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Wear 261 (2006) 922–931

Reduction of corrosion protection properties of organic coatings dueto abrasive damage produced by natural sands

S. Rossi ∗, F. Deflorian, M. RisattiDepartment of Materials Engineering and Industrial Technologies, University of Trento, via Mesiano 77, 38050 Trento, Italy

Received 30 September 2005; received in revised form 25 January 2006; accepted 31 January 2006Available online 10 March 2006

bstract

In many applications, the protective properties of organic coatings against corrosion require good resistance to abrasion. Mechanical damagean locally result in the reduction of the protective properties of paints.

This work focuses on the evaluation of abrasion damage produced by sands on organic coatings, using an electrochemical test and a modifiedaber test apparatus. The abrasive media were different natural sands, which were selected from seashores and deserts. These were chosen inunction of the dimension of the particles, their morphology and chemical composition.

The damage caused by the abrasive sands and the resulting reduction of the barrier effect were manifested and strongly dependent on the

haracteristics of the sands. In particular, the aggressiveness increased with the grain dimensions and with the reduction of the content of calciten the sand. The desert sands, which had rounded grains, exhibited lower abrasiveness than the sands whose grains had sharp edges. The lessggressive sands lead to a progressive reduction of the protective properties, with a reduction of the thickness with an uniform track on the wornurface; on the other hand, the more abrasive sands had a tendency to create non uniform tracks with larger defects.

2006 Elsevier B.V. All rights reserved.

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eywords: Paint tribology; Natural sands; Organic coatings; Modified Taber te

. Introduction

The use of organic coatings is one of the most common meth-ds for the corrosion protection of industrial components. In theast few years, the request for better protective performancend coating integrity has increased [1,2]. In addition, in manyndustrial applications, the protective properties against corro-ion have to be combined with a good resistance to abrasionnd particle impacts. This requirement is particularly importantor the automotive industry, naval, military and agricultural uses,nd all those applications that involve the possibility of abrasion,uch as in the presence of dust, sand or grains of hard mate-ial [3]. Further typical examples of systems undergoing severeamage due to abrasion wear are: milling of minerals, earth-oving equipment and use of farm equipments in hard soils.

dditional examples of less aggressive abrading conditions are:asonry ash and mineral-handling equipment, particles sliding

n chutes, ploughing sand [4,5]. These phenomena are not lim-

∗ Corresponding author. Tel.: +39 0461 882442; fax: +39 0461 881977.E-mail address: Stefano.rossi@ing.unitn.it (S. Rossi).

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043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2006.01.034

ctrochemical impedance spectroscopy

ted to a few geographical areas, in fact, although the locationslose to seashores or deserts represent about the 10% of totalmerged surface [6], tornados and desert storms can move greatuantity of abrasive sands. Moreover, winds can transport atreat distances the lightest sands, to areas very distant from therigin. These phenomena can give rise to cases of abrasion ofrganic coatings, due to the direct action of transported sand [7],r to cases of deposited sands, which act as third bodies [4,5,8,9].

Mechanical damage can locally result in the reduction ofrotective properties, hence the need to replace the coating. Forhis reason, it is important to evaluate the resistance to abrasionf the selected coating in the most accurate way.

Several tests aiming at the simulation of abrasive damageuring the service life of the components have been proposed10–12]. However, one of the most common tests is the Taberest [13,14]. An important factor is the simulation of the actualegradation phenomenon in a short time [15]. However, mostaboratory tests do not simulate well the actual service life

ehaviour of the components, giving rise to under- or overes-imation of the damage.

The Taber test is not only used to determine the abrasion resis-ance of coated systems, but in general, to estimate the protective

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roperties of different type of coatings. The typical experimen-al set-up is based on the use of two abrasive wheels, whose

ovement is activated by the rotational motion of the sample.he sample rotates at 60 turns per minute and from the inter-ction with the abrasive wheels the abrasive action is obtained12–14,16]. The usual damaged zone is a circular crown. Toncrease the abrasive action it is possible to select different abra-ive wheels and to impose loads ranging from 250 to 1000 g.valuation of the result is carried out by considering the mass

oss after 1000 rotations. Nevertheless, measurements of massnd coating thickness losses were not sufficient to appreciatehe actual reduction in the protective properties of organic coat-ngs in abrasive tests [12,17,18]. Consequently, electrochemicalests, after the Taber Abraser standard test, were used to evaluatehe decrease of protective properties [12,17,18].

