Electrolytic plasma processing for cleaning and metal …bennati.it/rik/ff/Doc/GDPE/Electrolytic plasma processing...E.I. Meletis et al. / Surface and Coatings Technology 150 (2002)

Surface and Coatings Technology 150(2002) 246–256

0257-8972/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž01.01521-3

Electrolytic plasma processing for cleaning and metal-coating of steelsurfaces

E.I. Meletis, X. Nie*, F.L. Wang, J.C. Jiang

Materials Science and Engineering Program, Mechanical Engineering Department, Louisiana State University, Baton Rouge, LA 70803, USA

Received 2 May 2001; accepted in revised form 8 September 2001

Abstract

Electrolytic plasma processing(EPP) involves electrolysis and electrical discharge phenomena and it is an emerging,environmentally friendly surface engineering technology. Electrolytic-plasmaymaterial surface interactions during processing canbe used for cleaning of metal surfaces, formation of diffusion layers andyor deposition of metal, ceramic and composite coatings.The present work was concerned with cleaning and deposition of metal coatings on steel surfaces for corrosion protection. Theeffects of processing parameters on(i) cleaning steel surfaces(oxides and contamination); and (ii) Zn and Zn–Al coatingdeposition were investigated. Surface roughness and oxygen content prior to and after cleaning were evaluated by profilometryand energy dispersive X-ray analysis(EDAX), respectively. The structure of the EPP cleaned outer surface layer as it evolvesafter the electrolytic–plasma interaction was studied by high resolution TEM. Morphology, microstructure, composition, adhesionand density of EPP-deposited Zn and Zn–Al coatings on cleaned surfaces were studied as a function of processing parameters.Corrosion properties of the cleaned and coated steels were evaluated by corrosion potential and potentiodynamic polarizationmeasurements. The results show that EPP can effectively produce clean surfaces and also metal and alloy coatings at highdeposition rates, and it has a great potential as a new plasma surface engineering technique.� 2002 Elsevier Science B.V. Allrights reserved.

Keywords: Corrosion; Cleaning; Coating; Zn; Zn–Al; Plasma electrolysis

1. Introduction

Metallic coatings(such as zinc and zinc–aluminum)can provide corrosion protection to the base metal froma corrosive environment by acting as sacrificial anodes(cathodic protection). Such protection requires a moreactive corrosion potential for the coating compared tothe substrate material. Under these conditions, the coat-ing corrodes preferentially and provides protection tobase metal even when exposed due to coating disconti-nuities or mechanical damage. Corrosion tests and dataof a zinc protective coating for steel have been devel-oped and widely used to describe its performancew1,2x.In the majority of applications for structural steel

(ship and marine structures, pipelines, bridge supports,industrial structures, etc.), cleaning is required prior tocoating of the surface. Cleaning is needed firstly becausesteel is covered by a layer of mill-scale(produced by

* Corresponding author: Tel.:q1-225-578-5808; fax:q1-225-578-5924.

E-mail address: [email protected](X. Nie).

the hot-rolling mill), other ‘soil’ and oil and greasecontamination. Secondly, adhesion to base metal is ofutmost importance to coating performance and is directlyrelated to the cleanliness of the surface and the abilityto develop an ‘anchor’ surface profile(surface withmicroscale roughness) providing a key or mechanicalinterlocking for the coating.Traditional methods of cleaning steel include: acid

pickling and sandyshotygrit blasting. These convention-al methods have major disadvantages(high energy cost,environmentally-unfriendly, disposal problems, unfavor-able surface profile, etc.). Electrolytic cleaning methodsare known, but they have not been successfully com-mercialized since in their present state they are unableto remove mill-scale and other heavy contaminants.Likewise, current metal coating methods have severallimitations. For example, electro- and electroless platingare relatively slow and may use environmentally hazard-ous chemicals. Dipping and metal spraying methods arefaster but the resulting coatings may have adhesionproblems during bending and formation in some cases.

