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Progress in Organic Coatings 69 (2010) 455–462

Contents lists available at ScienceDirect

Progress in Organic Coatings

journa l homepage: www.e lsev ier .com/ locate /porgcoat

e-painting with high-speed water jets: Paint removal process and substrateurface roughness

. Teimouriana, M.R. Shabgarda, A.W. Momberb,∗

University of Tabriz, Tabriz, IranRWTH Aachen, Department of Georesources and Materials Engineering, Aachen, Germany

r t i c l e i n f o

rticle history:eceived 16 May 2010eceived in revised form 12 August 2010ccepted 12 August 2010

a b s t r a c t

Although the use of water jets for paint removal processes is an accepted procedure, there are just afew studies known which discuss parameter optimization and surface topography in some detail. Thepaper investigates the effects of water jet kinetic energy and stand-off distance on the mass loss of an

eywords:ater jet

aint removalurface preparation

organic paint system applied to a steel substrate. It was shown that the material removal process wascharacterized by a combination of loading intensity and loading frequency. Water drops, formed in thewater jet at long stand-off distances, played a notable role. For rather high water jet energies, massloss exhibited high values at high stand-off distances. For lower water jet energies, however, maximummaterial loss values appeared at a critical stand-off distance. The transition water jet energy was 600 kJ.

e ste. Seco

urface roughness It could be shown that thpreparation by water jets

. Introduction

Surface preparation is a vital part of any process of corrosionrotection of steel structures by coatings and linings. Numerousurface preparation methods are available, mainly blast cleaningdry or wet), mechanical grinding and water jetting [1]. Recenteviews are provided in [2,3]. While blast cleaning is still the dom-nating method in new construction projects, water jetting hasound its way into the coating repair business because of a num-er of advantages [2,4]. An example for the application of water

etting for on-site paint removal is provided in Fig. 1. The water jet-ing method is accepted by coating and paint manufacturers, andnternational quality standards are available which promote thetilization of this technique [5].

One of the most important parameters of a steel substrate thatovern the function of a protective coating system is the profile, oropography, of the substrate. This issue is covered, among others,n [6] for corrosion protection applications. Profile parameters ofnterest include the average roughness (Ra), the maximum rough-ess (Rt) and the average maximum roughness (Rz), but usuallyz (Ry5) is specified prior to application of corrosion protection

oatings on steel substrates. It is well known that corrosion protec-ion system assessment parameters, such as pull-off strength, paintelamination or under rusting, are determined by the roughnessarameters of the steel substrates. Corresponding investigations

∗ Corresponding author. Tel.: +49 40 75271144.E-mail address: momber@muehlhan.com (A.W. Momber).

300-9440/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.porgcoat.2010.08.010

el substrate topography was not compromised due to secondary surfacendary blast cleaning, however, reduced the profile of the substrate.

© 2010 Elsevier B.V. All rights reserved.

have been performed in [7–10]. It is common sense to mentionthat the water jet paint removal process can reveal the originalprofile of a steel substrate established prior to the application ofthe paint. However, only one investigation is available that provedthis particular statement [11]; it considered the removal of a shopprimer only, but not the removal of a complete organic corrosionprotection system, whose removal is the major field of water jetapplications in the corrosion protection industry.

Detailed investigations into the material removal capacity ofwater jets in terms of organic paint removal are only a few. A recentsummary is provided in [2], where the effects of water jet parame-ters on paint removal and the state-of-the-art of process modelingare critically discussed. Most of the investigations dealt with ratherhigh water pressures, namely in excess of 2000 bar (200 MPa). Thisis because it is being believed that a sufficient substrate surfacequality can be achieved at such high water pressures only. The stud-ies dealt with the effects of individual water jet parameters, namelypump pressure, stand-off distance and nozzle traverse rate, but noattempts were made to establish an integral water jet parameterwhich covers the material removal capability of a given parameterconfiguration. The investigations also suffered from a lack in infor-mation about the composition of the paint materials used. Usually,only generic types of the paints were given.

