S-156-ENG 97

The role of duplex stainless steels in petrochemical heat exchanger

applications

Recommendations are for guidance only, and the suitability of a materialfor a specific application can be confirmed only when we know the actualservice conditions. Continuous development may necessitate changes intechnical data without notice.

Sandvik Steel has a quality system approved by internationallyrecognised organisations and holds ASME Quality SystemCertificate as Material Organization. Approval, by accredited thirdparty, to ISO 9001 is also held, as well as TÜV approvals to AD-W0/TRD 100, VdTÜV 1153 and KTA 1408, and other approvals.

CONTENTS

Introduction …………………………………………… 3

Organic acids ………………………………………….. 5

Chlorinated hydrocarbons …………………………….. 6

Cooling water, seawater cooling ……………………… 7

Selection criteria ………………………………………. 8– Localised corrosion– Organic acids

Reference deliveries …………………………………... 13

For more information seeS-120-ENG – Duplex stainless steels – fighting corrosion

worldwideS-33-41-ENG – Corrosion resistance of modern duplex

stainless steel in organic acid environ-ments

S-52-86-ENG – Duplex stainless steels for use in organicacid

S-51-57-ENG – SAF 2507 for seawater cooled heat ex-changers

S-91-57-ENG– Welding practice for the Sandvik duplexstainless steels SAF 2304, SAF 2205 andSAF 2507

S-91-59-ENG – Guidelines for tube-to-tubesheet joiningduring fabrication of heat exchangers insuper duplex stainless steel SAF 2507.

Sandvik, Sandvik SAF 2304, Sandvik SAF 2205 and Sandvik SAF 2507are trademarks owned by Sandvik AB

3

FROM WHALE OIL TOADHESIVESThe versatility of chemicals derivedfrom organic oils was first discoveredthrough the exploitation of whale oil inthe nineteenth century. Domestic light-ing was first made possible through theuse of this medium and was also thebasis for the establishment and growthof some of the worlds major chemicalcorporations. Thankfully the discoveryof another form of hydrocarbon in theshape of crude oil provided an alterna-tive source of domestic energy as wellas an abundant feedstock for the manu-facture of the majority of today’s con-sumable products. Fig. 1 summarisesthe derivatives of natural gas and crudeoil and the finished products that aremanufactured from them.

In today’s petrochemical market themost important hydrocarbon raw mate-rials following petroleum refining andnatural gas processing are ethylene,propylene, butadiene, benzene, mixedxylenes and methanol. Demand forthese products is expected to increase inproportion to world demand for con-sumer products.

RELIABLE HEAT TRANSFERHeat transfer is one of the fundamentalelements of hydrocarbon processing,

together with catalysis, thermal crack-ing, fluid flow and mass transfer.Despite technological advances in thearea of compact heat exchangers, shelland tube and air cooler type exchangersare the most popular pieces of equip-ment in bulk petrochemical operationsthanks to their ability to accommodatelarge heat loads and their ease of main-tenance and cleaning. This popularitywill continue long into the future.

Reducing corrosion of materials andreducing fouling of heat transfer sur-faces is a high priority in the hydrocar-bon processing industry. Corrosion andfouling can in many cases reduce on-stream time, increase maintenance andtherefore lower operating efficiency. Inthese times of strong competition with-in the industry and in the spirit of in-creased efficiency, in line with stif-fening environmental legislation, retro-fitting operations in petrochemicalplants often involve the addition of newor improved heat transfer equipment.Such investment can be difficult to jus-tify if the new equipment is still subjectto corrosion and fouling. There is there-fore a demand for cost effective solu-tions to the potential problems that canbe encountered in heat transfer equip-ment. This demand can be satisfied inmany cases by the specification and

INTRODUCTION

NATURAL GAS

PETROLEUM

ETHYLENE

PROPYLENE

BUTYLENE

BUTADIENE

BENZENE

TOLUENE

p-XYLENE

o-XYLENE

m-XYLENE

NAPHTALENES

AMMONIA

METHANOL

VINYL CHLORIDE

BUTADIENE

ETHYLENE DICHLORIDE

ETHYLENE GLYCOL

ETHYLENE OXIDE

METHACRYLATE

MALAIC ANHYDRIDE

PTHALIC ANHYDRIDE

ACRYLONITRILE

STYRENE

PHENOL

FORMALDEHYDE

UREA

MELAMINE

TOLUENE DIAOCYANATE

CARBON BLACK

OTHER ORGANIC CHEMICALS

ADHESIVES

DETERGENT AND SOAPS

EXPLOSIVES

FERTILIZERS

FILMS AND FIBRES

PAINTS AND COATINGS

PESTICIDES AND HERBICIDES

PHARMACEUTICALS

POLYMERISING

POLYMERS/RESINS

RUBBERS AND ELASTOMERS

SOLVENTS AND CLEANING COMPOUNDS

Fig 1.

The major corrosion problems in petrochemicalheat exchangers

• Organic Acids – Formic acid, acetic acid, terephthalic acid• Organic Chlorides – EDC, VCM, PVC and other chlorin-

ated hydrocarbons• Utilities - Cooling Water including Seawater Cooling• Product Purity - Contamination by carbon steels and other

non passive materials4

installation of Duplex Stainless Steelsin critical applications. Petrochemicaland chemical applications account forapproximately a quarter of the totalspending on heat exchange equipmentworld-wide. Almost two thirds of that isreportedly spent on shell and tubeexchangers and air coolers. By virtue oftheir application at elevated tempera-tures, heat exchangers can representsomething of a flashpoint when it con-cerns corrosion damage. It has becomecommon practice these days to retubeexisting carbon steel tubular heatexchangers with corrosion resistantmaterials in the event of premature fail-ure. This helps to increase plant life-times and reduce maintenance costs. Agreater awareness of the corrosive na-ture of today’s petrochemical applica-tions and the need to operate plant in ahighly efficient manner have also led tothe specification of new equipment inmore advanced materials.

