CORROSION AND MATERIAL SELECTION IN DESALINATION PLANTS

Anees U Malik and P.C. Mayan Kutty,

ABSTRACT

The construction of a desalination plant requires a wide spectrum of materials metals,-non-metals and composites which would not only satisfy the design and functionalrequirements of the particular component but should also have reasonable endurance tothe environment surrounding the plant installations. One of the prime considerationsduring the selection of construction materials for a particular unit or component is itscorrosion characteristics.

An MSF desalination plant is exposed to different and diversified kinds of environmentssuch as seawater, seawater-air and salt-air aerosols, corrosive gases, very fast or extremelyslow moving liquids, particulates contained in high velocity fluids or deposit-forming liquidsall of them create a number of corrosion related problems. Besides general corrosionand mechanically and chemically-induced erosion-corrosion, localized-corrosion such aspitting, galvanic and crevices are quite frequently observed. Cases of intergranular corrosion,selective leaching and stress-induced corrosion are also not uncommon. The flashchambers are subjected to highly aggressive corrosion due to flashing brine and Cl- attackwhereas heat exchangers face one of the severest environments and account for 70% ofthe failures resulting from pitting, crevice, impingement, dealloying andgalvanic corrosions.Other components which are prone to corrosion attack are water boxes, demisters,deaerators, venting systems, ejector condensers, pump valves, piping and the intakes. Inseawater reverse osmosis (SWRO) plants, high pressure piping, headers, connectors andmembrane containment vessels are prone to localized corrosion attack even though theoperating environments are much less severe than ih MSF.

Corrosion in desalination plants can cause a variety of undesirable consequences, includingloss of equipment, unplanned shutdowns, expensive repairs, leaks and contamination ofproducts as well as serious personal hazards. The criteria for selection of materials fordifferent sections of desalination plant are based upon the nahrre of the corrosive envi-ronment, equipment operating conditions, design features, desired plant life and costeffectiveness of the materials. Ferrous and non-ferrous metals and alloys, plastics, rubbers,ceramics, glass and composites including reinforcements are the materials to be consideredfor constructional purposes. In process industries like desalination plants corrosion

1 Presented to SWCC 0 & M Seminar, Al Jubail, April 1992.

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consideration out-weighs the other factors while carrying out the material selection for plantconstructions. Thus the selection and corrosion control are synonymous as far as safeoperation and maximum output from the plant facilities are concerned.

This presentation discusses the corrosion behavior of materials in environments surroundingthe desalination plants with special reference to the different sections of the plantwhich are vulnerable to corrosion failures. Furthennore, the occurrence of various formsof corrosion during the operation of a desalination plant is described, and the role of localattack is emphasized. The presentation also encompasses the criteria for the selection ofmaterials for different sections of the desalination plant which basically depend upon thenature of the corrosive environment and operating conditions. The short-comings of someof the materials currently employed are highlighted and the introduction of new materialseither in the existing plant as the possible replacements or in future plants is discussed.

1. CORROSION IN DESALINATION PLANTS

1.1 Introduction

The seawater desalination plants offer a multitude of corrosion problems due to theiroperation in relatively aggressive environments consisting of seawater, seawater-air andsalt-air aerosols, corrosive gases, very fast or slow moving liquids, particulates containedin high velocity fluids or deposit forming liquids. Corrosion rate data of metals in aeratedseawater at different velocities and at ambient temperature indicate highest metal lossin carbon steel and cast irons and lowest for stainless steels and titanium Table-l.Besides general corrosion, which is not a major concern, cases of localized corrosion inthe form of pitting and crevices corrosion are quite common in desalination plants. Infact, pitting accounts roughly 41% of the corrosion failures in MSF plants and crevicecorrosion under deposits is most troublesome. Impingement and cavitation are notuncommon and have been frequently noticed in heat exchanger tubes, pumps, valvesand liquid transmission pipes. Erosion- corrosion on water-side accounts 21% of thefailures and vapor side 14% of the failures. In MSF plants flash chambers are subjectedto severe corrosion due to flashing brine and Cl- attack whereas heat exchangers facemost aggressive environment and contribute 70% of the failures resulting from pitting,crevice, impingements, dealloying and galvanic corrosions. Due to deployment of dif-ferent types of metallic materials in the plant, galvanic interactions in presence of dis-similar metals are quite common and have a major influence on the material selectionTable-2. The other components in an MSF plant which are prone to corrosion attackare water boxes, demisters, deaerators, venting systems, ejector condensers, pumps,valves and pipings.

