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Applied Surface Science 264 (2013) 312– 319

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h om epa g e: www.elsev ier .com/ locate /apsusc

igh temperature oxidation behavior of AISI 304L stainless steel—Effect ofurface working operations

wati Ghosh ∗, M. Kiran Kumar, Vivekanand Kainaterials Science Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

r t i c l e i n f o

rticle history:eceived 30 August 2012eceived in revised form 3 October 2012ccepted 4 October 2012vailable online 12 October 2012

eywords:tainless steel

a b s t r a c t

The oxidation behavior of grade 304L stainless steel (SS) subjected to different surface finishing (machin-ing and grinding) operations was followed in situ by contact electric resistance (CER) and electrochemicalimpedance spectroscopy (EIS) measurements using controlled distance electrochemistry (CDE) tech-nique in high purity water (conductivity < 0.1 �S cm−1) at 300 ◦C and 10 MPa in an autoclave connectedto a recirculation loop system. The results highlight the distinct differences in the oxidation behavior ofsurface worked material as compared to solution annealed material in terms of specific resistivity andlow frequency Warburg impedance. The resultant oxide layer was characterized for (a) elemental analy-

urface workingigh temperature oxidationlectrochemical impedance spectroscopy

ses by glow discharge optical emission spectroscopy (GDOES) and (b) morphology by scanning electronmicroscopy (SEM). Oxide layers with higher specific resistivity and chromium content were formed incase of machined and ground conditions. Presence of an additional ionic transport process has also beenidentified for the ground condition at the metal/oxide interface. These differences in electrochemicalproperties and distinct morphological features of the oxide layer as a result of surface working were

ce of

attributed to the prevalen

. Introduction

Intergranular stress corrosion cracking (IGSCC) of austeniticrade 304L stainless steel (SS) in the core shrouds of boiling watereactors (BWR) is a dominant degradation mode [1–3]. Analysis ofhe cracked core shroud components did not show grain boundaryr carbides or Cr depletion associated with the regions exhibiting

GSCC, but dense dislocations bands and significant oxygen ingressere observed ahead of the crack tip [4–6]. At a number of locations

he cracks were found to initiate transgranularly up to a certainepth of the material and subsequently propagated intergranularly

nto the material [5–7]. Considering these observations it is diffi-ult to explain the core shroud (hard) weld cracking on the basisf either classical IGSCC or grain boundary Cr-depletion model ofrradiation assisted stress corrosion cracking (IASCC). One of theactors considered to play a key role in the stress corrosion crack-ng (SCC) susceptibility of a material is the nature of the surfacen the component that gets exposed to the high temperature envi-onment in the reactors. The final stages of fabrication processes

or most components in any industrial sector generally constitutef surface finishing operations like machining and grinding, whichesult in heavily cold worked surfaces. Such operations result in

∗ Corresponding author. Tel.: +91 22 25590473.E-mail address: swati364@gmail.com (S. Ghosh).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.10.018

heavily fragmented grain structure and presence of martensite.© 2012 Elsevier B.V. All rights reserved.

a wide variation between the surface and bulk properties of thematerial. It has been reported that typically, surface cold workingoperations on austenitic grade 304L stainless steel lead to the for-mation of a cold worked layer on the surface. The surface layerproduced due to surface cold working operations has very highlevels of residual stresses [8], sub-micron sized grain structure [9]and martensitic transformation [9] resulting in increased SCC sus-ceptibility [4,8–16]. The nature of the stainless steel surface againdictates the nature of the oxide film formed at high temperaturein an aqueous environment which is believed to play an impor-tant role in determining its SCC susceptibility. Hence oxidation ofaustenitic stainless steel in high temperature and high pressurewater has been a subject of interest for many researchers [17–29].The influence of composition, structure and morphology of oxidelayer on corrosion of stainless steel in high temperature water hasbeen reported [30–32]. In addition the effect of abrasion, pickling,grinding [33,34] and cold rolling [35,36] on the oxidation behav-ior of austenitic stainless steel in air has been studied. However,information on the effect of surface cold working operations likemachining and grinding on the oxidation behavior of austeniticstainless steel measured in situ in high purity water (conductiv-ity < 0.05 �S cm−1) environment is not present. Hence this study isthe first organized attempt to understand the high temperature oxi-

dation behavior of austenitic stainless steel subjected to machiningand grinding operations by in situ contact electrical resistance (CER)and electrochemical impedance spectroscopy (EIS) studies. Con-trolled distance electrochemistry (CDE) in high purity water at