MATERIALS IMPROVEPROCESSESIn addition to solving ongoing mainte-nance problems, the integration ofmaterials engineering into chemicalprocessing technology can also help toincrease the operating window of spec-ific pieces of equipment. Organic acid,methanol and urea production are justthree current examples of where thematerials of fabrication constitute anintegral part of the process technology.

This brochure describes the applica-tion of one group of stainless steels, theDuplex Stainless Steels, which was ini-tially developed to combat commonproblems associated with chloride bear-ing cooling waters and process fluids.Thanks to their alloying and micro-structure they can also offer a remarka-ble resistance to corrosive process envi-ronments, notably in the production of

organic acids, where equal or betterresistance to general corrosion thanhigh cost nickel base alloys has beendemonstrated.

Standard 300 series austenitic stain-less steels have found wide applicationin the petrochemicals industry particu-larly for use in piping systems andinstrumentation applications. Theirlimited use in heat exchanger applica-tions is due to susceptibility to stresscorrosion cracking (SCC) in chloridebearing media at temperatures greaterthan 50°C. Duplex stainless steels havea far greater resistance to SCC thanks totheir two phase, ferritic and austeniticmicrostructure. Sandvik’s presentDuplex range includes a family ofalloys, namely SAF 2304, SAF 2205and SAF 2507, that can be used to tack-le corrosion in a broad range of envi-ronments. The super-duplex SAF 2507can even be used in seawater coolingapplications. Not surprising since itwas this job that the material wasdesigned for. Due to the efficient use ofcritical alloying elements such as chro-mium, molybdenum and most notably;nitrogen these materials offer a cost ef-fective alternative to carbon steels, cop-per base alloys, brasses and bronzes.Duplex alloys can bridge the cost gapbetween these traditional materials andthe expensive, nickel based and titani-um alternatives while still giving theperformance level of the latter.

One of the great attractions of theduplex family of alloys is its compa-tibility with the other groups of alloyswith respect to fabrication. Physicalproperties such as the coefficient ofthermal expansion makes retubing ofcarbon steel exchangers possible withminimum modification. Also ”expan-sion only” joining into lower strengthtubesheet materials such as CuNi andAl-bronze is possible.

COMMON MATERIALS OF CONSTRUCTIONGeneral applications• AISI 304L• AISI 316L• AISI 317L

Demanding Environments• Nickel Alloys 600, 625• Hastelloy C-276• Titanium• Zirconium

SANDVIK SUGGESTSDuplex stainless steels show a remarkable resistance togeneral corrosion by organic acids, even outperformingsome of the nickel based alloys in many cases.However; the cost of these materials is close to theiraustenitic cousins. Results from both laboratory rese-arch and in plant testing described below prove thevalue of Duplex stainless steels as the most cost effecti-ve problem solver for organic acids production.

PRACTICAL EXPERIENCEIn plant testing in a terephthalic acid plantHigh alloy austenitics such as 2RK65 (904L) has beensuccessful in organic acid applications, but there is alack of references thus far for duplex stainless steels.Through laboratory testing it is possible to rank diffe-rent materials in terms of their corrosion resistance indifferent organic acid containing solutions. However; Itis not practically possible to simulate real process envi-ronments. The only real way to compare materials per-formance in process is through in plant testing. Thetable shows the respective corrosion rates of some stain-less materials seen in coupon testing in a European PTAplant using a process based on the oxidation of paraxy-lene in the presence of an acetic acid catalyst. Peoplefamiliar with this process will most likely recognise thespecific location of the coupons.

The table shows that corrosion from this processenvironment results in unacceptably high corrosion

rates in the austenitic stainless steel AISI 317L whereasSAF 2205 and SAF 2507 have very low rates.

5

PROCESSThe reactive acid group (-COOH) is responsible bothfor the wide use of these chemicals as well as theirresultant corrosive nature. They are slightly reducing innature and are often mixed with halides used as cata-lysts during their synthesis. The most corrosive of theorganic acids is formic, and the most widely used is ace-tic. Formic acid is also a common contaminant in aceticacid processing.

Pure Terephthalic Acid (PTA) is an important feed-stock in the manufacture of synthetic resins for plasticsmanufacture. Acetic acid is one catalyst used in the oxi-dation of paraxylene which is the most common methodof manufacturing PTA.

Materials Solution : Solution :75% acetic acid, 96% acetic acid, 25% H2O 3% H2OTraces of Br–, Co, Mn Traces of Br–, Co, MnCorrosion rate mm/y, Corrosion rate mm/y, T=175°C T=150°C

AISI 317L 0.44 0.67AISI 317L, welded 0.47 0.68SAF 2205 0.012 0.06SAF 2205, welded 0.027 0.072SAF 2507 0.004 0.011SAF 2507, welded 0.006 0.016

Reactor

Distillation

Acetic Acid

Air

Water

Steam

RawMaterial

Air Cooler(Condenser)

ORGANIC ACIDS

Fig 2. Acetic acid plant.

PROCESSIn the pure form organic chlorides are not corrosive butwhen they condense in heat exchangers, a certainamount of water is often present as a separate phase.The resultant hydrolysis when the two media mixresults in the formation of hydrochloric acid.Hydrochloric acid is one of the most corrosive mineralacids and even at trace quantities in solution can causerapid attack of even corrosion resistant alloys.

