acom No. 4-92

acomApplication of UNS S31254

(Avesta Sheffield 254 SMO®)Austenitic Stainless Steel

in Power Plantsby

Jan Olsson, Research & Development, Avesta Sheffield AB, S-774 80 Avesta, Sweden, andJames D. Redmond, Technical Marketing Resources, Inc., 3209 McKnight East Drive, Pittsburgh, PA 15237, USA

AbstractUNS S31254 six percent molybdenum austenitic stain-less steel has been successfully used in seawatercooled power plant condensers since September 1981.Utility applications include condenser tubing, heatexchanger tubing, tubesheets and service water pipingin nuclear power plants, and condenser tubing and heatexchanger tubing in coal fired power plants. The oper-ating environments and performance histories aredescribed.

IntroductionThe copper-base alloys, like copper-nickel and differenttypes of brass, were extensively used in seawatercooled condensers in power plants built in Finland,Sweden, and the United States during the nineteensixties and seventies. For example, all sixteen units ofthe six nuclear power stations in Finland and Swedeninitially used either copper-nickel or brass in theircondenser tubes.

The copper-base alloy condenser tubes had, however,inadequate corrosion resistance. Several test programswere performed in order to establish suitable replace-ment materials. Highly alloyed stainless steels and tita-nium were the strongest candidates. The overall bestresult for the stainless steels was achieved by AvestaSheffield 254 SMO® (UNS S31254) as it was completelyresistant in all field tests performed (1, 2, 3). Similarresults were obtained in a series of seawater exposuretests that involved both ambient seawater exposure forperiods of 2, 9, and 24 months and exposure to filteredseawater at 25°C (77°F) for two months (4, 5). Crevicecorrosion resistance in both seawater exposure testscorrelated well with the critical crevice corrosion tem-perature in 10% FeCl3 · 6H2O at pH 1. These resultsare summarized in Table 1 (page 2).

254 SMO is a registered trademark of Avesta Sheffield AB.Paper presented at the NACE annual conference and cor-rosion show, March 11-15, 1991, Cincinnati, Ohio. Copyright:NACE, Houston, TX.

AVESTA SHEFFIELD CORROSION MANAGMENT

4-1992

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Table 1: Results of crevice corrosion tests

Maximum penetration, mm

FilteredAmbient seawater seawater CCT*

UNS No. 2 months1) 9 months2) 24 months2) 2 months °C (°F)S31603 0.23 -3) - 1.41 -2.5 (27.5)N08700 0.00 0.09 0.11 1.49 15.0 (59.0)N08904 0.00 0.06 0.05, (0.08)4) 0.83 0 (32.0)N08925 - - - 0.55 22.5 (72.5)N08366 0.00 0.01, (0.02)4) 0.02 0.23 17.5 (63.5)N08367 - - - 0.18 32.5 (90.5)S31254 0.00 0.00 0.00 0.00 32.5 (90.5)

1) The temperature range was 15.6° to 25°C (60.1° to 77.0°F).2) The temperature range was 5° to 31°C (41.0° to 87.8°F).3) Not tested.4) Attack at fouling sites.

* Critical Crevice Temperaturein 10 % FeCl3 · 6H2O; pH 1.

MetallurgyS31254 is a fully austenitic stainless steel with a chemi-cal composition as shown below. ASTM and ASMEspecifications and code coverage for S31254 are sum-marized in Table 2.

Chemical composition of UNS S31254 (weight p.c.):

Cmax. Cr Ni Mo Cu N0.020 20 18 6.2 0.7 0.20

Table 2: S31254 specifications and codes

ASTMA167, A182, A240, A249, A269, A276, A312, A351,A358, A403, A409, A473, A479, A743 and A744.Pending in A 193, A 194.

ASMESection III, Division 1, Class 2 and 3

Code Case N-439: SA-182, SA-240, SA-249, SA-312,SA-358

Code Case N-440: Castings to SA-351Code Case N-441: Fittings to SA-403

Section VIII, Division 1, up to 400°C (750°F)SA-182, SA-240, SA-249, SA-312, SA-358, SA-409,SA-479Code Case 2036: Castings to SA-351

ANSIPending in B31.1

S31254 was the first high performance austenitic stain-less steel designed specifically to use nitrogen foradded corrosion resistance. Therefore, it has superiorresistance to chloride pitting and crevice corrosioncompared to earlier non-nitrogen alloyed austeniticstainless steel tubing such as N08366.