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1.2 Vapor-Space Corrosion

In MSF plants vapor space conditions are less well controlled and severe corrosion hasbeen observed in both acid and additive dosed plants at rates well in excess of thedesigned corrosion allowance. Apart from water vapor which is always present,uncondensable gases evolved from the flashing brine will be present. These gases aremainly CO2, O2, and N2. In some cases H2S and NH3 also would be present if seawaterfeed to the plant is polluted with decomposing organic materials. Also, where brineheaters are vented to the first stage, NH3 will be present from decomposition of boilertreatment compounds such as hydrazine. NH3 and H2S may cause severe corrosion onCu- base alloy tubes in heat recovery and reject bundles. Ammonia selectively corrodesCu-base alloys condenser tubes by forming soluble copper-ammine complex, this causesmetal loss and pitting in the tubes and the corrosion products can impede heat transferacross the surface of metal. The soluble complex can attack other materials of con-struction in the evaporator i.e. Cu plating of carbon steel. Bromine gas produced as aresult of seawater chlorination can cause severe localized problems. Acid treated MSFplants provide conditions suitable for bromine gas formation in decarbonator anddeaerators. Corrosion due to bromine liberation could be obviated by addition of adechlorinator such as Na2SO3 before acid injection.

The main source of CO2 is bicarbonate present in seawater. In additive-dosed plants,CO2 is released in high temperature stages along with the dissolved gases. For thisreason, top few stages are usually vented directly to vent system to remove gases asquickly as possible. In acid-dosing plants, most of CO2 is removed by passing throughdecarbonator before further degassing in the deaerator. However, some CO2 will stillbe present in the plant evolved through out the flashing process. Proportion of CO2 inadditive-dosed plants is much more significant than in acid-dosed plants. Figure 1 showsinfluence of CO2 presence and temperature on corrosion rate of carbon steel.

Feed to the plant and recirculating brine contain less than 20 ppb of dissolved oxygen(D.O.). Flashing brine will cause some oxygen to be evolved and this will pass into vaporspace with other gases. As MSF operates at reduced pressure air-in-leakage will occur.The air will pass into the vapor space adding to the amount of oxygen evolved from thebrine.

Corrosion products in the vapor space is usually black magnetic iron oxide. This itselfindicates that oxygen is more predominant than CO2 in causing corrosion. Air leakagerather than D.O. is the major source of oxygen in vapor space.

Change of material construction to 316L or similar SS appears to be the best solutionto minimize the effects of vapor space corrosion.

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1.3 Corrosion in Flash Charnhers

Carbon steel is the most common construction material for flash chambers, it is usedas such or cladded with stainless steel or Cu-Ni in early or all the stages. Epoxy coatinghas also been used.

Flash chambers are subjected to severe corrosion and potential metal failures. The roleof oxygen in the corrosion of metals of construction in MSF plants in general andevaporators in particular is quite complex. For example, oxygen accelerates corrosionof carbon steel by acting as a cathodic depolarizer whereas in stainless steels it retardsby growing and/or repairing oxide film responsible for passivity. Therefore, an oxygenlevel perhaps to the extent of a few ppb is desirable. Figure 2 shows corrosion rate of carbonsteel in seawater as a function of temperature at various oxygen concentrations. H2Sand NH3 can cause extensive damage inside the evaporator no matter how small theirconcentrations are. The interesting features of the corrosion of flash chamber (mildsteel) are as follows :

(1)

(2)

(3)

(4)

(5)

(i)(ii)

(iii)(iv)

Corrosion is maximum in the middle of the stages where the combined effects oftwo competing factors e.g. oxygen leakage and temperature are optimum.

Corrosion is usually most severe on the interstage walls and often one wall ismuch more attacked than the other.

Corrosion product is usually black magnetic oxide, Fe3O4.

The corrosion product is separated from the metal by a void. In case the disturbedsheets of corrosion products several mm in thickness fall away, an even metalsurface is left behind.

In some plants, blockage of demisters by corrosion products has caused plantshut down.

The major causes of the corrosion damage are :

High velocity of the brine flow affecting floor;Violent brine flashing (impingement) and collapsing of the flashing vapors(cavitation) on the walls;High chloride contents of brine;High Cu content of recirculating brine.

Breaking of the hard magnetite (formed at low D.O.) due to above factors results inthe formation of a cell in which base metal acts as an anode and magnetite as cathodeGates controlling flow of brine from stage to stage are made of stainless steel becauseof its high resistance to turbulent brine. In SS lined stages this poses no problem but

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in CS stages sometimes deep pitting occurs beneath the SS gates. Adjacent CS surfacesalthough showing slight general corrosion do not have these pits. Pitting is attributedto galvanic effect from SS. Electrically insulating the SS by fitting plastic sleeves andwashers to all fasteners connecting the SS to CS structure is most effective.

