Failure Evaluation in Desalination Plants - Some Case Studie

8/10/2019 Failure Evaluation in Desalination Plants - Some Case Studie

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FAILURE EVALUATION IN DESALINATION PLANTS - SOME CASE STUDIES1

Anees U. Malik, T.L. Prakash and Ismaeel Andijani

Research & Development CenterSaline Water Conversion Corporation

P.O.Box 8328, Al-Jubail 31951Kingdom of Saudi Arabia

ABSTRACT

In the desalination plants, the corrosion failure of components due to environmentalrelated factors constitutes a major part of the reported cases. This paper presents fewinteresting case studies of the failed components of SWCC desalination plants in theKingdom of Saudi Arabia.

It has been recognized that the biofouling of offshore structure of desalination plantscombined with sulfide contamination are responsible for Microbiologically InducedCorrosion (MIC) of components. A case study investigated on the failure of monel

bolts in the seawater intake pump has revealed severe MIC attack by sulfide reducingbacteria. Failure of materials by Stress Corrosion Cracking (SCC) in service due tocombined and synergistic interaction of mechanical stresses and corrosion reaction isnot an uncommon phenomenon in desalination plants. The second case study concerns

with the investigation on a large size intermediate bearing support block of a seawaterintake pump. The results of the study confirmed that the failure is due to SCC resultedfrom the retained residual stresses during component manufacture. Another case studyon the failure of steam impingement plate of the desal chamber also indicated that thefailure is due to SCC caused by the development of thermal stresses during platereinforcement. The methodology adopted and the analysis of material/corrosion

products carried out are detailed. The issues that must be addressed in order to controlcorrosion and/or failure are discussed.

INTRODUCTION

The failure analysis is a tribute to the society since it rewarded scientists and engineersby systematically identifying, exploring, understanding and finally solving theproblems. Although the word failure reflects in negative meaning, but the analysis of

1 Presented in IDA World Congress on Desalination and Water, Sciences, Abu

Dhabi, November 18-24, 1995.

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failure is most positive of disciplines since it benefits every one by improving productreliability and safety. Failure of components or material may stem from many originslike corrosion, faulty design, improper material processing, operational errors etc.In the desalinations cum power plants the failures of components due to corrosion andrelated environments constitute major part of the reported cases, This paper deals witha few interesting case histories of the failed components of the desalination plants.

1. FAILURE OF MAIN SEAWATER INTAKE PUMP BOLTS

It has been recognized that biofouling of offshore industrial structure combined withsulfide contamination is responsible for Microbiologically Induced Corrosion (MIC) ofmaterials. The bacteria are the first organism to colonize on immersed material by oneor more mechanisms. Therefore, intimate contact of marine bacteria with metalsurface is of consequence in corrosion process.

Several theories [1] have been putforth to explain MIC. The most conceivable ones arecathodic depolarization theory, galvanic cell theory and metabolite theory. It is

beyond the scope of the present paper to go in detail about the mechanisms of MIC. Inmetal corrosion bacteria produces in their microniche a local difference in theconcentration of protons and other cations that is substantially different from that of thegeneral biolilm [2]. These constitutes local electrochemical corrosion cells whosedimensions increase with increased colonialization.

A number of investigations have been reported on microfouling in aerobic andanaerobic environments by SBR on several alloys [3,4]. It has been suggested that

bacteria influence corrosion process through their capability to dissolve protective filmsover the metal surface.

The present study describes the failure analysis carried out on the failed bolts used inthe casings of a MSF plant seawater intake pump. They were in service for more than10 years. These bolts have developed cracks on their shafts and made them unfit forfuture service. Some bolts were of 25 mm dia. 100 mm length and few were of 18 mmdia, 75 mm length. It was found that several longitudinal and circumferential crackswere present on the shaft of the bolt including threaded portion [Fig. 1], All the boltmaterial were anaiysedis for their chemical composition by spectrochemical methods.the result of the analysis indicates that the material belonged to Monel 400 grade alloy(composition : Cu-32.68%, Fe - 1.46%. Al - 0.09%; Ni - Bal.%).

Metallography

Metallography was carried out on the longitudinal and transverse cross section of thebolt. It was observed that the most of the cracks have originated from the surface of the

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bolt and penetrated deep radially towards the central axis of the bolt. The grainboundaries have been affected more severely and the path of the cracks were totallyintergranular (Fig.2). The material exhibited recrystallized and equiaxed grainstructure with plenty of annealed twins. The average grain size of the material wasfound to be 140 m.

