The Effect of MEVVA Implanted Cr on the Corrosion Resistance of CrN

7/28/2019 The Effect of MEVVA Implanted Cr on the Corrosion Resistance of CrN

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Surface and Coatings Technology 172 (2003) 7278

0257-8972/03/$ - see front matter 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0257-8972(03)00318-9

The effect of MEVVA implanted Cr on the corrosion resistance of CrNcoated low alloy steel by cathodic arc plasma deposition

K.L. Chang , S. Han , J.H. Lin , J.W. Hus , H.C. Shih *a b a a a,

Department of Materials Science and Engineering National Tsing Hua University, Hsinchu 300, Taiwan, ROCa

National Taichung Institute Technology, Taichung, Taiwan, ROCb

Received 16 September 2002; accepted in revised form 3 February 2003

Abstract

Metal vapor vacuum arc (MEVVA) source implantation is a novel and profitable surface modification process. Cathodic arcplasma deposition of CrN coating has been applied on an industrial scale to improve the corrosion resistance of AISI 4140 steel.The effect of the Cr implanted by MEVVA (Crysteel) on the corrosion behavior of CrNysteel through the surface modificationwas investigated. Both AISI 4140 steel and its coated assemblies of Crysteel, CrNysteel and CrNyCrysteel were evaluated inaerated 0.1 N HCl solution. The composition and structure of the MEVVA implanted Cr and cathodic arc plasma deposited CrNon steel were studied by X-ray diffraction and secondary ion mass spectrometer. The polarization resistance (R ) of all samplespwas measured and compared with d.c. polarization resistance and electrochemical impedance spectroscopy. The microstructure ofcorroded samples was also examined by scanning electronic microscopy and electron probe X-ray microanalyser. The resultsindicated that the corrosion resistance of CrN coated steel was significantly enhanced by the MEVVA implanted Cr in CrN yCrysteel. 2003 Elsevier Science B.V. All rights reserved.

Keywords: Metal vapor vacuum arc; CrNysteel; CrNyCrysteel; Electrochemical impedance spectroscopy; Corrosion resistance

1. Introduction

Ion implantation has been used to improve surfaceproperties, such as corrosion, wear and oxidation resis-tance. The final surface microstructures of the ion-implanted steels are from stable to metastable or toamorphous states on the surface w1x. Metal vapor vacu-um arc (MEVVA) ion source is an attractive ionimplantation technique for surface modification w2,3x,

which may have advantages in improving surface hard-ness, wear and corrosion performance w4 6x.

Transition metal nitride coatings have been widelyused for the corrosion protection of steels. In aggressiveenvironments, the major corrosion problem of coatedsteels is the defects in the coating, e.g. porosity, cracks,pinholes that may form during the deposition process.These defects may form direct paths between the sub-strate and the exposed environment. The application of

*Corresponding author. Tel.: q886-3-571-5131, ext. 3845; fax: q886-3-571-0290.

E-mail address: [email protected] (H.C. Shih).

an intermediate layer between the substrate and coatingis used to limit the effect of the defects and to improvethe corrosion resistance of the substrate w7,8x. Theintermediate layer can be prepared by several techniques,such as electroplating, electroless plating and PVDmethods w9x. A duplex coating of (CrNyCrysteel) withthe electroplate chromium as the intermediate layer hasbeen investigated w10,11x. The MEVVA ion implantationwas used to modify the steel surface. One of the main

ideas is to introduce surface elements with a high affinityfor oxygen in reducing corrosion of steels using ionimplantation. This makes the passivation easier to pro-duce better corrosion resistance w12x. This study focusedon the effect of MEVVA Cr ion implantation on thecorrosion resistance of Crysteel. The corrosion resistanceof CrNyCrysteel was compared with that of CrNysteelby electrochemical approaches.

2. Experimental

AISI 4140 was used as the substrate with the follow-ing chemical composition in weight percentage: 0.39 C,


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73K.L. Chang et al. / Surface and Coatings Technology 172 (2003) 7278

Fig. 1. XRD pattern of steel, Crysteel, CrNysteel and CrNyCrysteel.

Fig. 2. SIMS depth profiles of Cr, Fe, O and N for (a) CrNysteel (b)CrNyCrysteel.

Fig. 3. R values for the steel, Crysteel, CrNysteel and CrNyCrysteelpas a function of immersion time.

0.24 Si, 0.71 Mn, 0.029 P, 0.04 Ni, 0.18 Mo, 1.06 Cr

and Fe bal. Disk-shaped specimens 3 mm thick and 15mm in diameter were machined for electrochemicalmeasurements, including electrochemical impedancespectroscopy (EIS) and d.c. polarization resistancemeasurement (R ). All specimens were ground with apseries of emery papers of 120, 400, 800, 1000 and 1200grit. Contamination on the surface was first ultrasoni-cally cleaned with acetone and ethanol, and then thesample was stored in a desiccator prior to the coatingoperation.

