GB2526822A

GB2526822A - Heat exchanger

Info

Publication number: GB2526822A
Application number: GB1409864.4A
Authority: GB
Inventors: Sven Morten Hesjevik
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2014-06-03
Filing date: 2014-06-03
Publication date: 2015-12-09
Also published as: GB201409864D0

Abstract

A heat exchanger for use in a subsea environment comprises a vessel bound by a steel wall having an inner surface and an outer surface. In use the inner surface is in contact with a fluid wherein heat is transferred across the steel wall from the inner surface to the outer surface. The outer surface is in contact with sea water. The outer surface is coated with an aluminium coating layer and is provided with a sacrificial anode. The aluminum coating acts as a corrosion protector and the anode has an electrode potential slightly lower than the coating so that the difference in PH generated during oxidation at the anode is small and calcerous deposits are not generated on the aluminium coating. The alumiium coating may be applied by thermal spraying. The coating may be porous and sealed with a siloxane sealant. The heat exchanger may be an electrical component such as variable speed drive, transformer, capacitor, or switch gear requiring cooling and comprises an outer metallic surface with the aluminium coating and anode.

Description

Heat Exchanger This invention relates to corrosion protection on heat exchangers located under seawater or at least. heat exchangers repeatedly sit bj ect to contact with sea water and hence at risk of corrosion. More particularly, the invention relates to the coating of a heat exchanging surthee, typically made of steel, with a layer of thermally sprayed aluminium and provided with a sacrificial anode in order to simultaneously prevent corrosion without detrimentally affecting the heat exchanging capabilities of the heat exchanging surFace.

Background

There is an increasing interest within oil and gas production in placing most of the production on the seabed. There is therefore the need for subsea heat exchangers. Heat from within a wide variety of subsea systems needs to be transferred by a heat exchanger into the sea.

Au ideal material for use in a heat exchanger is obviously a metal, in particular a steel surface. Steel readily absorbs heat within a system, readily transfers the heat across the steel and vents the heat into water on the outside of the system. Steel however, is subject to corrosion in seawater and must he protected from corrosion.

Normally. subsea steel structures are protected from corrosion using cathodic protection. The geometry of heat exchangers can be quite complex however and Lhese can be characterised by large surface area and narrow spacing in between surFaces. Ihis makes it difficult to provide sufficient cathodic protection and a coating is often needed.

To ensure corrosion protection of the outer surface of heat exchangers therefore, the outer steel surface may also be coated. A conventional coating is organic, in particular polymeric, and is applied to the steel surface before the heat exchanger is placed in situ. The presence however of an organic coating on the outcr steel surfacc in contact with the water limits its ability to transfer heat. In effect, the outer steel siirthce acting as the heat exchanger is coated with an insulator. These subsea heat exchangers rely on the cold sea waler absorbing the heat from within the exchanger and that ability is hindered if the outer surflice of the heat exchanger is coated with an insulating layer. It would be desirable therefore to avoid the need for this organic coating on the outer sur1ce of the steel. The organic coating is also costly so avoiding its use has both thnctional and economic benefit.

Cathodic protection is also a well-known tecimique to control the corrosion of a meia.l surface by making the metal surface the cathode of an electrochemical cell. A simple method of protection connects the protected metal, typically steel, to a more easily corroded "sacrificial metal' to act as the anode. The sacrificial metal then corrodes instead of the protected metal. As the a.node corrodes, it supplies electrons to the metal being protected thus preventing an anode reaction at the protected metal surface.

Cathodic proteclion systems protect a wide range of metallic structures in various environments, in particular in subsea environments.

Wherever cathodic protection is used subsea however, a problem encountered is the fonsiation of a calcareous coating or the formation of calcareous deposits on the protected surface. These deposits are mixtures of calcium carbonate, masnesium carbonate and magnesium hydroxide that are deposited on cathodically protected surfaces because of the increased pH adjacent to the surfhce. An increase in hydroxyl ions as a result 0f cathodic protection (either through oxygen reduction or hydrogen evolution) lea ds to a shift in the hydrogen carbonate/carbonate ratio increasing formation of carbonates. The result is the formation of metal carbonates and hydroxides on the cathode, so called calcareous deposits.

Even with a bare. metal sled surface exposed to sea water the cathodic protection current will increase the pH so that a calcaitous layer will precipitate.

