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Paper No.

522CORROSION97

CORROSION FAILURES IN PLATE HEAT EXCHANGERS

R.L,Turieeini, T. V. Brono, E, P. Dehlberg, end R. B, SetterhmdMetelhrrgical Consultants, hc.

P. 0, BOX88046Houston, TX 77288-0046

ABSTRACT

Corrosion failures in plate heat exchangers arc discussed with reference to equipment design, service conditions andmaterials of construction. Included are case histories that illustrate service experience,

Plateheat exchangersare chosenover ehell-end-htbcexchangersfor applications requiring superior heat transfer etlciencyand Compactness,and lower weight. Efforts to maximize their inherent advantages drive plate heat exchanger design toward the

usc of thin plate sections that require highly corrosion-resistant materials, and narrow flow passages that are conducive to fouling.

Plate heat exchangers in general require extensive scaling along the edges of the plate. Consequently, crevice corrosionmay occur under gaskets or adjacent to seal welds. Localized corrosion maybe either initiated or aggravated by the leaching ofharmful ionic species into crevices from polymer gasket materials. Stress-comosion failures are also encountered, particularly atcold formed corrugations incorporated into some designs to contain gaskets or to improve heat transfer cocff]cients,

Kcywcrdscrcviwcomosbn,stresscorrosion,SCC,gasket, seal weld, amalgam, liquid metal cracking, liquid metal embrittkrnent,LME, mercury, hydride, pitting, heat flux, heat transfer, heat exchanger, fluorocarbon, polymer, cold box

INTRODUCTION

Increased energy costs end conservation needs in recent decades have enhanced the attractiveness of efficient heatexchanger designs. Plate heat exchangers have more heat tremsferarea per unit volume then shell-end-tube heat exchangers,providing more ticient performance along with space end weight savings. The superior heat-trrmefer etliciency of plate heat

cxehengcm reduces water requirements in ccckre and eondmacrs and permits waste heat to be recovered economically with smaller

temperature dtierenees.

The effectsofmreh variablesse componentgeometry, heat flux and service fluid contaminants are important in promotinglocahzedattackinplate heat exchangers, Corrosion considerations are complex in this equipment, making past service experience

Copyright01997 by NACE International. Requests for permission to publish this manuscript in any form, in pad or in whola must be made in writing to NACEInternational, Conferences Oivision, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in Ihlspaper are solely those of the author(a) and are not necessarily endorsed by the Association. Printed in the U.S.A.

Ahmed Attyub - Invoice INV-639305-5DD9R3, downloaded on 3/4/2013 6:43:58 AM - Single-user license only, copying and networking prohibited.

-~vdusble inmticiP~gPmiblef~trre m=hh so that appropriate corrosion control meawes maybeimplemented.

DESIGN

Examples of a few plate heat exchanger rxmf@rations are illustrated in Figures 1 through 3. In the design in Figure 1,the heat exchangeelements are spiral-formed plates that conduct the opposing streams between the cylindrical outer wall and thecentralaxis. The plates are sealed at the edges to prevent the streams tbm mixing. One of the streams enters the heat exchangerat the center and leaves radially at the outer shell, while the second stream enters at the outer shell, flows counter-current to theopposing s@~ and exita axially at the center,

Figure 2 shows a multiple-pass plate-rmd-fm heat exchanger. Flow passages between plates are directed by sets of

~fi ~ fl~ated irrthe ~ drawing. The opposing strems in this example flow across one another, with the differentpasses separated by baftles. The flow streams are sealed at lined panels by means of gaskets.

A popular plate-and-frame heat exchanger car@uration, shown in Figure 3(a), employs a stack of plates mounted on

c-g b~s ~d held ~tw~n end COVHSby mew of impression bolts. Each plate is gasketed about its periphery to containthe tlow Gasketsalsoeontml the paths of the opposing streams which flow through ports cut into the comers of the plates. Cold-formed corrugations act to stitTenthe plates and to induce controlled turbulence for improved heat transfer. The flow schemeillustrated in Figure 3(b) is single-pass and counter-flow. Multiple-pass schemes can be provided by blinding the inlet ports atintermediate plates to redirect the flow.