2. Fundamentals of water jet cleaning processes

The load induced on a target impinged by a high-speed waterjet can be characterized through loading intensity and loading fre-

456 H. Teimourian et al. / Progress in Organic Coatings 69 (2010) 455–462

Nomenclature

dD drop diameterdN nozzle diameterEJ water jet kinetic energyI loading intensityLT nozzle traverse lengthM mass lossN loading frequencypI impact pressurepS stagnation pressurepW water pressureQ̇W volumetric water flow ratetE exposure timevD drop velocityvJ water jet velocityvT nozzle traverse ratex stand-off distancex0 optimum stand-off distancexC jet core length�W water density

Fb

qbMr(isawow

E

pezt

t

Fig. 2. General relationship between loading intensity and loading frequency.

provided the water jet kinetic energy had high values. This situa-tion would apply to the right region of the material removal curvesplotted in Fig. 2.

Table 1Process parameter variation for the water jet paint removal experiments.

Waterpressure inMPa

Water jetvelocity inm/s

Traverserate inmm/s

Volumetricflow rate inl/mina

Exposuretime in s

Jet kineticenergy inkJb

40 254 1.0 20 47.0 6241.5 20 31.3 4162.0 20 23.5 312

50 284 1.0 23 47.0 8971.5 23 31.3 5972.0 23 23.5 449

60 312 1.0 26 47.0 12171.5 26 31.3 8102.0 26 23.5 609

ig. 1. Paint removal with water jetting in practice (copyright: Muehlhan AG, Ham-urg, Germany).

uency [2,12,13]. The general approach is illustrated in Fig. 2. Theold lines characterize given levels of material removal (M1 and2). It can be recognized that a given material removal can be

ealized either if loading intensity is high and loading frequencyor impact number) is low, or if loading intensity is low and load-ng frequency (or impact number) is high. This expression featuresome relationship to liquid drop impact fatigue curves [14]. Oka etl. [15] developed a very similar conception for the evaluation ofater jet based surface treatment processes. The loading intensity

f an impinging water jet can be expressed by its kinetic energy,hich can be estimated with the following equation:

J = �W Q̇W

2v2

J tE (1)

The volumetric water flow rate depends mainly on nozzleressure, nozzle cross-section, nozzle fan angle and nozzle flowfficiency. The latter three parameters are summarized in a “Noz-

le Index” issued by the nozzle manufacturer. The local exposureime can be approximated as follows:

E = LT

vT(2)

Fig. 3. Water jet structure (adapted from [20]).

Values for the local exposure time and for the kinetic jet energyare listed in Table 1 for different parameter configurations. Eqs.(1) and (2) show that any increase in water jet velocity, and anyreduction in traverse rate, will increase the kinetic energy of animpinging water jet. It was shown in [16] that the material removalon a ductile-behaving material (low-carbon steel) exhibited a lin-ear relationship to the kinetic energy of an impinging water jet,

70 337 1.0 28 47.0 15291.5 28 31.3 10182.0 28 23.5 765

a Estimated from manufacturer’s nozzle index table.b Calculated with Eqs. (1)–(2).

H. Teimourian et al. / Progress in Organic Coatings 69 (2010) 455–462 457

Table 2Mechanical properties of the steel specimens.

Property Value

Density 7860 kg/m3

Elongation 33%Grain structure ASTM 8/100Rockwell hardness 24 HRBTensile strength 350 MPaYield strength 260 MPa

Table 3Chemical composition of the steel specimens.

Elements Mass percentage

Carbon 0.13Phosphorus 0.03Manganese 0.5

teTgizztcodblrw

x

djtzi

p

olsaertaap

idf

p

Ha

Table 4Parameters of the paint (coating) system.

Parameter Paint layer

1 2 + 3 4

Designation Zinc-richprimer

EPintermediate

PU top coat

Dry film thickness in �m 1 × 60 2 × 80 1 × 80Binder type Epoxy-

polyamideEpoxy-polyamide

Acrylic-isocyanate

All tests were performed on plain and painted low-carbon steelsamples. The mechanical and chemical properties of the substratematerial are listed in Tables 2 and 3. The specimens were cut awayfrom a standard plate by mechanical sawing; the dimensions were:

