CORROSION AND MATERIAL SELECTION IN DESALINATION PLANTS Anees U Malik and P.C. Mayan Kutty, ABSTRACT - of materials metals, The construction of a desalination plant requires a wide spectrum non-metals and composites which would not only satisfy the design and functional requirements of the particular component but should also have reasonable endurance to the environment surrounding the plant installations. One of the prime considerations during the selection of construction materials for a particular unit or component is its corrosion characteristics.
An MSF desalination plant is exposed to different and diversified kinds of environments such as seawater, seawater-air and salt-air aerosols, corrosive gases, very fast or extremely slow moving liquids, particulates contained in high velocity fluids or deposit-forming liquids all of them create a number of corrosion related problems. Besides general corrosion and mechanically and chemically-induced erosion-corrosion, localized-corrosion such as pitting, galvanic and crevices are quite frequently observed. Cases of intergranular corrosion, selective leaching and stress-induced corrosion are also not uncommon. The flash chambers are subjected to highly aggressive corrosion due to flashing brine and Cl- attack whereas heat exchangers face one of the severest environments and account for 70% of the 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. In seawater reverse osmosis (SWRO) plants, high pressure piping, headers, connectors and membrane containment vessels are prone to localized corrosion attack even though the operating environments are much less severe than ih MSF. Corrosion in desalination plants can cause a variety of undesirable consequences, including loss of equipment, unplanned shutdowns, expensive repairs, leaks and contamination of products as well as serious personal hazards. The criteria for selection of materials for different sections of desalination plant are based upon the nahrre of the corrosive environment, equipment operating conditions, design features, desired plant life and cost effectiveness of the materials. Ferrous and non-ferrous metals and alloys, plastics, rubbers, ceramics, glass and composites including reinforcements are the materials to be considered for 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 plant constructions. Thus the selection and corrosion control are synonymous as far as safe operation and maximum output from the plant facilities are concerned. This presentation discusses the corrosion behavior of materials in environments surrounding the desalination plants with special reference to the different sections of the plant which are vulnerable to corrosion failures. Furthennore, the occurrence of various forms of corrosion during the operation of a desalination plant is described, and the role of local attack is emphasized. The presentation also encompasses the criteria for the selection of materials for different sections of the desalination plant which basically depend upon the nature of the corrosive environment and operating conditions. The short-comings of some of the materials currently employed are highlighted and the introduction of new materials either 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 their operation in relatively aggressive environments consisting of seawater, seawater-air and salt-air aerosols, corrosive gases, very fast or slow moving liquids, particulates contained in high velocity fluids or deposit forming liquids. Corrosion rate data of metals in aerated seawater at different velocities and at ambient temperature indicate highest metal loss in 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 in the form of pitting and crevices corrosion are quite common in desalination plants. In fact, pitting accounts roughly 41% of the corrosion failures in MSF plants and crevice corrosion under deposits is most troublesome. Impingement and cavitation are not uncommon and have been frequently noticed in heat exchanger tubes, pumps, valves and liquid transmission pipes. Erosion- corrosion on water-side accounts 21% of the failures and vapor side 14% of the failures. In MSF plants flash chambers are subjected to severe corrosion due to flashing brine and Cl- attack whereas heat exchangers face most aggressive environment and contribute 70% of the failures resulting from pitting, crevice, impingements, dealloying and galvanic corrosions. Due to deployment of different types of metallic materials in the plant, galvanic interactions in presence of dissimilar metals are quite common and have a major influence on the material selection Table-2. The other components in an MSF plant which are prone to corrosion attack are 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 has been observed in both acid and additive dosed plants at rates well in excess of the designed corrosion allowance. Apart from water vapor which is always present, uncondensable gases evolved from the flashing brine will be present. These gases are mainly CO2, O2, and N2. In some cases H2S and NH3 also would be present if seawater feed to the plant is polluted with decomposing organic materials. Also, where brine heaters are vented to the first stage, NH3 will be present from decomposition of boiler treatment compounds such as hydrazine. NH3 and H2S may cause severe corrosion on Cu- base alloy tubes in heat recovery and reject bundles. Ammonia selectively corrodes Cu-base alloys condenser tubes by forming soluble