Applied Surface Science 255 (2009) 7056–7061

Initial oxidation of duplex stainless steel

Crtomir Donik a,*, Aleksandra Kocijan a, Djordje Mandrino a, Irena Paulin a,Monika Jenko a, Boris Pihlar b

a Institute of Metals and Technology, Lepi pot 11, SI-1000 Ljubljana, Sloveniab Faculty of Chemistry and Chemical Technology, Askerceva 5, SI-1000 Ljubljana, Slovenia

A R T I C L E I N F O

Article history:

Received 21 January 2009

Received in revised form 11 March 2009

Accepted 13 March 2009

Available online 24 March 2009

Keywords:

Duplex stainless steel

Stainless steel

Oxidation

XPS

Thickogram

Plasma oxidation

A B S T R A C T

Three different techniques were used to produce thin oxide layers on polished and sputter-cleaned

duplex stainless-steel samples. These samples were exposed to 10�5 mbar of pure oxygen inside the

vacuum chamber, exposed to ambient conditions for 24 h, and plasma oxidized. The oxide layers thus

produced were analysed using XPS depth profiling in order to determine the oxide layers’ compositions

with depth. We found that all the techniques produce oxide layers with different traces of metallic

components and with the maximum concentration of chromium oxide shifted towards the oxide-layer–

bulk-metal interface. A common characteristic of all the oxide layers investigated is a double-oxide

stratification, with regions closer to the surface exhibiting higher concentrations of iron oxide and those

more in-depth exhibiting higher concentrations of chromium oxide. A simple non-destructive

Thickogram procedure was used to corroborate the thickness estimates for the thinnest oxide layers.

� 2009 Elsevier B.V. All rights reserved.

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1. Introduction

Stainless steel materials (FeCr and FeCrNi-based alloys) areemployed in a wide range of modern applications due to theirability to withstand corrosive environments while maintaininggood mechanical properties. Their corrosion resistance originatesfrom Cr-rich oxide layer which serves as a barrier against iondiffusion between the alloy and the ambient phase. Custom steelgrades can be designed for specific applications by optimizing theirproperties throughout alloy composition. DSS 2205 (also known asW.Nr. 1.4462) investigated in this study is a 22% chromium, 5–6%nickel, 3% molybdenum, nitrogen-alloyed duplex stainless steelwith high general, localized and stress corrosion resistanceproperties in addition to a high strength and excellent impacttoughness. Duplex stainless steels are balanced with two phases,ferrite and austenite. This type of stainless steel is increasinglyused because of its several superior properties. The superiormechanical properties, resulting from both the balance of theduplex phases and the interstitial solid solution with a highnitrogen content, mean the alloy can be used in some particularfields for low-weight constructions. Its high resistance to bothgeneral and localized corrosion due to the high chromium,

* Corresponding author at: Institute of Metals and Technology, Surface

Engineering and Applied Surface Science, Lepi pot 11, SI-1000 Ljubljana, Slovenia.

Tel.: +386 1 4701 955; fax: +386 1 4701 928.

E-mail address: crtomir.donik@imt.si (C. Donik).

0169-4332/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2009.03.041

molybdenum and nitrogen contents, and especially its highresistance to pitting and crevice corrosion in chloride-containingmedia, extends its applications to pollution control, chemicalvessels, and the off-shore oil and gas industries, etc. It also has highcorrosion and erosion-fatigue properties as well as a lower thermalexpansion and a higher thermal conductivity than austenitic. Itsyield strength is about twice that of austenitic stainless steels,which allows the designer to save weight and makes the alloy morecost competitive than 316 L or 317 L. Alloy 2205 is particularlysuitable for applications covering the temperature range from�50 8C to 300 8C. Temperatures outside this range may also beconsidered, but there are some restrictions, particularly for weldedstructures [1–3].

Due to the mechanical and corrosion properties, which result ina weight reduction for constructions, thus contributing to adecrease in the total costs, and the widespread and versatile use ofthe alloy, duplex stainless steel is gradually displacing the stainlesssteels of the AISI 300 series.

