Wear 258 (2005) 629–640

Solid particle erosion–corrosion behaviour of a novel HVOF nickelaluminium bronze coating for marine applications—correlation

between mass loss and electrochemical measurements

K.S. Tan, J.A. Wharton, R.J.K. Wood∗

Surface Engineering and Tribology Group, School of Engineering Sciences, University of Southampton,Highfield, Southampton SO17 1BJ, UK

Received 1 October 2003; received in revised form 27 February 2004; accepted 28 February 2004Available online 28 October 2004

Abstract

This paper investigates the solid particle erosion–corrosion performance of an experimental high velocity oxy-fuel (HVOF) sprayedn e coatingc te coatingf cted to puree the effectso h describedt eviations ofe breakdownu tic energies,e e film whichr ctrochemicalm©

K

1

wisestso

evel-on a

al-stantf ap-

n or

sedtingVOF

valvedur-r allthe

0d

ickel–aluminium bronze (NAB) coating using conventional gravimetric techniques as well as in situ electrochemical analysis. Thonsists of HVOF powders from three alloys: stainless steel alloy, nickel-based alloy and aluminium bronze alloy. It is a candidaor marine applications as a cost effective replacement of existing castings and to improve component life. The coating was subjerosion, flow corrosion and erosion–corrosion tests. A jet impingement slurry erosion rig was used to carry out the experiments;f jet velocity were investigated. By gravimetric analysis the degree of synergy was evaluated and a constant was revealed whic

o what extent the presence of corrosion products/films reduces the erosivity and promotes negative synergy. Likewise, standard dlectrochemical current measurements are shown to reveal the presence of protective film formation under flow corrosion and filmnder erosion–corrosion conditions. Separation of the erosion-enhanced corrosion component revealed that at high erodent kinerosion-enhanced corrosion dominates and generates a positive synergy. At lower energies, this coating system forms a protectiveduces the contact conditions on impingement and a negative synergy results. Overall, correlations between the mass loss and eleeasurements have been established and were used to identify and quantify synergy.2004 Elsevier B.V. All rights reserved.

eywords:Erosion–corrosion; Synergy; Electrochemical noise; HVOF; Coating; Nickel–aluminium bronze

. Introduction

Seawater is used for fire-fighting and cooling systemsithin the marine, oil and gas production, and power generat-

ng industries. However, sand particles can be entrained in theeawater, which reduce the design life by a combination ofrosion and corrosion. If erosion and corrosion attack occursimultaneously, this can enhance the overall material wastagehrough a synergistic effect due to a coupling between ero-ion, a mechanical process, and the electrochemical processf corrosion.

∗ Corresponding author. Tel.: +44 23 8059 4881; fax: +44 23 8059 3230.E-mail address:rjw3@soton.ac.uk (R.J.K. Wood).

Organic and metallic coatings systems have been doped to resist erosion–corrosion whilst being appliedrelatively low cost substrate, thereby offering economicternatives to components made from high corrosion resialloys. Other advantages of coatings include the ease oplication and ability to recover components that are worcorroded.

High velocity oxy-fuel (HVOF) coatings have been uwidely in various engineering components for combawear and corrosion. Components that are coated by the Hmethod include propellers, pump impellers and casings,bodies/trim and pipe systems. Porosity that is introduceding the spraying process is a common phenomenon fothermal coatings. Such porosity is detrimental towards

043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2004.02.019

630 K.S. Tan et al. / Wear 258 (2005) 629–640

corrosion protection offered by the coating; electrolyte canpermeate the coating reducing the life of the coating and itssubstrate. It is, therefore, desirable to fabricate coatings thatcan reduce or prevent corrosive electrolytes from reachingthe substrate by forming insoluble corrosion products at thepores.

