SEM / EDX , XPS , CORROSION AND SURFACE ROUGHNESS CHARACTERIZATION OF AISI 316 L SS AFTER ELECTROCHEMICAL TREATMENT IN CONCENTRATED HNO

Original scientific paper In the paper there is described the AISI 316L stainless steel surface obtained after electrochemical treatment in concentrated HNO3. It was characterized by roughness parameters, corrosion protection behaviour and chemical composition of the surface layer. Generally used 2D and 3D roughness parameters describing the surface after the electrochemical treatment are the following: Ra = 0.737 μm and Sa = 1,13 μm as well as Rq = 0,895 μm and Sq = 1,37 μm. In case of corrosion studies the passive current density was equal to 2,3×10 ̶ 2 μA/cm, pitting potential was equal to Epit = 1140 mV vs. SCE and repassivation potential was 79 mV vs. SCE. In the surface layer there were detected compounds of iron, chromium, molybdenum, cobalt, manganese, as well as calcium (contamination). The high resolution XPS spectra/results data have shown that most of detected iron compounds can be described as Fe2O3/FeOOH and chromium compounds as Cr2O3/CrOOH. The molybdenum detected in the surface layer was mainly in valency states six (65 at%) and five (17 at%).


Introduction
Stainless steels are commonly treated by electropolishing considered as a standard electrochemical process/treatment [1 ÷ 6].The process is generally used in industry for finishing operations and is often carried out in the mixture of two acids, H 2 SO 4 and H 3 PO 4 [7 ÷ 10].After this process a treated part reveals smooth and bright surface with good corrosion resistance and improved mechanical properties [10 ÷ 16].It is of special importance when using that stainless steel as a biomaterial.Concerning improved corrosion resistance [12 ÷ 18], the steel parts may be chemically treated in concentrated nitric acid and that process is described in the literature [19 ÷ 21].On the other hand, there are no study results available on electrochemical treatment of stainless steels in concentrated nitrogen acid.
The use of an oxidizing acid, such as nitric acid, for passivation has two purposes.First of all, the acid dissolves any surface inclusions in the steel.Secondly, it assures a uniform, clean surface those results in the consistent formation of the passive chromium oxide film (and molybdenum film) [2].The passivation process enhances the chromium fraction in the passive film, as established before [4,12,16].The main mechanism for this process is selective dissolution of predominantly iron [3,4].In addition to the chemical process with a standard immersion in nitric acid solution the electrochemical treatment is proposed.
The aim of the study is to perform electropolishing of AISI 316L stainless steel in HNO 3 and investigate the surface properties after that operation.Specifically, the study of the corrosion behaviour of that steel in a typical Ringer's solution [7,13,16] was investigated.Thorough characterization of such obtained 316L steel surface may be of a great practical value.

Method 2.1 Material
For the study there were used samples of the AISI 316L austenitic chromium-nickel-molybdenum stainless steel.The addition of molybdenum to the stainless steel increases corrosion resistance and mechanical properties.Typically that sort of steel is very often used in food, pharmaceutical, marine, architecture applications, as well as applied for biomaterial.The bulk composition of the steel, by standard, and as-measured, is shown in Tab. 1. Second column of Tab. 1 presents composition provided by the manufacturer [15].
Ten samples of AISI 316L stainless steel as received of dimensions 60×30×1 (mm) were prepared for the studies.They were treated by electrochemical treatment in concentrated HNO 3 under defined conditions, with the current density used i = 65±5 A/dm 2 .The electrochemical polishing process was carried out for 3 minutes time in the electrolyte of temperature of 65 °C, with the temperature control of ±10 °C.The electrolytic cell was made of glass, containing up to 500 cm 3 of electrolyte.Afterwards the samples were rinsed in distilled water for about 2 minutes and dried in air for about 3 minutes.The surface roughness studies were carried by interferometric method with Taylor-Hobson Precision Talysurf CCI 6000 (Coherent Correlation Interferometry).The SEM/EDX measurements were performed on ASPEX EXPLORER™ apparatus.The XPS experiments were carried out in an ultra-high-vacuum system with a base pressure of about 10 −8 Pa.The XPS measurements, with the angle of 90°, were performed using a SES2002 electron energy analyzer with a monochromatized Al Kα (hν = 1486,6 eV) X-ray source (Gammadata-Scienta).A total resolution of about 0,6 eV was obtained for the presented spectra.In view of optimizing the signal-tonoise ratio, one XPS measurement cycle covered 100 sweeps.For the XPS analyses the CasaXPS 2.3.14 software was used [22 ÷ 24].The XPS spectra were analysed by using Shirley background and GL (30), LA(1,2; 4,8; 3), LA(1,3; 4; 5) line shapes [25 ÷ 29].
Corrosion studies after electrochemical treatment were carried out in a typical Ringer's physiological fluid, at 25 °C.The electrochemical system used for the corrosion measurements consisted of a potentiostat ATLAS 98 with the software IMP98, a platinum counter electrode Ept-101, and a saturated calomel reference electrode EK-101P.The corrosion measurements data were recorded every 5 mV with the rate of 0,1 mV/s.In the case of the potentiodynamic measurements the specimens were immersed in the same solution for 1 hour prior to the measurement in order to stabilize the surface at the conditions of open circuit potential.

