Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001,27-35. 27

UNDERSTANDING THE CORROSION BEHA VIOR OF35Ni19Cr ALLOYUSING X-RAYMICROANALYSIS

A. Wong-Moreno', D. López-López', L. Martínez2•

1 Instituto Mexicano del Petróleo, Blvd. Ruiz Cortines 1517-12, Fracc. Costa de Oro, 94299, Boca delRío, Veracruz, México, acwong@imp.mx,· dlopez@imp.mx

2 Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Av. Universidad S/N, Col.Chamilpa, CP. 6?210, Cuernavaca, Morelos, México, lorenzo@cejis.unam.mx

Abstraet

X-ray microanalysis of corroded specimens of 35Ni19Cr austenitic steel was performed in order to understand its oil-ashcorrosion behavior. Corrosion testing involved the exposure of the alloy at temperatures in the range of 600°C - 900°C, to asulfate-rich oil ash which is also constituted by low melting point sodium vanadates. The curve describing the corrosionbehavior as a function oftemperature exhibits two relative maximums at 715°C and around 800°C, suggesting that there is anevolution of corrosion mechanisms as temperature is increased. X-ray microanalysis of the corrosion products scale and of themetal subyacent to the interface metal/scale let characterize the corrosion mechanisms prevailing along the temperature range.It was concluded that the resultant corrosion behavior depends on both: the oxidation, oil-ash corrosion and sulfidationresistance ofthe alloy, and the stability ofthe oil ash, which determines the chemical compounds responsible for the corros ionprocess observed.

Resumen

Keywords: X-ray microanalysis; oil ash corrosion; 35Nil9Cr austenitic steel; sulfidation; high temperature oxidation

Se llevó a cabo un estudio por microanálisis de especímenes corroídos de acero austenítico 35Ni 19Cr con el fin de entendersu comportamiento de corrosión por cenizas de combustóleo. Los ensayos de corrosión involucraron la exposición de laaleación a temperaturas en el intervalo de 600°C - 900°C, a un depósito de ceniza con alto contenido de sulfatos alcalinos y quetambién está constituido por vanadatos de sodio de bajo punto de fusión. La curva que describe el comportamiento decorrosión en función de latemperatura exhibe dos máximos relativos a 715°C y alrededor de 800°C, lo cual sugiere que hay unaevolución de los mecanismos de corrosión operantes a medida que la temperatura se incrementa. Los resultados del estudio pormicroanálisis de la costra de productos de corrosión y del metal debajo de la interfaz metal/costra, permitió caracterizar losmecanismos de corrosión operantes a lo largo del intervalo de temperatura considerado. Se concluye que el comportamiento decorrosión resultante depende-tanto de la resistencia de la aleación a oxidación, sulfidación y corrosión por cenizas, como de laestabilidad de la ceniza de combustible, ya .que ésta determina el tipo de compuestos químicos responsables del proceso decorrosión.

Palabras Clave: Microanálisis por rayos X; cenizas de combustóleo; acero austenítico 35Nil9Cr; sulfidación; oxidación aaltas temperaturas.

1. Introduetion

High temperature corrosion behavior ofheat resistant al-loys is a very important aspect in the selection of material sfor very aggressive conditions. 35Ni19Cr alloy is a heat re-sistant austenitic alloy with a good combination of mechani-cal properties ami resistance to carburization, oxidation andthermal cycling. It exhibits good metallurgical stability and itdoes not show embrittlement caused by sigma phase. Car-burization is a material deterioration process that can affectthe performance offossii-frred boiler components that oper-ate at elevated temperatures, causing inclusive their failurein some cases (1-8). For that kind of components (hangers,

supports, flame stabilizers) alloys such as 304, 310, 309 aus-tenitic steels are typically used, but is evident the need of analloy with better resistance to carburization besides to oil-ash corrosion. Given the good resistance to carburization of35Ni 19Cr alloy, it was considered to determine its corrosion

_behavior by oil-ashes in order to know if it could be an alter-native alloy for this components. This paper shows theresults of corros ion testing of 35Crl9Ni alloy exposed to aboiler sulfated oil-ash at several temperatures in the range of600°C-900°C, and the results of a rnicroanalysis study car-ried out in order to understaná the corrosion behavior ofthis alloy:

28 A. Wong y col. /Revista Latinoamericana de Metalurgia y Materiales

Table 1. Chemical Composition of35Ni19Cr Alloy

UNSNo. C Si Mn P S Cr Ni Mo Co Cu Fe

N08330 0.03 1.20 1.75 0.016 0.003 19.28 34.96 0.02 0.34 0.05 Bal.