CORROSION CONSIDERATIONS

From a cmroaionstandpoint heat exchangem,likeboilers and other equipment that involve heat transfer, are distinguishedby a tendency for increased corrosivity due to heat flux effects at metal/fluid interfaces. Boundary layer conditions such as

~, ccmcentrationand otherfluidcharacteristicsat theheat transfer surfaces are different horn those calculated or measuredfor the bulk fluids. Boding maydamageprotectivefilmsor causelocally high concentrations of harmful solutes. Scales may depositcsrthe heat trarrslmsurface,creatingconcentrationcellsor producing locally high temperatures due to insulating effects. Cool metalsurfaces in contact with hot vapors maybe subject to corrosion by condensing acidic solutions.

Despite theirmany advantageaover the morecommonlyusedshell-and-tubeheat exchangers, plate heat exchangers presentcertain inherent corrosion difllculties that must be considered by the engineer involved in material selection.

In shell-and-tubeheat exchangers, carbon steel is a wmmon material for use in “non-comosive” fluids, e.g., hydrocarbonservices. Type 304 stainlesssteelor aluminum,however, are the minimum choice for plate heat exchangers, while titanium, highlyalloyed“super-austenitic” stainless steels or nickel-chromium-molybdenum alloys are otlen employed. There are several reasonsfbr thehigher degm of conservatism in plate heat exchanger material design. Plate heat exchangers are usually employed whereexwPti~ h~t tier etliciencyk~@d. TIwdegradationinheat transfer caused by the insulating effects of corrosion depositswould run counter to this consideration. Plates are less tolerant of material loss than tubes because plate elements are fabricatedto thicknesses of about 0.020 inch (0.05 mm) while tubes are typically 0.050 to 0.065 inch thick (1.3 to 2.6 mm).

Due to their compact construction, the flow passages in plate heat exchangers tend to be very natrow so there is muchpotentialfcr ftigby anapendedcormsinnproductsand scale, although filters may be employed to remove suapendcd solids. Thenecessity for clean fluid is one more reason for employing highly corrosion-resistant material not only for plate material but forupstream piping.

The necessity to avoid corrosion is compounded by the diff]cuky of maintaining plate heat exchangers. The greateraccessibilityof shell-arxi-tubeheat exchangerinternals in many cases allows in-place maintenance. Thus a leaking tube can simplybe ple atlerremovingtirehed and the heat exchanger quickly returned to service. Similarly, fouled tubes can be serviced byhy&ojetting or other means. In plate heat exchangers, on the other hand, there is no simple way to access or stop flow through aleaking plate element or to remove scales in place so that disassembly is generally required.

Plates must be sealed around their edges to contain process streams. These extensive sealing requirements provideumsiderable opportrrnity for crevice corrosion, further complicating material selection.

Polymergasketsprovide exceptionally tight, deep crevices that are highly conducive to crevice corrosion. Furthermore,certaingasketmaterials contain leachable, corrosive ionic species such as chlorides and fluorides that may promote either crevicecorrosion or stress corrosion.


Crevice corrosion and stress-corrosion cracking cart occur in water-odd, shell-and-tube heat exchangers atroller-expanded joints when cooling water is on the shell side. The potential for stress corrosion is especially severe in unventedverticalshell-and4ubebeat exchangerswhere a vapor space maybe present below the lower face of the tube sheet. At the sacrificeof mtne process etliciency,one can design around this problem by putting the water on the tube side. Unfortunately, in most plateheat exehangtmthe two fluid streams encounter identical materials and geometries so that a corrosion problem cannot be avoidedby switching streams.

Aluminumanditsalloysarestandard materials of construction for plate heat exchangers and associated piping employedin “coldboxes”used to refrigerate liquefied naturrd gas. These fluids are normally noncorrosive. Excellent thermal conductivity,along with the low cost, low density and the absence of a ductile-to-brittle temperature transition make aluminum and its alloyshighlyattractive. However,due to the susceptibility of aluminum to liquid metal cracking and amalgamation in mercury, care mustbe taken to avoid the introduction of hydrocarbons containing naturally occuming mercury into such units,

CASE HISTORIES

Thefollowingexamples of service failures in plate heat exchangers serve to illustrate many of the material selection andcorrosion considerations discussed above,

Crevice Corrosion

Plates are sealed by gaskets, seal wehis or brazed joints. This provides crevice areas and potential sites for localizedcorrosion.

