Positron beam and RBS studies of thermally grown oxide films on stainless steel grade 304

Applied Surface Science 333 (2015) 96–103

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Positron beam and RBS studies of thermally grown oxide films on stainless steel grade 304 P. Horodek a,b,∗ , K. Siemek b , A.G. Kobets a,c , M. Kulik a,d , I.N. Meshkov a a

Joint Institute for Nuclear Research, Joliot-Curie 6, 141980 Dubna, Moscow Region, Russia Institute of Nuclear Physics PAS, E. Radzikowskiego 152, 31-342 Krakow, Poland Institute of Electrophysics and Radiation Technologies NAS of Ukraine, Chernyshevsky St. 28, 61002 Kharkov, Ukraine d Institute of Physics, Maria Curie-Skłodowska University, Pl. Marii Curie-Sklodowskiej 1, 20-031 Lublin, Poland b c

a r t i c l e

i n f o

Article history: Received 4 September 2014 Received in revised form 27 December 2014 Accepted 18 January 2015 Available online 7 February 2015 Keywords: Positron beam Oxidation Annealing Stainless steel

a b s t r a c t The formation of oxide films on surfaces of stainless steel 304 AISI annealed at 800 ◦ C in vacuum, air and in flow N2 atmospheres was studied using variable energy positron beam technique (VEP) and Rutherford backscattering/nuclear reaction (RBS/NR) methods. In frame of these studies, Doppler broadening of annihilation line (DB) measurements were performed. For a sample heated in vacuum the oxide film ca. 8 nm is observed. For specimens oxidized in air and N2 the multi-layered oxide films of about a few hundred nanometers are recognized. The RBS/NR measurements have shown that the sample annealed in vacuum contains a lower quantity of oxygen while for samples heated in the air and N2 non-linear and rather linear time-dependency are observed, respectively. The thicknesses of total oxide films obtained from RBS/NR tests are in good agreement with the VEP results. Time evolution of the oxide growing was studied as well. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Austenitic stainless steel (ASS) is a material applied in many fields due to the wide range of useful properties such as corrosion resistance, formability etc. It is characterized by a high level of Cr (about 18 wt%) and 8 wt% of Ni. These additions improve anticorrosion properties of this group of stainless steels (SS). The presence of Cr on the level of 13 wt% in SS causes the saltatory variation of electrochemical potential and this is a broad lower limit for higher corrosion resistance [1]. Generally, chromium contained in ASS reacts with oxygen and as a result a thin passive layer of Cr2 O3 for protection from corrosion is formed on the surface. Protective properties of this layer deteriorate in higher temperature conditions or aggressive environments where the diffusion of iron and oxygen takes place. In this way the multi-layer system of oxygen with different chemistry and structure covers the surface. The formation of this oxide system, which depends on environment and temperature, is accompanied by stresses and appearing of vacancies leading to creation of cracks.

∗ Corresponding author at: Institute of Nuclear Physics PAS, E. Radzikowskiego 152, 31-342 Krakow, Poland. Tel.: +48 12-662-83-70. E-mail address: [email protected] (P. Horodek). http://dx.doi.org/10.1016/j.apsusc.2015.01.225 0169-4332/© 2015 Elsevier B.V. All rights reserved.

The problem of formation of oxide systems on the surface of stainless steel is still open, even though many studies using different methods were performed. It is known that the colour of films formed on SS depends on the temperature [2] or environment e.g. acid solutions [3]. The review of passive layer formed in aqueous conditions has been reported by Jin and Atrens [4]. They collected the literature data concerning the passive layer in SS formed in various conditions and obtained using different methods. They pointed out that the passive film includes different components and consists of a layer containing bound water or oxyhydroxide, oxide and metallic layers, which contact the interior of the alloy sample. A thin layer of hydroxide creates the outermost part of the oxide film in aqueous environments. Fe and Cr oxides form the major oxide products on the surface in various media. The thickness of passive layer is around 0.5–4 nm, decreases with increasing Cr concentration and it is related to the total oxidized Cr content on the surface. Additionally, it depends on the surface treatment method [5]. Using Conversion Electron Mössbauer Spectroscopy (CEMS) and Auger Electron Spectrometry (AES) Nomura and Ujihira [6] investigated the oxide films formed on the surface of ASS 304 annealed for 1 h at 400, 500, 600, 700 and 800 ◦ C. They estimated the thicknesses of passive layers between 20 nm and 50 nm. They also observed the oxide films consist of large particles of hematite ␣-Fe2 O3 in the outer layer which covers the spinel mixed NiO(Cr, Fe)2 03 in the inner area. Langevoort et al. [7] performed X-ray

