Materials for Seawater Application

8/6/2019 Materials for Seawater Application

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CORROSION RESISTANT MATERIALS FOR SEAWATER RO PLANTS1

Ata M. Hassan and A.U. Malik

A B ST R A C T

A survey of corrosion resistant materials employed in Seawater ReverseOsmosis (SWRO) desalination plants in Saudi Arabia and the GCC countries hasbeen carried out with special reference to stainless steels. The history of theapplication of these materials is reviewed along with the major corrosion problems

encountered in those plants with main emphasis on SWCC RO desalination plants.The two SS alloys 316L and 317L have been used in the construction of twoSWCC SWRO plants at the remote area plants of Al-Birk and Umm Lujj withno noticeable corrosion damage in either plant after their operation for over 5years and 3 years, respectively. Similarly, other SS alloys 317LN, 904L, 254SMOand 329 performed well, without any significant corrosion when they were usedin the construction of SWRO plants or parts there of in the GCC countries. Theseplants are in continuous operation for years. The paper also discusses the roleof different alloying additions in determining the corrosion resistance andmechanical properties of stainless steels relevant to SWRO desalination appli-cations. The possibility of some new cost-effective materials with emphasis onbetter corrosion resistance and mechanical properties are reviewed. Finally, likein MSF plants, the feasibility of using deaerated seawater feed in RO plants,hitherto not much used in RO plants is discussed.

INTRODUCTION

A proper selection of materials of construction is one of the importantfactors in the efficient and economic operation of a desalination plant. In SaudiArabia and other GCC countries, multistage flash (MSF) process involving themultistage distillation of seawater at low pressure and reverse osmosis (RO) basedon the separation of fresh water from seawater through a membrane under highpressure are the two main industrial processes used or desalination. Althoughthe bulk of water is produced from MSF plants, due to the simplicity in designand operation of RO plants and their lower operation and maintenance costs,there is an upsurge in the number of RO plants in recent years especially inpost 1980 era. Moreover, by comparison to MSF which was applied in seawaterdesalination during the late fifties, seawater desalination by RO process is arelatively new technology. The first major size SWRO plant was the 12,000m3 /d Jeddah SWRO plant, built in 1978-1979 by the Saline Water ConversionCorporation (SWCC), Saudi Arabia. Since that time numerous SWRO plants have

1 Published in Desalination, 74 (1989), pp 157-170.

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been built in the GCC countries with most of them in Saudi Arabia whichpresently has SWRO capacity in excess of 133,000 m3 /d, located mainly on theRed Sea coast ( 1).

A survey of literature on materials used in desalination plants indicates thatalthough numerous references are available on corrosion resistant materials relatedto MSF plants but only a limited references are available on similar informationsrelevant to SWRO plants (2-7). The failure of materials due to corrosion inSWRO plants is not uncommon, the failure can be attributed either to the useof improper material(s) or poor maintenance or both. In an RO system, thecorrosion products from a component material can block the membrane surfaceand adversely affect the efficiency of the operation. Improper selection ofmaterials for a high pressure RO system consisting of pumps, pipings and valvesand handling high levels of chlorides (22,000 to 36,000 ppm) and dissolvedoxygen (5-10 ppm), results in pitting, crevice and stress corrosion.

The early SWRO plants were built utilizing 316L in the construction of thehigh pressure part of the SWRO plant. With the advancement in stainless steel(SS), alloys of newer and more corrosion resistant austenitic and duplex SS alloysof high strength are being used along with 316L in the construction of SWROplants. These SS alloys include 317L, 317LN, 904L, 254SMO plus others. Duplex329 was also used in certain cases in the internal parts of the high pressurepumps. Use of high grade alloys with larger content of chromium andmolybdenum in their composition is intended to prevent and combat SS corrosionin seawater environment, mainly the pitting, crevice and stress cracking typecorrosion.

This paper surveys the use of stainless steel alloys and their performancein several SWRO plants in the Gulf, with special emphasis on SWCC SWROplants. The paper also presents a brief discussion on the role of different alloyingadditives in determining the corrosion resistance and mechanical properties of

SS in seawater environment under different feed conditions of pH, temperature,flow rate, and oxygen level.