The presence of calcareous deposits is not always a problem however as the calcareous deposits act as on oxygen barrier. The deposit layer therefore acts as a corrosion inhibitor however it also acts to prevent heat transfer again in the context of a heat exchanger. The carbonates and hydroxides are poor conductors of heat meaning that the ability of a surface to exchange heat is compromised when calcarcous deposits arc present.

Thus, both an organic coaling and calcareous deposits will lirniiheat ransfer and make a heat exchanger less elTective This invention relates to new ways of protecting heat exchanging surfaces against corrosion without recourse to an organic coating and such that the sw-lace is ideally not subject to calcareous deposits. The present inventors have determined that thermally sprayed aluminium, provided with a suitable sacrificial anode offers a possible solution to this problem Rather than use an organic coating therefore, the inventors propose applying an Al coating onto the heat exchanging surface. The Al coating acts as a corrosion protector but is also metallic and hence an excellent lheniial conduclor. That Al layer is then provided with a sacrificial anode which preferably has an electrode potential only slightly lower than that of the Al coating layer. In this way, the diference in pH generated during oxidation at the anode is so small that calcareous deposits are not generated and hence the Al surface can be kept free of calcareous deposits. The result therefore is a corrosion protected surface that is ideally free of insulating calcareous deposits and free of organic layers and which therefore acts most efficiently as a heat exchanger.

The use of thermally sprayed aluminium with cathodic protection as corrosion protection for steel in sea waler is described in the Masters thesis of Solveig Egtvedt, July 201 i. NTNU, Dept Materials and Science Engineering.

Summary of Invention

Thus, viewed from one aspect the invention provides a heat exchanger for use in a subse-a environment comprising a vessel bound by a steel wall having an inner surface and an outer surface, when in use said inner surface being in contact with a fluid wherein heat is transferred across said steel wall from said inner surface to sa.id outer surface, said outer surface being in contact with sca. water; wlicrein said outer surface is coated with an aluminium coating layer and said aluminium coating layer is provided with a sacrificial anode.

Viewed from another aspect the invention provides a heat exchanger for use in a subsca environment comprising a vessel bound by a steel svall having an inner surlace and an ouler surface, said inner surFace being in contact wilh a fluid wherein heat is transferred across said steel wall from said inner surface Lo said outer surfhce.

said outer surface being coated with an aluminium coating layer and said aluntiniwn coating layer being provided with a sacrificial anode.

Viewed from another aspect the invention provides a heat exchanger for use in a subsea environment comprising a vessel hound by a steel wall having an inner surface and an ouler surface, said inner surthce being in contact with a fluid wherein heat is transferred across said steel wall from said inner surface to said outer surface, said outer surface being coaled with an aluminium coaLing layer and said aluminium coating layer being provided with a sacrificial anode: wherein sa.id anode his an electrode potential lower than said steel wall and lower than the Al coating layer such that said anode is preferentially corroded in use under the sea.

Viewed from another aspect the invention provides a heat exchanger for use subsea comprising a vessel bound by a steel wall having an inner surface and an outer surface, wherein heat is transferred across said steel wall from said inner stuface to said outer surface, wherein said outer surface is thermally sprayed with an aluminium coating layer and wherein said aluminium coating layer comprises a sacrificial anode wherein said anode has an electrode potenhial lower than steel wall and lower than the Al coating layer such that in operat ion no calcareous deposits form on said Al coating layer.

Viewed from another aspecl the invention provides a heat exchanger system for use subsea comprising a vessel bound bya steel wall having an inner surface and an outer surface, said inner surface being in contact with a fluid from which heat is to be removed; wherein heat is transferred across said steel wall from said inner surface to said outer surface, said outer surface being coated with an aluminium coating layer and said aluminium coating layer bcing provided with a sacrificial anode: wherein the p1-1 increase experienced at Ihe Al coating layer surface caused by corrosion of the sacrificial anode is not large enough to cause precipitation of calcareous deposits on the Al coating layer.

Viewed from another aspect the invention provides the use of an aluminium layer provided with a sacrificial anode a.s a coating on a steel heat exchanging surface located under die sea.

Viewed from another aspect the invention provides the use of an aluminium layer provided with a sacrificial anode as a coating on a steel heat exchanging surface located under the sea where no calcareous deposits are formed in the Al coaling layer during operation of the heat exchanger.