The pickling of duplex stainless steel has proven to be muchmore difficult than that of the standard austenitic grade (AISI 300series). There is no complete agreement in the literature on thescale (high-temperature oxidation) dissolution mechanism inneutral pickling solutions [4–6]. During annealing, duplex stainlesssteel is heated in an annealing furnace up to 1050 8C and is kept atthis temperature for some time to soften the metal in order torelease the work hardening induced by the hot and cold rolling. Theelimination of the surface defects by the formation of an oxidescale is required to improve the corrosion resistance. The native

C. Donik et al. / Applied Surface Science 255 (2009) 7056–7061 7057

oxide layer on stainless steel is typically 1–3 nm thick. Its structureand chemical composition affect reactivity, adhesion of the surface,and thus the corrosion resistance. Insight into nano- and atomic-scale oxidation and segregation phenomena on alloy surfaceremains rather limited despite their fundamental and technolo-gical importance. Surface analytical studies related to thisprocesses on stainless steels have been conducted predominantlyex situ or of the initial oxidation of FeCr and FeCrNi single crystals[7,8] and simple polycrystalline alloys [9,10]. Recent articles alsopresent the investigation of austenitic stainless steel and duplexstainless steels low temperature oxidation [11,12]. These previousstudies conclude that the oxide films formed at low temperatureand oxygen pressure consist of mixed Fe–Cr oxides, and are duplexin nature, with an outer Fe-rich spinel covering an innerrhombohedral Cr2O3 layer [12–17].

The aim of the present study was to investigate the initialphases of oxide growth on duplex stainless steel. Thin oxide layerswere produced in several different ways: by the controlledexposure of polished and sputter-cleaned duplex stainless-steelsamples to approximately 10�5 mbar of oxygen, by the exposure ofsamples to ambient conditions (normal humidity and air pressure)for 24 h, and by applying oxide plasma oxidation at atmosphericpressure. X-ray photoelectron spectroscopy (XPS) profiles weremeasured for the oxide films formed on the surface. On thethinnest oxide layers a type of simplified ARXPS surface analysiswas performed to corroborate the thickness estimates [13,18–24].

2. Experimental

The bulk composition of the investigated material as deter-mined by analytical chemical methods is shown in Table 1. Thevalues are consistent with Duplex 2205 [1].

The samples were mounted in epoxy resin, ground with emerypapers up to No. 4000, and then polished with 1-mm diamondpaste. Samples of approximately 8 mm � 8 mm � 1 mm wereprepared. These samples were fixed onto sample holders for theAES/XPS investigations by means of UHV-compatible double-sidedsticky tape. The sputter cleaning of the samples was performedunder UHV conditions inside the main vacuum chamber of theAES/XPS apparatus, to which the samples had been introduced viaa fast-entry air lock. As we observed the absence of the oxygenpeak in the XPS spectra (at 530 eV binding energy) the sputtercleaning was complete. Thin oxide layers were produced on thesamples’ surfaces by exposing the sputter-cleaned samples toapproximately 10�5 mbar of oxygen inside the fast-entry chamber,which had been previously pumped down to 10�8 mbar. Thus, anapproximately 99.9% oxygen atmosphere was ensured in the fast-entry chamber. The exposure time was 10 min, corresponding toan exposure of several tens of Langmuirs (1 L = 10�6 Torr � s).

Another type of oxidation used in the study was exposing thepolished and sputter-cleaned sample to the ambient conditions(normal atmospheric pressure, 60% humidity, 20 8C) for 24 h, andthen reintroducing it into the AES/XPS apparatus for the analysis.

The third type of oxidation used in our study was performed in aplasma passivation chamber with an oxygen plasma at roomtemperature. An RF generator with an output power of about250 W and a frequency of 27.12 MHz was used. The electrontemperature and the ion density were estimated using a doubleLangmuir probe, which was immersed in the plasma. The electron

Table 1Chemical composition of the duplex stainless steel (at.%).

Cr Ni Mn Si P S C Mo N Fe

23.96 4.82 1.48 0.85 0.05 0.00 0.13 1.83 0.67 66.21

temperature was about 5 eV, whereas the ion density depended onthe pressure and was between 5 � 1015 m�3 and 10 � 1015 m�3.

As the RF generator was turned on the temperature of the DSSstarted rising due to the catalytic recombination of oxygen atomson its surface. The temperature of the material was measuredsimultaneously. As soon as the RF generator was turned off thetemperature decreased and dropped close to the room tempera-ture (approximately 22 8C) in several seconds. The maximumtemperature is dependent on the pressure inside the chamber.