Two of the most common processes for the chlorina-tion of hydrocarbons are direct chlorination using Cl2 oroxychlorination using HCl. Both processes rely heavilyon catalyst technology. Fig. 3 shows a simplified dia-gram describing oxychlorination used in the productionof ethylene dichloride (EDC), the most important feed-stock in the manufacture of PVC.

COMMON MATERIALS OF CONSTRUCTION Carbon steel is used extensively in these plants, both forcolumns and heat exchangers as well as piping systems.Carbon steel could be considered satisfactory providedthe moisture content in the process can be kept underclose control. In practice this can be quite difficult toachieve. Shut down periods can be particularly trouble-some when equipment temperatures drop below the dewpoint temperature of HCl.

SANDVIK SUGGESTS The presence of aqueous HCl constitutes the major cor-rosion problem that can be experienced in these plants.General corrosion of carbon steels, high risk of pittingand stress corrosion cracking in standard AISI 300 seri-es stainless steels and pitting of nickel based alloys arethe most likely failure modes. Nickel base alloys can beuseful when the corrosion mode is rapid and heavygeneral attack, however if the risk is for localised corro-sion then very often high performance duplex stainlesssteels have an advantage due to higher alloying withchrome, molybdenum and nitrogen. Nickel itself haslittle effect on resistance to pitting attack.

6

Reactor

EDC

Quench Tower

Pre-condenser

EDC for finalprocessing toVinyl Chloride

EthyleneAir

Hydrochloric Acid

CHLORINATED HYDROCARBONS

PRACTICAL EXPERIENCE Alkyl ChloridesThe resistance of the duplex stainless steels in HCl vari-es according to the composition of the material and thecharacteristics of the acid. In a German petrochemicalplant a heat exchanger used to condense branched alkylchlorides at 75°C was tubed with 22Cr duplex. Thealkyl chlorides themselves were not corrosive, howeverthe organic phase contained a certain amount of waterwhich condensed as a separate phase. The aqueous

phase absorbed some chlorides producing hydrochloricacid. As a result the tubes corroded in a few months. Theform of corrosion was preferential attack of either oneof the phases which is normal under conditions of gene-ral corrosion. The outlet pipe of the exchanger whichwas fabricated from the same material also corroded bythe same mechanism. In order to gain experience withanother material, the outlet pipe was replaced with SAF2507. This has now been in operation for three yearswithout sign of corrosion.

Fig 3. EDC plant.

COMMON MATERIALS OF CONSTRUCTIONSuitable materials for construction of water cooled heatexchange equipment must therefore have the correctcombination of properties to cope with the severity ofthe water while at the same time being compatible withprocess side conditions which may themselves be cor-rosive.

Traditional materials for cooling water service :• Carbon steel• AISI 304L/316L• CuNi, 70/30, 90/10• Admiralty brass

Main limitations• General corrosion of carbon steel• SCC of 300 series stainless steels• Erosion resistance of brass and CuNi• General corrosion of CuNi in the presence of

sulphides polluted waters

SANDVIK SUGGESTSSAF 2304, SAF 2205 in chloride containing coolingwater (non seawater applications)

• Excellent resistance to SCC• Excellent resistance to pitting up to specific limits

SAF 2507 for seawater cooling

• Excellent resistance to SCC• Excellent resistance to pitting corrosion on chlorin-

ated seawater• Excellent resistance to erosion corrosion• Excellent resistance to a wide range of process media.

All Sandvik's Duplex grades have excellent weldabilityand fabricability enabling tube to tubesheet joining bywelding, expanding or a combination of both.

7

PROCESSCooling waters can vary in salt content from virtuallynil in de-ionised and fresh water up to 1.8% in seawater.Water sources may also be polluted with sulphides,ammonia and carbon dioxide amongst others as well ascarrying entrained solids. All these factors adjust thecorrosivity of the water dictating that careful considera-tion must be given to which materials may be used ineach case.

By far the most challenging test for materials in cool-ing water applications is in the case of seawater coolers.

Seawater cooled heat exchangers are subject to con-stant maintenance due to various problems caused bythis particular cooling medium.

FACTORS AFFECTING CORROSIONPERMORMANCE OF MATERIALS FOR SEAWATER EXCHANGERS• High chloride content• Deposits and biofouling• Chemical treatments such as chlorination• Erosion due to entrained solids• Pollution by sulphides and ammonia

COOLING WATER, SEAWATER COOLING

PRACTICAL EXPERIENCERetubing seawater coolers with SAF 2507There are few materials solutions to seawater corrosionproblems available at reasonable cost. The kinds ofmaterials that can resist corrosion in seawater areexpensive and require special consideration when itcomes to manufacturing of equipment.

At a hydrocarbon processing plant in Singapore, landreclamation led to increased amounts of sand in the sea-water used for process cooling. At flowrates as low as 1 m/s heat exchangers tubed with admiralty brass start-ed leaking prematurely.

The two candidate materials for retubing the ex-changers were Titanium and SAF 2507.

Titanium tubes have been known to split duringheavy expansion during retubing operations. In addi-tion, it is necessary to apply cathodic protection in hea-der boxes to prevent galvanic corrosion of the brasstubesheet. Under such circumstances brittle titaniumhydrides can form in the tubing greatly reducing theintegrity of the equipment.

As a result SAF 2507 was the selected material andhas been in service since 1993.

8

CONSIDERATIONSDuplex stainless steels may be used inmany corrosive environments withinthe temperature range of approximately-50 to 300°C.