In a previous study [2], a modification of the standard Taberbraser apparatus was shown, which allowed the electrical con-

act between the sample substrate and the electrochemical appa-atus was presented. The geometry of the coated sample wasodified and the evaluation of the loss of protection propertiesas achieved without removing the sample from the abrading

ystem. The tests were carried out using a slurry composed of aon aggressive aqueous solution and the abrading sand. Follow-ng these procedures, it was possible to leave the coated samplen the platform of the Taber equipment and monitor the decreasef protection properties caused by abrasion. The problem of theeduction of abrasion efficiency of the abrasive wheels, typicalf the traditional Taber test, was also avoided [1,12,17].

The aim of the present work is the evaluation of the abrasionamage produced on organic coatings by the action of sand,y using electrochemical tests and a modified Taber test appa-atus. In order to obtain realistic information, several naturalands, chosen in function of particle dimensions, morphologynd chemical composition, were selected. Natural sands fromeashores and deserts can be considered among the most repre-entative media for this kind of phenomenon.

. Experimental

The materials used in this work were previously reportedn more detail [1,2,12,17]. Q-panels of carbon steelSAE 1008/1010; 0.13 max C, 0.25–0.60 Mn, dimensions0 cm × 10 cm) were powder coated with an epoxy-polyesteresin. Prior to the coating application, the samples were ironhosphor-degreased. The organic coating was cured at 210 ◦Cor 20 min. The mean coating thickness was 70 ± 10 �m, and itas measured using a Karl Deutsch Leptoskop 2040 thicknessauge.

Taber Abraser tests [13,14] were carried out using a modifiedpparatus, which allows electrical contact between the sampleubstrate and the electrochemical apparatus, avoiding electricalontact with the Taber Abraser metallic parts, as shown in a pre-

ious work [2]. As for previous studies [1,2], instead of abrasiveheels, a pair of rubber wheels and abrasive sands were used. Aodified test sample geometry was used to contain the abrasive

lurry [2]. Tabl

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The abrasive slurry was composed of the sand and a 0.6 wt.%a2SO4 aqueous solution, which is not aggressive and allows

he evaluation of the extent of damage without affecting theubstrate. Two imposed loads (500 and 1000 g) were chosen.

The natural sands used as abrasive media are listed in Table 1.he sands were selected in order to obtain information about thebrasive effect produced, in function of their chemical compo-ition, morphology and granulometry. The morphology of theands was examined using a scanning electron microscope TMP

SEM FEI (secondary electron image obtained in low vacuumode). In addition, the abrasive particles were analysed using-ray diffraction to obtain information about their crystalline

tructure.

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Fig. 1. ESEM micrographs of abrasive sands: (a) Cocoa; (b) Aswan; (c) V

1 (2006) 922–931

To evaluate the decrease of protection properties, electro-hemical impedance measurements (EIS) were carried out19–22] every 100 cycles with a signal amplitude of 30 mV,n the frequency range between 100 kHz and 10 mHz. The mea-urements were carried out at the free corrosion potential inhe case of defective coating; a potential of −400 mV Ag/AgClas imposed when the organic layer was protective. In the lat-

er case, when the free corrosion potential was not measurableecause of the insulation offered by the organic coating, in order

o avoid polarization, the EIS measurements were carried out atfixed potential, close to the predicted free corrosion potential

1,12]. In order to avoid external noise affecting the electro-hemical signal, the Taber apparatus and sample were placed in

igo; (d) Comacchio; (e) Djanet; (f) Kos Marmari; (g) Kos Kefalos.

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Faraday cage. A three electrodes electrochemical cell was usedith an Ag/AgCl (+207 mV SHE) as reference and a platinumire as counter electrode. The test area was 62 cm2. A PARC73 potentiostat and a Solartron 1255 frequency response anal-ser connected to a PC were used. The EIS data were fitted usingSIMP WIN program, based on Boukamp’s equivalent circuits

23].In order to highlight the morphology of the damage, after

he loss of protection properties, the damaged surfaces werebserved using optical and scanning electron microscopes TMPSEM FEI.