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00 ◦C combined with depth profile analysis and morphologicalharacterization of the oxides has been used in this study.

. Experimental

.1. Material and methods

The material used in this study was grade 304L austenitic stain-ess steel with a chemical composition: 0.023 C, 17.14 Cr, 9.13i, 0.29 Si, 0.99 Mn, 0.035 P, 0.004 S and balance Fe (all in wt%).he as received 304L stainless steel was in solution annealedondition. The surface of the solution annealed material was inickled and passivated condition. The stainless steel specimen inhis solution annealed condition was taken for grinding/machiningperations. Two different plates of solution annealed 304L stainlessteel having dimension 80 mm × 50 mm × 3 mm were subjected toa) machining and (b) grinding to remove a depth of 0.5 mm fromhe surface. The surface roughness produced by surface cold work-ng operation was measured by a surface profilometer. Solutionnnealed 304L SS had been used as a reference in each measure-ent. It was polished up to 0.1 �m surface finish using a diamond

aste.The surfaces of the plates in solution annealed, machined, and

round conditions were characterized using stereo microscopy.etailed microstructural characterization of the plates was donesing scanning electron microscope (SEM) – electron back scat-ered diffraction (EBSD) technique. For EBSD measurements, theross sections of the stainless steel samples in different conditionsere electro polished. The EBSD measurements were carried out on

FEI Quanta 200 HV scanning electron microscope with TSL–OIMystem. The measurements were made at an operating voltage of0 keV, using a step size of 0.20 �m. For crystallographic analysis,easurements with a confidence index of 0.1 or higher were used.

onfirmation of the phase transformation brought about by surfaceold working operations in 304L stainless steel was done by X-rayiffraction (XRD) measurements in a Panalytical MRD system. Highesolution (0.02◦ step size) �–2� (where � is the Bragg angle) scansere used.

.2. Exposure conditions

Exposure studies on solution annealed, machined, and ground04L stainless steel samples were carried out in an autoclave madef type 316L SS. The autoclave was connected to a recirculation loop.he detailed description of the recirculation loop has been reportedarlier [37]. The demineralized water was pressurized to 10 MPand circulated through the autoclave. The inlet water conductivityas maintained at < 0.10 �S cm−1 and the inlet dissolved oxygen

oncentration at <5 ppb (by mass) throughout the experiment.oupons of 304L stainless steel in solution annealed, machined, andround conditions having dimension of 15 mm × 15 mm × 2.5 mmere exposed to high temperature high purity water in the auto-

lave at 300 ◦C and 10 MPa for a period of 360 h. These specimensere attached to a specimen holder using zirconia (ZrO2) spac-

rs for electrical insulation. Further, for in situ electrochemicaleasurements, specimen of 3 mm diameter was exposed in a

pecially designed specimen holder for maintaining controlledistance between counter and working electrode (elaborated inection 2.3.2).

.3. Characterization methods

.3.1. Morphological characterization and elemental analysis ofxide films

The surfaces of oxides formed after 360 h of exposure on solu-ion annealed, machined, and ground type 304L stainless steel were

ience 264 (2013) 312– 319 313

examined ex situ (after exposure) under SEM. Elemental com-position of the surface film along its depth was assessed usingglow discharge optical emission spectroscopy (GDOES). A GDA 750instrument with a polychromator of 750 mm focal length, equippedwith 2400 grooves mm−1 grating was used. The instrument param-eters used were: excitation voltage 750 V, discharge current 20 mA,anode diameter 4 mm. The spectrometer was calibrated by a sput-tering rate correction calibration method using certified referencematerials.