247E.I. Meletis et al. / Surface and Coatings Technology 150 (2002) 246–256

Thus, there is a need at present for new, cost-effectivecleaning and coating methods that can produce quality,long-lasting surfaces.Plasma electrolysis is a relatively new surface engi-

neering technique, which includes an anodic process(such as plasma electrolytic anodic oxidation) w3,4x anda cathodic process(such as plasma electrolytic satura-tion) w5x. Plasma electrolysis has a similar configurationto a conventional anodic oxidation or electroplatingprocess. However, the applied electrode potential in allthe electrolytic plasma processes is much higher thanthat of the conventional processes. The bi-phaseelectrodeyelectrolyte system normally encountered inconventional electrolysis is transformed into a four-phaseelectrodeydielectricygas(plasma)yelectrolyte couple forthe anodic oxidation processing or a three-phaseelectrodeygas(plasma)yelectrolyte couple for thecathodic processingw6x. In the plasma dischargechannelyarea, plasma chemical interaction occurs, andoxides w3,4x or nitrides w5x can form on substratesurfaces to be treated.Plasma electrolysis can be quite versatile. For exam-

ple, it can be utilized in conjunction with PVDyCVDprocesses for improvement of tribological and corrosionpropertiesw4x, or with electrophoresis to form biomedi-cal ceramic coatingsw7x. In this work, plasma electrol-ysis was extended to cleaning mill-scales on steelsurfaces, and to depositing Zn and Al–Zn coatings onsteel substrates as well, for the purpose of corrosionprotection.Electrolytic plasma process(EPP) may have the

potential to overcome limitations of conventional meth-ods, both for the cleaning and metal-coating of steelw8,9x. In the present work, EPP was studied in relationto cleaning steel surfaces and developing zinc(Zn) andzinc–aluminum(Zn–Al) coatings.

2. Process description

Electrolytic plasma processing involves two charac-teristic phenomena(i) electrolysis of a liquid environ-ment by application of different electrical potentialsbetween the workpiece material and a counter-electrode;and (ii) the production of an electrical discharge at, orin the vicinity of, the workpiece surface. Although thedischarge phenomena associated with electrolysis werediscovered more than a century ago by Sluginovw10x,their importance as an emerging surface engineeringprocess were realized only very recentlyw11x. A typicalelectrode current–voltage characteristic curve of theelectrolytic cathodic process is shown in Fig. 1. Thenormal glow discharge zone U –U –U is usually2 29 3

selected as the work region, where the electrode currentdecreases but plasma intensity increases when the elec-trode voltage increases from U to U . The samples(as2 29

a cathode) can be either immersed in the electrolyte

(Fig. 1a), or dripped with electrolyte(Fig. 1b). How-ever, the corresponding slope of the electrode current–voltage curve in the U –U segment is different in2 29

these two cases. For the configuration shown in Fig. 1a(i.e. the workpiece is immersed in the electrolyte), thecurrent sharply drops from the highest point to thelowest point when the electrode voltage increases fromU to U . For the configuration in Fig. 1b(i.e. in this2 29

case, the electrolyte drips down to the workpiece throughsmall diameter holes on the cathodic electrode plate),the slope of the U –U line(i.e. current reduction rate)2 29

is much smaller than that of Fig. 1a. It is found that theplasma intensity increases when electrode voltageincreases, and when U is reached, a continuous enve-29

lope is formed on the sample surface, which results inrapid increase of the surface temperature with increasingelectrode voltage. The normal operation voltage in thiswork is within U –U . Although there is heating of the2 29

substrate due to plasma action, the bulk substrate tem-perature remains relatively low(less than 200–3008C)due to the simultaneous cooling action by the electrolytesolution. The local surface temperature adjacent toplasma bubbles, however, is very high, which is criticalto cleaning mill-scale and enhancing the metallurgicalconnection of the coating microstructure.There appears to be several distinct physical processes

involved in cleaning and subsequent coating using EPPw8,9,12x. While the electrolyte flows through the perfo-rated anode onto the workpiece(cathode) (Fig. 2), apotential is applied across the electrode gap and sets upa strong electric field. The hydrogen evolution reactionbegins to occur at the surface of the workpiece, creatingfine hydrogen bubbles. A strong electrical potential isestablished on the bubble adjacent to the workpiecesurface, and hydrogen plasma is thus generated in a thinlayer, just above the steel surface. Metal cations presentin the electrolyte film begin to migrate toward the steelsurface, but the large majority of these ions are attachedto the hydrogen bubbles. These ions are either originaladditions to the electrolyte or are generated from theanode metal by electrolytic decomposition.In particular, in the case of cleaning, the cations may