P. Horodek et al. / Applied Surface Science 333 (2015) 96–103

photoelectron spectroscopy (XPS) studies of samples of ASS 304 oxidized at pressures 10−6 –10−4 Pa and temperatures 27–527 ◦ C. The iron chromium was observed at the top of chromium oxide layer. At temperatures above 377 ◦ C the surface became rich in chromium, which preferentially oxidized at these conditions. Also Briggs and Seah [8] showed the formation of duplex oxide on the surface of SS, where ␣-Fe2 O3 was created in outermost layer before the inner oxide an iron-chromium. Green and Higginson [9] observed the differences in the oxide morphology of samples ASS 304 and 316L with ground (240 grit) and polished (1 ␮m) surface finishes and annealed at 750 ◦ C and 800 ◦ C for times of up to 24 h. On the ground surfaces the homogeneous film of oxides was produced. Due to the fast diffusion of chromium through grain boundaries and prior ferrite regions, the oxide islands appeared on the surface of polished specimens. Additionally, they gave a schematic diagram of the mechanisms of early oxide growth which is in agreement with Lloyd et al.’s [10] results according to which together with increasing time chromium can diffuse to the surface from the deeper volume of SS. The studies of oxidation of materials in different conditions are usually performed using electron microscopy, CEMS, AES and XPS. In this paper, we want to show that VEP can be also applied to problems connected with the corrosion of SS. Wu et al. [11,12] performed VEP studies of pure Fe and ASS 304 corroded for 1 h with various anodic potentials at room temperature. The corroded iron introduced both large-size defects (e.g. voids) as well as small-size ones, while in the case of SS only the small-size (vacancy-type) defects were recognized. In their experiment samples were oxidized in aggressive environments, whereas we intend to focus our studies around the influence of annealing time on the range of oxidation. The subjects of studies are samples of 304 AISI SS annealed in vacuum, air and in flow of N2 atmospheres in different times. Respect to results obtained by other methods [2–9] we confirmed the creation of multilayered oxide systems on the surfaces of samples heated in air and N2 . In comparison to studies mentioned above (chosen as the representative literature) the time dependency of growing oxide films for this kind of ASS and in these annealing times, as well as atmosphere conditions were not performed. Analysis of S parameter profiles obtained from VEP measurements allowed to get some characteristic about defects inside oxide films, thicknesses of layers and their evolution in the time. At the end, we applied Rutherford backscattering spectroscopy/nuclear reaction (RBS/NR) for evaluation of the thickness of the oxide films and content of oxygen atoms in the studied depths. 2. Materials and methods 2.1. Sample preparation Samples of ASS 304 AISI containing 0.04% C, 1% Si, 2.0% Mn, 17.0% Cr, and 9% Ni were prepared in the form of discs 20 mm in diameter and 2 mm thick. The surfaces were sequentially polished – first using silicon carbide waterproof abrasive paper with gradation in turn 500, 1000, 1200 and 2500 to remove any roughnesses, and next using polishing machine Tecmet 2000-MP21 V on the polishing cloth to receive the surface without visible scratches. After that, one sample was annealed in the furnace for 2 h at 800 ◦ C in vacuum 6 × 10−6 mbar. Then 2 groups consisting of 4 samples were heated in the same temperature for 2, 3, 4 and 8 h in air and in flow of 0.35 cm3 /s of N2 atmospheres. After annealing all samples were left in the closed furnace to cool down slowly to room temperature. The surfaces of samples oxidized in air and N2 were covered in different scale by a visible layer of oxides. The set of 9 samples prepared in the way described above were mounted in the holder for VEP measurements. Then they were placed in the chamber for RBS tests.

