ero

ihchu11,

C. Pitting corrosion

) arctros, chemectricrros

lloys hnumers, eachs in thnching

always contained one major element. The nanostructured HEAswere developed in an effort to break away from the traditional al-loy design.

According to Boltzmanns hypothesis [9], the congurationalentropy change per mole, DSconf, during the formation of a solid

treatment, and without softening for up to 200 h at 800 C. These al-loys may therefore have great potential for use in structural partsand tools that operate at high temperatures (600900 C) [15]. Pre-vious studies have focused on the mechanical properties and, moreimportantly, on the corrosion behaviour when applications ofthe alloys were considered. The HEAs (AlCoCrCu0.5FeNiSi), evalu-ated previously, exhibited narrow passive regions in both H2SO4and NaCl solutions [16,17]. Once corrosion had begun, it pro-ceeded faster than that of stainless steel because of the lack of aprotective passive lm. The purpose of this investigation was tostudy the effect of aluminium on the corrosion properties of the

* Corresponding author. Address: Department of Materials Science and Engi-neering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 300,Taiwan. Tel.: +886 3 5715131; fax: +886 3 5710290.

Corrosion Science 50 (2008) 20532060

Contents lists availab

Corrosion

.e lE-mail address: hcshih@mx.nthu.edu.tw (H.C. Shih).cooling rate of over one million degrees per second [3,4]. They con-rmed that an amorphous phase can be obtained if the cooling rateis sufcient to suppress the nucleation and growth of crystals.Since the 1970s, intermetallic compounds of TiAl [5], NiAl [6]and FeAl [7] in binary systems have attracted substantial atten-tion because they have an extremely high specic strength andthermal resistance. In 1988, the ground-breaking discovery wasmade that, by mixing together many metals of different atomicsizes, the melt can be frozen as a glass using the much slower cool-ing rate of one degree per second [8]. However, the designs of theaforementioned alloys remained limited by the fact that the matrix

has been called an HEA because its congurational entropy(DSconf = 1.612.20 R) exceeds that of an ordinary alloy (1.10 R)[10]. The HEAs exhibit simple solidsolution structures, ease ofamorphization and nanoprecipitation and promising propertiessuch as high hardness and superior resistance to temper softening,wear and oxidation [1114]. Among these, AlxCrFe1.5MnNi0.5 alloyshave a hardness that increases 50% from 300 to 450 Hv withincreasing aluminium content from 0 to 0.5 mol. The AlxCrFe1.5Mn-Ni0.5 alloys exhibit signicant age hardening. The maximum hard-ness of the Al0.3CrFe1.5MnNi0.5 alloy was 800 Hv after an aging1. Introduction

Several novel multi-component arecent years. These alloys comprisein equimolar or near-equimolar ratioment et al., discovered metallic glastained a goldsilicon alloy by que0010-938X/$ - see front matter 2008 Elsevier Ltd. Adoi:10.1016/j.corsci.2008.04.011ave been developed inous principal elementsat 535 at% [1,2]. Kle-

e early 1960s; they ob-the liquid alloy at a

solution from ve elements with equimolar fractions, is given bythe following equation:

DSconf k lnw RX5

i1xi ln xi R ln 5 1:61R; xi 15 1

where k is the Boltzmann constant; w is the number of mixedstates, and R is the gas constant. The multiple element systemB. EISC. Passivity

the alloy increased. 2008 Elsevier Ltd. All rights reserved.Effect of the aluminium content of AlxCrFthe corrosion behaviour in aqueous envi

C.P. Lee a, C.C. Chang a, Y.Y. Chen a, J.W. Yeh a, H.C. ShaDepartment of Materials Science and Engineering, National Tsing Hua University, Hsinb Institute of Materials Science and Nanotechnology, Chinese Culture University, Taipei 1

a r t i c l e i n f o

Article history:Received 18 February 2008Accepted 22 April 2008Available online 29 April 2008