THE RO PROCESS

The seawater reverse osmosis desalination is a separation process by whichthe pretreated saline feed is separated into a product stream and a concentrate(brine) stream after passing the feed over the surface of a semipermeablemembrane under high pressure. The product water containing less than 1.5%of the salt content in the feed, flows at a normal pressure and may requirepassage into a second stage RO plant (brackish water type) to lower its saltcontent even further. The final product is stabilized by the addition of hardness,mostly by lime and CO2 treatment. The reject water which contains about 50%more salt than the feed maintains nearly the same pressure as that in thepressurized feed with the exception of small pressure drops. In a nonfouling

membrane this drop is normally less than 1% of the feed pressure. The highenergy in the reject which otherwise would be wasted can be recovered by theutilization of an energy recovery system.

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Austenitic steels owe their corrosion resistance to a passive film of Cr2O3formed on the steel. These steels are, however, prone to pitting and crevicecorrosion in certain waters notably those containing chlorides and/or in stagnantuntreated seawater. The resistance to pitting and crevice can be improved bythe addition of Mo to the SS. Since Mo is a ferrite former, it is necessary toincrease the Ni content to maintain an austenitic structure. The presence of N,a strong austenitic former, also improves remarkably the pitting and crevicecorrosion of stainless steels. Nitrogen also increases the proof stress, so thatthinner sections can be used to withstand the high pressure in RO plants. Therelative effect of Cr, Mo and N on crevice corrosion are assessed on the widelyused PREN, Pitting Resistance Equivalent (7);

PREN = Cr% + 3.3Mo% + 16 N%

According to this empirical equation the pitting resistance of SS alloys

increases as their content of these three alloying elements is raised. One partof Mo and N, however, has 3.3 and 16 times the effect of pitting resistance ofone part of Cr, respectively. The nominal composition of some standard stainlesssteels that are mentioned in this study is given in Table 1, which also providesdata on their mechanical properties, PREN, etc.

The 316L has been the most commonly used material for brackish or seawaterRO plants. The alloy has been used in pumps and high pressure pipings. Athigh velocity of feed water, the corrosion is less frequent in pumps and pipings,however, failures in the forms of pitting and crevice corrosion are some timeobserved at flanges and seals of the pumps and at or close to the weld or heataffected zones in the high pressure pipings. The stress corrosion cracking (SCC)is not a common phenomenon below 7OoC and since SWRO plants work at amuch lower temperatures therefore SCC is not of much interest. Besides 316L,the other austenitic stainless steels used for RO applications are 317L and 904L(UNSO8904) (8 - 13). In RO plants, 904L has been suggested as a better alternativeto 316L because of its superior corrosion resistance. The 317L has a much higherstrength than 316L. 317LN exhibits about 40% higher strength than 317L,therefore the former is of special interest for high pressure sections of RO plantsusing large diameter pipes.

Although reasonably long life is expected from stainless steel grades suchas 316L or 317L in RO applications provided high velocities of the feed waterare maintained and the design of the RO module does not encourage forma-tion of crevices. Even under these conditions, high pressure piping, headers,connectors and membrane containment vessels are prone to crevices corrosionattack. In such cases, the use of relatively less costlier material like 317L mightnot be economical in the long run and the use of higher grade stainless steelscontaining larger amounts of Cr, Mo or Ni is desirable.

In recent years, the manufacturers introduced a large number of other alloyswhich are practically immune to crevice and pitting corrosions and have muchsuperior strength. Table 1 lists the compositions of some commercially availablehigh grade alloy steels such as 20Cb-3, 254SMo, AL-6XN, C276, 20MOD,Cronifer 1925 HMo which are austenitic SS. A limited number of alloys suchas 2205, 255 and 329 are duplex (austenitic-ferritic). Super ferrit Remanit 4575

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and MONIT are the ferritic steels which are suggested for high pressure RO.piping. Amongst high alloy steels, 254SMO has been in use in some SWROplants in Middle East. An equivalent replacement to 254SMO will be: AL6X,AL6XN, Cronifer 1925 HMo. All those SS alloys have PREN in excess of 42.It is also possible to use the nickel-base alloys 625 and Hastelloy C-276 withPREN of 50 & 68, respectively. Ni-base alloys such as Inconel 625 or HastelloyC-276 are virtually immune to pitting and crevice corrosion in seawater buttheir high costs overshadow their good corrosion and mechanical properties.

