Surface and Coatings Technology 168(2003) 156–160

0257-8972/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž02.00861-7

Double glow surface alloying of low carbon steel with electric brushplating Ni interlayer for improvement in corrosion resistance

Xu Jiang *, Xishan Xie , Zhong Xua, a b

High Temperature Materials Labs, School of Materials Science and Engineering, University of Science and Technology, Beijing, PR Chinaa

Research Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, PR Chinab

Received 19 June 2002; accepted in revised form 26 November 2002

Abstract

The present study concerns double glow plasma alloying of low carbon steel with predeposited Ni(by electric brush plating)to enhance corrosion resistance of a single alloying layer without electric brush plating interlayer. The composition andmicrostructure of the alloying layer were analyzed using scanning electron microscopy and X-ray diffraction. The results indicatedthat the composite layer consists of singleg phase, but the single alloying layer without predeposited Ni interlayer(by brushplating) consisted of ag matrix and several precipitates(M C andm phase). Owing to thermal effects during the double glow6

plasma alloying process, Ni could diffuse toward the substrate, which taken as a barrier layer for carbon was effective to decreasethe content of carbon in the composite alloying layer and at the same time formation of interface diffusion between the brushplating layer and the substrate could improve the adhesion strength. Corrosion resistance of the composite alloying layer wasinvestigated by an electrochemical method in 3.5% NaCl and 5% HCl solution, and 20-h immersion tests in 20% H SO and2 4

20% HCl solution. The experimental results showed that the corrosion resistance of composite layer was better than that of thesingle alloying layer. Thus, it was concluded that double glow plasma surface alloying of low carbon steel with electric brushplating Ni interlayer was an appropriate technique to enhance the corrosion resistance as compared with single double glowsurface alloying.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Double glow; Multi-element surface alloying; Composite alloying layer; Electric brush plating

1. Introduction

Surface alloying technique is an effective way toenhance the surface properties of metal by applyingother elements in the form of diffusion layers. Thedouble glow plasma surface alloying technique is anupdated technology in the field of surface alloyingw1,2x.By comparison with the other advanced surface alloyingtechnologies, such as ion implantation and laser alloying,the double glow technique is cheaper for many potentialusers. The technology employs a low temperature plas-ma produced by glow discharge. By using this technique,an alloying layer with special physical, chemical prop-erties can be obtained on the surface of metallic mate-rials. Major research has been accomplished over the

*Corresponding author.E-mail address: xujiang@sina.com.cn(X. Jiang).

past 10 years in exploring the area of corrosion resis-tance with Xu-Tec process. In stead of using bulk nickel-base alloy, the Xu-Tec process uses only a fraction ofthose rare and expensive elements to form surface alloywith high resistance to corrosion. A Ni–Cr–Mo–Nballoying layer, similar to Inconel625, has been formedon the three kinds of substrate(304 stainless steel, lowcarbon steel and pure iron) w3,4x. Recently, the authors,have successfully obtained a Ni–Cr–Mo–Cu alloyinglayer formed on the two kinds of substrates, e.g. AISI304 stainless steel and low carbon steel by means of thedouble glow plasma surface alloying techniquew5x. Theexperimental results indicate that the corrosion resistanceof surface alloying layer formed on AISI 304 stainlesssteel substrate is better than that of alloying layersformed on low carbon steel. This probably was becausethe content of carbon in the substrate plays a veryimportant role in the corrosion resistance of the alloying

157X. Jiang et al. / Surface and Coatings Technology 168 (2003) 156–160

Fig. 1. Sketch of double glow plasma surface alloying devise.

Fig. 2. Chemical composition of composite alloying layer.

Fig. 3. Chemical composition of the single alloying layer.

layer. To improve the corrosion resistance of alloyinglayer formed on the low carbon steel, the compositealloying layer with electric brush plating Ni interlayer,which has microstructure homogeneity within the sur-face alloyed layer and high corrosion resistance, hasbeen obtained.