An implanted chromium layer was applied using aMEVVA ion implanter at an extraction voltage of 50kV. The Cr ion energy, ion doses and current density

were 100 keV, 2=10 ionsycm and 6 mAycm , respec-17 2 2

tively. The substrate temperature was measured by a K-type thermocouple, and the substrate was heated fromroom temperature up to 100 8C after implantation.Subsequent deposition of the CrN film was conductedin a cathodic arc plasma deposition system. A Crinterlayer prior to the CrN deposition in order to providebetter adhesion between the substrate and its coating.During the deposition process, the substrate was biasedwith a d.c. voltage ofy150 V at an arc current of 60A, which gave a substrate temperature of 350 8C underthe N partial pressure of 2.7 Pa for a deposition time2

of 70 min.The microstructure was observed by scanning elec-tronic microscopy (SEM), and the crystallographic phas-es were determined by X-ray diffraction (XRD) Cu Karadiation (MAC Sci., model MXP-18). The specimenswere investigated using uy2u diffraction mode rangingfrom 30 to 908. The Cr atom distributions were deter-mined by secondary ion mass spectrometer (SIMSCameca IMS4F). The electrochemical corrosion analysiswas controlled by an EG&G 273 potentiostat, using athree-electrode cell configuration. A saturated calomelelectrode was used as the reference and a platinum sheetas the counter electrode. AISI 4140 steel and the three

coated assemblies (i.e. Crysteel, CrNysteel and CrNyCrysteel) were immersed in 400 ml aerated 0.1 N HClsolution for corrosion tests. The corrosion current and


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74 K.L. Chang et al. / Surface and Coatings Technology 172 (2003) 7278

Fig. 4. (a) Corrosion potential and (b) corrosion current density ofthe CrNyCrysteel in 0.1 N HCl solution with immersion time.

Fig. 5. Cross-sectional SEM images after EIS tests (a) CrNyCrysteeland (b) CrNysteel.

the polarization resistance R were evaluated based onpthe linear polarization method over a small potentialrange of"15 mV with respect to the corrosion potential(E ). For the polarization resistance determination, acorrsweep rate of 10 mVymin was employed. The EISmeasurements were analyzed using a frequency analyzer(EG&G 5301). The impedance spectra were recordedin a frequency range between 100 kHz and 10 mHz.The amplitude of sinusoidal signals was 10 mV around

the free corrosion potential E . The X-ray mapping ofcorrcorroded specimens was performed by electron probeX-ray microanalyser (EPMA JXA-8800M).

3. Results and discussion

3.1. Crystallographic analysis

The XRD spectra of CrNysteel and CrNyCrysteelhave been compared and consistent with the JCPDSdatabase of CrN phase (No. 11-0065). In the uy2u

mode, as shown in Fig. 1, the presence of the (2 2 0)and (2 0 0) peaks, and a preferred orientation of CrN(2 2 0) of the CrN structure can be clearly observed.However, the XRD spectrum of Crysteel, i.e. Fig. 1(2)which is similar to that of substrate, i.e. in Fig. 1(1),does not provide any information related to the Cr

phase. The results of SIMS measurements are used onCrNyCr (implanted)ysteel shows the depth profiles of

Cr, Fe, O and N in Fig. 2a. It is evident that the54 57 16 14

composition of Cr and N is uniform along the thickness(;2 mm) of the coatings. The coatingysubstrate inter-face indicates the thickness of CrN coatings is approxi-mately 2 mm. The contribution of implanted chromiumcan be seen more clearly at the coatingysubstrate inter-face of Cr distribution (Fig. 2b) when compared withthat in Fig. 2a. The distribution of Cr implanted layer


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Fig. 6. Nyquist plots as a function of immersion time in a 0.1 N HClsolution: (a) CrNysteel and (b) CrNyCrysteel.

Fig. 7. Bode plots as a function of immersion time in a 0.1 N HClsolution: (a) CrNysteel and (b) CrNyCrysteel.

varies with the implantation depth. From the depthprofile the maximum implantation depth is ;200 nm.