In stainless steel-lined flash chambers, severe localized corrosion (pitting, crevice andintegrannular) has been observed. The liner plates are joined one another by seamwelds and to the mild steel by stainless steel rivets. Micropitting and rust bleeding wereprominently appeared at welds, rivets and rivet crevices. Corrosion was most severe atwelds and heat affected zones. In SS-lined CS flash chambers the most effective methodof controlling corrosion at the junction is to reduce the effective cathodic area by paintingthe SS but painting of CS to control corrosion is not recommended. Holidays in thecoating on SS will not result in a significant increase in attack, however, if CS is paintedthen attack is intensified at holidays and pitting rates of 10 mm/year can be experienced.

Corrosion of stainless steels is most likely to occur during shut down where slow movingor stagnant high chloride brine and crevices (formed by salt and/or dirt deposits) onthe floors, walls and other appurtenances of the flash chamber and D.O. produce mostfavorable environment for initiating and propagating corrosion process. Avoidance ofair leakage, complete drainage followed by flushing with distilled water are the pre-cautions to do away with corrosion. Deaeration is of prime importance in pretreatmentin order to control corrosion in evaporators. Dissolved oxygen in deaerator affluentmust be less than 20 ppb and CO2 less than 3 ppm.

In SWCC desalination plants, the use of stainless steel cladded mild steels in the flashchambers of first few stages followed by mild steel in other stages provided reasonablysatisfactory performance. Only in a few cases some minor corrosion problems havebeen reported. Table 3 lists the materials used in flash chambers of different SWCCdesalination plants in the Kingdom.

A corrosion allowance of 12 mm below the demisters and 7.0 mm above the demistersis usually accepted for a design life of 25 years.

1.4 Heat Exchangers

Heat exchanger tubings represent the single largest procurement item in an MSF plantand not surprisingly more than 70% of the corrosion failures in desalination plants areattributed to heat exchange tubes. Heat exchangers tubes handle two fluids of completelydifferent properties (seawater and condensing vapors). It is one of the severest envi-ronments from the point of view of corrosion.

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Over 85% of failure of copper alloy tubes in salt water condensers, heat exchangers andMSF equipments are caused by a condition known as tube inlet erosion/corrosion. Thistube inlet damage is almost always located in the first six inches (150 mm) of the tubeinlets and is caused by circulating water flow conditions. Severe damage often resultsin perforation. As the water entrained particles and air enter the tubes from the waterbox area it is highly turbulent with an infinite number of varying velocities. As it movesalong into the tubes, this turbulent flow rapidly becomes laminar and by the time it is4 inches to 6 inches into the tubes, the intensity of condition causing erosion/corrosionhas completely subsided or disappeared. Use of prefabricated plastic or nylon inserts,coating, metallic inserts or shield/seal are the methods employed to combat tube inletcorrosion. Shield/seal method involving thinned walled superalloy shield may providethe best protection.

90/10 Cu Ni, 70/30 Cu Ni, 66/30/2/2 Cu/Ni/Fe/Mn and Ti are the materials used forheat exchanger tubes. The choice of the most suitable copper alloy depends upon thesystem (brine heater, heat recovery or heat rejection) to be considered. Tables 4 andTable 5 list the various heat exchanger materials used in the different MSF desalination plantsin the Kingdom.

Brine Heater

It operates at high temperature, with a strongly scale forming solution, as these scalesare normally very hard and difficult to remove by chemical means, mechanical cleaningis resorted to which in turn requires an intrinsically harder material. 70/30 Cu /Ni ormodified 66/30/2/2 has been used as tube material in most of the plants but in JubailPhase-I, Ti tubing (ASTM338) has been used. In some older plants (Jeddah Phase IIto IV, Haql) 90/10 Cu / Ni wa used.

Heat Recovery

This section normally has its first 3 or 4 stages operated at high temperatures, whichmay cause attack on the outer surface of the tubes due to non-condensable gases (e.g.CO2). Inside the tube, however, the water is deaerated and virtually oxygen free. Dueto its least vulnerability towards corrosion, 90/10 CuNi has been used in the recoverysection in most of the desalination plants. Only Jubail (Phase-I) has used Ti tubing.

Plugging or choking in the pipe section of heat recovery system is quite common whichmay cause corrosion.