Study results
In Fig. 1 there are shown the SEM photo and EDX spectrum of surface after electrochemical treatment in concentrated HNO 3 .The structure of the treated steel surface is revealed and, on the basis of the EDX results one may easily notice there is a possibility to determine the chemical composition of the obtained surface.In that case there were detected the biggest signals from iron, chromium and nickel.
In order to investigate in detail the chemical composition of surface layer formed after electrochemical treatment in concentrated HNO 3 , the XPS spectroscopy as survey scan (Fig. 2) was used.On the basis of that method, in surface layer there were more chemical elements than only iron, chromium and nickel detected, with such additional elements like oxygen, molybdenum, cobalt, manganese, as well as calcium as contamination.The survey of XPS study has shown more information about the surface layer than that from EDX, but still not enough.In that situation there were used the high resolution XPS measurements for iron, chromium and molybdenum; they are presented in Figs. 3 ÷   The Fe 2p spectrum was fitted by two peaks: one line for iron metal Fe-M and the second one for a compound part Fe-X.Shape lines for iron spectrum fitting from GL(30) (metal and compounds spectrum part) and LA(1.2,4.8,3)(metal part) were selected.On the basis of results shown in Fig. 3 and Tab. 2 it is visible that the atomic percent of iron metal is in the range of 4,3 ÷ 4,4 at% and percent of iron compounds amounts for 95,6 ÷ 95,7 at%.
Fitting by two peaks is not the best method, but with well-chosen line shapes it should provide essential information about the predominance of oxides, hydroxides as well as phosphates and sulphates of iron.Therefore two peaks analysis for binding energy 711,5 eV was performed.In the presented data the most probable chemical iron compound appears to be a mixture of Fe 2 O 3 /FeOOH.In Fig. 5 and Tab. 4 there are shown the XPS results of molybdenum Mo 3d spectrum.The molybdenum metal was fitted by LA(1,1;.2,3;.2) and molybdenum compounds by GL (30).It was detected that in surface layer there were mainly molybdenum compounds in valency states six (65 at%) and five (17,4 at%).The other Mo valency states (2, 3 and 4) as well as metal part of molybdenum spectrum were in the range from 2,4 at% to 5,4 at%.In Figs. 5 and 6 there are shown the results of surface topography analysis i.e. 3D and 2D surface simulations, texture direction, Abbott-Firestone curve, roughness profile as well as 2D and 3D roughness parameters with Gaussian filter and cut-off equal to 0,08 mm (Tabs.5 and 6).There is not visible one texture direction what can be observed on the surfaces after mechanical polishing.Obtained surface is uniform, characterized by 2D and 3D roughness parameters: Ra = 0,737 µm, Rq = 0,895 µm and Sa = 1,13 µm, Sq = 1,37 µm.
In Fig. 8 there is shown an exemplary curve of potentiodynamic corrosion measurements of surface after electrochemical treatment in concentrated HNO 3 .The average of passive current density equals 2,3×10 −2 µA/cm 2 .The pitting corrosion resistance corresponding to pitting potential (1140 mV vs. SCE) is higher than that after a standard electrochemical polishing (E pit = 600 mV vs. SCE), and re-passivation potential equals 79 mV vs. SCE.The dark dense area between 560 mV and 800 mV in Fig. 8 indicates the unstable pits, which undergo to repassivation.
In Fig. 9 there are given SEM photos of pit development in Ringer's solution with the pits of increasing diameters.They were taken on ASPEX EXPLORER™ device from a typical sample after electrochemical corrosion studies.

