Materials Science and Engineering A290 (2000) 145–154

Synergistic effect of cavitation erosion and corrosion of variousengineering alloys in 3.5% NaCl solution

C.T. Kwok a, F.T. Cheng a, H.C. Man b,*a Department of Applied Physics, The Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, China

b Department of Manufacturing Engineering, The Hong Kong Polytechnic Uni6ersity, Hung Hom, Kowloon, Hong Kong, China

Received 4 February 2000; received in revised form 22 March 2000

Abstract

The cavitation erosion and corrosion characteristics of various engineering alloys including grey cast iron, steels, copper-basedalloys and stainless steels were studied by means of a 20 kHz ultrasonic vibrator at a peak-to-peak amplitude of 30 mm in distilledwater and in 3.5% NaCl solution at 23°C. The contributions of pure mechanical erosion, electrochemical corrosion, and thesynergism between erosion and corrosion to the overall cavitation erosion–corrosion in 3.5% NaCl solution were determined. Itwas found that in 3.5% NaCl solution, the effect of corrosion on the overall cavitation erosion–corrosion was most pronouncedin mild steel and grey cast iron, and negligible in stainless steels. The stainless steels only suffered pure mechanical erosion in 3.5%NaCl solution in the presence of cavitation owing to the unfavourable local environment for pit growth. © 2000 Elsevier ScienceS.A. All rights reserved.

Keywords: Cavitation erosion and corrosion; Synergism; Mechanical properties; Steels; Grey cast iron; Copper-based alloys

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1. Introduction

Cavitation is the growth and collapse of cavities orbubbles inside a liquid caused by the sudden changes ofinternal pressure in the liquid. During the collapse ofbubbles, a large pulse of stress is generated and this isoften accompanied by a high velocity jet of liquid thatimpinges the solid surface in contact with the liquid.The rapid repetition of the stress pulses would inducelocalised fatigue failure and subsequent material re-moval. Cavitation erosion refers to damage due tofatigue processes occurring in the surface of the mate-rial itself. If cavitation attack causes accelerated re-moval of an existing passive film or corrosion productlayer, and if the overall rate determining mechanism isthe rate of replacement growth of the film or layer, thenthe resulting type of loss of material is referred tocavitation erosion–corrosion.

There have been many attempts to correlate cavita-tion erosion resistance to a single or a combination of

mechanical properties of a metal. These mechanicalproperties include ductility, hardness, ultimate tensilestrength, yield strength, ultimate resilience, engineeringstrain energy, percentage of elongation [1] and productof fatigue strength coefficient and cyclic strain-harden-ing exponent [2]. Hammit [1] has found the followingempirical relations for a wide range of materials:

(MDPR)−1=k(UR)

(MDPR)−1=k(HB)1.8

(MDPR)−1=k(SE)0.738

(MDPR)−1=k(UR×HB)0.72

(MDPR)−1=k(UR×Y)0.659

where MDPR is the mean depth of penetration rate, aninverse measure of the cavitation erosion resistance;UR is the ultimate resilience; HB is the hardness; SE isthe strain energy; Y is the Young modulus and k is aconstant which takes different values for the relationsabove.

In addition, Tichler et al. [3] found that the relationbetween MDPR and the true ultimate tensile strengthfor various kinds of copper–nickel alloys and steels wasas below:

* Corresponding author. Tel.: +852-27666629; fax: +852-23625267.

E-mail address: mfhcman@polyu.edu.hk (H.C. Man).

0921-5093/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.PII: S0921 -5093 (00 )00899 -6

C.T. Kwok et al. / Materials Science and Engineering A290 (2000) 145–154146

(MDPR)−1=k(TS*)2.3

where TS* is the true ultimate tensile strength and k isa constant.

