Review of Nitriding in Ammonia Plants

Nitriding that occurs in the synthesis section of ammonia plantsis reviewed based on experience, a literature survey, and reportedincidents from operating plants.

S.Y. Sathe and T.M. O'ConnorM.W. Kellogg Company, Houston, TX 77046

Nitriding is one of the factors considered inthe design of ammonia plant equipment, espe-cially the ammonia converter, converter ef-fluent waste heat recovery exchangers, start-up heaters and the associated piping. Con-siderable experimental and plant test rackinvestigation data has been reported and isavailable in the literature on a wide varietyof steels and test conditions. But much ofthe data is not easily accessible because ofits age or language, we have summarized thisbody of published knowledge. This paperreviews and generalizes the previouslyreached conclusions so as to provide someuseful guidelines to design and operatingengineers. The guidelines address materialsselection, inspection and precautions forwelding to minimize problems due to nitrid-ing.

Nitriding occurs relatively slowly, itsembrittling effects are not obvious in theearlier years of operation but are just nowbeing experienced in some plants. Nitridinghas gained attention in recent years, as theaverage service age of ammonia plants is in-creasing. Many plants have now been in oper-ation for over ten and a significant numberover 20 years.

The increasing number of converter re-vamps, which involves reuse, modification orwelding to existing components necessitatesan improved understanding of nitriding.Thus, detection of nitrided surfaces by in-spection and requirements for welding arealways necessary and will be discussed.

Two stainless steel converter exchangersare known to have failed due to through ni-triding of the thin wall tubes. Failure ofthermal barriers, wire screens, and tie wireshave also occurred. Cracking in start-upheater tubes was reported in the last AIChEAmmonia Symposium (l).

Figure 1 shows three typical convertercomponents which have been damaged by ni-triding. These are a thermal barrier, screenand interchanger tubes. A close-up view ofthe screen in Figure 2 shows severe crackingboth in cross-section and longitudinally.This screen is TP 304 S.S., rather than theInconel 600 specified for the service.Figure 3 is a photomicrograph of the samescreen. Nitriding extends completely throughthe wire.

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A thin sheet metal thermal barrier (TP304 S.S.) shown in Figure 4 is severelycracked and buckled. Buckling is probablycaused by the increased volume of the ni tri ri-ed layer. Figure 5 is a cross-section of thebarrier showing a thin, un-nitrided layer inthe middle of the sheet. Increased nitridingis observed under the crack tip. This isvery typical of cracks due to nitriding. Thesame pattern is observed in the tube cross-section illustrated in Figure 6.

Controlled Nitriding

There are many different commercialnitriding processes used to intentionallyintroduce nitrogen into the surfaces of steelparts. Diffused nitrogen produces a veryhard, thin surface layer which provides im-proved resistance to wear, galling and fa-tigue.

In the gas-nitri ding process used forhard surfacing applications treatment isusually carried out in a pure anhydrous NHj,controlled atmosphere in the region of 500°C(932°F). At this temperature the ammoniacatalytically dissociates when in contactwith the hot steel surface, allowing absorp-tion of atomic nitrogen. A level between 15and 30% dissociation is maintained. Drymolecular nitrogen is inert towards mostmetals and alloys.

The high hardness óf the surface layerresults from nitriding. Distortion or crack-ing that may accompany quenching in heattreatments is not common in commercial ni-triding. The white layer (iron nitrides) isgenerally considered to be undesirable inmost applications where nitriding is used.This is because, both the hardness and depthof nitrided layer depend upon the amount ofnitride forming elements (substantial addi-tions of aluminum and chromium) in the steeland, for alloy steels not intended specifi-cally for nitriding, on the core hardness ofthe steel. Nitriding heat treatments areinvariably accomplished at temperatures belowthe eutectoid transformation temperature ofthe steel. The relatively low nitridingtemperatures limit the depth to which nitro-gen can diffuse. Corrosion resistance ofnitrided carbon and low-alloy steels canapproach that of the martenistic stainlesssteels.

Mechanism

Nitriding may be considered as a high

temperature corrosion process. It occurs asa result of dissociation of NH3 and diffusion(adsorption) of atomic nitrogen into thesteel surface from long term exposure attemperatures over 700°F. Nitriding is a timeand temperature dependent phenomenon, whichresults in the development of a thin, hard,non-uniform, brittle surface layer called anitrided case.

