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THE CORROSION BEHAVIOR OF CHROMIUM
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NITRIDE FILM ON AISI 4140 AND H13 STEELS
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Running head: The corrosion behavior of CrN film
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Pornwasa Wongpanya1*, Thipusa Wongpinij2, Pat Photongkam2,
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Chonthicha Keawhan2, Sarayut Tunmee3 and Nirun Witit-Anun4
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School of Metallurgical Engineering, Institute of Engineering,
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Suranaree University of Technology, Nakhon-Ratchasima, 30000 Thailand
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Tel. 0-4422-4486; Fax. 0-4422-4482; E-mail: pornwasa@sut.ac.th
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Synchrotron Light Research Institute (Public Organization),
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Nakhon-Ratchasima, 30000, Thailand
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Department of Materials Science and Technology, Nagaoka University of Technology,
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16031-1 Kamitomioka-machi 940-2188, Japan
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Department of Physics, Faculty of Sciences, Burapha University, Chonburi 20131
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Thailand
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*Corresponding author, email: pornwasa@sut.ac.th
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Abstract
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The chromium nitride film (CrN) was prepared on steel substrates,
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AISI 4140 and H13 with variation of substrate roughness, using reactive DC
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magnetron sputtering technique at room temperature in controlled partial
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pressures of Ar and N2 gas atmosphere. The films were polycrystalline
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confirmed by X-ray diffraction analysis. The corrosion behavior was
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investigated using an electrochemical technique in 3.5 wt% NaCl solution with
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various pH values at room temperature. The results indicated that CrN coated
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AISI H13 present better corrosion resistance than CrN coated AISI 4140 at
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pH 2 and 7, but worse at pH 10. In addition, the rate of corrosion decreased
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with a reduction of surface roughness. The chemical states of corrosion
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product were clearly identified by spatially resolved secondary electron yield
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X-ray absorption spectroscopy (XAS) with X-ray photoemission electron
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microscopy (X-PEEM). It was found that formation of Cr2O3 is formed on non-
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corroded CrN film and AISI H13 substrate, whereas Fe2O3 is formed on
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corroded surface and AISI 4140 substrate.
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Keywords:
chromium
nitride,
corrosion
behavior,
electrochemical
technique, X-ray photoemission electron microscopy, X-PEEM
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Introduction
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The metal-nitride films are extensively employed as a protection film for
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surface improvement of engineering parts. They do not only represent high
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hardness and high wear resistance, but they also protect the substrate from corrosion
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in liquid environments (Darja et al., 2004; Zhou et al., 2003). Chromium nitride
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(CrN) is inexpensive comparing to other nitrides, such as .TiN, TiAlN and TiAlSiN,
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and its performance in terms of corrosion and wear resistance is generally applied
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to many applications, e.g. auto mobile parts, cutting tools and molds. Many research
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(Keawhan et al., 2012; Wongpanya et al., 2014) demonstrated that CrN coated
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steels with difference of substrate roughness significantly improved their corrosion
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resistance. They reported that the CrN coated steels exhibited better corrosion
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resistance than the uncoated steels and the corrosion rate can be reduced by
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decreasing roughness of steel substrate. In addition, the corrosion behavior of CrN
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coated steels drastically changed with pH of solution in particular for the acid
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solution (Christine and Alan, 2005). It is however, it still remains in question about
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how type of steel substrate do affect to corrosion behavior of CrN film. In this paper,
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it was focused on the evaluation of the corrosion resistance of CrN coating on AISI
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4140 and H13 steels. The corrosion resistance of the CrN coated and uncoated steels
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in 3.5 wt% NaCl solution were tested by an electrochemical method. Since some
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engineering parts were used in liquid environments, they were used in contact with
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both acid and alkaline solutions. Therefore, corrosion was tested in solution at pH
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2, 7 and 10 that is more realistic approaches toward solving the problem of metal
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corrosion. After electrochemical testing, the corroded surfaces of CrN coated and
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uncoated steels were investigated by scanning electron microscopy (SEM). And,
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X-ray photoemission electron microscopy (X-PEEM) has been used to evaluate the
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corrosion products on the corroded surface of CrN coated steels.