97

2.2. PAS technique PAS (positron annihilation spectroscopy) is a sensitive method for detection of structural defects at the atomic scale such as vacancies, vacancy clusters, dislocations, and nanometer-scale voids [13,14]. The basis of this technique is positron-electron annihilation process. Positron e+ (as the antiparticle of electron e− ) after implantation into the matter slows down to the thermal energy during ∼1 ps, diffuses and annihilates with a random electron. During this process masses, energy and momentum of participating particles are converted into energy and momentum of photons, and in most cases two quanta with energy ca. 511 keV are emitted. Detection of annihilation rays can be applied for recognition of the state of annihilating positron as free in a defect free structure or bound in the case of annihilation in a lattice imperfection such as vacancy or dislocation. There are mainly three experimental techniques within the framework of PAS: angular correlation of emitted gamma quanta, positron lifetimes and the Doppler broadening (DB) of annihilation line measurements. In our studies we applied the last method, which is based on the detection of annihilation line 511 keV which is Doppler broadened due to the motion of annihilating pair. It results mainly in a momentum of electrons which annihilate with positrons. Low momentum electrons occupy vacancies or other open volume defects. Positron annihilation with such electrons or at vacancies results in a less broadened line than with high momentum electrons present in core or interstitial regions. This method can determine defect concentration in the studied material. More details are reported here [15–17]. In practice, the analysis of annihilation line brings down to evaluation of two important parameters. The first one, called S parameter, is defined as the integral of central part of the annihilation line normalized to total integral of the line centred at 511 keV. It is experimentally proven that this parameter is extremely sensitive to the presence of open volume defects, like a vacancy or its clusters and jogs at dislocation line, where positrons are localized. The second parameter, namely W parameter, is calculated as the ratio of the area under wing part of annihilation line to the whole area under the entire annihilation line. Both W and S parameters provide information about the presence of one kind or more kinds of defects and are commonly used in positron beam technique (VEP). In this paper we present results obtained from slow positron beam working at LEPTA facility [18,19] at Joint Institute for Nuclear Research in Dubna, Russia. Nowadays, positron beams offer the possibility to detect changes closer to surface in comparison to classic measurements based on positrons emitted directly from a radioactive source [20]. Varying the energy one can locate positrons on the given depth from the surface. We used a slow positron flux (3 mm in diameter) where positrons were emitted from 25mCi activity of 22 Na source and moderated onto frozen neon (7 K) under the pressure 10−9 Torr. The intensity of flux was about 3 × 105 e+ /s. Moderated positrons were accelerated to the demanded energy within the range between 50 eV and 35 keV. The annihilation gamma quanta were registered by HpGe detector with the energy resolution of 1.2 keV at 511 keV. 2.3. RBS technique The Rutherford backscattering spectrometry (RBS) is a method widely used in surface analysis. Its principle is based on the elastic two-body collision. It is assumed that a particle of nucleus mass m1 (proton or ␣ particle), moving with the constant velocity, scatters elastically with the stationary particle of mass m2 . The kinetic energy of mass m1 is larger than the binding energy of atoms in the target. Conservation of kinetic energy and conservation of

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momentum can describe the interaction between two bodies. For m1 < m2 the ratio of the projective kinematic energy after elastic collision E1 to the one before Eo is defined as the kinematic factor K [21]. Its value depends only on the masses of the colliding bodies and the scattering angle . It is expressed by the formula: m22 − m21 sin2 

1/2

+ m1 cos 

m1 + m2

.