be sodium(i.e. sodium carbonate solution) which willparticipate in the process but ultimately remain insolution. The ions due to adsorption gradually concen-trate on the hydrogen bubble surface and the bubble isthus converted into a small capacitor. The electricalfield between the positive ions at the bubble surface andthe negatively charged steel surface ionizes the hydrogengas in the bubble, resulting in a high temperature plasma.This occurs quickly once hydrogen starts forming. Thelifespan of the average hydrogen bubble is less than 1ms and the plasma exists for 1–10ms w8,11x. Theplasma is continuously forming at this high rate overthe entire steel surface. The nature of the plasma bulbgeneration and extinction results in local surface melting

248 E.I. Meletis et al. / Surface and Coatings Technology 150 (2002) 246–256

Fig. 1. Current–voltage dependence in electrolytic plasma discharge.

and also creates forceful pressure disruptions at thesurface associated with bubble collapsing and shockwave production. The net effect of these processes isthe removal andyor reduction of the milling-scale at thesteel surface and the formation of circular wavelets andspheroids. Also, the freezing that follows local meltingcauses a quenching effect that may result in ultra finegrained(or even amorphous) structures, as shown laterin this work.In the coating process, some metal ions may find

their way to the steel surface by diffusion through theelectrolyte but the predominant modes of transport areion acceleration through the plasma and ion bubbleadsorption transport where ions are carried to the steelsurface by riding the surface of the hydrogen bubble asit collapses. Both of the latter processes eliminate theboundary layer diffusion and result in high depositionrates.

3. Experimental

The present work involved characterization and studyof corrosion behavior of surfaces and coatings cleanedand deposited, respectively, by EPP. Low carbon steel(AISI 1010) was selected as the substrate material. Alaboratory unit was utilized to produce the aforemen-tioned cleaned surfaces and coatings according to theprocessing conditions shown in Table 1. These condi-

tions were selected based on previous experience withEPP and the desire to systematically study the effect ofprocessing parameters on the characteristics of thecleaned surfaces and produced coatings. Zn coatingswere produced under two different process arrange-ments. In the first, a stainless steel anode was used andZn deposition occurred entirely from the electrolyte(Zncoating I) and in the second, a Zn anode was utilizedthus Zn was deposited from the electrolyte and theanode(Zn coating II).After processing, specimen surfaces were examined

by scanning electron microscopy(SEM) to characterizethe surface morphology. The microstructure of the outersurface layer evolved by the electrolytic–plasma inter-action was studied by focused ion beam(FIB) methodsw13x and high resolution transmission electron micros-copy (TEM) (JEOL 2010) on cross-sections of EPPcleaned surfaces. Metallographic cross-sections werealso prepared to determine coating thickness, densityand uniformity. Density was assessed by taking micro-hardness measurements(Knoop at 10g ) on coatingf

cross-sections. Energy dispersive X-ray analysis(EDAX) was conducted in conjunction with SEM(Hita-chi S 4500) to analyze the composition prior to andafter EPP cleaning. Wavelength dispersive spectroscopy(Jeol JXA 733 super electron probe microscope) wasconducted(at 15 keV accelerated voltage and 10 nAbeam current) to analyze Zn and Al content in the Zn–


Fig. 2. Schematic illustration of the electrolytic plasma process.

Table 1Electrolytic plasma processing parameters used for cleaning and coating steel surfaces

Parameters Cleaning Zn coating I Zn coating II Zn–Al coating

Voltage(V) 170–200 180, 200, 220 180, 200 180, 200, 220Current density(A ycm )2 0.23–0.45 0.11–0.78 0.3–0.5 0.8–1.2Anode material Stainless steel Stainless steel Zn plate Stainless steelElectrode gap(mm) 8, 12, 16 8, 12, 16 8, 12, 16 8, 10, 12Electrolyte(wt.%) NaHCO3 ZnSO4 w20% ZnSOq80% Al (SO ) x4 2 4 3