3.2. Electrochemical behavior

3.2.1. Polarization resistance

The polarization resistances (R ) of steel, Crysteel,pCrNysteel and CrNyCrysteel resulting from linear polar-ization measured in 0.1 N HCl as function of immersiontime are presented in Fig. 3. The R value of Crysteelpis obviously higher than that of bare steel, indicatingthat the corrosion resistance has been improved by theCr implanted steel surface. The R of CrNysteel decreas-pes more rapidly than that of CrNyCrysteel as immersiontime increases, showing that CrNyCrysteel possesses abetter capability for preventing the localized galvanicattack between the coating and the substrate. The R ofpCrNyCrysteel indicates that the Cr implanted layer is

effective in reducing the corrosion. The variation inE and i of CrNysteel and CrNyCrysteel in 0.1 Ncorr corrHCl solution with immersion time is shown in Fig. 4.The i is an important parameter to understand thecorrkinetic mechanism of corrosion reactions. The i ofcorrthe CrNysteel is higher than that of CrNyCrysteel (Fig.

4b) and the E of the CrNysteel decreases for the firstcorr12 h of immersion, and approaches a steady state valueofy0.55 V thereafter, as compared to y0.54 V for the


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Fig. 8. SEM micrograph and its X-ray mappings of Cr, Cl and Fe: (a) CrNysteel and (b) CrNyCrysteel.

CrNyCrysteel (Fig. 4a). It is apparent CrNysteel suffersmore corrosion than CrNyCrysteel coatings w13x. Thecross-sectional SEM images after the immersion testsshow the appearance of corrosion products at the coat-ingysubstrate interface, as shown in Fig. 5. A part ofCrN coating has been spilled off due to the local

expansion of the corrosion products in a pit (;400 mm)on CrNysteel. The corrosion mechanism of the CrNcoated steel may be comprehended by the generation ofpits on the substrate, resulting from the dissolved anodesthat were produced due to the coating defects on theexposed substrate steel. During a progressive corrosionprocess, new pits initiated and existing pits may spreadlaterally. Finally, these pits link up together to form alarge pit w14x. The corrosion of steel takes place insidethe coating, where a higher i of CrNysteel is producedcorrw15x. It may be concluded from these observations thatthe Cr layer in CrNyCrysteel provides a beneficial effect

on the corrosion resistance of the steel.

3.2.2. Electrochemical impedance spectroscopy behavior

EIS can provide useful information on the corrosionprocess of the PVD coated steel exposed in an aggressiveenvironment w8,16x. The Nyquist impedance diagramsat the free corrosion potential for CrNysteel and CrNyCrysteel are presented in Fig. 6. The polarization resis-

tance (R ) values were also evaluated by the EISpbehavior. The solution resistance (R ) was estimatedsfrom the impedance in a high frequency range, whilethe sum of R and the R was estimated from thep simpedance in the low frequency range. The differencebetween these two impedance values results in R , whichpis inversely proportional to the corrosion rate. The Rpvalues from EIS confirm the trends of results from linearpolarization. The impedance values of CrNysteel andCrNyCrysteel decrease with the immersion time.

Fig. 7 shows the bode plots of CrNysteel and CrNyCrysteel as a function of immersion time. A second

time constant can be clearly resolved at lower frequen-


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Fig. 8 (Continued).

cies of CrNysteel Bode-phase plot after 108 h immersion(Fig. 7a). The different shape of Bode-phase plot afterlonger time of immersion is associated with the presenceof local detachment of the coating due to the accumu-lation of corrosion products at the coatingysubstrateinterface w17,18x. The Bode-phase plot of CrNyCrysteel(Fig. 7b) shows a much less resolved time constant at

lower frequencies after longer time of immersion. TheX-ray mapping of the elements Cr, Fe and Cl of thecorroded CrNysteel and CrNyCrysteel are illustrated inFig. 8a and b, respectively. From the EPMA observationon CrNysteel, the corrosion products plugged the defectsin CrNysteel to detach the coating of CrN (Fig. 8a).Such mechanism is in agreement with the EIS behavior.When compared with the CrNysteel, the CrN coating ofCrNyCrysteel degrades more slowly (Fig. 8b). Theimplanted Cr in the CrNyCrysteel assembly is appar-ently more efficient in reducing the corrosion of theCrNysteel assembly. The performance of the coated

steels against corrosion is strongly affected by thecoating defects w17,19,20x. The Cr implanted layerpromotes the surface passivity of the steel substrate andphysically to isolate the structure defects of the CrNcoatings.

4. Conclusions

Cr implantation using MEVVA is an effective methodto improve the surface passivity of the steel substrateand to physically cut off the paths originating from thedefect structure of the CrN coating. The R valuespevaluated either from d.c. polarization or from EISshowed that the Cr implanted steel provided a betterprotection against corrosion of steels in 0.1 N HClsolution.

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The Effect of MEVVA Implanted Cr on the Corrosion Resistance of CrN