(1)

R (E) ∝ (Z1 Z2 )2

m22

− m21

2

sin 



1/2

+ m1 cos 

m2 sin4  m22 − m21 sin2 

1/2

2

,

(2)

where Z1 and Z2 are the atomic numbers of incidence particles and target’s atom. The value of R (E) for light elements is less than for atoms with heavier nucleons. The method of Rutherford backscattering spectrometry and nuclear reaction (RBS/NR) is used to increase the sensitivity of measurements. In this situation the energy of beam of the incident particles’ increase. Detailed description of RBS and RBS/NR methods is presented in [21]. Similarly to the works [22], in this paper all results have been obtained using the O16 (␣,␣)O16 nuclear reaction [23]. The RBS/NR measurements of studied samples were performed using ␣ particles with energy range between 3.035 MeV and 3.060 MeV. The angle between normal to the surface of the samples and the incidence beam was in the range from 30◦ to 75◦ . The energy resolution of used detector was 15 keV. The concentrations of the oxygen atoms were calculated with the help of SIMNRA code [24]. 3. Results and discussion 3.1. PAS results The S parameter on the function of positron incident energy is presented in Fig. 1. The black circles represent the dependence for the ASS sample, called a reference sample (RF) which was annealed for 2 h in the vacuum 6 × 10−6 mbar before VEP measurements. The S parameter for RF decreases with positron implantation energy and in the region of higher energies saturation occurs. This is a typical dependence which is observed for SS in VEP investigations [11,25]. Other points represent measurements of the ASS samples annealed for different times in air (Fig. 1a) and N2 atmosphere (Fig. 1b). The white squares come from the samples heated for 2 h,

0.17

0.53

1.06 reference 2h 3 h Air 8h

0.54

2

In this way mass m2 can be calculated and hence nucleus can be identified. It is also assumed that one particle collides with one nucleon while it is entering and going out, and loses energy on the interaction with electric fields. The particles of the incident beam directed on the sample’s surface penetrate the region near the surface. The electric field of these incidence charges interacts with the one formed by electrons and nuclei of target’s atoms. The energy of a  penetrating leaving particle is lost and its kinetic energy is Eo < Eo . Next collision takes place between two bodies and the energy of scattered particles is lower than E1 . The energy loss of leaving particles is due to both the transfer of energy during collision as well as the process of interaction and braking fields formed by electrons along with nuclei of target’s atoms. The number of backscattered particles is proportional to the number of incident particles multiplied by the solid angle of the detector, the differential cross-section connected with the mean energy  (E) and concentration of atoms in target. It also depends on the incident angle of the beam. If the energy of incident particles is lower than 3 MeV, then  (E) is described with the equation of Rutherford R (E). It has the following expression:



0

S parameter



estimated positron implantation range [µm]

0.56

0.52

0.50

0.48

0.46

0.44 0

10

20

30

40

positron energy [keV] b)

estimated positron implantation range [µm] 0

0.17

0.53

1.06

0.56 reference 2h 3h N 2 8h

0.54

S parameter

E1 K= = E0

a)

0.52

0.50

0.48

0.46

0.44 0

10

20

30

40

positron energy [keV] Fig. 1. The dependency of S parameter on positron implantation energy obtained in VEP experiment for four samples of ASS 304 AISI: the RF sample annealed for 2 h in the vacuum 6 × 10−6 mbar (black circles) and three samples heated in the air (a) and N2 (b) for 2 h (white squares), 3 h (grey triangles) and 8 h (dark grey diamonds). The solid lines present the best fits of experimental data points using VEPFIT code. On the top axis the average positron depth is presented.