The source of atomic nitrogen in a syn-thesis loop is dissociated NH3- DissociatedN2 from air or feed stocks containing ammoni-ated compounds can cause nitriding in otherservices. At temperatures between 400°C(750°F) and 600°C (1100°F) thermal dissocia-tion of ammonia occurs into atomic nitrogenand hydrogen, the dissociation being catalyz-ed by metal surfaces. Increased temperaturepromotes nitriding perhaps because of agreater degree of dissociation or higherdiffusivity. Undissociated ammonia practi-cally does not interact with metal.

The nitride forming elements are: Fe,Cr, Al, Ti, Mot V, W. The elements with lessaffinity (noble) towards atomic nitrogen are:C, Ni, Si, B. The various types of metallicnitrides of interest that were identifiedare: ON, CrN2, 2TiN, 2VN, 2A1N, 2Fe4N(Gamma-phase), Fe2N (Epsilon-phase)

Nitriding of metal is accompanied by anincrease in its volume. The nitrided layeris in compression and exerts tensile stresseson the base metal, which increases hydrogenpermeability and its embrittlement.

Internal nitriding rate is controlled bythe volume of atomic nitrogen in the surfacelayers. This quantity may be insufficient tonitride the nobler major constituent butsufficient to nitride the reactive minorelements. An external layer will form onlywhen the effective pressure of atomic nitro-gen beneath the metal-gas interface exceedsthe required concentration of atomic nitro-gen.

Many metals corrode at high tempera-tures. The rate of attack is rapid at thebeginning, but the rate slows with time.This relationship is quite accuratelydescribed by a parabola. The parabolic rateis in accordance with the simple diffusiontheory which indicates that as the scale getsprogressively thicker, the rate of reactionbetween metal and nitrogen decreases. Diffu-sivity of nitrogen in a given metal is lowerthan the corresponding oxygen diffusivity.

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In the oxidation process the most commonobservation on iron base alloys is amultiple-layer oxide scale formation. Theiron tends to diffuse outward to form Fe3U4and F62Û3 and the oxygen diffuses inward toform the innermost layers of FeO. Internaloxidation or what is also called "sub-scale"or sub-surface corrosion occurs only in alloysystems where the alloying elements in thesteel have widely different "affinities" foroxygen. This difference depends on the rela-tive stabilities of the compounds formed. Inother words, there is a sub-surface zone of"partial oxidation". The more easily oxi-dized atoms of the alloy are oxidized, butthe more noble atoms are unattacked.

Low Alloy Steels

Ferritic steels up to 12% Cr suffer fromheavy nitriding and flaking if exposed tometal temperatures over 371°C (700°F) in thesynthesis environment. Nitriding depth pro-gresses with time and increased temperature,causing microstructural changes, high hard-nesses and preferential grain boundarynitriding with resultant formation of sub-surface cracks and embrittlement.

Carbon steels lose carbon and adsorbnitrogen throughout their cross section.Numerous cracks and fissures are found whichaffect their mechanical properties, espe-cially ductility. Carbon steel mainlysuffers from decarburization, loss of ductil-ity and strength and microfissuring due toreaction with atomic hydrogen present in themixture, also additionally generated by thedisassociation of

Low chrome alloys adsorb nitrogen pri-marily at the surface, but the core is alsosubstantially affected. Cracking in theheavily-nitrided outer layer is thereforeprone to propagate through the embrittledcore.

In the mixed N2-H2 environment (atomicnitrogen/hydrogen), the chromium carbidephase (CryCs) present In the low alloy steelsmay be attacked by atomic nitrogen to form astable chromium nitride (CrN/Cr2N) andrelease carbon to react with H2 to formmethane. Steels with moderate chromium con-tent suffer both from nitrogen and hydrogenattack under synthesis conditions and thus,their useful life is very limited.

When Cr content is raised to levelsgreater than 12% the nitrided case becomesthinner. The nitrogen is concentrated,

especially in the outer layer of the case.The nitrogen forms a high chromium nitridecase and slows down the further absorption ofthe gas. The chromium carbide phase (CrçsCs)in these steels, with higher than 12%chromium content, is more stable thanchromium nitrides and therefore, the reactiondoes not occur as rapidly. The presence ofcarbides to which chromium can segregate doesaffect the development of nitrided hardness.Thus, carbide morphology plays a significantrole in resistance towards nitriding andaccompanying hydrogen attack.