The resistance of S31254 to pitting, crevice corrosion,and stress corrosion cracking in chloride-containingwater has been described extensively (6 to 25) and insummary the results can be described as excellent.

Light walled tubing such as condenser tubing is weldedby the gas tungsten arc welding (GTAW) process. Nofiller metal is added and the tubes receive a full solutionanneal. Heavy walled tubing and pipe are welded withan overalloyed filler in order to compensate for thesegregation of molybdenum, which occurs when weld-ing highly alloyed austenitic stainless steels (9, 26).

Service ExperienceCondenser TubingThe first full scale installation of S31254 condensertubing took place in January 1982 when Imatran VoimaOy in Finland retubed one of the condensers at theNaantali Power Station on the Finnish west coast. Thecondenser was originally tubed with aluminum brass(OD 23x1 mm) in a Muntz metal tube sheet, but thetubes had, after 9 years of service, leaked severely andreplacement was necessary. Dezincification was theprimary form of corrosion.

Eighty-two hundred (66,000 m, 23x0.7 mm) S31254tubes were successfully rolled into the Muntz metaltubesheets despite the rather thin ligaments betweenthe holes and significantly higher hardness of the stain-less steel (27).

No cathodic protection of the tubesheets was appliedbecause it was believed that the cast iron water boxeswould supply sufficient protection. However, continueddezincification was observed after one year of serviceand a cathodic protection system was designed andinstalled. No further corrosion of the tubesheet hasoccurred.

The cooling water, which is Baltic Sea brackish water,has a chloride level of approximately 3500 ppm andreaches a maximum temperature of 35°C (95°F).

The plant operator reported no problems in the tubereplacement process, no leaks, and high plant avail-ability (27). These favorable experiences lead to aseries of additional installations in Finnish plants.

During the years 1985-89, 285,000 m, (935,000 ft.), ofS31254 condenser tubing (OD 28 x 0.8 mm) were instal-led on three different occasions in Unit # 1 and in Unit# 2 of Loviisa nuclear power station on the Finnish south

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Table 3: S31254 utility steam condenser installations

Utility: Helsinki City Imatran Voima Oy Imatran Voima Oy Imatran Voima OyEnergy Works (Finnish Power Co.) (Finnish Power Co.) (Finnish Power Co.)

Generating station: Hanasaari Naantali #1 Loviisa #1 Naantali #2Location: Finland Finland Finland FinlandType of fuel: Coal Coal Nuclear CoalOutput: 160 MW 270 MW 445 MW 270 MWYear installed: 1985, retube 1982, retube 1984, partial retube 1986, retubePrevious alloy: Al-brass Al-brass, 9 years 70-30 copper-nickel Al-brassReason for replacement: Pitting and Dezincification Erosion-corrosion Pitting and

erosion-corrosion erosion-corrosionTubesheet material: Muntz Muntz Type 321 MuntzTube-to-tubesheet joint: Rolled Rolled Rolled and Rolled

seal weldedTotal length: 65,827 m 66,000 m 94,500 m 66,000 m

(215,969ft.) (216,535ft.) (310,039ft.) (216,535ft.)Cooling water source: Baltic Sea Baltic Sea Seawater Baltic Sea

Utility: Imatran Voima Oy Imatran Voima Oy Tennessee Valley Thailand Power Imatran Voima Oy(Finnish Power Co.) (Finnish Power Co.) Authority (TVA) Authority (Finnish Power Co.)

Generating station: Loviisa #2 Loviisa #2 Johnsonville #7 Krabi #3 Loviisa #2(Cond. #4)

Location: Finland Finland New Johnson- Krabi Province, Finlandville, TN Thailand

Type of fuel: Nuclear Nuclear Coal Lignite NuclearOutput: 445 MW 445 MW 150 MW 20 MW 445 MWYear installed: 1986, retube 1989, retube 1989, air removal 1990 1990, retube

sectionPrevious alloy: 70-30 copper- 70-30 copper- 90-10 copper- Al-brass 70-30 copper-

nickel nickel nickel nickelReason for replacement: Erosion-corrosion Erosion-corrosion Erosion-corrosion Pitting Erosion-corrosionTubesheet material: Type 321 Type 321 Brass Naval brass Type 321Tube-to-tubesheet joint: Rolled and Rolled and Rolled Rolled Rolled and

seal welded seal welded seal weldedTotal length: 95,000 m 94,572 m 6,584 m 30,000 m 95,477 m

