Analysis of oxide layers on stainless steel (304, and 316) by conversion electron IVl6ssbauer spectrometry

Transcription

1 JOURNAL OF MATERIALS SCIENCE 25 (1990) Analysis f xide layers n stainless steel (304, and 316) by cnversin electrn IVl6ssbauer spectrmetry KIYOSHI NOMURA, YUSUKE UJIHIRA Faculty f Engineering, The University f Tky, Hng 7-3-1, Bunky-ku, Tky 113, Japan Stainless steels f type SUS304 and SUS316 were chemically treated and heated at varius temperatures, and the xide films frmed n the surface were analysed by Auger electrn spectrmetry and cnversin electrn M6ssbauer spectrmetry. The utermst xide layers f stainless steels were enriched with irn and chrmium after heat treatment belw 600 ~ C and abve 700 ~ respectively. It was fund that at least tw magnetic cmpnents f irn species were present in the xide layers f stainless steel heated belw 600 ~ C and that the fine particles f irn xide are prduced in the inner xide layers f the samples prepared by heating at temperatures higher than 700 ~ C. Only paramagnetic irn species were detected in the xide layers f the stainless steel prepared by chemical treatment. The structures f the xide layers prduced by thse heat and chemical treatments are prpsed. 1. Intrductin Stainless steel is ne f the mst useful materials in industry, architecture, and daily use. It is knwn that the heat treatment and chemical xidatin f stainless steel at under certain cnditins imprve the crrsin resistance f the substrate by frming clured films [1]. Clured stainless steel f type SUS304 is knwn t becme glden at 400 ~ C, range at 500 ~ C, red t purple at 600 ~ C, purple at 700 ~ and blue abve 700 ~ C. X-ray phtelectrn spectrmetric (XPS) studies f the clured films, which were frmed by the immersin f stainless steel in chrmic acid and sulphuric acid slutin, have been reprted by Ansell et al. [2]. Many researchers have studied passivating films f 1 t 5nm thick n irn and stainless steel. Thick xide films f 57Fe were initially studied by electrplating enriched 57Fe n the surface f nnenriched irn [3]. In situ study f passive xide film n irn was undertaken by O'Grady [4]. Recently, Stewart et al. [5] applied cnversin electrn M6ssbauer spectrmetry (CEMS) t the analysis f the xide layer n the enriched 57 Fe thin fil f stainless steel (type 310) which was xidized at high temperatures (> 700 ~ C) and reprted that ~-Fe203 was detected in the chrmiumdepleted layers. Hwever, very few M6ssbauer spectrmetric studies have been carried ut n the frmatin f xide films n stainless steel which was systematically xidized by a heating and/r chemical prcedure. We recgnized experimentally that it takes t much time t measure CEM spectra f xide films n stainless steel cntaining a natural abundance f 57 Fe. Even a thin xide film smetimes became charged while detecting cnversin electrns. While imprving the detectr fr CEMS, we have s far analysed the chemical structure f xide layers [6], phsphate layers [7] and xalate layers [8] f steels. We als /90 $ Chapman and Hall Ltd. reprted recently that an xide film n stainless steel can be used as a ph sensr [9]. The ph respnse f this xide film, hwever, depended n the preparatin cnditin f xide film. Oxide structures f stainless steel analysed by CEMS and Auger electrn spectrmetry (AES) are reprted in this paper. 