Nitrogen in nitrided steels is in theform of iron nitride and not as a pressuregenerating gas. This contrasts with methanewhich forms when hydrogen reacts with carbonin the carbides. Nitrogen tends to make thesteel harder and more brittle and alsoreduces corrosion resistance in some environ-ments. Methane tends to cause cracking dueto internal pressure build-up during cool-down of equipment.

Austenitic Stainless Steels

Austenitic stainless steels behave dif-ferently than ferritic steels. They developa very thin case with very high concentra-tions of nitrogen and less reduction ofcarbon content. The core changes signifi-cantly less and, therefore, gives betterperformance than low alloy steels. Also,austenitic stainless steels are virtuallyimmune to high temperature hydrogen attack(decarburization).

TP 321 S.S., which is stabilized with astrong carbide forming element titanium hasthe hardest case with high N2 content. Thisis because titanium is a stronger nitrideformer than columbium or molybdenum. Thetitanium also prevents decarburization of thenitrided layer.

TP 316 S.S. has shown poor resistance tonitriding in some instances (2), perhapsbecause of the presence of the nitride-form-ing element molybdenum, which contributes tothe development of a friable or exfoliatingscale.

Because of their chromium content allstainless steels can be nitrided to somedegree. Austenitic S.S. of the 300 seriesare non-magnetic alloys, which cannot behardened by heat treatment; consequently, thecore material remains relatively soft.

The resistance to nitriding of austeni

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tic stainless steel is enhanced by an oxidefilm naturally existing on the metal, becauseoxides have no solubility for nitrogen. Onlywhen the protective oxide layer is disturbedor destroyed, diffusion of atomic nitrogen isfacilitated.

Materials Selection Guideline

Materials for ammonia synthesis musthave resistance to hydrogen attack andnitriding under synthesis conditions. Theprimary factors in material selection toavoid hydrogen attack and nitriding are ope-rating temperatures hydrogen partial pressureand chromium and nickel content of the alloy.The partial pressure of ammonia and minoralloying elements (Ti, Nb) are of secondaryimportance.

The synthesis gas composition is mainlyhydrogen and nitrogen in a ratio of 3:1, andammonia concentration ranges between two and20 mole percent or higher, this is a poten-tial nitriding environment in the temperaturerange over 700°F (371°C). Selection of mate-rials for the ammonia converter internals andother components in the synthesis loop tominimize nitriding can be based on the fol-lowing guidelines:

Below 700°F (371°C): Low alloy steels

2-l/4Cr-lMo, 5Cr-l/2Mo, or 9Cr-lMo

Temperature range 700°F (371°C) to 950°F(4820C):

Austenitic Stainless Steels:

TP 304, 321 or 347 S.S., Alloy 800

Temperatures greater than 900°F (482°C):

Alloy 800, Inconel 600

Thicker sections in the hot zone (up to950°F) in the converter, such as bed supportsand partition plates of austenitic stainlesssteels with proper corrosion allowance tocompensate for the expected nitriding metallosses, have performed satisfactorily. Theyare generally used due to economic considera-tions. This corrosion allowance could be atotal of 1/16" (1.6mm), or 1/32" (0.8mm) oneach exposed side.

For thinner parts, such as thermalbarriers, screens, interchanger tubes andseal strips, materials with higher nickelcontent should be employed for obtaining long

service life. Materials such as alloy 800 oralloy 600 offer a combination of strength,resistance to nitriding and chloride stresscorrosion cracking and do not suffer hydrogenembrittlement under ammonia synthesis condi-tions. Depth and rate of nitriding are par-ticularly significant for the thinnest parts(screens, tie wires), because the nitridingcan completely penetrate through the sectionmaking the part extremely brittle. Highnickel alloy 600 has shown good resistance tonitriding and should be used for these compo-nents.

Fortunately, operating conditions insynthesis loops seldom exceed 950°F (510°C),due to temperature limitations of the cata-lyst. Above this temperature, the corrosionrate for austenitic stainless steel is signi-ficant and expensive high nickel alloys maybe required.

Nitriding Depths & Rates - Discussion

The two major factors affecting the rateand depth of nitriding are metallurgy andtemperature. Analysis of the data is simpli-fied by considering two groups of metallurgy.One is the nickel bearing alloys ranging from18-8 stainless steel through pure nickel.The second group of metals have chrome as theprimary alloy, ranging from 1 to 28 percent.