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Materials and Methods
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Sample preparation
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Firstly, AISI 4140 and H13 steels were prepared with dimensions of
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10x10x2 mm3. The chemical compositions investigated by atomic emission
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spectroscopy were shown in Table 1. They were heated for 30 minutes at 850oC
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(AISI 4140) and 1025oC (AISI H13); subsequently they were oil–quenched and
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tempered. The sample were polished with SiC paper at various grit numbers, e.g.
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180, 600 and 1200 for preparing difference of substrate roughness. The
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microstructures of these steels were studied by light optical microscopy. They were
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ground, polished and etched for the metallographic microstructure evaluations. An
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etchant solution used to reveal microstructure was prepared from a 2% nital solution
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composes of 2 ml nitric acid and 98 ml ethyl alcohol. The microstructure of AISI
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4140 steel were found to consist a mixture of ferrite and pearlite, whereas the
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microstructure of AISI H13 steel were compose of ferrite and carbide in the
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normalizing condition as shown in Figure 1 a) and b). After heat treatment, the
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microstructure of AISI 4140 and H13 steels had been changed to be martensite and
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retained austenite as shown in Figure 1 c) and d). Before CrN coating, the substrate
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samples were rinsed by distilled water, acetone and were air-dried. The DC
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magnetron sputtering process was employed to deposit CrN film on these samples
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with deposition conditions as shown in Table 2. List of samples for corrosion
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evaluation dependent on steel types and surface roughness was shown in Table 3.
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Corrosion Testing
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The corrosion behavior of uncoated and CrN coated samples was evaluated
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by electrochemical technique in accordance with ASTM standard G44-99 (ASTM
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G44-99, 2005). Experiments were performed using a potentiostat analyzer. An
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electrochemical cell contains three electrode cells including samples as a working
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electrode, graphite as counter electrode and silver/silver chloride (Ag/AgCl with
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3.0 M KCl) as a reference electrode. The samples were polarized to potential
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between -900 mV and +200 mV at a scan rate of 1.0 mV/s in 3.5 wt% NaCl solution
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with pH value of 2, 7 and 10 at room temperature. The pH of solution was adjusted
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by addition of hydrochloric acid (HCl) and sodium hydroxide (NaOH). The
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polarization curves were used to study the corrosion behavior of samples. They
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were carried out in order to obtain important corrosion parameters, e.g. corrosion
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potential (Ecorr) and corrosion current density (Icorr). The corrosion rate (CR) was
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calculated from the polarization curve according to Faraday’s law and was
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evaluated by the Tafel extrapolation technique.
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Structural and Surface Characterizations
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The crystallographic phase structure of CrN films before corrosion testing
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was studied by the X-ray Diffraction. After the testing corroded surface was
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characterized by scanning electron microscope (SEM) and X-ray photoemission
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electron microscopy (X-PEEM)
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Results and Discussions
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The Crystallographic Structure of CrN film
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Figure 2 shows the X-ray diffraction pattern of CrN film deposited on AISI
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4140 and H13 steels. The XRD analysis reveals the films deposited on both
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substrates were polycrystalline with four diffraction peaks of CrN (111), (200),
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(220) and (311) planes at 2θ angles of 37.38º, 43.50º, 63.20º and 76.20º,
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respectively. The dominant orientation of CrN film in this study is the (111) plane.
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The strong orientation in the (111) plane is also reported by Ruden et al. (2013).
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Corrosion Behavior
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A Comparative study of corrosion resistance of uncoated and CrN coated samples
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Figure 3 shows examples of polarization curves of AISI 4140-180, AISI
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H13-180, CrN/4140-180, CrN/4140-1200, CrN/H13-180 and CrN/H13-1200
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samples tested in 3.5 wt% NaCl solutions at pH 2, 7 and 10. In Table 4, the corrosion
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parameters of each sample was evaluated from the polarization curves and based
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on Tafel equation (Uhlig, 1971). It is obvious the CrN film highly impacts on
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corrosion properties of the AISI 4140 and H13 steels in particular the corrosion
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potential (Ecorr) and corrosion current density (Icorr). From a comparison of the
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corrosion potential (Ecorr) of the CrN coated samples and uncoated samples, it is
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found that the CrN coated samples have higher corrosion potential (Ecorr) than the
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uncoated samples. In addition, it is found that CrN coated samples show lower
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corrosion current density (Icorr) than the uncoated samples (Cunha et al., 1999).
