Indian Journal of Engineering & Materials Sciences Vol. 6, August 1999, pp . 21 3-218

Ellipsometric and electrochemical studies of the surface films on AISI 304 SS in acidic KeNS solution

A Bose & M K Totlani

Su rface Engineering Secti on, Materials Processing Division, Bhabha Atomic Research Centre, Mumbai 400 085 , Indi a

Received 5 August 1998; accepted 8 furz e 1999

Surface ti lms formed on solution annealed and sensi tised AISI 304 SS in 0.5 M H2S04 solution in the absence and presence of 0.0 I M KCNS have been evaluated for their refractive index and thi ckness using ellipsometry. The results are correlated to the protectiveness of the surface films formed in thi s environment, as determined by electrochemical potentio­ki llectic reacti vati on (EPR) tests. It has been observed that surface films (189-363 A th ick) formed on the sensiti sed AlSI 304 SS ill the ac idic so lution without 1.nd with KCNS are non-protective, whereas those formed 011 the solution annealed SS (123- 134 A) ill the same environment are protecti ve in nature.

Austenitic stai nl ess stee l forms a protect ive surface film, re ferred to as pass ive film , which pro tects the stainless steel surface from the attack of aggress ive environment. However, austenitic stainless steels lose passivity when ex posed in the temperature range of

SOO-800De, frequentl y encountered during we lding. Thi s process, known as sensitisat ion, results because of prec ipitation o f c hro mium carbide along the grain boundaries and the eventual localised depl eti on of chromium l

'). T he ex tent of sensiti sation or the loss of the passivity is dete rmined by an e lectrochemical test ca lled Electroc hemical Potentiokinetic Reac ti vation (EPR) tes t; the test essenti a ll y consists of exposi ng the test pieces in 0.5 M H2S04 + 0 .0 I M KeNS solu­ti on6

.R

. The protec ti ve nature of the passi ve film is c losely re lated to its thickness .

Thi s paper g ives the results of the study on the cor­re lati on be t ween the change in thi ckness of the pas­sive fi lm on A ISI 304 SS because of sensit isat ion, and its performance in EPR-electro lyte. Thickness has been determ ined hy e llipsometry, using a Gaertner Ellipsometer mode l L 11 9, emp loy ing mercury green light (5461 A )9.

Experimental Procedure The che mi ca l composition of AISI 304 SS used is

given in Table I . A rectangular 3 mm thick plate of

thi s stainl ess stee l was solution annealed at 1100 DC for 30 min in argon gas and then water quenched. It has been reported by Latani sion and Staehle 10 that at

11 00 DC only the austenite phase is present , that gets reta ined by water que nching.

Samples of size lO x 10 mm were cut from this sol ution annealed pl ate; the samples were then sealed in quartz glass in argon gas at approx. 0 .2 times the atmospheric pressure and were g iven sensitis ing heat

treatment at 650 DC for 6 h. The sealed samp les were then water quenched maintaining water at room tem­perature to ensure no furt her sensiti sation . These heat treatment conditions were chosen based on the T ime­Temperature-Sensiti sation (T-T-S) diagram of the AISI 304 S.S . reported by Novak et al.7 .

Electrochemical-Potentiokinetic-Reactivation (EPR) test

The EPR test was performed as per the procedure reported earlier6

. The conditions used are summarised in Tabl e 2 and the essenti a l features o f curve are given in Fi g. I.

Table I--Chemical composition of AISI 304 SS

Cr Mil C S p N Mo Si Cu Fe

10. 25 O.OG 0.020 008 O. I 0 Balance

I X.60 0.0 10 0.028 0.40

214 INDIAN 1. ENG. MATER. SCI. , AUGUST 1999

Table 2--Experimental conditions of EPR test

Electro lytes

Deaeration Temperature Speci men fi ni sh Initi al potent ial (OCP) Vertex potenti al Ver1 ex delay Scan rate

: a) 0.5 M H2S04

b) 0.5 M H2S04 + 0 .0 I M KCNS : NitrogenlArgon ( 150 cc/min .)

: 30DC : I mi cron (di amond paste) :-400 mV (vs SCEj : +200 mV (vs SCE) : 2 min. : 100 mY/min.