Discussion
In presented paper, there is shown the compact surface film characterization of AISI 316L austenitic stainless steel after electropassivation in HNO 3 .It is a novel approach to the subject of surface finishing.The most interesting findings in the article are the analysis of chromium compounds especially of the sixth level oxidation compounds as well as molybdenum states of oxidation on the background of iron compounds analysis.
To illustrate the changes after the treatment, the SEM photos of the sample after proposed electro-passivation with EDX surface analysis (Fig. 1) plus additional information of 3D surface roughness measurements (Figs. 6 and 7, and Tabs. 5 and 6) are given.In order to characterize the corrosion resistance, there were shown the polarization characteristics (Fig. 8), which comprehensively describe the properties of the surface layer formed.The authors' previous results [7,13] obtained on 316L SS after a standard electropolishing in the Ringer's solution indicate the level of pitting corrosion resistance of about 600 mV versus SCE.In their analysis of AISI 316L surface after electropolishing and passivation in HNO 3 the Henkels [30] show that the difference between the pitting potentials of only electropolished and electropolished plus passivated 316L SS sample was about 100 mV.Other researchers find [21] that nitric acid passivation can increase the pitting potential of AISI 316 stainless steel.Following this reasoning the steel surface after passivation in HNO 3 should be about maximum in the range of (700 ÷ 800) mV vs. SCE.In comparison, the data in this paper show that the pitting potential is equal to 1160 mV only after electro-passivation treatment in HNO 3 (mechanical/ abrasive polishing was used as the pre-treatment only).On the basis of literature data obtained and current knowledge in the field of surface finishing, our results should be considered as much better and the method innovative.

Conclusion
In the article there was described the surface obtained after electrochemical treatment in concentrated HNO 3 .It was characterized due to roughness, corrosion protection and chemical composition of the surface layer.The obtained roughness was equal to Sa = 1,13 µm (Ra = 0,737 µm), Sq = 1,37 µm (Rq = 0,895 µm).The corrosion resistance in the Ringer's solution was described by a passive current density (i pass = 2,3×10 ̶ 2 µA/cm 2 ), pitting corrosion potential (E pit = 1140 mV vs. SCE) and repassivation potential (E repass = 79 mV vs. SCE).The XPS spectroscopy was used to find the composition of the surface layer after the treatment performed.On the basis of obtained data, there were detected the following chemical elements: iron, chromium, molybdenum, cobalt, manganese, as well as calcium (contamination).The high resolution XPS spectra have shown that iron compounds can be described as Fe 2 O 3 /FeOOH and chromium compounds as Cr 2 O 3 /CrOOH.Concerning Mo as the alloying element of AISI 316L SS, it was detected that in surface layer there was mainly molybdenum in valency states 6 (65 at%) and 5 (17,4 at%).The Mo valency states such as zero, two, three and four were in the amount range from 2,4 at% to 5,4 at%.

Figure 1 Figure 2
Figure 1 SEM/EDX results of AISI 316L SS after electrochemical treatment in concentrated HNO3

Figure 8 Figure 9
Figure 8 Polarization curve of AISI 316L SS after electrochemical treatment in concentrated HNO3

Table 1
Chemical composition of AISI 316L stainless steel

Table 4
Analysis of Mo 3D spectrum

Table 5
2D roughness parameters according to ISO 4287 Ra 0,737 µm Ra: Arithmetic mean deviation of the roughness profile Rq 0,895 µm Rq: Root-Mean-Square (RMS) Deviation of the roughness profile Rp 1,72 µm Rp: Maximum peak height of the roughness profile Rv 1,77 µm Rv: Maximum Valley Depth of the roughness profile Rt 5,95 µm Rt: Total height of roughness profile Rz 3,49 µm Rz: Maximum height of roughness profile