Nickel–aluminium bronzes (NAB) are widely used forpropulsion and seawater handling systems in naval platforms.Nickel–aluminium bronzes offer high strength, high corro-sion resistance and are available in a number of differentforms. Steady state corrosion rates for nickel–aluminiumbronzes are in the order of 0.025 mm/year in seawater con-ditions [1]. This good corrosion resistance is attributed to astable and protective surface film, consisting of two layers,an inner layer of cuprous oxide (Cu2O) and an outer layer ofalumina (Al2O3) [2]. Damage or depassivation of the surfacefilm occurs by exposure to high velocities, cavitation and par-ticle impingement. This can lead to localised damage of thisprotective surface film causing pitting and selective phasecorrosion. The strong ability of nickel–aluminium bronze toquickly repassivate results in a low corrosion rate; hence it isused widely under erosion–corrosion conditions.

2. Synergy

ion–c ssedb

S

w toe ands intot -e on,s

S

sionfi[ ity oft sionm ites,a ion,( , (iii)l (iv)s pacte n rate[ possi-b valo h ex-p s( ltingi e

in the number of stress concentration defects resulting frommicropitting and (viii) detachment of plastically deformedflakes on the metal surface due to stress corrosion cracking.Most of the above mechanisms, if dominant, would be ex-pected to lead to positive synergy. However, in some instancesnegative synergy can occur. Possible mechanisms which re-duce erosion rates (−�E) are (ix) increased work hardeningdue to corrosion mechanisms, (x) shot-peening[7] by highvelocity sand particle impacts or (xi) the presence of a soft orloosely adherent corrosion film or (xii) blunting of the cracktips by lateral dissolution retarding the speed of crack propa-gation. The reduction in corrosion rates (−�C) could resultfrom rapid corrosion film growth, scaling or the formation ofa passive film reducing corrosion rates dramatically.

3. Erosion–corrosion of protective coatings

Many investigations have covered the erosion–corrosionbehaviour of metallic and polymeric based protective coat-ings[8–12]. In general, plastic deformation was observed forductile coatings, while brittle coatings eroded by crackingand chipping mechanisms. Coatings usually consist of bothductile and brittle constituents and the erosion behaviour isdominated by the major component. When a particular coat-i ium,m thes be-h . Att pro-c tingl atinga uringe

–c ings,p apourd rro-s ut us-i osiono thec VOFN n in3 t rig,r , fol-lT ials’b aledt ed ind s onHs tallicb par-t

r ofH cor-

Synergy is defined as “the difference between erosorrosion and the sum of its two parts” and can be exprey Eqs.(1) and (2)

= T − (E + C). (1)

here T, C, E and S are gravimetric terms relatingrosion–corrosion, pure corrosion (in situ), pure erosionynergy, respectively. Synergy can be broken downwo components,�E and�C, where�E is the corrosionnhanced erosion and�C is the erosion-enhanced corrosiee Eq.(2).

= �E + �C. (2)

Erosion can mechanically strip the protective corrolm creating fresh reactive corrosion sites and producing�C3], dependent on the rate of repassivation and the integrhe film formed. Other possible erosion-enhanced corroechanisms include (i) local acidification at erosion sccelerating corrosion rates and prohibiting film formatii) increased mass transport by high turbulence levelsowering the fatigue strength of a metal by corrosion andurface roughening of the specimen during particle imnhanced mass transfer effects increased the corrosio

4]. Corrosion-enhanced erosion mechanisms are alsole (�E). The�Ewear rate could be due to (v) the remof work hardened surfaces by corrosion processes whicose the underlying base metal to erosion mechanism[5],vi) preferential corrosive attack at grain boundaries resun grain loosening and eventual removal[6], (vii) the increas

ng is exposed to abrasive particles in a corrosive medass transfer effects between the coating/liquid and

ubstrate/liquid can affect the slurry erosion–corrosionaviour of the coating due to increased oxide formation

he same time, porosity introduced during the coatingess will allow the liquid to permeate through the coa

eading to possible galvanic corrosion between the cond substrate, thus accelerating the material wastage drosion.