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Thus, a corrosion rate (CR) of CrN coated samples is lower than that of uncoated
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samples for all substrates and pH solutions as shown in Figure 4. This implies that
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the corrosion resistance is better since the CrN coated on the surface of substrate
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(Liu et al., 1995; Walter et al., 2011). Those results indicate CrN coated samples
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have better corrosion resistance than that of the uncoated samples. Corrosion
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resistance of CrN coated samples is attributed to the presence of chromium (Cr) and
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nitrogen (N) atoms within CrN film. After CrN film reacted with the corrosive
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solution, the chromium atoms were oxidized to chromium oxide. It is well known
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that the chromium oxide is a stable oxide film and it is able to inhibit corrosion
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(Lippitz and Hubert, 2005; Ibrahim et al., 2002; William Gripset et al., 2006). For
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nitrogen atom, the nitrogen atoms in the CrN film are dissolved into the solution in
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the form of nitrogen anions (N-) during corrosion, and then they repel chloride
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anions (Cl-) away from the surface of sample. Moreover, nitrogen anions (N-)
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combines with hydrogen ion (H+) in the solution to form the ammonium (NH4+) as
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shown in Equation (1), which results in increase pH of solution. As a result,
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corrosion attack from the solutions decreases. (Chyou and Shih, 1991; Grabke,
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1996).
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N ( s)  4H  (aq)  3e  NH4 (aq)
(1)
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Effect of steel types on corrosion behavior
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Figure 4 shows corrosion rate of uncoated and CrN coated samples
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dependent on steel types. It is obvious that the AISI H13 and CrN/H13 samples
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show lower corrosion rate than the AISI 4140 and CrN/4140 samples, respectively,
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at pH 2 and 7. This might be due to chemical compositions of substrate as shown
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in Table 1. The AISI H13 steel (5.40%Cr) has chromium content higher than that
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of the AISI 4140 steel (0.81%Cr). This means that chromium oxide was developed
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on the surface of the AISI H13 easier than that of the AISI 4140. As a result, the
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AISI H13 and CrN/H13 samples have better corrosion resistance than the AISI
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4140 and CrN/4140 samples, respectively.
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In contrast, the uncoated AISI 4140 shows lower corrosion rate than the
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uncoated AISI H13 at pH 10 as shown in Figure 4. The solution at pH 10 was
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adjusted by addition of sodium hydroxide (NaOH) that results in higher amount of
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OH- and Na+ as shown in Equation (2). The iron (Fe) in the AISI 4140 substrate are
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dissolved to be Fe2+ and the Fe2+ further combines with hydroxide ions (OH-) to
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form iron (II) hydroxide (Fe(OH)2) as shown in Equation (3).
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NaOH ( s)  Na (aq)  OH  (aq)
(2)
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2Fe( s)  O2 ( g )  2H2O(l )  2Fe2 (aq)  4OH  (aq)  2Fe(OH )2 ( s)
(3)
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According to Pourbaix diagram of iron (Beverskog, B. and Puigdomenech,
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I., 1996), at pH 10 and potential between -0.6 V and 0.0 V, Fe are dissolved to be
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Fe(OH)2, Fe3O4 and Fe2O3. This can confirm that at pH 10, the Fe(OH)2 is easily
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initiated on AISI 4140 steel. It is believed that the Fe(OH)2 can prevent corrosion
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but it is unstable film. And, it can be oxidized to be an iron oxide (Fe2O3). While,
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the AISI H13 steel has chromium oxide, it thus inhibits the diffusion of OH-into
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substrate. As a result, Fe(OH)2 is difficultly formed. With all above information, it
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can be concluded that the uncoated AISI 4140 steel has high corrosion resistance
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than the uncoated AISI H13 steel at pH 10. However, formation of oxide has to be
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further evaluated by X-ray photoemission electron microscopy (X-PEEM)
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combined with X-ray absorption spectroscopy (XAS).