copper-ammine complex, this causes metal loss and pitting in the tubes and the corrosion products can impede heat transfer across the surface of metal. The soluble complex can attack other materials of construction in the evaporator i.e. Cu plating of carbon steel. Bromine gas produced as a result of seawater chlorination can cause severe localized problems. Acid treated MSF plants provide conditions suitable for bromine gas formation in decarbonator and deaerators. Corrosion due to bromine liberation could be obviated by addition of a dechlorinator 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 this reason, top few stages are usually vented directly to vent system to remove gases as quickly as possible. In acid-dosing plants, most of CO2 is removed by passing through decarbonator before further degassing in the deaerator. However, some CO2 will still be present in the plant evolved through out the flashing process. Proportion of CO2 in additive-dosed plants is much more significant than in acid-dosed plants. Figure 1 shows influence 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 vapor space 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 the brine. Corrosion products in the vapor space is usually black magnetic iron oxide. This itself indicates that oxygen is more predominant than CO2 in causing corrosion. Air leakage rather 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 solution to 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 used as such or cladded with stainless steel or Cu-Ni in early or all the stages. Epoxy coating has also been used. Flash chambers are subjected to severe corrosion and potential metal failures. The role of oxygen in the corrosion of metals of construction in MSF plants in general and evaporators in particular is quite complex. For example, oxygen accelerates corrosion of carbon steel by acting as a cathodic depolarizer whereas in stainless steels it retards by growing and/or repairing oxide film responsible for passivity. Therefore, an oxygen level perhaps to the extent of a few ppb is desirable. Figure 2 shows corrosion rate of carbon steel in seawater as a function of temperature at various oxygen concentrations. H2S and NH3 can cause extensive damage inside the evaporator no matter how small their concentrations are. The interesting features of the corrosion of flash chamber (mild steel) are as follows :
(1) Corrosion is maximum in the middle of the stages where the combined effects of two competing factors e.g. oxygen leakage and temperature are optimum.
(2) Corrosion is usually most severe on the interstage walls and often one wall is much more attacked than the other.
(3) Corrosion product is usually black magnetic oxide, Fe3O4. (4) The corrosion product is separated from the metal by a void. In case the disturbed sheets of corrosion products several mm in thickness fall away, an even metal surface is left behind.
(5) In some plants, blockage of demisters by corrosion products has caused plant shut 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; (iii) High chloride contents of brine; (iv) High Cu content of recirculating brine.
(i)
(ii)
Breaking of the hard magnetite (formed at low D.O.) due to above factors results in the formation of a cell in which base metal acts as an anode and magnetite as cathode Gates controlling flow of brine from stage to stage are made of stainless steel because of its high resistance to turbulent brine. In SS lined stages this poses no problem but 8
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in CS stages sometimes deep pitting occurs beneath the SS gates. Adjacent CS surfaces although showing slight general corrosion do not have these pits. Pitting is attributed to galvanic effect from SS. Electrically insulating the SS by fitting plastic sleeves and washers to all fasteners connecting the SS to CS structure is most effective. In stainless steel-lined flash chambers, severe localized corrosion (pitting, crevice and integrannular) has been observed. The liner plates are joined one another by seam welds and to the mild steel by stainless steel rivets. Micropitting and rust bleeding were prominently appeared at welds, rivets and rivet crevices. Corrosion was most severe at welds and heat affected zones. In SS-lined CS flash chambers the most effective method of controlling corrosion at the junction is to reduce the effective cathodic area by painting the SS but painting of CS to control corrosion is not recommended. Holidays in the coating on SS will not result in a significant increase in attack, however, if CS is painted then 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 moving or stagnant high chloride brine and crevices (formed by salt and/or dirt deposits) on the floors, walls and other appurtenances of the flash chamber and D.O. produce most favorable environment for initiating and propagating corrosion process. Avoidance of air leakage, complete drainage followed by flushing with distilled water are the precautions to do away with corrosion. Deaeration is of prime importance in pretreatment in order to control corrosion in evaporators. Dissolved oxygen in deaerator affluent must 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 flash chambers of first few stages followed by mild steel in other stages provided reasonably satisfactory performance. Only in a few cases some minor corrosion problems have been reported. Table 3 lists the materials used in flash chambers of different SWCC