Table 2. Chemical analysis ofthe residual oil ash (weight %) and its main compounds.

V Na K Al Fe Ni Ca Mg S C S04 VlNa+Smolar

25.65 12.36 0.43 0.58 2.89 1.95 1.02 0.71 12.00 0.04 30.17 0.55

pH (1 g/100mI H20): 3.8~

Main compounds (XRD):Na1l3V1I3VS/30S; Na2S04; Nas V24063 ; NaxV20S, x = 0.7 - 1.0; NaSV12032 ; FeV04

2. Experimental

Test specimens: Chemical composition ofthe alloy stud-ied is shown in Table l. l5x12xl.5 mm test specimens werecut from sheets. The samples were ground to a 600 grit SiCfinish, degreased, weighed and cleaned in acetone prior totesting.

Gil ash deposit: Samples were exposed to a residual oilash with high sodium sulfate content, whose characteriza-tion is shown in Table 2. This oil ash is a representativesample of fuel oil-ash deposits collected directly from theprimary superheater banks of a 84 Mw utility boiler. Theboiler burns heavy high sulfur fuel-oil with high contents ofsodium and vanadium. Besides the oil-ash chemical analy-sis, Table 2 also lists the V/(Na+S) ash molar ratio, which isconsidered as a corrosivity index [9,10], and the major com-pounds identified by X-ray diffraction analysis: Sodium sul-fate, sodium vanadil vanadate ofVlNa=6 (m.p. ?625°C), lowermelting point vanadates with high sodium content and lowerVlNa ratio (m.p. »535°C), and the corros ion product FeV04(m.p. 816°C). The deposits were ground to 1OO-meshbeforethey were contacted with alloys.

Test Conditions: Corrosion crucible tests were conductedfor 250 hours under isothermal conditions in electric fur-naces. The specimens were totally packed in oil-ash depositpowder contained in silica crucibles at nine temperatures inthe range óf 600°C - 900°C. The amount of deposit addedwas 500 mg per cm? of initial area of the specimen. Theatmosphere used was static air. At least four specimens ofeach alloy were exposed at each test temperature. It has beenshown that crucible tests, one ofthe most simple laboratoryprocedures, are very useful for observing the effect of someofthe variables involved in high temperature corrosion pro-

cesses [11]. The comparison ofmaterials performance in aqualitative or semiquantitative way under different condi-tions (temperature, composition of the corrosive agent, at-mosphere) by this technique is totally reliable[12]. Besidesthe corrosion crucible tests oxidation testing in static air wascarried out at temperatures in the range of 560°C-950°C, inorder to compare with oil ash corrosion. The oxidation re-sults confirmed the good oxidation resistance ofthis alloy atelevated temperatures.

Post Corrosion Examination: After testing, three cor-roded specimens per temperature were descaled accordingthe ASTM G 1 standard and the weight change and the thick-ness loss of three specimens per temperature were deter-mined. The fourth corroded sample from each test was cross-sectioned, mounted in conductive bakelite and examined as-polished (polished without water) by scanning electron mi-croscopy and microanalysis to analyze the chemical compo-sition, morphology and distribution of reaction products,and to determine the characteristics and depth of any sub-surface corrosive attack. Elemental X-ray mapping was per-formed using a Microspec WDX-3PC system connected to aZeiss DSM960 scanning electron microscope. Furthermore,EDX line profiles were performed using an EDAX DX Prime60 system, to characterize the metallic degradation sufferedby the alloy and understand its corrosion behavior.