A stainl ess stee l wire 30 mm long was spot welded to one side of the samp le to provide e lectrical contact. The ent ire assemb ly was then mounted in superfast cold setting res in MET-SET IO I by placing it in a rOllnd mould of dia 20 mm and depth 20 mm. The mounted sample was poli shed to I -flm diamond paste fin ish and ri nsed in di stilled water and then in ace tone before testing. A fresh solution of the elect ro lyte was used in each test. A bright pl at inum sheet and a satu­rated calomel electrode (SCE) were used as counter and reference electrodes respectively.

A potentiodynamic curve was obtained by a poten­tial sweep at a scan rate of 100 mY Imin from open ci rcuit potential (OCP) to + 200 mY (vs SCE) . After a 2 min holding at +200 mY, a back scan at the same rate was conducted up to the OCP to obtain the reac ti vati on C li I' ve.

The grain size was determined as per the procedure described in ASTM E l liI The grain size number of the solution annealed sample was 3.5.

Ell ipsometry Test

Ellip:--ome try tes t was done us ing a Gaertner ellip­~ometer Model L119. emploYlIlg Mercury green light (5461 A ) ')

Ell ipsometric measurements of the sol ution an­neal ed and sensitised S.S . specimens were carried out ill s itll in 0.5 M H2S04 wi thout and wi th the addition of 0.0 I M KCNS at OCP and after ho ldi ng for 2 min at passivat ion potential (at +200 mY vs SCE).

An electrochemical cell with a three electrode as­sembly (working electrode, reference electrode­saturated calomel electrode and platinum as counter electrode) was connected to a potentiostat fo r main­ta ini ng the work ing e lectrode at a desired potential. Two glass fl anges at an angle of 130° to each other, with POll1t of intersection at the centre of the cell wi th opti ca ll y fla t windows, were attac hed to the cell to enable li ght beam to fall hori zontall y on the speci­men; spec Imen is mounted vert ically at the centre of the ce ll .

] 0 0+-

C CI)

0 0.. b

CI)

>

Log

Fig. I--Character istic parameters o f EPR curvcs . Cp--Activation charge (area under abc); C,,-Reacti vation charge (area under ade) : I,,-Max imum peak current den ~i t y reactivation: and Ip--Criti ca l pass ivation cu rrent densi lY

At eac h tc:--t condit ion, polari ser (P) and ana lyser (A) va lu es we re recorded at a fixed compensator azi­mu th of + 45° and at a fixed angle of incidence + 6SO; fo r ext incti on setting a sensiti ve photomultipler as­semb ly was lI sed. The surface characterist ics of the fil m fo rmed arc thcll compu ted using the foll owi ng re lat ions:

Relative phase retardatil n, /j, = 90 - 2P Rela ti ve alllplitude reduction, \jf = A

.. . ( I )

T he thi ckness of the surface fil m (I ) is computed by using Raman and Ramdas' fo rmul a l 2

, whi ch is essenti all y modifi ed Drude' s fo rmula 12. t '.

t = - K A I/ ~ - I ll ~ . J[ . ~ . (1/ ; - I ~l ~ - n~ )

.. . (2)

where K is the co-efficient of ell iptic ity, 112 and 11., are the refractive iildices of the film and the substrate respec tively.

The co-efficien t of ell iptici ty (K) is determined by the re lati on" "!> :

. ~1 1 1 1

K = SIO Q. tan 2P -V 1 + 11 ; tan - ¢I - 11 ;

4 . n; · sin ¢,.tan ¢t .. . (3)

where, Q = 45° and ¢I = angle of incidence on the

surface oxide fi lm. n2 is determined from the for­mula l

? as follows:

~ .

BOSE & TOTLANI: ELLIPSOMETRIC AND ELECfROCHEMICAL STUDIES OF THE SURFACE FILMS 215

2 2 .2 .1.[1 tan 2<p, .coS 22lj1.] n2 = n, sm 0/, + ----'-'-----''----(1 + sin 2lj1cos~y

. . . (4)

In computing t by Eq. (2), the values of K and n 2

were calculated using Eqs (3) and (4) based on read­ings recorded in the present study and the value of n3,

i.e., the refractive index of the substrate AISI 304 S.S., has been taken as 2. I 5 from the work reported by Matsuda et al.'8

Results and Discussion The values of 112 , as calculated from Eqs (4) and

(5) and K values as per Eq. (3) are given in Table 3. Utili sing these values, the values of surface fi lm thickness, both for solution annealed and sensitised AISI 304 S.S . have also been calculated and are also given in Table 3. The values of film thickness of AISI 304 S.S. and other stainless steel as reported in the literature are given in Table 4 for comparison.