Heat Reject Section

The seawater flowing in heat rejection tubes is natural, very often chlorinated (againstbiofouling) and may also contain suspended solids and sulfides. Ti metal tubing(ASTM338) has been used in most of the plants. Jeddah (II to IV) and Shoiba have

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used 90/10 Cu/Ni and 70/30 Cu/Ni, respectively in heat reject section.

In heat reject section, corrosion problems are quite common. Corrosion in vapor spacedivision wall above demister, general corrosion, deposit attack and choking have beenreported.

Cupronickel alloys Table 6 have shown good performance under high water speed,ammonia contamination, presence of suspended solids, higher temperatures etc. Inmodified alloys addition of small amounts of Fe and Mn strengthens the metal matrix,thus counteracting impingement corrosion that leads to continuous removal of theprotective corrosion layers formed on copper alloys during initial service. It has beenfound that the additions not only provide stability to surface protective film but alsoincreases its self healing propensity. Therefore, CuNi Fe Mn, the so-called modifiedalloys are finding increasing use as heat exchanger materials replacing the plaincupro-nickel alloys.

Spaces between tube rolled on tube plates are active sites for deposition and creviceattack. Rust bleeding appears from joints of 90/10 tubes and tube sheets made of CSlined with lmm 90/10.

Titanium possesses excellent anti corrosion and heat transfer properties but there areproblems of crevice corrosion and hydrogen adsorption in high temperature salt water.Addition of noble metal like Pd improves corrosion resistance. Thus Ti-.15Pd andTi-.05Pd-.3Co have excellent resistance toward crevice corrosion.

Cathodic Protection (C.P.) has been shown to yield some benefit in reducingimpingement attack on tube ends, as well as waterbox or tube plate corrosion due togalvanic coupling in seawaters; whenever continuous monitoring of cathodic corrosionis unwarranted, sacrificial anode system is preferred over impressed current systems.Iron anodes are preferred over Zn since it interferes less with the formation of protectivefilm on copper base alloys but at the same time releases significant amounts of ferrousions to act as additional protectant of the tube surface Table 7. The water boxes whoseprimary function is to maintain same velocity and water distribution in all tubes cansuffer if there is stagnation of water around anodes.

1.5 Distillate System

Distillate system has to handle aggressive fluids due to dissolution of non-condensableCO2 and other gases. The parts affected are :

(i) Trough (SS316L)(ii) Evaporator sheet (CS or cladded CS)(iii) Product water or distillate piping (316L or FRP)

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Pitting has been observed on distillate tray, distillate plate and distillate pipe. In distillatesystems, corrosion tends to be most severe above the distillate transfer troughs wheredissolved gases transferred from high temperature stages are assumed to accumulate.

1.6 Ejectors

The materials used in the construction of ejectors and ejector condensers in the SWCCdesalination plants are given in Tables 8 and 9.

Severe corrosion has been observed in 316L air ejector systems of MSF plants due tothe presence of bromine in non-condensable gases. Use of 904L appears to be lessproblematic since it is resistant to this attack.

1.7. Ejector Condensers

Barometric type (direct connections) are usually employed. The components vulnerableto corrosion attack are : body, nozzles and piping. Body of the ejector condenser (CScladded with SS, Ti, Cu-Ni) sometimes showing metal loss has now been replaced byFRPin new plants. Nozzles (SS 304 or 316) and condenser piping (316L or Cupro-nickel)are most affected by pitting, the attack is more prominent at the welded seams. On theafter condenser (steam/vapor inlet pipe) severe Cl- induced S.C.C. was noted whichwas attributed to high operating temperature and salt deposition. The 316L pipe wasreplaced by Incoloy 825. Presence of 0.1-0.2 ppm chloride in the uncondensed gases issufficient to produce corrosion. Cupro-nickel and SS are replaced by 254 SMO as anejector condenser material in some of the plants in Qatar and Abu Dhabi due to theirpoor performance.

1.8 Venting System

Failures of 304 venting pipes between inter and after condenser due to severe localizedpitting was observed in Jeddah Phase-II which was attributed to Bromine attack. SS316L is the most commonly employed pipe material for venting system.

1.9 Pumps

Seawater intake, brine recirculation, brine blow down and make-up pumps of thedesalination plants are affected by corrosion or erosion-corrosion.

In desalination plants, vertical lift pumps are usually employed. AISI 316L (castequivalent CF3M) is the most widely used material for pump internal, impeller, shaft,sleeves etc. due to its excellent corrosion resistance under fast flowing condition, andgood weldability. Furthermore, any damage to impeller can be readily repaired.

