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Vacuum 72 (2004) 489–500

*Correspondin

Mechanical Eng

Beijing 100084, C

E-mail addre

0042-207X/$ - see

doi:10.1016/j.vac

Ni-based superalloy surface alloying by double-glow plasmasurface alloying technique

Xu Jianga,b,*, Xu Zhangc, Xishan Xieb, Zhong Xud, Wenjin Liua

a Laser Processing Research Center, Mechanical Engineering Department, Tsinghua University, Beijing 100084, Chinab University of Science and Technology Beijing, Beijing 100083, China

c Beijing Normal University, Beijing 1000875, ChinadTaiyuan University of Technology, TaiYuan 030024, China

Abstract

The double-glow surface alloying technique, also called the Xu-Tec/Xu-Loy process, is a novel technique in the field

of surface alloying. This technique allows alloy layers with unique physical, chemical and mechanical properties, such as

nickel-based alloy layers, stainless-steel layers and age-hardened surface high-speed steel layers to be formed at the

surface of treated metallic materials. In this paper, recent research of the application of the double-glow plasma surface

alloying technique in the formation of corrosion resistance alloy layers is briefly reviewed. The results of a study of Ni–

Cr–Mo–Nb and Ni–Cr–Mo–Cu corrosion-resistant alloying layers as well as composite alloying layers with an electric

brush plating Ni interlayer are reported.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Double glow; Composite alloying layer; Ni–Cr–Mo–Cu; Ni–Cr–Mo–Nb

1. Introduction

It is well known that corrosion of most metalliccomponents is related to the surface properties ofmaterials. The techniques of surface treatmentsthat result in special properties have stimulatedmuch interest for increasing hardness, corrosionresistance, etc. A new surface treatment techniqueknown as the double-glow plasma surface alloyingtechnique, also called the Xu-Tec/Xu-Loy process,was patented in 1985 [1]. The double-glow surfacealloying technique is a unique and hybrid plasma

g author. Laser Processing Research Center,

ineering Department, Tsinghua University,

hina.

ss: xujiang73@sina.com.cn (X. Jiang).

front matter r 2003 Elsevier Ltd. All rights reserv

uum.2003.11.001

surface treatment process which is the evolution ofboth plasma nitriding and sputtering techniquesand was developed in response to the need forhigher-quality alloy layers on the surface of low-cost materials. This technology employs a low-temperature plasma produced by a glow dischargeto drive source material atoms of one or moreelements to be sputtered and then diffused into thesubstrate’s surface. The depth of the alloying layercould vary from several micrometers to 500 mm,with alloying elements in a concentration of fewpercent to 90% or more. Mono-element alloyingof alloying elements of Ni, Cr, Mo, W, Ta, Al, Ti,etc. [2] and multi-element alloying of alloyingelements of Ni–Cr, W–Mo, W–Mo–Cr–V, etc.have been studied [3,4]. Compared to ion implan-tation or laser surface alloying, the double-glow

ed.

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X. Jiang et al. / Vacuum 72 (2004) 489–500490

plasma technique is cheaper for many potentialusers. Much research has been accomplished overthe past 10 years in exploring the area of corrosionresistance with the Xu-Tec process. From thepoint of view of corrosion resistance and techno-logical feasibility, Ni–Cr–Mo–Nb corrosion resist-ing alloy (Inconel 625) was first selected as a sourcematerial for supplying the alloy element [5]. Inorder to further increase the corrosion resistanceof alloying layer, a high molybdenum content ofNi–Cr–Mo–Cu was used as a source material forsupplying the alloy elements [6]. Based on theanalysis of experimental results, a novel compositetechnique which consists of brush plating surfacetreatment and double-glow plasma surface alloy-ing was developed [7], and the corrosion resistanceof the alloying layer formed on the low-carbonsteel can obviously be improved using thiscomposite surface technique.

2. Experimental method

The Xu-Tec process is performed in a vacuumchamber. Fig. 1 indicates the general principle ofthe Xu-Tec process. There are three electrodes: theanode and two negatively charged members, thecathode (workplace) and the source electrode. The

Fig. 1. Schematic diagram of the Xu-Tec unit.

source electrode is made up of the desired alloyingelements. With the two power supplies turned on,both the cathode and source electrode are sur-rounded by a glow discharge. One glow dischargeheats the substrate to be alloyed while the secondglow strikes the source electrode materials forsupplying desired alloying elements. The desiredalloying elements travel toward the substrate anddiffuse into the substrate materials surface formingan alloying layer.