3. Corrosion Results

Corrosion as a function oftemperature is shown in Figure1. It can be compared the oxidation resistance of35Ni19Cralloy with its performance under exposure to oil ashes. Cor-rosion caused by oil ashes is 2-4 orders of magnitude higherthan tbat resulting from high temperature oxidation. It was

Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001.

found two relative maxima, at 715°C and around 800°C, asthe specimens exposed to 785°C and 805°C were completelyconsumed. This fact is indicated by the arrows on top oftheir respective data in the graph (the data includedi in thegraph for these temperatures were estimated from their initialweight, but obviously the real corrosion amount should havebeen higher than those values). The presence of the peaksal 715°C and around 800°C suggested the occurrence of ad-ditional corros ion processes to that of metallic dissolutioncaused by molten vanadium compounds. It is well knownthat the curve describing corrosion by sodium vanadates orvanadium pentoxide is exponential as temperature is in-creased [13, 14]. Evidence ofthe processes involved in thecorrosion by sulfate-rich oil ashes will be shown in the nextection.

10.000 _ .

l: -AR .tO +Vj(Na+S)= 0.55 ;;\1() 1,000 \

i I ~~ ~ .o 100 i t----t....... !

C) Ig 10~

~ '1~ 0.11

I0,01 -li----,----,---~---,----l500

35Ni19Cr Allay

600 , ·700 800 900 1.000

Temperature eC)Fig. 1. Oxidation and oil-ash corrosion of 35Ni19Cr austenitic

stee1 as a function of temperature.

4. Microanalysis Results

Figure 2 shows the magnitude of the subsurface attackand if it consisted in internal oxidation (O), sulfidation (S) orboth (0, S). The depth of internal attack has been added tothe thickness loss in order to estimate the total metal cor-roded. The arrows on top ofthe bars corresponding to 78SOCy 805°C mean that all the specimens exposed at these tem-peratures were completely consumed. As it was mentionedin the last paragraph the metal loss at these temperatureswas higher than 1.5 mm and this is the meaning of the arrowson top of their respective bars. The scale of the vertical axiswas lirnited to 0.6 IImi in arder to.show the magnitude oftheinternal attack ofthe results obtained at the other tempera-tures. As it can be observed from this Figure, the sulfate-rich oil ash causes sulfidation from 675°C, and it is also de-veloped an internal oxidation process at temperatures above785°C.

29

0.3

tSJ internal altack ls o~sD Ihickness Ioss - ~

~es:

SO,SS es:::'! i

es:: ! j 0,5 ,---¡ ,

~

!I

~,...... _- ¡

I

J; ~

Ii '---'

, , :! , ;

°TE 0.5

E...•.I/l 0.4IIJO

I

! t~ 0.2i

10.1 I[

011ooQQe

(.) (.) (.) Uo o o oLO LO Q LO1'- ,.. LO CIO(j) 1'- 1'- 1'-

'rernper ature

Fig. 2. Thickness loss and interna1 attack depth of 35Ni19Cr ex-posed to an oi1ash with V/(Na+S)=O.55. O: interna1 oxidation; S:su1fidation.

~(.)oLOQco

ooLO('Ij

co

ooQQ0\

UoLO('Ij(j)

Figure 3 shows the zone near to the interface metal/scaleof specimens exposed to 675°C, 715°C, 750°C and 900°C.Chrornium sulfides are the black phases at the subjacentzone to the metal/scale interface of the specimens exposedto 675°C, 715°C and 750°C. They precipitated at the chro-mium-depleted zone. The corrosion front is almost uniformat 675°C, but evidence of metallic dissolution is observed at715°C and 750°C.Figures 4 and 5 shows in more detail thezone near to the interface of specimens exposed to 900°Cand 785°C, respectively.

As it was mentioned before, at 785°C and 805°C the corro-sion rate was catastrophic and evidence of simultaneousvanadium corrosion and intergranular sulfidation caused byhot corrosion was found in additional specimens exposedduring only 125 hours, Figure 3 shows a characteristic fea-ture of these specimens in the group of three rnicrographscorresponding to 785°C. The backscattered electron imagewas taken from the bottom of the Cr-depleted zone in themetallic matrix and its Cr and S maps shows that chrorniumsulfides have been oxidised releasing sulfur to continue thesulfidation process. Therefore they act as nucleation sitesfor the selective oxidation of chrornium at grain boundaries,confirrning that grain boundaries are the fastest diffusionpaths for chrornium [15, 16].At 900°C there was also observedinternal oxidation and precipitation of some Mn sulfides par-ticularly at the bottom of the Cr-depleted zone as can beseen in Figure 4. This figure shows the combined effect oftwo corrosion mechanisms by oil ashes: sulfidation causedby hot corrosion and corrosion induced by vanadium com-pounds, which at this temperature causes internal oxidationand also intergranular corrosion.