14% 21%, 24%, 27% ZnSO 24%4 10%, 11.5%, 13%Flow rate(l ymin) 3–3.8 1.9–3.8 1.9–3.8 1.9–3.8Temperature(8C) 75 73 73 70Treatment time(s) 13 32 32 32

Al coatings. Profilometry was performed after process-ing to characterize surface topography. Coating adhesionwas determined by pull tests. Tapered Al pins(2.7-mmhead diameter) were bonded to the coating with anepoxy (cured at 1508C for 30 min). The adhesionstrength was estimated from the force required to pullthe pin from the specimen.Corrosion potential measurements and anodic polari-

zation tests were performed to characterize the generalcorrosion behavior of the clean steel surfaces as pro-duced by EPP. Tests were also conducted on the back-ground low carbon steel(cleaned by grinding with 600grit paper) for comparison purposes. Anodic polarizationtests were also carried out on the Zn and Zn–Al coatingsproduced by EPP. Such tests were also performed onhot-dip galvanized steel(denoted as: galvanized) spec-imens that were supplied by the International Lead ZincOrganization(ILZRO).The first type of experiments involved corrosion

potential measurements for clean steel and backgroundsteel as a function of time in tap water for a totalduration of 8=10 s. The second type of experiments4

involved anodic polarization scans that were conductedfor all surfaces and coatings in two different environ-ments: tap water and 3.5% NaCl solution using anEG&G Corrosion Measurement System(Model 273).In these tests, the potential was first allowed to stabilizeand then scanned from a value of approximately 200–300 mV below the open circuit potential(OCP) to a

value of approximately 800 mV above OCP at the scanrate of 0.2 mV s , while current was continuouslyy1

recorded. The corrosion rate was calculated from thesetests using the Tafel extrapolation method. All corrosionpotentials were measured with respect to a saturatedcalomel electrode(SCE).

4. Results and discussion

4.1. Effect of processing parameters

The processing parameters used for the EPP cleaningand coating treatments are listed in Table 1. The EPPcleaned surfaces were characterized in terms of surfacemorphology, roughness and oxygen content. The char-acteristics of the cleaned surface with the lowest oxygencontent are presented in Table 2. Similarly, coatingswere assessed in terms of adhesion, density(microhard-ness), roughness, composition, thickness, uniformity andcontinuity. Table 2 presents the range of these charac-teristics and those of the best coatings(based on astatistical analysis of performance data).Fig. 3a and b shows the surface morphology in plan

view and cross-section, respectively, of an EPP-cleanedsteel specimen. The observed topography is a directresult of the interaction between physical processestaking place during EPP. As noted earlier, a hightemperature plasma is created within the small sizehydrogen bubbles in the thin electrolyte layer on the


Table 2Characteristic data of the cleaned surfaces and coatings

Cleaned surface Zn coating I Zn coating II Zn–Al coatings

Thickness – 12mm 15mm 18mmDeposition rate – 0.37 0.47 0.56(mmys) (0.28–0.53) (0.41–0.47) (0.41–0.56)

Roughness 2.5mm 3.5mm 2.5mm 2.3mm(R )a (1.8–3.4mm) (2.2–8.2mm) (1.5–2.5mm) (0.8–3.8mm)

Element content 1.38 wt.% O2 Zn Zn 3.0 at.% Al, Zn bal.(1.38–7.3 wt.% O)2 (0.2–3.0 at.% Al, Zn bal.)

Adhesion strength – )70 )70 )70(MPa) (20 to)70) ()70) (30 to)70)

Hardness(GPa) 1.9 (bulk steel) 0.97 1.06 1.30

Note: Values in parentheses show the range of characteristic data determined for all samples treated under the parameters described in Table1. Adhesion values)70 MPa show that failure occurred in the epoxy.

surface of the workpiece during EPP. This causes local-ized melting but also shock waves from the collapsingof the bubbles and thus, strong surface forces on smallmolten patches of steel. The action of hydrogen is ableto reduce oxides present at the surface and results in aclean surface. As the present experimental results show,O content was reduced dramatically in all EPP-cleanedspecimens(Table 2). Typical EDAX spectra prior toand after cleaning are shown in Fig. 4. Once the plasmahas dissipated, the temperature drops rapidly causingfreezing of the produced features on the surface. Theabove processes leave an essentially ‘renewed’ surface,which is virtually clean. The surface consists of forceimpact craters and spheroidal projections at the rim ofthe impact craters producing a highly desirable profilefor a subsequent coating treatment. In the present study,the lowest O content was observed in the specimencleaned under the following EPP conditions: 12-mmelectrode gap, 170-V electrode voltage and 3.5 lyminelectrolyte flow rate.Fig. 3c and d shows the morphology of the Zn coating

