Revista Latinoamericana de Mewlurgiu y Materiales, Vol. 10 Nos. J & 2 (J990) 43

Hot corrosion products formed in as-cast 25Cr-20Ni refractory stainless steels

Luis María Gribaudo*Instituto de Desarrollo Tecnológico para la ~dustria Química,INTEG, Guemes 3450, 3000 Santa Fe, ArgentinaThe composition of the corrosion products formed after exposing the refractory 25Cr-20Ni stainless steel lo aggressive media al 700°C was studied by severalanaIyticalmethods. The externaI and internal chemically different layers that conformthe scale were observed. Other corrosion products, internally formed in theattacked alloys, were also characterized.

*The author has performed the experimental work at the LEE-ENSSEG, Domaine Universitare, 3840 I SI. Martin d 'Heres, France

INTRODUCTION

Many operations and processes in chemical engineering areperfonned al temperatures above 500"C, which usually isthe lowest temperature limit of hot corrosion. The oxidationa~high temperatures comprises a set of complex reaetionsinvolving oxygen in atmospheres whieh also contain mix-tures ofinertand/orreacting gasessuch as C02, 502,MH3, erc.The materials used for chemical equipment under aggressiveconditions are a great number of specially designed alloyswhich often have Ni as the main component; Hastelloy,Inconel , Incoloy, Chromel, ete, are trade marks of wellknown superalloys.Some Fe-base superalloys having refraetory properties arealso used in the petrochemical industry, being the AISI 310HC stainless steel an example. The simplest compositíonof the corresponding corrosionresistantcastíng is the HK40,following the ASTM specification, which contains Cr 25wt%, Ni 2Owt%, CO, 4 wt % and Fe balancing.The behaviour of the staínless steel pieces facing oxidaliveor corrosive environments are generally associated with theproperties of a superficiallayer, usually adherent and protec-tive, and essentially fonned by ehromium oxide.In this work, the general characteristics and composition ofthe scale and other products yielded in as-cast 25Cr-20Nistainless steels exposed to several hot corros ion environmentsare presented. The main motivation for this study was to knowthe performance of this kind of steels when they are used inthe fabrication of grids of coolers in the cement industry. Inthis case the pieces are in contact with the clinker, which hasacomposition expressed in weightof oxides of23% Si02, 4%Ah03, 4,5% Fe203, 67% CaO and 1,5% S03. As the mainaggressive agents here are the sulphates, some of the testswere perfonned with melted alkalme sulphates, some of thetests were performed with melted alkaline sulphates.

EXPER~NTAITECHNIQUES

Corrosion tests on alloy specimens (25Cr -20NI steels with 0,4wt%ofCand different W and Si contents) were performedunder severa! conditions: atmospheres of argon, air oroxygen; corrosion media of clinker, sulphates or mixtures ofsulphates and chlorides: temperature of 700 "C and exposuretimes of 24, 120, 138 and 1000 hours.Details of equipments and methods are presented in [1] .Synthetic carbides Crsfec, Cn-FesCs.and CrlóFC6WCweremanufactured and exposed to sulphates in order to cJarify mecorrosion mechanism interpretation.Several identification techniques were used to determinethe composition of the seales on me specimens and the othereorrosion products: X-Ray diffraction patterns using Debye-Scherrer technique with a cobalt target, glow-discbargeoptical spectrometry (GOOS), seanning electronic micro-scope (SEM) with energy dispersive X-Rayanalysis (EDS),electronmicroprobe analysis (EMA) with pattems of elementdistributions and chemical analysis.

AIlOY MICROSTRUCTURES

A dendritic pattem is generally found when the HK 40 alloysolidifies: the matrix is austenite and me eutectic zone is formedby austenite and Cr-rieh carbides as is shown in fig.1. Thereare no grain joints but interdendritic spacings.The rnatrix is a solution phase involving all me components; thetype of carbide depends upon the alloy composítíon [2].Raising me Ni content or adding Si, the formation of M7C3carbides is promoted; if the Cr content increases or W isincorporated, the formation of N23C6carbides is favoured.·When the Si and W contents are 2% and 2,8% respectively,M6C carbides appear at the end of the solidification.If the alloys are exposed for long time periods to temperaturesabove 70Cf'C, M23C6 carbides are precipitated from solidsolulion in the austenitic matrix.The eutectic zones present different morphologies accordingto the type of carbides that precipitate during the solidifieationprocess: the M7C3 carbides are irregular and separate blades,the M23Cócarbides fonn agglomerated islands, fig. 2.The austenitic matrix adjacent to the solidification carbides isa Cr-depleted zone which can fall till 63 % of the global Crcontent of the alloy; the width of this depleted zone can be aslarge as 5,3 um.