When considering which duplexstainless steel to use in a particular heatexchanger application, the main con-cern is resistance of the material tolocalised pitting corrosion.The parameters affecting the pittingtendency of a given stainless steel canbe defined as :• temperature• chloride content• oxidant content• pH• sulphide content• inhibiting ion content• flow rateBrief consideration of these parametersenables further simplification for gradeselection;

Oxygen is the most common oxidantfound in natural waters. Its content va-ries between 0 - 9 ppm between boilingand 20°C. The corrosivity of the watersdrops considerably when the oxygencontent drops clearly below 1 ppm.

Chlorine is another oxidant which iscommonly added to seawater ex-changers to mitigate against biofouling.Its effect is to considerably increase theelectrochemical potential and thusincrease the severity of the environ-ment. Only materials with an excep-tionally high resistance to pittingshould be used in systems containingchlorinated seawater.

The pitting resistance is impaired bystagnant solutions. High flowrates ofchloride containing water in tubularheat exchangers will keep the surfacesclean both from deleterious species atpitting sites and from fouling whichcould otherwise reduce heat transfer.

The following 4 factors are the mostcritical in assessing the probability ofpitting attack :1. high electrochemical potential2. high chloride content3. low pH

4. high temperature

To predict whether pitting corrosionwill occur within a given set of envi-ronmental parameters it is necessary torelate the Critical Pitting Temperature(CPT) of the material in that environ-ment to the Maximum Tube wallTemperature (MTT) that will be expe-rienced in the exchanger. The MTT canbe calculated using the following rela-tion :

SELECTION CRITERIAFOR DUPLEX STAINLESS STEELS IN HEAT EXCHANGERS

Temp

Hot Fluid Cold Fluid

Tube Wall

MTT

T(h)

T(c)

r(o) r(f,o) r(w) r(f,i) r(i)

1/U

(C)

Fig 4. Temperature drop through a tube wall from a hot to cold me-dium.

Where :U = overall heat transfer coefficienth = individual heat transfer coefficientR = overall heat resistance (1/U)r = individual heat resistance (1/h)o/i = outside/inside of tubef = foulingw = tube wallT(h) = temperature of hot fluidT(c) = temperature of cold fluidMTT= Maximum Tube wall Temper-

ature

1 1 1 1 1 1(1) – = ––– + ––– + –––– + –––– + –––

U h(o) h(w) h(f,o) h(f,i) h(i)

1(2) – = r(o) + r(f,o) + r(w) + r(f,i) + r(i) = R

U

r(o)+r(f,o)(3) MTT = T(h) – –––––––––– T(h) – T(c)

R

Thus, if the MTT is maintainedbelow the CPT of the material in agiven set of conditions then the risk oflocalised corrosion may be disregard-ed. Should some of the required infor-mation not be available, a reliable, butconservative, estimation can also bemade simply by using the temperatureof the warmest corrosive fluids on theshell or tube side.

Many years of experience have ena-bled Sandvik to formulate a method oflaboratory testing for pitting resistancethat has correlated well to workingconditions when compared with practi-cal experiences.

CRITICAL PITTINGTEMPERATURE CURVESA rapid method of testing for the criti-cal temperatures at which localised pit-ting corrosion takes place has beendeveloped by Sandvik and utilises apotentiostat that simulates the oxidis-ing nature of chloride containing pro-cess fluids and cooling waters. Theapplied potential maintains the con-stant oxidising power of the solution inwhich the materials are tested. With aconstant potential applied, the tempe-rature of the solution is increased by5°C increments until localised corro-sion is determined. This is defined asthe temperature at which the currentdensity measured on the surface of thesample rises above a value of 10 µA/cm2.The method has been substantiated bycomparing the results of the testingwith data collected from real heatexchanger applications.

The following step by step methodshould be used :1. Define chloride content and MTT.2. Define pH, presence of oxidising

species and species that may act asinhibitors.

3. Estimate oxidising character of thesolution preferably by measuringthe corrosion potential.

4. Check the diagrams and judge theapplicability of the steels underconsideration. Remember resultsfrom testing are likely to be conser-vative.

Fig 7. Critical pitting temperatures (CPT)for SAF 2205 and SAF 2304 in various con-centrations of sodium chloride at+300 mV vsSCE, neutral pH. 9

SAF 2507

10 15 20 25

Cl–,%3 6 9 15

SAF 2205

25 Cr – Duplex

Temperature,°C

40

50

60

70

80

90

NaCl, weight-%

125

Fig 5. Critical pitting temperatures (CPT)for SAF 2507 and SAF 2205 in various con-centrations of sodium chloride at +600 mVvs SCE, neutral pH.

Fig 8. SCC resistance for SAF 2507, SAF2205 and SAF 2304 inoxygen-bearing neu-tral solutions.

Fig 6. Critical pitting temperatures (CPT)for SAF 2507 and SAF 2205 in 3% NaClsolutions with varying pH at +600 mV SCE.

Fig 9. Isocorrosion diagram for SAF 2507,SAF 2205 and SAF 2304 in sulphuric acid(0.1 mm/year).

80 100 20 40 600 0 20

40

60

80

100

120

Temperature, °C

SAF 2507

SAF 2507

Boiling point curve

AISI316L

SAF2304

SAF2205

904L

AISI316L

SAF2304

H2SO4, weight-%

SAF 2205

1 2 3HCl, weight-%

0 4 5

904L

6Mo+N

Boiling point curve

SAF 2507

AISI 316L

Temperature, °C

100

80

60

40

20

5 4 3 2 1 pH

SAF 2205

25 Cr – Duplex

SAF 25076Mo + N

80

90

70

60

50

40

Temperature °C

Fig 10. Isocorrosion diagram for SAF 2507and SAF 2205 in hydrochloric acid (0.1mm/year).