Brush plating of Ni is an electroplating processthat is performed with a hand-held or portableplating tool. The plating tool is soaked in theplating solution and then the plating material (lowcarbon steel) is deposited by brushing the platingtool against the base material. The plating solutionwas delivered to the work area by a porous,absorbent cover wrapped over the anode of theplating tool. The composition of electric brush Nisolution was:

NiSO4 � 7H2O

253–255 g/l NH3 �H2O(25%) 100–110 ml (NH4)3C6H5O7 55–56 g/l CH3COONH4 22–23 g/l (COONH4)2 �H2O 0.1–0.2 g/l

Electric brush plating was operated at a workingvoltage of 12–14 V. The Ni plating layer with adeposition thickness of 20 mm was obtained. Thecomposite process was accomplished after thedeposited sample was used as substrate to bealloyed.

A superalloy Inconel 625 (the composition is Cr2–23, Mo 8–10, Nb 3.5–4.5, Ni>58 in wt%) plate(150� 50� 3 mm) and Hastelloy C-2000 was usedas the source electrode for supplying alloyingelements, respectively. Low-carbon steel 1020,industrial pure iron and 304 stainless-steel plates(80 mm� 25 mm� 3mm) were used as substratematerials. The processing parameters were: sourceelectrode voltage, 1050 V; substrate voltage, 250 V;working pressure, 35 Pa; and parallel distancebetween the source electrode and the substrate,15 mm and treatment time of 3 h.

The chemical compositions and microstructureof the surface layer were analyzed by a LEO-1450scanning electron microscopy (SEM) and X-ray

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X. Jiang et al. / Vacuum 72 (2004) 489–500 491

energy-dispersive spectroscopy. The phase struc-ture identification was determined using D/Max-RB X-ray diffraction (XRD) studies using CuKa

radiation. A Hitachi H-800 TEM was used toidentify the phase.

Potentiodynamic anodic polarization curveswere obtained at a sweep rate of 20 mV/min,starting from the time when the open circuitpotential becomes steady after immersion of thespecimen for about 10 min. A saturated calomelelectrode and a platinum sheet were used asreference and counter electrodes, respectively. Anelectrolyte used was 3.5% NaCl and 5% HClsolution open to air at 25�C. The chemicalcompositions of the passive film on the surface ofthe two kinds of substrates of alloying layer werestudied by F-5600 type XPS and its XPS experi-mental parameters were as follows: a MgKa targetwas used with a power of 300 W, a pass energy of23 eV, analysis area of 0.8 mm2 and the vacuum inthe test chamber exceeds 5� 10�8 Pa. All bindingenergies were corrected for the sample chargingeffect with reference to the C1s line at 284.6 eV.The depth profiles were achieved by etching withargon ions of 4 keV energy. During the depthprofiling experiment, the measured sample currentwas 25 mA, and the etched area 1� 1 mm2. Datawere acquired for Ni2p (846–885 eV), Cr2p (572–589 eV), Mo3d (225–240 eV), Fe2p (704–716 eV)and C1 s (276–294 eV) regions. The deconvolutionof the spectra was achieved by fitting the data witha Gaussian/Lorentzian-combination peak shapewith variation in peak full-width at half-max-

distance from thesurface(¦ Ìm)

60.00

40.00

20.00

0.00 20.00 40.00distance from th

0.00 20.0

0.00

dist

ribut

ion

of a

lloy

NiC

r M

o N

bFe

(%) 80.00

60.00

40.00

20.00

0.00dist

ribut

ion

of a

lloy

NiC

r M

o N

bFe

(%)

NiCrMoNb

(a) (b)

Fig. 2. The alloy elements distribution of Ni–Cr–Mo–Nb alloyed la

stainless steel.

imum, position and height being determined by aniterative program. The peak identification wasperformed by reference to an XPS database [8].