A hot alkylate product that had a high chloride content was used to preheat boiler feed water in a spiralplatebeat exchanger of the general type shown in Figure 1, The process (alkylate) stream was contained by welding the plates atthe edges. The original Type 304 stainless plate set had failed by chloride stress corrosion and was replaced witlr Nickel-CopperAlloy UNS N04400, which is not subject to this mode of attack.

Numerous leaksoccurred in the nickel-copperalloyreplacementplates at corrosion pits that initiated on the process streamside. Some pitting occurred at spacer pin welds but the majority of the leaks initiated in the crevices formed by fillet seal welds.Thiscrevicecorroaiu illustratedin Figure 4, was most severe in weld fusion zones but the adjacent base metal was also attacked.

WhileNickel-CopperAlloyUNS N04400 is resktsnttochloridestresscorrosion, this alloy is subject to pitting and crevicecotmsicmin chloridesolutions. Manyhighlyalloyed stainless steels have shown better resistance to localized corrosion in chloridesolutions and have been successtid as replacement materials for services that produce chloride pitting and crevice corrosion inNkkel-Ccppw AlloysUNS N04400 and N055tM. A material upgrade to a 6-percent molybdenum, super austenitic stainless steel,e.g., UNS S31254 or N08926, would provide superior crevice-comosion resistance without increasing material cost.

. seals made withpolymergasketsprovidetight,deep crevicesthat are more vulnerable to crevice corrosionthatsmetal-to-metal seals. Crevice comosion occurred under PTFE gaskets employed to seal the edges of a Oracle 1 (unalloyed)titaniumsheetthat linedan externalsteelpanel. This under-gasket corrosion is illustrated in Figure 5. The service fluid was wastewater at temperatures between 212 “F (100”C) and 230”F (110° C), with a pH from 2 to 4, and having chloride concentrationsbetweea2000 and 4000 pptn Crevicecorrosionallowedthe wastewater to leakbetween the titanium liner and a carbon steel panel.This produced corrosion in the carbon steel as weIl as the inner surface of the titanium liner, which became perforated.

The corrosionpredrrctsat the edgesof the plates were primarily titanium oxide, with about 2 percent each of chloride andsulfbr. The efkct of halides in promoting creviw corrosion in titanium alloys at elevated temperatures is well known. The sulfurmay have been present in the water as sulfate ions, which also have been found to induce crevice corrosion.

Figure 6 is a photornicrograph of a section taken through one of the plate edge areas. Titanium hydride needles werepresent within the grains. Hydrogen is produced by the reduction of hydrogen ions at local cathodes. Hydrogen atoms are thenabsorbed into the titanium, see Figure 7, which has a strong chemical athity for hydrogen.

Once 10CSIwrmosioninitiated, attack was accelerated by autoeatalytic effects that commonly occur in crevice and pittingcorrosionprocesses, i.e., pH depression due to hydrolysis reactions involving metal ions, and the migration into crevice areas ofadditional negatively charged halogen ions attracted by positively charged metal ions.

In heat exchangers,basic creviee-comosionprocesses maybe augmented by additional solute concentrating mechanisms.

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Besides producingdmcttsionally severe crevices, polymers may intluence crevice corrosion by altering crevice environments.Ffucmpofymcrsand polymers containing chlorides can induce crevice corrosion by releasing halogen ions. Gasket materials that

aPP~ be l~a cnnd~ive to Crevicecorrosion include EPDM, PVC, asbestos, epoxy, nybnsod silicone rubber. Graphite andgraphite-impregnated gaskets appear to inhibit crevice corrosion under certain conditions. In the case history discussed above, achangeto a more benignChISSof gasketmaterialahnrrhibe mrrsi&red. EPDM has relatively good resistance to moderately elevatedtemperatures and non-oxidizing acidic environments and would be a candidate material,

Case EMmQkd. The stagnant conditions within crevice environments produce poor heat transfer and locally high&np@rcs. Harmfulsolutessuch as chloridescan coneenkatc in the crevices as a result of boiling, Shortly after start-up, leakageoccurred in Type 304 stainless steel plates in a heat exchanger used to heat a rich diglycol amine (DGA) solution from 163 “F~2.8”C) to 201‘F (93.9”C). The rich DGA, which contained dissolved carbon dioxide, then entered an amine stripper columnwhrmthecarbondioxidc wssresnoved The returninglesnDGA then entered the hot side of the heat exchangerat270”F(132”C).