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Effect of substrate roughness on corrosion behavior
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It clearly demonstrates that substrate roughness of substrate effects on their
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corrosion rate as shown in Figure 4. The samples prepared under the rougher surface
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roughness (XX-180 and XX-600) have higher corrosion rate (CR) than the samples
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prepared under the finer surface (XX-1200) for all pH solutions. This is due to the
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fact that the rough surface leads to more defects in CrN film and also results in
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lower complete coverage of the samples by the coating. Figure 5 shows examples
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of image surface scanned by SEM of CrN/4140-180, CrN/4140-600 and CrN/4140-
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1200 before corrosion test, it shows that the CrN/4140-180 sample has more defects
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than the CrN/4140-600 and CrN/4140-1200 samples, respectively.
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Effect of pH of solution on corrosion behavior
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It is obvious that the corrosion rate decreases as the pH of solution increases
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as shown in Figure 4. The highest corrosion rate is observed at pH 2 because of
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addition of hydrochloric acid (HCl). The Cl- ions are adsorbed at the surface of CrN
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coated and uncoated samples, then they penetrate and attack the samples especially
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at the defects of CrN film such as porosity (Hui et al., 2003; Mazhar et al., 2001).
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In addition, high concentration of hydrogen (H+) ion in solution at pH 2 promotes
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hydrogen evolution causing higher corrosion rate (Keawhan et al., 2012).
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At pH 7 and 10, hydroxide ions (OH-) in solution can react to iron in the
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substrate to form unstable hydroxide film which can against corrosion
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(Beverskogand Puigdomenech, 1996). The passive region of CrN coated samples
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is clearly observed in polarization curves as shown in Figure 3 b) and 3 c). This is
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due to the fact that CrN film reacts with OH- in the solution at pH 7 and 10;
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subsequently chromium hydroxide (Cr(OH)2) is produced. And this chromium
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hydroxide enhances corrosion resistance. As a result, all of CrN coated samples
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show corrosion resistance at pH 7 and 10 better than at pH 2.
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Surface Microstructure Analysis
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Since surface of uncoated and CrN coated samples are attacked and
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destroyed by aggressive solution; corrosion products have been generated. The
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certain corrosion products, especially oxide formation, were analyzed with
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scanning electron microscope (SEM) and X-ray photoemission electron
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microscopy (X-PEEM) in order to evaluate which element or compound is
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responsible for corrosion resistance. In this study, the samples tested in solution at
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pH 2 were selected to evaluate because such samples show the highest corrosion
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rate. Figure 6 shows secondary electron images of uncoated (AISI 4140 and H13)
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and CrN coated (CrN/4140 and CrN/H13) samples after corrosion test at pH 2. The
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uncoated samples show more degradation than the CrN coated samples. In addition,
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it is found that the CrN/4140 sample shows more crack surface of film than the
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CrN/H13 sample. As a result, the corrosion products on surface of the AISI 4140
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and CrN/4140 samples are higher than those of the AISI H13 and CrN/H13 samples,
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respectively.
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To identify the compound which is responsible for corrosion resistance of
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CrN coated samples, the CrN/H13 and CrN/4140 samples immersed in solution at
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pH 2 were selected to be investigated by spatially resolved X-ray absorption
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spectroscopy using X-PEEM. The X-PEEM technique can visualize corrosion
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surface and the image intensities were relevant to X-ray absorption coefficient.
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These help to characterize the surface by the shapes and sizes of the corroded area.