3. Results and discussion

3.1. Ni–Cr–Mo–Nb alloying layer

The new Ni–Cr–Mo–Nb alloying layer wasformed on surfaces of three substrates (pure iron,low-carbon steel and 304 stainless steel), by usingInconel 625 alloy as the source material. Thedistribution of alloying elements in the surfacealloying layer formed on those substrates is shownin Fig. 2. The gradient alloying layers arecomposed of alloying elements (Ni, Cr, Mo, Nb)-enriched layer, in which the composition is similarto that of superalloy Inconel 625, and theconcentration of alloying element changes gradu-ally with the depth direction of the surface alloyinglayer.

The typical microstructure of surface alloyedlayers formed on three different substrates bydouble-glow plasma surface alloying is shown inFig. 3. The surface alloying layer is continuous andcompact. The phase structure of surface-alloyedlayers formed on the surface of three kinds ofsteels (pure iron, low-carbon steel and 304 stainlesssteel), is shown in Fig. 4. The microstructure of thesurface alloyed layer is g matrix and precipitates ofcarbide NbC and the Laves phase. More carbideprecipitates in the alloying layer on the substrate

esurface(¦ Ìm)0 40.00

distance from thesurface(¦ Ìm)0.00 10.00 20.00 30.00 40.00

80.00

60.00

40.00

0.00

dist

ribut

ion

of a

lloy

NiC

r M

o N

bFe

(%)Ni

CrMoNb

NiCrMoNb

(c)

yer formed on: (a) pure iron, (b) low-carbon steel, and (c) 304

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Fig. 3. Microstructure of surface alloyed layer formed on: (a) pure iron, (b) low-carbon steel, and (c) 304 stainless steel.

Fig. 4. XRD spectra of the surface alloyed layers formed on

different substrates.

X. Jiang et al. / Vacuum 72 (2004) 489–500492

of low-carbon steel, because of its higher carbonconcentration.

The potentiodynamic polarization tests of sur-face alloying layer on three kinds of substrates

have been investigated in comparison to thenickel-base superalloy Inconel 625 and 304 stain-less steel in 3.5% NaCl and 5% HCl. The resultsare shown in Table 1.

In 3.5% NaCl solution, the passive currentdensity ip of the surface alloying layer formed onthe 304 stainless steel was lower than that ofInconel 625, and one order of magnitude lowerthan that of 304 stainless steel. The passive currentdensity ip of the surface alloying layer formed onthe industrial pure steel and low-carbon steel arealso lower than that of Inconel 625 and 304stainless steel. The pitting potential Eb of surfacealloying layer formed on stainless steel is 150 mVhigher than that of the Inconel 625, and 560 mVhigher than that of 304 stainless steel. The pittingpotential Eb of surface alloying layer formed onthe surface of industrial pure iron is also higherthan that of Inconel 625 and 304 stainless steel.The pitting potential Eb of surface alloying layerformed on the surface of low-carbon steel is lowerthan that of Inconel 625, and also higher than thatof 304 stainless steel.

In 5% HCl solution, the pitting potential ofsurface alloying layer formed on the surface ofstainless steel is higher than that of Inconel 625and 660 mV higher than that of 304 stainless steel.The passive current density ip of the surfacealloying layer formed on the 304 stainless steel islower than that of Inconel 625. The passive currentdensity ip of the surface alloying layer formed onthe pure iron and low-carbon steel are higher thanthat of Inconel 625, but obviously lower than thatof 304 stainless steel.

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Table 1

Electrochemical properties of different surface alloying layers in 3.5% NaCl and 5% HCl solution (Ni–Cr–Mo–Nb alloying layer)

Specimen 3.5% NaCl solution 5% HCl

Pitting potential

(mV)

Passive current density

(mA/cm2)

Pitting potential

(mV)

Passive current density

(mA/cm2)

Alloying layer on stainless steel 770 0.316 980 7.94

Alloying layer on pure iron 830 0.630 900 19.9

Alloying layer on low-carbon steel 410 0.707 700 272

Inconel 625 620 0.794 900 15.8

304 stainless steel 210 1.99 320 501.18

0 5 10 15 20 25 30 35 40

0

10

20

30

40

50

60 Ni Cr Mo Cu

cont

ent o

f Ni,

Cr,

Mo,

Cu(

%)

0

10

20

30

40

50

60

cont

ent o

f Ni,

Cr,

Mo,

Cu(

%)

distance form the surface/µm distance form the surface/µm

0 5 10 15 20 25 30 35 40

Ni Cr Mo Cu

(a) (b)

Fig. 5. The alloy elements distribution of Ni–Cr–Mo–Cu alloyed layer formed on: (a) low-carbon steel, and (b) 304 stainless steel.