Bardal and co-workers[13,14] carried out erosionorrosion tests on a range of metallic and cermet coatroduced by a range of coating processes (chemical veposition, thermal spray and HVOF). Erosion, flow coion and erosion–corrosion experiments were carried ong a rotating disc apparatus, in synthetic seawater. Corrf the metallic binder in the cermet coatings underminedarbide particles, leading to enhanced material loss. Hi–Cr–Si–B–C coatings subjected to erosion–corrosio.5% NaCl solution, using a submerged jet impingemenesulted in surface roughening mechanism due to erosionowed by corrosion of less noble coating constituents[15].his lead to removal of ‘unsupported protruding matery further sand particle impingement. Microscopy reve

hat selective corrosion at the splat boundaries resultislodgement of splat particle by erosion. Similarly, testVOF WC–Co–Cr coatings reported by Perry et al.[16]howed that preferential corrosion occurred at the meinder followed by removal of the hard carbides by sand

icle impingement.The aim of this work was to investigate the behaviou

VOF nickel aluminium bronze coatings under erosion,

K.S. Tan et al. / Wear 258 (2005) 629–640 631

Table 1Chemical compositions of materials used in the current investigation

P S Mn Al Ni Fe Cr Mo Si C Cu Notes (mixing ratio)

9.5 – 1 – – – – Bal D1004 (89%)– 12 Bal 17 2.5 1.0 0.1 – D1003 (6%)5 Bal – – – – – – D4008NS (5%)9 5 4.5 – – – – Bal Cast NAB alloy8.71 5.55 4.50 1.02 0.15 0.06 0.01 Bal NAB coating

0.05 0.05 1.6 Bal 0.5 0.25 BS 4360 steel

The table also shows the three types of HVOF powders used for fabricating the HVOF NAB coating.

rosion and erosion–corrosion conditions. Whilst undertakingthese experiments, a correlation between conventional massloss measurements and electrochemical noise techniques wassought for identification and quantification of the synergisticeffects.

4. Experimental procedures

4.1. Test materials

A novel HVOF NAB coating with a composition similarto that of cast NAB alloy was used in the current investi-gation. The coating was deposited onto BS 4360 steel sub-strates by using a commercial Diamond Jet HVOF sprayingsystem. The NAB coating was formulated from three typesof commercially available HVOF powders.Table 1comparesthe chemical composition of the HVOF NAB coating, bulkNAB alloy and BS 4360 steel.Fig. 1shows a SEM transversesection of the as-sprayed NAB coating with a coating thick-ness nominally 300�m. The coating was built up by semi-molten splats, spreading up to 80�m in the horizontal direc-tion. Unmelted particles were observed in the transverse sec-

tion. The coating/substrate interface was well-bonded, withsome alumina grits used for substrate preparation present atthe interface. The NAB coating has a heterogeneous appear-ance, consisting of the different types of HVOF powders used.EDX spectra obtained from the lighter phases revealed that itconsists of aluminium bronze particles. Darker phases werefound to be stainless steel particles and Ni5Al alloy particles.As seen inFig. 1, the as-sprayed surface was rough, thus inorder to obtain reproducible surfaces prior to the experiments,the NAB coating and 4360 steel surfaces were ground withSiC paper and polished with a diamond suspension to a mir-ror finish (Ra < 0.1�m). Before and after testing, specimenswere washed, degreased and kept in a desiccator for 3 daysprior to gravimetric mass loss measurements. The mass lossmeasurements were made using a precision balance with arange of 205 g and an accuracy of±0.02 mg.

4.2. Methodology

The slurry jet impingement rig has been previously de-tailed elsewhere[14,17]. Modifications have been made toaccommodate a silver/silver chloride (Ag/AgCl) referenceelectrode (RE) and a platinum counter electrode (CE), see

owing

Fig. 1. SEM of HVOF NAB coating transverse section sh coating morphology, particles/splats and pores within the coating.

632 K.S. Tan et al. / Wear 258 (2005) 629–640

Fig. 2. The data collection section of the jet impingement rig used forerosion–corrosion experiments.