These relations are empirical in nature and onlyprovide prediction to a certain degree for a narrowgroup of materials. Owing to the repetitive, dynamic,stochastic and localised nature of the stress pulsesproduced by cavitation, the cavitation erosion resis-tance of a material should be regarded as an indepen-dent material property on its own and not derivablefrom others [4]. Moreover, when the cavitating liquid isalso corrosive, such as one containing chloride ions orsulphide ions, the material loss is not purely mechanicalin nature because corrosion also comes into play. Thesimultaneous existence of mechanical erosion and elec-trochemical corrosion is a common scenario for engi-neering alloys used in marine environments, such aspump impellers and valves. The situation is furthercomplicated by the fact that the effects of erosion andcorrosion are in general not additive owing to theinteraction between them. The overall damage arisingfrom erosion and corrosion including the interactionbetween them is termed cavitation erosion–corrosion.The relative significance of corrosion, erosion, and theinteraction between them depend on the material/envi-ronment system. In fact, the interaction in the case ofmetallic materials can be synergistic or inhibitive [5].The nature of the interaction is determined by a num-ber of factors, the more important ones being thepassivity of the metal surface, the adherence of thecorrosion product, the metallurgical state of the metal,the significance of the diffusion of dissolved oxygen, thepresence of aggressive ions and the intensity of cavita-tion. These factors would determine the mode of corro-sion and the rate of erosion–corrosion loss [6–8].Tomlinson and Talks [7] studied the cavitation ero-sion–corrosion of various types of cast iron in 3%NaCl solution, and found that the fractional weightloss due to pure corrosion ranged from 1 to 10% whilethat due to corrosion-induced erosion ranged from 16to 90%. In the study of the synergistic effect of cavita-

tion erosion and corrosion for copper and cupro-nickelin seawater, Wood and Fry [8] concluded that syner-gism was most marked when cavitation erosion oc-curred in the presence of mild corrosion, and wasresponsible for 50% of the overall penetration depth.However, studies on the relative importance of erosion/corrosion synergism in stainless steels, an importantclass of engineering alloys, have not been reported. Itwas pointed by Vyas and Hannson [5] that for stainlesssteels, because of the existence of the passive film,cavitation might accelerate or decelerate corrosion, de-pending on the intensity of cavitation and the metallur-gical state of the stainless steel specimens. Fractionalcontributions of corrosion and synergism to overallmass loss were not given. Moreover studies on thesynergism between erosion and corrosion were usuallycarried out under different conditions by different au-thors, making results difficult to compare.

In view of the reasons pointed out in the brief reviewabove, the present study aims at assessing the relativeimportance of erosion, corrosion and the synergismbetween them in the overall cavitation erosion–corro-sion damage of various engineering alloys, includingstainless steels, in 3.5% NaCl solution at 23°C.

2. Experimental details

2.1. Materials and specimen preparation

The alloys studied include super duplex stainless steelZeron 100, austenitic stainless steels AISI 304 and316L, copper, brass (Cu–40%Zn), bronze (Cu–10%Sn),mild steel 1050, tool steel W1 and grey cast iron. Thenominal chemical compositions for these alloys areshown in Table 1. The raw materials were in the formof round bar and machined to flat-end studs of diame-ter 12.7 mm and thickness 3.4 mm for cavitation testand threaded for attachment to the tip of the trans-ducer horn of the cavitation facility. The specimens forelectrochemical corrosion test were in the form of discs

Table 1Nominal chemical compositions of various alloys (wt.%)

WSiAlloys SPNaCaSnZnCuMnMoNiCrFe

–bal. – – – –––– –––Copper –0.1bal. –Brass –––0.1– – – – 40.1– ––

–––9.7–bal.––– –––Bronze ––0.2 – – 0.4 0.8 – – 0.5Mild steel 1050 –bal. 0.04 0.05 0.2 –0.15 0.2 0.1 0.4 – – – 0.7Tool steelW1 –bal. – – – 0.15

––0.20.6–3.6––Cast iron 0.40.8––0.2bal.17.6 11.2 2.5SS 316L 1.4bal. 1.4 0.4 – 0.03 – – – 0.4 –18.4bal. –SS 304 0.30.10.1–0.08–1.72.11.6–8.7

0.80.30.1–0.20.03–0.50.7bal. 0.6SS Zeron 100 4.07.225.6

a Maximum values.

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Table 2Mechanical properties and stacking fault energy of various alloys

Alloys 0.2% Yield(HRB) TensileHardness EngineeringMicrostructure Ultimate Young’s SFEcElongation(mJ m−2)modulus (%)resiliencebstrength strain energyastrength

(N mm−2)(N mm−2) (N mm−2)(N mm−2)

– – – – 6 –79Grey cast iron 110Graphite in pearlite24 8815 25 –151881Mild steel 1050 Ferrite and pearlite 96 621

Tool steel W1 –Martensite with very – – – – –81 –fine carbidedistributiona 66 7 4431 28 5539 285Copper 275

7 8510 24 592Brass 34045274a+b14 7304 32 –Bronze a+b 73 539 510 14512 9094 45 25239316L 55061984Austenite

587 258 17 9820 46 15304 94Austenite 619255 14 14 190 39 –645768103Zeron 100 Austenitic island in

ferritic matrix

a Total area under stress–strain curve.b Area under elastic portion of stress–strain curve.c Stacking fault energy.