The XPS depth profiles of the oxide layers were measured with aVG Microlab 310F SEM/AES/XPS. For all the XPS measurements MgKa radiation at 1253.6 eV with anode voltage � emission curren-t = 12.5 kV � 16 mA = 200 W power was used. The Ar+ energy of3 keV for a 1-mA ion current over an 8 mm � 8 mm area wassimilar to the ion-beam parameters used for the pre-oxidationsputter cleaning of the sample, apart from the raster area beingcloser to 10 mm � 10 mm in this case. This is not inconsistent withsome calibration measurements performed on metallic and oxide-type samples as well as with some reference data for sputteringrates for Fe and Cr and their oxides [19,20,25–30].

The spectra were acquired using the Avantage 3.41v dataacquisition & analysis software supplied by the manufacturer. Thethickness of the passive layer was estimated by argon-ionsputtering for different time intervals and other non-destructivetechniques Thickogram and ARXPS [22–24]. The referencesputtering rate was 0.01 nm/s, and this was calculated relativeto Ta2O5 as the intensity of the oxygen signal decreased to a half[31,32]. Casa XPS software [33] was used to process the data. TheXPS peaks were separated into contributions from the differentspecies, which were quantified separately. The Shirley background[34], the Scoffield cross-sections [35–37] and the standard peakparameters [35,37] were used in the evaluation and quantificationof the spectra. The components binding energy differences werefixed at values from the literature and the full with at halfmaximum (FWHM) for the oxide peaks were fixed to be the same.The FWHM used for the metallic component was taken from the Crand Fe peak measured after surface oxides were removed. Thethree and five oxides peaks, respectively, were expected to bepresent based on previous works [10,14,38–42].

3. Results and discussion

Fig. 1 shows high-resolution XPS scans of the Cr 2p3/2 and Fe2p3/2 transitions with the corresponding metallic and oxidecomponents.

Fig. 2 shows both the oxide and metallic components for the Cr2p3/2 and Cr 2p1/2 transitions inside the oxide layer, but onlymetallic components remain inside the bulk of the material. Thesetypes of spectra sets were used to construct the depth profiles.

Fig. 3a shows an XPS depth profile of the oxide layer after theoxidation of the 2205 DSS with the 90 L oxygen exposure in thepreparation chamber. The relative atomic concentrations (where100% corresponds to a total content of metals) of the metallic andoxidized species of a particular metal after consecutive sputteringcycles are used in the depth profiles. The stoichiometry of the oxideis approximate, at best, due to the ion-sputtering-induced effects,such as the reduction of the oxidation state [16,31,38–40].

XPS depth profiles were also measured on the 2205 DSS aftermuch longer oxygen exposures, up to 10,800 L (Fig. 3b), withoutobtaining substantially different results. This suggested that anincrease in the pressure and/or different atmospheric conditions,e.g., humidity were necessary to produce a thicker oxide layer[8,10,41–43].

The profiles in Fig. 3a and b show that these oxide layers are ill-defined, with slowly changing concentrations of oxidized andunoxidized metals from the surface of the oxide layer towards the

Fig. 1. Examples of fitted Cr 2p3/2 and Fe 2p3/2 XPS spectra.

Fig. 2. Set of high-resolution XPS spectra of Cr 2p transitions as used in deriving the

depth profiles in Fig 3.

C. Donik et al. / Applied Surface Science 255 (2009) 7056–70617058

bulk of the sample. Also, large fractions of unoxidized metals arepresent at the very surface of the oxide layers. These concentra-tions may be partially overestimated, since the oxide-layerthickness estimations obtained from the profiles in Fig. 3a and bare 2–3 nm, which is of the order of the probing depth of the XPSused for the analysis, so there is a possibility of unoxidized metalsfrom the bulk influencing the measurement. Even in these poorlydefined oxide layers a two-oxide structure can already be resolved:a larger iron-oxide concentration at, and close to, the surface, withchromium oxide being the predominant oxide more in-depth,towards the layer–bulk interface. This can also be seen in the Feox/Crox entries in Table 2.

Fig. 3c shows an XPS depth profile of the DSS 2205 oxidizedunder ambient conditions for 24 h. A thicker layer, estimated to be14–15 nm, was obtained. The oxide layer and the layer–bulkinterface are better defined than in the samples oxidized in oxygen.The two-oxide structure of the oxide layer is well developed andthe relative concentrations of unoxidized metals at the surface aremuch lower than in the oxygen oxidized samples. This can also beseen by comparing the corresponding entries for the Feox/Femet,Crox/Crmet and (Feox + Crox)/(Femet + Crmet) relative atomic concen-tration ratios in Table 2.