Table 3--Ellipsometry resul ts, givi ng the film thi ckness of AIS! 304 SS

Environment 'f' (A) P t<,= " 2 Co-efficient of Thickness (/) degree (90-2P) elli pticity (K) A

0.5M H2S0 4(OCP) 28.07 94.5 1 -99.02 1.05 -0.0 15208 123

0.5 M H2SO4 28.20 94.9 -99.80 1.05 -0.0 18448 134 Solution (at + 200 mY ) Annealed

0.5 M H2S04 +0.0 1 M 29.65 95.85 - 101.70 1.04 -0.018772 183 KCNS (at OCP)

0.5 M H2SO4 29.74 95.95 - 101.90 1.04 -0.018772 187 +0.0 1 M KCNS (at +200 mY )

0 .5 M H2S04(OCP) 29.98 96.0 1 - 102.Q2 1.04 -0.018967 189

0.5 M H2SO4 30. 10 96.03 -102.06 1.04 -0.0 19032 190 (at +200 mY )

Sensitized 0.5 M H2SO4 31.15 96.0 - 103.07 1.03 - 0.0 19232 250 (650"C/ +0.01 M KCNS 535 6 h) (at OCP)

0.5 M H2SO4 32.13 97.04 - 104.08 1.02 --0.0 19032 363 +0.0 1 M KCNS (at +200 mY )

Table 4-Comparison of the reported values of film thickness of AIS l 304 S.S. and other stainless steel

Material Environment Thickness Technique used Reference A

AIS! 304 SS I M Na2S04 (PH=5.8) 60 XPS 19

AIS I 304 SS 5 N H2S04 + 0.5 N NaCI 100 AES 20

Fe-17Cr-3 Mo IN H2S04 + 70 AES 2 1 0.04 M CI - ions by HCI addi ti on

Fe-IOCr-IO 0.14 N Boric acid + 62 Cou lometric technique 22 0. 15 N sodium borate (p H = 8.4)

216 I DIAN J. ENG. MATER. SC I , AUGUST 1999

Film thickness of solution annealed AISI 304

Solution annealed AISI 304 S S shows a film th ickness of 123 A in 0.5 M H2S04 so lution at OCP i.e.-420 mY (vs SCE). And the fi lm thickness is 134 A at the pass iva ting potent ial of +200 mY (vs SCE) . The film thickness value reported in the lit­erature fo r AISI 304 S.S. in H2S04 solution is 100 A, where film thickness was determined by Auger Elec­tron Spectroscopy (AES)21. It is likely that decompo­sition of some components of the film (such as hy­droxide layer or chemisorbed water molecu le) in the high vacuum in AES can lead to lower thickness val ue by the AES technique.

With the addition of 0.0 I M KCNS to the 0.5 M H2S04 so lution, the film thickness increases to 183 A at OCP and is very nearly the same ( 187 A) at the passivat ing potential of +200 mY (vs SCE).

Film thickness of sensitised AISI 304 S.S.

The sensiti sed (a t 650 DC for 6 h ) AISI 304 SS shows a film thickness of 189 A in 0.5 M H2S04 at OCP (Table 3). Th is thickness is 66 A hi gher than that of the so lution annealed S.S. in the identical en­vironme nt. When 0.0 I M KCNS is added to the 0.5 M H 2S0~, the surface film thickness grows to

250 A at OCP, and 363 A at passivating potential of +200 mY (vs SCE).

Passive film on S.S. req uires a balance between film breakdown and film repair. In the absence of oxygen and ox idi sing conditions, a defective passive film is li kely to be fo rmed that wil l give easy access to corrosive spec ies through it; this will lead to thick­ening of the surface fil m that will not be protect ive.