grey triangles for 3 h and dark grey diamonds for 8 h. The profiles for samples annealed for 4 h were not shown in order not to make the readability of dependencies worse. There are well visible differences between S parameter profiles for RF and samples heated in different atmospheres. Generally, the S parameter for specimens annealed in air and N2 declines with positron implantation energy, then the region of increasing S parameter occurs and at the end for higher energies decreases as for RF. In this way two energy ranges are well marked. The first one appears in the range of significant dissimilarities, while the second one is found in the zone of S parameter on the similar level as for the RF sample. This behaviour can be explained as the formation of multilayer system of oxides on the surface of a sample as a result of heating in air or N2 . In this way, the values of S parameter in the first region, which is characterized by other chemical composition and consists of defects, are different in comparison to the one region which is substrate – ASS. In Fig. 1a, where the S parameter profiles for RF and annealed samples in air are compared, it is well visible that the location of minimum and maximum in the oxide films depends on the duration of annealing and moves in the direction of higher positron

P. Horodek et al. / Applied Surface Science 333 (2015) 96–103

(3)

and taking into consideration the density of ASS equal 7.8 g/cm3 as well as the energy E range of implanted positron between 0.05 keV and 35 keV, the mean investigated depth can be estimated to be from 0.02 nm to 1.38 ␮m. The axis of approximated positron implantation depths was placed at the top of figures. The aim of the analysis was getting the approximated thickness of oxide layer and the positron diffusion length given as L+ =

3

4

5

6

7

8

9

AIR N2

L+ [nm]

16 12 8 4 0

b) thickness of the 1st layer [nm]

80 60 40 20 0

c)

1000 800 600 400 200 0

A n E 



2

20

D+ bulk , 1 + bulk C

(4)

where D+ is positron diffusion coefficient,  bulk – positron life time at nondefected structure, C – defect concentration and  is the positron-trapping coefficient at a given defect. The L+ will be shorter for positrons trapped and annihilated in the defect in comparison to L+ for almost defect free RF sample. In the case of RF a two-layer model containing oxide film and the bulk was used during fitting in VEPFIT. The positron diffusion length in the 8 ± 1 nm thick oxide layer is 4.2 ± 0.9 nm, while in the bulk it is about 94 ± 3 nm. Qiang-mao et al. [28] evaluated L+ in A 508-3 steel on the level of 100 nm. Wu and Jean [29] found that the thickness of the oxide film for 304 and 316 stainless steels is 20 nm and 5 nm and positron diffusion length is 11 nm and 5 nm, respectively. Positron diffusion length in the bulk for 304 and 316 steels was found to be 78 nm and 59 nm. Thus, the obtained values correspond to these reported by other authors. It should be emphasized, that according to the literature [6–8], under the conditions of high temperature the multi-oxide film consists of ␣-Fe2 O3 at the top of the layer and spinel containing mainly Cr2 O3 and other kinds of Fe, Cr, Ni oxides. Changes in thickness, structure and composition of oxide film during heating could not apply the two-layer model for fitting the profiles of samples annealed in different atmospheres. At least four-layers model should be taken into account. The first layer corresponded to the ␣-Fe2 O3 , the second and third ones to spinel and the last one to the bulk. In such a case the fitted dependencies reproduce the experimental one. Because we are not able to recognize individual phases, we intend to focus on giving the diffusion length and the thickness of the first layer (in guess hematite) and the total thickness of the

d) total thickness of the oxide layer [nm]

z¯ =

time [h] 1

a)

thickness of the spinel [nm]

implantation energies it means deeper from the surface. Fig. 1b shows samples heated in flow of 0.35 cm3 /s of N2 atmosphere as well as the RF one. In comparison with the specimens annealed in air, here the behaviour of the S parameter is similar. However, the profiles are flatter, i.e. the differences between the minimum and maximum of each sample are smaller both within the samples themselves and in comparison with the other ones. It is well known that the presence points defects in a sample induces the increase of the S-parameter value, because the increase of annihilation with low momentum at open volume defects. Then it is expected that the S(E) profile for samples with defects would be located above for the reference, only with residual defects sample. This is in metals and SS. However, the presence of oxides layers on the surface of the samples involves annihilation with oxygen atoms and additional defects in this layer for which the value of the Sparameter is different than in SS. In this way values of S parameter for oxidized samples are lower at the beginning of the profile. The description of profiles presented in Fig. 1 as well as those gained after annealing for 4 h in air and N2 atmospheres (invisible in Fig. 1) were performed using VEPFIT code [26]. The best fits for all samples are shown with solid black lines. The fitting procedures were performed with the Makhov function with parameters for pure iron: m = 1.766, n = 1.692 and A = 2.62 ␮g cm2 keVn which defines the positron implantation profile [27]. Using