Figure 7 shows the strong relationshipbetween temperature and rate. This datareflects the performance of Type 304, 316,321, 347 and similar austenitic stainlesssteels. The pressure ranges from atmosphericto 11,000 psi, and ammonia concentrationvaries from two to 20%. Test periods rangefrom one to 27 years. In spite of thesevariables, a clear trend with temperatureexists. Although a relationship with totalpressure or partial pressure of NH3 probablyexists, no correlation of these parameterscould be achieved with this data. Effects ofpressure are especially difficult to deter-mine because no single investigation haspresented data at varying pressure or ammoniaconcentration with other conditions heldconstant. Conversely, many authors havevaried the temperature, alloy composition,sample preparation, and other factors.

Figure 8 shows the variation in nitrid-ing rate for 304, 316, 321, 347 and 310stainless steel. These curves are based onthe data points in Figure 7, except for the310 stainless steel. Sufficient data is notavailable to provide comparable curves forhigher nickel alloys. The data for these two

118

figures is limited to that with test periodsgreater than 1 year, with a maximum period of21 years. Additional data is available forshort periods of less than one year. Thisdata, however, reflects very high initialrates that are less useful for predicting thelong term performance of stainless steel.Limiting the curves to data for more than ayear appears to adequately mask any effectsof the high initial rates. The inclusion ofactual operating data for periods of 18 and21 years indicates that nitriding continuesindefinitely at rates similar to those in thefirst few years. It does not appear thatnitriding is arrested at any specific depth,as suggested by some authors. Depths of upto 90 mils have been observed in stainlesssteels, however, depths greater than 50 milsare unusual.

The rate of nitriding for the variousstainless steels shows improvement with theaddition of stabilizing elements and increasein nickel content compared to TP 304, whichhas the minimum nickel and no stabilizingelement. For instance, TP 310 shows a consi-derably lower rate than 18-8 stainlesssteels, although bulging of the nitridedlayer on 310 S.S. has been reported. TP 316shows consistently good results for the longterm data used in the plots. Moran (2),however, reports very poor performance of TP316 S.S. in his short term experiments.Takehara(3) reports that TP 321 and 347,which have stabilizing elements (Ti, Cb/Nb),show ho decarburization in the nitridedlayer, while TP 304 does. The condition ofthe base metal should also be considered.Tanimura(4) found a 20 to 30% reduction inimpact properties of the base metal.

The very beneficial effect of nickel isillustrated in Figures 9, 10 and 11.Increasing the nickel content above that of18-8 stainless can reduce the rates by anorder of magnitude. Figure 9 shows long termdata at temperatures typical of synthesisloops. Figure 10 has short term data atsimilar temperatures. Figure 11 shows that asimilar relationship exists for very shortterm data at very high temperatures. Thislatter data suggests that nitriding potentialmay exist in secondary reformers and associ-ated waste heat boilers. Although this hasnot been reported, brittleness and bucklingof internal liners has occurred and is typi-cal of nitrided metal.

The low chrome alloys have less avail-able data and are more difficult to analyzedue to scatter and extremely high initial

nitriding rates. Although Marsch (5, 6) hasshown the increase in depth of nitriding withtime, it was not possible to do so with thecollection of data from all other sources.It appears that chrome alloys nitride to asignificant depth very quickly and then theincrease is gradual with time. The shortesttime for which data was found was 0.1 years.Nitrided depths of at least 8 mils werealready present after this time period. Amore useful relationship, given the limita-tions of the data, appears to be nitridedepth as a function of temperature, ignoringthe factor of time. Figure 12 shows theincrease in depth with increased temperaturefor 2-l/4Cr-lMo and SCr-1/2 Mo. The timeperiod for each data point is included todemonstrate the poor correlation with time.The performance of these two levels ofchromium appears to be virtually the same.However, Figure 13 illustrates a clear reduc-tion in nitrided depth with increasing chromecontent. The four data points fromTakehara(3_) for l-l/4Cr-l/2Mo demonstrate theadvantage of alloying elements such as Ti,Nb, and V. The sample without these alloyingelements had a nitrided depth of 80 mils,against 50 to 60 mils for the alloy contain-ing samples. This is very similar to thetrend noted for 18-8 stainless steel with Tiand Nb.

The performance of the chrome alloys is,of course, much worse than that of chromtumnickel alloys. At 800°F the rate for stain-less steels is about 1 mil per year.2-l/4Cr-lMo and 5Cr-l/2Mo at this temperatureare nitrided to 20 or 30 mils in a period asshort as 0.1 years.