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Table 3Rate of corrosion, cavitation erosion resistance and cavitation erosion–corrosion resistance

Distilled waterAlloys 3.5% NaCl solution

icDensity C Rc T=MDPR Rec E=MDPR Re

(mm h−1)(10−3 mm h−1) (h mm−1)(g cm−3) (mm h−1)(h mm−1) (h mm−1)(mA cm−2)

3.27 4.95 202Grey cast iron 0.387.15 2.63 0.28 3.57Mild steel 1050 7.85 12.00 15.90 649 0.13 8.33 0.03 33.33

6.50 8.60 116 0.157.84 6.67Tool steel W1 0.11 9.098.94Copper 2.00 2.65 377 0.64 1.56 0.62 1.618.53Brass 1.20 1.68 595 0.29 3.45 0.26 3.85

1.00 1.31 763 0.208.80 5.00Bronze 0.20 5.008.00SS 316L 0.38 0.43 2326 0.07 14.29 0.07 14.29

0.38 0.45 2222 0.067.90 16.67SS 304 0.06 16.670.21SS Zeron 100 0.247.84 4167 0.01 100.00 0.01 100.00

of diameter 12.7 mm and thickness 3.4 mm. The surfaceof the specimens were all polished by 600 grit siliconcarbide paper. The mechanical properties of the alloysobtained from the stress–strain curves are shown inTable 2.

2.2. Ca6itation testing

Cavitation erosion experiments were carried out us-ing an ultrasonic cavitation facility with a 550 Wultrasonic probe conforming to ASTM Standard G32-92 [9]. Cavitation tests were carried out in distilledwater and 3.5% NaCl solution at a constant tempera-ture of 2391°C and a vibratory frequency of 20 kHzand a peak-to-peak amplitude of 30 mm. Each test wascompleted in 4 h, which included eight intermittentperiods every 30 min. After each test period, the weightloss of the specimens was measured. The mean depth ofpenetration (MDP) and mean depth of penetration rate(MDPR) were used to express the cavitation erosionloss and the loss rate of materials and they are calcu-lated as below:

MDP (mm)=DW/(10rA) and

MDPR (mm h−1)=DW/(10rADt)

where DW is the weight loss at each time interval in mg,Dt is the time interval in hours, A is the surface area ofspecimen in cm2 and r is the density of specimen in gcm−3. In addition, the cavitation erosion resistance indistilled water (Re) or the cavitation erosion–corrosionresistance in 3.5% NaCl solution (Rec) is defined as thereciprocal of the mean depth of penetration rate over aperiod of 4 h i.e.:

Re (h mm−1)= (MDPR)−1 or

Rec (h mm−1)= (MDPR)−1

2.3. Electrochemical measurements

Polarisation studies were carried out to investigatethe electrochemical corrosion behaviour of various al-loys in 3.5% NaCl solution. Potentiodynamic polarisa-tion scans were carried out using an EG&G PARC 273corrosion system according to the ASTM StandardG5-94 [10]. The solution was maintained at 2391°Copen to air. A constant potential scan rate of 5 mV s−1

was used in all experiments. All potentials were mea-sured with respect to a saturated calomel electrode(SCE, 0.244 V versus SHE at 25°C) as the referenceelectrode. Two parallel graphite rods served as thecounter electrode for current measurement. All datawere recorded after an initial delay time of 30 min forthe specimen to reach a steady state. The corrosioncurrent densities were determined from the polarisationcurves using the Tafel extrapolation method. The cor-rosion or penetration rates (C, as shown in Table 3) ofvarious alloys were calculated by the following equa-tion according to the ASTM Standard G102-89 [11]:

corrosion rate=C=K(ic/r)EW (in mm h−1)

where K is a constant=3.73×10−4 mm g mA−1 cm−1

h−1; ic is the corrosion current density in mA cm−2; r

is the density of the alloys in g cm−3 and EW is thealloys equivalent weight which is defined as follows:

EW=�%ni fi

Wi

�−1

where fi is the mass fraction; ni is the valence and Wi isthe atomic weight of the ith element in the alloys. EWis thus dimensionless.

