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Microstructure and Corrosion Performance of Carbonitriding Layers on Cast Iron by Plasma

Electrolytic Carbonitriding

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2009 Chinese Phys. Lett. 26 086805

(http://iopscience.iop.org/0256-307X/26/8/086805)

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CHIN. PHYS. LETT. Vol. 26,No. 8 (2009) 086805

Microstructure and Corrosion Performance of Carbonitriding Layers on Cast Ironby Plasma Electrolytic Carbonitriding *

PANG Hua(庞华)1**, LV Guo-Hua(吕国华)1, CHEN Huan(陈睆)1, WANG Xin-Quan(王鑫权)3,ZHANG Gu-Ling(张谷令)4, YANG Si-Ze(杨思泽)1,2

1Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences,Beijing 100190

2Fujian Key Laboratory for Plasma and Magnetic Resonance, Department of Aeronautics, School of Physics andMechanical and Electrical Engineering, Xiamen University, Xiamen 361005

3College of Science, Changchun University of Science and Technology, Changchun 1300224College of Science, Central University for Nationalities, Beijing 100081

(Received 1 June 2009)The surface carbonitriding of cast iron is investigated in an aqueous solution of acetamide and glycerin. Mi-crostructure, chemical and phase composition and corrosion performance of the carbonitriding layers are inves-tigated by scanning electron microscopy, energy dispersive spectroscopy and x-ray diffraction, as well as poten-tiodynamic polarization testing. X-ray diffraction results show that the carbonitriding coatings are composed ofmartensite, austenite(𝛾-Fe), Fe2C, Fe3C, Fe5C2, FeN and 𝜀-Fe2−3N. After the plasma electrolytic carbonitridingtreatment the corrosion resistance of cast iron is clearly improved compared to the substrate, and the coatingsproduced at 350V for 30 s give the best corrosion resistance.

PACS: 68. 55.−a, 81. 65. Kn

Cast iron has been widely used in various ap-plications because of its good machining ability andlow cost. A number of engine parts (e.g., engineboxes, cylinder heads, cylinder liners, and lathe bed)and water/oil supply pipes are made from cast ironand the surfaces of the components usually needgood corrosion resistance and mechanical properties.Therefore, there is a demand for surface improvementfor mechanical performance and corrosion protection.Plasma electrolytic carbonitriding (PEC/N) is a rela-tively novel surface modification technique, which op-erates at ambient temperature and pressure.[1−6] Theunrestrained size and geometry and shorter process-ing time compared to vacuum-plasma processes makeit desirable for industrial use. The PEC/N treatmentson low carbon steel, mild carbon steel, and stainlesssteel have been investigated.[4−9] Nie et al.[10] usedthe plasma electrolytic nitriding (PEN) method forthe surface modification of cast iron and steel. Theyinvestigated hardness and tribological and wear be-havior of the PEN-treated samples. The studies men-tioned above usually used aqueous solutions of urea,ethanolamine or ammonium chloride in the PEC/Nprocess. In this work, we use an aqueous solution ofacetamide and glycerin as the electrolyte and investi-gate the influence of treatment time on microstructureand corrosion performance of the PEC/N layers.

Rectangular substrates of cast iron with dimen-sions of 15 mm× 10 mm× 1 mm were used in thisstudy. All the testing substrates were ground with

300–1200 grit SiC paper to a roughness of 𝑅𝑎 ≈ 0.06and scrubbed away by acetone before the plasma elec-trolytic carbonitriding processing. The PEC/N pro-cesses were carried out in a custom-built plasma elec-trolysis saturation (PES) coating synthesis system,which mainly consists of an insulated electrolyte bathwith a cooling system and a pulsed high-voltage dcpower supply. One output of the power supply is con-nected to a carbon rod, and the other to a substrate.A voltage pulse of 350 V was selected during the pro-cess. The duty cycle was 50% and the frequency was100 Hz. Different treatment times of 30, 45 and 60 swere chosen (denoted as sample 1, 2 and 3, respec-tively). An aqueous solution of acetamide (C2H5NO),glycerin (C3H8O3) and other additives (sodium chlo-ride was added primarily for adjustment of electricalconductivity) was chosen as the electrolyte.

The phase compositions of the layers were investi-gated using an XD-3 x-ray diffractometer (XRD) withCu 𝐾𝛼 radiation and scanning velocity of 2∘/min.The cross-section morphologies were studied by us-ing a scanning electron microscope (SEM, HITACHS-4200). The energy dispersive spectroscopy (EDS)attachment of the SEM was used for semi-qualitativeelemental analysis of the cross-section. The corrosionperformances were carried out using an M283 electro-chemical measurement system in 3.5 wt% NaCl solu-tion with the saturated calomel electrode (SCE) usedas the reference electrode.

In the PEC/N process, when a certain voltage was

*Supported by the National Natural Science Foundation of China under Grant No 10675165, and the Project of InternationalCooperation and Exchanges NSFC-KOSEF (No 10811140326).

**Email: hpang@aphy.iphy.ac.cnc○ 2009 Chinese Physical Society and IOP Publishing Ltd

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applied to an electrolysis cell, gas bubbles arose fromthe sample and a gas film formed around the sample.Then when the applied voltage was higher, there weremany microplasma discharge areas on the sample sur-face, and the near C and N atoms around the samplewere activated and diffused into the sample. The SEMmicrograph of the cross-section for the sample thatwas treated at 350 V for 60 s is shown in Fig. 1. It canbe seen that the growth speed of the carbonitridinglayer was very fast, and the thickness of the layer wasabout 20µm. The distribution of chemical elementsacross the layer is shown in Fig. 2. It is obvious thatC and N contents near the surface are the largest inthe whole layer, and their contents decrease graduallyfrom the surface to the substrate. EDS studies of thecoated layer verify the diffusion of carbon and nitrogenfrom the surface into the inner part.