STAINLESS STEEL ALLOYS USED IN SWRO PLANTS IN THE GULF

Table 2 lists several SWRO plants, their capacity, date of commissioningand type of SS used in their construction. The austenitic 316L and 317L havebeen the choice material for many seawater RO plants built by SWCC in SaudiArabia. The 316L have been used in the construction of the 12,000 m3 /d Jeddah

SWRO plant and the 2,270 m3 /d Al-Birk SWRO plant. The 317L was used atthe 4,400 m3 /d Umm Lujj SWRO plant and the 56,800 m3 /d Jeddah 1 Reha-bilitation SWRO plant. All these plants are located in the Western region of theKingdom at the Red Sea. In the Ras Tanajib SWRO plant, built by ARAMCO,Saudi Arabia, the 317LN (DIN 1.4439) was used in all high pressure pipes andvessels. All other parts were made of 904L (DIN 1.4539). Carbon steel withrubber lining was used in a few other special parts (9).

Various stainless steel materials were used in the 3,000 m3 /d SWRO pilotplant in Doha, Kuwait. One SWRO line, capacity 1,000 m3 /d, utilized SS 317Lin high pressure pipes, 316Ti (DIN 1.4571) in valves and duplex stainless steel329 (DIN 1.4460) in high pressure pump (10). In the second line, capacity 1,000m3 /d, 904L (DIN 1.4539) was used in the high pressure pipes & valves whileDuplex 329 was used in the internal parts of the pump that are in contact withseawater (11). In the third line, also capacity 1,000 m3 /d, rubber lined carbon

steel was used in the high pressure pipes (12). The high grade stainless steelalloy 254 SMO (UNS S31254) was used in the 46,000 m3 /d Ras Abu Jarjur SWROplant (13). The feed TDS is about 13,000 ppm & contains H2S.

PERFORMANCE EVALUATION OF SS ALLOYS IN SWRO PLANTS

Table 2 lists the various SWCC SWRO plants examined in this study, theircapacity, type of membrane and SS used in the high pressure desalination partof plants. It also summarizes their corrosion performance which was establishedfrom their operation and maintenance data and also from visits made to theplants. Except for damage in bucket surface of the Pelton Wheel energy recoverysystem used at Umm Lujj plant, no noticeable corrosion occurred at the Al-Birkplant, constructed from 316L, nor at Umm Lujj plant in which 317L was usedin plant construction. The first plant has been in operation over 5 years whilethe second plant was in operation about 3 years. The two plants are spotlesslyclean and in appearance they resemble a newly constructed, recently commis-

sioned plant. Because the two plants provide the only source of drinking waterto both towns they are well-maintained and well-operated on a continuous basis:Umm Lujj plant operates at full capacity year around. Al-Birk plant which hasexcess capacity, has only one of the two trains in continuous operation yearround. Moreover, each of the two plants is well-designed and equipped with a

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good flushing system thus avoiding saline water stagnation in the units duringshut-down or stoppage. Evidently the two SS alloys 316L and 317L have sufficientcorrosion resistance to SWRO operation. The good plant operation and pro-grammed maintenance along with plant operation at pH greater than 6, relativelylow temperature 20-35oC and low water recovery (less than 30%) are importantfactors that also contributed to their excellent corrosion performance in suchsaline water environment.

By contrast to the good corrosion performance of the above two plants, the12,000 m3 /d Jeddah plant which was the first major SWRO plant to be built,now about 10 years old, showed corrosion in some parts of the plant. Corrosionoccurred in some parts of the 316L high pressure feed pipes, external coupling joints and some parts of the module end-caps. Very little corrosion was observedin the 316L header, the distribution pipes, and the 316L pipes connector to themodule.