desalination plants in the Kingdom. A corrosion allowance of 12 mm below the demisters and 7.0 mm above the demisters is 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 plant and not surprisingly more than 70% of the corrosion failures in desalination plants are attributed to heat exchange tubes. Heat exchangers tubes handle two fluids of completely different properties (seawater and condensing vapors). It is one of the severest environments 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 and MSF equipments are caused by a condition known as tube inlet erosion/corrosion. This tube inlet damage is almost always located in the first six inches (150 mm) of the tube inlets and is caused by circulating water flow conditions. Severe damage often results in perforation. As the water entrained particles and air enter the tubes from the water box area it is highly turbulent with an infinite number of varying velocities. As it moves along into the tubes, this turbulent flow rapidly becomes laminar and by the time it is 4 inches to 6 inches into the tubes, the intensity of condition causing erosion/corrosion has completely subsided or disappeared. Use of prefabricated plastic or nylon inserts, coating, metallic inserts or shield/seal are the methods employed to combat tube inlet corrosion. Shield/seal method involving thinned walled superalloy shield may provide the best protection. 90/10 Cu Ni, 70/30 Cu Ni, 66/30/2/2 Cu/Ni/Fe/Mn and Ti are the materials used for heat exchanger tubes. The choice of the most suitable copper alloy depends upon the system (brine heater, heat recovery or heat rejection) to be considered. Tables 4 and Table 5 list the various heat exchanger materials used in the different MSF desalination plants in the Kingdom. Brine Heater It operates at high temperature, with a strongly scale forming solution, as these scales are normally very hard and difficult to remove by chemical means, mechanical cleaning is resorted to which in turn requires an intrinsically harder material. 70/30 Cu /Ni or modified 66/30/2/2 has been used as tube material in most of the plants but in Jubail Phase-I, Ti tubing (ASTM338) has been used. In some older plants (Jeddah Phase II to IV, Haql) 90/10 Cu/Ni wa used. Heat Recovery This section normally has its first 3 or 4 stages operated at high temperatures, which may 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. Due to its least vulnerability towards corrosion, 90/10 CuNi has been used in the recovery section 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 which may cause corrosion. Heat Reject Section The seawater flowing in heat rejection tubes is natural, very often chlorinated (against biofouling) 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 space division wall above demister, general corrosion, deposit attack and choking have been reported. Cupronickel alloys Table 6 have shown good performance under high water speed, ammonia contamination, presence of suspended solids, higher temperatures etc. In modified alloys addition of small amounts of Fe and Mn strengthens the metal matrix, thus counteracting impingement corrosion that leads to continuous removal of the protective corrosion layers formed on copper alloys during initial service. It has been found that the additions not only provide stability to surface protective film but also increases its self healing propensity. Therefore, CuNi Fe Mn, the so-called modified alloys are finding increasing use as heat exchanger materials replacing the plain cupro-nickel alloys. Spaces between tube rolled on tube plates are active sites for deposition and crevice attack. Rust bleeding appears from joints of 90/10 tubes and tube sheets made of CS lined with lmm 90/10. Titanium possesses excellent anti corrosion and heat transfer properties but there are problems of crevice corrosion and hydrogen adsorption in high temperature salt water. Addition of noble metal like Pd improves corrosion resistance. Thus Ti-.15Pd and Ti-.05Pd-.3Co have excellent resistance toward crevice corrosion. Cathodic Protection (C.P.) has been shown to yield some benefit in reducing impingement attack on tube ends, as well as waterbox or tube plate corrosion due to galvanic coupling in seawaters; whenever continuous monitoring of cathodic corrosion is unwarranted, sacrificial anode system is preferred over impressed current systems. Iron anodes are preferred over Zn since it interferes less with the formation of protective film on copper base alloys but at the same time releases significant amounts of ferrous ions to act as additional protectant of the tube surface Table 7. The water boxes whose primary function is to maintain same velocity and water distribution in all tubes can suffer if there is stagnation of water around anodes. 1.5
Distillate System
Distillate system has to handle aggressive fluids due to dissolution of non-condensable CO2 and other gases. The parts affected are : (i) (ii) (iii)
Trough (SS316L) Evaporator sheet (CS or cladded CS) 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 distillate systems, corrosion tends to be most severe above the distillate transfer troughs where dissolved 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 SWCC desalination plants are given in Tables 8 and 9. Severe corrosion has been observed in 316L air ejector systems of MSF plants due to the presence of bromine in non-condensable gases. Use of 904L appears to be less problematic since it is resistant to this attack. 1.7.