. JJ ), h d Goswaml and Staehle-- and Antropov-' ave reporte that the reasons fo r pass ivity of austenitic S.S. 304 in H2S04 ac id sol ution is due to chromium oxy­hyd roxides in the passive film which lead to protec­tive passive film. Asami et a/.

14 have also reported that main consti tuent of the pro tecti ve passive fi lm formed on Fe-Cr alloys containi ng > 13 % Cr in deaerated I M H 2S0~ so lution is hydrated chromium oxy hydrox icle. while on low chromi um alloys under simi lar condit ions the fil m is co mposed of FeO, (OH)y. IlH 2U, wh ich is not protective. Our studies on the E.S.CA. 0f the ~ urface film on AISI 304 S.S. fo rmed ill H 2S0~ so lution confirms this observat ion25

E.S.CA. characterisation of the fi lms formed in 0.5 M H 2S0~ + 0.0 I M KCNS solution shows absence of chromium oxy-hydroxicle in the pas ~ i\'e film, thus

Tahle 5-Resu lts o f e lec trochemical potentioicinelic reactivati on (EPR) test. Scan !'ale: 100 mY I min

M:.llerial El cclru lylc Open E LTIl H':.l1 ( ·flt lr.;al Ep'J... ... s,\'c I passive Reac- Peak Remarks AISI 30-l ci rcuit mY A/cm2 mY A/cm2 tivatiun stan current SS poten- potential density

tia l (mY mY on reacti-v, SCE) vation

A/cm2

05M - -1 30 -340 9.0x I O-~ - 140 1.2x 10-5 Protective H, SOJ passive fil m

Solution annealed 05M - 400 0 5.0x I0-2 0 3.0x I0-5 Protective ~amp le I I, SOJ + passive 111 m

O.OIM KCNS

05 M - 460 - 300 I.lxIO-J -140 1.9x I 0-5 -ISO 2.Sx I 0-5 Unprotective

H, SOJ passive film, as it breaks

Scmit l- down in re-

zed saln- verse scan

pic O:'i M - 400 0 4.0x I0- 2 0 5.0xIO---{' -40 4.0x I0-5 Passive film is

(650"C/f, H, SOJ+ more unpro-

h/W Q) O{) 1M tec ti ve. as the

KCNS reacti vation chal ge i.e. area under the reacti vation cu rve is more

~'

BOSE & TOTLANI: ELLIPSOMETRIC AN D ELECTROCHEMICAL STUDIES OF THE SURFACE FILMS 217

c learly impl yi ng the non-protective nature of the fi lm in this environment. This, in tum, leads to thickening of the passi ve film. Pass ive fi lm formed on sensitised AISI 304 S.S. in O.S M H2S04 + 0 .01 M KCNS is hi ghly defective. thus giving an easy access of corro­s ive species through it and thickening of the fil m to 363 A at passivating potential of +200 mY vs SCE.

EPR tests

The results of EPR tests are given in Table 5, and EPR scan is given in Fig.l . It is seen from Fig.1 that the passive film over the solution annealed 304 S.S . does not breakdown in the reactivation step, i.e., re­verse scan indicating protectiveness of the passive film. However, the sensit ised 304 stainless steel's pass ive film breaksdown in the reverse scan clearly impl yin g the unprotectiveness of the passive film . As shown by our earl ier work6

, the degree of unprotec­ti veness is best represented by the reactiv~ti o n charge, i.e., the area unde r the reactivation curve, as shown in Fig. I . Accordingly the resu lts have been represented in the present work by the reactivation charge, i.e., the areas under the cu rves shown by the hatched areas in Figs 3a and 3b.

Solution annealed AISI 304 S.S.

The EPR curves for the solution annealed AISI 304 S.S. in O.S M H2S04 and O.S M H2S04 +0.01 M KCNS are gi ven in Figs 2a and 2b respectively; the curves show the protectiveness of the pass ive film.

It can be seen from Table 5, which gives the e lec­trochemical characteristics of the EPR curves, that addition of O.OIM KCNS in the corrosive environ­ment (O.S M· H2S04) has delayed the arrival of the

::'Xl

140 A I S I 304 (SoM ion annealed )

m w 60

Scon rote-XX) mV/min .