The good corrosion resistance of 317L in SWRO plants was also reportedafter two years of plant operation at Doha-Kuwait SWRO plant, in which thehigh pressure feed pipes in one unit of the 3 X 1000 m3 /d units were constructedfrom this SS alloy. The plant has 97% availability (10). Similar behaviour wasalso established for 317LN after one and one half years operation at Tanajibplant. The plant which employs a deaerator also has high availability andmaintained good productivity (9). Although some valves exhibited minorerosion-corrosion, the 904L alloy, with molybdenum content in the alloy of 4.5%compared to 3-4% in 317L alloy and 2-3% in 316L (See Table l), also showsgood corrosion resistance, after 2 years of plant operation, in SWRO plant atDoha- Kuwait where it IS used in high pressure pipes and valves (11). Thealloys also performed well at Tanajib plant where it was used in high pressurevalves and the internal part of the high pressure pump (9).

After two years of operation no corrosion has been observed in Ras AbuJarjur SWRO plant in which the SS 254SMO, with Molybdenum content of 6.5%,was utilized in the high pressure pipes and pumps. Alloys 20 which was usedin the construction of valves in the same plant did not show any corrosion either.Some pitting corrosion due to cavitation, however, was observed in wheel bucketsof the energy recovery system (13,14). Use of Duplex 329 in high pressurepumps at Doha-Kuwait SWRO plant proves satisfactory in resisting corrosionafter 2 years operation (11,12).

From the above discussion it can be concluded that the numerous SS alloys:316L, 317L, 317LN, 904L, 254 SMO and 329 which were used as describedabove in various SWRO plant in the GCC countries performed well againstcorrosion. With good plant design and good operational and maintenanceprocedures it should be safe to use any of those alloys in the construction ofSWRO plants even at high TDS in seawater, i.e., 43,000 ppm. The question that

arises is why should SWRO plant builders and users select 254 SMO or 904Lrather than 316L when their price ratio varies considerably in the order of 2.7to 2.45 to 1, respectively? (For SS cost comparison see Table 1, final column).Truely the former alloy will have greater pitting, crevice corrosion and stresscorrosion resistance than 316L but does that justify this price differential? Thesuppliers argue that with higher strength of 254 SMO it is possible to use a much

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thinner material of construction when using this alloy than when using 316L or317L which have lower yield strength (See Table 1). Calculations made by SSsuppliers showed that when this factor is taken into consideration the pricedifferential in SS material between 254 SMO & 317L drops significantly. Sofar SWCC has been specifying 317L in its latest plants, e.g., the 56,800 m 3/dJeddah SWRO plant, the 4,400 m3 /d Umm Lujj SWRO plant. This is a goodcompromise choice since 317L is an improvement in properties over 316L andis much cheaper than 254 SMO which has higher Molybdenum and Nickelcontent.

USE OF DEAERATORS IN R.O. PLANTS

The role of oxygen is found to be of key importance in corrosion ofdesalination plants equipment in general and MSF in particular. Oxygeninvariably acts as a cathode during a corrosion process involving an electro-

chemical cell and therefore, always has a significant effect on corrosion rates.Virtually complete removal of oxygen is a prime step in saline water treatmentby flash distillation using acid treatment. Even for MSF plants operated at lowtemperature of 90.6oC, such as Jubail Phase II MSF plant, feed deaerationfollowed by SBS injection is carried out to remove dissolved oxygen from theseawater make-up feed. Proper degasification is essential to minimize corrosionof evaporator internals, to prevent carbonate scale and to minimize condensorfouling by non condensable gases. Generally, in MSF plants the level of dissolvedoxygen is kept to minimum (less than 20 ppb) by using a deaerator followed bydeoxidizer like sodium bisulfite. However, a very low concentration (perhapsa few ppb) of oxygen may be required to heal a ruptured oxide film if it occurs.Although the deaerator forms a vital unit of a MSF plant, it has only sparselybeen considered for SWRO plants. In one SWRO plant a vacuum deaerator hasbeen used in the Aramco SWRO plant at Tanajib, Arabian Gulf Coast, with atotal capacity of 0.6 mgpd (9). The deaeration provided a residual oxygen of