Keywords:A. AlloyB. Polarization

a b s t r a c t

High-entropy alloys (HEAsnamic polarization and elein H2SO4 and NaCl solutionof aluminium decreases. T>1000 mV in acidic environwhich represented the elethe general and pitting co

journal homepage: wwwll rights reserved.1.5MnNi0.5 high-entropy alloys onnmentsa,b,*

300, TaiwanTaiwan

e a newly developed family of multi-component alloys. The potentiody-chemical impedance spectroscopy of the AlxCrFe1.5MnNi0.5 alloys, obtainedlearly revealed that the corrosion resistance increases as the concentrationAlxCrFe1.5MnNi0.5 alloys exhibited a wide passive region, which extendednts. The Nyquist plots of the Al-containing alloys had two capacitive loops,al double layer and the adsorptive layer. SEM micrographs revealed thation susceptibility of the HEAs increased as the amount of aluminium in

le at ScienceDirect

Science

sevier .com/locate /corsc i

AlxCrFe1.5MnNi0.5 alloys, which have a wide passive region in H2SO4and NaCl solutions.

2. Experimental

2.1. Test materials

the morphology of the corroded surface of the alloy specimenwas investigated using a scanning electron microscope (SEM,JEOL-5410).

3. Results and discussion

3.1. Potentiodynamic polarization

Fig. 1 plots the potentiodynamic polarization behaviour of theAlxCrFe1.5MnNi0.5 alloys with various aluminium contents (x = 0,0.3 and 0.5) in 0.5 M H2SO4 solution. The AlxCrFe1.5MnNi0.5 alloyshave a wide passive region (DE > 1000 mV), which indicates a ten-dency of the alloys to passivate. Table 2 summarizes the electro-chemical parameters associated with the general corrosionbehaviour of the AlxCrFe1.5MnNi0.5 alloys in H2SO4 solution. Thecorrosion potentials (Ecorr) of the AlxCrFe1.5MnNi0.5 alloys were

2054 C.P. Lee et al. / Corrosion Science 50 (2008) 20532060Elements Al, Cr, Fe, Mn and Ni in the form of granules with puri-ties of over 99 wt% were used as rawmaterials. Each of the alloys inthis HEA family had ve components in the form of AlxCrFe1.5Mn-Ni0.5, consisting of 1.5 mol of iron, 0.5 mol of nickel, 1 mol of chro-mium, 1 mol of manganese, and either 0, 0.3 or 0.5 mol aluminium.These elements were melted by the arc melting process at a cur-rent of 500 ampere in a water-cooled copper hearth. Melting andcasting were conducted in a vacuum of 0.01 atm. following purgingwith argon three times. The alloy was repeatedly melted and solid-ied so as to yield a completely alloyed state and to improve itschemical homogeneity. The ingots were approximately 50 mm indiameter and 20 mm in thickness. Table 1 presents the chemicalcompositions of the AlxCrFe1.5MnNi0.5 alloys in weight percentage.An alloy cylinder, used for measuring electrochemical characteris-tics, was obtained by cutting the bulk material using an electric arcline. Each test specimen was then cold-mounted, using an epoxyresin, to expose an area of 0.5 cm2. Before electrochemical mea-surements were made, all specimens were mechanically polishedusing a series of 2401200 SiC grit papers and cleaned in acetoneand distilled water.

2.2. Electrochemical measurements and surface morphology

Both electrochemical polarization (d.c.) and electrochemicalimpedance spectroscopy (EIS) were performed in a typical three-electrode cell with a specimen as the working electrode. AnAg/AgCl electrode (3 M KCl) with E = 0.208 VSHE was used as thereference electrode and a platinum sheet with a much greater area(4 cm2) than that of the specimen was used as the counter elec-trode. All of the potentials in this work are presented on the stan-dard hydrogen scale (SHE). The test solution was deaerated bybubbling puried nitrogen gas before and throughout the electro-chemical tests to eliminate any effect of dissolved oxygen. Poten-tiodynamic polarization curves and EIS were plotted after thespecimen was allowed to corrode freely for 30 min, the time neces-sary to reach a quasi-stationary value of the open circuit potential(OCP). Then, the specimen was cathodically polarized to a potentialof 0.2 V for 300 s to reduce the possible existing surface oxides.The potentiodynamic tests were performed at a scan rate of1 mV s1 from an initial potential of 0.5 V to a nal potential of1.5 V versus the OCP. This scan rate was found to be convenientand sufciently slow to prevent any distortion of the potentiody-namic polarization curves. The potential was controlled and thecurrent was measured using a potentiostat (AUTOLAB PGSTAT30).The EIS was carried out at the OCP with a sinusoidal potentialamplitude of 10 mV, running from 10 kHz to 10 mHz, using anAUTOLAB PGSTAT30/FRA system from ECO CHEMIE. Followingthe polarization experiment, the specimen was cleaned using dis-tilled water, and then dried in nitrogen. Immediately thereafter,