Welding Precautions and Inspection Guideline

The purpose of inspection is to deter-mine the physical condition and to .examineany damage or deterioration of internal com-ponents of the ammonia converter. Thisinspection could be performed only duringcatalyst change or any major repair orreplacement. Components to be inspectedinclude the basket, screens, piping, supportgrids, thermal barriers, thermowells, gasdeflection plates, distributors, seal strips,interchanger tubes and tube-sheets.

Techniques used in performing an inspec-tion for nitriding are:

1. Visual examination2. Magnet check/Ferrite-scope3. Hardness measurements4. Ultrasonic thickness measurements

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Stainless Steel Components

1. The nitrided surface of austeniticstainless steel shows a characteristic dull,dark gray appearance. It is a thin, hard,adherent surface layer.

2. The nitrided layer is usually mag-netic. A simple check with a permanentmagnet can be made in detecting nitridedsurfaces of stainless steels, which arenormally non-magnetic.

3. The nitrided layer shows higherhardness values in the range of 400-500 (DPHor BHN). Portable testers could be used forhardness checks. The nitrided layer could beremoved by light grinding and polishing untilthe base metal hardness indicates a readingin the range of 170-190 BHN (Brinell), whichcorresponds to the nominal hardness value ofthe austenitic stainless steel material.

4. The nitrided depth can be estimatedas the thickness of the scale in which themicrohardness is approximately 50 pointsabove the hardness of the unaffected basemetal (5), and also by metallography.

5. Ultrasonic thickness measurementscan be performed to determine the depth ofpenetration by ni tri ding and the remainingsound metal thickness and to detect cracks.

Low Alloy Steel Components

If the existing components are con-structed of low alloy steels (start-up heatertubes, transfer piping), it is necessary toperiodically inspect these components thor-oughly because of the likely aggressivenitriding attack combined with the possibili-ty of hydrogen damage.

The surveillance program should includevisual inspection, liquid penetrant or mag-netic particle examination, hardness testingand examination of the internal subsurface,using ultrasonic and radiographie techniquesas required.

Welding Precautions

Nitriding of austenitic stainless steelsdoes not greatly affect the bulk propertiesof the steel below the nitrided surfacelayer. The heat of welding or thermalcutting could liberate nitrogen from thenitrided layer and can cause difficulties,unless removed by light grinding of about1-2 m of thickness on either side of the

exposed surface.

Summary

Cases of catastrophic failures or majordamage due to nitriding have not been report-ed in the literature. Nitriding of low alloysteels and austenitic stainless steels isknown to very gradually occur when exposed tothe mixed hydrogen and nitrogen environmentof ammonia synthesis. The effect of nitrid-ing and hydrogen attack is more pronounced onthe low alloy steels. Nitriding is similarto oxidation, which is a surface phenomenon,but less damaging than the high-temperaturehydrogen attack, which affects the entirethickness of the metal. The common materialsof construction, especially the austeniticstainless steels and high nickel alloys, haveperformed well when used within their tempe-rature limits. Guidelines have been present-ed for materials selection to min.imize ni-triding based on temperature. Proper tech-niques for inspection and welding have alsobeen discussed.

Figure 1. Screen, tube and thermal barrier samples(0.9X).

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Literature Cited

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4. Tanimura Masayuki and Nakazawa Toshio,"On the Corrosion Embrittlement ofCommercial Steels Caused by HydrogenAttack and Nitriding Attack in anAmmonia Converter Corrosion Engineering(Tokyo), Vol. 21, No. 5, p. 209-218 May1972.

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Figure 3. Longitudinal section of an individual wireshowing cracking and the nitrided conditionthroughout 0.5% nital etch (150X).

Figure 2. Portion of the screen showing the Figure 4. Portion of the thermal barrier showing apresence of transverse and longitudinal cracks (20X). network of cracks (3.5X).

123

Figure 5. Section across a crack in the thermalbarrier showing nitriding extending inward from bothsurfaces. Nitriding is deepest beneath the crack tip0.5% nitai etch (150X).

„**%x »"*»;*>'V:

• > f4 ̂ "

Figure 6. Transverse section of one of the tubes.Nitriding is deepest beneath the crack tip 0.5% nitaletch(150X).

Data Source100

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" 0 Takeharax Marsch» TanimuraACFIv VerrnaOWahl• Aberg> McDowellv Comlnco

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.