Surf&c dcpds on the rich aminesidesof the plates covered numerous pits in the gasket areas. A cmrosion pit is shownin Figure 8. An electron microprobe analysis of the associated &posits showed that the particle indicated by the arrow waspotassium cbforide. The chforide contaminant had become concentrated sutlcient]y to produce rapid pitting under the gaskets.

Stress Corrosion

Plate andtlarne heat exchangctsofthc type illuatmtedin Figure 3 usually contain cold-formed corrugations, The resultingresidual stresses may induce stress corrosion should the appropriate environmental conditions exist in either fluid stream,

Ca e Historv 4. A plate-and-thrne heat exchanger employing titanium A 265- Grade 11 (Ti-O.2% Pd) plates andflucmmbons elastomergasketswas used as a cmsntertloweconomizerthatheated water contaminated with 0. 1’XOchlorinated or8anic~ h $Q”F(32”C) to 180°F (82.2°C). The waterpH wcs Ixtween 1 and 2. The returning water was free of the organiccontaminants but also had a low pH.

Intergranufarstress-corrosioncracks initiated under the gaskets on both sides of the plate. Cracks were observed at boththe fsd andCOOIends of the heat exchanger. Crevice corrosion also occurred under the gaskets at the hot ends of the plates in bothstreams. Figore 9 shows cracking along with crevice corrosion under a port gasket. Figure 10 is a scanning electron microscopeimage of SSSintcrgrsttulm stress-comosion fkscture surface.

Electron microprobe analysis revealed fluorides within stress-corrosion cracks and in plate-surface corrosion deposits.The fluorides were probably leached from the fluorocarbon elastomer gaskets.

~. Figure 11shows a hole formed in a titanium plate as a result of stress-corrosion cracking at cold-fonncdcorrugations. The arrows indicate the location of additional small cracks. Typical brittle branched cracks were present in thetmnawse section shown in Figure 12, while Figure 13 is a scanning electron flactograph showing a cleavage-like pattern typicaloftmnagmnufarstress-comosion cracks in titanium alloys. Cracking was due to stress corrosion that initiated at surfaces exposedto sour water mntain@ diaaolval chloridesat 250”F (121“C), with a pH below neutral. This stream was heated by a counter-flow,stripped-wateratresmat 260”F (127 “C).

Referenceto Figure 7 indicatesthat crevicecorrosion could occur at temperatures of250°F(1210 C) and higher at neutralpH in unafloyed titanium. It is likely that the titanium plate was operating in the region of borderline passivity. Materials thatcxpaknw so active-passive transitionare often subject to stress corrosion under such conditions. A material upgrade is in order,Figure 7 shows that Titanium-Grade 12 has far more stable passivity in brine at depressed pH and elevated temperatures thanunalloyed titanium, making it an obvious candidate for a plate material in this heat exchanger.

Fouling

Due to their large heat transfer surfaces and narrow flow passages, plate heat exchangers are subject to flow blockagecaused by scale deposition or suspended corrosion products.

A Type 304 stairdesssteelplate faifedin another “lean-rich amine heat exchanger that heated a diethanol- @EA) solution from 105“F (40.6 “C) to 208°F (97.8 “C). The carbon dioxide-loaded DEA solution contained 2300 ppmcftfnridcathat ran casntcr-crumnt to a hot lean DEA solutionthatenteredtheexchangerat240“F(1I6”C)andexitedat137“F(58.3”C).Friableimncarbonatescalecoatedtheopposingplatesurfacesh therichDEAstream.The scale build-up was sutlcientto bfockthe flowpassages in some areas but no corrosion was found under the scale. Erosion-comosion occurred at the hot end of

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the plate%downatran flonr the scaledeposits,and was attributed to the flashing of carbon dioxide out of solution. Carbon dioxidenormally remains in solution until the rich DEA enters an amine stripping column downstream from the heat exchanger. Thefldsing ofearbon dioxidewithin the heat exchanger therefore suggested possible improper process design. A local pressure dropdue to flow blockage and turbulence may have contributed to the premature flashing.