Stress corrosion cracking

General corrosion

N08028/Sanicro 28

SAF 2205

AISI 316/316L

AISI 304/304L

0.0001 0.001 0.01 0.1 1 10 Cl–, weight-%

SCC

No SCC

904L SAF 2507 No cracking

Temperature,°C

300

250

150

100

50

0

SAF 2304

SAF 2205

AISI 304L

SAF 2304AISI 316L

Pitting

No pitting

CPT, 300 mV SCE, °C

Cl–, weight-%0.01 0.02 0.05 0.10 0.20 0.50 1.0 2.0

80

60

40

20

0

10

PRACTICAL ASPECTS TOSELECTING MATERIALSWhile it has proved effective to selectmaterials for heat exchanger applica-tions based on arbitrary laboratory testresults, it is also necessary to considercertain aspects of operating heat ex-changers that cannot be suitably repre-sented in the laboratory testing.

DepositsThe most important consideration isperhaps the potential for the build up ofdeposits in or on the tubes. Such depos-its may emanate from the process side,for example from tenacious hydrocar-bons and process slurries. Coolingwater sources may contain sand orsediment that can lie in horizontallymounted exchangers when operated atlow flowrates. Due consideration mustbe given to the possibility of crevicesforming under these deposits whichmay lead to corrosion taking place attemperatures lower than the CPT of theselected material.

Probably; two of the most potenttools in selecting material for theupgrading of heat exchangers are :• Previous experience with other

materials.What has been the mode of failureof the previously installed unit andwhere have the problems occurred?This information can be gatheredduring inspection.

• What material solutions have beenused successfully elsewhere?

Much information is available fromSandvik reference lists, which presentdata from plants worldwide whereduplex alloys have a proven trackrecord over a period of time.

These two items of information,used in combination with the exchang-er operating parameters, technical datasheets and corrosion tables will enableeffective materials specification to bemade.

Organic acids are widely used in thechemical industry of today. One impor-tant reason is the reactive carboxylicgroup (-COOH) in organic acids, whichforms a base for manufacturing ofvarious chemical compounds, rangingfrom drugs to plastics and fibres.

The most common organic acids are:

Formic acid HCOOH

Acetic acid CH3COOH

Propionic acid CH3CH2COOH

Butyric acid CH3CH2CH2COOH

Fatty acids, comprising six or morecarbon atoms are also relatively com-mon, examples of these are stearic acidand tall oil fatty acids. Another com-mon acid is terephthalic acid, which isused as an intermediate in polyesterfibre processing.

CORROSIVITYOrganic acids are usually slightlyreducing and they can, especially ifimpurities are present in the processsolution, be relatively corrosive. As arule, the corrosivity of organic acidsincreases as the size of the acid mole-cule decreases, which means that for-mic acid is the most aggressive of allpure organic acids. Acetic acid is lesscorrosive in its pure form, but in pre-sence of impurities the corrosivenessmay increase substantially. Acetic acid

is also present as a solvent in the manu-facturing of terephthalic acid, wheresevere corrosive conditions may occur.Propionic acid and butyric acid are lessaggressive but at high temperatures orwith presence of impurities they maygive corrosion problems on low alloyedstainless steels. Fatty acids are lessaggressive, but the acids may be corro-sive during processing, if lower mole-cular weight acids are remaining infatty acid mixtures, or if the fatty acid isanhydrous.

Impurities are present in organicacids as catalysts or contaminants suchas chlorides, ferric or cupric ions andalso as air or peroxide. Other examples

11

CORROSION IN ORGANIC ACIDS

SAF 2205

A 625SAF 2507

C-276 C-22 C-40

0.2

0.4

0.6

0.8

1.0

Corrosion rate, mm/year

Fig 12. Corrosion rates of various alloys inboiling 40% formic acid + 2000 ppm Cl. Testtime 1+3+3 days.

Boiling point curve

HCOOH, weight-%

316L

SAF 2507

80 100 0 20 40 60

Temperature, °C

100

80

60

40

20

304L

904LSAF 2304

Fig 11. Isocorrosion diagram for SAF 2507and SAF 2304 in formic acid.

40

60

80

100

120

80 1000 20 40 6020

CH3COOH, weight-%

304L

316L SAF 2507

Temperature, °C

Fig 13. Isocorrosion diagram 0.1 mm/year, inacetic acid. Shaded area represents risk oflocalized attacks on 304L steel. The curve forthe higher alloyed steels coincides with theboiling point curve.

are formic acid or acetic anhydridewhich may be present in acetic acidprocessing. If cupric or ferric ions arepresent, the conditions become moreoxidising, and the corrosiveness mayin fact decrease under certain condi-tions. On the other hand, simultaneouspresence of chlorides may give a riskof pitting or stress corrosion cracking.This implies that it may be difficult tojudge the performance of a certainmaterial if the process contains impuri-ties, and as a rule organic acids arealmost never handled in their comple-tely pure state.

MATERIALS OF CONSTRUC-TIONSince organic acids are reducing, thematerials of construction should eitherhave high contents of Ni and Mo, orthey should be easily passivated. Theformer group is characteristic forNickel base alloys, whereas the latteris a typical feature of a duplex stainlesssteel.

Standard austenitic stainless steelsmay be used in organic acids at low tomoderate temperatures and concentra-tion ranges. In more corrosive condi-tions, upgrading has to be done tohigher alloyed stainless steels, such asthe duplex stainless steels, or nickelbase alloys. Other materials such ascopper and aluminium may be usedbut they are very sensitive to specificprocess conditions. For instance, cop-per is attacked in presence of aerationor cupric ions, whereas under deaerat-ed conditions the resistance may bevery good, even above the atmosphericboiling point.