2.3.2. In situ electrochemical characterizationThe in situ characterization of the oxide films was done using

CER and EIS techniques. The detailed description of the CDE setuphas been given by Saario et al. [37]. The 3 mm discs required forfitting into the CDE holder [37,38] were prepared from stainlesssteel samples in solution annealed, machined, and ground con-ditions. Iridium (Ir) tip with a diameter of 3 mm was used as acounter electrode for both CER and CDE-EIS measurements per-formed in situ during the exposures in the recirculation autoclavesystem. The EIS measurements were done in a two-electrode con-figuration in which the Ir probe is connected to both the referenceand the counter electrode terminals. The CER technique is based onthe measurement of the electrical resistance across a solid–solidcontact surface using direct current [37–39]. During the measure-ment, the surfaces of the Ir and the steel sample were brought incontact and pulled apart to a distance of 10 �m at regular inter-vals. The surfaces of the working electrode and the Ir electrodecome in contact with the environment when the probes are pulledapart. When the surfaces were brought into contact, a direct currentwas passed through the contact surfaces and the resulting poten-tial was measured in order to determine the resistance (CER) ofthe surface film. All the resistance measured in this arrangement isdue to the surface film (oxide) formed on the working electrodeas Ir remains inert in the environment and does not contributeto the resistance. Application of potential to the sample was notfeasible as the measurements were done in high purity water (con-ductivity < 0.10 �S cm−1). The resistance of the film was recordedwith increasing time of exposure until a film with stable CER valuewas formed. The Ir tip was then positioned at 10 �m from thesample surface by the help of a stepper motor arrangement andimpedance spectra of the film were recorded. In the EIS measure-ments, an a.c. voltage perturbation of 20 mV was applied over afrequency ranging from 10−3 to 106 Hz. The response in current wasrecorded in terms of changes in impedance and phase angle shift.All the impedance measurements in this study have been carriedout at the open circuit potential. The spectra thus obtained weremodeled using equivalent circuit approach. The kinetic parame-ters (corresponding to processes contributing towards oxidation)derived from the modeling were used to compare the oxidationbehavior of all the three surface conditions of type 304L SS. Theuniqueness of this study lies in the fact that all the impedancemeasurements done were in high purity demineralized water (con-ductivity < 0.1 �S cm−1) which is the first of its kind reported in theliterature and which has been possible by the CDE arrangementwhich allows the measurements to be taken at a very small dis-tance from the sample surface (∼10 �m) thereby minimizing thesolution resistance and hence the ohmic drop.

3. Results

3.1. Microstructure before oxidation

Fig. 1 shows the stereo micrograph of the solution annealed,machined, and ground surfaces used in the present study. The sur-face roughness of machined surface was 0.58 �m and that of ground

314 S. Ghosh et al. / Applied Surface Science 264 (2013) 312– 319

Fig. 1. Stereo micrograph of 304 L stainless steel before exposure to high tem-pc

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Fig. 2. EBSD of the cross section of 304L stainless steel in ground condition (a) show-ing heavily fragmented grains near the surface and (b) phase contrast map showing

erature and high pressure in (a) solution annealed (b) machined and (c) groundondition.

urface was 0.75 �m. Results of the EBSD measurement of the crossection of the plates in ground condition are shown in Fig. 2. Fig. 2bresents phase maps overlapped with the image quality (IQ) whichepresents the contrast of the EBSD patterns for cross section oftainless steel specimen in ground condition. Similar studies hadeen carried out on the machined plate and reported earlier [8]. Inoth the cases of machined and ground stainless steel, it is foundhat extensive grain fragmentation took place in the near surface

egion of austenitic stainless steel as a result of surface cold workingperations (Fig. 2a). Moreover, austenite undergoes phase transfor-ation to produce both stress and strain induced martensite as a

presence of martensite in the work hardened layer.

