extreme conditions or circumstances).
• Check the shipyard and contractors are capable of meeting the specifications.
• Ensure compatibility between all products, systems and materials. Discuss with
relevant supplier companies and experts at an early stage.
• Ensure that the products can be applied at the selected shipyard: take account of
Health & Safety regulations, solvent emissions, worker training, etc.
• Ensure the coatings can be maintained by the crew and do not need specialised
equipment or conditions or experience to apply.
Metallic coatings
Metallic coatings come a close second to organic coatings in the arsenal of anti-corrosive
measures. The most common metallic coating is galvanising using zinc. This is a form of cathodic
protection using the zinc coating as both a barrier in the same way as an organic coating and
also as a favourable galvanic couple. Galvanised items can also be coated with organic coatings
to further increase the service lifetime but great care has to be taken with surface preparation
otherwise early de-lamination of the organic coating may occur.
nineteen
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Most commonly, galvanising is carried out by immersing the items in a bath of molten zinc.
Zinc coating thicknesses of up to 120μm can result if this process is carried out correctly.
Galvanised items used on warships may have a service lifetime in excess of 10 to 15 years
before the zinc loses its protective properties.
Painting the galvanised items may extend
further the lifetime of the zinc coating.
Smaller items or sheet steel can be
galvanised using electro-deposition processes.
When special anti-corrosive properties
are required from a surface, specialist
processes such as flame spraying may
be an economic choice over more
corrosion resistant alloys.
Preparing pipes for galvanising
Cathodic protection
Sacrificial cathodic protection uses the principle of galvanic corrosion to provide protection to the
chosen structure by the dissolution of another metal. Zinc or aluminium anodes are usually used
to protect steel structures, while zinc anodes may be used to protect aluminium hulls. Sacrificial
anodes may be used in tanks and holds and on exterior surfaces such as hulls and jetties.
The use of anodes to provide passive cathodic protection is very common as a backup to ICCP
system on the outer hull. Anodes need to be carefully sited around the steering gear and the
propellers to give a good current distribution and to work in harmony with the coating system.
Passive cathodic protection systems are very useful in cargo holds, cargo tanks, ballast tanks and
in areas where galvanic couples can occur and cause high corrosion rates. Anodes are commonly
used in a mixed metal situation in heat exchangers and inter-coolers associated with propulsion
and air-conditioning systems.
ICCP systems use inert (non-dissolving) anodes together with reference electrodes as part of a
feedback system. This allows manual or automatic control over the protection provided.
These systems are used on exterior surfaces only as they generate gases that could be dangerous
in confined spaces. It is important that the cathodic protection system works together with a
coating scheme and does not cause paint breakdown. Careful choices and balances of both
elements are required for optimum performance of both systems.
twenty
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Sacrificial anode in a ballast tank.
Impressed current anode during refurbishment.
In military applications cathodic protection systems on the outer hull may lead to undesirable
electrical fields in the water surrounding the vessel increasing its susceptibility to detection by
others. Special cathodic protection systems are designed that have a number of external anodes
distributed along a length of the hull. They are also used in conjunction with electronic systems
that reduce the electro-chemical field produced by the turning propeller shafts and the propellers
themselves. The amount of current drawn from such systems depends on the quality of the
coating on the hull and its through life integrity.
Should the integrity of the coating degrade then
the amount of current taken from the ICCP
system will increase, raising the likelihood of the
vessel being detected. Good coating quality
management at the outset is the best way to
reduce this risk. When designing out corrosion
at the new building and procurement stage,
very careful consideration should be given to
balancing the needs of cathodic protection
systems and organic coatings.
An impressed current reference electrode.
twenty one
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
MANUFACTURE AND CONSTRUCTION
“Quality means conformance to requirements, not goodness…
‘do it right the first time’. Traditionally, the standard used is
‘acceptable quality level’ or ‘that’s close enough’.
These are a commitment to errors.”
Philip B Crosby, Quality: the changing of minds, 1986
Before construction begins, it is essential that all parties involved agree on the methods, conditions
and time scales involved in construction. Adequate inspection by trained personnel and good
record keeping are also necessary, in case of future disputes.
Construction processes
Incoming raw material checks
Surface preparation standards
QC procedures
Shop primer line
Sub assembly
Block assembly
Weld and edge preparation
Coating application and curing
Partial fitting out
Erection on dock or slipway
Sea trials
Final fitting out
Modifications and repairs
Delivery
Service life
Poor surface preparation can result in early failure
of the paint.
Coating guarantees
Construction processes
During the construction of a ship, many or all of the processes listed above are involved. Each
process presents its own corrosion challenge both during construction and once in service.
When considering coatings as an anti-corrosive strategy for new warships, it is extremely
important to operate a “get it right first time” policy.
twenty two
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Conditions far more favourable to the
application of coatings exist at the new
construction stage than as part of a
maintenance and repair procedure. This is
because the surface cleanliness, surface
contamination, surface profile and the initial
steel quality of the substrate surfaces have a
greater impact upon the long term integrity of
the coating than the quality of the coating itself.