(Zn coating I) that exhibited the best characteristics.This coating was developed by using an 8-mm electrodegap, 200-V electrode voltage, 24% ZnSO solution and4

3.6 lymin electrolyte flow rate. The coating exhibitedexcellent adhesion()70 MPa) and SEM observationsshowed a dense and continuous structure with a thick-ness of 12mm with no discontinuities at the coatingysubstrate interface(Fig. 3d). It is interesting to note thesimilarities in the surface morphology between thecleaned steel surface(Fig. 3a), and the Zn coating(Fig.3c). It is apparent that the plasmaysurface interactionscreate similar effects during coating(i.e. localized melt-ing) producing a very similar surface morphology.Microhardness measurements at cross-sections of thiscoating revealed high values(0.97 GPa) consistent withSEM observations. Hardness of bulk Zn is approximate-ly 0.8 GPA but in view of freezing expected afterlocalized melting during EPP, the coating grain size is

expected to be in the nanoscale(or even amorphous)thus possessing a higher microhardness as observedexperimentally. Finally, the results show that surfaceroughness can be controlled to a certain extent by theprocessing parameters and that of the aforementionedbest coating was found to be approximately 3.6mm.The second type of Zn coating(Zn coating II) was

deposited by the same processing conditions as theaforementioned Zn coating I but using a Zn anodeinstead of a stainless steel anode. Compared to Zncoating I, the best coating in this case exhibited a higherdeposition rate(0.47mmys), a somewhat higher density(microhardness, 1.06 GPa) and lower roughness(2.5mm). Also, a relatively large but uniform grainyclusterstructure was present on the surface(Fig. 3e). Obser-vations at cross-sections also showed a dense anduniform coating(Fig. 3f). It should be noted that whena Zn anode is utilized during EPP, erosion of the anodetakes place under the electric field thus activating anadditional deposition mechanism. This is expected toresult in higher deposition rates, higher density andsmoother surfaces and is consistent with the experimen-tal measurements of thickness, microhardness and rough-ness. It is important to note that in both of these coatingtreatments the deposition rates achieved are significantlyhigher than those obtained by conventional electroplat-ing processes.Regarding the Zn–Al coatings, the WDS analysis

showed that under the EPP conditions used only smallamounts of Al could be incorporated into the coating.The maximum Al content reached 3.0 at.%. This maybe considered significant in view of the fact that Aldeposition by conventional electroplating is not possible.Surface and cross-section morphology of the best Zn–Al coating obtained in the present study is presented inFig. 3g,h, respectively. This coating was developedunder a 12-mm electrode gap, 220-V electrode voltage,10% electrolyte concentration and 2.9 lymin flow rate.A uniform, large grainycluster was evident on the


Fig. 3. Surface(a, c, e and g) and cross-section(b, d, f and h) SEM micrographs of(a, b) the cleaned steel,(c, d) Zn coating I,(e, f) Zn coatingII and (g, h) Zn–Al coating, respectively.

surface. A high deposition rate was also achieved in thiscase(0.56 mmys). The coating exhibited good conti-nuity, with some voids due to the large cluster structureas observed in the coating cross-section. Microhardnessmeasurements on coating cross-sections showed highvalues indicative of high coating density. The micro-hardness values obtained were significantly higher thanthose observed in pure Zn. This more than likely is dueto solid solution strengthening effects.For all of the Zn and Zn–Al coatings, the anchor

profile can be observed at the interface between thecoatings and steel substrates. SEM analysis also showed

that both Zn coatings were dense with good uniformityand no voids present at the substrateycoating interface.Closer observations showed that the Zn–Al coatingexhibited more irregularities on its top surface and somevoids in the coating structure. The reasons for suchdifferences in topography are not very clear at present.However, the high electrode current density during theZn–Al coating treatment(Table 1) indicates that theprocess is at the initial stage of U –U(Fig. 1), which2 29

can lead to an unstable and weak plasma. The sinteringeffects due to meltingycondensing in plasma dischargechannel on the Zn–Al coating are not as strong as those


Fig. 4. EDAX spectra from the steel substrate(a) prior to and(b) after EPP cleaning.

during the Zn coating treatment. Thus, the Zn–Alcoating is expected to exhibit a larger cluster size. Theseeffects are under further investigation at present.