S. Ghosh et al. / Applied Surface Sc

Fig. 3. The XRD spectra of 304L stainless steel in (a) solution annealed, (b) machineda

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having ≤5 ppb (by mass) O2 for 360 h is presented in Fig. 4. Thematerials, in general, showed duplex oxide morphology. This istypical of the high temperature oxide formed on stainless steel ashas been reported previously by Kim and Takeda and Shoji et al.

nd (c) ground condition.

esult of machining [8] and grinding operations (Fig. 2b). The for-ation of martensite has been confirmed by XRD studies. The X-ray

pectra for solution annealed, machined and ground stainless steelre shown in Fig. 3. In case of all three specimens, standard peaksorresponding to austenite phase (�) were present. However, inddition to the austenite, peaks corresponding to martensitic phaseere observed in case of both surface machined and ground speci-ens. The peak at 2� value of 96.1◦ in Fig. 3b for machined specimen

′

orresponds to strain induced martensite (˛ ). Further, in case of theround specimen, in addition to austenite (�) and strain inducedartensite (˛′), peaks corresponding to stress induced martensite

ience 264 (2013) 312– 319 315

(ε) phase (at 2� values of 41.8◦ and 48.7◦ in Fig. 3c) were alsoobserved.

3.2. Ex situ characterization of the oxide

3.2.1. Oxide morphologyThe surface morphology, as observed by SEM, of the oxide scale

formed on type 304L stainless steel in solution annealed, surfacemachined, and ground conditions exposed to demineralized water

Fig. 4. SEM image of the oxide morphology on (a) solution annealed, (b) machinedand (c) ground 304L austenitic stainless steel after exposure to high temperatureand high pressure conditions.

3 face Science 264 (2013) 312– 319

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18,40–46]. The duplex morphology of oxide had a compact innerayer of chromium rich spinel and an outer layer enriched with ironut depleted in chromium (as shown by GDOES analysis, Section.2.2) similar to reported literature [23,39]. The outer oxide layer inoth solution annealed and machined specimens had a bimodal dis-ribution of particles having loose faceted ones of size ∼2.5 �m andlosely packed particles of ∼0.5 �m. Similar observation of refine-ent of the outer oxide layer on type 316L stainless steel in high

emperature water under dynamic loading has been reported byakeda et al. [47]. On the other hand, the ground sample showed

high density of only large oxide particles throughout the surfacever the compact inner layer of oxide (Fig. 4c) with the grindingarks clearly visible.

.2.2. Elemental depth profile (GDOES analysis) across the oxideThe elemental profiles across the depth of the oxides formed

ver solution annealed, machined, and ground stainless steel spec-men after 360 h exposure at 300 ◦C are shown in Fig. 5. It ismportant to mention here that the concentrations of the individ-al metallic constituents like Ni and Cr have been normalized to theotal concentration of metallic elements. This is done in order toxclude the influence of oxygen on the depth profiles of the metal-ic elements in the oxide [47,48]. In all the three cases, the oxideas a duplex structure which is a characteristic feature of high tem-erature oxide formed on austenitic stainless steel having an inner

ayer rich in Cr and a thinner outer layer containing mainly Fe [23].ven though Ni is not remarkably enriched in either of these films,ore of it was found in the inner layer (Fig. 5). The inner layer is

ompact (as shown by SEM images of the oxide morphology andhickness measurement from GDOES studies). It is composed ofhromium rich spinel oxide. But the outer layer is less compact.ts composition is similar to magnetite and its structure is reportedo be inverse spinel [17–29].