(311,680ft.) (310,277ft.) (21,600ft.) (98,425 ft.) (312,245ft.)Cooling water source: Seawater Seawater Kentucky Brackish Seawater

Reservoir,Tennessee River

Total installed length: 611,960 m (2,007,743 ft.)

coast. The original tubes were 70-30 copper-nickel, butthey suffered erosion-corrosion. The tubesheets areType 321 (S32100) stainless steel, which enabled sealwelding following the rolling-in process. Some weldingtechnique development work was required for the firstretrofit. Once this was accomplished, service experi-ence has been excellent with no leaks.

The cooling water is taken from the Gulf of Finland,which is part of the Baltic Sea, and consequently, thewater composition does not deviate much from that ofthe Naantali water. The chloride level is, however,slightly lower, approximately 3000 ppm. Periodicinspections have shown excellent performance by theS31254 condensers.

In 1985, approximately 66,000 m (216,535 ft.) of alu-minum-brass tubes were replaced by S31254 tubesin the Hanasaari power station in Helsinki. The tubeswere rolled into Muntz metal tubesheets, see Figure 1(page 4). The chloride level of the water is approxi-mately 3000 ppm and it can sometimes contain hydro-gen sulphide (3).

Then in 1986, a second installation took place at Naan-tali power station. The quantity, dimensions, and waterchemistry were identical, and so far, so has the serviceexperience.

In 1989, the Tennessee Valley Authority (TVA) retubedthe air removal section of Johnsonville Unit # 7 in NewJohnsonville, Tennessee, with S31254. The unit hadoriginally been tubed in 90-10 copper-nickel whichfailed due to erosion-corrosion. The S31254 condensertubes were rolled into a brass tubesheet. The KentuckyReservoir and the Tennessee River provide the coolingwater for Johnsonville # 7.

In September 1990, 30,000 m (98,360 ft.) of S31254condenser tubing was installed in the Krabi Power plantin Thailand.

There are now nine utility steam condenser installationsof S31254 with approximately 611,960 m (2,007,743 ft.)of installed tubing. Details on these installations aresummarized in Table 3. All of these installations are con-sidered highly successful at this time.

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TubesheetsThe first large scale application of S31254 tubesheets ina power plant took place in September 1981 at theSwedish nuclear power station Ringhals Unit # 1. Cool-ing at this station is made with North Sea water with achloride content between 15,000 and 17,000 ppm. It ischlorinated during the summer season, May to Septem-ber, with a residual chlorine level of approximately 1ppm. The maximum water outlet temperature is 35°C(95°F). Both aluminum-brass and Naval brass tube-sheets had previously suffered corrosion at this locationand had to be replaced.

Twenty-four S31254 tubesheets were installed in thetwo main condensers. Their dimensions were3700x2400x37 mm (12x8 ft. x 1.5 inch), see Figure 2.Titanium tubes were rolled into the tubesheets withoutdifficulty and seal welding was not employed. Becauseof the joint design and the use of dissimilar metals, theunit has been monitored carefully for signs of galvaniccorrosion. Tubes have been pulled periodically toenable examination of the crevice area, but neitherleaks nor indication of corrosion have been detected(28). This experience indicates that S31254 can be usedsuccessfully using rolled joints with titanium tubes.S31254 tubesheets will also perform well with eitherrolled or seal welded joints with similar tubes.

Service Water PipingAt the oldest Swedish nuclear power station, Oskars-hamn Kraft Grupp AB, the initial service water pipingwas made of stainless steel, SS 2333 (Type 304,S30400), SS 2347 (Type 316, S31600) and SS 2343(Type 316,2.75 min. Mo) and rubber-lined SS 2333. Theservice water, which is taken from the Baltic Sea, has achloride content of approximately 4000 ppm and thedesign temperature is 50°C (122°F). After several yearsof service, leaks occurred due to pitting and crevice cor-rosion so partial replacement with S31254 was decided.

In 1984, 1985, 1987 and 1989, pipe and fittings with ODranging from 60.3 to 408 mm (2.4 and 16 in. respect-ively), totalling 270 m (885ft.) were installed, see Figure3. The system was opened in 1990 to make a guaranteeinspection after 5 years of service and all inspectedsurfaces including the flanges were without corrosionattack.