Table 1Chemical composition (wt%) of the AlxCrFe1.5MnNi0.5 alloys

Element Al(26.96)(%)

Cr(51.99)(%)

Fe(55.84)(%)

Mn(54.94)(%)

Ni(58.69)(%)

CrFe MnNi 0.00 23.63 38.03 24.99 13.331.5 0.5Al0.3CrFe1.5MnNi0.5 3.55 22.79 36.68 24.10 12.86Al0.5CrFe1.5MnNi0.5 5.78 22.26 35.83 23.54 12.56determined to fall from 194 to 206 mV as the aluminium con-tent increased from 0.3 to 0.5 mol; the corrosion current densities(icorr) of the Al0.3CrFe1.5MnNi0.5 and Al0.5CrFe1.5MnNi0.5 are2.39 103 and 5.08 103 A/cm2, respectively; the icorr valuedropped to 6.86 104 A/cm2 when aluminium was eliminatedfrom the alloy, as in CrFe1.5MnNi0.5. Furthermore, the critical cur-rent density (icrit) increased with the aluminium content, implyingthat the barrier to passivation increases with the aluminium con-tent. The passive current density (ipass) of the CrFe1.5MnNi0.5 alloyis lower than that of the Al-containing alloys. Hence, adding alu-minium to the AlxCrFe1.5MnNi0.5 alloys reduces the resistance togeneral corrosion in H2SO4 solution when the alloys are in the pas-sive state.

The as-cast CrFe1.5MnNi0.5 (four-component, aluminium-free)alloy is a face-centered cubic (fcc) solidsolution with an a-FeCrstructure, according to the XRD patterns (Fig. 2) [18]. However,as the aluminium content increased, the peak intensities of thebody-centered cubic (bcc) phase increased. The as-cast Al0.3Cr-Fe1.5MnNi0.5 (ve-component) alloy is composed of mixed fcc/bccphases whereas a single bcc structure was observed for the alloycontaining 0.5 mol aluminium. The ferritic stainless steels (suchas type 430 stainless steel) are essentially ironchromium alloyswith structures mostly of the bcc a-iron type. However, the austen-itic stainless steels (such as type 304 stainless steel) are ternaryironchromiumnickel alloys, and their structures are of the fccc-iron type. Indeed, the austenitic stainless steels normally havegreater corrosion resistance than the ferritic stainless steels [19].This fact may suggest why the CrFe1.5MnNi0.5 alloy (fcc structure)is more resistant to corrosion than are the AlxCrFe1.5MnNi0.5(x = 0.3 and 0.5) alloys (bcc structure).Fig. 1. The effect of aluminium on the potentiodynamic polarization curves of theAlxCrFe1.5MnNi0.5 (x = 0, 0.3, 0.5) alloys in 0.5 M H2SO4 solution.

l in 0

icrit (

1.262.365.548.19

cienThe width of the passive region (DE) for the CrFe1.5MnNi0.5 alloyis 1227 mV. However, as the aluminium content in the AlxCr-Fe1.5MnNi0.5 alloys increases from 0.3 to 0.5 mol, the DE falls from

Table 2Electrochemical parameters of the AlxCrFe1.5MnNi0.5 alloys and the 304 stainless stee

Ecorr (mVSHE) icorr (A/cm2)

CrFe1.5MnNi0.5 229 6.86 104Al0.3CrFe1.5MnNi0.5 194 2.39 103Al0.5CrFe1.5MnNi0.5 206 5.08 103304 stainless steel 186 7.45 105

Fig. 2. XRD patterns of the as-cast AlxCrFe1.5MnNi0.5 alloys. [18] *indicates the e-xistence of the a-FeCr phase.