99

1000 800 600 400 200 0 1

2

3

4

5

6

7

8

9

time [h] Fig. 2. Positron diffusion length L+ in the 1st layer (a) and thicknesses of the 1st layer (b), spinel (c), total oxide film (d) obtained from VEPFIT code in dependency on annealing time obtained from fitting experimental points. The black points represent samples heated in the air, while the white ones – N2 .

oxide layer determined from the VEPFIT code. These values can be obtained directly from VEPFIT programme which gives the boundaries of each layer. The thickness of spinel was evaluated as the difference between the boundary of first and third layer. The aim of dividing of spinel into two layers was getting a better fit. For the reason of complicated structure the deeper analysis was not performed. Obtained thicknesses of total oxide films we can compare with the RBS studies. The values of parameters as the function of annealing time are presented in Fig. 2. In Fig. 2a the positron diffusion length in the outer layer versus annealing time for samples oxidized in different atmospheres is presented. In both cases L+ decreases with heating time but there are differences between these groups of specimens. For samples annealed in air decreasing of diffusion length is rather linear. 2 h-annealing in N2 atmosphere gives higher value of L+ in comparison to the sample heated in air. Longer time of annealing makes positron diffusion length obtained for specimens annealed in N2 much shorter in confrontation with air. Additionally, in this case the linear dependency does not appear. In the case of samples annealed in air the positron diffusion lengths are in the range between 47 and 56 nm in the second layer, between 48 nm and 69 nm in the third one and for samples oxidized in N2 between 34 nm and 44 nm and 68 nm and 80 nm, respectively. The deviation from all values was on the level of 15%. Additionally, any visible tendencies in changes of L+ between annealing time were not observed for this reason they are not depicted in the figure.

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b) 0.54

0.54

0.52

0.52

0.50

surface

oxides

bulk

0.48

0.46

S parameter

S parameter

a)

surface

0.005

0.010

0.015

oxides

bulk

0.50

0.48

0.46

AIR

RF 2h 3h 8h

N2

0.005

0.020

0.010

0.015

0.020

W parameter

W parameter

Fig. 3. W parameter versus S parameter for studied samples: the RF sample annealed for 2 h in the vacuum 6 × 10−6 mbar (black circles) and three samples heated in the air (a) and N2 (b) for 2 h (white squares), 3 h (grey triangles) and 8 h (dark grey diamonds). There were presented only chosen points to show the main tends in this plot. The marked area represents the investigated region of oxides.

Similar dependencies are reflected in Fig. 2b where the thicknesses of the first layer versus annealing time are shown. Longer heating time leads to reduction of the thickness. In the case of 2 hoxidation at 800 ◦ C this layer is deeper for N2 flux atmosphere but for longer heat treatment, similar as Fig. 2a, the thickness is strongly reduced and as a result it is smaller in comparison to the samples heated in air. In Fig. 2c and d the thicknesses of the spinel and total oxide films in dependency on the annealing time are visible, respectively. The thicknesses presented in Fig. 2d were obtained directly from fitting procedure while the thicknesses of spinel (Fig. 2c) were calculated as the difference between the whole oxide layer and the first one. In both cases the thicknesses depend on the time of annealing as well as on the kind of atmosphere. The thicknesses of the spinel and oxide layer increase linearly with the annealing time for samples heated in air, while for N2 atmosphere the linear

mean implantation depth [µm]