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Figure 7 shows X-PEEM images and XAS spectra of CrN/H13 sample, having two
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areas, e.g., area 1 as an intact CrN film (Cr- and N-enrichment) and area 2 as a CrN
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film removed which is deep down to the AISI H13 substrate (Fe-enrichment). The
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Four elements, e.g., Cr, N, Fe and O were investigated at the photon energy of 560
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- 600 eV, 390 - 420 eV 700 - 730 eV and 515 – 550 eV for Cr L3,2-edge, N K-edge,
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Fe L3,2-edge and O K-edge, respectively (Guiot et al., 1999; Frazer et al., 2003;
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Hocking et al., 2006; Wasinger et al., 2003; Esaka et al., 1997; Schedel-Niedrig et
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al., 1998). These four elements were evaluated since Cr and N are main components
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of CrN film while Fe is a main component of AISI H13 substrate and O is a main
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component of oxide generated as a result of corrosion reaction. It is obvious that
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the CrN film (area 1) showed high intensity of Cr L3,2-edge and N K-edge, while
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the low intensity of O K-edge and Fe L3,2-edge were detected. At the CrN film
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removed (area 2), it showed high intensity of Fe L3,2-edge, Cr L3,2-edge and O K-
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edge, while the low intensity of N K-edge were detected. These implied it would be
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Cr oxided for both areas. The oxidation of Cr to be chromium oxide (Cr2O3) is a
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vital key for corrosion protection of CrN film (Cunha et al., 1999; Schedel-Niedrig
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et al., 1998). The XAS spectrum of Fe L3,2-edge was detected at area 2; this reveals
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the oxidation of Fe to be rust (Fe2O3) (Hocking et al., 2006). Fe(OH)2, is unstable
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do it cannot be detected. It confirms that it was transformed to Fe2O3.
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To study the effect of substrate steels on corrosion of CrN film, X-PEEM
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images and XAS spectra of CrN/4140 and CrN/H13 samples after tested at pH 2
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were comparatively evaluated. The area 2, where is the CrN film was removed due
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to the corrosion reaction, of two samples were analyzed in particular for the Cr and
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Fe as shown in Figure 8. It is obvious that both samples show the spectra of Fe L3,2-
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edge, but the CrN/H13 sample shows lower intensity of Fe L3,2-edge than the
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CrN/4140 sample. In addition, only the CrN/H13 sample shows the spectra of Cr
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L3,2-edge. This can be used to confirm that Cr2O3 developed at surface of AISI H13
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substrate. Therefore, the CrN/H13 sample has better corrosion resistance than the
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CrN/4140 sample. Moreover, it is found that the corroded area of CrN/4140 sample
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is deeper than that of CrN/H13 sample as shown in Figure 8 a).
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Conclusions
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The corrosion behavior of CrN films deposited on AISI 4140 and H13 substrate
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was studied. The preferential orientation of CrN film is in the (111) plane. For the
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corrosion behavior, the CrN coated samples exhibited better corrosion resistance
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than the uncoated samples for all pHs. In addition, it is found that the CrN coated
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AISI H13 shows better corrosion resistance than the CrN coated AISI 4140 for pH
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2 and 7. At pH 10, the CrN coated AISI 4140 are better corrosion resistance than
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the CrN coated AISI H13. In addition, the samples prepared under the rougher
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surface roughness have higher corrosion rate than the samples prepared under the
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finer surface. X-PEEM images and XAS measurements have proven to be great
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techniques to evaluate corroded surface and to identify the chemical state of
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corrosion products. XAS spectra suggests Cr2O3 on CrN film and AISI H13
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substrate, and it showed Fe2O3 on corroded surface and AISI 4140 substrate.
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Acknowledgements
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The authors gratefully acknowledge the financial support of this work by Office of
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the National Research Council of Thailand (NRCT), Suranaree University of
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Technology with the Contract No. 61/2553 and Synchrotron Light Research
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Institute of Thailand under contract No.2552/05. The authors wish to thank
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scientists of BL3.2Ub at SLRI for their support during the experiments.
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Table 1. Chemical composition of AISI 4140 and H13 steels (in wt.%) determined by
atomic emission spectroscopy
Grade
C
Cr
Si
Mn
Mo
V
Fe
AISI 4140
0.47
0.81
0.26
0.78
0.19
-
Bal.
AISI H13
0.42
5.40
0.48
0.35
1.12
0.81
Bal.