Fig. 3d shows an XPS depth profile of the plasma oxidized DSS.The oxidized Cr and Fe profiles are rather similar to the sampleoxidized for 24 h under ambient conditions (Fig. 3c). The maindifference is the absence of the unoxidized Cr and Fe species at thetop of the oxide layer. The formation of an oxide film, with an innerregion essentially formed of chromium oxide and an outer regionmainly consisting of iron oxide, was also observed as in all theother oxide layers. Some numerical parameters for these oxidelayers are given in Table 2, where it can be seen that while thisoxide layer is broadly similar to the one prepared with 24 h ofambient oxidation, some features are much enhanced: theconcentration of the unoxidized metal species in the topmostpart of the oxide layer is approximately 200 times lower than theconcentration of their oxidized counterparts, while there isvirtually no unoxidized iron in this stratum of the oxide layer.

Mo and Ni profiles were also measured for all types of samplesand are shown in Fig. 3a–d. While roughly corresponding to theirbulk values for DSS 2205 inside the substrate, Ni concentrationseems to increase slightly towards the interface and to decreasetowards concentrations comparable to the detectability limittowards the top of the oxide layer. Since this was observed inthinner (Fig. 3a and b) as well as in thicker (Fig. 3c and d) oxidelayers, it is possible that there is actual Ni present in the oxide layer

and not only attenuated substrate Ni signal observed through theoxide layer. Mo has been reported to increase corrosion resistanceof stainless steels, especially in synergy with nitrogen [44–46]. Animportant corrosion inhibition mechanism proposed for Mo isformation of Mo oxides in oxide layers [44].

Fig. 4 shows the ratios for the total (unoxidized + oxidizedspecies) Fe and Cr concentrations. The ratio c(Fe)/c(Cr) = 2.98 forthe untreated DSS 2205 is also shown in Fig. 4. Again, the similarnature of the oxide layers produced by the 90 L and 10,800 Loxygen exposures can be observed, as well as the double-oxidestratification of both and the Cr-enrichment of the in-depth regionsof the layers. Very similar shapes of Fe/Cr curves were obtained foroxide layers produced by 24 h of ambient exposure and by plasmaoxidation. They also exhibit double-oxide stratification and Cr-enrichment, much more pronounced than for the first twosamples. The increased thickness of the samples can be easilyobserved from the curves, as can the Cr-depleted region of the bulk,after the oxide-layer–bulk interface, where c(Fe)/c(Cr) approaches4. These large Cr-depleted regions are a counterpart to the large Cr-enriched regions in the oxide layers, which are thick compared tothe oxide layers produced by oxygen exposure. An additionaldistinctive factor in the formation of the oxide layers by theoxygen-exposure procedures could be a very low partial pressureof water vapour, thus hindering the formation of the FeOOH and

Fig. 3. XPS depth profile at 90 L exposure in the preparation chamber (a), at 10,800 L exposure in the preparation chamber (b), of the sample oxidized in ambient conditions for

24 h (c) and of the sample oxidized in plasma (d).

C. Donik et al. / Applied Surface Science 255 (2009) 7056–7061 7059

Fe2O3, which are the main species formed on stainless steel undernormal conditions [16,39,42,47–49]. Therefore, while an iron oxideat the surface, chromium oxide inside, type of stratification canalso be observed in these samples it is much less pronounced thanin the samples prepared by oxidation in ambient conditions or inplasma.

To corroborate the thickness estimations for the oxygen-exposure-produced oxide layers based on sputtering times andsputtering rate, the Thickogram method [24] for evaluating thethickness of the thin layers from the XPS measurements of the

Fig. 4. Fe/Cr atomic ratios calculated from XPS depth profiles for oxide layers

prepared by 90 L exposure in the preparation chamber, 10,800 L exposure in the

preparation chamber, oxidation in ambient conditions for 24 h and plasma

oxidation. Horizontal line at 2.98 represents calculated Fe/Cr atomic ratio in bulk

DSS 2205.