2. Experimental details Sheets f stainless steel (type SUS304 and SUS316) were degreased in acetne, and heated in air fr 1 h at 400, 500, 600, 700 and 800 ~ C. SUS316 stainless steel cntained 3 wt % Me in additin t similar cnstituents f SUS304 stainless steel with 18 wt % Cr, 8 wt % Ni, and Fe balance. The ther xide films were als prepared by immersing stainless steel in 2.5 reel dm 3 chrmic acid and 5.0 reel dm -3 sulphuric acid slutin at 70 ~ C. In rder t analyse these xide-cated samples by CEMS, 25 mci 57C(Rh) surce and a gas flw cunter [10] were used. Care was taken t prevent a sample frm becming electrically charged during the measurement by cntact f the substrate with the cunter wall when the sample was munted in the detectr. Distributins f a hyperfine field f partial data were calculated using Hesse's methd [11]. The intensity rati f magnetically split peaks was suppsed t be 3:3:1:1:3:3 althugh this methd is nt really suited fr the M6ssbauer spectrum in which several kinds f irn xides are included. Sme data were fit t the sum f Lrenzian using the fitting prgram which was laded n a persnal cmputer. Depth prfiles f sme samples were measured by AES t supprt the analysis f M6ssbauer data. The thickness f xide films n stainless steel was rughly estimated by the weight change f stainless steel and by the AES peaks cupled with argn in etching, presuming that the xide flms were cmpsed f unifrm Fe

2 800 " "'/i'll _1. I I I I I I I I I i I I f I [ I ] I I J~ TOO "g,il ~ I I I ~ I I I t i i I I I L I I L I I I 600 '0 '1 I I I I I I I I I I I I I I I I I I I I xi 500 "c ~. Ul g,% ~ ~ *. -.~ IP 9. ~" ~s%~"~'ii r ~.. ".~" ';~...,:....) \', -..,.,-.. ~ ~ 9.~-,.- '. ~,3,',~ "~.'. "~, J I '' 9 ~" I,~,~ "'t.. % I I I I I I I I I I I I I I I I I I I l I 400 * t I I I I I I I I I I I I [ I 1 I I I I I [ I Vetc=ty (turn sec -1 ) Figure 1 CEM spectra f xide films n SUS304 stainless steel heated fr 1 h at 400, 500, 600, 700 and 800 ~ C. 3. Results and discussin 3.1.Oxidatin in dry air Oxide films n SUS304 Cnversin electrn M6ssbauer (CEM) spectra f SUS304 sheets, which were heated at 400, 500, 600, 700 and 800~ fr 1 h, are shwn in Fig. 1. The thickness f the xide films was estimated t be 20 t 50nm n increasing the heating temperature t 700 ~ C. The intensity f a singlet f austenitic stainless steel was extremely high cmpared with that f irn xide peaks, because the irn cntent in the xide layer was far less than that in the bulk, and the xide film was thin cmpared with the range f 7.3keV electrns in irn (ca. 300 nm). Data were pltted by the expansin t five times higher than the smthing line f the whle peaks. In the samples heated belw 600 ~ C, magnetically split peaks were clearly bserved. The magnetic hyperfine field f irn cmpunds prduced in the xide films increased with increasing heating temperature as shwn in Fig. 1. Hwever, the intensity f the internal magnetic hyperfine field did nt always increase in accrdance with the increase in heating temperature. This suggested that the irn cntent changed in the xide films. Frm the AES depth prfile f the sample heated at 500 ~ C as shwn in Fig. 2, it was fund that irn was cncentrated n the utermst surface, and that irn, chrmium and nickel were present in the inner xide layers. Furthermre there was a smth transitin between these layers and the bulk. The sample heated at 500~ was measured at dry ice temperature and the CEM spectrum was analysed by Lrenzian fitting as shwn in Fig. 3b. At least tw irn species f ismer shift (I S) = 0.37ram sec -~, quadruple splitting (QS) = 0.02 mm sec-1, hyperfine field (Hin)= 530kOe and f IS = 0.2mmsec -1, //in = 350kOe were fund. The frmer species was 1 2.-'2_ 8 Fe 5 2 Ni I J / I a Sputtering time (sec) Figure 2 AES in-depth prfiles f SUS304 stainless steel heated at 500~ fr 1 h. Thickness/sputtering time ~- 10 nm/60 sec. 1746

