BMTDSL Corrosion Resistant Ship Whitepaper

8/12/2019 BMTDSL Corrosion Resistant Ship Whitepaper

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WHITE PAPER:

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 repairthe 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.


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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


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THE FEASIBIL ITY OF A CORROSION RESISTANT SHIP

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 and application.

Influence of environmental factors.

Material suitability, alone 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.

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INTRODUCTION


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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 worlds 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 ofweapon 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.


THE COST OF MARINE CORROSION


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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.



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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.

CORROSION IN THE MARINE ENVIRONMENT



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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.

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Generic corrosion susceptible areas

Outer hull

Ballast tanks

Fuel tanks

Fresh, grey, black water tanks

Bilges

Pipe work and cooling systems

Holds and storage tanksBoilers and engines

Rudders

Propellers

Bearings

Flanges

Valves

Pumps

Void spaces

Sea chests

StabilizersCorrosion 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.


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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 40C, aqueous corrosion reaction rates can double with every 10C to 20C 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 arenecessary 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.

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Warship specificcorrosion susceptible areas

Flush deck fittings

Guardrail stanchionsLadders

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

Corroded flange and pipe internal


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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 isnt 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 vessels 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 prevalentin the marine environment

Crevice corrosion

Uniform corrosion

Microbiological corrosion

Hyrdogen embrittlement

Pitting corrosion

Erosion corrosion

Galvanic corrosion

High temperature 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

Crevice corrosion


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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 concentrationcell 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 ofcomponents 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 Pitting corrosion


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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 downmolecules 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

hydrogen embrittlement.

Galvanic corrosionGalvanic 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.

Galvanic corrosion between the copper deposits

and the hull


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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 propellers rotational motion may result in a high relative velocityof 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.



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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.

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:

CONCEPT AND DESIGN CONSIDERATIONS

This confined space and shape provided inadequate access for surface preparation and painting

and would always be a location for corrosion and coating breakdown.

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 nave understanding of the

forces at work often lead to the acceleration of corrosion in other areas.


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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 environmentalfactors

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.

Corrosion managementstrategies

Repair of damage during construction.

Cathodic protection via sacrificial anodes and

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.


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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.

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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;

daily heat/cool cycling; proximity to heated pipes or engine rooms.

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,

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

crevices can cause localised corrosion. The presence of contaminants in inert gas, which is introduced into fuel tanks to

prevent explosions, can sometimes accelerate corrosion.

Individual areas of the warship will be exposed to local corrosive conditions, such as

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.


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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 ships 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 theimportance 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.


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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.

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.

SPECIFYING FOR CORROSION PREVENTION


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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.


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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 qualitymanagement 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.

Sacrificial anode in a ballast tank. Impressed current anode during refurbishment.

An impressed current reference electrode.


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twenty two


MANUFACTURE AND CONSTRUCTION

Quality means conformance to requirements, not goodness

do it right the first time. Traditionally, the standard used isacceptable quality level or thats 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

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.

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 curingPartial fitting out

Erection on dock or slipway

Sea trials

Final fitting out

Modifications and repairs

Delivery

Service life

Coating guaranteesPoor surface preparation can result in early failure

of the paint.


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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, surfacecontamination, 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.


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twenty four

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.

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 buildingcontract 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 withquality 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.

The surface quality of raw materials can be

extremely variable.

Surface preparation of welds, cut edges and damagedshop primer needs to be defined carefully.



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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.

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.

Plates can become contaminated with oil and grease.


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WHAT TANGIBLE ACTIONS CAN BE TAKEN

BY PROJECT TEAMS?

Consider corrosion at the concept and design stages taking expert advice ateach stage to avoid major problems. Treat corrosion as with any other risk andmanage it accordingly.Corrosion may be considered under the following three headings:

Structural design

Material selection

Corrosion management

Improve awareness of the importance of avoiding or managing corrosion throughprovision of an education programme for relevant project personnel, such asdesigners, inspectors, officers and crew.

Assess the feasibility of the design in the context of the construction location

all shipyards differ in skills, capabilities and environment.

Review ship design and materials selected / proposed and consider setting up(where feasible) a testing protocol before making final specification or purchasedecisions for items such as materials, coatings and corrosion control systems.

Obtain independent checks on the project through all stages to ensure corrosionsusceptibilities are managed effectively.

Assess conditions in the shipyard before agreeing the paint specification; engagingwith independent coatings specialists will help ensure that paint specificationsare 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 whichthey 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 contaminatedor 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 thatsome of these coatings can determine the service lifetime of critical components andultimately the lifetime of the ship.

Identify parts or surfaces that are vulnerable to corrosion attack and apply preventivemeasures, where possible.

Remember that good surface preparation is essential.

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DO NOT - Assume anti-corrosion measures are maintenance free or will last forthe lifetime of the ship.

DO NOT attempt to reduce costs by accepting unfinished edges or poorquality 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 (includingtemperature, humidity, coating interval times, etc).Reduced standards = reduced performance.

DO NOT assume that all coatings are compatible or interchangeable or will providethe same 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 madeunder more controllable conditions.

DO NOT assume that an Impressed Current Cathodic Protection system is alwayscorrect. Have the data checked periodically by an independent corrosion specialist.

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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, [email protected]

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

Documents

BMTDSL Corrosion Resistant Ship Whitepaper