Viewed front another aspect the invention provides a process for the removal of heat from a fluid comprising providing a subsea heat excik tiger comprising a vessel bound by a sieel wall having an inner surface and an outer surface, said inner surface being in contact with a fluid from which heal is to be removed; allowing heat to be transferred across said steel wall from said inner surface to said outer surface which is in contact with sea water; said outer surface being coaled with an aluminium coating layer and said alnrnmium coating layer being provided with a sacrificial anode; wherein the p1-I increase experienced at the Al coating layer surface caused by corrosion of the sacrificial anode is not large enough to cause precipitation of calcareous deposits on the Al coating layer.

Detailed Description of the Invention

This invention relates to the corrosive protection of the heat exchanging surface of a heat exchanger located under the sea. The production of oil and gas necessitates the use of heat exchangers located under the sea. The outer heat exchanging surface of those heat exchangers, i.e. the surface that is exposed to seawater and that enables heat to be transferred from a warmer to a colder side is subject to corrosion. This invcntion relates to a ncw method for protecting that surface against corrosion without hindering the ability of thai surface to act as a heat exchanger.

The present inventors have realised that the application of an aluminium based metallic coating on the outer heat exchanging surface that is exposed to the sea enables the formation of a surface that can be both corrosion resistant and an excellent heat exchanger.

The Al coating layer comprises aluminium. It should be at least 75 wt% Al, such as at least 80 wt% Al. Ideally. Al fonts 90 wt% or more, such as 92 wt% or more of the Al coaling layer. The Al layer can be "commercially pure" At, typically therefore with a purity of 99 wt9o or more, such a.s 99..S wt% or more, even 99.99 wt% or more. Alternatively, the Al can be formed as an alloy with other metals.

The term alloy implies herein the presence of I wt% or more of non Al components in the coating layer.

Other metals which can be present alongside Al in an Al alloy coating layer include Zn, Mg, Fe and Si. Non Al components of the coating preferably form no more than 10 wt?/o of the alloy in total. It is preferred ifno single non Al metal component of the alloy forms more than 5 wt% of the alloy. If the content of non Al metals is too high then these can form separate phases within the alloy and therefore reduce the strength of the coating layer and limit its corrosion protection. These phases can form areas of high corrosion on the coating surface leading to a breakdown in the natural aluminium oxide layer that provides Al surfaces with their ability to resist corrosion.

Fe may be present at amounts up to 0,5 wt% of the Al coating layer. Si may be present at amounts up to 0,5 wt% of the Al coating layer. The most abundant non Al metal might be Mg. Mg may be more present at amounts up to 5.0 wt% of the Al coating layer. One advantage associated with the use of Mg as an alloying metal is that it is more active than Al. It will corrode faster than Al, so any Mg particles present on the surface of an Al/Mg coating layer might be preferentially corroded leaving a surface purer in Al. Purer Al is generally more resistant to corrosion. In effect therefore the Mg particles within the alloy might act themselves like a sacrificial anode.

It will he appreciated that ally Al alloy may contain small amounts ola variety 01 other components such as Cu, Mn, Cr, and Ti. Each of these metals, if present, will preferably form less than 0.1 wt% of die Al coating layer. There might be for example 0.05 wt% of Cu and 0.05 wt% of Cr present in die Al coating layer.

The use of Si in the alloy is especially preferred as Si increases corrosion resistance in an alkaline environment. As cathodic protection vil increase the p11 around Lhe Al coating layer surface relative to sea water itself, the use of Si helps prevent any breakdown of the natural thin aluminium oxide layer that forms on top ofAl layers. All Al surfaces have a tiny surface conleni ofAl oxide which is why Al is well known to offer corrosi on protection.

It is preferred if the Al coating layer is applied by thermal spraying - (Thermally sprayed aluminium (ISA)). It is most preferred if [lie Al layer is thermally coated onto the steel layer of the heat exchanger. Thermal spraying is a coating process in which melted (or semi-molten) material is sprayed onto a surface.

The feedstockt' (coating precursor) is heated by electrical plasma or arc or by chemical means and accelerated towards a surface to be coated typically using a gas sich as air.

The feedstock is typically fed to (he apparatus in powder or wire form, heated to a molten or semi-molten state and accelerated towards substrate in the form of micrometer-sue particles. Resulting coatings are made by the accumulation of numerous sprayed particles.