The mass fractions of various alloys are obtainedfrom their typical compositions in Table 1. In addition,the corrosion resistance Rc is defined as the reciprocalof the corrosion rate, i.e.:

C.T. Kwok et al. / Materials Science and Engineering A290 (2000) 145–154 149

Rc (h mm−1)= (C)−1

In addition, free corrosion potential of specimens in3.5% NaCl solution was measured under quiescent andcavitation conditions.

3. Results and discussions

3.1. Ranking of Re of 6arious alloys in distilled water

The graphs of cumulative mean depth of penetrationas a function of time for various alloys eroded indistilled water and 3.5% NaCl solution are shown inFig. 1(a) and (b) respectively. Within 4 h of exposure,the incubation, acceleration and attenuation periodswere observed for cast iron, mild steel, tool steel,copper-based alloys whereas only the incubation andacceleration periods were observed for stainless steels.This clearly shows that the former group of alloys areless resistant to cavitation erosion than that of the latterone. The corresponding values of cavitation erosionresistance in distilled water (Re) and cavitation ero-

sion–corrosion resistance (Rec) in 3.5% NaCl solutionare shown in Table 3. The following ranking of alloysin distilled water can be established:

Zeron 100\1050\304\316L\W1\bronze

\brass\grey cast iron\Cu.

This ranking in pure mechanical erosion (i.e. in dis-tilled water) can be explained by the microstructure andmechanical properties of the alloys.

The microstructure of grey cast iron consists ofgraphite flakes in a matrix of lamellar pearlite as shownin Fig. 2(a). Grey cast iron is poorly resistant tocavitation erosion because of its brittle graphite flakewhich acts as stress concentrators and of its poor workhardening capacity [7]. Also, the presence of porosity isdetrimental to the resistance to cavitation erosion.

Ferrite is present in a higher proportion than pearlitein mild steel 1050 as shown in Fig. 2(b). Its cavitationerosion resistance is even higher than austenitic stain-less steels 304 and 316L due to its high mechanicalstrength as shown in Table 2.

The microstructures of copper and brass are shownin Fig. 2(c) and (d). Copper has the worst cavitationerosion resistance because of its inferior mechanicalstrength and high stacking fault energy (55 mJ m−2

[2]). For brass, the planar slip mode as a result of lowstacking fault energy (5 mJ m−2 [12]) enhances itscavitation erosion resistance. Bronze has higher tensilestrength properties and as a result, higher cavitationerosion resistance than brass.

The response of the austenitic stainless steels to cavi-tation erosion is similar to their quasi-static mechanicalbehaviour. Austenite fails by ductile fracture mecha-nism. Their low stacking fault energy (25 mJ m−2 [2])prevents recombination of partial dislocations necessaryfor cross-slip and thus planar slip is the predominantmode of deformation [13]. Austenitic stainless steel 304has higher cavitation erosion resistance than austeniticstainless steel 316L owing to its higher martensitictransformability and work hardenability, and lowerstacking fault energy, although both steels have similarmechanical strength [13]. The super duplex stainlesssteel Zeron 100 has the highest cavitation erosion resis-tance because of its superior mechanical properties andhigh work hardening rate [14].

3.2. Ranking of Rec of 6arious alloys in 3.5% NaClsolution

The ranking of the alloys in 3.5% NaCl solution is asfollows (Fig. 3):

Zeron 100\304\316L\1050\W1\bronze

\brass\grey cast iron\Cu.

The ranking is similar to the previous one for Re

except that the position of 1050 is changed. In aFig. 1. Cumulative MDP as a function of time for various alloyseroded in (a) distilled water and (b) 3.5% NaCl solution at 23°C.

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Fig. 2. Microstructure of (a) grey cast iron;, graphite flake (A) in a matrix of lamellar pearlite (B); (b) mild steel, ferrite and pearlite (B); (c) copper;(d) brass (Cu–40%Zn), a and b phases; (e) austenitic stainless steel 316L and (f) super duplex stainless steel Zeron 100.

chloride environment, the cavitation erosion–corrosionresistance of 1050 is much reduced because of its poorcorrosion resistance. Thus, corrosion and synergisticeffect become dominant and this will be discussed in thenext part.