Good quality coatings, such as epoxies and
polyurethanes, offer excellent protection for a
considerable number of years when applied on
a high quality surface. However, if the same
coatings are applied on to poorly prepared substrates then coating failure due to blistering and
delamination may occur within a few months of the vessel entering service.
Throughout the building of the vessel, the conditions under which the coatings are applied are
crucial in determining the service life and cost effectiveness of the coatings scheme. Weather
conditions such as fog, rain and other high humidity conditions may lead to coating delamination
between coats. Low temperatures during coating application may lead to poor adhesion between
coats and airborne contamination may lead to inter-coat blistering.
Welding and other forms of rework during the construction process may lead to coating damage
especially with warships that have an extensive fitting out period.
Clear planning of this period in order to minimise all forms of coating damage and corrosion
damage due to exposure to the elements and the effects of mechanical abrasion is essential to
give good through life performance without inconvenient and expensive repairs being necessary
at a later stage.
Good planning of a coatings maintenance procedure starts at the design stage as it is often areas
that are physically difficult to access that tend to break down first. It is often far more economic
to apply a very good coating on to a well-prepared surface when that surface is easily accessible
than to try to effect repairs when the surface has been covered by insulation, wiring or pipe work.
Void spaces and other inaccessible areas should be coated for life.
Choosing mechanically strong or abrasion resistant coatings for areas subject to wear and impact
damage should be considered for all surfaces that will be in contact with personnel, mooring ropes
and in contact with jetties and tugs.
twenty three
THE FEASI B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Incoming raw material
Often corrosion problems arise with the new materials supplied to the shipyard. Many materials
pick up surface contaminants during their manufacture and subsequent transportation. Care
should be taken to remove surface contamination prior to installation or preparation and painting.
Stainless steel components will often need
thoroughly cleaning and passivating prior to
installation otherwise severe surface pitting
can occur.
The surface quality of raw materials can be
extremely variable.
Copper and copper based alloys commonly
arrive with carbon films on the surface. These
can act as good cathodes and may lead
to high corrosion rates. This is particularly
common with pipe work, valves and pumps.
Steelwork may arrive in a heavily pitted
condition that will cause subsequent problems
during blasting and painting operations.
Surface preparation standards
Surface preparation standards and other
material cleanliness issues should be
addressed at the time that the ship building
contract is placed. Clear, unambiguous
standards on how surface preparations are
to be applied should be agreed before
construction starts, to avoid disputes later.
Quality control procedures
Quality control procedures should be in place
for all processes that may affect either the
surface quality or the anti-corrosive properties
of all the materials in service. Together with
quality control procedures, the procedures
should be in place for rectification or
replacement of components that have become
damaged in such a way that their anti-corrosive
service lifetime has been compromised.
twenty four
Surface preparation of welds, cut edges and damaged
shop primer needs to be defined carefully.
T H E F E A S I B I L I T Y O F A C O R R O S I O N R E S I S TA N T S H I P
Shop primer line
The shop primer line is often referred to as primary surface preparation. It is at this point that the
quality of the coatings applied to the block sections is set.
If material contaminated with oil or grease passes through the shop primer line without
contaminants being removed properly, the coatings applied on top will be severely compromised.
Close attention should be applied to inspection of the plates as they emerge from the automated
blasting process and also to the thickness and quality of the shop primer applied.
Sub-assembly stage
At the sub-assembly stage the hull
components are coated with shop primer,
which may then become contaminated or
damaged. Welds and edges need careful
attention with regard to smoothness and lack
of porosity or other irregularities. Often the
welding and cutting processes may introduce
surface contamination in the form of oils,
weld fume or footprints. Overhead cranes
and hand held air tools are common sources
of oil contamination.
Plates can become contaminated with oil and grease.
Coating application and curing
Coating application and curing processes are extremely sensitive to both temperature and
atmospheric moisture variations so, even if the coatings are properly applied to a well prepared
surface, major problems may still be encountered in the presence of poor atmospheric conditions.
Low temperatures at any time during the coating curing process may inhibit the coating
cross-linking and result in early failures. Low temperatures may also result in condensation on
the surface. This condensate or other sources of high humidity may result in the curing agents
malfunctioning in the coating and reacting with the atmospheric moisture instead. This may
result in inter-coat adhesion failures.
Erection stage and fitting out
The individual blocks that make up the structure are welded together at the erection stage of
construction. Substantial coating damage invariably occurs at this stage due to damage from
scaffolding and general wear and tear from the workers welding the blocks together.
twenty five
These block join up welds will need special
attention and extra inspection as they are
hard to execute and to paint. Often the
work is carried out under poor conditions of
illumination and cleanliness.
Warships, in common with other vessels
invariably undergo modifications and
rectification as the final commissioning process
and fitting out stages are completed. These
processes may often have an adverse effect
on the subsequent corrosion performance of
the items in question. Minimising the amount
of damage to metallic and organic coatings
at this stage needs careful planning and
implementation. Good repair procedures,
that are properly carried out, are essential to
ensure that the coating guarantee is
not compromised.