Thermal spraying can provide thick or thin coatings depending on the process and feedstock, over a large area at high deposition rate. The thickness of the Al layer in the present invenlion is preferably 20 micrometers to 5 mm. such as 100 microns to 2 mm.

It is obviously preferred if the whole of the outer surface of the heat exchanging layer is coated with the Al coating layer.

It will be clear that the nature of the coating precursor and in particular the non Al metal present in the coating precursor reflect the desired metal contents in the final coating layer There should not be any change in the elemental malce up of the coating precursor during spraying.

The use of the Al coating enables cathodic protection to be effective. At the same Lime heat transfer will not be limited as the Al coating acts as a heat conductor.

The inventors have surprisingly found that calcareous deposits can he avoided on the Al layer. Using a sacrificial anode which ha.s a. lower electrode potential than the Al layer then deposits can be minirnised. Ideally however the anode potential is just low enough to ensure that corrosion occurs on the anode and not the cathode bul. high enough that. the pH at tFe cathode will not rise to a level where calcareous deposit precipitate.

It is crucial therefore in avoiding calc.areous deposits that the pH at the Al coaling layer surface is not increased relative to the p1-1 of the sea waterto a level high enough to cause calcareous deposition.

The pH of seawater is typically 7.5 to 8.3. Ideally, the pH at the Al coating surface is 9.5 or less, especially 9.3 or less. That pH should be measured directly at the Al coating layer surface, e.g. as close as possible to the surface without touching it. Anode

The Al layer must be provided with a sacrificial anode. It will be appreciated that the anode must be iii direct contact with the Al layer but it need not form a coating on the Al iayer. The size of the anode is governed by the length of time the cathodic protection must be effective for and can be determined based on various factors such as the sea. conditions and so on. Anodic corrosion is dependent on sea temperature for example as higher sea temperatures lead to fluster corrosion.

In the application olpassive cathodic protection, a galvanic anode, i.e. a piece of a more electrochemically "active" metal, is attached to the vulnerable Al metal surthce where it is exposed to an electrolyte. In this case therefore, the anode is attached to the Al coating layer and the electrolyte is seawater. Galvanic anodes are selected because they have a more "active" voltage (i.e. a more negative electrochemical potential) than the metal of the target structure. For effective cathodic protcction, the potential of the Al surface is polarized (pushed) more negative until the surface has a. uniform potential. At that stage, the driving force for the corrosion reaction with the protected surface is removed. The galvanic anode continues to corrode, consuming the anode rnalerial until eventually it must be replaced.

Polarization of the Al layer is caused by the electron flow from the anode to the cathode, so the two metals must have a good electrically conductive contact.

This is simply achieved in the present invention through direct contact of the anode and the Al layer. The driving force for the cathodic protection current is the difference in electrochemical potential between tile anode and the cathode.

Galvanic or sacrificial anodes are made in various shapes and sites. The anodes of the present invention are preferably alloys of nnc, magnesium and aluminum. It is particularly preferred if the anode comprises Al. Anodes of particular interest are typically formed from Al with In arid/or Hg.

In order to minimise calcareous deposits, the Al coating Layer is ideally provided with an anode that has a potential very similar to the coating layer but just slightly lower. Ideally, the difference in potential is such that calcareous deposits do not form. It will be appreciated that the sacrificial anode acts to prevent corrosion of the Al coating layer as without this layer, the Al layer would act as a sacrificial anode towards the steel.

Steel used to form the heat exchanging vessel wall is typically polarised to around -600 to -650 mY (vs Ag/AgCI). Ii is preferred ilthe Al coating layer has a polarised electrode potential of -950 to 1050 mY (vs Ag/AgCI), such as about -l000mV (vs Ag/AgCI). It is preferred if the anode has an electrode potential of around -1050 to 1150 mY (vs Ag/AgCl), such as about -llOOmV (vs Ag/AgO).

The difference in poLential between anode and. caLhude may be less than 200 mY such as 150 my or less. In order to ensure that the anode is preferentially sacrificed, it is also important however that the difference in electrode potential is at least 25 mY (vs Ag/AgC1).

Ideally, the electrode potential difference between Al coating layer and steel is at least 50 mY. such as at least 100 mY. such as at least 150 mY vs Ag/AgCl.