Silicon 0.03Aluminium 0.1Titanium 0.04Micro-alloys 0.01

Loading frequency of an impinging water jet can be expressedhrough the stand-off distance, because stand-off distance is anxpression of the jet structure, and through total exposure time.he longer exposure time, the more drops impinge the surface at aiven stand-off distance. Usually, the structure of a water jet flow-ng through still air can be subdivided in axial direction into threeones: a jet core zone, a jet transition zone and a final zone (dropletone). This is illustrated in Fig. 3. In the cone-shaped jet core zone,he flow properties, such as stagnation pressure and jet velocity, areonstant along the jet axis. In that zone, the loading regime consistsf a stationary loading part due to jet core impingement, and of aiscontinuous loading part due to impinging water drops. The ratioetween stationary and discontinuous loading is dependent on jet

ength, or stand-off distance. The length of the core zone can beelated to the nozzle diameter of round nozzles in the followingay [17]:

C = (20 . . . 100) dN (3)

It can be seen that core zone length can vary notably, mainlyue to variations in water pressure and nozzle geometry. For fan

et nozzles, as used in this study, the core zone is usually smallerhat for round nozzles [13]. The load generated on the target in thatone is dominated by a static stagnation pressure, generated by thempinging water jet core, which can be estimated as follows:

S =�W v2

J

2(4)

In the transition zone, however, notable water drop formationccurs also on the jet axis due to internal turbulences, and theoading regime is dominated by the discontinuous loading due toubsequent water drop impingements. The impinging drops addhighly dynamic component to the jet, very similar to a fatigue

ffect. This aspect was in details investigated in [18] for the mate-ial removal of low-carbon steel with water jets, and in [15] forhe treatment of aluminium alloys by water jets. The spray zonet the circumference of the jet, formed due to external frictionnd air entrainment, does not contribute to the material removalrocess.

The loading intensity of impinging liquid drops can be character-zed by their impact velocity, or by the impact pressure generateduring the impingement. This pressure can be approximated as

ollows [19]:

I = cW vD �W (5)

ere, cW is the shock wave velocity of the water (which can bepproximated with the speed of sound in water: 1450 m/s), vD is

Solid content by weight in % 90 ± 1 75 ± 1 60 ± 5Density in g/cm3 2.70 1.50 1.15Mixing ratio by weight 100:10 100:20 100:25Zinc weight% in dry film 80 – –

the drop velocity, and �W is the liquid density. It can be seen thatthe impact pressure does not depend on drop size. The drop velocitycan be approximated as follows for fan jet nozzles [20]:

vD = 0.9(

2 pW

�W

)1/2(6)

A combination of Eqs. (4) and (5) delivers the following relation-ship:

pI

pS= 2 cW

vD/J(7)

Therefore, the impact pressure generated during drop impinge-ment is higher than the stagnation pressure in the core zone aslong as the jet flow velocity does not exceed the value 2cW (whichis about 2 × 1450 m/s).

Diameters and velocities of liquid drops both depend on thestand-off distance, respectively on jet length. Values for eitherparameter drop if stand-off distance increases [15,20]. The diame-ters of water drops formed in a water jet exiting a fan jet nozzleat water pressures between p = 40 and 60 MPa were found torange between dD = 30 and 300 �m, while the majority of all dropshad diameters between dD = 30 and 100 �m [20]. Drop veloci-ties, measured with a laser anemometer device, ranged betweenvD = 270 and 320 m/s [20].

3. Experimental set-up and procedure

Fig. 4. Steel substrate after primary blast cleaning.

458 H. Teimourian et al. / Progress in Organic Coatings 69 (2010) 455–462

tic energy, stand-off distance and paint mass loss.

5(matw9macfwnopTapTrwdushpf

Table 5Process parameters for paint system removal experiments.

Parameter Value

Pressure (MPa) 40–50–60–70Stand-off distance (mm) 60–70–80–90

Fig. 5. Relationships between water jet kine

00 mm × 70 mm × 5 mm. Primary blast cleaning of the samplesbefore the paint system was applied) was performed with a com-

ercial blast cleaning unit with an air pressure of p = 0.5 MPa (5 bar)nd a nozzle diameter of dN = 8 mm. Copper slag with a mean par-icle size of dP = 1.0 mm was used as blasting agent. All specimensere blast cleaned at an exposure time of tE = 400 s, at an angle of