X. Jiang et al. / Vacuum 72 (2004) 489–500 493

3.2. Ni–Cr–Mo–Cu alloying layer

Hastelloy C-2000 alloy is designed to resist anextensive range of corrosive chemicals, includingsulfuric, hydrochloric, and hydrofluoric acids.Unlike previous Ni–Cr–Mo alloys, which areoptimized for use in either oxidizing or reducingacids, C-2000 alloy extends corrosion resistance inboth types of environments. The combination ofmolybdenum and copper provides outstandingresistance to reducing media, while oxidizing acidresistance is provided by a high chromium content.In order to further increase the corrosion resis-tance of the alloying layer, we choose the excellentcorrosion resistance alloy, i.e. Hastelloy C-2000 asa source material in the double-glow process.

Gradient materials are formed on the surface oflower-carbon steel and 304 stainless steel afterplasma surface alloying treatment. The contentdistribution of alloying elements in the surface

alloying layer formed on two kinds of substratesobtained from the experimental parameters areshown in Fig. 5. The microstructures of the surfacealloyed layer formed on two kinds of substrates bydouble-glow plasma surface alloying are shown inFig. 6. The surface alloying layer is continuous andcompact. The phase structure of surface alloyedlayers formed on the surface of two kinds ofsubstrates is shown in Fig. 7. Fig. 7(a) presents atypical XRD profile obtained from the alloyinglayer formed on the low-carbon steel. It may benoted that carbide M6C and m phase emerge herein addition to the strong austenite (g) peak. It canbe seen from Fig. 7(b) that phases in the surfacealloyed layer formed on the 304 stainless steelconsist of g matrix and precipitated m phase andthe amount of m phase is very little. Comparedwith the alloying layer on the substrate of 304stainless steel, more carbide precipitates in thealloying layer on the substrate of low-carbon steel

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Fig. 6. Microstructure of surface alloyed layer formed on: (a) low-carbon steel, and (b) 304 stainless steel.

0

80

60

40

20

100

µµµµ

γ(11

1)γ(

111)

γ(20

0) γ(22

0)

γ(31

1)

γ(22

2)

Substrate:Low carbon steel

Substrate:Stainless steel

γ(22

0)

γ(31

1)

γ(22

2)

M6C

M6C M6C

M6C M

6CM6C

0

80

60

40

20

100

120

cps

cps

20 40 60 80 100

µµ

µ

2θ/(°)

20 40 60 80 100

Fig. 7. XRD spectra of the surface alloyed layers formed on

different substrates.

Fig. 8. The TEM morphology of: M6C (a), and m phase (b) and

diffraction pattern.

X. Jiang et al. / Vacuum 72 (2004) 489–500494

with a higher carbon concentration. The morphol-ogy and diffraction patterns of precipitated phasein alloying layer (M6C phase and m phase) byTEM are shown in Fig. 8. Fig. 8 shows that themorphology of M6C phase and m phase is block

and needle shaped, respectively. The chemicalcomposition of the M6C and m phases is listed inTable 2. The results indicate that the two kinds ofprecipitate phases have a high content of molyb-denum and is harmful to corrosion resistance ofthe alloying layer.

The potentiodynamic polarization tests of thesurface alloyed layer on two kinds of substrateshave been investigated in comparison to thenickel-based superalloy Hastelloy C-2000 and304 stainless steel in 3.5% NaCl. The results areshown in Table 3.

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Table 2

Chemical composition of M6C and m phase (wt%)

Precipitate phase Ni Cr Mo Cu Fe

m phase 19.71 14.26 44.77 1.43 19.83

M6C phase 14.32 12.47 57.17 5.09 10.94

matrix 44.22 14.55 7.26 3.45 30.53

Table 3

Electrochemical properties of different surface alloying layers in

3.5%NaCl

Specimen 3.5% NaCl solution

Pitting potential

(mV)

Passive current

density (mA/cm2)

Alloying layer on

stainless steel

910 10.02

Alloying layer on

low-carbon steel

190 31.62

Hastelloy C-2000 600 10.00

304 stainless steel 190 56.79

0 10 20 30 40 50 60-10

0

10

20

30

40

50

60

70

80

90 Ni Cr Mo

Cu Fe

Con

tent

of t

he a

lloy

Ni,

Cr,

Mo,

Cu,

Fe

(wt%

)

Distance form the surface (µm)

Fig. 9. Chemical composition of composite alloying layer.