C.P. Lee et al. / Corrosion S1176 to 1114 mV. Furthermore, a transpassive region exists for theAlxCrFe1.5MnNi0.5 alloys in H2SO4 solution. The transpassive break-down of stainless steels occurs near the oxygen evolution potentialwhere the chromium-rich passive lm is unstable. Above thebreakdown potential, water is unstable and is oxidized to oxygengas (H2O! 1=2O2 2H 2e, E0 = 1.23 V) [20]. The transpassivedissolution of Cr from the underlying alloy through the passive lm(2Cr 7H2O! Cr2O27 14H 12e, E0 = 0.30 V) is also expectedin a sulfuric acid medium [21]. A wealth of information is availableon the corrosion behaviour of 304 stainless steel that is exposed toH2SO4 solutions [22]. Accordingly, a comparison of the corrosionbehaviour of HEAs with that of the conventional ferrous alloys,such as 304 stainless steel, is of interest. Table 2 indicates thatthe Al-free CrFe1.5MnNi0.5 alloy has a wider passive region thanthe 304 stainless steel. However, the Ecorr values of the AlxCr-Fe1.5MnNi0.5 alloys are more active than that of the 304 stainlesssteel (186 mV), and the icorr values of the AlxCrFe1.5MnNi0.5 alloysalso exceed that of the 304 stainless steel (7.45 105 A/cm2) in0.5 M H2SO4.

Although the formation of an oxide lm is effective in protect-ing the HEA, when localized damage of this passive lm occurs,pitting corrosion advances rapidly. Fig. 3 presents the potentiody-namic polarization tests of the AlxCrFe1.5MnNi0.5 alloys in 1 M NaClsolution. Because pitting initiates at the pitting potential (Epit), Epitmay be used as an index of resistance to pitting corrosion: a noblervalue of Epit is associated with an increased resistance to pitting[23]. A sharp increase in the anodic current demonstrates the sus-tained localized breakdown of the passive lm. In the presentstudy, the Epit values of the Al0.3CrFe1.5MnNi0.5 and Al0.5CrFe1.5Mn-Ni0.5 alloys were about equal, and they were signicantly lowerthan for the Al-free CrFe1.5MnNi0.5 alloy. As the aluminium contentin the AlxCrFe1.5MnNi0.5 alloys increased from 0 to 0.5 mol, the Epitfell from 19 to 123 mV. Therefore, the addition of aluminium tothe AlxCrFe1.5MnNi0.5 alloys reduces the resistance to pittingcorrosion.

The potentiodynamic polarization curves for the Al-free and Al-

.5 M H2SO4 solution

A/cm2) Epp (mVSHE) ipass (A/cm2) DE (mVSHE)

102 55 3.14 105 1227 102 12 7.39 105 1176 102 47 6.82 105 1114 104 22 8.05 106 1178

Fig. 3. Comparisons of the potentiodynamic polarization curves for the AlxCr-Fe1.5MnNi0.5 (x = 0, 0.3, 0.5) alloys in 1 M NaCl solution.

ce 50 (2008) 20532060 2055containing alloys obtained in 0.5 M H2SO4 with various concentra-tions of NaCl (00.5 M) are plotted in Fig. 4. The potentiodynamicpolarization curves of the AlxCrFe1.5MnNi0.5 (x = 0 and 0.3) alloys