Time: 1.5 to 21 YearsPressures: NIL to 11,000 psiAlloys: TP 304, 316,321, 347

4P*.]20 pis

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.5

.1

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Figure 7. Nitriding rate vs. temperature for 18-8 typestainless steels.

30

20

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5

OJn

CCutcB

.5

304

'316

310

347

Based on Data From Figure 7

800 900 1000Temperature "F

1100

Figure 8. Rate of nitriding vs. temperature.

124

2000

1500

1000

500

(OocO)

£Z

100

50

10

lOSO'C(1922-F)

Date SourceA Steinkusch

O 10 20 30 40 50 60 70 80 90 100

% Nickel

Figure 9. Rate of nitriding vs. nickel content: long-term data (greater than one year).

2015

10

accu>

.5

.3

Data Source> McDowellO Aberg•*• Tanimura

0 10 20 30 40 50 60 70 80 90 100% Nickel

Figure 11. Nitriding rate vs. nickel content at hightemperatures for short periods (less than 200 hours).

200

150

100

50

10

525°C(977°F)

J_ l

10 20 30 40 50 60

% Nickel

70 80 90 100 400 500 600 700 800

Temperature, °F

900 1000 1100

Figure 10. Rate of nitriding vs. nickel content: short-term data (less than one year).

Figure 12. Nitrided depth vs. temperature: 2% Crand 5 Cr.

125

200

150

100

50

=E.Sa.

~ 10

6.90

Hydrogen Partial Pressure, MPa abs

10.34 13.79 17.24 20.734.548.3

Controlling Al,oy5Mechanism '

Data Source* Ihrig0 Takehara* lanlmura® Cihal

l t l l

450-520-C(842.963-F)

*Some samples nave additionalalloying elements.

Nitridlng S.S. toHigh Nickel

Hydrogen 2V.CrAttack« toNllridlng S.S.

Hydrogen C.S.&1'ACr

1500 2000 2500 300050007000

Hydrogen Partial Pressure, Lb/ln1 abs

Figure 14. Metallurgical selection for resistance tonitriding and hydrogen attack.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28% Chrome

Figure 13. Depth of nitrided layer vs. chromecontent.

T.M. O'Connor

DISCUSSION

JAN BLANKEN, DSM Fertilizer, Ijmuiden, Holland: Youmentioned in your paper that the nitriding is caused byammonia. You also mentioned that you haven't been ableto find a correlation between the ammonia content or thepartial pressure of ammonia and the rate of nitriding. Ourmetallurgist, Kees van Grieken, had done a lot of workabout 20 years ago. He developed a formula to calculatethe rate of nitriding, in which he had the partial pressureof ammonia. As far as I can remember, when checkingthe remnant life of the tubes in the feed-effluent exchanger122C, he calculated that the nitriding depths on the insideof the tube where the ammonia content was 12% of 2 mmand 0.5 mm on the outside of the tube where the ammoniacontent is 2%. My question is, referring to Figure 6 in yourpaper, which shows the depths of nitriding of the feed-effluent exchanger, did you compare the depths of nitridingoutside and inside of those tubes?M. O'CONNOR, M.W. Kellogg: I agree that the volumepercent, the partial pressure, or some other parameterdealing with ammonia concentration is appropriate. Thepoint is, gathering data from so many sources, there's stilla fair amount of scatter, so it is hard to determine fromthe data that's readily available, particularly becausepressure data are not typically presented. I agree, with these

samples, you see the different nitrided depths on the twosides of the tubes. Presumably, the temperature of the metalon the two sides is virtually identical. The trend is alwaysthat the higher concentration side seems to have a greaterdepth. I agree that the trend must exist, but it can't bequantified in a general sense from this data.

JAN BLANKEN, DSM Fertilizer: I will try to procure a copyof the paper for you, I will translate it from Dutch beforeI send it to you.

MAT! TIIVEL, CIL: I'd like to confirm what Jan Blankenhas said: "ICI has also done tests on nitriding and thereis definitely a correlation between ammonia percentageand the rate of nitriding." I think you will find that nitridingwill progress at the rate equal to the square root of thetime, until the brittle layers flake off and new metal isexposed. The process will then repeat itself.

M. O'CONNOR, M.W. Kellogg: We tend to believe thatyou reach a constant rate at some point As the layer getsthicker, it also deteriorates, allowing diffusion. Some of thedata we were showing was mixed with periods of one to20 years and one can't tell any effect. Now, if you get into1 - or 2-month data, it's very different.

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