The inner layer is considered to slow down corrosion reactions.he oxide film thickness has been estimated by taking the oxygenignal and setting the film/metal interface at the distance at whichhe oxygen signal dropped to 50% of the value at the surface [47,48].he thickness of the oxide film in the case of solution annealedtainless steel is higher (∼1.2 �m) than in the cases of machined∼0.6 �m) and ground (∼0.32 �m) SS. The maximum concentrationf Cr in the inner layer of oxide formed over machined (Fig. 5b) andround (Fig. 5c) stainless steel is ∼74% higher than that for solu-ion annealed steel (Fig. 5a). The highest chromium concentrations observed for the oxide formed on the ground specimen surfaceollowed by that formed on machined specimen surface.

.3. In situ electrochemical characterization

.3.1. Contact electrical resistance (CER)The CER vs. time plot for solution annealed, surface machined,

nd ground 304L stainless steel measured at 300 ◦C and 10 MPa inigh purity water is shown in Fig. 6. An exposure period of 24 h waseeded to obtain a stable CER value of the film. The contact resis-ance increased fast and stabilized at 155 m� cm2, 163 m� cm2 and42 m� cm2 for solution annealed, ground and machined type 304Ltainless steel specimen respectively. These values are in the rangef resistance obtained for iron by Bojinov et al. [49] in high temper-ture water. The contact electric resistance measurements togetherith measurement of the thickness of the oxide film yields the spe-

ific resistivity of the oxide film produced over type 304L stainlessteel in machined, ground and solution annealed conditions. The

pecific resistivity of the film formed under different conditions haseen derived from the measured resistance values to be 0.13 � cm,.236 � cm and 0.512 � cm for solution annealed, machined, andround 304L stainless steel specimen respectively. The oxide

Fig. 5. In-depth elemental analysis of the oxide formed on (a) solution annealed, (b)machined and (c) ground 304L austenitic stainless steel by GDOES.

produced over the ground sample has the highest specific resistivityfollowed by machined and solution annealed SS.

3.3.2. Electrochemical impedance spectroscopy (EIS)The EIS spectra at the open circuit potential for the solution

annealed, surface machined, and ground type 304L stainless steelspecimen in deoxygenated, demineralized water at 300 ◦C are

S. Ghosh et al. / Applied Surface Sc

Fig. 6. Contact electric resistance of the oxide film formed on solution annealed,machined and ground 304L austenitic stainless steel at 300 ◦C and 10 MPa in dem-ineralized water environment.

Fig. 7. The electrochemical impedance spectra (phase angle vs. frequency) mea-s1m

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ured in situ in demineralized water containing < 5 ppb (by mass) O2 at 300 ◦C and0 MPa using controlled distance electrochemistry technique for solution annealed,achined and ground 304L austenitic stainless steel.

hown in Fig. 7. The phase angle vs. frequency plot for the solutionnnealed and the machined steel shows two time constants. Theseime constants are in broad analogy with the previous EIS resultsn iron, ferritic and austenitic steels [50]. The in situ impedancetudies on the oxide revealed the electronic and ionic transportroperties of the oxide film formed for surface worked condition vs.he solution annealed condition. The phase angle vs. frequency plotor the solution annealed and the machined stainless steel showedwo time constants that are typical of austenitic steels but theresence of an additional time constant for the ground condition

ndicated the presence of a Warburg-type ionic transport processt the metal oxide interface. The Warburg impedances obtainedor the oxide film formed over solution annealed, machined, andround type 304L stainless steel are 0.048 � cm2, 0.042 � cm2, and.035 � cm2 respectively. The Warburg impedance is minimum inase of ground type 304L SS which indicates the presence of addi-ional diffusion process in ground condition.

. Discussion

The present study highlights the effect of surface cold workingperations like machining and grinding on (a) microstructure (b)xidation behavior of the surfaces in high temperature and highressure water and (c) the nature (morphology and composition)

ience 264 (2013) 312– 319 317

of oxide formed on 304L stainless steel. There is a drastic change inthe surface microstructure of 304L stainless steel as a result of thesurface cold working operations. EBSD results of the cross sectionof ground 304L stainless steel (Fig. 2) showed heavy grain fragmen-tation in the near surface layers. Moreover, high volume fraction ofmartensite is formed as a result of machining and grinding opera-tions. The altered microstructure on the surface together with thegeneration of high magnitude of tensile residual stresses as a resultof surface cold working operations had played a key role in drasti-cally increasing the susceptibility of 304L stainless steel to chlorideinduced transgranular stress corrosion cracking [8,12].