FORMIC ACIDFormic acid is the most aggressive ofthe organic acids. At temperatures upto 40°C Sandvik 3R12 may be used,whereas at higher temperatures 3R60may be used if the concentration islow. At concentration exceeding 10%and temperatures above 80°C specialstainless steels must be used to keepthe corrosion rate low. The duplexstainless steels Sandvik SAF 2304,SAF 2205 and SAF 2507 have goodresistance within certain limits, at tem-peratures above 100°C SAF 2507 isthe preferred material, except in boil-ing solutions in the concentrationrange 40-90%, where the temperaturemust be limited to approximately100°C. In boiling 40-90% formic acidmaterials such as Hastelloy C-4 oralloy 625 may be used.

The presence of impurities in formicacid may increase corrosiveness sub-stantially, this is especially true for fer-ric chloride and copper chloride, wherecorrosion rate increases substantially.

In presence of chlorides but no oxidi-sing agents such as the ferric or cupricions, SAF 2507 shows a performancesimilar to or slightly better than alloy625.

An isocorrosion diagram for variousalloys in formic acid is shown in figu-re11. Figure 12 shows corrosion ratesof various alloys in 40% formic acidwith an addition of chlorides.

ACETIC ACIDAcetic acid in its pure form is lessaggressive than formic acid. If noimpurities exist Sandvik 3R60 can beused up to the boiling point at all con-centrations, with the exception of con-centrated anhydrous acetic acid, wherethe solution will be more aggressive.In anhydrous acetic acid and in acidcontaining impurities such as chlo-rides, formic acid or acetic anhydride,higher alloyed materials such as SAF2507 or Hastelloy C-4 are preferredmaterials of construction. Figure 13shows the isocorrosion diagram in ace-tic acid. Figure 14 shows the corrosionrate of various alloys in concentratedacetic acid with an addition of aceticanhydride and chlorides.

TEREPHTHALLIC ACIDWhen terephthalic acid is manufac-tured, the intermediate stages containcatalysts such as MnAc and NaBr andacetic acid is used as solvent (oxida-tion of paraxylene). In some caseschloride contaminants may be present.Terephthalic acid is very aggressiveand there is a risk of corrosion onstandard stainless steels. A high resist-ance to general corrosion and stresscorrosion cracking are criteria thathave to be fulfilled for materials ofconstruction. Both 3R12 and 3R60have limited resistance in the process,and therefore special grades such asthe duplex SAF 2205 and SAF 2507are recommended.In the plant testing in a terephthalicacid plants has been performed onSAF 2205 and SAF 2507, showingvery low corrosion rates, see page 5.

12

5 10 15 25 30HCOOH, weight-%

0 20


0.20

0.15

0.10

0.05

0

6Mo+N

50% acetic acid

AISI 317LAISI 316L

N08028Sanicro 28

SAF 2205

SAF2507 No attack

Fig 15. Corrosion rates of SAF 2507 andSAF 2205 in boiling mixtures of 50% aceticacid and varying proportions of formic acid.Test time 1+3+3+ days.

A 625 C–4 C–22 C–276


0

0.05

>0.1 mm/ year

SAF 2507

SAF 2205

Fig 14. Corrosion rates of various alloys inacetic acid with anhydride and 200 ppmchlorides.

ORGANIC ACIDSApplication Chloride salts

Country Italy

Size 38.1 x 1.65 mm

Service conditions Tube side Glutamic acid16–20% Cl, 1.8–3% SO4

(2–),Mg, Na, K, Ca, pH 4–5Temperature max 85°C (185°F)

Shell side Steam at max 115°C (239°F)

Previous experience 316L tubes failed due to SCC.

Sandvik SAF 2205 Tubes in service since 1983 with excellent performance.

Application Caprolactam

Country Italy

Size and quantity 19.05 x 1.65 mm, 11 600 m (38000 ft)

Service conditions Tube side Diathermic oilInlet 325°C (617°F)Pressure 15.3 bar (222 psi)

Shell side Benzoic acid + underproductsInlet 285°C (545°F)

Previous experience TP316L and 317 failed due to corrosion and fretting at baffle plates. Severe fouling on shell side.

Sandvik SAF 2205 Two reboilers installed in early 1984.

Application Reboiler/Condenser

Country USA

Size 25.4 x 1.65 mm, test tubes

Service conditions Tube side Methanol vaporising at 120°C (248°F)

Shell side Overhead gases condensed to 20-25% acetic acidT = 120-130°C (248-266°F)

Previous experience AISI 316L, AISI 317L and Hastelloy C276 corroded.

Sandvik SAF 2507 Installed in 1991. In service inspection 1994 showed no attack.

CHLORINATED HYDROCARBONSApplication EDC/VCM

Country Norway

Size 20 x 2 mm

Service conditions HCl Tower overhead condenser.

Previous experience 3RE60 failed due to pitting in the haz of badly executed welds.

Sandvik SAF 2205 In service since 1984.

Application EDC/VCM

Country Sweden

Size and quantity 3" Sch 40, 250 m

Service conditions Tube side WaterTemperature max 90°C (194°F)Pressure 2 bar

Shell side Emulgator, vinylacetate, ethylenvinyl chloride, organic acids, etc.Temperature up to 80°C (176°F)Pressure max 120 bar

Previous experience Unknown.

Sandvik SAF 2205 Installed in 1985.