4.2. Characterization of EPP-cleaned outer surfacelayer

In an effort to obtain more information on possiblemetallurgical changes at the outermost surface region,FIB and TEM studies were conducted on the cleanedsteel surfaces. A FIB cross-section into the surface ofEPP-cleaned specimens was made and high-resolutionimaging was obtained in secondary electron mode. Fig.5a shows an overall view of the specimen surface andcross-sectional area. It is evident that the bulk steelstructure has a relatively large grain size)10 mmwhereas the outer surface layer exhibits a very fine

grain structure. Fig. 5b is a high magnification image ofthe nodule cross-section that is shown in the centerportion of Fig. 5a. Two very interesting observationscan be made. The bulk of the nodule exhibits a finegrain structure with a size of approximately 0.5mm. Atthe surface of the nodule there is a thin layer with anultra-fine structure. This evidence strongly suggests thatquenching effects during freezing of locally meltedsurface material produce a fine microstructure at thesurface layer. The grain structure at the latter outersurface layer could not be resolved with secondaryelectron imaging and thus it was further investigated byhigh resolution TEM.Thin foils were prepared from the cross-sectional area

of EPP-cleaned surfaces. The results showed the pres-ence of a distinct outer surface layer that developedafter cleaning on the entire specimen surface(Fig. 6a).


Fig. 5. FIB secondary electron image showing(a) overall morphologyof surface and cross-sectional area and(b) high magnification of thenodule marked in(a) revealing a small grain structure in its interiorand an ultra-fine structured layer at its surface.

Fig. 6. (a) Cross-section TEM image of the EPP-cleaned steel.(b)Selected-area electron diffraction(SAED) pattern taken from the sur-face layer of a thickness of approximately 150 nm showing a fine-grained polycrystalline structure and(c) SAED pattern from the baseFe showing thew311x zone diffraction pattern of bcc Fe.

Fig. 7. A representative high-resolution TEM image from the surfacelayer in Fig. 6a showing a polycrystalline structure. The grain size isin the range of 10–20 nm in diameter.

The thickness of the surface layer was found to varybetween 150 and 250 nm. Electron diffraction analysisfrom the surface layer showed that it consists of an ultrafine-grained structure(Fig. 6b), where the basic steelcontains the original large grain structure(Fig. 6c). Highresolution TEM showed that the grain size in that outerlayer is between 10 and 20 nm(Fig. 7). Plasmadischarge developing in the hydrogen bubbles at thematerial surface during cleaning seems to have twobasic effects. Firstly it reduces andyor removes oxidesand mill scale from the surface and second, locallymelts the surface layer. Due to fast cooling rate of thethin molten surface layer a fine grain size develops.Thus, the end result is a clean, nanostructured surface.

4.3. Corrosion behavior

Fig. 8 presents the variation of the corrosion potentialof EPP-cleaned steel and background steel(cleaned bymechanical grinding) as a function of time. Backgroundsteel initially shows a small tendency for passivation(increase in potential) but its potential is graduallymoving in the active direction and stabilizes at approx-


Fig. 8. Variation of corrosion potential of steel in tap water as a func-tion of time.

Fig. 9. Anodic polarization behavior of steel in(a) tap water and(b)3.5% NaCl solution.

Table 3Summary of corrosion properties of steel from anodic polarizationtesting

Materialycondition Corrosion potential Corrosion ratemV (SCE) mA ycm2

Tap water NaCl sol. Tap water NaCl sol.