X. Jiang et al. / Vacuum 72 (2004) 489–500 495

It can be clearly seen from Table 3 that thepassive current density ip of the surface alloyinglayer formed on 304 stainless steel is equivalent tothat of Hastelloy C-2000, and lower than that of304 stainless steel in 3.5% NaCl solution; thepitting potential Eb of the surface alloying layerformed on the 304 stainless steel is 310mV higherthan that of Hastelloy C-2000, and 720 mV higherthan that of 304 stainless steel; the passive currentdensity ip of the surface alloying layer formed onthe low-carbon steel is lower than that of 304stainless steel, and higher than that of Hastelloy C-2000; the pitting potential Eb of surface alloyinglayer formed on the low-carbon steel is exactly ofthe same amount as that of 304 stainless steel, andlower than that of Hastelloy C-2000.

3.3. Ni–Cr–Mo–Cu composite alloying layer

The corrosion test results from the Ni–Cr–Mo–Cu and Ni–Cr–Mo–Nb alloying layer indicate thatthe corrosion resistance of the alloying layer is to ahigh degree relative to the content of carbon in thesubstrate. The higher content of carbon of thealloying layer on the low-carbon steel must have

resulted in a higher amount of carbide precipita-tion and microstructural inhomogeneity within thesurface alloying layer. To enhance the corrosionresistance of the Ni–Cr–Mo–Cu alloying layer ona low-carbon steel substrate, a predeposited Niinterlayer was adopted by electric brush platingbefore double-glow plasma alloying process. Thiscomposite surface alloying layer is formed by ahigh-temperature diffused process producingstrong metallurgical bonding with the (carbonsteel) substrate which may be attributed directlyto the diffusion and mixing of atoms at the inter-face.

The distribution of alloying elements in thecomposite surface alloying layer is shown in Fig. 9.It is evident that the content of Ni, Cr, Mo, Cu inthe alloying layer decreases as the distance fromthe surface increases, but the Fe element in thealloying layer increases as the distance from thesurface increases because of diffusion of the alloyelement between the alloying layer and substrate.The gradient alloying layers are identically com-posed of alloying elements (Ni, Cr, Mo, Cu)-enriched layer, in which the composition is similarto that of Hastelloy C-2000, and transition layer tothe substrate. But there are differences between thecomposite alloying layer and single alloying layer(Fig. 5(a)). In Comparison with the compositealloying layer, the distribution of elements in thesingle alloying layer is sharper and the thickness of

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Table 4

Electrochemical properties of alloying layers in 3.5% NaCl

(substrate: low-carbon steel)

Specimen 3.5% NaCl solution

Pitting potential

(mV)

Passive current

density (mA/cm2)

Composite alloying layer 900 12.59

Single alloying layer 190 31.62

X. Jiang et al. / Vacuum 72 (2004) 489–500496

the alloying layer is about 10 mm thinner than thatof the composite alloying layer.

The typical microstructure of the compositealloying layer is shown in Fig. 10. XRD spectra(Fig. 11) show that the phase structure of thecomposite alloying layer consists of a single gphase. Table 4 shows the potentiodynamic polar-ization curves of the composite alloying layermeasured in a 3.5 NaCl solution open to air at25�C. The polarization curve of the alloying layerwithout an electric brush plating Ni interlayer(single alloying layer) is shown for comparison. Ina 3.5% NaCl solution, the pitting potential Eb of

Fig. 10. SEM photographs of cross-section composite alloying

layer.

alloying layer

20 40 60 80 100

0

2

4

6

8

10

2θ/(°)

γ

γ

CP

S

Fig. 11. XRD patterns of composite alloying layer.

the composite alloying is 710 mV higher than thatof a single alloying layer without a predeposited Nilayer. The passive current density ip of thecomposite alloying layer is also lower than thatof a single alloying layer.