1.2 Scanning Electron Microscopy (SEM)

The SEM studies have been carried out on bolt material. Specimens both from soundas well as cracked sections were prepared for the studies. The cracked surface sampleswere prepared by cutting a notch exactly opposite side of the cracked surface of the boltshaft and gently hammering in the direction such that to expose the cracked surface.The SEM of the polished sample indicated the grain boundary attack. Interestingly, thegrain boundaries near the cracked regions contained white nodular deposits typicallyof bacterial colonies (Fig.3). These deposits when analyzed by Energy DispersiveSpectroscopy (EDS) with SEM were found to be rich in sulfur containing compounds ofnickel and copper (Fig.4). The deposits found near crack tip when analyzed by EDSwere found to be rich in sulfur and the deposits over the crack surface had also revealedsulfur enrichment. The mode of crack propagation was entirely through intergranularregions(Fig.3b).

1.3 Discussion

The evidences gathered during the course of this investigation suggest that the bolt havefailed mainly due to intergranular corrosion during its service. The intergranularcorrosion in this alloy could be due to Strerss Corrosion Cracking (SCC) or MIC TheSCC in the subject bolt is a remote possibility because SCC would occur in materialwith history of cold work. The bolt material has showed no evidence of cold work asnoticed by the presence of annealed twins in the microstructure of the alloy. Thehardness measurement made on the alloy also indicated that there is no stored energy ofcold work which could have manifested in increasing the hardness of the material. Theaverage hardness value was found to be 146 VHN, which indicates that the materialwas in the softened condition. Hence SCC may not be the reason for the observed

cracking of bolts in service.

The presence of sulfur and oxygen in the corrosion products at the grain boundaries(Fig.4) and at crack tip and considering the long exposure of bolt material to marineenvironment, it is likely that the intergranular corrosion attack on the grain boundarieswas by MIC. The only conceivable source of sulfide in this case is bacterial activity ofSulfate Reducing Bacteria (SRB). The sulfides which are the product of SRB can cause

breakdown of the passive film layer on the Monel 400 alloy, thus making it susceptibleto intergranular corrosion by chlorides.

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The role of sulfide in the breakdown of passive film layer in Monel 400 have beenexamined in the solution containing different chloride, sulfate and sulfide ionconcentration. Potentiostatic polarization techniques have been used for this purpose.The experiments were performed in an universal buffer (pH = 6.5) mixture of

phosphoric acid, acetic acid, boric acid and sodium hydroxide.

The electrochemical measurements were carried out on EG &G Potentiostat-Model 273using Monel 400 electrode. A saturated colomel electrode (SCE) was used as referenceelectrode. Polarization resistance measurements at 0.1 mv/min were made at ambienttemperature. The anion concentration was determined spectrophotometrically. Theresults have been tabulated in Table 1.

Table 1

Potentiostatic Polarization Data

Electrolyte E corr PR I corr CRS.No. (mv> (Wcm*) (@/cm*) (mpy)

1 Buffer + Chloride of : (i) 0.4M -172 6.89 3.15 1.39(ii) 0.5M -174 2.7 7.88 3.49(iii) 0.6M -186 2.47 8.77 3.86

2 Buffer +0.6Mchloride (i) 0.064M -586 4.48 4.85 2.14+ sulphide of (ii)0.094M -540 6.4 3.29 1.50

(iii) 0.13M -641 1.3 16.68 7.35

3. Buffer +0.6Mchloride (i) 0.01M -332 8.48 2.56 1.13+sulfate of (ii) 0.015M -211 11.68 1.86 0.81

(iii) 0.02M -197 8.47 2.57 1.134 Buffer + 0.6Mchloride + 0.02M

sulfate+ 0 13M sulfide -528 11.93 1.82 0.8

5 Buffer+ 0.02M sulfate+ 0.13M -599 9.59 2.26 0.997

From the comparison of results obtained, it is apparent that although chloride inducesthe breakdown of the passive film, the breakdown potential is strongly dependent onthe sulfide ion concentration in the presence of chloride. In presence of sulfide, thevalue of corrosion current obtained is 2 times the current which was recorded in absenceof sulfide.

1.4 Conclusion

The observed cracking of monel 400 alloy bolt in service is due to MIC by SRB. Thesulfides which are the product of SRB can cause the breakdown of passive film makingit susceptible to intergranular attack by chlorides present in the environment.