30

rscale,¡.

'1'Cr-dePlfed zone

TAUoy,¡.

r

Bottom ofCr-depleted zone

Alloy ~

A. Wong y col. /Revista Latinoamericana de Metalurgia y Materiales

Fig. 3. Internal attack of the subjacent zone to metal/scale interface in specimens of 35Ni 19Cr alloy exposed at temperaturesbelow and above 785°C, temperature at which corrosion was catastrophic. Microanalysis ofthis zone revealed remarkable matrixCr-depletion and the presence of chromium sulfides. At 7K5°C and 900°C it was also identified internal metallic oxides. Themicrograph corresponding to 785°C was taken from the bottom of the Cr-depleted zane. The network of internal chromiumoxides in this zone revealed chromium sulfides were oxidized releasing sulfur to continue the sulfidation process into the alloy.

(Note the scale at the micrographs as they were taken at different rnagnifications).

Fig.4. Backscattered electron image ofthe interface scale/metal (a and b) and x-ray mappings tbat show the role of'vanadium and sulfur inthe oil-ash corrosion of35Nil9Cr alloy exposed at 900YC. The specimen was mounted. in cross secrion and ob erved as-polished in ordertosee the neighborhood ofthe metal/scale interface (on the top of the image is found tbe cesrcsi sca1e). It can be observed internaloxidation (from Cr and Mn maps) and intergranular corrosión (see V map inside the aIloy the presence of some Mn sulfides,particularly at thebottorn ofthe Cr-depleted zone. (a) SEI image, (b) er-K« S-Kamap, (e) V-Ka map, (f) Fe-

Ka map, (g) Ni-Ka map.

Revista Latinoamericana de Metalurgia y Materiales, Vol. 21, N° 2,2001.

The X-ray maps ofV, Cr, S and Mn suggest that most ofe internal corrosion products are at grain boundaries (Cr

aad Mn sulfides, Cr and Mn vanadates and chromium oxide).Figure 5 shows somé features of the cross section of

i19Cr alloy exposed 125 hours at 785C. At this tempera-mre, the alloy.exhibited catastrophic corrosion rates in the

O-hour tests. Microanalysis and XRD results showed thatmetallic oxides and metallic vanadates mainly constitute theextemal scale. Scale fissuring is related to the formation of anon- protective scale, responsible ofthe catastrophic corro-sien exhibited by this alloy, A vanadium-rich layer at the

.metal/scale interface, and the presence of metallic oxides (Cr,Fe, Ni) that have re-precipitated above it, isthe result ofthecorrosion mechanisminvolves an accelerated oxidation pro-cess.

31

Cr and S X-ray maps obtained from the Cr-depleted zoneare evidences ofthe occurrenceof a sulfidationprocess fol-lowed by oxidation. It can be observed inside the.alloy zones(see the arrow) containing chromium oxide that are a conse-quence of the massive precipitation of sulfides that are oxi-dized releasing the sulfur for continuing the sulfidation pro-cess into the alloy. The high sulfur concentration in the bot-tom ofthe sulfidized zone is evidence ofthe auto-sustainingnature of the sulfidation process. Therefore, the high corro-sion rates experimented by the alloy at 785°C and 805°C re-sulted from the simultaneous occurrence of sulfidation andoxidation, being the first one, favored by the high Ni contentofthe alloy.

Fig. 5. 35Ni19Cr alloy exposed to oi1ash with V/(Na+S)=O.55 at 785°C by 125 h urs. The scale and the zone exhibiting internal attack (BSEimage) were typical ofthe developed under the most corrosive conditions (785°C and 805°C). It can be observed inside the alloy zones (seethe arrow) containing chromium oxide that are a consequence of the massive precipitation of sulfides that are oxidized releasing the sulfur forcontinuing the sulfidation process into the alloy. The high sulfur concentration in the bottom ofthe Cr-depleted zone is evidence ofthe auto-sustaining nature ofthe sulfidation process. V has been incorporated to the oxide scale near to the metal/scale interface, making it even lessprotective. The region marked with a B (right comer) is the mounting material.