CORROSION FEATURES

The corroded surfaces on the alloys show generalized andlocal attacks, fig. 3. The local damage outlines the eutecticzones of the material.The degradation is very small when the specimens are exposedunder pure oxygen atmospheres but when me clinker orsulphates are added as a corrosive medium, both, general andlocalízed attacks are very importantand praelically independ-ent of the surrounding atmosphere. Transverse section speci-mens after tests are presented in fig.4.The local attaeks on the euteetic zones are connected to me typeof carbides. Steels with M7C3 are corroded at the Cr-depletedzones surrounding them, the carbides remain non-attaeked andisolated. In steels with M23C6, these precipitates are degradedtogether with the adjacent matrix. In the last case the damagedeutectic zone remains sponged. . /The same features are found for the more prolonged test, but itmust be mentioned that the local attacks in the steels with M7C3carbides advance more quickly.

CORROSION PRODUCTS

The products of attacks, when clinker or sulphates are presentin the eorrosion tests, form a superficial scale which looks asshown in fig. 5. The local corros ion on me eutectie zones areemphasized in the pictures due to me accumulation of plates.After EDS analysis the products of the plates are iron andchromium oxides, where chromium oxide IS predominant ayer

44 LatillAmeric,'clII .loumal of Metallurgy and M"teri"L~. Vol. toNos. 1 & 2 (l990)

Fig. 1. Dendritic solídificatíon pattems l/1Il1 Cr and W distributions in HK 40 steeL~.EMA images.

10p.mt----t

Fig. ¿. VtJJen'lIt rartnüe morphotogies in the eutectic rones. SEM micrographs.

Revista Latinoamericana de Melil/urg;l/ y Mlller;l//es, Vol. 10 Nos. 1 & 2 (1990) 45

Fig •.l Corroded sutfaces. Steels with M,C, (leftfana M"C6('tight) carbides. SEM images.

21 fLmI t

Fig. 4. Corrosion attacks (transverse sect;fHu)SteeLv w;th M,C, (left) a/III M1JC6 (right) carbides. SEM images.

Fig.5. SEM micrographs of the extemal sea/e on corroded sperimens. Steels with M,Cj

(left and middlc) and M,-,C, (rigth) catbides.

46 Latinñmerican Ioumal alMek¡/turgy and Matenals, Vol. 10 Nos. J & 2 (J990)

l outer 'ayer~-1 I1 IW S Cr Fe

~,---I_-Fig. 6. SEM micrographs and EDS spectra ni the two lay« ... ", the

attacked synthetic carbüies.

me matrix and iron oxide contentis higher in the interdendriticzones.In fact, me products due lo me general corros ion over thematrix form two sub-scales or 1ayers. According lo GDOS,SEM, EMA and X-Ray diffraction analysis me outer scale isalways composed of oxides, specially chromium ~U1dferricoxides, and the inner scale is fonned by nieke1 sulphide [1].The outer oxide scale has two parts: a chromium-rích layer(outside) and an iron-rich layer (inside) with some inclusions

.\.,;.- /" /'

14p.m1------<

of metallie sulphides. The inner sulphide scale also containsoxide inclusions. The analysisofthenickel sulphideshows thatthe ratio between Ni and S atoms is 1.5, so it can be specifiedas NbS2. The iron oxide is mainly Fe2Ú3. When silicon ispresent in me alloy composítíon, SiChcan be detected withouta preferential localization in the scale.Sintered carbide samp1es after corrosion tests in sulphatemedium present a general attack. Sulphates can penetrateinside tbe pores and again an inner and an outer scale ofcorrosion products are observed, fig. 6. The composition ofthese products are in accordance with the results obtained forme alloys: the electron energy dispersive spectra determinesme existence of an extemallayer (where oxides are predomi-nant) and an intemallayer (where su1phides are predominant).The corros ion products formed inside the damaged zones,which penetra te me metallic specimens, are of difficultcharacterization.Nevertheless, from me distribution patterns of elements andconcentration profiles obtained by EMA, me main productsseem to be specially nickel sulphide and other metallicsulphides, lig.7. If the alloy contains W, the tungsten sulphidealso appears.The presenceof chlorides in me corroded media accelerateswidely tbe material degradation, special1y me generalizedcorrosion, a" can be appreciated in fig. 8. Chlorides are me newproducts of me attack and now me oxygen compounds aredispersed over all the corroded zones.