2. Experimental method

Brush plating of Ni was an electroplating process thatis performed with a hand-held or portable plating tool.The plating tool is soaked in the plating solution andthen the plating material(low carbon steel) is depositedby brushing the plating tool against the base material.Plating solution was delivered to the work area by aporous, absorbent cover wrapped over the anode of theplating tool. The composition of electric brush Nisolution was:

NiSO Ø7H O 253–255 gyl4 2

NH ØH O (25%) 100–110 ml3 2

(NH ) C H O 55–56 gyl4 3 6 5 7

CH COONH 22–23 gyl3 4

(COONH ) ØH O 0.1–0.2 gyl4 2 2

Electric brush plating was operated at a workingvoltage of 12–14 V. The Ni plating layer with a depositthickness of 20mm was obtained. The alloying processof as-deposited sample was performed in a double glowsurface alloying device. The principle sketch was shownin Fig. 1. Three electrodes were designed in the vacuumchamber: the anode and two negatively charged mem-bers, the cathode where the work piece was located, andthe source electrode where the desired alloying elementswere placed. Both cathode and source electrode weresurrounded by glow discharges as the powers wereturned on. One glow discharge heated the substrate tobe alloyed while the second glow stroke the sourceelectrode. The desired alloying elements were sputteredby the second glow from the source electrode, travelingtoward the substrate. Subsequently the alloying elementsdeposited onto and diffused into the surface of the

substrate, forming a functionally graded surface in whichthe alloying elements were gradually tapered in thedepth direction. A superalloy Hastelloy C-2000(thecomposition is Cr 23, Mo 16, Cu 1.6, Ni 59 in wt.%)plate (130=50=4 mm ) was used as the source elec-3

trode materials for supplying alloying elements. Lowcarbon steel sample(80=25=3 mm ) were used as3

substrate materials. The processing parameters were:source electrode voltage, 1050 V; substrate voltage, 250V; working pressure, 35 Pa; and parallel distancebetween the source electrode and the substrate, 15 mmand treatment time of 3 h.The chemical compositions and microstructure of the

surface layer were analyzed by a LEO-1450 scanningelectron microscopy(SEM) and X-ray energy dispersivespectroscopy. The phase structure identification was

158 X. Jiang et al. / Surface and Coatings Technology 168 (2003) 156–160

Fig. 4. Cross-section line scanning of different elements of the com-posite alloying layer.

Fig. 5. Cross-section line scanning of different elements of the singlealloying layer.

Fig. 6. SEM photographs of cross-section the composite alloying layer(a); the single alloying layer(b); and Ni brush plating layer(c).

determined using DyMax-RB X-ray diffraction(XRD)studies using CuKa radiation.Potentiodynamic anodic polarization curves were

obtained at a sweep rate 20 mVymin, starting from amoment when the open circuit potential become steadyafter immersion of the specimen for approximately 10min. A saturated calomel electrode and a platinum sheetwere used as reference and counter electrodes, respec-tively. An electrolyte used was 3.5% NaCl and 5% HClsolution open to air at 258C.

3. Results and discussion

3.1. Chemical composition of composite surface alloyedlayer

The distribution of alloying elements in the composite

surface alloying layer and compared alloying layerwithout electric brush plating Ni interlayer are shown inFigs. 2 and 3. It is evident that the content of Ni, Cr,Mo, Cu in two kinds of alloying layers decreases as thedistance from the surface increases. The gradient alloy-ing layers are identical composed of alloying elements(Ni, Cr, Mo,Cu)-enriched layer, in which the composi-tion is similar to that of Hastelloy C-2000 and transitionlayer to the substrate. But there are something differentbetween the composite alloying layer and the single

159X. Jiang et al. / Surface and Coatings Technology 168 (2003) 156–160

Fig. 7. XRD patterns of the brush plating layer(a); the compositealloying layer(b); the single alloying layer(c).

Fig. 8. Polarization curves of composite alloying layer and contrastalloying or plating layer formed on steel 20 in 5% HCl solution.

alloying layer. Comparison with the composite alloyinglayer, the distribution of elements in the single alloyinglayer is sharper and alloying layer thickness is approxi-mately 10mm thinner than that of composite alloyinglayer. Owing to the thermal effects during double glowplasma alloying, the interface diffusion layer is formedand, thus, the adhesion of the surface alloying layer onthe substrate is excellent. Figs. 4 and 5 show the resultsof line scans of different elements of two kinds of thealloying layer. It is noted that the content of carbon inthe composite alloying layer is obviously lower thanthat of the alloying layer without the electrical brushplating Ni interlayer and carbon stack was formed atthe substrate side of the interface between the alloyinglayer and substrate as a result of that non-carbideelements such as Ni, Cu diffuse forward into the matrix

so as to prevent the carbon from diffusion toward thealloying layer.