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1.5 Recommendation

In the light of the above, it is recommended to use material containing chromium orchromium and molybdenum as they have better resistance to MIC [5]. The nickelcopper alloy containing chromium (alloy UNS No. 6600) or alloy containing chromiumand molybdenum (alloy UNS No. 10276) would be a good choice for sea water service.

2. FAILURE OF INTERMEDIATE BEARING SUPPORT OF ASEAWATER INTAKE PUMP

Engineering materials when subjected to combined action of mechanical stresses andcorrosion reaction would fail in service due to development of cracks. Such failures areknown as Stress Corrosion Cracking (SCC). The type of loading, nature ofenvironment and the grade of material are responsible for SCC to occur. Generally,tensile stresses are required to cause SCC and are usually below the yield stress of thematerial. They are externally applied, some time the residual stresses present in thecomponent can also cause SCC. The environments that cause SCC are generallyaqueous solution, Typically, SCC of an alloy is due to the chlorides or other reactivespecies present in the environment. In SCC the cracks initiate and propagate at a slowrate until the stresses in the remaining part of the uncracked area exceeds the fracture

strength. Many different mechanisms have been proposed [6,7] to explain the SCCbehavior of materials. The following two case histories enumerated below belonged tofailure by SCC.

An investigative study has been carried out on an intermediate bearing support blockwhich was cracked during service. The intermediate bearing support block belonged tothe main seawater intake pump of a Desalination Plant. The cracks have beendeveloped at the rim and arm joint prematurely. The photograph of the component isshown in Fig.5. The arrow marks in the photograph indicates the region where cracksappeared during service. It was found by dye fluorescent non-destructive testingtechnique that the other two arms and rim joints were devoid of any cracks.

2.1 Chemical Composition

The material of the component was analyzed for its chemical composition byspectrochemical methods, The material used for the component were confirmed to be ofductile Ni-Resist Cast Iron, grade ASTM-A439 type D2 and had compositon : C -l.8%, Mn - 1.2%, Si - 0.2%, Ni - 27.8%, Cr - 1.7%, Fe - Bal.

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2.2. Metallography

The samples for metallographical examination have been prepared by cutting theportion of cracked parts initially by power hacksaw followed by milling. The generalmicrostructure of the sample is shown in Fig.6.The microstructure showed spheroids ofgraphite and even distribution of carbide networks essentially made up of chromiumcarbides in austenitic matrix. The grain boundaries have been revealed by etching in2% Nital + 3% Picral etchant for 30 to 60 seconds. The average grain size of thematerial was found to be 280 mm. The grain boundaries appeared irregular and wavydue to the pinning of grain boundaries by carbides during grain growth. Themetallographic examination of the cracks has revealed that the main cracks were

propagated along the grain boundaries. The branching of cracks have taken place oftenacross the grains. The main crack and the sub-cracks branching are shown in Fig.6b.

2.3 Scanning Electron Microscopy (SEM) & Energy Dispersive X-rayAnalysis (EDX)

The etched samples containing cracks have been examined in scanning electronmicroscope. It was found that the cracked regions contained essentially oxides rich iniron and small amounts of chlorides and sulphates. The SEM picture of the sample

showed main cracks associated with transgranular branching (Fig.7). The EDXspectrum of the deposits found along the cracked surface is shown in Fig.8. The typicalbranching of cracks from the main cracks associated with corrosion suggests that thematerial had failed due to stress corrosion.

2.4 Discussion

It is well known that the ductile Ni-Resist cast iron have fairly good corrosion resistanceproperties particularly to marine environments. It is reported in the literature that thecorrosion rate or the pitting rate of Ni-Resist Cast Iron is independent of velocity [8] indeoxygenated seawater. The average corrosion rates [9] have been reported to bearound 40 mm/year at flow rates of 8 ms -1. In spite of the excellent corrosionresistance to seawater, several failure of cast components of Ni-resist cast iron have

been reported in the past. The reason of failures have been attributed to many factorssuch as carbide content of the alloy [l0], the sea exposure temperature, faultyinoculation procedures, improper heat treatments, etc.

It is seen from the investigation carried out on the component that the severalmicrocracks have been originated at the region where there is considerable variation inthe section size. These areas are however, more vulnerable for the development of highresidual stresses during manufacturing process. Hence, proper care has to be exercisedin the design and also in the heat treatment to relieve the stresses soon after the casting.