3.3. Contribution of erosion, corrosion and theirinteraction to ca6itation erosion–corrosion

When an alloy is exposed to a cavitating liquid whichis corrosive, both mechanical erosion and electrochemi-cal corrosion will contribute to the removal of materialfrom the surface. The overall cavitation erosion–corro-sion rate T may be expressed as:

T=E+C+S

where E is the erosion rate; C is the corrosion rate andS is the contribution arising from the synergistic effectof erosion and corrosion.

T and E in the present study are given by values ofMDPR obtained from cavitation tests in 3.5% NaClsolution and in distilled water respectively, since dis-tilled water is non-corrosive and cavitation loss is en-tirely of mechanical origin. The corrosion rate C isobtained from the potentiodynamic polarisation curves(Fig. 4). Values of T, E and C for various alloys,expressed as mm h−1, are listed in Table 3 togethervalues of the corresponding resistances Rec, Re and Rc.

C.T. Kwok et al. / Materials Science and Engineering A290 (2000) 145–154 151

The relative contributions of erosion, corrosion andsynergism to the overall cavitation erosion–corrosionare given in Table 4. It can be observed that thecontribution of corrosion varies widely for differentalloys. In this respect the alloys studied may be dividedinto three groups: (I) grey cast iron, mild steel and toolsteel; (II) copper, brass and bronze; and (III) stainlesssteels. Among these three groups, the presence of chlo-ride ions has the greatest effect on the cavitation dam-age in group (I) alloys, while group (III) alloys are leastaffected. In fact the contribution of corrosion andcorrosion–erosion synergism amounts to 28.5% for castiron and 88.1% for mild steel, comparable with theresults of Tomlinson and Talks [5], whereas for stain-less steels, corrosion does not play a noticeable role.

The difference in behaviours of these groups of alloyin NaCl solution is closely related to the nature andproperties of the corrosion (or reaction) productsformed, and the effects of cavitation.

For steels and cast iron, the surfaces which hadundergone cavitation attack in 3.5% NaCl solutionbecame tarnished. The test solution became brownrapidly and contained black deposits. In distilled water,the surfaces of the specimens were not tarnished andremained clean and the solution was relatively clear.This observation showed the difference in corrosivity ofdistilled water and 3.5% NaC1 solution with respect tosteels and cast iron, and that the corrosion product wasnon-adherent in a cavitating environment and henceunprotective.

For copper and copper alloys, owing to the goodcorrosion resistance as indicated by the low corrosioncurrent densities in 3.5% NaCl solution (Fig. 4(b)), theeffect of corrosion and synergism was much lower thanfor group (I) alloys. The effect of cavitation on the freecorrosion potential of brass is shown in Fig. 5(b).Cavitation shifted the free corrosion potential of brassin 3.5% NaCl solution in the active direction for about60 mV. The effect of cavitation on the corrosion be- Fig. 4. Potentiodynamic polarisation curves of (a) cast iron and steels;

(b) copper, brass and bronze; (c) various stainless steels, in 3.5%NaCl solution (open to air) at 23°C.

Fig. 3. Cavitation erosion resistance (Re) and cavitation erosion–cor-rosion resistance (Rec) for various alloys.

haviour in general and on the corrosion potential inparticular depends on the two competing effects ofcavitation, namely, corrosion film or product detach-ment and increase of mass transport [5]. Even forsimilar alloys the overall effect depends on experimentalconditions. In fact, similar to the present study, Trethe-way et al. [15] observed a noble shift in corrosionpotential for a copper manganese–aluminium bronze inseawater, while Al-Hashem et al. [16] noted an activeshift for a cast nickel–aluminium bronze in seawater.

As pointed out in the introduction above, the relativeimportance of corrosion and corrosion erosion syner-gism in the cavitation erosion–corrosion of stainless

C.T. Kwok et al. / Materials Science and Engineering A290 (2000) 145–154152

Table 4Contribution of erosion (E), corrosion (C), interaction between corrosion and erosion (S) of various alloys in 3.5% NaCl solution

Alloys Rate of damagea % of damage

Sb T E/T C/TC S/TE

0.28 0.005 0.095 0.38 73.7 1.3 25.0Grey cast iron0.086 0.13 21.50.016 12.30.03 66.0Mild steel 1050

0.0090.11 0.021 0.15 78.6 6.4 15.1Tool steel W10.017 0.64 96.8Copper 0.50.62 2.70.0030.028 0.29 98.70.002 0.7Brass 9.60.260.009 0.20 95.0Bronze 0.50.20 4.50.001

�0 0.07 �100�0 �0SS 316L �00.07SS 304 �00.06 0.06 �100 �0 �0�0

�0 0.01 �100 �0 �0�00.01SS Zeron 100

a Note that damage rates less than 10−3 mm h−1 are neglected in this Table.b S=T−(E+C).

