May 21st – 23rd 2014, Brno, Czech Republic, EU
XPS STUDY OF SURFACE LAYER FORMED ON AISI 304L SS AFTER HIGH-CURRENTDENSITY ELECTROPOLISHING
Krzysztof ROKOSZ1a, Tadeusz HRYNIEWICZ1b, Sławomir RZADKIEWICZ1-2c
1Division
of Surface Electrochemistry and Technology, Faculty of Mechanical Engineering,
Koszalin University of Technology, Racławicka, Koszalin, Poland, EU,
‘2Scientific Circle of Faculty of Mechanical Engineering, Faculty of Mechanical Engineering,
Koszalin University of Technology, Racławicka, Koszalin, Poland, EU,
[email protected], [email protected], [email protected]
Abstract
In the paper, there are presented XPS results of surface layer formed after electrochemical polishing of
AISI 304L (EN 1.4307) stainless steel at high current density of 2000 A/dm 2. For the investigation, electrolyte
based on orthophosphoric (H3PO4) and sulfuric (H2SO4) acids in proportions 3:2, was used. The obtained
results have shown, that high current density considerably influences the composition of passive layer
formed after electrochemical treatment. It was found that after the High-Current-Density Electropolishing
(HDEP) at current density equal 2000 A/dm 2 the chromium compounds to iron compounds ratio, which was
obtained, was equal to 3. In the passive layer there were detected chromium compounds on the third oxide
stage, i.e. chromium oxide Cr2O3 (49.5 at %), chromium hydroxides: CrOOH, Cr(OH)3 (15.3 at%) and
chromium phosphates and sulfates: CrPO4, Cr2(SO4)3 (5.7 at%) as well as chromium on the sixth oxide stage
CrO42 (5.8 at%).
Keywords: High-Current-Density Electropolishing HDEP; AISI 304L (EN 1.4307) stainless steel; XPS
1.
INTRODUCTION
The austenitic AISI 304L (EN 1.4307) stainless steels have been used in many industries. In case of using it
as a biomaterial, e.g. for implants and/or coronary stent production the big current density, i.e. 2000 A/dm2
can be applied. Authors of the work, in many their studies, investigated the passive layers’ chemical
compositions and mechanical properties of metals [1-9] and alloys [10-29] after a standard electrochemical
polishing EP [1-12] as well as after magnetoelectropolishing MEP [12-29]. For some specific cases, now a
new approach to the electrochemical polishing has been realized by applying a high current density (HDEP)
of about 2000 A/dm2. This way a new surface layer, due to the modified stainless steel chemical
composition, can be obtained. In this paper the new XPS results, together with their analysis, are presented.
The most important constituents of such obtained passive layer have been revealed.
2.
METHOD
2.1
Material
The AISI 304L (EN 1.4307) stainless steel samples served for the study, with the material composition
presented in Table 1. The chemical composition of the examined steels by spark spectrometer
SPECTROMAXx were performed. The samples were cut off a cold-rolled metal sheet of the stainless steel
after plate rolling so that the austenitic structure was retained. They were prepared in the form of rectangular
specimens of dimensions 5 × 30 mm cut of the metal sheet 1 mm thick.
May 21st – 23rd 2014, Brno, Czech Republic, EU
Table 1 Composition of AISI 304L stainless steel, in wt%
C
Cr
Mo
Ni
Mn
Si
Cu
Co
V
W
N2
Fe
0.04
18.10
0.22
8.15
1.25
0.44
0.29
0.17
0.09
0.02
0.08
bal.
2.2
Set up and parameters
The electrolytic polishing operations were performed at the current density of 2000±10 A/dm2. The main
elements of the High Current Density Electropolishing (HDEP) set-up were: a processing cell, a dc power
supply Telzas PDN 24-48-(60)/30(25), the electrodes and the connecting wiring. The studies were carried
out in the electrolyte of initial temperature of 202 C, with the temperature control of 5 C. Generally the
finish electrolyte temperature was 755 C. The time of the electrochemical treatment was 20 s. For each
run, the electrolytic cell made of glass was used, containing up to 500 cm 3 of electrolyte.
2.3
XPS studies
The XPS measurements on HDEP (1000 A/dm 2) electrochemically polished AISI 316L stainless steel
samples were performed on the SCIENCE SES 2002 instrument using a monochromatic (GammadataScienta) Al K(alpha) (hν = 1486.6 eV) X-ray source (18.7 mA, 13.02 kV). Scans analyses were carried out
with the analysis area of 1 × 3 mm and a pass energy of 500 eV with the energy step 0.2 eV and step time
200 ms. The binding energy of the spectrometer has been calibrated by the position of the Fermi level on a
clean metallic sample. The power supplies were stable and of high accuracy. The experiments were carried
out in an ultra-high-vacuum system with a base pressure of about 6∙10-10 Pa. The XPS spectra were
recorded in normal emission. In view of optimizing the signal-to-noise ratio to about 3.2, one XPS
measurement cycle covered 10 sweeps. For the XPS analyses the CasaXPS 2.3.14 software with Shirley
background type, and Gaussian-Lorentzian GL(30) shape options were used [30-34].