13

REFERENCE DELIVERIES

14

Application PVC

Country Italy

Size 20 x 2 mm

Service conditions PVC granules

Previous experience Previous material failed due to erosion corrosion.

Sandvik SAF 2205 SAF 2205 installed in various exchangers since 1985, replacing old material.

Application EDC/VCM

Country Taiwan


Service conditions Tube side Cooling waterTemperature norm. 145°C (293°F)Pressure norm 3 barTemperature max 200°C (392°F) Pressure max 16 bar

Shell side EDC, C2H2, O2, HCl, CO, CO2, N2,CuCl2, Al2O3 catalystOutlet 250-300°C (482-572°F)Inlet 100-250°C (212-482°F)Pressure max 2 bar


Sandvik SAF 2205 Delivered September 1987.

Application VCM

Country Norway

Size and quantity 1" and 2" Sch 80, 150 + 60 m (490 + 195 ft), plate and flanges

Service conditions Fluidized bed in oxy-chlorination reactorTemperature 170°C (338°F)

Previous experience C-steel and Alloy 825 eroded after 6-12 months.

Sandvik SAF 2205 Installed in 1982, and improved service life compared with previous materials.

Application Condenser

Country USA

Size 22.25 x 1.25 mm

Service conditions Tube side Brackish waterTmax = 65°C (149°F)

Shell side Gases with methylene chloride, some HCl and moisture condensing at 65°C (149°F)

Previous experience AISI 316L showed pitting attacks through the tube wall after 8–12 months.


SEAWATER COOLINGApplication Seawater Cooler

Country France

Size 19.05 x 1.24 mm

Service conditions Tube Side Chlorinated seawaterT 20-50°C (68-122°F)

Shell Side Ethylene oxideTin 100°C (212°F)

Previous experience Replacement of bimetallic tubing.

Sandvik SAF 2507 Delivered 1991. Tubes still operating perfectly in March '97.

15

Application Shipboard Seawater CondensersCountry United KingdomSize 19.05 x 1.65 mmService conditions Tube Side Seawater up to 50°C (122°F)

Shell Side Liquified Petroleum Gas at 18 barPrevious experience Coated LT C-steel and 316L both failed within 12 months.

Particular problems due to sand in the cooling water in shallow waters.

Sandvik SAF 2507 First order 1992. Due to good performance SAF 2507 specified for the whole fleet of 9 vessels.

Application Product CoolersCountry IndonesiaSize 25 x 2 mmService conditions Tube Side Intermittently chlorinated seawater

Tin 29°C (84°F)Tout 39°C (102°F

Shell Side Reformed gasTin 113°C (235°F)Tout 40°C (104°F)

Previous experience New project.Sandvik SAF 2507 Tubes delivered 1996 to fabricator. Plant start up 1997.

Application Seawater coolersCountry Singapore Size 19.05 x 1.65 mmService conditions Tube Side Chlorinated seawater

Tmax 45°C (113°F)Shell Side Low concentration organic acids

Previous Eexperience New plant. Sandvik SAF 2507 SAF 2507 selected in preference to 90/10 CuNi because a high risk

of erosion corrosion due to sand in the coolant. Plant start up Jan. 97.

Application Seawater CoolersCountry ArubaSize 19.05 x 1.65 mmService conditions Tube Side Seawater polluted with sulphides

Shell Side HydrocarbonsPrevious experience CuNi failed within 2 months due to the sulphide polluted

seawater. Sandvik SAF 2507 Specified in preference to Titanium due to possibility of

retaining tubesheets and baffles in CuNi. Tubes delivered 1995.

Application Effluent CoolerCountry AustraliaSize 19.05 x 1.65 mmService conditions Tube Side Hydrocarbons + 1.7% H2S, traces of

ammonia and water.Tin 138°C (280°F)Tout 40°C (104°F)

Shell Side SeawaterTin 27°C (81°F)Tout 37°C (99°F)(estimated metal skin temperature 44°C (111°F)

Previous experience AISI 317 failed due to localised corrosion in 2 weeks. A second bundle failed after 6 months.

Sandvik SAF 2507 SAF 2507 installed Sept 89. Still in perfect order when inspected in November 1991.

16

OTHER AREASApplication Ethanol

Country Italy

Service conditions 1) Vacuum separator: solution of ethanol, NaCl (max 22.4%) and water at 82°C (180°F)

2) Separator (3rd effect): solution of ethanol, NaCl (max 17%) and water at 63°C (145°F)

3) Heat exchanger (3rd effect): tube side has ethanol with max 17% NaCl and H2O and temperature of 63°C (145°F). Ethanol+steam on shell side.

4) Separator-distillator: Tube side with 14% NaCl and water at60°C (140°F). (4th effect) on the shell side similar solution at 44°C (111°F).

Previous experience New plant.

Sandvik SAF 2205 Delivered in 1983.

Application Methanol

Country Canada

Size and quantity 19.05 x 1.65 mm, 8000 m

Service conditions Tube side Methanol with wet CO2About 10 ppm chloridesInlet 241°C (465°F)Outlet 144°C (291°F)

Shell side Methanol with Na2CO3About 70 ppm chloridesInlet 85°C (185°F)Outlet 115°C (239°F)

Previous experience Severe SCC occured on Tp 304 tubes. Only 5 to 6 weeks life reported.


Application Ethylene glycol

Country Netherlands

Size and quantity 19.05 x 2.03 mm, 20,600 m

Service conditions Tube side Steam 11.7 bar (170 psi)Temperature 225°C (437°F)

Shell side Acid bleach stream containing glycols and saltTemperature 175°C (347°F)Pressure 4.2 bar (61 psi)

Previous experience C-steel failed every two years due to corrosion from shell side.