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If proper stress relief heat treatment is not carried out, local internal stresses close to theyield strength of material would make the material prone to stress corrosion cracking(SCC). The branching of sub-cracks from the graphite nodules in transgranualr mode(Fig.6b) and the presence of macrocracks connecting the graphite (Fig.7a) stronglysuggest that the local residual internal stresses accumulated at those region wherecracks were initiated. The metallography carried out on the sound portion of thecomponent suggests that there is no material inhomogeneity in the component. The

macrohardness measurements taken near the cracked regions as well as the soundportion of the components were also found to be uniform. Therefore, the observedcracks could be due to internal stresses developed during the casting process and thematerial has not been sufficiently stressrelieved by heat treatment.

2.5 Conclusion

The development of cracks in the component are predominently due to the combinedeffect of stress and corrosion leading to Stress Corrosion Cracking (SCC). The reasonof SCC is due to the presence of residual stresses at the regions where there is largevariation in section sizeof the component.

2.6 Recommendation

It is suggested that care must be exercised in the design aspects to minimize thevariation in section size to avoid accumulation of residual strcsscs (at arm and rim Jointregions). Proper stress relief heat treatment to be carried out to ensure that all theaccumulated stresses are fully relieved after the heat treatment. We suggest to make useduplex stainless steel such as NORIDUR 9.4460 [9] as an alternative material for thiscomponent. These steels exhibit excellent pitting and SCC resistance to seawaterapplications.

3. FAILURE OF STEAM IMPINGEMENT PLATE IN THE BRINEHEATER

Another investigativc study, has been carried out on a steam impingement plate in thebrine heater compartment. The failed plate was of size 250 cm x 150 cm x 0.6 cm

thick.

The plate had contained perforations to allow the steam impinging on it into the heatexchanger tubes. The plate was positioned in the brine heater compartment by weldingthe opposite sides of the plate to the circular outer shell which is made out of carbonsteel. The plate was reinforced by angular iron strip from the bottom. Cracks haddeveloped at the welded portion of plate and shell joints and also at both the sides ofwelding near the middle portion of the plate where there is a central reinforcementunderneath by angular strip. The impingement plate was so positioned that the steam

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would directly impinge on the plate at the center during the operation. A schematicpicture of the plate with the places of cracks developed is showed inFig .9.

3.1 Chemical Composition

The plate samples were analysed for their chemical composition by spectrochcmicalmethod. The plate material was confirmed to be of AISI 304 grade stainlesr steel alloy

with the composition : Cr-18.3%, Ni-9.04%, C-0.041%, Fe - Bal

3.2 Metallography

The samples containing visible cracks have been subjected to metallography. Thegeneral microstructure of the sample is showed in Fig. 10a.The microstructure revcalcdtypical austenitic structure containing plenty of twins with in the grains. The averagegrain size was found to be around 50 mm. The cracks appeared to have originated at

places close to weld regions and also at regions approximately 1 cm away from weldzone. The cracks which are noticed away from weld zones had propagated intransgranular mode and showed branching from main cracks (Fig. l0b). At few placesclose to weld zone the propagation of cracks appeared to have propagated inintergranular mode with less branching. The propagation of cracks in trans and

intergranular modes suggests that the plate might have failed due to stresses which aretensile in origin and may also due to intergranular corrosion.

3.3 Scanning Electron Microscopy (SEM)

The SEM has been carried out on the fractured surfaces and also on the polished andetched samples. The fractured surface of samples near weld zone showed extensivecracking along grains indicating intergranular mode of failure with extensive fissuringalong grains. The cracks observed in SEM from the polished samples representingregions away from weldments revealed transgranular mode of cracks propagation. TheEnergy Dispersive Spectrum taken at the crack tip during SEM showed nocompositional degradation in the matrix as evidenced in the spectrum (Fig. 11). Thequantitative analysis done on the elemental spectrum was found very close to that of thechemical composition of the alloy.

3.4 Discussion

The intergranular corrosion in the weldments of AIS1 304 stainless steel [12] is awell-known problem if proper care is not exercised during welding. The postweld heattreatment is absolutely necessary to avoid weld decay occurring near the weld zones.The evidence of severe cracking all along the zones close to weld clearly suggest thatthe problem is with the AISI 304 material. During welding the areas near welds whichexperience temperature of 600 - 850C is likely to favour chromium carbide

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precipitation reaction at the grain boundaries. This reaction would deplete chromiummetal concentration in the areas adjacent to grain boundaries thus making it susceptibleto cracking due to corrosion in course of time. The observed separation of plate bycracking is likely due to this phenomenon.