Thermally coated substrates may need to be sealed or over coated as thermally spraycd coatings tend to be slightly porous. A sealant can thcrcfore be applied to fill in pores on the thennally coated substrate surface. Ideally, there should not he a measurable overlay of sealant on the Al coating layer after application. The sealant layer is ideally therefore less than 100 microns in thickness Sealing of the coaling is also important to ensure that die coating is dense and that the component being protected is separated from the corrosive media.

The sealant should be of low viscosity to penetrate into the pores and seal them, without necessarily adding thickness to the protective system. The sealant can be sprayed on the aluminium coating as soon as possible after the coating has dried and cooled in order to smooth the surface texture and hillder contamination of the Al surface. An unsealed coating will also be sealed to some extent with corrosion products filling the pores. hut this process may take some time and may allow sea svater access to the steel below before the pores are filled.

Sealants of interest. are typically siloxane based.

It has been found. that the presence of the sealant reduces current density demand vs. a non sealed substrate. The presence of a sealant also markedly reduces corrosion rate.

A flu-ther benefit of the use of an Al coating layer on the steel heat exchanging surface is that the current demand. is reduced, sometimes significantly relative to a bare steel surilice. This should in theory lead to less rapid corrosion of the sacrificial anode. The current demand reduction also increases over time. The longer the heat exchanger remains under water the larger ihe differential between current demand of the Al coating layer vs the steel heat exchanger layer becomes.

The nature of the actual heat exchanger is not limited. This invention can be applied to any subsea. heat exchanging surface. The hea.t exchanger should a vessel, such as a pipe, comprising a warm fluid from which ii. is desired to remove heat.

ihe warm fluid passes through the vessel and hence the warm fluid is preFerably in contact with the inner surface of the heat exchanging steel wall of the vessel so that heat from the warm fluid is absorbed by the steel. Heat passes through the steel wall and then through the Al layer and is absorbed by the cold sea. in coniact with the surface of the Al layer.

Ideally however, the heat exchanger comprises multiple warm fluid conducting vessels some or all of which arc coatcd with the Al coating layer of thc invention. It is also preferred if the fluid conducting vessels within the heat exchanger are adapted in some way to maximise their surface are& such as coiled For efficiency, heat exchangers are designed to maximiie the surface area olthe wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger's performance can also be affected by the addifion of fins or corrugations, which increase surface area and may channel fluid flow or induce turbulence -In subsea fluid processing therefore the surrounding seawater is used to cool fluid flowing in a vessel. The vessel can be configured with a plurality of bends or combining a plurality of such vessels in a parallel configuration in order to achieve large contact area between the vessel and the water, and thus a high heat transfer rate between the fluid in the vessel(s) and the surrounding water.

The heat exchanger vessel ina.y therefore be cylindrical for example. It may be provided with fins to increase surface area. It may be coiled and so on. Heal exchangers of interesl may therefore be based on the double pipe desiwi, shell and tube heat exchanger, plate heat exchanger, plate and shell heat exchanger, adiabatic wheel exchangers, plate fin exchangers. pillow plate exchangers, fluid heat exchangers, waste heat recovery units, phase change heat exchanger, spiral heat exchangers, and so on.

The heat exchanger may be associated with a variable speed drive or generally associated with electrical equipment that needs cooling. The shape therefore of the heat exchanger and in particular the heat exchanging surfaces thereof are not limited. However, it is most preferred if the heat exchanger comprises a housing in which are located multiple fluid conducting vessels. The vessels are bound by the heat exchanging steel layer having inner and outer surfaces.

Ihese vessels are exposed to sea water in use and the coating of the invention is ideally present on all such sea water exposed vessels.

In processes where the flow-rate, the temperature. or characuristics of the tluid flowing through the pipe varies, the above described arrangement can involve challenges for the operator, since he cannot control the exact cooling rate. Varying temperatures of the surrounding seawater can also imply corresponding challenges.

Possihlc remedies for such challcngcs can be to control the flow rate of thc fluid in the pipe(s) or to flow the fluid through longer or shorter lengths of the pipe by the control of appropriately arranged valves. The heal exchanger might Lherefore be provided with a plurality of parallel branches. Ifa higher heat transler rate is needed, more branches can be connected. Correspondingly, if less heat transfer rate is needed, the operator can disconnect one or more branches.

in some embodiments, the outer Al coating layer might be exposed directly to the set In other embodiments however, the vessel may be enclosed by a housing enclosure comprising a sea water inlet and a sea water outlet. Furthermore, the hea.t exchanger may be provided with means for controlled through-flow of surrounding sea water from the sea water inlet to Lhe sea water oulleL With such a sabsea heat exchanger, an operator is able to control the heat transfer i-ate between the fluid flowing through the vessel and the sea water. This can be performed for instance by varying the pump speed or by controlling a throttling valve to control the through-flow of sea waler.