0.2 ppm in the feed water. For elimination of the residual oxgen after thevacuum deaeration, a dosing pump injects approx. 20 - 30 ppm of NaHSO3 intothe main pipe down stream of the aerator. Main objective for using the deaeratoris the avoidance of destruction by oxidation of the polyether composite membranewhich is highly sensitive to oxygen and chlorine. Removal of oxygen bydeaeration also minimizes the corrosion problems in materials and reducedconsumption of sodium bisulfite. Even after elimination of oxygen from water,the alloy 317LN was chosen for high pressure piping while in other parts 904Lwas used.

No conclusion can be drawn from this plant on the effect of deaeration onthe corrosion performance of SS alloys material in oxygen free feed. In thisplant the prime objective of deaeration was the complete removal from the feedof oxygen which is destructive to this type of membrane. The observed goodcorrosion performance of the two alloys used in Tanajib SWRO plant operatingwith deaerated feed was also observed for the same alloys when used in other

plants without deaeration (10,ll). A more informative experiment, however,will be in utilizing 316L or 317L in two similarly constructed SWRO plants butone-with deaerator and the second without. As already has been demonstratedin MSF plants, definitely, feed deaeration should allow the use of lower gradeSS alloys in SWRO plants as well.

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CONCLUSIONS

A survey of corrosion resistant materials employed in Sea Water ReverseOsmosis (SWRO) desalination plants in Saudi Arabia and GCC countries showsthat the conventional SS alloys: 316L, 317L, 317LN, 904L, 254SMO and 329used in the construction of RO plants do not show any significant corrosionprovided the plant design & operation are made correctly. Most of these plantsare in operation for more than 5 years. Some new cost effective materials withbetter corrosion resistance and mechanical properties offer potential for appli-cations in new generation SWRO plants.

The relative effect of Cr, Mo and N on Crevice Corrosion can be assessedfrom the widely used PREN, Pitting Resistance Eauivalent.

PREN = Cr % + 3.3 Mo% + 16N %

Use of Deaerators in RO Plants

Although the deaerator forms a vital unit of a MSF plant it has only sparselybeen considered for SWRO plants. Like MSF plants, feed deaeration shouldmost probably allow the use of low grade SS alloys in SWRO plants. Moreinformation regarding the usefulness of deaerator in SWRO plants can be obtainedby utilizing 316L, 317L or similar SS steels in two similarly constructed ROplants but one with deaerator and second without.

REFERENCES

1.

2.

3.

4.

5.

6.

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8.

9.

10 .11 .

12 .

1988 IDA Worldwide Desalting Plants Inventory, June 1988.

J.W. Oldfield and B. Todd, Desalination 55( 1985) 261-280, also see NUCCINo.4-84

R.M. Kain, Paper 200, NACE, Corrosion, 81, also see ACOM, No. 1-1988.

S. N o r d i n and J. Olsson, Desalination 55 (1985), 334 also see ACOM No.

T.J. Glover, MP, July 1988: Presented at Corrosion/88, Paper No. 323in St. Louis, Ma.

S. Nordin & J. Olsson, Desalination 62 (1987), 235-244.

G. Herbsleb, Werkstaffe und Korrosion, 33 (1982), 334.

A.M. Hassan, S. Al-Jarrah, T. Al-Lohibi, A. Hamdan, L.M. Bakheet andM.I. Al-Amri, Performance of SWCC SWRO Plants, Paper for presen-tation at the IDA 4th World Congress, Kuwait, Nov 1989.

N. Heyden, Desalination, 52 (1985), 187-199.

J. Grigoleit and B. Schottler, Desalination, 63 (1987), 217-223.W. Nieszen, Desalination, 63( 1987), 247-262.

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13. M.Al-Arrayedh, B. Ericsson and M. Ohthani, Desalination, 55 (1985),319-342.

14. M. Al-Arrayedh, B. Ericsson, M.A. Saad and H. Yushioka, Desalination,65(1987), 197-230.

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Materials for Seawater Application