It is observed in the SEM and also in the metallography that several cracks were seen inregions quite away from the welds and these cracks are of transgranular type (Fig. 10b).The AISI 304 stainless steel can fail in transgranular mode normally when it is

subjected to high tensile stresses. The reinforcement of the plate by angular strips bywelding generated high degree of thermal stresses which are tensile in nature had alsoresulted in cracks formation during service.

3.5 Conclusion

All the observation made in this investigation on the failure of the impingement platesuggest that there are likely two main reasons for failure. First reason being theintergranular corrosion arising out of welding of AISI 304 stainless steel. The secondreason being the development of thermal stresses during plate reinforcement andsubsequent welding to shell had resulted in the failure due to stress corrossion cracking.

3.6 Recommendation

It is recommended to use stainless steel type AISI 321 ( Ti stabilized ) or AISI 347 (Nbstabilized ) to overcome weld decay problems. It is also suggested that after welding, athorough weld anneal heat treatment to be carried out on the plate to relieve all thethermal stresses before it is put into actual service.

REFERENCES

1.

2.

3.

4.

5.6.

Hamilton, W.H. and S. Marwell, Biological and corrosion activities ofsulphate reducing bacteria with in natural biofilms. Biologically InducedCorrosion, NACE-8. Ed, Stephen C. Dexter. (1986) NACE : p. 132.Costerntorn, J.W. and G.G. Goescy, The microbial ecology, Biologically

Induced Corrosion - NACE, Ed. Stephen C. Dexter, 1986. NACE : p. 224.Lee, W. and W.G. Characklis, Corrosion of mild steel under anaerabic

biofilm, Corrosion, 49,3 : p. 186-199.Eashwar, M., P. Chandrasekharan and G. Subramaniam, Marine microbialfilms and the corrosion of steel, B. Electrochem, 1988. 4(2) : p. 118-l 19.Wagner, Patricia and Brenda Little, Metal Performance, Septa. 1993. p. 66.

Newman, R.C and R.P.M. Procter, Silver Jubilee Review - Stress CorrosionCracking; 1965-1990, Br. Corrosion Jour., 1990. 25 (4) : p. 259-269.

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7.

8.

9.10 .11.

12.

Jones. R.H., Stress Corrosion Cracking, Metals Handbook 9th Edition, Ed.ASM International, Vol. 13; 1957. p. 145-162.Malik, A.U., S.Basu, I.N. Andijani, N.A. Siddiqui and S. Ahmed, BritishCorrosion Journal, 1993. 28 (3) : pp. 209-216.May, T.P., J.F. Mason and W.K. Abbot, Mat. Prot., 1996 1. 1: p. 40.Dawson , J.V. and B. Todd, BCIRA. Journ., 1987. l-9.Technical data KSB Akiengesselschaft Frankonthal. Germany on Cast Ferritic- Austenitic Stainless Steel,NORIDVR, July 1993. 9.4460.Steigerweld, Robert, Metallurgically Influenced Corrosion, Metals Handbook.9th Edition. Edtd. ASM International, Vol. 13, 1987. p. 123.

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Figure 1. Photograph of bolts. Arrows indicate the longitudinal and

circumferential cracks developed during service.,

Figure 2. Photomicrograph of bolt showing intergranular cracks x 100

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Figure 3. Scanning electron micrographs of bolt: (a) Polished surface-note

white deposits surrounding the grain (b) Cracked region showing

complete intergranular separation.

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--

Figure 4. Energy Dispersive Spectroscopy of grain boundary deposits.

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Figure 5. Photograph of main seawater pump intermediate

bearing support. Note: Arrow indicates the place

of cracking.

Figure 6. Photomicrographs of intermediate bearing support. (a) General

microstructure. X 100 (b) Main cracks and branching of cracks.

X 100

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Figure 7. Scanning Electron Micrographs showing (a) Intergranular cracks.

(b) Transgranular cracks.

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Figure 8. EDX analysis of deposits at the cracks. Arrow in the inset

represents the area of the analysis.

\ h Perforated sheet

Cracks

Angular reinforcement

strip from bottom

Figure 9. Schematic Picture Of Impingement Plate.

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Figure 10. Photomicrographs of impingement plate. (a) General

microstructure. X 100 (b) Transgranular cracks in the

microstructure. X 100.

Figure 11. EDX analysis spectrum at the crack tip. Arrow in the inset

represents the areaof analysis.

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Failure Evaluation in Desalination Plants - Some Case Studie