Fig. 2. The ejector assembly, used for sand particle intake,was located downstream to prevent erosion damage on thecounter and reference electrodes. A valve situated near theejector allowed the sand intake to be completely isolated un-der flow corrosion conditions. Electrochemical current noise(ECN) measurements were made using a Gamry InstrumentsPC4-750 potentiostat and ESA400 software. The ECN forthe flow corrosion and erosion–corrosion conditions were ob-tained after steady state flow corrosion and erosion conditionswere achieved (i.e. between 3 and 4 h of a 5 h experiment).The ECN was measured by potentiostatically controlling theworking electrode at the open-circuit potential (measured im-mediately before the ECN). The sampling rate was fixed at2 Hz. The flow corrosion test electrolyte consisted of a 3.5%NaCl solution, while the slurry was a 3.5% NaCl solutionwith a 3% (w/w) silica sand concentration. The silica sandparticles, shown inFig. 3, were sub-rounded with a meandiameter of 235�m and sub-angular with a mean diameterof 135�m. For pure erosion condition, a−200 mV cathodicprotection was applied based on the potentials observed fromerosion–corrosion conditions. Three velocities, 3.0, 5.0 and6.7 m s−1, were used for investigating the effects of jet veloc-ity in affecting the synergistic behaviour. All tests were donein duplicate and the results were found to be within±5%.

F thec

5. Results and discussion

5.1. Gravimetric mass loss measurement

The mass loss rate (mg/h) between flow corrosion, pureerosion and erosion–corrosion for the 4360 steel and the NABcoating are shown inFigs. 4 and 5, respectively. In addition,the figures show the summation of pure erosion and flowcorrosion (E+C), as well as synergy (S) which is the dif-ference betweenT andE+C. This assumes thatC, E, andT occurred uniformly over the specimen surface. The flowcorrosion rates for both steel and NAB coating remained rel-atively constant over the velocity range tested. Whereas, forerosion and erosion–corrosion the mass loss appears to in-crease with jet velocity, with a marked increase at 6.7 m s−1.The increasing material loss underEandTcan be attributed tothe increase in erodent kinetic energy (Ek) which will producegreater plastic deformation/cutting wear[18] and for theTcondition freshly exposed metal will also result in higher cor-rosion rates. For the 4360 steel and NAB coating, both the ero-sion and erosion–corrosion rates are similar at low velocities;however, at 6.7 m s−1 the mass loss for erosion–corrosion is10% greater than for pure erosion. In order to quantify theamount of synergy, theSpercentage (S/T) for the steel andNAB coating can be plotted against jet velocity, seeFig. 6.P −1

i d att

riall ult ofe ni e-h ucest ions.A er-e n-e idefi siona mea-sw rec plexs igherv sp ga-t el. Itw muchh ns[ (i.e.E

in-ip r ofs -e her ents

ig. 3. SEM of the 235�m sub-rounded silica sand particles used inurrent work, showing the angular tips and blunt areas.

ositive synergy was found at 6.7 m s(11% for NAB coat-ng and∼3% for steel) while negative synergy was founhe two lower velocities.

For the erosion–corrosion tests the majority of mateoss usually results from mechanical damage as a resrosion. Hence, when the rates ofT andE are similar, show

n Table 2, negativeSvalues can be found. This type of baviour implies that the presence of corrosion films red

he erosivity of the sand under erosion–corrosion condits seen inTable 2, flow corrosion can also increase the diffnce betweenT andE+C values, resulting in negative syrgy. This is especially true when substantially thick oxlms or corrosion products are formed on the flow corrond erosion–corrosion surfaces, affecting the mass lossurements. Results similar to those presented inFigs. 4 and 5ere reported by Bardal et al.[8]. Synergy experiments wearried out on WC–Co based HVOF coatings and dutainless steels, using a rotating slurry pot tester and at helocity ranges. It was shown that between 14 and 30 m−1,ositive synergy existed for the HVOF coating while ne

ive synergy was observed for the duplex stainless steas considered that the duplex stainless steel has aigher critical flow velocity under flow corrosion conditio

19], hence contribution of corrosion was not significant/T ratio is closer to 1).