Whilst the invention has been described with reference to the use of a dedicated sacrificial anode, the use of an Al coating layer is also compatible with impressed current cathodic protection (ICCP). In ICCP. a direct current is applied to a cathodic surface from an auxiliary anode. The current flow makes electrons pass to the cathodic. surface preventing corrosion whilst the anode corrodes, Moreover, the techniques described herein are suitable for use with heat exchangers that are subject to marine conditions intermiliently even if the heal exchanger is not located under the sea at all times. For example, heat exchangers that might he covered and uncovered through the movement of the tides or heat exchangers located in places where they might be subject to waves and splash in roLigh sea are covered here, Whilst the invention has generally been described with reference to heat exchangers, the principles of the invention can be extended to any subsea surface where heat needs to be transferred from a hot surface into the sea. For example, elecirical components can often heat up during use and it would be useful if the heat of those components could be dissipated. By providing an electrical component with an Al coating layer and a sacrificial anode as hereinbefore described, it is cnvisagcd that hcat can also bc removed from clcctrical components.

Electrical components which might benefit from this treatmeliL uclude variable speed drives transformers, capacitors, switch gears and so on.

Thus, viewed from a fl.rrther aspect the invention provides an electrical device for use in a subsea environment comprising an electrical component provided within a metallic housing, said housing having an inner surface and an outer surface, when in use said inner surface is subject to heat from said electrical component, wherein heat is transferred across said metallic housing from said inner surface lo said outer surface, wherein in use said outer surface is in contact with sea water; wherein said outer surface is coated with an aluminium coating layer arid said aluminium coating layer is provided with a sacrificia.l anode.

The metallic housing is preferably steel.

The invention will now be described with reference to the following non limiting examples and figures.

Existing technology as well as the invention is illustrated in Figure 1. In figure 1 a steel wall layer is provide with an organic coating. This limits heat transfer. In figure lb. the steel va1l is covered in calciferous deposits. These too limit heat transfer through the steel wall. In figure Ic, the steel wall is coated with ISA and provided with a sacrificial anode. Heat transler is then maximised whilst avoiding corrosion.

Examples

Steel panels are coated with TSA in order to determine whether corrosion thereof could be prevented according to the invention.

Steel wuh therma//v sprayed alum/nuni The specimens were made out of a steel plate sprayed with thermally sprayed aluminum. Wire arc spraying was the method being used and the coating was 100 -jim thick. The alloy was AIMgS, with 95% Al and 5 % Mg. After the coating was applied, the plate was cut in pieces of 35 x 60 mm. Next, the hack and sides of the specimens were painted with an organic coating, Carbomasl.ic 1 g FC, to avoid exposure of the bare steel.

Steel with thennallv spt'ayed alwninurn sea/ed Since appliance of sealant on TSA is recommended practice for applications submerged in seawater, a silicon sealer was applied on two of the TSA specirnens This wa.s in order to compare the I.wo cases, sealed and unsealed ISA. The organic sealant will gradually degrade. resulting in the same reactions as in the absence of sealant.

The specimens described above were exposed to natural seawater. The water, with a temperature at 8-12 °C. was filtered before entering the submersion bath where the specimens were mounted. The circulation of the seawaler was so slow, that the conditions in the tub can be described as nearly stagnanL The specimens were exposed for 6 weeks.

Due to practical reasons, the specimens were divided in two tubs; the sealed TSA samples were placed in one tub, while the unsealed ISA specimens were placed in the other. The specimens were coupled to a potensiostat and polariied down to -lO5OniV vs AgiAgCl. A resistance of 10 ohms was connected between the working electrodes (the specimens) and the platinum counter electrodes. The current was calculated by measuring the potential drop over the resistance and applying Ohm's law.

After 6 weeks exposure the specimens were taken out and rinsed in water and acetone. Subsequently they were examined visually in the LVFESEM (Low vacuum field emission scanning electron microscope) Zeiss Supra 55VP and the FESE.M (field emission scanning electron microscope) Zeiss Ultra, 55 Limited Edition.