40 mm

Substrate

PES coating

Fig. 1. SEM micrograph of cross-section for PEC/N at avoltage of 350V for 60 s.

Carbon

Nitrogen

Iron

0 10 20 30

0

20

400

1

2

3

0

10

mm

40 mm

Fig. 2. EDS elemental distribution for the layer producedby PEC/N at 350V for 60 s.

The phase compositions of cast iron and thePEC/N layers produced on cast iron at a voltage of350 V for different treatment times are presented inFig. 3. It can be seen from the diffraction patternsthat there is a clear difference between untreated andtreated samples. Before the PEC/N process, therewas only 𝛼-Fe in the surface of the substrate. Afterthe PES process for 30 s, some new phases, austen-ite (𝛾-Fe), Fe2C, Fe3C, Fe5C2, FeN and 𝜀-Fe2−3Ncan be found on the carbonitriding layers. The pres-ence of these new phases suggests that carbon andnitrogen elements have permeated the substrate and

the surface temperature of the substrate has attainedthe phase transformation temperature of austenite.[11]

However, the phases of 𝜀-Fe2−3N and Fe2C at 2𝜃 val-ues of 29.953∘, 30.016∘ and 31.755∘ disappeared withincreasing treatment time. This is the reason that,when the treatment time is 30 s, the temperature ofthe substrate is low, nitriding is the main process,while when the treatment time increases and the tem-perature of the substrate is high, carburizing is themain process. Moreover, the presence of Fe3C, Fe5C2

and 𝜀-Fe2−3N suggests that the concentration of car-bon and nitrogen in the surface layers exceeds the sol-ubility limits of carbon and nitrogen in cast iron.[11]

20 30 40 50 60 70 80 90

Fe5C2

ε-Fe2-3N

Fe2C Fe3C

FeN α γ-Fe-Fe

? ?

?

? R

ela

tive inte

nsi

ty (

arb

.

units)

2θ (deg)

Cast iron

60 s

45 s

30 s

Fig. 3. XRD spectra of cast iron and PEC/N layerstreated for 30 s, 45 s and 60 s.

Fig. 4. Potentiodynamic polarization curves for cast ironand PEC/N layers.

The potentiodynamic polarization curves for thetreated and untreated samples are shown in Fig. 4.The corrosion potential 𝐸corr, corrosion current den-sity 𝑖corr, and polarization resistance 𝑅𝑝 at the cor-rosion potential are determined from the potentiody-namic polarization curves by the software CorrView.The electrochemical corrosion parameters are listed inTable 1. After the PEC/N treatment, the corrosionpotential 𝐸corr of all the coatings becomes higher while

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CHIN. PHYS. LETT. Vol. 26,No. 8 (2009) 086805

the corrosion current density 𝑖corr becomes lower thanthose of the substrate, which means that the corrosionprocesses are restrained by the PEC/N coatings. It isconcluded that the corrosion resistance of cast iron isimproved by the PEC/N process.

Table 1. Electrochemical corrosion parameters of cast iron andPEC/N layers.

Specimen𝐸corr 𝑖corr 𝑅𝑝

(mV) (10−6 A/cm2) (103 Ω·cm2)

Cast iron −0.64875 9.2482 2.78Sample 1 −0.49169 2.6483 3.75Sample 2 −0.53175 12.287 2.13Sample 3 −0.50131 7.3419 3.73

Comparing the corrosion performance of thePEC/N layers for different treatment times, it canbe found that the corrosion potential of sample 1 in-creased from −649 mV (for the untreated substrate)to −491 mV, which shows the best performance inall corrosion tests. Sample 1 shows the best per-formance in all corrosion tests, which may be dueto the presence of 𝜀-Fe2−3N at 2𝜃 values of 29.953∘,30.016∘ and 31.755∘. The temperature determinesthe treatment: if the temperature is high, carbur-izing is the main process during the PEC/N treat-ment. On the other hand, if the temperature is low(500–600∘C), nitriding is the main process during thePEC/N treatment. Below 400–500∘C, monophasedlayers of expanded or transformed austenite (𝛾N) and𝑆-, 𝑚- or 𝜀′-phase with a high corrosion resistancecan be produced; this effect has been found in DCplasma nitriding,[12,13] ion nitriding[14] and in plasma-immersion ion implantation.[13,15] Thus, with increas-ing treatment time, the N element content decreased,and accordingly the corrosion resistances of the lay-ers were degraded; namely, samples treated at 350 Vfor 30 s possess the best corrosion resistance in alltests. Thus, in a few minutes, carbonitrided coat-ings with corrosion protection are prepared by thePEC/N method. Moreover, without vacuum systems,the PEC/N technique presents high treatment effi-ciencies and the possibility of continuous treatment

of workpieces.In conclusion, the plasma electrolytic carbonitrid-

ing process can be used for surface modification of castiron in an aqueous solution of acetamide and glycerin.SEM study combined with EDS shows the presenceof carbon and nitrogen in the coated layer. More-over, from surface to substrate the concentrations ofcarbon and nitrogen of the layers gradually decrease.For XRD analysis, the PEC/N coatings are composedof austenite (𝛾-Fe), Fe2C, Fe3C, Fe5C2, FeN and 𝜀-Fe2−3N. Corrosion performance testing shows that thecorrosion resistance of the coatings produced by thePEC/N process is greatly improved. When the sub-strate is processed at 350 V for 30 s, the coating givesthe best corrosion resistance.

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