Ejector Condensers
Barometric type (direct connections) are usually employed. The components vulnerable to corrosion attack are : body, nozzles and piping. Body of the ejector condenser (CS cladded with SS, Ti, Cu-Ni) sometimes showing metal loss has now been replaced by FRPin 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 the after condenser (steam/vapor inlet pipe) severe Cl- induced S.C.C. was noted which was attributed to high operating temperature and salt deposition. The 316L pipe was replaced by Incoloy 825. Presence of 0.1-0.2 ppm chloride in the uncondensed gases is sufficient to produce corrosion. Cupro-nickel and SS are replaced by 254 SMO as an ejector condenser material in some of the plants in Qatar and Abu Dhabi due to their poor performance. 1.8
Venting System
Failures of 304 venting pipes between inter and after condenser due to severe localized pitting was observed in Jeddah Phase-II which was attributed to Bromine attack. SS 316L 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 the desalination plants are affected by corrosion or erosion-corrosion. In desalination plants, vertical lift pumps are usually employed. AISI 316L (cast equivalent CF3M) is the most widely used material for pump internal, impeller, shaft, sleeves etc. due to its excellent corrosion resistance under fast flowing condition, and good 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 condition of stagnancy in seawater. In these pumps, casing of Ni resist is used to provide protection to 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 cavitation damage 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 corrosion fatigue failures have been observed in the shafts of brine recycle pumps of the desalination plants. 1.10 Valves General corrosion, pitting, impingement and cavitation are the common mode of corrosion failure. Body and disc of the throttling valves in down stream are affected by impingement showing horse shoe shaped pits. Ball valves made of chrome-plated carbon steel and which were used in water transmission line show pitting and general corrosion. 1.11
Intake System
Failures have occurred in pipelines carrying seawater due to rebar corrosion or in RCC structures exposed to seawater environment. Splashing or intermittent drying and soaking 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 in desalination plants. Type of corrosion observed and the materials and systems affected are stated as follows: Type of Corrosion Pitting and crevice corrosion
Materials and Systems Involved 316L distillate pipeline due to dissolved CO2, O2, high Clor stagnancy.
13
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Impingement
Mild steel/cast iron/epoxy lined pipeline due to water hammer or turbulence.
General corrosion and pitting
316L vent pipes due to non-condensable gas attack
Rebar corrosion
CCP or PCP product water pipe lines. General corrosion due to the formation of electrochemical cells between rebar and air/O2/CO2 through concrete cover.
1.13
Reverse Osmosis Plants
For a high pressure seawater reverse osmosis (SWRO) plant consisting of pumps, piping and valves and handling high level of chloride (22,000 to 36,000 ppm) and dissolved oxygen (5-10 ppm), improper selection of materials and/or laxity in operation may result in pitting, crevice or stress corrosion. Austenitic stainless steels are the conventional materials used for high pressure inlet piping leading to RO membrane module, brine rejection pipe, product water outlet pipe and high pressure pumps. High velocities of the feed water and design of RO does not encourage formation of crevices. Even then high pressure piping (closed to weld or heat affected zones), headers, connectors, flanges, seals of pumps and membrane containment vessels are prone to crevice corrosion attack. SCC is not a problem in RO because of working at lower temperatures (below 70 o C). The performance of materials in different SWRO plants in the Kingdom and outside is given in Table 10. Austenitic 316L, 317L and 904L (in order of increasingly good performance) are suitable materials for SWRO high pressure piping under conditions of good quality plant maintenance and efficient flushing system. However, these alloys are prone to crevice corrosion attack in case stagnant conditions are developed or deposits are formed in the 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 considerations in the selection of materials. Plain carbon steels, copper-base alloys, stainless steels, 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 which have important relevance as construction materials for seawater desalination plants. Table 12 gives a cost comparison of various pipe materials on weight basis assuming carbon steel cost index as 1. Fibre glass reinforced plastics (FRP), rubbers, various types of epoxy, vinyl, polyurethane based coatings, concrete and reinforced concrete are some of the non-metallic and organic based materials utilized in the plants. Various types of materials used in marine applications are classified in Table 13 and Table 14 provides a classification of materials used in desalination plant equipments. The classification 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 materials because of the fact that these materials have adequate mechanical properties and excellent fabricability and are inexpensive and most abundantly available. They have moderate to good corrosion resistance if serviced under coated or cathodic protection condition. Carbon steels have high general corrosion rates particularly when high