The high sulfur concentration in the bottom of thesulfidized zone is evidence of the auto-sustaining nature ofthe sulfidation process. Therefore, the high corrosion ratesexperimented by the alloy at 785°C and 805°C resulted fromthe simultaneous occurrence of sulfidation and oxidation,being the first one, favored by the high Ni content of thealloy.

EDX line profiles ofFe, Cr, Ni, Si, S, V, O were obtainedfrom the cross- sectioned samples exposed to each test con-dition. Only for purposes of illustrating the zone where thelinescans were taken, Figure 6 shows the cross section ofthe specimens exposed to 600°C (a), 715°C (b), 750°C (e) and900°C (d), and the respective line along the elementallineprofiles were measured. This line was drew on the

32 A. Wong y col. /Revista Latinoamericana de Metalurgia y Materiales

backscattered electron image using the plus marks available,however these marks do not represent the exact sites wherethe analysis were made. The distribution ofpoints for.analy-sis was regular in the zone far from the metal/scale interface,both in the alloy and in the scale.

However, in both the depleted zone and the scale nearestto the metal/scale interface, the analysis was made more me-ticulously. In those zones, the number of points analyzedwas higher, in order to get a more detailed profile because ofthe relevance of these zones for the understanding of theglobal corrosion process. It is worth to mention that theanalysis was not made using the spot mode, but using a 2rnicrons width rectangle. In this way, it was obtained anaverage concentration of each element in the zone coveredby the rectangle, which was parallel to the interface.

Figures 7 and 8 show two ofthe profiles acqüired, along azone near to the metal/scale interface of the specimens ex-posed at 600°C and 715°C. The vertical axis indicates theconcentration ofthe element. Note that the axe scales aredifferent in order to make visible the variations registered.By comparing both profiles, it is pretty clear that a differentcorrosion process is occurring at 715°C, as is suggested bythe Cr (and Fe) depleted zone and the precipitation of sulfurinside the alloy. The Cr and S profiles clearly show the pre-cipitation of chromium sulfides below the metal/scale inter-face. It can also be observedthe incorporation ofu 5% V tothe oxide scale lattice near to the interface, as well as a highconcentration of sulfur in it, making it more defective.

Fig.6. BSE Images ofthe areas where the linescans were carried out at four ofthe test temperatures. The center ofthe 2 microns width

rectangles used for the analysis was located on the line drawn using plus symbols.

Revista Latinoamericana de Metalurgia y Materiales, Vol.21, N° 2,2001.

--scale metal~

Ni

Fe

Si

Cr

s

.A_

l¡1I \ .. ~

~

v

«)

= ='J'l =..• ..•, ,==..• ='J'l..•

Distance to Interface (Microns)

Fig. 7. Line profiles obtained from the specimen exposed at 600°C.

(weight %).

33

f-scale metal-e

Ni

Fe

s

2 Óv

~,10 -

==..•, ==, =le, ij, =~, = ==..• =~..•Distance to Interface (Microns)

Fig. 8. Line profiles obtained from the specimen exposed at 715°C.

(weight %).

34 . A. Wong y col. /Revista Latinoamericana de Metalurgia y Materiales

--scale rnetal-7

eooc

4 o3 O

1 f 5C

no e

91l0C

el el i ~el ~ i ~ siliiI !i'I

N 'i!" .•. .•.. ••• Nr I I

Distance to Interface (Microns)

Fig.9 Effect oftemperature on Cr line profiles of35Nil9Cr steelexposed to an oil ash with V/(Na+S)=O.55. (weight %).

Microanalysis also showed matrix Cr-depletion aroundsulfides due to the sulfidation process.In Figure 9, the Cr line profiles obtained from the specimensexposed to 600°C, 675°C, 715°C, 750°C and 900°C are com-pared, in order to show, as temperature is increased, the evo-lution of the depth of the zone where oxides and sulfidesprecipitated inside the metal. The Cr levels -reached as aresult of matrix depletion can also be eompared. It is clearthat if the matrix is becoming depleted in chromium as a re-sult ofthe oil ash corrosion process, which.intrinsically in-volves accelerated oxidation besides sulfidation at severaltemperatures, it will develop a less protective scale. Fur-therrnore, the presence ofV and S in the scale also reducesits protectiveness.