In order to reveal the effects of flow corrosion in determng positive/negative synergy effects, ratios ofS/TandE/Tarelotted inFig. 7, which also contains data from a numbeimilar studies[8,9,20–28], using a similar definition of synrgy, i.e. whereC is corrosion under flowing conditions. Tesults were mostly obtained from mass loss measurem

K.S. Tan et al. / Wear 258 (2005) 629–640 633

Fig. 4. Synergy for 4360 steel at different velocities. Conditions: 3% w/w sand concentration, 3.5% NaCl solution at 20◦C.

Fig. 5. Synergy for NAB coating at different velocities. Conditions: 3% w/w sand concentration, 3.5% NaCl solution at 20◦C.

Table 2Synergy results obtained from mass loss measurements for 4360 steel and NAB coating

Material/velocity Mass loss (mg) Percentage

T E C E+C S E/T (%) S/T (%)

BS 4360 steel3.1 m s−1 18.5 20.3 10.8 31.1 −12.6 109.7 −68.15.0 m s−1 27.5 27.5 11.7 39.2 −11.7 100.0 −42.66.7 m s−1 (8 h test) 100.5 88.1 9.6 97.7 2.8 87.7 2.8

NAB coating3.1 m s−1 4.9 4.6 2.5 7.1 −2.2 93.9 −44.95.0 m s−1 18.3 16.9 2.6 19.5 −1.2 92.3 −6.66.7 m s−1 49.1 39.7 3.9 43.6 5.5 80.9 11.2

634 K.S. Tan et al. / Wear 258 (2005) 629–640

Fig. 6. Comparison of synergy percentage expressed as percentage of erosion–corrosion mass loss.

from synergistic based experiments (i.e. whereT, E andCmass loss measurements were made). As seen inTable 3, thecollated data covers a wide range of studies from simple slurryjet and pot experiments, to cavitation tunnels and two-bodyabrasion. The studies cover a wide range of experimental con-ditions: velocity ranges between 3.0 and 29.3 m s−1; erodentsize between 63�m (silica sand) and 6.4 mm (pea gravel);sand concentrations from 0.03 to 5.0%; and test electrolytesincluded potable water, seawater (natural and synthetic e.g.3.5% NaCl) and synthetic mine water.Fig. 7 demonstratesthat irrespective of the experimental conditions, target ma-terials and experimental techniques, the transition betweenpositive and negative synergy occurs at the same position.This observation allows for the determination of a synergyconstant (SC) defined by a zero synergy and theE/T ratio be-

ing 0.8. The effect ofE/T ratio on the extent of synergy relatesto the degree of corrosion affecting the total material loss un-der erosion–corrosion (T). When the ratio is greater than 0.83,the presence of corrosion products/films reduces the erosiv-ity and generates negative synergy or antagonism. This canbe a result of passive film formation or thick oxide forma-tion, protecting the surface from further oxidation and subse-quent materials loss giving a negative�E. AnE/T ratio lowerthan 0.8 indicates high degree of interaction between erosionand corrosion, and is associated with an actively corrodingsurface. Under such conditions, both erosion-enhanced cor-rosion and corrosion-enhanced erosion contributes toward apositive synergy value.

Using SC the extent of synergy can be assessed (whetherpositive or negative) for materials under a range of operating

ion–co

Fig. 7. Comparison betweenS/T ratio andE/T ratio for abras rrosion, erosion–corrosion and cavitation–corrosion conditions.

K.S. Tan et al. / Wear 258 (2005) 629–640 635Ta

ble

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and

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used

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N/A conditions provided the pure erosion and erosion–corrosion

rates are known. This estimation technique is also applica-ble for various types of systems that undergo combined me-chanical and chemical degradation (i.e. abrasion–corrosion,erosion–corrosion and cavitation–corrosion). As seen inTable 2andFig. 7, transitions from positive to negative syn-ergy can occur for both the carbon steel and NAB coating.This shows that the synergy can be affected by operatingconditions such as erodent type, target materials, impact ki-netic energy, and solids concentration. These variables canaffect the overall corrosion behaviour, possibly in the formof passive film rupture/removal[29,30], mass transfer[31]and permeation of the sub-viscous layer/oxide film that ex-ists on the surface[32–34].