3 z~ g Ca) _ :%y." ~ ~ 9.. ~. I t I t i I a t J t 1 i I r I I J I I i ~_j-.--. *~..,,. I I I I [ ] I I I I I i t i I t J I ~ ) I I I 1 I i Vetcity (ram sec -1) Figure 3 Dryqce temperature CEM spectra f xide films n SUS304 stainless steel heated fr 1 h at (a) 900~ and 500 ~ C. attributed t :~-Fe203 and the latter t a pr crystalline spinel cmpund such as NiO(Cr, Fe)203. Only paramagnetic peaks were bserved at rm temperature in samples heated abve 700~ as shwn in Fig. 1. The chrmium cntent tended t increase in the utermst surface and the irn cntent t decrease in the sample heated at 700 ~ C, as shwn in the AES prfile f Fig. 4. Althugh the CEM spectra f xide film f SUS304 sample heated at 900 ~ C gave a dublet at rm temperature, a brad sextet (IS = 0.025mmsec -l, QS = 0.08mmsec -1, and average //in = 300 koe) and a dublet (IS = 0.27 mm sec -l, QS = 0.86mmsec ~) were bserved at dry-ice temperature as shwn in Fig. 3a. The dublet bserved at rm temperature was cnsidered t be due t super- paramagnetism f fine irn xide beneath the Cr203 layer. It may als be mixed with chrmium, nickel and irn t frm as (Fe, Cr)203 and NiO (Fe, Cr)203 at the initial xidatin. When the sample heated at 400~ fr 1 h was further heated at 700 ~ C, large particles f a-fe203, haematite, were bserved n the chrmium xide layer by scanning electrn micrscpy, c~-fe203 is frmed as a result f decmpsitin f these intermediate mixed cmpunds when the stainless steel is further heated fr a lng time. It is cnsidered that the grwth f irn xide beneath the chrmium-xide layer destrys the stacking and unifrmity f chrmium xide layers at high temperature. 3, 1,2, Oxide films n SUS316 CEM spectra f heated SUS316 between 400 and 900~ are shwn in Figs 5 and 6. The peak intensity f irn prducts was higher than that f SUS304 althugh the xide layers n SUS316 were a little thicker than xide layers n SUS304. The surface clur f SUS316 was nt as shiny as that f heated SUS304. Three irn species, which had hyperfine fields f 280, 340, and 530 koe, were fund in the sample heated at 400 ~ C. These were cnsidered t be due t Fe(III)/(Fe(II) pairs in ctahedral sites and Fe(III) in tetrahedral sites f spinel cmpunds such as (Fe, Ni)O(Fe, Cr)203, and Fe(III) in clumdm structure f ~-Fe203 and (Fe, Cr)203. The distributin f the internal magnetic field f irn xide prduced n SUS316 is shwn in Fig. 7. The hyperfine fields f the intermediate irn cmpunds increased and the intensity decreased with increasing heating temperature. The schematic changes f xide films n SUS316 as a functin f heating temperature were similar t thse f SUS304. Hwever, a larger amunt f irn remained at the utermst surface n SUS316 than n SUS304 heated belw 600 ~ C. Because mlybdenum is easily xidized, the xidatin f the SUS316 surface is cnsidered t prceed while the diffusin f irn int the bulk and f chrmium int the surface is l 2 l 0 c" _d - ~ Fe v c- 4.--, c- 0 1 O Sputtering time (sec) Figure 4 AES in-depth prfiles f SUS304 stainless steel heated at 700~ fr I h. Thickness/sputtering time ~ 10nm/60sec. 1747