peaks from the layer and the substrate at a controlled tilt (thusdetermining the emission angle) of the sample was used [46]. The(d/l) values, where d is the is the oxide-layer thickness and l is theattenuation length, obtained by the Thickogram calculationmethod at different emission angles u are presented in Table 3.Indices 1–3 refer to different pairs of overlayer/substrate peaks: 1 –O 1s/Cr 2p3/2 metallic, 2 – Cr 2p3/2 oxide/Cr 2p3/2 metallic and 3 – Fe2p3/2 oxide/Fe 2p3/2 metallic. The average values, hd/li, are

Table 2Femet/Crmet, Feox/Crox, Feox/Femet, Crox/Crmet, and (Feox + Crox)/(Femet + Crmet)

relative atomic concentration ratios at the surface of the oxide layer, inside the

oxide layer and in the substrate bulk. ‘‘Inside’’ is defined as the depth of the

maximum oxidized chromium concentration.

90 L 10,800 L Air 24 h Plasma

Femet/Crmet

Surface 1.94 1.61 1.04 0.00

Inside 2.46 3.08 3.63 1.85

Bulk 2.98 2.61 4.37 3.68

Feox/Crox

Surface 2.06 1.77 3.78 2.62

Inside 0.68 0.37 0.64 0.87

Bulk 0.00 0.00 0.00 0.48

Feox/Femet

Surface 0.90 0.97 11.22 !1Inside 0.39 0.20 0.73 2.37

Bulk 0.00 0.00 0.00 0.01

Crox/Crmet

Surface 0.85 0.89 3.07 55.6

Inside 1.63 1.66 4.16 5.06

Bulk 0.03 0.03 0.10 0.07

(Feox + Crox)/(Femet + Crmet)

Surface 0.88 0.94 7.22 �200

Inside 0.71 0.56 1.47 3.32

Bulk 0.01 0.01 0.02 0.02

Table 3(d/l) values for different emission angles u obtained with the Thickogram

calculation method.

u (8) (d/l)1 (d/l)2 (d/l)3

0 1.8 2.2 1.2

15 1.8 1.7 1.5

30 1.5 1.8 1.3

45 1.4 1.7 1.4

60 1.3 1.4 1.1

75 0.6 0.8 0.6

hd/li0–608 1.6 1.8 1.3

C. Donik et al. / Applied Surface Science 255 (2009) 7056–70617060

calculated over the u range 0–608 only, since this is theapplicability range of the method. In all cases they are close tothe values measured at u = 458, where the Thickogram method ismost accurate. It should be noted that for 2 and 3 a simplercalculation method than the Thickogram could be employed, sincethe overlayer and the substrate peaks in these cases are at nearlyidentical energies, which makes the corresponding attenuationlengths practically the same [34,36]. The attenuation lengths, l, for1–3 inside the oxide layer were estimated with the CS2 [37]equation as 2.5 nm, 1.7 nm and 1.4 nm, respectively, thus givingestimates (using u = 458 values) for the oxide-layer thickness, d, of3.5 nm, 3.1 nm and 1.8 nm.

4. Conclusions

Two different classes of oxide layers were obtained. The firstis very thin, of the order of 2–3 nm, obtained with a 10�5 mbaroxygen exposure, independent of the total exposure time, evenwhen the times differ by two orders of magnitude. XPS spectrafrom these thin oxide films allow us to use Thickogram foradditional calculations of the thickness of thin oxide films whichcorroborate the thickness measured with XPS depth profiles. Thesecond is of the order of 14–15 nm, obtained by exposure inambient conditions or plasma oxidation. A more detailedanalysis shows that despite the broad similarities betweenthe ambient- and plasma-oxidized samples there are alsodifferences in certain numerical parameters (like the ratios ofunoxidized versus oxidized iron, or metal in general) that allowus to denote the plasma-oxidized sample as more ‘‘thoroughly’’oxidized. A common characteristic of all the oxide layersinvestigated is a double-oxide stratification, with regions closerto the surface exhibiting higher concentrations of iron oxide andthose more in-depth exhibiting higher concentrations ofchromium oxide. This is less developed but still clearlyobservable in the oxygen-exposure-produced oxide layers, butis very pronounced in the thicker layers. One is tempted to viewthe oxygen-exposure-produced layers as a good model for theearliest phases of any oxidation. To investigate this advancedexposure a chamber with the possibility of raising exposurepressure into mbars could be used. Another possibility forfurther studies is to use ARXPS for more accurate depth profilingof the thinnest oxide layers.

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