each show a passive region where o/o log iipass

1 in a Cl-free solu-tion. In all cases, the effect of adding NaCl to the H2SO4 solutionis to shift Ecorr and Epit to more active values. Table 3 indicates thatthe icorr and ipass values increase, while DE changes only slightly asthe chloride content increases from 0 to 0.1 M. However, at thecritical chloride concentration of 0.25 M, ipass increases to valuesthat are two orders of magnitude higher than that observed inthe Cl-free solution. The existing passive lm breaks down whenthe applied potential reaches the pitting potential, resulting in thenucleation and formation of pits at discrete locations on the metalsurface [24]. The presence of chloride in an acid solution generallyincreases potentiodynamic anodic currents at all potentials; how-ever, the most notable feature here is the sharp increase in currentat Epit. The current at the Epit increases markedly as the amount ofchloride in the sulfuric acid increases, regardless of whether or notaluminium is present. Moreover, as the concentration of chlorideincreases from 0.1 to 0.25 M, DE falls from 1150 to 495 mV forAl-free CrFe1.5MnNi0.5 alloy, and from 1127 mV to almost no pas-sive region for the Al-carrying Al0.3CrFe1.5MnNi0.5 alloy. Addingaluminium apparently reduces the ability to develop a passive lmon the alloy surface.

3.2. Cyclic potentiodynamic polarization

A cyclic polarization technique was used to determine whetherthe AlxCrFe1.5MnNi0.5 alloys suffer from pitting corrosion in Cl-containing acid. Cyclic polarization measurements were made ata scanning rate of 10 mV s1. The potential scan began at 0.75 Vand continued in the anodic direction until the potential reached

2056 C.P. Lee et al. / Corrosion Science 50 (2008) 205320600.95 V, at which value the potential scan was reversed andreturned to where the polarization began. Fig. 5 plots the cyclicpolarization curves of the AlxCrFe1.5MnNi0.5 alloys for (a) x = 0and (b) x = 0.3, in H2SO4 solution containing 0.25 M of chloride.The black and gray arrows next to the forward and reverse anodicbranches indicate the potential scan directions. Negative hysteresisis said to occur when the current density of the reverse scan is lessthan that of the forward scan whereas positive hysteresis is said tooccur when the reverse scan exceeds the forward scan [25]. Thenegative hysteresis in the reverse scan of the cyclic polarizationcurve (Fig. 5a) indicates that the CrFe1.5MnNi0.5 alloy is not suscep-tible to localized corrosion, and that the passive lm repairs itself.The positive hysteresis of the cyclic polarization curve (Fig. 5b)demonstrates that pitting of the Al0.3CrFe1.5MnNi0.5 alloy can be in-duced in a Cl-containing solution. The area contained within a po-sitive hysteresis loop is related to the amount of pit propagationthat occurs during the cycle, according to Wilde [26]. The small

Fig. 4. Potentiodynamic polarization curves for (a) CrFe1.5MnNi0.5 and (b) Al0.3Cr-Fe1.5MnNi0.5 alloys obtained in 0.5 M H2SO4/x NaCl (x = 00.5 M) solutions.

Table 3Electrochemical parameters of the CrFe1.5MnNi0.5 and Al0.3CrFe1.5MnNi0.5 alloys in 0.5 M H

CrFe1.5MnNi0.5

Ecorr (mVSHE) icorr (A/cm2) ipass (A/cm2) Epit (mVSHE) DE

0.5 M H2SO4 221 6.86 104 3.14 105 1172 120.5 M H2SO4 + 0.10 M NaCl 242 2.06 103 7.09 105 1180 110.5 M H2SO4 + 0.25 M NaCl 238 4.60 103 4.75 103 589 40.5 M H2SO4 + 0.50 M NaCl 240 9.75 103 5.78 103 475 3

a Passive region observed was so small that it can be neglected.area of the hysteresis loop of the Al0.3CrFe1.5MnNi0.5 alloy showsthat the nucleated pits do not continue to grow substantially.