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Immersed parts of impeller are subject to pitting and crevice corrosion under conditionof stagnancy in seawater. In these pumps, casing of Ni resist is used to provide protectionto SS; Ni resist has adequate corrosion resistance to give long life to the equipment.

316L SS has commonly been used for impellers in brine recirculation or intake pumps,the impeller shows pitting due to stagnation or galvanic corrosion. In other pumps,Al-bronze has been used for impellers.

Discharge column of brine recycle pump made of Ni-Resist (D-2 type) shows cavitationdamage due to water hammer and SCC due to high Cl-, high operating temperature(~50°C) and internal stresses present in the material. Cl- induced SCC and corrosionfatigue failures have been observed in the shafts of brine recycle pumps of the desali-nation plants.

1.10 Valves

General corrosion, pitting, impingement and cavitation are the common mode of cor-rosion failure.

Body and disc of the throttling valves in down stream are affected by impingementshowing horse shoe shaped pits.

Ball valves made of chrome-plated carbon steel and which were used in water trans-mission line show pitting and general corrosion.

1.11 Intake System

Failures have occurred in pipelines carrying seawater due to rebar corrosion or in RCCstructures exposed to seawater environment. Splashing or intermittent drying andsoaking in seawater environment are the main cause of the failures.

1.12 Piping

A wide variety of corrosion problems have been observed in pipings employed indesalination plants.

Type of corrosion observed and the materials and systems affected are stated as follows:

Type of Corrosion Materials and Systems Involved

Pitting and crevicecorrosion

316L distillate pipeline due to dissolved CO2, O2, high Cl-or stagnancy.

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Impingement Mild steel/cast iron/epoxy lined pipeline due to waterhammer or turbulence.

General corrosionand pitting

316L vent pipes due to non-condensable gas attack

Rebar corrosion CCP or PCP product water pipe lines. General corrosiondue to the formation of electrochemical cells between rebarand air/O2/CO2 through concrete cover.

1.13 Reverse Osmosis Plants

For a high pressure seawater reverse osmosis (SWRO) plant consisting of pumps, pipingand valves and handling high level of chloride (22,000 to 36,000 ppm) and dissolvedoxygen (5-10 ppm), improper selection of materials and/or laxity in operation may resultin pitting, crevice or stress corrosion.

Austenitic stainless steels are the conventional materials used for high pressure inletpiping leading to RO membrane module, brine rejection pipe, product water outlet pipeand high pressure pumps.

High velocities of the feed water and design of RO does not encourage formation ofcrevices. Even then high pressure piping (closed to weld or heat affected zones), headers,connectors, flanges, seals of pumps and membrane containment vessels are prone tocrevice corrosion attack. SCC is not a problem in RO because of working at lowertemperatures (below 70oC).

The performance of materials in different SWRO plants in the Kingdom and outsideis given in Table 10.

Austenitic 316L, 317L and 904L (in order of increasingly good performance) are suitablematerials for SWRO high pressure piping under conditions of good quality plantmaintenance and efficient flushing system. However, these alloys are prone to crevicecorrosion attack in case stagnant conditions are developed or deposits are formed inthe piping system due to operation and/or maintenance problems.

2. MATERIAL SELECTION

2.1 Introduction

Corrosive environment and plant operating conditions are the two important consid-erations in the selection of materials. Plain carbon steels, copper-base alloys, stainlesssteels, cast irons and titanium are the main metallic materials used in the construction

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of desalination plant. Table 11 lists nominal composition of some marine alloys whichhave important relevance as construction materials for seawater desalination plants.Table 12 gives a cost comparison of various pipe materials on weight basis assumingcarbon steel cost index as 1. Fibre glass reinforced plastics (FRP), rubbers, varioustypes of epoxy, vinyl, polyurethane based coatings, concrete and reinforced concreteare some of the non-metallic and organic based materials utilized in the plants. Varioustypes of materials used in marine applications are classified in Table 13 and Table 14provides a classification of materials used in desalination plant equipments. The clas-sification is based on cost and performance.

2.2 Plain Carbon and Low Alloy Steels

Carbon steels and low alloy steels arc the most commonly used construction materialsbecause of the fact that these materials have adequate mechanical properties andexcellent fabricability and are inexpensive and most abundantly available. They havemoderate to good corrosion resistance if serviced under coated or cathodic protectioncondition. Carbon steels have high general corrosion rates particularly when highvelocities are involved and therefore, if these materials are used without protection,require a large corrosion tolerance and additional allowance for the design stresses.