Distinct differences in the oxidation behavior and in the natureof oxide film formed as a result of different material conditionshave been understood from the present study. Stellwag [23] hasexplained the mechanism of formation of the duplex oxide filmon austenitic stainless steel in high temperature water. The innerlayer of oxide in case of austenitic stainless steel is formed by solidstate growth process and the outer layer by the precipitation ofmetal ions released from the corroding surface [23]. Similar obser-vations on the duplex nature of high temperature oxide formed ontype 304L stainless steel have been reported in a number of recentstudies [40–45] but very few of these studies reported the oxida-tion behavior of surfaces which have been given prior cold workor subjected to surface finishing operations [43,45,46]. The currentobservation of bimodal particle size of the outer Fe rich oxide layerhas also been reported earlier by Takeda et al. [47] on type 316Lstainless steel in high purity water under dynamic loading. Theelemental analysis with depth of the oxide film formed over 304Lstainless steel in solution annealed, machined, and ground condi-tions (Fig. 5) yielded information on the differences in the oxidefilm properties in the three cases. The differences in the three casesare as follows: (a) the thickness of the oxide film in case of solu-tion annealed stainless steel is higher (∼1.2 �m) than in case ofmachined (∼0.6 �m) and ground (∼0.32 �m) stainless steel. (b) Themaximum concentration of Cr in the inner layer of oxide formedover machined and ground stainless steel is ∼74% higher than thatfor solution annealed steel. The highest chromium concentrationis observed for the oxide formed on the ground specimen surfacefollowed by that formed on machined specimen surface. This obser-vation can be explained from the presence of a strained surfacelayer for machined and ground specimens which enables higherdiffusion of Cr from the metal matrix to the oxide as has been alsoreported in previous studies [5,35,36,40,46]. The work hardenedsurface layer present on type 304L stainless steel has been char-acterized in detail in previous studies [8] and is found to containhigh levels of plastic deformation visible in the form of (a) highconcentration of slip bands, (b) heavily fragmented grain structureresulting in sub-micron grain size and (c) the presence of martens-ite. The specific resistivity of the oxide film was derived from theelectrical resistance value of the oxide obtained from CER mea-surements (Fig. 6) and the thickness of the oxide film produced ineach case. The specific resistivity of the film in solution annealed,machined, and ground 304L stainless steel specimen was found tobe 0.13 � cm, 0.236 � cm and 0.512 � cm respectively. The oxideproduced over the ground sample had the highest specific resis-tivity followed by machined and solution annealed stainless steel.The specific resistivity of a film is a measure of the resistance to thediffusion of ions across the film. The higher the specific resistivityof the film, the lower is the permissible diffusion of ions throughit. This is supported by the GDOES results (Fig. 5) which indicatedthat surface cold working resulted in the formation of a film havinghigher chromium. Chromium enriched oxide film is more protec-

tive in nature and restricts diffusion of ions through the film. EISstudies (Fig. 7) have highlighted the presence of an additional ionictransport process for the ground specimen probably at the metaloxide interface. The possible reason for an additional ionic transport

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rocess in the case of ground 304L stainless steel is probably theresence of very high magnitude of tensile residual stresses onhe ground surface (∼1100 MPa [7]) over which the oxide forms.he presence of high magnitude of tensile residual stresses in theetal matrix imparts instability to the oxide formed and results

n a higher rate of dissolution of metallic ions at the metal oxidenterface.