Auxiliary Heat ExchangersS31254 has seen extensive successful application inshell and tube type heat exchangers cooled withbrackish and seawater at chemical plants and refiner-ies. However, at the Naantali power station on theFinnish east coast, (see Condenser Tubing above), aplate heat exchanger oil cooler, made of S31254,suffered penetrating crevice corrosion after only 6months of service. The failure was unexpected since asimilar heat exchanger had been in service for 2 yearsin a neighboring plant, an oil refinery, without any cor-rosion.

The only difference in service conditions was the tem-perature, the oil cooler at the power plant had a tem-perature of 50°C (122°F) while the cooler at the refineryhad a temperature of 40°C (104°F) (27). No chlorinationof the cooling water was performed. It had also beenreported elsewhere that plate type heat exchangersare prone to crevice corrosion when exposed to seawater (29, 30). This usually occurs at the gaskets be-tween the plates which is a very severe crevice site.

Figure 1S31254 austenitic stainless steel condenser tubes in a Muntzmetal tubesheet at the Hanasaari power station in Helsinki,Finland.

Figure 2S31254 austenitic stainless steel tubesheet, 37 mm (1.5 inch)thick, at the Ringhals nuclear power station in Sweden. Tita-nium tubes were rolled into the S31254 tubesheet.

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Figure 3S31254 austenitic stainless steel service water piping at theOskarshamn Kraft Grupp AB nuclear power station in Sweden.

Other Utility ApplicationsIn 1987 Long Island Lighting Co. installed S31254bolting material for under sea fiberglass piping manwaycovers. High strength and a design requirement for a 40year life were factors in the selection of the boltingmaterial.

Carolina Power & Light Co. experienced crevice cor-rosion failure of Type 316 (S31600) in 30 days in thecomponents of a tube cleaning and debris filter systemat its Brunswick Unit 2 station. These components werereplaced with S31254 in 1978. Similar 316 componentsof the Unit 1 station were replaced in 1979. Both havegiven excellent service since then.

Conclusions• S31254 is an excellent material for tubes and tube-

sheets in power plant condensers where brackishwater or seawater is used as a coolant.

• S31254 is an excellent material for service waterpiping handling brackish water or seawater.

• S31254 should be used with caution for plate typeheat exchangers handling brackish water.

• S31254 should not be used for plate type heatexchangers handling seawater.

• Based on chemical process industry experience,S31254 can be considered for a variety of auxiliarypower plant heat exchangers of the shell and tubedesign.

References1. S. Henrikson, Proc. International Colloquium on

Choice of Material for Condenser Tubes, Avignon,p. 31, 1982.

2. S. Henrikson, Proc. 9th Scandinavian CorrosionCongress, Copenhagen, p. 349, 1983, Korrosions-centralen.

3. M. Aninali and S. Yläsaari, Proc. 8th InternationalCongress on Metallic Corrosion, Mainz, Vol. 2 p.1284, 1984.

4. A. P. Bond, H.J. Dundas, Materials Performance,Vol. 23, No. 6, p. 39, 1984.

5. H.J. Dundas, A. P. Bond, Materials Performance,Vol. 24, No. 10, p. 54, 1985.

6. J. Olsson, Proc. International Conference onEvaluation of Materials Performance in Severe Envi-ronments, Kobe, November 1989, The Iron & SteelInstitute of Japan.

7. J. Olsson and M. L Erbing, Proc. 4th World Congresson Desalination and Water Re-use, Kuwait, Novem-ber 1989, IDA/KSIR.

8. B. Wallén, Proc. 2nd BSE-NACE Corrosion Confer-ence, Bahrain, p. 140, 1981.

9. A. Garner, CORROSION/82, Paper No. 195, (Hous-ton, TX: NACE, 1982).

10. P.E. Manning, CORROSION/82, Paper No. 176,(Houston, TX: NACE, 1982).

11. J. Olsson and B. Wallén, Proc. International Confer-ence on Underwater Technology, Bergen, Norway,p. 469, 1982, Veritas.

12. J. Olsson and B. Wallén, Proc. 1st World Congresson Desalination and Water Re-use, Florence, Italy,Vol. 1 p. 241, 1983, IDA.