3.
RESULTS
In Fig 1 the XPS results of iron (Fe 2p), chromium (Cr 2p), manganium (Mn 2p), nickle(Ni 2p), carbon (C1s),
oxygen (O1s), phosfourus (P 2p), and sulfur (S 2p) spectra of AISI 304L surface electropolished at
1000 A/dm2 are presented. It follows from the results, that in the studied surface layer there are both
chromium metal part and compounds part as well. There are also visible the clear peaks of manganese and
nickel as well as of phosphorus and sulfur. The last one, with a high amount of oxygen in the passive layer,
except of oxides and hydroxides, can form also iron and/or chromium sulfates and phosphates. In view of
better understanding of the surface layer chemical composition the Cr 2p3/2 and Fe2p3/2 fittings were
performed, that is displayed in Fig. 2. On the base of obtained XPS data (Fe 2p, Cr 2p, Mn 2p, Ni 2p, O 1s,
P 2p, S 2p, C 1s) the chemical composition of the studied surface layer was determined. In the surface layer,
1.8 at% of iron, 3.2 at% of chromium, 1.2 at% of manganese, 0.3 at% of nickel, 7.2 at% of phosphorus, 2.2
at% of sulfur, 47.1 at% of oxygen, and 37 at% of carbon were found. On the base of those results the totalchromium-to-total-iron ratio equaling 1.8 and the phosphorous-to-sulfur ratio equaling 3.3, were determined.
The ratio of chromium-compounds-to-iron-compounds equaling 3 was also found. Chromium compounds on
the third oxide stage, i.e. chromium oxide Cr2O3 (49.5 at %), chromium hydroxide: CrOOH, Cr(OH)3 (15.3
at%) and chromium phosphates and sulfates: CrPO4, Cr2(SO4)3 (5.7 at%), as well as chromium on the sixth
oxide stage, CrO42 (5.8 at%), were detected, that is given in Table 2. In case of iron analysis, the 52.2 at%
of iron metal (Fe0), 11.2 at% of iron oxides (FeO and Fe2O3), and 23 at% of iron hydroxides (FeOOH), as
well as over 11 at% of Fex(SO4)y, Fex(PO4)y, were found, that is shown in Table 3.
May 21st – 23rd 2014, Brno, Czech Republic, EU
Fig. 1 XPS results of iron (Fe 2p), chromium (Cr 2p), manganese (Mn 2p), nickel (Ni 2p), carbon (C 1s),
oxygen (O 1s), phosfourus (P 2p), and sulfur (S 2p) spectra of AISI 304L surface electropolished at
2000 A/dm2
May 21st – 23rd 2014, Brno, Czech Republic, EU
Fig. 2 Fitting of Cr 2p3/2 and Fe 2p3/2 spectra of AISI 304L SS surface electropolished at 2000 A/dm2
Table 2 XPS fitting results of Cr 2p3/2
BE [eV]
574.1
557.7
576.7
577.5
578.5
578.9
577.0
577.6
579.6
FWHM
1.3
1.0
1.0
1.0
1.0
1.0
2.1
2.1
1.6
Cr 2p [at%]
23.7
17.3
16.8
9.1
3.9
2.4
15.3
5.7
5.8
Chromium
Compounds
Cr0
CrOOH
Cr(OH)3
(Cr3+)
CrPO4
Cr2(SO4)3
(Cr3+)
(CrO4)2
Cr6+
Cr2O3
(Cr3+)
Table 3 XPS fitting results of Fe 2p3/2
BE [eV]
706.7
709.8
711.7
713.2
714.8
716.5
FWHM
1.1
1.8
1.6
1.5
0.7
0.7
Fe 2p [at%]
52.2
11.2
23.0
8.9
2.6
2.2
Iron
Compounds
Fe0
Fe2O3, FeO
FeOOH
Fex(SO4)y, Fex(PO4)y, sat(Fe2+)
May 21st – 23rd 2014, Brno, Czech Republic, EU
4.
CONCLUSIONS
After electrochemical polishing of austenitic AISI 304L SS (EN EN 1.4307) at current density of 2000 A/dm2
in surface layer the iron, chromium, manganese, nickel, carbon, oxygen, phosforus and sulfur in the amount
of atomic percent: 1.2, 3.2, 1.2, 0.3, 37, 47.1, 7.2, 2.2, respectively, were detected. On the base of binding
energies of the analysed peaks it can be concluded that in the passive layer there is a lot of oxides,
hydrooxides and iron-chromium-manganese phosphates and sulfates. In passive layer the iron (FeO, Fe2O3,
FeOOH, FeSO4, Fe2(SO4)3,, Fe3(PO4)2, FePO4) and chromium (CrOOH, Cr(OH)3, Cr2(SO4)3, CrO42)
compounds were detected.
ACKNOWLEDGEMENTS
The BerlinerLuft company, especially Bogusław Lackowski, PhD is acknowledged for delivering
samples for the studies. The Authors acknowledge Assoc. Prof. Gregor Mori, DSc PhD of
Montanuniversitaet Leoben, Austria, for providing bulk chemical composition of the AISI 304L
stainless steel used in the studies.
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