Results The thickness of the calcareous deposit on the ISA samples or the aluminum aHoy samples is not possible to measure, as no typical calcareous deposit structure was found. Whilst c/c ininUnis coating is arguably present, the TSA sprayed surface can be regarded as free of calcareotis deposits in comparison rn an uncoated steel surface with anodic protection.

Potentiostatic polarisation curves can be used to show the development of current density needed to keep the potential of the specimens at -1050 mY vs Ag/AgC1 over time.

The current density demand for the unsealed ISA specimens polariied to - 1050 mV vs Ag/AgCl at 5-12 °C seawater temperature was in the order of4() mAim2 after one month. That is 4 times lower than for steel specimens. After 4 months the current demand was 25 niA/m2.

Sealed ISA specimens showed even lower current density demands, at 10 mA'm2.

Claims

Claims L A heat exchanger for use in a subsea environment comprising a vessel bound by a steel wall having an inner surface and an outer surface, when in use said inner surface is in contact with a fluid wherein heat is transferred across said steel wall from said inner surface to said outer surface, when in use said outer surface being in contac.t with sea water: wherein said outer surface is coated with an aluminium coating layer and said aluminium coating layer is provided with a sacrificial anode.
2. A heat exchanger for use in a subsea environment comprising a vessel bound by a steel wall having an inner surthce and an outer surface, said inner surface being in contact with a fluid wherein heat is transferred across said steel wail from said inner surface to said outer surface.said outer surface being coated with an aluminium coating layer and said aluminium coating layer being provided with a sacrificial anode.
3. A heat exchanger for use in a subsea environment comprising a vessel bound by a steel wall having an inner surface and an outer surlh.ce. said inner surface being in contact with a fluid wherein heat is transferred across said steel wall from said inner surface to said outer surface, said otiter surface being coated with an aluminium coating layer and said aluminium coating layer being provided with a sacrificial anode; wherein said anode has an electrode potential lower than said steel wall and lower than the Al coating layer such that said anode is preferentially corroded in use under the sea.
4. A heat exchanger for use subsea comprising a vessel bound by a steel wall having an inner surface and an outer surface, wherein heat is transferred across said steel wall from said inner surface to said outer surface.wherein said outer suriace is thermally sprayed with an aluminium coating layer and wherein said aluminium coating layer comprises a sacrificial anode wherein said anode has an electrode potential lower than steel wall and lower tha.n the Al coating layer such that in operation no ca.lcareous deposits form on said Al coating layer.
5. A he& exchanger system for use subsea comprising a vessel bound by a steel wall having an inner surface and an outer surface, said inner surface being in contact with a fluid from which heat is to be removed: wherein heat is transferred across said steel wall from said inner surface to said outer surface, said ouier surface being coated with an aluminium coating layer and said aluminium coating layer being provided with a sacrificial anode; wherein the p1-I increase experienced at the Al coating layer surface caused by corrosion of the sacrificial anode is not large enough to cause precipitation of calcareous deposits on the Al coating layer.
6. A heat exchanger as claimed in any preceding claim wherein the Al coating layer comprises at least 90 wt% Al.
7. A heat exchanger as claimed in any preceding claim wherein the Al coating layer is a thermally sprayed Al coating layer.
8. A heal exchanger as claimed in any preceding claim wherein the Al coating layer comprises Mg and/or Si ions in addition to AL
9. A heai exchanger as claimed in ally preceding claim wherein the Al coating layer is sealed with a sealant.l0 A heaL exchanger as claimed in any preceding claim wherein the potential difference between anode and the Al coating layer is between 25 to 200 mV. (vs Agi'AgCI).11. Use of an aluminium layer provided with a sacrificial anode as a coating on a steel heat exchanging surthce located under the sea.12. Use of an aluiniiuuni layer provided with a sacrificial anode as a coating on a steel heat exchanging surface located under the sea where no calcareous deposits are formed iii the Al coating layer during operation of the heat exchanger.13. An electrical device for use in a subsea environment comprising an electrical component provided within a metallic housing, said housing having an inner surface and an outer surface, when in use said inner surface is subject to heat from said electrical component, wherein heat is transferred across said melallic housing from said inner surface to said outer surface.iyherein in use said outer surface is in contact with sea water; wherein said outer surface is coated with an aluminium coating layer and said aluminium coating layer is provided with a sacrificial anode.