Carbon steels or low alloy steels can be safely used in marine environments if the cor-rosion control is provided either by barrier coatings or cathodic protection, C.P. (im-pressed currents or sacrificial anodes) combined with organic coatings. Cathodicprotection is efficient and most beneficial providing good corrosion resistance. However,C.P. systems work only when current can access the surface that needs protection;therefore, corrosion control is more difficult in splash zones or above.

Carbon steel has been widely used in flash chamber construction. Since the problemof corrosion is mainly confined to the first few stages due to relatively high temperaturesand high turbulence, better corrosion resistant materials such as 90/10 Cu-Ni orAISI 316SS have been utilized. The latter has frequently been used as lining or claddingmaterials for the carbon steels. In higher stages (usually above stage 3), carbon steelflash chambers have been used. 90/10 Cu-Ni appears to have better performance than316 SS since the former does not undergo pitting and crevice corrosion under stagnantconditions - a condition sometimes encountered during shut down and is also not subjectto stress corrosion cracking in presence of Cl-, high oxygen (oxygen leakage) or low pH(CO2 dissolution).

2.3 Stainless Steels

Stainless steels are quite close to ideal materials for desalination plant construction.Stainless steels have excellent resistance to general corrosion and erosion - corrosioneven at high water velocities. The alloys have good mechanical properties, easy fab-

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ricability and weldability, easy availability and relatively moderate cost. The generalcorrosion rates of stainless steels in flowing seawater above a velocity of about 1m/s (3ft/s) and upto nearly 40m/s (130 ft/s) is negligible.

Conventional austenitic SS 304 and 316 are the most common constructional materials,their main assets are high strength, ductility, workability, weldability and good resistanceto general corrosion. Their main weakness is localized corrosion in Cl- containingsolutions and pitting under static condition. Mo-containing 316 has better corrosionperformance and is more widely used. The application of 316 L in desalination plant’sequipments include flash chambers, demisters, trays and troughs, ejectors condensers,venting pipes, pump internals, valves etc.

Addition of Mo or nitrogen to stainless steels increases their resistance to pitting as wellas strength. Pitting resistance equivalent is defined as:

PREN = % Cr + 3.3% MO + 16%N.

In general, higher the value of PREN, the greater the resistance to pitting. Addition ofnitrogen not only increases pitting/crevice corrossion resistance, and possibly resistanceto SCC but also increases strength substantially without sacrificing ductility. Anothereffect of nitrogen is the suppression of intermetallic phase precipitation which allowsprocessing and welding of heavy sections.

Stainless steels with improved corrosion resistance such as 317, 904 and Alloy 20 havealso been used in place of 316. However, all these alloys are subject to local corrosionattack, the severity of attack though is much smaller.

Ejector condenser piping, nozzles and venting pipes of MSF plants made of 316L showextensive pitting due to uncondensable gases including chlorine and bromine or deposits.High pressure pipings of RO system made of 316L or similar alloys face problems dueto crevice or pitting corrosion. High alloys stainless steels such as 254 SMO (UNSSS1243) have been utilized to replace conventional 304 or 316 steels. These alloys exhibitalmost total resistance toward crevice or pitting corrosion. High cost appears to be themain limitation in using these alloys. Figures 3 and Figures 4 show crevice corrosion resistanceof a range of stainless steels and nickel-base alloys in ambient seawater.

The other high stainless steels which have promising future as a desalination materialare austenitic SS AL6X (Fe -20Cr - 24 Ni-6.5Mo) and nitrogen - containing AL6XN;ferritic SS such as Sanicro 28, AL29-4C, Monit and Sea-Cure; and duplex SS Ferraliumalloys 255 and alloy 2205. Ferritic stainless steels have exceptional resistance to SCCalong with good pitting/crevice corrosion resistance in aggressive environments. Theseare proven materials for saline water applications. Besides cost, tendency of ferriticmaterial to form hydride under cathodically protected system is their main shortcoming.

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The duplex steels exhibit 50-50 austenitic - ferritic microstructures, have the charac-teristics of near-immunity to chloride SCC (from ferritic steels) and toughness, strengthand ease of fabrication (from austenitic steels).

2.4 Copper-base Alloys

Copper-base alloys have found their applications in desalination plants due to their goodantifouling characteristics and excellent heat exchange properties. The main limitationis their sensitivity towards polluted water (sulfide and ammonia - containing waters)and high water velocities.