. Results and discussion

.1. Characterization of the abrasive sands

The grain morphology of the various sands was observed toe very different. The sands collected in Cocoa beach showedrains with sharp edges (Fig. 1a). On the contrary, the sandsrom Aswan exhibited rounded grains (Fig. 1b). The other sandshowed an intermediate morphology (Fig. 1c–g). The sand fromjanet (Fig. 1e) was characterised by the presence of several

arge grains (up to 800 �m), although the present figure is onlyepresentative for the shape of the grains and the largest grainsre not visible in this micrograph. The range of grain dimensions,or each sand, was observed to be very wide in some cases, suchs the sand from Vigo, or very narrow as for the sand fromomacchio. The morphology of both desert sands were similar,ut the sand from Djanet showed a wider range of dimensions.he range of sand dimensions was very wide and this aspect wasonsidered in the discussion of the results of the abrasion tests.able 1 summarises all the characteristics of the sands.

X-ray diffraction analysis revealed the different compositionf the sands (Table 2). The sand from Aswan was composedf almost pure quartz, whereas that from Comacchio showedhe highest amount of calcite, with a conspicuous percentagef feldspars. The feldspars content was high in the sand fromreece. It should be noted that the hardness of quartz is higher

7 on the Mohs scale) than calcite (3 on the Mohs scale) [24,25].he hardness of feldspar is 6–6.5 on the Mohs scale.

.2. Abrasive effect of the sands

In a previous work [2], the possibility of studying the pro-ection loss of an organic coating by observing the changes in

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able 2and compositions

Quartz (%) Calcite (%)

1 Aswan, Egypt, Africa 96 42 Djanet, Algery, Africa 903 Cocoa, Florida, US 80 104 Vigo, Spain, Europe 84 15 Kefalos, Greece, Europe 40 206 Comacchio, Italy, Europe 43 327 Marmari, Greece, Europe 35 7

1 (2006) 922–931 925

he corrosion potential was evaluated. It was concluded thathe evaluation of the protection properties was not quantifiedith this experimental technique. For this reason the use of aifferent testing method was proposed [2]. In fact, the electro-hemical impedance spectroscopy (EIS) method was believed toe able to evaluate and quantify the decrease of protection prop-rties of an organic coating [19,26]. In accordance with otherork, every 100 abrasion cycles the platform was stopped (sta-

ionary conditions are necessary for EIS measurements) and anmpedance measurement was carried out. The new experimentalet-up allowed keeping both the sample and the solution withhe abrasive sand in the Taber platform.

EIS measurements allowed to evaluate very well the fall oforrosion protection properties of organic coating caused bybrasion action. This aspect was very well highlighted consider-ng the impedance plots reported in Fig. 2. This figure shows thempedance plots (Nyquist representation), after different test-ng periods, with an applied load of 500 g, for the sand fromomacchio. The initial values exhibited a capacitive behaviour,ue to the high protection properties of the paint. During thebrasion action, a semicircle appeared and a decrease of the totalmpedance was observed, indicating the reduction of protection.

The pore resistance of the coating (Rp), obtained by fittingmpedance data, was used to gain information on the extent ofamage [1,2]. In previous works, the fitting methods were dis-ussed in more detail [1,2,12,17]. The value of pore resistance ofn organic layer, 106 � cm2, was used as a threshold value, abovehich the coating could still be considered protective [1,27].ig. 3 shows the changes in the pore resistance at increasingbrasion cycles, with an applied load of 500 g, using the dif-erent sands. All the samples showed an initial high resistance>1011 � cm2), indicating the good protection properties of theaint and the absence of defects.

After a few cycles, the resistance of all samples decreasedbruptly, indicating extensive abrasion damage. In particular,fter 100 cycles, the samples tested using sands from Vigo andos Marmari reached a value of the coating resistance that was

ower than the threshold value, indicating a marked loss of pro-ective properties. These two sands were the most aggressive.ess aggressive than the previous cases were the sands fromocoa and Kos Kefalos, which exceeded the protection limitfter 300 and 400 cycles, respectively. These were followed by

he sand from Aswan, where the damaged coating lost its pro-ectiveness after 1000 cycles. The specimens tested with theands from Comacchio and Djanet lost their protectiveness after

Magnesite (%) Feldspar (%) Clay (%)

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926 S. Rossi et al. / Wear 261 (2006) 922–931

er of cycles, using the sand from Comacchio and an applied load of 500 g.