Sandvik SAF 2205 Delivered in 1984

Application Methanol

Country New Zealand

Size and quantity 19.05 x 1.65 mm, 14 000 m

Service conditions Tube side Methanol with wet CO210 ppm chloridesInlet 241°C (465°F)Outlet 144°C (291°F)

Shell side Methanol with Na2CO370 ppm chloridesInlet 85°C (185°F)Outlet 115 (239°F)

Previous experience Tp 304 tubes failed after 5-6 weeks due to chloride induced SCC from the shell side where carbonate scale is formed.

Sandvik SAF 2205 Heat exchanger commissioned in 1983. SAF 2205 was selected in preference to type 26Cr-1Mo steel.

17

Application Cyanamid acrylamide

Country Netherlands

Size and quantity 19.05 x 1.65 x 1900 mm, 4430 m

Service conditions Tube side Air + steamInlet 20°C (68°F)Outlet 35°C (95°F)Pressure 0.01 bar (0.14 psi)

Shell side Brackish waterTemperature 27-34°C (80-93°F)Pressure 2.5 bar (36 psi)

Previous experience Tp 304L failed due to pitting and SCC.


Application Overhead condensers

Country Japan

Size 19.05 x 1.2 mm

Service conditions Tube Side Seawater Tin 30°C (86°F)Tout 35°C (95°F)

Shell Side Hydrocarbons, H2, N2, CO2, H2STin 127°C (261°F)

Previous Eeperience Duplex alloy containing 3% Mo suffered from pitting corrosion.

Sandvik SAF 2507 Tubes delivered 1995.

Application Phenol formaldehyde reactor condenser

Country Australia


Service conditions Tube side VapourTin 95° (203°F)Tout 90°C (194°F)

Shell side WaterTin 26°C (79°F)Tout 40°C (104°F)50 ppm Cl–


Sandvik SAF 2304 Installed in May 1988.

Application Phtalic anhydride crystallisation

Country Italy

Size and quantity 33.7 x 1.5 mm, 8418 m + 42000 m

Service conditions Heating coils in crystallisation reactorTube side Diathermic oil 50°C (122°F)

Shell side Reactor is fed with naphtalene, 20°C (68°F), which is heated up to 50°C (122°F) at which temperature reaction takes place.

Previous experience Carbon steel had a service life of 5 years in 33.7 x 2 mm.


Application Steam condensing vessel

Country United Kingdom


Service conditions Tube side Cooling water

Shell side Steam

Previous experience Carbon steel failed due to steam erosion.

Sandvik SAF 2304 In service end of 1987. SAF 2304 was chosen due to its resistance to SCC.

18

Application Maleic anhydride - heat exchanger

Country USA


Service conditions Secondary cooler after the reactorTube side Maleic anhydride

Tin 204-232°C (400-450°F)

Shell side Steam condensate containing some chlorides

Previous experience AISI 316 failed from SCC after 2-3 years service.


Application Butyl rubber



Service conditions Transfer line for moist butyle rubber cement crumb from the dryers to the storage vessels.Temperature 60-70°C (140-158°F)Cl– tracespH 10-14

Previous experience AISI 316 regularly failed due to SCC.

Sandvik SAF 2304 In service since July 1986.

Application Fermentation of hydrocarbons

Country Germany

Size and quantity 25 x 2 x 2450 mm, 135 m

Service conditions Tmax 120°C (248°F), 1/2 h per dayTnorm 40°C (104°F)Cl– 200-500 ppmpH 3.5-4.5

Previous experience AISI 316Ti has been tested but failed due to corrosion fatigue.

Sandvik SAF 2304 Delivered 1987. SAF 2304 was chosen due to excellent test results.

Application Phenol

Country Germany

Size and quantity Heat exchanger tubing

Service conditions Tube side SteamShell side 90% phenol and water

About 60 ppm chloridesSlightly acidicTemperature 70°C (158°F)

Previous experience Tp 321 and 316Ti stainless steel failed due to pitting and SCC after 3-4 weeks.

Sandvik SAF 2205 Tubes were installed in early 1984.

Application Glycerol

Country Italy

Size and quantity 1" Sch 5 & 10, 240 m

Service conditions Tube side 80% glycerol + water with about 20 000 ppm chlorides.Temperature max 105-110°C (230°F)Flow rate 1 m/s (3.3 ft/s)

Shell side SteamTemperature max 140°C (284°F)

Previous experience C-steel failed after about 2 years.

Sandvik SAF 2205 Installed in 1982. Plant on stream 1986.

19

Application Polypropylene glycol

Country Korea

Size and quantity 1 1/2" Sch 10S, 240 m

Service conditions Tube side Fresh water with 5 ppm chloridesInlet is ambient

Shell side Polypropylene glycol with some chloridesInlet max 130°C (266°F)

Previous experience Tp 304 failed due to stress corrosion cracking after a short time. Replacement in 316 also failed due to the same reason.

Sandvik SAF 2205 Coil with SAF 2205 pipe has been in service since 1982.

Application Reboiler

Country USA

Size 19.05 x 1.24 mm

Service conditions Tube side SteamT = 127°C (260°F)

Shell side Isopropyl alcohol, water and solidsT = 80-105°C (176-221°F)


Application IPA/IPE Reboiler



Service conditions Tube side Mixture of isopropylacetat, isopropyl-ethylene and some water

Shell side Steam, 120°C (248°F)

Previous experience AISI 316 failed due to SCC from shell side.

Sandvik SAF 2304 In service end of October 1987.

Documents

S-156-ENG 97