4 600~ i,~. I I I I I I I r I I I I i i i i I I I I l I 500 ~ i ' I I I I I 1 I I I I I~. I I I I. I I I i 1 i i e:j v c g u 400~ i"- i h i, q I I f I I I I I [ ( I I I I I l I I I r f Vetcity ( mm sec -11 Figure 5 CEM spectra f xide films n SUS316 stainless steel heated at 400, 500 and 600 ~ C. retarded by the frmatin f mlybdenum xide. A superparamagnetic dublet was bserved in the rmtemperature CEM spectra f the samples heated at a temperature higher than 700 ~ C. The dublet indicated that irn species existed almst in the same fine 900 ~ 600 ~ OOOO= OO~ ~ ~ OOO e 500 ~ 00~176 ~ ~ c ~_ ~ ~ ~ ~ c ~ Q. ~176 ~ %0O 0 400~ ~ ~ OO~ %000OO ~! I I I I T I I I L i I Hin (k0e) Figure 7 Distributin f hyperfine field f xide films n SUS316 stainless steel heated fr 1 h at 400, 500 and 600 ~ C. particles f chrmium and irn mixed xides as n SUS304. Magnetic splitting was bserved in CEM spectra f the samples heated at 900~ fr 2h. c~- Fe20 3 is frmed as the final.prduct f the crrsin. Schematic structures f xide films n SUS304 and SUS316 stainless steels are cnsidered t be as shwn in Fig. 8. There is n inherent difference f xide structures between SUS304 and SUS316, althugh the utermst xide layer f SUS316 cntained mre irn than that f SUS304. Mlybdenum xide was distributed thrughut the whle xide layers f SUS316. The transient xidatin f these Fe-Cr-Ni-(M) stainless steels was similar t that f Fe-Cr allys reviewed by Hmma [12] Wet xidatin f stainless steel Stainless steels were treated in a slutin f 2.5 tl dm -3 CrO 3 and 5mldm -3 H2SO 4 fr several tens f minutes. Ansell et al, [2] reprted, frm the results f their X-ray phtelectrn spectrscpic study, that the clured film n stainless steel, prepared by immersin in a chrmate slutin, is cmpsed principally f irn, chrmium and xygen and the species q I I I I I I I 800 ~ --I I ~ I I I I I I (a) ~ e203 4~ :" "" ".,"' - E -; ". "" -_'/. /' --";,'~F%N~)O(Cr, Fe)20 s - ---_ ---- ~-< c.~ni U] i.... Me*. a r 0 (3 I I I I I 1 I I 700 ~ I I I! I I I 1.~z=~_._=_~. _ ~._-:'_~_._a_. NiO<Cr, Fe)20 S J,r-~ ~...I._.~,.~,, ~,,~.-~.~ ~Fe20 s --- Metal Ic) ~ 'I ;, ~ I ''--4"-,,, / CROON I I I I _1 I_ I l Vetc~ty (mm sec-;) Figure 6 CEM spectra f xide films n SUS316 stainless steel heated at 700, 800 and 900 ~ C. i ;, r,, ',,, r t -, r Metl Figure 8 Schematic structure f xide films n stainless steel heated at (a) lwer temperature and higher temperature than 700 ~ C, and (c) xide films frmed in a chrmate slutin

5 Fe C r ~. ~,~ 0 1 O0 200 ; Sputtering time (see) Figure 9 AES in-depth prfiles f SUS304 stainless steel treated in a chrmate slutin at 70~ fr 25 min. Thickness/sputtering time -~ 10 nm/60 sec. present in the film are invariant with thickness. That is, in the clured regin, the intensity rati f Fe/Cr remained reasnably cnstant althugh the rati decreased n ageing frm the heated clured specimens. It was fund frm AES results as shwn in Fig. 9 that chrmium was slightly enriched and irn decreased slightly n the utermst xide films and that nickel existed unifrmly in the xide films. Sulphur was detected in the xide layers because a treatment slutin cntained many sulphate in. In the chemical state f irn in these films, nly a dublet (IS = 0.4 mm sec- ~, QS = 1.0 mm sec-~) appeared in the CEM spectra bth at rm and dry-ice tern- peratures as shwn in Fig. 109 This irn prduct was cnsidered t be irn xyhydrxide cntaining chrmium and nickel. The intensity f the dublet decreased frm 20% t 10% with increasing immersin time althugh the chemical state f irn in the xide films remained the same. The xide films n SUS316 treated in a chrmate slutin gave the same CEM spectra as thse n SUS304 as shwn in Fig. 11. The IS and QS f the dublet were 0.34 and 1.10 mm sec-~ respectively. The intensity f the dublet tended t decrease with the temperature f the chrmate slutin. Mre irn was included in xide films f SUS316 than in thse f SUS304. Mlybdenum was detected unifrmly in the (a) (a) ql 'E,4 c g r i I I I I I I I I I I 1~ I I I I r I I, I l Jl 03 ) I r [ I E! i '7 q I I ~ I I I r Velcity (turn sec -1) Figure lo CEM spectra f xide films n SUS304 stainless steel treated in a chrmate slutin at 70~ fr 15min. Measuring temperature: (a) rm temperature and dry-ice temperature. I r I "r I I ~ J I I t f r P [ p r Velcity (ram sec -1) Figure II CEM spectra f xide films n SUS316 stainless steel treated in a chrmate slutin fr 20 min at (a) 60~ and 70 ~ C. 1749