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Fig. 4. comparison between sand granulometry (maximum dimensions) andabrasion resistance (load 500 g).

Fig. 2. Impedance plots (Nyquist representation) after periodic numb

100 and 1200 cycles. Considering the least aggressive sands,omacchio and Djanet, a stabilisation of the resistance valuesas observed. This fact is probably due to the formation of newefects in different areas without reduction of coating resistance.n fact, it is statistically less probable that the sand grains extendhe previously formed defects with following decrease of resis-ance value [1]. It is important to underline that the coatingesistance is sensitive to the defect depth connected with thectual corrosion protection and not to the number of defects.

hen the sands were very aggressive, the formation of defectsroduced a quick reduction of resistance.

It is interesting to compare the relationship between the abra-iveness order of the sands and their grain maximal dimension,s showed in Fig. 4. With increase of grain dimensions, the sandsecame more abrasive. The only exception was the sand fromjanet, probably because of its rounded morphology.

Examining the results of the abrasion test with the imposed

oad of 1000 g (Fig. 5), a marked loss of protection propertiesf the organic coating was observed, after a few cycles, for all

ig. 3. Resistance as a function of cycles for different sands; a load of 500 g waspplied.

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ig. 5. Resistance as a function of cycles for different sands; a load of 1000 gas applied.

S. Rossi et al. / Wear 26

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ig. 6. comparison between the chemical composition (% calcite) and the num-er of abrasion resistance cycles, with an applied load of 1000 g.

he sands. The abrasiveness trend of the sands is very similaro that observed with the lower load. Nevertheless it is possibleo observe that the effect of granulometry resulted less deter-

inant. With this load the stabilisation of the resistance valuesas not present, probably due to the more aggressiveness of testarameters.

Only by considering the effect of the least abrasive sands, itas possible to highlight several changes.In general, the influence of chemical composition on the abra-

ive capability is strongly related to the hardness. Consideringhe hardness of the chemical components of the used sands,alcite showed the lower values. Quartz and feldspar showedigher hardness values. For this reason, in order to evaluate theffect of the chemical composition on the protection propertiesoss, it was very interesting to consider the calcite percentage.ig. 6 shows the calcite presence compared with the abrasionesistance of studied sands. This aspect confirmed the goodgreement between the hardness of sands and the abrasiveness28]. The sands with a higher percentage of calcite resulted less

brasive. A good agreement between the quantity of calcite inhe sand composition and the aggressiveness of the sands wasbserved. Also in this case, the sands from the desert differedrom the general trend, due to their rounded morphology. Sev-

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Fig. 7. Resistance as a function of cycles, under 500 an

1 (2006) 922–931 927

ral authors previously reported the influence of the roundnessf the abrasive particles on the damaging effects [29–31]. Inact, in spite of the quartz composition, both sands (Aswan andjanet) showed a slight abrasive effect. In particular, the sand

rom Djanet showed the lowest abrasiveness.In order to investigate the influence of the applied load on the

ecrease of protection properties of the coating, it is interestingo consider Fig. 7. The Rp trends, as a function of the appliedoads, are shown for the least aggressive sands (Comacchio).he load influenced the results: in fact, the number of cyclesecessary to exceed the protection limit (106 � cm2), with theigher load was lower (3 0 0) than those necessary for the 500 goad (9 0 0). The increasing number of cycles necessary to loosehe protection from the coating, using a lower load, was alsobserved in the trend of the sands from Aswan and Djanet (com-arison between the data shown in Figs. 3 and 5).

For more abrasive sands, such as from Vigo, the number ofycles for protection loss did not change with increasing load.n order to confirm the little influence of the load in the casef the most abrasive sand, an experiment, with an applied loadf 250 g, was carried out. Likewise, the number of cycles toxceed the threshold did not change. However, it was possibleo observe that the resistance values remained higher, with theowest imposed load.