Liquid Metrd Attack

Naturallyeaurring mer-aay is occasionallyfound in rmrefmed hydrocarbons. Mercury causes cracking in highly stressedalUminumalloymmponents due to liquid metal rsnbrittlemcnt (LIME)and also forms so amalgam with aluminum. Amalgamationinitiates at sites where the protective aluminum oxide film is danraged, afier which material wastage proceeds at extremely rapidrates.

~asc Historv 7. An ethylene processing plant was shut down when, shortly rifler it began processing an unrefinedfcedstock,mcrcusywas found in a strainer in Type 304 stainless steel piping just upstream from a cold box. The cold box, whichrefrigerated the fluid to - 270”F (- 168”C), contained a plate-and-tin heat exchanger with Ahnnimmr 3003 plate elements andwelded Ahuninum Alloy 5086-0 piping.(z)

Cracking was found at two circumferential girth welds in eight-inch Aluminum Alloy 5086-0 piping in the cold box.Figure 14 is a photomicrograph of a section through the fusion zones of one of the welds, showing an interdendritic LME crack,A sixamingelectronmicmacopeexaminationshowedthe ti-achuEsurfaces to be wetted with mercury. Figure 15 shows microscopicglobules deep inai& the crack, along with crazed (“mud-cracked”) aluminum oxide-corrosion products. Cracking also occurredinbase metalareas adjacentto thewelds, in the form of exfoliation attack that progressed along planes parallel to the pipe surfaces.

The coldbox was completelyreplaced. The aluminum plate-and-fin heat exchanger was not disassembled for inspectionb to the hazard of mercurycxpo~ to personnel. The potential for large quantities of mercury to condense from the refrigeratedfluidand accumulateis evident. Analysis indicated about 40 ppb of mercury in the feed stock. The flow rate of feed stock throughthis piping and heat exchanger is 200,000 pounds per hour, This is equivalent to an input of 70 pounds of mercury per operatingyear.

CONCLUSIONS

Like most plant components, plate heat exchangers are designed to meet expected service conditions at the lowestlcng-tcnn cosL However,even when the tics Meis 1sssthan economical,losses are limited when equipment deterioration occursin a predictable manner and can be tracked by inspection. Failures that are sudden and occur in a unexpected manner causeunack&ded shutdowns,leavingplantsunpmpamdto properlyreplace failed components, thereby aggravating lost production costs.suchhihSreSarelxmmcSS in heat transferequipment,andthis discussionhas focussed on failures specific to plate heat exchangers.It is intmded in the above discussion to illustrate the benefits of specific service experience in anticipating failures that often resulttlurn lecalized phenomena or anomalous service conditions.

REFERENCES

1. R W. ShrrtzandD. E. Tlrornas,“Cormsiorrof Titaniumand Titanium Aloys,” Metals Handbook, Ninth Edition, Vol. 13,ASM International, 1987, pp. 669-706.

2. J. J. EnglishandD. J. Duquette, “Mercury Liquid Embrittlement Failure of 5083-0 Aluminum Alloy Piping,” Handbook9fCaSS H@trtcs m

,..Failure - Vol. 2, ASM International, 1993, pp. 207-213.


FIGURE 1- Cut-away view of spiral plate heat exchanger.

/

FIGURE2- Multiple-pass,cross-flowplate-and-tinheatexchanger.

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(a) Gasketed Plate-and-Frame Assembly (b) Exnkxfed View of Plate Pack Showing Flow Paths Between Plates

FIGURE 3- Plate-andhrne heat exchanger.


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14

12No hwkogen pickup Hydrogen pickup

No cm ‘osion \

10 \

4

2

n

“38 93 149 204 260 315Temperature, ‘C

FIGURE7-Temperatum’pHlimits for titanium alloys in NaCl brines based on Iaboratog andfield experience, horn Reference 1.

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FIGURE 12- Transgranular stress comosion in titanimn

FIGURE13-Scanning electron microscope fiactograph oftransgrsnular stress-corrosion fracture surface in titanium

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