(.) 20 ui

'" 0

> -20 > -60 E - - 100 :2 - 140 C .,

-'80 ;£ - 220

- 260

-300

-340

-380

-420

-460

16 6 ,6:5 ,~

Current density. A / cm2

Fig. 2a--EPR curve of solution annealed sample in 0.5 M H2S04

pass ivity. In the case 0.5 M H2S04 solut ion, the pas­sivity has been achieved at - 140 mY, wh ile in the case of 0.5 M H2S04 + 0.0 I M KCNS it has been achieved at a mY. Thus, the charge consumed or, the metal dissolution is more in the latter case and hence the formation of th icker passive film (183 A ) as compared to the fo rmer case (123 A).

Similarly I cri' in the latter case is higher (5.0 x 10.2

Ncm2) as compared to the fo rmer case (9.0 x 10.

4

Ncm2).

Sensitised (650 "C fo r 6 h) AISI 304 SS The EPR curves for sensitised AISI 304 S.S . (at

650 °c for 6 h) in 0.5 M H 2S04 and 0.5 M H2S04 + 0.0 I M KCNS solu tions are shown in Figs 3a and 3b respectively. In both these figures, it can be seen that the passivity is breaki ng down in reverse scan, the hatched area in the curves, i.e., the reactivation charge, is more in Fig. 3b than that in Fig. 3a.

This clearly indicates that the thickness of the sur­face film formed wi ll be more in the latter case than that in the fo rmer, and that the thickness of the film formed in the fo rmer case will be more than that formed in case of solu tion annealed AISI 304 in 0.5 M H2S04 without and with 0.0 I M KCNS . That thi s is so has been shown by ellipsometric study of the surface films , reported in earlier part of this paper, thus supporting the observati ons of EPR study.

Conclusion I Thick.ness of the protective passive film formed

on AISI 304 S.S. in deaerated 0 .5 M H2S04 at OCP and a passivating potential or' + 200 mY (vs SCE) is 123 A and 134 A respectively.

2 Addition of a a I M KCNS to the 0 .5 M H2S04

solution, has increased the thickness of the film

2CXl

'60

' 20 80

40

0

-40

> -110

E -'20 11 -160

j -200

of -240

-ZIIO

-~

",

~ s s. (saul on onn.aled ) o ~ .. "250. + 00'" KeNS ScO"'i rol. - 100 mV / "'l tI

Curren' density. A / cm2

Fig. 2b--EPR curve of solution annealed sample in 0.5 M H2S04

+ 0.01 M KCNS

218 INDIAN J. ENG. MATER. SCI., AUGUST 1999

200 r------:::-------A~,SI~304:-:-:cS.::cS.(::-s.r..-t ... -.t-:6!)()=.C:-:-/ 6I>c-:-) --,

'40 0 5'" ~ Sca"I rot. -1OOmV/",in.

-460_~.~==~~=====-~---~---~---J '0 'O~ 10· 103 if} -,

10

Q.nw\t dONi ty . A / cm2

Fig. 3a--EPR curve of sensitised sample in 0.5 M H2S04

formed under the identical conditions to 183 A at OCP and 187 A at + 200 mY (SCE) .

3 The sen g.~t~~&d AISI 304 S .S . has comparatively higher film thickness of 189 A in 0.5 M H2S04 at OCP and 250 A in 0 .5 M H2S04 + 0 .01 M KCNS so­luti on at O .C.P ; the value increases to 363 A at +200 mY (vs SCE)

4 The film thickness determined by ellipsometery using modified Raman and Ramdas relation agrees reasonabl y we ll with the thickness determined by other tec hniques like AES, XPS and coulometry.

5 The re is good corre lation be tween the results of EPR and e llipsometry.

Acknowledgement The auth ors are th ankful to Dr S Banerjee, Associ­

ate Direc tor. Mate ri a ls Group and Dr. A K Suri, Head, Mat e ri a ls Process ing Division , BARC, for the encouragement received during this study. Authors are grate ful to Dr P K De, M aterial Science Division, BARC, for hi s va luable sugges tion s for the electro­chemical tes ts (EPR). The authors are al so indebted to Dr S K Ghos h, C hemi stry Divi sion , BARC, for his co-operati on in the optics of e llipsometry. Thanks are a lso due to Dr S Deb, So lid State Physics Division , BARe and to Dr G Mukhopadhyay, Physics Depart­ment , LIT, Bombay, Mumbai for va luable di SCUS- I sions on e llipso metry .