Background steel y325 y535 3.51 11.8EPP-cleaned steel y196 y385 0.14 9.5

imatelyy590 mV. On the contrary, the potential of theEPP-cleaned steel is significantly higher than that of thebackground steel during the entire testing period. Uponexposure, the potential of the EPP-cleaned steel movesin the noble(passive) direction at a relatively high rateand stabilizes at approximately 50 mV(approx. 250 mVhigher than its initial potential). Also, a comparisonbetween the stabilized potentials(after 80=10 s) shows3

that the EPP-cleaned steel has a potential that is approx-imately 640 mV higher than that of the backgroundsteel. These results indicate that the surface cleaned withthe EPP method exhibits passivation that remains stableduring the test duration while the plain steel tends to besignificantly more active. Visual examinations after test-ing were consistent with the above results since thebackground steel surface was completely rusted wherethe surface of the EPP-cleaned steel seemed intact.During EPP treatment, hydrogen plasma develops athigh temperatures, and besides oxides is expected toalso remove C from the steel structure. Thus, the presentpotential measurements are consistent with the formationof a clean, pure iron surface layer(with no presence ofFeyC microgalvanic cells).Fig. 9 presents the anodic polarization behavior of

steel in tap water and 3.5% NaCl solution and Table 3summarizes the corrosion properties calculated fromthese tests. Consistent with the previous results in tapwater, the EPP-cleaned steel exhibited a more noblecorrosion potential and passivated for the entire potentialrange tested. On the contrary, the background steelexhibited breakdown at a potential of approximately 170mV. It should be noted that at even lower potentials, thebackground steel exhibits a more active behavior(highercorrosion current density) compared to EPP-cleanedsteel. Also, the results show that the corrosion rateexhibited by the EPP cleaned steel in tap water is

significantly lower (approx. 25 times lower) than thatexhibited by the background steel. These results areconsistent with the presence of a pure iron layer at thesurface of EPP-cleaned steel.The NaCl solution is an aggressive environment and

the behavior in terms of corrosion rate, is similar forboth types of steel surfaces. However, the results showthat the corrosion potential difference remains at thesame levels as in tap water indicating that even in the


Fig. 10. Anodic polarization behavior of various coatings in(a) tapwater and(b) 3.5% NaCl solution.

NaCl solution, the EPP-cleaned steel surface is thermo-dynamically more stable than the background steel.Fig. 10 presents the anodic polarization behavior of

the EPP Zn and Zn–Al coatings and the hot dipgalvanized coating in tap water and 3.5% NaCl solution.In tap water, all three coatings exhibit activation polar-ization and comparable corrosion rates with the EPPcoatings possessing a little higher corrosion potential.The behavior in NaCl solution is along the same lineswith the EPP coatings exhibiting somewhat highercorrosion potential. Again all three coatings show acti-vation polarization at potentials close to the OCP. Thehot dip galvanized coating exhibited a little lowercorrosion rate that can probably be attributed to theparticular surface topography of the EPP coatings. Thepresence of craters or discontinuities at the surfaceproduced a larger surface area than the nominal areaused to estimate the current density and therefore theactual corrosion current density may be smaller. Thus,the present results demonstrate that the EPP method iscapable of producing sacrificial coatings for corrosion

protection while at the same time produces a ‘passive’steel outer surface that can serve as an additionalcorrosion resistance layer.

5. Conclusions

The results showed that EPP can effectively cleansteel surfaces that possess highly desirable anchor pro-files. The produced surfaces are nanocrystalline andexhibit a passive behavior. The latter is a direct outcomeof the physics involved in the cleaning process andresults in a significantly improved corrosion resistancecompared to base steel. This feature of enhanced lon-gevity of the cleaned surface is important when consid-ering the quality of subsequent coating operations andthe allowable time lag prior to coating. The EPP methodis also capable of producing adherent, dense and uniformZn and Zn–Al coatings that can function as sacrificialanodes protecting the underlying steel while the passivesteel outer surface layer can provide an additionalcorrosion resistance mechanism. The present resultsshow that EPP is an emerging, high potential cleaningand coating technology.

Acknowledgements

Financial support for this work was provided by theLouisiana Transportation Research Center and Depart-ment of Transportation(Award No. DTFH 61-00-X-00008). The authors would like to thank Mr EdwardDaigle (Metals Technology Inc.) for assisting with theelectrolytic plasma treatments and Dr Michael Phaneuf(Fibics Corp.) for conducting the FIB work.

References

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