a) 0

0.17

0.53

increasing is broken. Additionally, the thickness of these films is bigger for the samples annealed in air. The behaviour of parameters depicted in Fig. 2 can be explained by the oxide growth with the heating time. Below we bring up this subject on the basis of available knowledge. As it was mentioned, the surface of ASS as a result of annealing is covered with ␣-Fe2 O3 on the outermost layer and spinel rich in the chromium oxide in the inner layer [6–8]. It was observed [30–33] that the temperature of 800 ◦ C is closed to the limit above which the chromium oxide is a predominant layer because of diffusion of chromium from the bulk. In this way, with the reference of quoted information for samples studied here, annealing at 800 ◦ C leads to formation of the multi-oxide films on the surface, where at the top ␣-Fe2 O3 containing defects appeared. The longer time of annealing causes growing of spinel rich in chromium and disappearing of hematite

mean implantation depth [µm]

b)

1.06

0

0.020

0.17

0.53

1.06

0.020

N2

0.018

0.018

0.016

0.016

W parameter

W parameter

AIR

0.014 0.012 0.010

0.014 0.012 0.010

0.008

0.008

0.006

0.006

0.004

reference 2h 3h 8h

0.004 0

10

20

energy [keV]

30

0

10

20

30

energy [keV]

Fig. 4. The dependency of W parameter on positron implantation energy obtained in VEP experiment for four samples of ASS 304 AISI: the RF sample annealed for 2 h in the vacuum 6 × 10−6 mbar (black circles) and three samples heated in the air (a) and N2 (b) for 2 h (white squares), 3 h (grey triangles) and 8 h (dark grey diamonds). The marked area represents the investigated region of oxides.

P. Horodek et al. / Applied Surface Science 333 (2015) 96–103

energy [MeV] a)

20

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

reference 15

10 Col 41 vs Col 42

RBS N2

b)

c)

normalized yield, arbitrary units

5

0 20

2h 15

10

5

0 20

O

8h

15

Cr

10

Fe 5

0 0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

energy [MeV] Fig. 5. Normalized RBS/NR spectra (grey lines) collected using a 3045 keV He+ ion beam and the scattering angle 170◦ for samples ASS 304 AISI annealed at 800 ◦ C: (a) in vacuum 6 × 10−6 mbar for 2 h and in the air: for 2 h in (b) 8 h in (c). The solid black lines are the best fit of these spectra.

101

on the outer layer. The thickness of the outer layer is reduced and decreased positron diffusion length shows the increase of defects such as vacancies or dislocations induced by the large oxide particles destroying the layer. The reduction of iron oxides by chromium to FeCr2 O3 and finally to Cr2 O3 was observed in iron–chromium–nickel systems by Sequeira et al. [34]. The linear increasing of the thickness of the total oxide film, presented in Fig. 2d, is in agreement with Betz et al.’s [35] observation at higher temperatures. In contrast, Saeki et al. [36] found a non-linear time dependency of growing oxide thickness at 1000 ◦ C but they did not find the parabolic dependency. The protection of the N2 shield atmosphere from oxidation is clearly visible in Fig. 2d. Annealing in condition of N2 reduced the thickness of oxide films and gave rise to the linear dependency of the growth of the oxide layer. The difference is especially marked in the case of heating for 8 h where the thickness of the oxide layer on the sample oxidized in N2 is twice as thin as in the specimen annealed in air. Additionally, the dependencies of positron diffusion length as well as the thickness of the first layer versus annealing point out higher concentration of defects in this region. Information about the types of defects could be obtained by plotting W parameter as a function of S parameter. It is presented in Fig. 3. The points do not represent all data from the experiment, but they were assorted to show the main tends. In the case of the RF sample W and S points for individual measurement are located around the straight-line. As the kinetic energy of implanted positrons is increased, they move from the left to right, reaching a minimum S value. For specimens annealed in air (Fig. 3a) and N2 (Fig. 3b) atmospheres three regions can be recognized. The area comes from the surface at the top, the region connected with the presence of the oxide films in the middle part of the plot and the bulk area at the bottom where points lie around the straight-line. In Fig. 4 the dependencies of W parameter versus positron incident energies for studied samples are depicted. In this case profile for RF (black circles) arises and saturates for higher energies. For specimens annealed in air (Fig. 4a) as well as N2 (Fig. 4b) the hatched regions represent oxide films on the surface. In these regions the

Fig. 6. The dependencies of oxygen concentration (a) and thickness of the oxide layer (b) obtained from RBS/NR studies on annealing time for samples ASS 304 annealed at 800 ◦ C in air (black squares) and in flow of 0.35 cm3 /s of N2 (white squares) atmospheres.