velocities 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 corrosion control is provided either by barrier coatings or cathodic protection, C.P. (impressed currents or sacrificial anodes) combined with organic coatings. Cathodic protection 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 problem of corrosion is mainly confined to the first few stages due to relatively high temperatures and high turbulence, better corrosion resistant materials such as 90/10 Cu-Ni or AISI 316SS have been utilized. The latter has frequently been used as lining or cladding materials for the carbon steels. In higher stages (usually above stage 3), carbon steel flash chambers have been used. 90/10 Cu-Ni appears to have better performance than 316 SS since the former does not undergo pitting and crevice corrosion under stagnant conditions - a condition sometimes encountered during shut down and is also not subject to 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 - corrosion even 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 general corrosion rates of stainless steels in flowing seawater above a velocity of about 1 m/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 resistance to general corrosion. Their main weakness is localized corrosion in Cl- containing solutions and pitting under static condition. Mo-containing 316 has better corrosion performance and is more widely used. The application of 316 L in desalination plant’s equipments 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 well as 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 of nitrogen not only increases pitting/crevice corrossion resistance, and possibly resistance to SCC but also increases strength substantially without sacrificing ductility. Another effect of nitrogen is the suppression of intermetallic phase precipitation which allows processing and welding of heavy sections. Stainless steels with improved corrosion resistance such as 317, 904 and Alloy 20 have also been used in place of 316. However, all these alloys are subject to local corrosion attack, the severity of attack though is much smaller. Ejector condenser piping, nozzles and venting pipes of MSF plants made of 316L show extensive 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 due to crevice or pitting corrosion. High alloys stainless steels such as 254 SMO (UNS SS1243) have been utilized to replace conventional 304 or 316 steels. These alloys exhibit almost total resistance toward crevice or pitting corrosion. High cost appears to be the main limitation in using these alloys. Figures 3 and Figures 4 show crevice corrosion resistance of a range of stainless steels and nickel-base alloys in ambient seawater. The other high stainless steels which have promising future as a desalination material are 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 Ferralium alloys 255 and alloy 2205. Ferritic stainless steels have exceptional resistance to SCC along with good pitting/crevice corrosion resistance in aggressive environments. These are proven materials for saline water applications. Besides cost, tendency of ferritic material 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 characteristics of near-immunity to chloride SCC (from ferritic steels) and toughness, strength and ease of fabrication (from austenitic steels). 2.4
Copper-base Alloys
Copper-base alloys have found their applications in desalination plants due to their good antifouling characteristics and excellent heat exchange properties. The main limitation is 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 an advantage in marine applications. Most copper alloys can be coupled to each other without serious acceleration of galvanic corrosion. However, stainless steels or titanium tubes in copper alloys systems usually require cathodic protection to prevent accelerated corrosion of copper alloys. Copper alloys perform well with increasing water velocity, upto a critical velocity that characterizes each alloy. At high water velocities, the layer of protective film on the metal is stripped away by shear stresses. The critical shear stress varies with velocity-copper alloys can tolerate higher nominal velocities in the piping system as the diameter increases. Aluminium brass has excellent resistance to corrosion by clean seawater but not suitable for use in polluted waters. Cupronickel alloys exhibit better resistance both at high liquid velocity (erosion-corrosion) and at low liquid velocity (pitting attack), ammonia contamination, suspended solids etc. 90/I0 Cu-Ni is the suitable material for heat recovery section of the desalination plants where the unit works at relatively high temperature and attack on the tubes outside is suspected due to non-condensable gases such as NH3/CO2. Inside the heat exchanger tubes of recovery section, however, the seawater 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 sections which 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 and failure has been reported in periods as little as 6 months. Modified 70/30 Cu-Ni alloys containing Mn2 and Fe2 is better choice due to its property of not only providing stability to 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 heater tubes. Titanium self passivates in air and the passive film is not attacked by heavily polluted water or seawater. A good combination of properties including high strength to weight ratio, zero thickness allowance for corrosion and excellent performance at high water velocities makes titanium a good material for high performance heat exchangers.