Calculations of synergy based on gravimetric measure-ments can be affected by inaccuracies caused by incorporat-ing the mass of corrosion products. The gravimetric measure-ment errors from the current work can add up to nearly 50%of the synergy, inevitably producing scatter in the data. Upperand lower limits of scatter (±50%) were calculated based onthe trend line and plotted as dotted lines, it was shown thatmost of the results fall within these limits, seeFig. 7. Datascatter could also be a result of the following:

1. Estimation of corrosion rates from Faradaic mass loss cal-culations or static corrosion experiments. Faradaic mass

sur-oint

-own-nsfer

ents

2 nts.ctedthe

. Fur-entthe

3 imensti-

andt the

4 coat-entstimerme-d a

eenp min-i ed.

loss estimation may not be accurate because of thefaces being polarised anodically at the stagnation pwhile the free jet region remains cathodic[23]. This can result in an internal circuit, resulting in reduced current flinto the counter electrode[35]. Static corrosion rates canot be used for synergy calculations because mass trareactions are higher under flowing conditions[31]. Theseeffects will also induce scatter in the ECN measuremdiscussed later in this paper.

. Use of cathodic protection for pure erosion experimePure erosion under cathodic polarisation can be affeby the deposition of calcareous deposits, affectingmass loss measurements and synergy calculationsthermore, hydrogen bubble evolution and embrittlemcould occur during cathodic polarisation, increasingwear rate and complicating the synergy calculation.

. The existence of surface films/scale on the specsurface before weighing, due to interests in invegating the corrosion products from flow corrosionerosion–corrosion. These products can also affecmass loss measurements.

. In coated systems, permeation of electrolyte into theing/substrate interface during flow corrosion experimcan also affect the mass loss results. Experimentalshould be kept short enough to prevent electrolyte peation into the interface but sufficiently long to recormass loss value.

In order to determine the exact transition point betwositive and negative synergy, such errors have to be

mised so that a more accurate trend line can be obtain

636 K.S. Tan et al. / Wear 258 (2005) 629–640

Fig. 8. ECN standard deviation of flow corrosion and mass loss at various jet velocities for the NAB coating and 4360 steel. Conditions: 3.5% NaCl solutionat 25◦C.

5.2. Standard deviation analysis

Fig. 8 compares the mass loss results and ECN standarddeviation (σC) under flow corrosion conditions, for both theNAB coating and 4360 steel. TheσC for the NAB coating ap-pears to be relatively insensitive to fluid flow, consistent witha material that can form a protective oxide film. This is incontrast with the 4360 steel which will undergo uniform cor-rosion. TheσC for the steel shows higher degree of sensitiv-ity, since increasing the fluid velocity will increase the avail-ability of oxygen (increased mass transport of the cathodicreactant), thus increasing the corrosion rate[36–39]. Simi-larly, the mass loss results versusσT under erosion–corrosion

conditions are presented inFig. 9. Combinations of jet flowvelocities and erodent particle sizes were used to achieve arange of erodent kinetic energies (0.02–0.41�J). TheσT forthe NAB coating (0.05–0.3�A) shows a clear linear relation-ship with mass loss indicating that the higher kinetic energysand particles have a stronger tendency to result in surfacefilm breakage, exposing fresh coating surface and increasingelectrochemical activity[30,40,41].

Previous work has revealed that material removal on thecoating surface is mainly caused by the cutting action of im-pinging sand particles[18]. In contrast,σT for 4360 steel ap-pears to be uncorrelated for the erosion–corrosion condition.The values fluctuated at around 0.25�A, similar to those ob-

F t vario lts obf

ig. 9. ECN standard deviation of erosion–corrosion and mass loss a

rom two different sand sizes (235 and 135�m), 3% w/w sand concentration, 3.

us jet velocities for the NAB coating and 4360 steel. Conditions: resutained

5% NaCl solution at 25◦C.