3.3. Electrochemical impedance spectroscopy

Electrochemical impedance is a powerful tool in studying corro-sion and passivation processes. EIS provides more information thanthe other electrochemical techniques about the electrochemicalprocesses that occur at the surface. Fig. 6 shows the effect of alu-minium on the Nyquist plot of the AlxCrFe1.5MnNi0.5 (x = 0, 0.3and 0.5) alloys in 0.5 M H2SO4. The Nyquist plot for the Al-freeCrFe1.5MnNi0.5 alloy includes one capacitive loop from high tomedium frequencies and one inductive loop at low frequencies.The capacitive loop is related to the double layer capacity, andthe presence of the inductive loop reveals that the alloy surfaceis partly or totally active [27,28]. Moreover, for the higher alumin-ium-containing Al0.3CrFe1.5MnNi0.5 and Al0.5CrFe1.5MnNi0.5 alloys, adifferent behaviour was observed, in that the electrochemical

2SO4 with different concentrations (00.5 M) of NaCl solutions

Al0.3CrFe1.5MnNi0.5

(mVSHE) Ecorr (mVSHE) icorr (A/cm2) ipass (A/cm2) Epit (mVSHE) DE (mVSHE)

18 194 2.39 103 7.39 105 1164 117650 219 2.48 103 1.21 104 1156 112795 231 6.05 103 1.78 103 250 a52 250 1.04 102 3.95 102 257 a

Fig. 5. Cyclic polarization curves for (a) CrFe1.5MnNi0.5 and (b) Al0.3CrFe1.5MnNi0.5alloys in 0.5 M H2SO4 + 0.25 M NaCl solution.

cienimpedance diagrams included two capacitive loops, which are typ-ically related to the presence of an adsorption layer and chargetransfer across the metal-electrolyte interface [29].

Appropriate models of the impedance were developed to t thetest data, which helped to evaluate the parameters, and thus char-acterize the corrosion process. Fig. 7a and b presents the experi-mental and simulated Nyquist and Bode plots, respectively, ofthe CrFe1.5MnNi0.5 alloy. An equivalent electrical circuit was de-signed to best-t the experimental results for the electrode, as dis-played in Fig. 7c. Such an impedance dispersion can becharacterized using an equivalent circuit Rs(Cdl[Rt(RLL)]), where Rsis the resistance of the solution; Rt is the charge transfer resistance,and Cdl is the double-layer capacitance. RL is the resistance that isassociated with the inductive processes, and L denotes the pseu-do-inductance [30]. Capacitance is replaced by a constant phaseelement (CPE) to compensate for the nonhomogeneity in the sys-tem [31]. ZCPE is related to the impedance, and is given by

ZCPE Y10 jxa 2where, Y0 is the proportionality factor, j is the imaginary unit, x isthe angular frequency, and a is the phase shift. The CPE exponent

Fig. 6. The effect of aluminium on the Nyquist plot of the AlxCrFe1.5MnNi0.5 (x = 0,0.3, 0.5) alloys in 0.5 M H2SO4 solution.

C.P. Lee et al. / Corrosion Sa is a measure of the capacitance dispersion with values between1 (ideal capacitance) and 0.7 (highly dispersed capacitance, suchas at porous electrodes). For a = 0, ZCPE represents a resistance withR = Y01; for a = 1, it represents a capacitance with C = Y0; for a = 0.5,it represents a Warburg element, and for a = 1, it represents aninductance with L Y10 [32].

The Nyquist plot for the aluminium-containing Al0.3CrFe1.5Mn-Ni0.5 alloy, presented in Fig. 8a, consists of two capacitive loops.Fig. 8a and b displays the corresponding experimental and simu-lated Nyquist plot and Bode plots, respectively. The experimentalimpedance spectra were tted by the equivalent circuitRs(Cdl[Rt(CadRad)]), as shown in Fig. 8c. The second capacitive loopof the Al-containing alloy in the Nyquist plot was regarded as beingcaused by the formation of an adsorptive lm on its surface. Radand Cad represent the resistance and constant phase elementassociated with the adsorptive characteristics on the surface ofthe Al-containing alloy. The adsorptive complexes formed in anacid medium are produced by the dissolution of elemental alumin-ium in the HEA.