References Bain E C. Aborn R H & Ru therford J J B, Trans Am Soc SU'l'! Treat. 21 (1933) 4R I.

2 Mulford R A. Hal l E L & Briant C L, Corrosion. 39 (1983) 133 .

.1 Au ~ t K T. Armijo J S & Westhrook J H, ASM Trans Qllart . 59 ( 1966) 544.

4 Clarke W L & C lrlso n D C. Mater Pel!orm , 19 (1980) 16. 5 CLlrkc W L. Cowan R L & Walker W L, " In tergranlliar

w ~ V> - -eo > > - , 21) E

~ --'60

i -200

~ -240

-280

-320 .[:'4..

-)60 . ..., ... ... -400

,0' .0'

3045.$ (SoluI IOl'! annea..a l + s.n.itINd 01

6~C . 6h O ~!Iot H-zso.+OD1'" KeNS ScOft '01 . '00 ",VI _A.

,64 1() !

Current cMnsity , A / cm2

Fig. 3b--EPR curve of snesiti sed sample in 0.5 M H2S04 + O.OIM KCNS

Corrosion (!( Sta inless Alloys" A.S.T.M , STP-656, edited by SteigerwJld R P (Phil adelphia A.S.T.M, Pennsylvani a), 1978, 99.

6 Bose A & De P K, Corrosion , 43 ( 1987) 624. 7 Novak P . Stefec R & Franz F, Corrosion, 3 1 (1980) 344. 8 Pednckar S & Smi alowska S, Corrosion , 36 (1980) 565. 9 Archer R J, Manl/al 0 11 Ellipsolll etry, (Gaertner Scientific

Co rporati on, Chi cago, 111 -60614. USA). 10 Latani sion R M & Staehle R W , "S C C oj Fe-Ni- Cr AL­

lo."!!. Proc Co nI' on Fundamaental Aspects of Stress Corro­sion Crack in g. NACE. U SA, 1967, 21 4.

I I ASTM /Jook oj Standards. (ASTM, Philadelphia, Pennsylva­ni a. USA) II ( 1980) 186.

12 Ral1l:1n C V & Ramdas L A, Phil Mag, 3 (1927) 220. (Sci­en tifi c Paper~ of C V Raman, Vol I, pp 233-236, Indian Academy of Sciences, Oxford University Press, New Delhi, Indi a) .

13 Drude P , 'Theory oj optics', ( 1902) ,English translation pub­li shed by Dover Publications Inc , New York, USA, (1959) 126, 295 .

14 Drude P , Anll Phys Cizem, 39 ( 1890) 48 1. 15 Drude P , Ann Phys Cho n, 36 (1889) 532, 865. 16 Ellipsolll eiry ill the Measurement oj SUlfaces alld thin fi lms,

edited by Passagli a E , Stromberbg R R & Kruger J, Nat ional Burean of Standards Symposium Proceed ings, Washi ngton, USA, NBS Miscell aneous Publication - 256, 1963.

17 Vasicek Antonin, ibid, 25- 39. 18 Matsuda S, Sugimoto K & Sawada Y, Trans JIM, 18 (1977)

66. 19 Rossi Antonell a & Elsener Bernh ard, Proc 12th IllIerna­

tionai Corrosion Congress, (NACE Intern ational, Houston, USA) , 1972, p. 2 120.

20 Smialowski SzkJ arska Z & Lukomski N , Corrosion, 34 (1978) 177.

21 Schneider A , Ku ron D , Hofmann S & Kirchheim R , Cor-1'0.1' Sci, 3 1 (1990) 19 1.

22 Goswami K N & Staehle R W, Electrocilell1 Acta , 16 (1971 ) 1895.

23 Ant ropov L I, Theort!licai Electrochemistry, Translated by A Bek nazarov (Mir Pu bli shers, Moscow), 1977.

24 Asami K, Hashimoto K & Shimodai ra S , Corros Sci, 18 (1 978) 151.

25 Nature of Passive Ii lms formed on AISI 304 S S, in Acidic KCNS Soillfion-E SeA SII/dy (to be publ ished).