Copper and copper alloys occupy a position midway in the galvanic series, which is anadvantage in marine applications. Most copper alloys can be coupled to each otherwithout serious acceleration of galvanic corrosion. However, stainless steels or titaniumtubes in copper alloys systems usually require cathodic protection to prevent acceleratedcorrosion of copper alloys. Copper alloys perform well with increasing water velocity,upto a critical velocity that characterizes each alloy. At high water velocities, the layerof protective film on the metal is stripped away by shear stresses. The critical shearstress varies with velocity-copper alloys can tolerate higher nominal velocities in thepiping system as the diameter increases.

Aluminium brass has excellent resistance to corrosion by clean seawater but not suitablefor use in polluted waters. Cupronickel alloys exhibit better resistance both at highliquid velocity (erosion-corrosion) and at low liquid velocity (pitting attack), ammoniacontamination, suspended solids etc. 90/I0 Cu-Ni is the suitable material for heatrecovery section of the desalination plants where the unit works at relatively hightemperature and attack on the tubes outside is suspected due to non-condensable gasessuch as NH3/CO2. Inside the heat exchanger tubes of recovery section, however, theseawater is deaerated and virtually oxygen free and therefore, no danger of corrosion.

Cu-Ni 70/30 is the most appropriate choice for brine heaters and heat rejection sectionswhich are most prone to corrosion due to the presence of suspended solids, sulfides,chlorine in seawater,etc. In such a situation various types of attack are possible andfailure has been reported in periods as little as 6 months. Modified 70/30 Cu-Ni alloyscontaining Mn2 and Fe2 is better choice due to its property of not only providing stabilityto protective film on the alloy but also self healing to the broken film.

In spite of its high cost Ti has replaced Cu/Ni alloys for heat rejection and brine heatertubes. Titanium self passivates in air and the passive film is not attacked by heavilypolluted water or seawater. A good combination of properties including high strengthto weight ratio, zero thickness allowance for corrosion and excellent performance athigh water velocities makes titanium a good material for high performance heatexchangers.

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Titanium has the tendency towards crevice corrosion especially at high temperatures(above 80oC). Addition of noble metals like Pd appears to improve crevice corrosionresistance of Ti. Thus Ti-.15 Pd and Ti-5Pd-.3 Co have excellent resistance towardscrevice corrosion. Use of Ti tube can create a potential galvanic problems to the tubeplates. Ti is incompatible with Teflon and therefore, it cannot be used as sealing ringsfor Ti tubes otherwise it can undergo pitting corrosion. In such a situation Ni-Al bronzeand high grade stainless steel can be used. Titanium can also form hydride in cathodicallyprotected system. Such a situation should be avoided.

It is evident from the foregoing discussion that modified 66/30/2/2 Cu/Ni/Fe/Mnalloys and titanium are the best materials for heat exchangers.

2.5 Nickel-base Alloys

Nickel-base alloys have good resistance to general aqueous corrosion, pitting, SCC andwear and are therefore, ideal materials for seawater applications. However, their highcost limits their use to applications where reliability is crucial. Hastelloy series of alloyssuch as Hastelloy C, Hastelloy C-276 and Hastelloys C-22, Inconel 625, Incoloy 825 andalloy 718 are some of the alloys which have high degree of pitting/crevice corrosionresistance, corrosion fatigue strength, tensile strength and resistance to chloride ion-SCC. These alloys offer attractive candidature for seawater/desalination applicationsif cost is not the major consideration.

2.6 Composite Materials

Recent advances in reinforced plastics make them very attractive alternatives for metals,alloys or coatings which are either prone to some type of corrosion attack and/or tooexpensive. Plastic-coated and rubber-lined steels and fibre-reinforced plastics have infact found applications in some selected components of desalination plant such as intakepipe, deaerator, decarbonator, venting system, blow down and make up pumps, coatingson water transmission steels or concrete pipes etc. Fibre glass-reinforced plastics lookedpromising due to their light weight, ease of fabrication, high strength, dimensionalstability and above all their excellent erosion/corrosion resistance. These materialsappear to fulfil the condition of moderate cost, high reliability and low maintenancecost. The components where such materials might find applications include evaporator,demister, distillate system, piping etc. Recent developments in conductive plasticmaterials using high aspect ratio fillers (brasses, Al, Ni-plated mica, stainless steel fibres)produced plastic formulations that combine the desired conductivity of materials withprocessing ease and economy of plastics. The materials have thermal conductivity manymagnitude higher than the case polymer. It will not be surprising if the next generation

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of heat exchangers are fabricated from these materials. These material could be strongerand immune to corrosion and erosion, possessing good heat transfer properties andshould be cheaper than the traditional heat exchanger materials.