358
17
359
360
Table 2. The deposition conditions of CrN coating (Keawhan et al., 2012;
Wongpanya et al., 2014)
Ar flow rate (sccm)
6
N2 flow rate (sccm)
9
Base pressure (mbar)
5.0 x 10-5
Working pressure (mbar)
3.5 x 10-3
Ar pressure (mbar)
3.3 x 10-3
N2 pressure (mbar)
5.7 x 10-4
Current (A)
800
Voltage (V)
-456
Deposition time (min)
45
Target-to-substrate spacing (cm)
15
Coating thickness (nm)
914
361
362
18
363
Table 3. List of samples for corrosion test
Substrate
AISI 4140
AISI H13
SiC paper
Uncoated samples
CrN coated samples
No. 180
AISI 4140 - 180
CrN/4140-180
No. 600
AISI 4140 - 600
CrN/4140-600
No. 1200
AISI 4140 - 1200
CrN/4140-1200
No. 180
AISI H13-180
CrN/H13-180
No. 600
AISI H13-600
CrN/H13-600
No. 1200
AISI H13-1200
CrN/H13-1200
364
19
365
Table 4. Corrosion parameters obtained from the polarization curves of uncoated and
366
CrN coated samples
pH
Samples
AISI 4140
pH2
AISI H13
CrN/4140
CrN/H13
AISI 4140
pH7
AISI H13
CrN/4140
CrN/H13
AISI 4140
pH10
AISI H13
CrN/4140
CrN/H13
SiC paper
Ecorr
(mV)
-571
-595
-559
-514
-578
-473
-538
-520
-518
-509
-500
-499
-535
-535
-598
-478
-527
-534
-485
-442
-400
-361
-407
-291
-582
-595
-559
-450
-352
-482
-465
-421
-419
-442
-389
-386
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
No.180
No.600
No.1200
367
20
Icorr
(µA/cm2)
175.00
67.00
52.00
120.00
55.00
42.92
75.00
20.00
12.50
26.00
18.07
7.24
32.00
21.00
12.00
18.17
7.55
4.36
16.00
8.50
6.50
7.00
6.18
1.33
18.00
12.50
7.00
35.06
15.25
16.05
9.00
4.50
2.20
14.00
6.50
3.70
CR
(mm/year)
1.9356
0.7786
0.6043
1.3946
0.6392
0.4992
0.9382
0.2502
0.1563
0.3252
0.2240
0.0897
0.3719
0.2440
0.1394
0.2113
0.0878
0.0507
0.2001
0.1063
0.0813
0.0875
0.0766
0.0165
0.2091
0.1564
0.0813
0.4078
0.1773
0.1867
0.1126
0.0563
0.0275
0.1751
0.0813
0.0460
368
369
370
Figure 1. Microstructures of the substrate samples; a) AISI 4140, b) AISI H13, c)
AISI4140 after heat treatment and d) AISI H13 after heat treatment
371
21
372
373
Figure 2. X-ray diffraction pattern of CrN film on AISI 4140 and H13 steels
22
374
375
376
Figure 3. The polarization curves of samples in 3.5 wt% NaCl solution at a) pH 2, b)
pH 7 and c) pH 10
23
\
377
378
Figure 4. Comparative corrosion rate of the uncoated and CrN coated samples
379
24
380
381
382
Figure 5. Secondary electron images of surface of CrN/4140-180, CrN/4140-600 and
CrN/4140-1200 samples before corrosion test.
383
384
25
385
386
Figure 6. Secondary electron images of uncoated (AISI 4140-180 and AISI H13-180)
387
and CrN coated (CrN/4140-180 and CrN/H13-180) samples after corrosion
388
test in 3.5 wt% NaCl solution
389
26
390
391
Figure 7. X-PEEM images and XAS spectra of the CrN/H13-180 sample after
corrosion test in 3.5 wt.% NaCl solution at pH 2
27
392
393
Figure 8. Comparative X-PEEM images and XAS spectra of the CrN/4140-180 and
394
CrN/H13-180 samples after corrosion test in 3.5 wt.% NaCl solution at pH 2
28