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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 corrosion resistance of Ti. Thus Ti-.15 Pd and Ti-5Pd-.3 Co have excellent resistance towards crevice corrosion. Use of Ti tube can create a potential galvanic problems to the tube plates. Ti is incompatible with Teflon and therefore, it cannot be used as sealing rings for Ti tubes otherwise it can undergo pitting corrosion. In such a situation Ni-Al bronze and high grade stainless steel can be used. Titanium can also form hydride in cathodically protected system. Such a situation should be avoided. It is evident from the foregoing discussion that modified 66/30/2/2 Cu/Ni/Fe/Mn alloys 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 and wear and are therefore, ideal materials for seawater applications. However, their high cost limits their use to applications where reliability is crucial. Hastelloy series of alloys such as Hastelloy C, Hastelloy C-276 and Hastelloys C-22, Inconel 625, Incoloy 825 and alloy 718 are some of the alloys which have high degree of pitting/crevice corrosion resistance, corrosion fatigue strength, tensile strength and resistance to chloride ionSCC. These alloys offer attractive candidature for seawater/desalination applications if 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 too expensive. Plastic-coated and rubber-lined steels and fibre-reinforced plastics have in fact found applications in some selected components of desalination plant such as intake pipe, deaerator, decarbonator, venting system, blow down and make up pumps, coatings on water transmission steels or concrete pipes etc. Fibre glass-reinforced plastics looked promising due to their light weight, ease of fabrication, high strength, dimensional stability and above all their excellent erosion/corrosion resistance. These materials appear to fulfil the condition of moderate cost, high reliability and low maintenance cost. The components where such materials might find applications include evaporator, demister, distillate system, piping etc. Recent developments in conductive plastic materials using high aspect ratio fillers (brasses, Al, Ni-plated mica, stainless steel fibres) produced plastic formulations that combine the desired conductivity of materials with processing ease and economy of plastics. The materials have thermal conductivity many magnitude 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 stronger and immune to corrosion and erosion, possessing good heat transfer properties and should be cheaper than the traditional heat exchanger materials. 3.
CONCLUSION
An analysis of the corrosion behavior of materials in desalination plants reveals that the overall performance of materials is satisfactory. The existing materials of construction employed in the evaporator and distillate systems of different desalination plants show little corrosion. Carbon steel cladded with SS 316L or CuNi appears to be the best choice for flash chambers. Besides materials selection, controlled operation and good maintenance and strict adherence to shut down procedures would be the other factors for 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 in brine heater and heat reject section is perhaps the ideal combination to obtain the best results. The problem of local corrosion attack is most frequently experienced in almost every component 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 and uncondensable gases, proper flushing and keeping an inert atmosphere during shut down, mechanical or chemical cleaning of deposits and maintaining C.P. where necessary. Even then in some instances such as venting pipes and ejector condensers where SS 316L is a customary material, corrosion was unavoidable and could bc prevented only by using some highly alloyed steels such as 254 SMO. In reinforced concrete pipe lines which are used in intake and water transmission systems, the rebar corrosion can best be avoided by using alternate materials such as FRP, epoxy coating or fusion bonded epoxy 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 wider choice 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. 5. D.C.Agarwal et al: “6% MO Austenitic Stainless Steel Selection for Offshore Applications”, 23rd Annual Offshore Technology Conference, May 1991, OTC 6598, pp 341-354. 6 . Ata M.Hassan and A.U.Malik: “Corrosion Resistant Materials for Seawater RO Plants”, Desalination, 74, 157-170 (1989). Ata M.Hassan et al: “Performance Evaluation of SWCC SWRO Plants”, Proc. 7. IDA World Conference, Vol. 1, August, 1991, Washington DC. A.U.Malik: “Case Histories on the Failure of Pipe Lines in Desalination Plants”, 8. Proc. IDA World Conference Vol.I August 1991, Washington DC. Edward J.Kubel Jr.: “Curbing Corrosion in Marine Environments”, Advanced 9. Materials andprocess, November, 1988. 10. W.R.Herda: “Materials for Seawater Systems”, VDM Nickel - Technologie AG (1988). 11. J.W.Oldfield and B. Todd: “Corrosion Problems caused by Bromine Formation in MSF Desalination Plants”, Desalination, 38,233 (1981). 12. H.Saricimen et al: “Performance of Austenitic Stainless Steels in MSF Desalination Plant Flash Chambers in the Arabian Gulf’, Desalination, October, 1990. Dr. N.Nada: “Venting Pipes Corrosion in MSF Acid Treated Plants”, Topics in 13. Desalination, SWCC, 1986. 14. Saleh G.A1 Zahrani, B.Tood and J.W.Oldfield, “Bimetallic Joints in MSF Desalination Plants”, Topics in Desalination, SWCC, 1986. 15. M.AAhmed Al-Mudaiheem and Hiroshi Miyamura : “Construction and Commissioning of Al-Jubail Phase-II Desalination Plant”, Topics in Desalination, SWCC, 1986. 16. Arshad H. Khan: “Desalination Process and Multistage Distillation Practice”, Elsvier (1986). 17. A.U.Malik and Mayan Kutty: “Investigation on the Failure of SS 316L Product Water Line in a Seawater Desalination Plant”, Proc. Fifth Middle East Corrosion Conference, Oct. 1991, Bahrain, pp 298-313. 18. Metal Hand Book Vol. 13, Corrosion, ASM (1987).
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