3. CONCLUSION

An analysis of the corrosion behavior of materials in desalination plants reveals that theoverall performance of materials is satisfactory. The existing materials of constructionemployed in the evaporator and distillate systems of different desalination plants showlittle corrosion. Carbon steel cladded with SS 316L or CuNi appears to be the bestchoice for flash chambers. Besides materials selection, controlled operation and goodmaintenance and strict adherence to shut down procedures would be the other factorsfor fine performance of the materials in the desalination plants. In heat exchangers,use of modified CuNi (66Cu 30Ni 2Fe 2Mn) alloy in heat recovery section, titanium inbrine heater and heat reject section is perhaps the ideal combination to obtain the bestresults.

The problem of local corrosion attack is most frequently experienced in almost everycomponent of desalination plant. It results from stagnancy, deposition, dealloying,galvanic couplation, dealloying and vapor space attack. The local attack (pitting, crevice)can be avoided in most of the cases by minimising dissolved oxygen level of brine anduncondensable gases, proper flushing and keeping an inert atmosphere during shutdown, mechanical or chemical cleaning of deposits and maintaining C.P. where nec-essary. Even then in some instances such as venting pipes and ejector condensers whereSS 316L is a customary material, corrosion was unavoidable and could bc preventedonly by using some highly alloyed steels such as 254 SMO. In reinforced concrete pipelines which are used in intake and water transmission systems, the rebar corrosion canbest be avoided by using alternate materials such as FRP, epoxy coating or fusion bondedepoxy or polyurethane.

There is an emerging field of superior materials borne out from fibre glass/plastic,ceramic/ plastic and metal/ceramic composites which would perhaps offer a widerchoice of material selection for next generation of desalination plants.

SUGGESTIONS FOR FURTHER READING

J.W.Oldlield and B. Todd: “Vapor Side Corrosion In Desalination Plants,”Desalination 66, 171-184 (1987).N.M.Volta: “Copper Alloy Tube for MSF Desalination Plant”, Desalination 66,245256 (1957).B.Todd: “Selection of Materials for High Reliability Seawater Handling System”,Supplement to Chemist a 2 July 1987, pp 14-22.B.Todd and J.W.Oldfield: “Reverse Osmosis - Which Stainless Steel to Use,“A

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vesta Corrosion Mangement (acorn) No. 1-2, 1991.D.C.Agarwal et al: “6% MO Austenitic Stainless Steel Selection for OffshoreApplications”, 23rd Annual Offshore Technology Conference, May 1991, OTC6598, pp 341-354.Ata M.Hassan and A.U.Malik: “Corrosion Resistant Materials for Seawater ROPlants”, Desalination, 74, 157-170 (1989).Ata M.Hassan et al: “Performance Evaluation of SWCC SWRO Plants”, Proc.IDA World Conference, Vol. 1, August, 1991, Washington DC.A.U.Malik: “Case Histories on the Failure of Pipe Lines in Desalination Plants”,Proc. IDA World Conference Vol.I August 1991, Washington DC.Edward J.Kubel Jr.: “Curbing Corrosion in Marine Environments”, AdvancedMaterials andprocess, November, 1988.W.R.Herda: “Materials for Seawater Systems”, VDM Nickel - Technologie AG(1988).J.W.Oldfield and B. Todd: “Corrosion Problems caused by Bromine Formationin MSF Desalination Plants”, Desalination, 38,233 (1981).H.Saricimen et al: “Performance of Austenitic Stainless Steels in MSF Desali-nation Plant Flash Chambers in the Arabian Gulf’, Desalination, October, 1990.Dr. N.Nada: “Venting Pipes Corrosion in MSF Acid Treated Plants”, Topics inDesalination, SWCC, 1986.Saleh G.A1 Zahrani, B.Tood and J.W.Oldfield, “Bimetallic Joints in MSFDesalination Plants”, Topics in Desalination, SWCC, 1986.M.AAhmed Al-Mudaiheem and Hiroshi Miyamura : “Construction and Com-missioning of Al-Jubail Phase-II Desalination Plant”, Topics in Desalination,SWCC, 1986.Arshad H. Khan: “Desalination Process and Multistage Distillation Practice”,Elsvier (1986).A.U.Malik and Mayan Kutty: “Investigation on the Failure of SS 316L ProductWater Line in a Seawater Desalination Plant”, Proc. Fifth Middle East CorrosionConference, Oct. 1991, Bahrain, pp 298-313.Metal Hand Book Vol. 13, Corrosion, ASM (1987).

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