.3. Analysis of the abraded surface after the loss ofrotection properties

The tracks produced by each sand were different, however,everal characteristic relationships between the properties of theands and the tracks produced were observed. In fact, it wasoticed that the less aggressive sands, (such as sands with a largemount of calcite) with grain dimensions lower than 1 mm (e.g.omacchio), were able to produce uniform tracks on the organicoating, which were comparable to the tracks produced by tradi-ional abrasive wheels (Fig. 8). Deep grooves were not observed

nd the thickness was uniformly and gradually reduced, in agree-ent with the electrochemical data. Examinations by ESEM

onfirmed these observations; in fact, the surfaces were uni-ormly damaged, without visible macro-defects (Fig. 9). A small

d 1000 g load, using the sand from Comacchio.

928 S. Rossi et al. / Wear 261 (2006) 922–931

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ig. 8. Abraded surface using the sand from Comacchio, under 1000 g load andfter 600 cycles.

rea, of bare substrate, was observed, where the paint had beenompletely removed. No significant differences were observedn the morphology of damage as a function of the appliedoads.

The surface of the samples abraded using the sands fromjanet (slightly aggressive) appeared very similar to the pre-ious case. The surface was uniformly abraded, without deeprooves. Nevertheless, the damaged area was not as clearlyefined as in the case of the sand from Comacchio. A simi-ar morphology was also observed on the samples exposed tohe sand from Aswan (Fig. 10). The external area, uniformlyamaged, was clearly visible; however, the internal area hadn uneven appearance, with visible scratches. The most dam-ged surface was observed using the electron microscope. Manyragments, produced by breakage of the grains of sand, werebserved. Possibly, this phenomenon increased the aggressive-

ess of the sand.

The surface abraded using the sand from Kefalos wasore damaged than the previous samples. The damaged area

ig. 9. Abraded surface using the sand from Comacchio, under 1000 g load andfter 600 cycles.

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ig. 10. Abraded surface using the sand from Aswan, under 1000 g load andfter 600 cycles.

as generally uniform; however, several localised defectsere observed. Several areas of bare substrates were exposed

Fig. 11), and the organic coating was completely removed.amage was observed also on the metallic substrate, induced by

he grains of sand. Fragments of the sands were, again, observedn the steel surface.

The sands of dimension larger than 1 mm (Cocoa, Vigo andarmari) induced, in few cycles, damage on the organic coat-

ng that in some cases was visible without a microscope. Theseefects were large cuts, grooves and indentations and had theotential to reduce, rapidly, the protection properties of the coat-ng. On the specimens exposed to the sand from Cocoa, the trackas not uniform and the thickness of the damaged paint wasifferent from one place to another. The damage was not uni-orm also between the external and the internal zone (Fig. 12).n the latter, several, isolated scratches were present. Imposing

smaller load, the difference was reduced, probably becauseore cycles were necessary for the loss of protection proper-

ies. In fact, the sand grains were active for a longer time on the

ig. 11. Abraded surface using the sand from Kefalos, under 500 g load, after00 cycles.

S. Rossi et al. / Wear 261 (2006) 922–931 929

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fsswere observed on the damaged surfaces (Fig. 14).

As a function of the shape of the grains, a different damagingprocess appeared to take place. The rounded sands had the ten-dency to deform the paint, under the action of the rubber wheels,

ig. 12. Abraded surface using the sand from Cocoa, under (a) 1000 g load,fter 300 cycles and (b) 500 g, after 700 cycles.

aint. Several defects were observed on the bare steel substrateFig. 13).

A general trend, for the generation of discrete defects, cane proposed: initially the grain of sand removed a small part ofhe organic coating. The steel substrate was no longer protected;

hen, the grains increase the dimension of the defects. In additiono the abrasion of paint, the bare steel was damaged also by themall fragments, generated by breakage of sand grains. Thisspect was clearly visible by consideration of the dimension of

ig. 13. Abraded surface using the sand from Cocoa, under 1000 g load, after00 cycles.

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ig. 14. Abraded surface using the sand from Marmari, under 1000 g load, after00 cycles.

he scratches on the substrate and the fragments found embeddedn the sample surface.

The morphology of the damaged surfaces using the sandsrom Marmari and Vigo appeared very similar. In few cycles,everal deep defects were produced, which reached the metalubstrate. In this case, a higher number of fragments of sand

ig. 15. The sands from Comacchio (a) and Aswan (b) after imposition of a000 g load and after 300 cycles.