13. C.W. Kovach and L.S. Redmenski, CORROSION/84, Paper No. 130, (Houston, TX: NACE, 1984).

14. T. Hodgkiess, P. Habilmatis and R. Ramsey, COR-ROSION/84, Paper No. 24, (Houston, TX: NACE,1984).

15. B. Wallén and T. Abrahamsen, Proc. StainlessSteels, Gothenburg, p. 463, 1984, Chalmers.

16. B. Todd and J. Oldfield, Trans. Institute of MarineEngineers, Vol. 97, p. 139, 1984, IME/BSE.

17. N. Nilsen and B. Espelid, CORROSION/85, PaperNo. 320, (Houston, TX: NACE, 1985).

18. B. Wallén and T. Abrahamsen, Proc. 4th Asian-Pacific Corrosion Control Conference, Tokyo,Japan, Vol. I p. 523, 1985.

19. S. Nordin, B. Ericsson and B. Wallén, Proc. 2ndWorld Congress on Desalination and Water Re-use,Bermuda, Vol. ll, p. 247, 1985, IDA.

20. B. Todd and J.W. Oldfield, Proc. 2nd World Con-gress on Desalination and Water Re-use, Bermuda,Vol. ll, p.261, 1985, IDA.

21. H. Diekmann, Werkstoffe und Korrosion, Vol. 37, p.130, 1986.

22. S. Nordin and J. Olsson, Proc. 3rd World Congresson Desalination and Water Re-use, Cannes, Vol. Ill,p. 235, 1987, IDA.

(cont.)

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References (cont.)

23. B. Wallén and S. Henrikson, CORROSION/88, PaperNo. 403, (Houston, TX: NACE, 1988).

24. T. Drugli, T. G. Rogne, J. Johnsen and S. Olsen, COR-ROSION/89, Paper No. 410, (Houston, TX: NACE,1989).

25. B. Wallén, A. Bergquist and J. Olsson, Proc. 11thScandinavian Corrosion Congress, Stavanger,Paper No. F-3,1989, Högskolesenteret i Rogaland.

26. M. Liljas, B. Holmberg and A. Ulander, Proc. Stain-less Steels, Gothenburg, p. 323, 1984, Chalmers.

27. E. Lecklin, Imatran Voima Oy, personal communica-tion.

28. S. Henriksson, Offshore Materials and Corrosion,Paper No. 4, Oslo, 1988, NITO/NiDI.

29. R. Steen Hansen, Offshore Materials and Corrosion,Paper No. 3, Oslo, 1988, NITO/NiDI.

30. O. Hagerup, Offshore Materials and Corrosion,Paper No. 2, Oslo, 1988, NITO/NiDI.

AddendumAll nuclear power plants in Sweden, in total twelve units,with a capacity of 9,700 MW, were originally equippedwith condenser tubes of copper alloy—mainly aluminumbrass. However, these proved to be insufficiently resis-tant to corrosion and a total refurbishing programmeinto titanium tubes was initiated during the 1980s.

After some years, the titanium tubes also showed signsof attack—not as a result of seawater, but from erosioncorrosion caused by high velocity droplets on the steamside of the condenser tubes. According to a report fromthe Swedish Power Board (Vattenfall), eleven out of thetwelve plants have experienced this type of attack. Theouter sets of tubes therefore need to be replaced. Thecondenser of the twelfth unit, Ringhals I, is expected tolast for the remainder of its service life, due to a lowersteam velocity. The same type of problem has beenreported by other plants, mainly in France, but also inFinland and Germany.

Material tests initiated by the Swedish Power Boardsimulating the same conditions showed that AvestaSheffield 254 SMO had a much better resistance to ero-sion corrosion than titanium. Consequently, 254 SMOwas selected for the first refurbishing carried out at theOskarshamn II plant in 1991. In total, some 50,000metres of tubes were replaced in the four sections ofthe condenser.

Further tests have shown that an even better degree ofresistance could be achieved by using Avesta Sheffield654 SMO (UNS S32654), and therefore this 24% Cr,22% Ni, 7.3% Mo type stainless steel can also be con-sidered a candidate for future retubing of power plantcondensers.

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Comments and correspondence can be directed toSten von Matérn, Technical Editor, Avesta Sheffield AB,S-77480 Avesta, Sweden. Tel. +46(0)226-81800.Telex 40976 AVESTA S, telefax +46(0)226- 81299.

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