In order to gain further insight into the natureof the passive film, XPS is employed to analyze thechemical composition and structure of an electro-chemically passive film of the alloying layers. TheXPS survey spectra of two kinds of alloying layers(Ni–Cr–Mo–Cu and composite Ni–Cr–Mo–Cualloying layer) after the anodic polarization testsin 3.5% NaCl solution are shown in Figs. 12(a)and (b), respectively. It can be seen from Figs.12(a) and (b) that there are Ni, Fe, Mo, Cr, O, Cchemical elements on the surface of the specimenafter contamination is cleared away, but chemicalelement of copper does not exist, which needs to beinvestigated further, which is possible relative tothe preferential dissolution of copper in corrosiveacid (this is a chemical process in corrosive acid,some reactive elements tend to dissolve, comparedwith other alloy elements). The composition andthickness of the passive films were determined byXPS depth profiling using argon ion. Fig. 13 showsthe depth distribution of all elements in the twokinds of layers as registered by XPS analysis. Theirconcentration has been calculated from the in-tensity of Ni2p, Cr2p, Fe2p, Mo3d, O1s and C1speaks. It is shown in Fig. 13(a) that the atomicconcentration of O and Mo elements is very steadyor decreases slowly with increasing sputteringtime; and before sputtering, Cr, Fe and Ni contentamount to 12.67, 9.64 and 11.47 at%, respectively,and after sputtering for 10 min, increasing slightlyto 14.25, 11.02 and 15.53 at%, respectively. Theanalysis of passive films formed on the single

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1000 800 600 400 200 00.0

1.4x106

1.2x106

1.0x106

8.0x105

6.0x105

4.0x105

2.0x105

-Ni3

p-N

i3S

-Mo3

d

-C1S

-NiL

MM

-NiL

MM

-O1S

-FeL

MM

-Cr2

P3

-FeL

MM

-Cr2

p1

-FeL

MM-F

e2p3

-Fe2

p1-O

KLL

-Ni2

p3-N

i2p1

-CK

LL

-MoM

NV

C/S

0.0

1.6x106

1.4x106

1.2x106

1.0x106

8.0x105

6.0x105

4.0x105

2.0x105

C/S

Binding Energy (eV)

1000 800 600 400 200 0

Binding Energy (eV)

-O2S

-Fe3

p-Ni3

p

-Mo3

d

-C1S

-NiL

MM

-NiL

MM

-O1S

-FeL

MM

1-C

r2p3

-Cr2

p1-F

eLM

M2

-FeL

MM

3-F

e2p3

-Fe2

p1-O

KLL

-Ni2

p3-N

i2p1

-CK

LL

(a) (b)

Fig. 12. XPS survey spectra of passive film formed on: (a) composite alloying layer, and (b) single alloying layer.

0 2 4 6 8 105

10

15

20

25

30

35

40

Mo2p

Ni2p

Fe2pCr2p

O1s

C1s

Ato

mic

Con

cent

ratio

n (%

)

5

0

10

15

20

25

30

35

40

Ato

mic

Con

cent

ratio

n (%

)

Sputter Time (min)0 2 4 6 8 10

Sputter Time (min)

Mo2p

Ni2p

Fe2p

Cr2p

O1s

C1s

(a) (b)

Fig. 13. Depth composition profile of passive film formed on: (a) composite alloying layer, and (b) single alloying layer.

X. Jiang et al. / Vacuum 72 (2004) 489–500 497

alloying layer (Fig. 13(b)) shows that the atomicconcentrations of O, Cr and Mo elements isobviously lower than that of the compositealloying layer, and the contents of Ni and Feelements is higher than that of the compositealloying layer. It must be noted that the depth of Oelement in the passive films represents the thick-ness of passive films. Therefore, the thickness ofthe passive film formed on the composite is thickerthan that of the single alloying layer. The valencestate spectra of Cr, Fe, Ni and Mo after sputteringfor 2min were fitted, and a valence state analysiswas conducted. Curve fittings of Cr2p, Fe2p, Ni2p

and Mo3d high-resolution spectrum after sputter-ing for 2min is shown in Figs. 14 and 15,respectively. For passive films formed on thecomposite alloying layer, after sputtering for2 min, the Cr spectrum can be fitted with threemain peaks (Fig. 14(a)). The respective bindingenergies at approximately 574.14, 576.43 and578.50 eV with corresponding contributions of17.10%, 60.26% and 22.64% are attributed tometallic Cr, trivalent Cr oxide (Cr2O3) andhexavalent Cr oxide (CrO3). The Mo spectrumcan also be fitted with three main peaks(Fig. 14(b)). The respective binding energies at

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Fig. 14. Cr2p(a), Mo3d(b), Fe2p(c), and Ni2p (d) XPS spectra from the passive film formed on the composite alloying layer.