Aluminium forms an adsorptive layer of Al(OH)ad on the metalin a sulfuric acid solution by the following mechanism [33]:

Als H2O! AlOHad H e 3ce 50 (2008) 20532060 2057AlOHad 5H2OH ! Al3:6H2O 2e 4For both alloys (CrFe1.5MnNi0.5 and Al0.3CrFe1.5MnNi0.5), the

agreement between the experimental and the simulated data isgood. Table 4 gives the simulated values for the equivalent circuitelements. The change in the adsorptive capacitance, Y0ad, can beused as an indicator of a change in the layer thickness, d. The reci-procal capacitance of the adsorptive layer, 1/Y0ad, is directly pro-portional to its thickness [34]. Accordingly, the expression for layercapacitance is based on the Helmholtz model [32]:

Y0ad Cad e0ed S 5

where, d is the thickness of the adsorptive layer, e0 is the per-mittivity of a vacuum (8.85 1014 F/cm), e is the dielectric con-stant of the medium, and S is the surface area of the electrode.

Fig. 7. The (a) Nyquist and (b) Bode plots and (c) the equivalent electrical circuitrepresentative of the electrode interface for the CrFe1.5MnNi0.5 alloy in 0.5 M H2SO4solution. Scattered points are the experiment data and solid lines show the modelt.

electrolyte. Both indicate that a porous corrosion product coversthe alloy [27,35]. The CPE exponents aad obtained by tting theEIS data are 0.72 and 0.78 for the Al0.3CrFe1.5MnNi0.5 and Al0.5Cr-Fe1.5MnNi0.5, respectively. The values are consistent with a porousadsorptive lm. The Al0.5CrFe1.5MnNi0.5 alloy has lower Y0ad val-ues than that of the Al0.3CrFe1.5MnNi0.5 alloy, revealing that thethickness of the adsorptive layer increases with the amount of alu-minium in the alloy. Rt is 24.0 Xcm2 for the Al-free CrFe1.5MnNi0.5

2

2058 C.P. Lee et al. / Corrosion Science 50 (2008) 20532060Accordingly, the large capacitance thus obtained results fromeither a large dielectric constant of the layer that covers the surfaceor a high effective interface area between that surface layer and the

Fig. 8. The (a) Nyquist and (b) Bode plots and (c) the equivalent electrical circuitrepresentative of the electrode interface for the Al0.3CrFe1.5MnNi0.5 alloy in 0.5 MH2SO4 solution. Scattered points are the experiment data and solid lines show themodel t.

Table 4Equivalent circuit elements values for EIS data corresponding to Al0CrFe1.5MnNi0.5 (Al0), Al0in 0.5 M H2SO4 solution

Rs (Xcm2) Rt (Xcm2) Y0dl (sadlX1 cm2) adl Ra

Al0 1.9 24.0 5.9 105 0.93 Al0.3 1.6 18.7 6.3 104 0.93 22Al0.5 1.4 21.0 6.0 104 0.9 82304s 2.2 205.0 3.5 104 0.89 alloy, and declines to 18.7 and 21.0 Xcm for the alloys containing0.3 and 0.5 mol of aluminium, respectively. The Rt of the 304 stain-less steel (205.0 Xcm2) exceeds that of the AlxCrFe1.5MnNi0.5 alloys.Accordingly, the corrosion resistance of the 304 stainless steel ishigher than that of the AlxCrFe1.5MnNi0.5 alloys. This result is con-sistent with the electrochemical parameters obtained from thepotentiodynamic polarization curves, as presented in Table 2.