3.2. Microstructure and phase analysis of the surfacealloyed layer

Typical microstructures of the composite alloyinglayer, alloying layer without electric brush plating Niinterlayer and electric brush plating are shown in Fig.6. The two kinds of alloying layer are continuous andcompact. XRD spectra(Fig. 7) show that the phasestructure of the composite alloying layer consists ofsingle g phase, but the single alloying layer withoutpredeposited Ni interlayer(by brush plating) consists ofa g matrix and several precipitates(M C andm phase).6

More carbide precipitated in the single alloying layer onthe substrate of low carbon steel, because of its highercarbon concentration. The XRD data represented thatthe electric brush plating Ni layer deposited on the lowcarbon steel is a mixture of crystalline and amorphousstructure.

3.3. Corrosion test results

Fig. 8 and Table 1 show the potentiodynamic polari-zation curves of the composite alloying layer measuredin a 5% HCl and 3.5% NaCl solution open to air at 258C. The polarization curves of alloying layer withoutelectric brush plating Ni interlayer are shown for com-parison. The passive current densityi of compositep

alloying layer is lower than that of alloying layer withoutelectric brush plating Ni interlayer, and one order ofmagnitude lower than that of alloying layer withoutelectric brush plating Ni interlayer in 5% HCl solution.The pitting potentialE of composite alloying layer isb

160 X. Jiang et al. / Surface and Coatings Technology 168 (2003) 156–160

Table 1Electrochemical properties of alloying layers in 3.5% NaCl and 5% HCl solution(substrate: low carbon steel)

Specimen 3.5% NaCl solution 5% HCl solution

Pitting Passive current Pitting Passive currentpotential(mV) density(mAycm )2 potential(mV) density(mAycm )2

Composite alloying layer 900 12.589 990 31.622Single alloying layer 190 31.622 45.91 316.228

Table 2Corrosion results of 200-h immersion tests in 20% H SO and 20% HCl2 4

Specimen Corrosion rate in 20% Corrosion rate in 20%H SO solution(gym h)22 4 HCl solution(gym h)2

Composite alloying layer 0.0612 0.0848Single alloying layer 0.193 0.2359

944.1 mV higher than that of alloying layer withoutpredeposited Ni layer(by electric brush plating). Thelower corrosion resistance of alloying layer without apredeposited Ni interlayer must have resulted from ahigher amount of precipitation, and the matrix near thedetrimental(M C and m phase) phase is prone to be6

attacked by aggressive ions. During double glow proc-essing, the electric brush plating Ni interlayer whichtaken as barrier layer of carbon is effective to decreasethe content of carbon in the composite alloying layer,and has eliminated the precipitation of detrimental(M C6andm phase) phases and enhanced the corrosion resis-tance of alloying layer.In a 3.5% NaCl solution, the pitting potentialE ofb

composite alloying is 710 mV higher than that ofalloying layer without predeposited Ni layer. The passivecurrent densityi of composite alloying layer is alsop

one-third of that of alloying layer without a predepositedNi interlayer.The results of the 200-h immersion test in 20%

H SO and 20% HCl solution(Table 2) show that the2 4

corrosion rates of composite alloying layer are superiorto that of alloying layer without predeposited Niinterlayer.

4. Conclusions

1. An electric brush plated Ni interlayer which taken asa barrier layer of carbon is effective to decrease thecontent of carbon in the composite alloying layer.

2. The structure of the composite alloying layer consistsof single g phase, but the alloying layer withoutpredeposited Ni interlayer(by brush plating) consistsof a g matrix and several precipitates(M C and m6

phase).3. The composite alloying layer has better corrosionresistance than that of alloying layer without prede-posited electric brush plating Ni interlayer in 3.5%NaCl and 5% HCl solution.

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