6

[1] A. Cambruzzi, S. Rossi, F. Deflorian, Reduction on protective proper-ties of organic coatings produced by abrasive particles, Wear 258 (2005)1696–1705.

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1 (2006) 922–931

[2] S. Rossi, F. Deflorian, M. Risatti, Modified Taber apparatus and new testgeometry to evaluate the reduction of organic coatings corrosion protectiveproperties induced by abrasive particles, Surf. Coat. Tech., 2006, in press.

[3] A.K. Bhakat, A.K. Mishra, N.S. Mishra, S. Jha, Metallurgical life cycleassessment through prediction of wear for agricultural grade steel, Wear257 (2004) 338–346.

[4] L. Bourithis, G. Papadimitriou, Three body abrasion wear of low carbonsteel modified surfaces, Wear 258 (2005) 1775–1786.

[5] Q. Shao, Continuum model of abrasive layer in an abrasive wear test, Wear259 (2005) 36–43.

[6] A. Laurent, Desire de desert Sahara, le grand revelateur, EditionsAutrement, Paris, 2000.

[7] L. Lian-You, G. Shang-Yu, S. Pei-Jun, L. Xiao-Yan, D. Zhi-Bao, Windtunnel measurements of adobe abrasion by blown sand: profile character-istics in relation to wind velocity and sand flux, J. Arid Environ. 53 (2003)351–363.

[8] L. Du, B. Xu, S. Dong, H. Yang, W. Tu, Study of tribological characteristicsand wear mechanism of nano-particle strengthened nickel-based com-posite coatings under abrasive contaminant lubrication, Wear 257 (2004)1058–1063.

[9] H. Wang, W. Xia, Y. Jin, A study on abrasive resistance of Ni-based coatingswith a WC hard phase, Wear 195 (1996) 47–52.

10] I.M. Hutchings, Abrasive and erosive wear tests for thin coatings: a unifiedapproach, Tribol. Int. 31 (1998) 5.

11] A.G. Roberts, Paint, abrasion resistance, in: G.G. Sward (Ed.), Paint TestingManual, ASTM, Philadelphia, USA, 1972.

12] S. Rossi, F. Deflorian, L. Fontanari, A. Cambruzzi, P.L. Bonora, Electro-chemical measurements to evaluate the damage due to abrasion on organicprotective system, Progr. Org. Coat. 52 (2005) 288–297.

13] Standard ASTM D 4060-95, vol. 06.01, American Society for Testing andMaterials, Philadelphia, US, 1995.

14] Taber Industries, Operating Instructions for Taber Models 5131 and 5151,Digital Abrasers with LED Readouts, Taber Industries, North Tonawanda,NY, USA, 1994.

15] J.H. Tylczak, J.A. Hawk, R.D. Wilson, A comparison of labora-tory abrasion and field wear results, Wear 225–229 (1999) 1059–1069.

16] A.G. Roberts, Paint abrasion resistance, in: G.G. Sward (Ed.), Paint TestingManual, ASTM, Philadelphia, USA, 1972.

17] S. Rossi, F. Deflorian, A. Cambruzzi, L. Fontanari, P.L. Bonora, Evaluationof mechanical damage of organic coatings using Taber test and EIS mea-surements: the influence of test parameters, in: Proceedings of the Eurocorr2003, Budapest, Hungary, 28 September–2 October, 2003 (paper 263 onCD).

18] S. Rossi, F. Deflorian, A. Cambruzzi, L. Fontanari, L. Fedrizzi, P.L. Bonora,The abrasion resistance of a protective organic system, Eur. Coat. 80 (2004)7–19.

19] A. Amirudin, D. Thierry, Application of electrochemical impedance spec-troscopy to study the degradation of polymer-coated metals, Progr. Org.Coat. 26 (1995) 1–28.

20] F. Deflorian, L. Fedrizzi, S. Rossi, P.L. Bonora, Organic coating capacitancemeasurement by EIS: ideal and actual trends, Electrochim. Acta 44 (1999)4243–4249.