K.S. Tan et al. / Wear 258 (2005) 629–640 637

Fig. 10. The relationship between ECN standard deviation ratio for erosion–corrosion (T) and flow corrosion (C) vs. mass loss for pure erosion (E). Mean sandparticle sizes 235 and 135�m, 25◦C. 3.5% NaCl solution for flow corrosion and erosion–corrosion.

served for the flow corrosion conditions, seeFig. 8. Given thatthe steel does not form a protective oxide layer, the corrosionactivity will be independent of the erodent kinetic energy.It is possible for the mechanical erosion processes to resultin increased surface roughness[42,43], hence larger surfacearea and subsequently increasing theσT [44]. However, thiswas not reflected in theσT values in the current investigation.The increase in the NABσT values for the erosion–corrosionconditions reveals the effects of erosion-enhanced corrosion(�C) possibly caused by the removal of surface film or crack-ing at the splat boundaries. The effects of�C, separated byelectrochemical methods, can possibly be used for correlationwith mass loss under pure erosion conditions—separation of�C is discussed later.

Fig. 10 shows the relationship between theσT/σC ratioand pure erosion mass loss at various kinetic energies. TheσT/σC ratio represents erosion-enhanced corrosion. For theNAB coating,�C shows a logarithmic relationship with theerosion rate with a power law coefficient of about 0.5. As the4360 steel does not produce a protective oxide film, the effectsof enhanced corrosion is minimal and the standard deviationratio values are close to unity.Fig. 10indicates that pure ero-sion mass losses can be estimated from a system undergoingerosion–corrosion, based on the standard deviations calcu-lated from ECN measurements. This type of measurements temsu im-p tingt viouro rvicee

i-c ticlei ond-i

ted against synergy for the NAB coating. Most of the datapoints were found in the negative synergy region, suggestingthat sand particles do not have sufficient energy to result insurface film rupture. Sasaki et al. have reported a thresholdkinetic energy of 0.03�J for passive film rupture in stainlesssteels[29]. In the current work, positive synergy was foundonly at 0.41�J. The protective layer on NAB alloys is ap-proximately 800 nm[2], compared with 10 nm for stainlesssteels, possibly indicating that much higher kinetic energiesare required to rupture the protective layer. In the presenceof protective films, theσT values did not deviate much fromthose under flow corrosion conditions. This type of behaviourindicates insufficient or negative contributions of�C and/or�E results in negative synergy. Under these conditions, botherosion and corrosion processes are then considered as antag-onistic. A possible transition region is shown at the vicinitywhere the standard deviation ratio value is close to 10; syn-ergy levels were shown to shift from negative to positive. This

F gy (p peyer[

hows promise for materials that can passivate or systilising corrosion inhibitors, undergoing sand particleingement conditions. However, this method of evalua

he erosion rate should be used with care as the behaf engineering materials is often dependent on the senvironment.

As theσT/σC ratios indicate the addition of electrochemal activity (corrosion rate) on the surface under solid parmpingement, it can also be utilised to reveal a correspng increase in synergy.Fig. 11shows theσT/σC ratios plot-

ig. 11. Relationship between the standard deviation ratio and synerS)ercentage for the NAB coating and that of recent work carried out by S

45].

638 K.S. Tan et al. / Wear 258 (2005) 629–640

Fig. 12. The effects of erosion-enhanced corrosion (�C) in affecting the percentage of synergy for the NAB coating.

transition can be caused by the following processes:

1. Increased surface film removal by higher energy particles,leading to more exposed surfaces and an increase in cor-rosion rate. This is a result of erosion-enhanced corrosion(�C) processes.

2. Higher kinetic energy sand particles can remove the workhardened and corroded plastic deformation lips at a higherefficiency, leading to corrosion-enhanced erosion (�E)processes.