0◦, and with a stand-off distance between nozzle exit and speci-en surface of x = 200 mm. The samples after primary blast cleaning

re shown in Fig. 4. After primary blast cleaning, all specimens wereleaned with dry compressed air to remove any rust, grease, or dustrom the surfaces to be evaluated. Then, a zinc-rich epoxy primeras applied to the samples with an approximate dry film thick-ess of DFT = 60 �m. Two layers of an epoxy polyamide paint andne layer of aliphatic polyurethane paint were applied to the sam-les with a dry film thickness of about DFT = 80 �m for each layer.he thicknesses of the individual paint layers were measured withcoating thickness gauge, Electromatic, “CECK-LINE 2000”. The

roperties of the individual paint materials are listed in Table 4.he samples were weighted before and after the secondary paintemoval process. The mass balance used was a “METTLER PM16-N”ith a precision of ±0.1 g. In order to control and adjust stand-offistance and nozzle traverse rate, a mechanism was made with

se of a cross-table, an electrical gear box and an inverter. Theecondary water jetting experiments were carried out by using aigh-pressure system type “WOMA Z-225”. The ranges of processarameter variation are listed in Table 5. For the nozzle type usedor the experiments (Form 19), the nozzle index was 1.15. Based on

Nozzle traverse rate (mm/s) 1.0–1.5–2.0Traverse length (mm) 47Impact angle 90◦

Nozzle diameter (mm) 1.5Nozzle type Fan jet

this index, the volumetric flow rate through the nozzle has beenobtained from flow charts issued by the water jet equipment man-ufacturer. The values for the volume flow rates estimated with thisprocedure are listed in Table 1.

Nine specimens were cut away from a standard plate withdimension of 150 mm × 150 mm × 5 mm for the profiling tests.Three samples were reserved for the profile evaluation of the pri-mary blast cleaning process, and six samples were painted underthe conditions mentioned in the previous section. In order to inves-tigate the effects of the secondary paint stripping processes onthe substrate surface profile, three specimens were de-paintedby water jetting, and three of them were de-painted by blast

cleaning. Test conditions of the secondary blast cleaning wereequal to those mentioned in the previous section, with the excep-tion of a shorter exposure time of tE = 40 s. Paint removal withwater jet was performed with parameters different from thoselisted in Table 5. The parameters were as follows: p = 100 MPa,

H. Teimourian et al. / Progress in Organic Coatings 69 (2010) 455–462 459

pain

vtmoeHlwTrr

4

4

iitiisi

Fig. 6. Relationships between stand-off distance, local exposure time and

T = 1.0 m/s, x = 90 mm. It can be recognized that this parame-er configuration provided a high-energy input into the substrate

aterial, and it characterized a worse-case scenario in termsf possible profile modification. In order to evaluate the gen-rated surface profiles, a mechanical profilometer type “Taylorobson” with a cut-off length of 0.8 mm was used. The traverse

ength of the measured profile was 40 mm, and the roughnessas measured at three different locations for each specimen.

he following roughness parameters were measured: averageoughness (Ra), maximum roughness (Rt) and average maximumoughness (Rz).

. Results and discussion

.1. Effect of water jet kinetic energy on mass loss

The effects of varying values for the kinetic energy of the imping-ng water jets on the mass loss of the paint system are illustratedn Fig. 5 for different stand-off distance values. The experimen-

al results show that the mass removal increased as jet energyncreased. This is an effect which would be expected from the phys-cal point of view. The more energy is available locally at the erosionite, the more materials shall be removed. In the range of the exper-mental points, the relationship could be fitted well with a linear

t mass loss. 1 – p = 40 MPa; 2 – p = 50 MPa; 3 – p = 60 MPa; 4 – p = 70 MPa.

regression as follows:

M ∝ EJ (8)

This result agreed with results obtained in [16] for the water jetprofiling of low-carbon steel. However, the precision of the linearcorrelation between mass loss and kinetic jet energy depended onjet energy and on stand-off distance as expressed by the r-numbersin Fig. 5. The correlation was high for kinetic energy values up toEJ = 600 kJ, and it started to deteriorate if this value was exceeded.In terms of stand-off distance, the correlation was weak (r = 0.81)for the longest stand-off distance of x = 90 mm over the entire jetenergy range (Fig. 5d). This very special result showed that staticloading intensity was not the only parameter governing the paintremoval capability of an impinging water jet. Other effects, orparameters, must be considered. Such additional effects seem to bethe loading frequency, the target is loaded at by the impinging jet,or a dynamic component of the loading intensity. As already men-tioned in Section 2, a high-frequency load is added to the water jetat rather long stand-off distances due to the impingement of waterdrops formed in the jet. These drops may contribute to the mate-

rial removal in particular if the static component of the loadingintensity is rather low.