.1. Implications for SCC

The understanding of oxidation behavior of surface worked 304Ltainless steel may be helpful in comprehending the instances ofGSCC experienced in the BWR during service. It may also be relatedo the transgranular stress corrosion cracking (TGSCC) behavior ofurface worked steels previously studied in laboratory [8,12]. Pre-ious studies had shown that surface cold working operations likeachining and grinding drastically increase the SCC susceptibil-

ty of stainless steel in chloride environment [8,12]. The nature ofhe oxide formed on the surface of austenitic stainless steel at highemperature high pressure (HTHP) conditions have been found tolay a key role in determining its SCC susceptibility [1–5,33,34,46].owever, an exhaustive study by Breummer and co-workers [5]ad shown that the nature of cracks produced in service in BWRore shrouds are very different from those generated in the labo-atory during crack growth rate (CGR) tests. Cracks generated inervice are filled with a lot of oxide (probably due to long time ofxposure as compared to laboratory CGR tests) and the crack tipsxhibited a blunted ‘finger like’ attack (contrary to the sharp naturef crack tips in the CGR tests). The cracks also showed the pres-nce of locally “dealloyed” zones of Fe and Cr. Alloy compositionseasured at the crack tips were 40 wt% Fe, 4 wt% Cr and 55 wt%i (immediately ahead of the crack front) versus approximately0 wt% Fe, 19 wt% Cr and 9 wt% Ni in the bulk material [5]. Theseeatures of in-service SCC in BWR core shrouds showed similarityo the SCC and oxidation behavior of surface worked stainless steeliscussed in the present study and are as follows: (a) crack blunting

s a characteristic feature of SCC for surface worked 304L stainlessteel at room temperature [8] where shallow cracks initiate earlyn the surface and propagate through the highly worked surfaceayer but get arrested on reaching the ductile austenitic matrix,b) the oxides produced on the surface of ground and machined04L stainless steel have much less Fe content (drops to ∼48 wt%e for ground and to ∼52 wt% Fe for machined) as compared toolution annealed 304L stainless steel (∼60 wt% Fe). This observa-ion suggests that oxidation of materials after surface cold workingperations also brings about local dealloying of Fe. Also, it is worthoting that both in the cases of machining and grinding a thinnerxide film is formed on the surface which is richer in chromium andxygen as compared to the oxide formed on the solution annealed04L SS. The oxide formed over the inner walls of the cracks and athe crack tip showed similar enrichment of chromium and oxygen5]. The higher oxygen concentration both at the crack tip and on theurface of machined and ground 304L stainless steel is attributedo stress/strain assisted diffusion. In addition the high densities ofefects present near the surface of machined and ground 304L SS,uch as very high grain boundary area [8,9,12] (as grain size nearhe surface is very small due to grain fragmentation) and dislo-ations within the strain-localized band, provide a quick path forxygen diffusion. When oxygen partial pressure, at some concen-ration points reaches a critical value for formation of oxide, theelective oxidation takes place leading to the formation of brit-le phases like chromia on the surface. Both the crack tip and the

orked surface are high stress regions and have high defect density.hereas crack tip stresses are the driving force for crack propaga-

ion, residual stresses on the surface are the driving force for cracknitiation resulting in formation of micro-cracks. Stresses present

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ience 264 (2013) 312– 319

in service result in the growth of the micro-cracks and coalescenceof these micro-cracks leading to crack propagation [46].

5. Conclusions

The studies on the oxidation behavior of 304L stainless steelsurfaces in machined, ground and solution annealed conditionsshowed that surface cold working brings about major changes inthe oxidation behavior of stainless steel surfaces and the nature andmorphology of the oxide film formed. The oxides formed in caseof machined and ground conditions had higher specific resistivityand were richer in chromium content. The thickness of the oxidefilm formed after a similar exposure period (360 h) was the highestfor solution annealed condition followed by machined and groundconditions. Presence of an additional ionic transport process hasalso been identified for the ground condition at the metal/oxideinterface. The differences in the oxidation behavior of the surfaceworked materials were attributed to the presence of a highly workhardened layer with fragmented grains and high volume fractionof martensitic phase formed as a result of machining and grindingoperations.

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