Nevertheless, contributions by flow corrosion and�C re-main embedded within the expression ofσT/σC ratio, sepa-ration of these individual entities is necessary so that furtherunderstanding of synergy can be gained. Flow corrosion canbe separated by both mass loss and electrochemical measure-ments, as seen inFigs. 5 and 8. The figures reveal relativelystable flow corrosion mass loss rates across the velocity rangetested, indicating that material loss under erosion–corrosioncould be dominated by�C. The effect of additional corro-sion as a result of erosion (�C) is subsequently quantified inEq.(3):

�C = σT − σC. (3)

rt 0.02ag -c l lossb -eu siond .

becomes simply,

S = −�E. (4)

Eq.(4) indicates that the occurrence of negative corrosion-enhanced erosion (�E) is possible at lower kinetic energies.This could be due to the presence of a continuous protectivelayer, acting as an interface between the target material andthe impinging sand particles and reducing the overall rate ofmaterial removal. As shown previously inFig. 12, �Cvaluesincreased with kinetic energy. It is highly possible at highEk that a continuous protective layer cannot be sustained onthe surface. The denuded regions will be actively corrodingthus enhancing erosion and leading to a positive�E. Thecombined effect contributes towards a positive synergy asgiven by Eq.(2).

The exact transition from negative to positive synergyis still currently unidentifiable by means of electrochemicalmeasurements. It was explained previously forFig. 11that apossible transition occurs at the region where theσT/σC ratiovalue is close to 10. It is possible that corrosion is sufficientat this point to initiate the onset of +�E, resulting in a sud-den shift towards positive synergy. The calculation ofσT/σCratio could ultimately be used as a monitoring procedure toestimate the critical level of synergy at predetermined operat-ing conditions. This has a similar function to the synergisticc assl

6

for-m iumb sitive

Fig. 12shows the contribution of�C towards synergy fohe NAB coating between the kinetic energy range ofnd 0.41�J as a function of the gravimetric ratio ofS/T. Theraph reveals an increase of both�C and synergy with inreasing erodent kinetic energy, revealing that materiaecomes dominated by�C at highEk. At lower kinetic enrgies, negative synergy is a result of a reduction inE (−�E)nder erosion–corrosion or mass gain under flow corroue to the growth of oxide films or scaling issues, i.e. Eq(2)

onstant that was previously obtained from gravimetric moss measurements (Fig. 7).

. Conclusions

Gravimetric analysis of the erosion–corrosion perance of an experimental HVOF sprayed nickel aluminronze coating and BS 4360 steel revealed that a po

K.S. Tan et al. / Wear 258 (2005) 629–640 639

synergy occurred at highest erodent kinetic energies for bothmaterials. At lower energies, repassivation/depassivation ki-netics plus the formation of corrosion products lead to nega-tive synergy.

Analysis of published results, as well as the current work,reveals anE/T ratio of 0.8 when there is zero synergy. Thus,whenE/T> 0.8 corrosion has a negative effect (reduces over-all mass loss) due to a continuous oxide film formation whichprotects the surface. Whereas, whenE/T< 0.83 a high degreeof interaction occurs between erosion and corrosion and is as-sociated with an actively corroding surface at denuded filmsites.

Standard deviation of electrochemical current measure-ments reveals the presence of protective film formation un-der flow corrosion conditions and also film breakdown un-der erosion–corrosion. Various correlations are identified be-tween the gravimetric trends and electrochemical currentnoise. TheσT/σC ratio as a function of erosion mass lossrevealed a correlation between the level of erosion and cur-rent noise levels for the NAB coating but not for the non-passivating steel surface. This approach enabled the separa-tion of�C, which was shown to increase with erodent kineticenergy and synergy. This infers that at low erodent energies,�E is larger than�C and is negative. At higher erodent en-ergies,�C becomes larger and�E positive, resulting in ap

be-t rfacea rates,f dardd ravi-m

A

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ter,

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isticment6.

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ositive synergy.Overall, this analysis has successfully differentiated

ween erosion–corrosion mechanisms of a passivating sund a non-passivating surface. This work also demonst

or the first time, a possible correlation between staneviation ratios of electrochemical current noise and getric data associated with erosion–corrosion.

cknowledgements

The authors would like to acknowledge the Universitouthampton and Dstl for their financial support of thisearch. Also our thanks go to the University of Barcelonaupplying the experimental HVOF nickel aluminium brooatings and to Dr. Terry Harvey for his help towardsompletion of this paper.

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