Another issue to be mentioned is that a linear relationshipbetween jet energy and mass loss cannot be applied to the entirejet energy range for physical reasons. The curves exhibited in Fig. 5

460 H. Teimourian et al. / Progress in Organic Coatings 69 (2010) 455–462

F rimarc

mmFiisoBpnTatr

4

asltltietoc

teaiclwkgMplab

ig. 7. Results of substrate surface roughness measurements. a – untreated (0); pleaning (I); secondary water jetting (II).

ust snap off at very low values for the kinetic energy, and theyust cross the abscissa either at the zero point (as marked in

ig. 5) or at a very low threshold energy value. The graphs plottedn Fig. 5 suggest a transition point which separated the functionsnto two branches. The first branch was characterized by a ratherteep progress of the curve at very low values for the kinetic energyf the jet (very low jet velocity or very short exposure time).eyond the transition point, in the range of the second branch, therogress of the curve dropped. The transition point was less pro-ounced in the case of the highest stand-off distance x = 90 mm.he transition points were at about EJ = 200 kJ for the conditionspplied in this study. The precise investigation and interpreta-ion of these transition points is an interesting topic for furtheresearch.

.2. Effect of stand-off distance on mass loss

The effects of variations in the stand-off distance on mass lossre illustrated in Fig. 6 for given values of water pressure and expo-ure time. Two trends could be distinguished in dependence on theoading intensity, or kinetic energy. If the kinetic energy was lowerhan a certain threshold value for the kinetic jet energy, the massoss function exhibited a maximum at a particular stand-off dis-ance value. These results agreed very well with results obtainedn [12,13,21]. These authors found this trend for the removal ofpoxy paint systems from steel substrates with water jets at highraverse rates. Similar results were reported in [22] for the removalf epoxy-based paints, and in [23] for the removal of epoxy-resinoatings.

The optimum stand-off distance for maximum paint removal inhe present investigation was found to be x0 = 80 mm for the low-nergy water jets. A kinetic energy threshold could be noted atbout EJ = 600 kJ. This number agreed with the number estimatedn the previous section. Below this threshold, a dynamic loadingomponent did not contribute to the mass removal process foronger stand-off distances x > 80 mm, while a dynamic component

as very helpful even at the very long stand-off distance if theinetic jet energy exceeded the threshold. These relationships areeneralized in Fig. 2, showing two material removal lines (M1 and

2) for a given paint material. It can be seen that the dynamic com-

onent, or loading frequency N1/N2 (at the ordinate), can be keptow if the loading intensity I1/I2 (at the abscissa) has high values,nd vice versa. This type of failure nomogram is also known fromlast cleaning processes [3].

y blast cleaning (I); secondary blast cleaning (II). b – untreated (0); primary blast

The experimental results very well fit into a model for the paintremoval with water jets proposed in [24]. This model subdividesthe material removal process into three stages: damage accumula-tion, rapid erosion of the upper part of the coating, slow erosionof the coating near the substrate. Material removal starts if thedamage accumulation (loading frequency, or number of impingingdrops) exceeds a threshold value. The results plotted in Fig. 6 show,that a particular number of impinging water drops was requiredin order to pre-weaken the material. This is illustrated in Fig. 2.In order to remove a material quantity M1, a drop number N1is required for a given loading intensity I1. The loading intensityof the drop impingement can be assessed with Eq. (5). Based onvD = 250 m/s (reported in [20]), Eq. (5) delivers an impact stress ofabout pI = 360 MPa.

The optimum stand-off distance, where the maximum paintremoval efficiency was achieved, would correspond to a relative jetcore length (xC/dN) of 80 mm/1.5 mm = 53, which is in the range ofEq. (3). The optimum stand-off distance is assumed to be a constantof the paint system, if all other parameters are kept unchanged. Theresponse of each paint material to water jet erosion is character-ized by a balance of static and dynamic loading components. Thisis illustrated in Fig. 2 for one paint material and two given materialremoval values (M1 and M2). The static component is characterizedby the jet core, and the dynamic component is characterized by theimpinging water drops. This balance may mainly be determined bythe capability of the paint system to dissipate impact energy andto suppress crack propagation. If the energy dissipation capacityis low, or if the damage accumulation limit is low, the optimumstand-off distance x0 may shift to low values.