DISCUSSION AND CONCLUSIONS

The oxidation of alloys at high temperatures has been thesubjectof many experimental ~U1dtheoretical studies publishedin the literature.Conceming tne products developed on attacked steels, it hHSbeen stated that Cf203 brings an efticient protective scale.There is a minimwn Cr content necessary to form a continuous

Fig.7. Element distrihution pattems (EMA) in corroded attoys.

Revista Latinoamericana de Melalurgitl y Materiales, Vol. 10 No s. 1 &: 2 (1990) 47

, .', ""

,)'. ~.. .•....

Fig.8. Distribution IIf elements after corrosion under chloride-added media. EMA images.

layer but the value is dependent on the type of atmosphere.Nagarayan et al. [3] have shown that the Cr203 formed in thepresence of sulphur does not result in a continuous layer. Thesame authors and Atkinson [4] have found tharsi contents uplo 2 wt% does not form a SiCh barrier in Fe-Cr alloys.Stott et al. [5] have determined that silicon is a less effectiveaddition than aluminium for developing a continuous heal-ing layerbecauseShO growsat a lowerrate than Al.Oi. Stouand Wei [6] mention that the formation oí' internal oxideprecipitatesof Ah03 orSiCh beneath thescale andstressescanresulto The stresses woukí be relieved hy the deformation ofthe substrate.Tests performed on Samples of2OCr-25Ni Nb stabilized steelshowed a marked deterioration 01' oxidation propertiesunder sulñdizing atmospheres [7]. Evans et al. [8] havestndied the oxidation behaviour of 2OCr-25Ni alloys and statethat the formation 01' internal oxides mercases the tendencyfor oxide rupture ami spalling with the subsequentenhancement of local attacks. A residual xilica film on thesurface of the metal acts as a diffusion barrier [9], and Evanset al. [10] found that a silicon content of 0,92 wt% gives aminimum for the parabolic rate constant 01' oxide thicknessgrowth.Determinations of the scale composition after hot corrosíonin environments containing salts are less cornmon in theliterature; Jakel and Schwenk [11] show two diíferent zonesof the scale formed over the surtace of Fe-Cr stainless steels,being the outer rich in chromium and iron oxides and the innercomposed of a mixture of sulphides and oxides. Their resultsare in agreement with those presented here.As a general result of this work it is possible to assure that thechange in composition of the 25Cr-20Ni alloys does notproduce any significant difference on the struct ure of the scalewhen they are exposed to sulphates al high ternperature. Byadding Si, the formation 01' siliea is possible but it seems lo

produce no beneticial effect on the layer. The W compoundsremain generally inside 01'the attacked eutectie zones withoutadditional effects.If chlorides are mixtured with sulphates the damage is greaterand the residual producís are dispersed covering the wholeattacked zones of the alloys.

REFERENCESl. L. M. Gribaudo and J. J. Rameau, Corros. Sci. 24 (19R4) 291.2. L. N. Gribaudo, F. Duran" and M. Durand-Charre, Nern. Sci. Rey. Met,Paris 80 0983) 211.3. V. Nagarayan. R.G. Niner and A. V. Levy, J. Electrochem Soc, 1290982)782.4. A. Atkinson, Corros. Sci .22 (1982) 87.5. F. H. Stou, G. J. Gabriel, F. L Wei and G. e Wood, Werk.<loffe undKorrosion 38 0')87) 521.6. F. H. Stoll ami F. L Wei, Oxido MeL 31 (19R9) 36<).7. R. C. Lobb ami H. E. Evans, Oxido Met. 16 (1981) 371.8. H. E. Evans, D. A. Hilton, R. A. Holrn and S. J. Webster, Oxido Met. 12097X) 473.9. H. E. Evans, D. A. Hi I10n, R. A. Holm and S. J. Webster, Oxido MeL 140980) 235.10. H. E. Evans, D. A. Hilton, R. A. Holm and S ..1. Webster, Oxido Me!. 19(1982) 1.11. U. Jakel and W. Shwenk, Werkstoffe un" Korrosion 26 (1975) 521.