X. Jiang et al. / Vacuum 72 (2004) 489–500498

approximately 228.33, 230.43 and 232.66 eV withcorresponding contributions of 50.87%, 27.12%and 21.99% are attributed to metallic Mo,quadrivalent Mo oxide (MoO2), and hexavalentMo oxide (MoO3). The Fe spectrum can be fittedwith three main peaks (Fig. 14(c)). The respectivebinding energies at approximately 706.86, 709.02and 711.54 eV with corresponding contributions of42.39%, 39.19% and 18.42% are attributed tometallic Fe, divalent Fe oxide (FeO) and trivalentFe oxide(Fe2O3). It can be seen from Fig. 14(d)that the Ni element is in the form of metallic Ni.Fig. 15 shows the high-resolution XPS scans fromthe passive film formed on the single Ni–Cr–Mo–Cu alloying layer Cr2p region. After an electro-chemical test, three main peaks were observed at574.11, 576.25 and 578.13 eV (Fig. 15(a)) withcorresponding contributions of 50.22%, 33.38%

and 16.40% attributed to metallic Cr, trivalent Croxide (Cr2O3) and hexavalent Cr oxide (CrO3).Same as that of passive film formed on thecomposite alloying layer, Ni element is in theform of metallic Ni (Fig. 15(b)) Fe region. Therespective binding energies at approximately706.96, 709.08 and 711.54 eV with correspondingcontributions of 40.06%, 37.74% and 22.20%attributed to metallic Fe, divalent Fe oxide (FeO)and trivalent Fe oxide (Fe2O3) (Fig. 15(c)). Butdifferent from passive film formed on the compo-site alloying layer, Mo element is in the form ofmetallic Mo (Fig. 15(d)).

It can be concluded that the passive film formedon the composite alloying with electric brushplating interlayer consists of Cr2O3, CrO3,MoO3, MoO2, FeO and Fe2O3 and the passivefilm formed on the single alloying layer without

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Fig. 15. Cr2p (a), Ni2p (b), Fe2p(c), and Mo3d (d) XPS spectra from the passive film formed on the single alloying layer.

X. Jiang et al. / Vacuum 72 (2004) 489–500 499

electric brush plating interlayer consists of Cr2O3,CrO3, Fe2O3 and FeO. The amount of oxide andthickness of oxide film of the former are greaterthan that of the latter, which is very important forproviding effective protection of the materialsurface from corrosion. The poor corrosionresistance of a single alloying layer is due to thehigher amount of the carbide precipitation andintermetallic compound m phase. The passive filmnear the detrimental phase is prone to assault byaggressive ions. The composite alloying layerpossesses an excellent corrosion resistance as aresult of microstructure homogeneity.

4. Conclusions

1. New Ni–Cr–Mo–Nb and Ni–Cr–Mo–Cucorrosion-resistant surface alloying layers areformed on low-cost substrates such as pure

iron, low-carbon steel and 304 stainless steelby the double-glow plasma surface alloyingtechnique.

2. The composite alloying layer has better corro-sion resistance than that of the alloyinglayer without predeposited electric brushplating Ni interlayer in 3.5% NaCl solution.Thus, it is concluded that double-glow plasmasurface alloying of low-carbon steel with electricbrush plating Ni interlayer is an appropriatetechnique to enhance the corrosion resistance ascompared with a single double-glow surfacealloying.

3. The passive film formed on the compositealloying consists of Cr2O3, CrO3, MoO3,MoO2, FeO, Fe2O3 and metallic Ni and thepassive film formed on the alloying layerwithout electric brush plating interlayer consistsof Cr2O3, CrO3, Fe2O3 and FeO and metallic Niand Mo, but the thickness of passive layer is

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X. Jiang et al. / Vacuum 72 (2004) 489–500500

obviously less than that of the compositealloying layer and the amount of Cr2O3,MoO3, MoO2, Fe2O3 and FeO of the formeris notably higher than that of the latter.

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