The almost linear relationship between stand-off distance andmass loss for the high-energy water jets (EJ > 600 kJ) can beexplained through the increase in dynamic cyclic loading. For thepaint system, which was able to locally dissipate energy, a cyclicload, characterized by a certain frequency (drop number) and acertain load intensity (impingement stress), was required in orderto pre-weak the material and to remove material. The high kineticenergy of the water jet was either a result of a high jet velocity or oflong exposure time. The impinging drops, therefore, generated highstresses (due to high impingement velocity), and their number was

sufficient enough (due to long exposure time). This situation is illus-trated through the point M1 in Fig. 2. The situation was different forthe cases of low-energy water jets (I2 < I1), where the mass removaldropped for stand-off distances x > 80 mm. Although the loadingregime was still characterized by a high number of impinging water

H. Teimourian et al. / Progress in Organ

Fb

dwdn(stfresttl

Related Products: Visual Assessment of Surface Cleanliness. Part 4: Initial Sur-

ig. 8. Results of 3D surface topography measurements. a – primary blast cleaning;– secondary blast cleaning; c – secondary water jetting.

rops (N1), the impingement stress, generated at the paint surface,as lower than for the high-energy jets (I2 < I1). Therefore, the con-ition M1 in Fig. 2 was violated. In order to meet the M1-line, theumber of impinging water drops with a given impingement stressI2) would need to be increased from N1 to N2. This was not pos-ible, and the material removal dropped from M1 down to M2 (oro a line between the M1-line and the M2-line). Another argumentor the reduction in mass loss at higher stand-off distances is theeduction in impact forces at rather high stand-off distances. Okat al. [15] have shown that the impact force of an impinging jet core

tarts to drop notably at stand-off distance of about x = 90 mm. Forhe case EJ < 600 kJ, the following situation appeared for x > 80 mm:he loading intensity (impingement stress, impact force) was tooow.

ic Coatings 69 (2010) 455–462 461

4.3. Substrate surface profile

The results of the roughness measurements for different surfacepreparation conditions are plotted in Fig. 7. All results obtained foruntreated conditions (0) were taken to 1. Three-dimensional topog-raphy images of the prepared substrates are provided in Fig. 8.As expected, primary blast cleaning (I) increased any roughnessparameter compared to the untreated substrate (0). Secondaryblast cleaning (II), as a result of the removal of the paint sys-tem from the substrate, deteriorated the roughness values, whichis well illustrated in Fig. 7a and in Fig. 8b. This phenomenon isfrequently referred to as “overblasting” in the manufacturing lit-erature [25]. Overblasting effects during blast cleaning processeshave been reported, among others, in [25–27]. The background ofthis phenomenon is discussed in more detail in [3]. Of particularinterest is the topography of the substrate after the paint removalwith water jetting (II). It can be seen in Fig. 7b and Fig. 8c, thatthe roughness values of the substrate were not compromised inthat case. The values for the roughness parameters Ra, Rz and Rt

remained unchanged after water jetting, compared with the val-ues for the substrate surface before the paint systems have beenapplied (primary blast cleaning). These results support the oftenmentioned statement that water jetting restores the original pro-file rather than add a profile to the substrate [2]. Moreover, theresults agree with results obtained in [11] for the removal of shopprimer from pre-blast cleaned steel substrates with water jets.

5. Summary and conclusions

It was shown that the water jet material removal process wascharacterized by a combination of loading intensity and loading fre-quency. For low stand-off distances, a linear relationship betweenwater jet energy and mass loss can be supposed. Water drops,formed in the water jet at long stand-off distances, played a notablerole during the material removal process. For rather high waterjet energies, mass loss exhibited high values at high stand-off dis-tances. For lower water jet energies, however, maximum materialloss values appeared at a critical value for the stand-off distance. Atransition water jet energy of EJ = 600 kJ was found for the parame-ter range investigated in this study. It could be shown that the steelsubstrate topography was not compromised due to secondary sur-face preparation by water jets. Secondary blast cleaning, however,compromised the profile of a pre-blast cleaned substrate.

Acknowledgments

The work was supported by Tabriz Petrochemical Company,Tabriz, Iran. The authors are thankful for this support. The authorsalso wish to thank Mr. D. Lafferty and Dr. M. Hashish for their helpand guidance.

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[

[

[

[

[[[[

[

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[

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