Fig. 5. Free corrosion potential against time for various alloys in 3.5% NaCl solution at 23°C under quiescent and cavitating conditions.

C.T. Kwok et al. / Materials Science and Engineering A290 (2000) 145–154 153

Fig. 6. Electrochemical test for stainless steel 316L (a) free corrosionpotential in 0.1 M KNO3 and (b) potentiodynarnic polarisationcurves in 0.1 M KNO3 and 3.5% NaCl solution (open to air) at 23°C.

The effect of cavitation on the free corrosion poten-tial for Zeron 100, 316L and 304 in 3.5% NaCl solutionis shown in Fig. 5(c–e). In all cases the free corrosionpotential was shifted in the active direction when cavi-tation was present, similar to the results of Al-Hashemet al. [16]. However, the active shift under cavitationdoes not mean that the alloy was actively corroding. Infact, there was no difference between the rate of massloss for stainless steels in the cavitation tests in distilledwater and in 3.5% NaCl solution. When cavitation wason, the passive film was destroyed together with ero-sion. However, the metal surface repassivated rapidly.Thus at steady state, the metal surface was covered bya passive film which was thinner than that under quies-cent condition. This was further supported by the factthat the values of the free corrosion potentials of thestainless steels under cavitation were still in the passiverange. It is well known that chloride ions will inducepitting corrosion in stainless steels in quiescent condi-tions. Under cavitation, stirring of the electrolyte andfilm destruction occur simultaneously [5], both upset-ting the local environment for pit growth. Thus theeffect of cavitation remains mechanical even in thepresence of chloride ions. As a further support of thispoint the free corrosion potential of 316L in 0.1MKNO3 solution was monitored under quiescent andcavitating conditions, with the result shown in Fig. 6(a).The polarisation curves of stainless steel 316L in bothNaCl and KNO3 solutions are shown in Fig. 6(b). Theeffect of chloride ions in inducing pitting under quies-cent condition is obvious in contrast the nitrate ions.The 0.1 M KNO3 solution functioned as a supportingelectrolyte, meaning that it was conducting but notcorrosive. The free corrosion potential dropped undercavitation, similar to the case of NaCl solution. Thus ina cavitating liquid chloride ions do not play an activerole in the erosion or corrosion process in the case ofstainless steels.

4. Conclusions

The alloys studied in 3.5% NaCl solution at 23°C areranked in terms of Rec as follows:

Zeron 100\304\316L\1050\W1\bronze

\brass\grey cast iron\Cu

The ranking of resistance of the alloys to cavitationerosion–corrosion may be explained in terms of themicrostructure, mechanical strength, work hardenabil-ity, martensitic transformability, stacking fault energyand also corrosion resistance. Corrosion only plays aminor role in the overall cavitation erosion–corrosion(0.1–1.8%) for copper-based alloys. For the steels andcast iron, corrosion/erosion synergism is very signifi-cant, amounting to 78% of the overall cavitation ero-

steels has not been reported in the literature. On theother hand, the contribution of corrosion and syner-gism in liquid–solid impingement erosion in 3.5% NaClsolution have been reported [17]. Unlike the results inthe present study, these authors reported a very sub-stantial contribution of corrosion–erosion synergism(17.8%) for stainless steel 316L. Thus it seems thaterosion–corrosion induced by liquid–solid impinge-ment is different from cavitation erosion–corrosion.Another reason for the high fraction of contributionfrom synergism might originate from the way of mea-suring pure erosion in these works. Instead of usingdistilled water to eliminate corrosivity, cathodic protec-tion was employed. It is difficult to suppress corrosionand hydrogen evolution simultaneously. As the lattermay exert a cushioning effect and hence reduce me-chanical erosion, the relative contribution of synergismmay be increased.

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sion–corrosion loss in mild steel. Corrosion and corro-sion/erosion synergism plays a negligible role for stain-less steels.

Acknowledgements

The authors would like to acknowledge the ResearchCommittee of the Hong Kong Polytechnic Universityfor the provision of a research grant (No. 340/737) andWeir Metals (UK) for the supply of Zeron 100.

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