21] G.P. Bierwagen, Reflections on corrosion control by organic coatings,Progr. Org. Coat. 28 (1996) 43–48.

22] J.M. McIntyre, H.Q. Pham, Electrochemical impedance spectroscopy;a tool for organic coatings optimisations, Progr. Org. Coat. 27 (1996)201–207.

23] B. Boukamp, A non linear least squares fit procedure for analysis of immit-tance data of electrochemical systems, Solid State Ionics 20 (1986) 31–44.

24] American Federation of Mineralogical Societies, http://www.amfed.org/t mohs.htm, 2005.

25] H. Kim, T. Kim, Measurement of hardness on traditional ceramics, J. Eur.

Ceramic Soc. 22 (2002) 1437–1445.

930 S. Rossi et al. / Wear 2

without producing cuts and grooves. In this way, the paint wasremoved or, sometimes, adhered to the grains. The phenomenoninduced a decrease of thickness that was due to the indentationsproduced by the grains on the paint. On the contrary, grains withsharp edges produced localised defects, which uncovered thesubstrate in many cases. Nevertheless, during abrasion the grainsof the sand became fragmented, producing new, sharp and abra-sive surfaces [30]. This statement is supported by the presenceof grain fragments on the worn surfaces. Moreover, examina-tion of the sand grains after 300 cycles, with an imposed loadof 1000 g was carried out. The sand grains were all fragmented,with a subsequent reduction of dimensions, observable for exam-ple for Comacchio and Aswan sands in Fig. 15 (see comparisonof the sands before tests shown in Fig. 1). This phenomenonproduced two different effects: in the presence of the roundedsand grains, fragmentation increased the aggressiveness. On thecontrary, for the sands with sharp edges, fragmentation reducedthe dimension of the grains and, therefore, the possibility ofcreating large/deep grooves.

4. Conclusion

Measurements of the coating resistance, carried out by elec-trochemical impedance spectroscopy, after different number ofabrasion cycles, without removing the samples from the Taberapparatus platform, allowed an overview of the abrasiveness ofdifferent natural sands and the effect on the decrease of protec-tive properties of an organic coating to be obtained.

The damage caused by the abrasive sands, and the result-ing reduction of the barrier effect, was marked and stronglydependent on the characteristics of the sands. In particular, theabrasiveness increased with increasing grain dimensions andwith decreasing amount of calcite in the sand, which generateda decrease in hardness.

The sands from the desert, with round-shaped grains, wereless abrasive than the sands with sharp-edged grains. In the caseof these sands, the chemical composition and the grain dimen-sions appeared less important.

The influence of different imposed weights was marked, inparticular for the less abrasive sands.

The less aggressive sands lead to a progressive reduction ofthe protective properties, together with a reduction of the thick-ness that induced a uniform track on the worn surface. The moreabrasive sands had the tendency to create uneven tracks, withlarge defects that, eventually, gave rise to the failure of the pro-tective properties, after few cycles.

During the abrasion process, the sand grains fragmented andgenerated new, sharp-edged abrasive surfaces. The fragmenta-tion of rounded grains increased their abrasivity, whereas theeffect of the sharp grains was to reduce their abrasivity.

References

26] J.R. Macdonald, Impedance Spectroscopy, Wiley, New York, USA, 1987.27] M. Bethencourt, F.J. Botana, M.J. Cano, R.M. Osuna, M. Marcos, Lifetime

prediction of waterborne acrylic paints with the ac–dc–ac method, Progr.Org. Coat. 49 (2004) 275–281.

ar 26

[

[

S. Rossi et al. / We

28] D.A. Kelly, I.M. Hutchings, A new method for measurement of particleabrasivity, Wear 250 (2001) 76–80.

29] K. Elalem, D.Y. Li, Variation in wear loss with respect to load and slidingspeed under dry sand/rubber-wheel abrasion condition: a modelling study,Wear 250 (2001) 59–65.

[

[

1 (2006) 922–931 931

30] N.B. Dube, I.M. Hutchings, Influence of particle fracture in the high-stressand low-stress abrasive wear of steel, Wear 233–235 (1999) 246–256.

31] G.B. Stachowiak, G.W. Stachowiak, Wear mechanism in ball-cratering tests with large abrasive particles, Wear 256 (2004) 600–607.