6 xide layers f SUS316, althugh the intensity f flurescent mlybdenum X-ray tended t decrease with increasing immersin time. Ensling et al. [13] studied the xide layers prduced n austenitic and martensitic steels after treatment under biler cnditins by CEMS and reprted that Fe3_.,.O 4 frmed n Fe-20Cr-10Ni steel and Ni,.Fe3_,.O4 n Incly 800 (Fe-20Cr-33Ni) with increasing immersing time. The three different irn species, namely Fe 3+ in ctahedral sites (//in = 532kOe), Fe 3+ in tetrahedral sites (//in = 493 koe), Fe2+/Fe 3+ pairs in ctahedral sites (Hin = 471 koe), were detected in the xide spectrum f Incly. Chrmium was depleted in the uter zne f the xide layer. These xide layers were different frm ur samples treated in a chrmate slutin. 4. Cnclusins Oxide films frmed n stainless steels, which were heated belw 700 ~ C r treated in a chrmate slutin, prduced varius shiny clurs. Oxide films prduced by the tw preparatin methds described abve culd be distinguished simply by detecting the irn state in the films using CEMS. The irn xide n heated stainless steel shwed magnetic hyperfine fields while the ther shwed paramagnetic peaks. The structure f the xide layers differed with the preparatin techniques. At least tw xide layers were bserved n the thermally treated stainless steels, while multi-xide layers culd nt be frmed clearly by chemical treatment. The thick xide films, prepared by heating belw 500 ~ C, were similar t the passive thin film bserved by ESCA, because in the passive layer the air-frmed film had a smewhat thicker irn xide layer at the tp surface [14]. The films n stainless steel, heated abve 800~ were nt s prtective against inner crrsin. It is easily understd by bservatin f the CEM spectra that fine xide particles prduced in the xide layers grew with increasing xidatin time and heating temperature, and the large xide particles destryed the prtective stacked xide films. The elements included in the bulk f the stainless steel are knwn t be segregated in deep layers by heating. The structure f the xide layers frmed n stainless steel by heating in air is cnsidered t be determined as a result f the cmpetitive xidatin. and segregatin rates f each element and the rientatin f the crystalline xides. Acknwledgement The authrs thank Mr J. Tanaka fr help with lading the M6ssbauer prgram n a persnal cmputer. References 1. R. NEMOTO and K. HORIOKA, Irn Steel 71 (1985) S R. O. ANSELL, T. DICKINSON and A. F. POVER, Crrs. Sci. 18 (1978) A. M. PRITCHARD and C. M. DOBSON, Nature 224 (1969) W. E. O'GRADY, J. Eleetrchem. Sc. 127 (1980) I. STEWART and M. J. TRICKER, Crrs. Sci. 26 (1986) K. NOMURA and Y. UJIHIRA, J. Mater. Sci. 19 (1984) Idem, J. Anal. Appl. Pyrl. S (1983) Idem, J. Mater. Sci. 18 (1983) Idem, Anal. Chem. 60 (1988) Idem, Hyperfine I~teractin 41 (1988) J. HESSE anda. RUBARTSCH, J. Phys. E7(1974) T. HOMMA, Hymen Kagaku (J. Surf Sei. Sc. Jpn) 9 (1988) J. ENSLING, J. FLEISCH, R. GRIMM, J. GRUBER and P. GUTLICH,Crrs. Sci. 18 (1978) S. JIN and A. ATRENS, Appl. Phys. A42 (1987) 149. Received 16 January and accepted 24 August