5. Discussion

It was concluded that, besides high temperature oxida-tion and dependingon temperature, in therange of tempera-ture of 600°C - 900°C, vanadium corrosion (molten salt COITO-

sion), sulfidation or both took place. Sulfidation involvesthe development of a scale that contains sulfides. The kinet-ics of this process is faster than the kinetics of oxidation,due to the scales containing sulfides are les s protective ne-cause they are more defective and sulfides have meltingpoints lower and are less stable than oxides [17]. As the timeelapses, sulfur ions may diffuse to the metal/oxide interface,increasing the sulfur potential there and, when the activity ishigh enough, internal sulfidation occurs.Matrix chromium depletion depends on the prevailing COITO-

sion mechanism, and microanalysis showed that it could reachfigures as high asJ.5% and 2% at 715°C and 900°C, respec-tively (Figure 9). Cr-depletion is a common proeess in aus-tenitic alloys due to their chromium diffusion coefficient islow [15].

At temperatures lower than 675°C, corrosion by sodiumvanadates rules the process and no Cr-depletion occurs,because metallic dissolution (caused by the molten vana-dates) is taking place. However, it is abrupt at the sulfidizedzone at the range oftemperature where both sulfidation andvanadium corrosion take place. At 715°C the scale containsmetallic sulfides in the Cr.O, layer, which explains its non-protective nature, as the ionie transport through sulfide-eon-taining scales is faster than in Cr.O, scales [17]. At 900°C,high temperature oxidation has a preponderant role, and matrixCr-depletion is gradual at the internal oxidation zone, onlyincreased in the neighborhood of oxidized grain bound-anes.

EDX line profiles showed that sulfidation extent is deeperthan internal oxidation at 785°C and.805°C, while at 825°Cand 900°C oxidation was more relevant than sulfidation.

Mn as alloying element promotes the forrnation of spineloxides, which enhances the incorporation of sulfur to theoxide scale and its subsequent diffusioa to the metal[l7].

Revista Latinoamericana de Metalurgia y Materiales, Vol.21, N° 2,2001.

Under sulfidizing conditions, the results confirmed thatMn can precipitate as sulfides in the metallic matrixenhancing the diffusion of sulfur into the alloy. '

6. ConcIusions

Fram X-ray microanalysis results, it can be conc1udedtbat the prevailing corrosion mechanisms, as a function of

mperature, are as following:_ T < 715°C:

Corrosion is induced basically by molten vanadates; highmperature oxidation is also involved as the vanadates en-

ce the oxygen transport to the metal.T", 715°C:

ulfidatioa process caused by the formation of eutecticsh as Me-MeS, which melt at temperatures araund 700°C

added to the corrosion process induced by molten vana-es

750°C < T < 825°C:This is the range of temperature the corrosion rate reach

maximum. In it there is a synergistic action of scdiumvanadates and sodium sulfate, which besides vanadium cor-

ion causes a simultaneous sulfidation/oxidation processrhat involves Cr-depletion of the metallic matrix. It couWbe related with the classical concept for low temperaturehot corrosion."T_ 825°C < T< 884°C (Na2S04 decomp. Temp.) :In this temperature range it takes place the synergistic ac-ñon of sodium vanadates and sodium sulfate, but the attack- moderated by high temperature oxidation of the alloy.Tberefore, the oxide scale developed by the alloy protects itin certain grade fram the attack of molten compounds, re-sulting in a decrease of corrosion rateos. T> 884°C:

At this temperature, the decomposition of Na SO oc-d

. 2 4urs, pro ucmg Na20, which acts as an inhibitor, and hence

decreasing sulfidation attack. Therefore the corrasion abovethis temperature is basically vanadium corrosión stronglymoderated by high temperature oxidation

Furthermore, regarding the corrosion resistance of35Ni19Cr alloy, fram these results is concluded that in spiteof its good carburization and oxidation resistance, it seemsuot to be a suitable material when the oil ashes are sulfate-rich, particularly around 800°e.

Acknowledgments

The authors acknowledge the economic support givenby CONACYT and the PADEP-UNAM (México) for thedevelopment of this research.

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