Poor housekeeping may result in blasting grit becoming
embedded in coatings.
Service life
Very careful inspections should be carried out within the first year of service as corrosion problems
that result from the vessel building and commissioning processes may often show up relatively
early in the service lifetime of the vessel.
Coatings problems such as delamination and blistering often show up within a few months of the
items coming into contact with water. Damaged areas of coatings, together with cracks showing
through coatings may often be observed easily because of the rust staining coming through
defects. Any claims on coating guarantees are much easier to progress if they are the result of
early inspections.
twenty six
twenty seven
WHAT TANGIBLE ACTIONS CAN BE TAKEN
BY PROJECT TEAMS?
3
Consider corrosion at the concept and design stages taking expert advice at
each stage to avoid major problems. Treat corrosion as with any other risk and
manage it accordingly.
Corrosion may be considered under the following three headings:
• Structural design
• Material selection
• Corrosion management
3
Improve awareness of the importance of avoiding or managing corrosion through
provision of an education programme for relevant project personnel, such as
designers, inspectors, officers and crew.
3
3
Assess the feasibility of the design in the context of the construction location –
all shipyards differ in skills, capabilities and environment.
3
3
Obtain independent checks on the project through all stages to ensure corrosion
susceptibilities are managed effectively.
3
3
3
3
3
3
3
3
3
Review ship design and materials selected / proposed and consider setting up
(where feasible) a testing protocol before making final specification or purchase
decisions for items such as materials, coatings and corrosion control systems.
Assess conditions in the shipyard before agreeing the paint specification; engaging
with independent coatings specialists will help ensure that paint specifications
are optimised and agreed before work commences.
Design vessels to incorporate features that facilitate maintenance and inspection.
Design vessels to withstand the extreme corrosion inducing conditions in which
they may be required to operate.
Avoid high humidity and provide adequate ventilation whenever possible.
Put in place practices to manage the incidences of steel becoming contaminated
or surfaces damaged during the manufacturing process.
Use good design to minimise stress and strain, especially at sensitive locations.
Include vapour phase inhibitors or dry air in sealed spaces for long term protection.
Apply suitable coatings: organic, inorganic or metallic, as necessary. Be aware that
some of these coatings can determine the service lifetime of critical components and
ultimately the lifetime of the ship.
Identify parts or surfaces that are vulnerable to corrosion attack and apply preventive
measures, where possible.
Remember that good surface preparation is essential.
twenty eight
7
7
7
7
DO NOT - Assume anti-corrosion measures are maintenance free or will last for
the lifetime of the ship.
7
7
7
7
DO NOT assume that all coatings are compatible or interchangeable or will provide
the same performance.
DO NOT attempt to reduce costs by accepting unfinished edges or poor
quality blasting.
DO NOT minimise the inspection processes during ship construction and fit out.
Corrosion initiating at these times can determine the service lifetime.
DO NOT allow any variations from the paint manufacturers’ specifications (including
temperature, humidity, coating interval times, etc).
Reduced standards = reduced performance.
DO NOT forget that corrosion combined with stress can cause accelerated rates
of metal loss.
DO NOT expect paint repairs carried out at sea to perform as well as those made
under more controllable conditions.
DO NOT assume that an Impressed Current Cathodic Protection system is always
correct. Have the data checked periodically by an independent corrosion specialist.
twenty nine
About the PublisheRS
BMT Defence Services is the leading independent centre of excellence for naval design and
through life support in Europe. Based in Bath and Weymouth, the company has platform design
expertise in surface warships, submarines and auxiliaries. A wide range of government and
industry customers rely on BMT Defence Services for its systems engineering and information
systems expertise. Over 200 naval architects, marine engineers, engineering consultants and
support staff are continually engaged in the development of technically complex, highly
integrated systems.
Web site: www.bmtdsl.co.uk
Contact: Tim Marchant, tmarchant@bmtdsl.co.uk
The company is a wholly owned subsidiary of BMT Group Ltd, the assets of which are vested in
an Employee Benefit Trust. This ensures that all BMT companies are independent of equipment
manufacturing or shipbuilding interests and thus able to offer truly impartial design and
engineering advice.
Amtec Consultants Ltd is an independent corrosion, coating and cathodic protection consultancy,
specialising in all aspects of vessel construction from design, through building, to service and
major repairs in later life. Amtec Consultants also investigates failures, handles claims, manages
joint research projects and undertakes impartial coating testing. Amtec operates on a global basis
and provides short notice response for owners, P&I clubs, charterers, law firms and others.
Web site: www.amteccorrosion.co.uk
Contacts: Dr Jane Lomas and Dr Les Callow
COPYRIGHTS AND ACKNOWLEDGEMENTS
© BMT Defence Services Limited 2009. © Amtec Consultants Ltd 2009.
All trade and service marks are acknowledged as the intellectual property of their respective owners.
Where not otherwise indicated, all images are the property of Amtec or BMT Defence Services
and may not be reproduced without prior permission
thirty