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values of W parameter are higher than in RF. For energies allowing positron implantation in oxides, the broadening of 511 keV line is observed and values of S parameter decrease while W parameter increases. On the broad of the layer S parameter consists information from two layers which are penetrated with different weight. In fact, the values of W and S parameters obtained from fitting of W(E) as well as S(E) profiles using VEPFIT could give information about defect state between layers. Unfortunately, in the case of W(E) profiles VEPFIT code did not recognize three layers for oxidized samples very well because of complicated shape and higher level of deviation of measurement points. In this way wider characteristic of defectiveness, especially between layers in dependency on time, is not possible using DB. 3.2. RBS/NR results In Fig. 5 the examples of normalized RBS/NR spectra of ASS 304 AISI samples (grey lines) are depicted. Fig. 5a represents the results for the RF sample annealed for 2 h at 800 ◦ C in the vacuum. In Figs. 4c and 5b spectra for samples annealed at the same temperature in air for 2 and 8 h, respectively, are visible. The peak edges observed near the energy 1.05 MeV and 2.3 MeV correspond to the scattered ␣ particles on nucleus of oxygen and iron atoms, respectively (Fig. 5c). The peak connected with the presence of oxygen is not visible for the RF. It appears for samples annealed in air and its intensity rises with the time of annealing. It confirms the dependency of oxidation on the time of heating. The decrease of counts in the spectra in the region from 2.00 MeV to 2.15 MeV, especially visible in Fig. 5c, can be explained by growing concentration of Cr. The theoretical model consisting of plano-parallel homogeneous uniform layer on the homogeneous substrate was used in the analysis. The oxygen concentration as well as the thicknesses of the oxide layers visible in Fig. 6 were evaluated using SIMNRA code [24]. Fits were tagged by solid black lines in Fig. 5. The oxygen concentration found in the RF sample was lower than 0.05% in the layer containing 400 atoms/cm2 . For remaining samples, the oxygen concentration increases with the time of annealing and is higher for the specimens heated in air (see Fig. 5a). In this case (black squares) a non-linear trend is visible, while for N2 (white squares) the distribution is rather linear, at least in the studied range of time. The approximated thicknesses of oxide layers were presented in Fig. 6b with squares: black and white for air and N2 atmospheres, respectively. The thicknesses of oxide films obtained from RBS/NR method agree very well with VEP results (see Fig. 2d). Annealing in air produces the thicker oxide layer than heating in N2 . Additionally, for both cases the linear growth of the oxide films in the function of annealing time is confirmed. 4. Conclusions The VEP results showed the multilayered character of oxides formed on the surfaces of 304 AISI annealed in air and N2 atmospheres. It was found that the thickness of the outer layer, recognized on the basis of other studies as ␣-Fe2 O3 , decreases with the heating time and induces the diffusion of Cr atoms from the substrate and as a result of extension of spinel. For samples oxidized in air, the non-linear time-dependency of oxygen concentration was found. The linear dependency of growing oxides on annealing time, obtained using DB as well as RBS/NR studies, was well visible. Heating in the protective atmosphere of N2 to temperature of 800 ◦ C protects samples from oxidation. It is possible that the speed of oxidation slows down especially for longer heating time uninvestigated in these studies. It occurs in dependency of oxygen concentration and thickness of oxide film versus annealing time

where the tendency due to a big variation and small amount of points cannot be absolutely determined.

Acknowledgement All works were supported by RFBR grant No. 12-02-00072.

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