3.4. SEM photomicrographs of corroded surfaces

Fig. 9 presents the SEM microstructure of the AlxCrFe1.5MnNi0.5alloys after they had been anodically polarized beyond the break-down potential (>1.25 V) in 0.5 M H2SO4. Fig. 9a depicts almostno pitting, except for some general material dissolution on the sur-face of the CrFe1.5MnNi0.5 alloy. However, a small amount of shal-low attack (25 lm in diameter and 510 lm in depth) occurred atthe surface of the Al0.3CrFe1.5MnNi0.5 alloy (Fig. 9b). Additionally,Fig. 9c shows an example of the localized corrosion (45 lm indiameter and 50 lm in depth) of the Al0.5CrFe1.5MnNi0.5 alloy,which is larger and deeper than that of the Al0.3CrFe1.5MnNi0.5 al-loy. Fig. 9d displays this localized corrosion of Al0.5CrFe1.5MnNi0.5alloy at higher magnication, and shows a large number of holeson the surface. Aluminium tends to form a porous oxide lm inH2SO4 and is involved in the galvanic attack of the weaker andmore porous oxide region; when the Al content of FeAl alloys isless than 20 at%, the passivation behaviour of alloys deterioratessignicantly [36]. Electron conguration theory may explain theminimum Al content that is required to ll the Fe atomic level dand thereby modify the surface characteristics [37]. However, theAl contents of the Al0.3CrFe1.5MnNi0.5 and Al0.5CrFe1.5MnNi0.5 alloysare 6.98 and 11.11 at%, respectively, which are signicantly lowerthan 20 at%. Accordingly, the passive lms on the surfaces of thesealloys are unstable, with breakdown throughout the period of thepolarization test.

Fig. 10 presents the SEM images of the CrFe1.5MnNi0.5 and theAl0.3CrFe1.5MnNi0.5 alloy obtained after they had been anodicallypolarized beyond the breakdown potential (>1.25 V) in H2SO4 solu-tion containing 0.25 M chloride. Pitting is a localized attack that re-sults in relatively rapid penetration at small discrete areas. Thepotentiodynamic polarization curves (Table 3) demonstrate thatthe Epit value changes from 589 to 250 mV, and the DE valuechanges from 495 mV to almost zero in H2SO4 solution containing0.25 M chloride, as a result of alloying the CrFe1.5MnNi0.5 alloy with0.3 mol aluminium. Hence, typical micrographs show that the pitson the CrFe1.5MnNi0.5 alloy are fewer and narrower than thoseformed on the Al0.3CrFe1.5MnNi0.5 alloy.

.3CrFe1.5MnNi0.5 (Al0.3), Al0.5CrFe1.5MnNi0.5 (Al0.5) alloys and 304 stainless steel (304s)

d (Xcm2) Y0ad (saadX1 cm2) aad RL (Xcm2) L (Hcm2)

8.2 3.4.5 2.6 102 0.72

.0 1.8 102 0.78

Fig. 9. SEM micrographs for the AlxCrFe1.5MnNi0.5 alloys with aluminium content (a) x =anodic polarization exceeded the breakdown potential (>1.25 VSHE) in 0.5 M H2SO4.

Fig. 10. SEM micrographs of (a) CrFe1.5MnNi0.5 and (b) Al0.3CrFe1.5MnNi0.5 alloyafter anodic polarization exceeded the breakdown potential (>1.25 VSHE) in H2SO4solution containing 0.25 M chloride.

C.P. Lee et al. / Corrosion Science 50 (2008) 20532060 20594. Conclusions

The potentiodynamic polarization curves of the AlxCrFe1.5Mn-Ni0.5 alloys in acidic solution exhibit activepassive corrosionbehaviour, yielding an extensive passive region (DE > 1000 mV).The corrosion current density and passive current density of theCrFe1.5MnNi0.5 alloy are signicantly lower than those of theAl0.3CrFe1.5MnNi0.5 and Al0.5CrFe1.5MnNi0.5 alloys. Therefore, theAl-free alloy is more resistant to general corrosion than that ofthe Al-containing alloy in acidic environments. Moreover, thealloying of aluminium in the AlxCrFe1.5MnNi0.5 alloys impairs thepitting resistance in chloride environments, in that the pittingpotentials for the Al-containing alloys are signicantly lower thanthat for the Al-free CrFe1.5MnNi0.5 alloy. Additionally, the negativehysteresis seen in the cyclic polarization curve conrms that theCrFe1.5MnNi0.5 alloy is not susceptible to localized corrosion inthe Cl-containing environments tested in this study, and thatthe passive lm is generally believed to repair itself.

For the aluminium-containing Al0.3CrFe1.5MnNi0.5 and Al0.5Cr-Fe1.5MnNi0.5 alloys, the Nyquist plot has two capacitive loops,which are typically related to the presence of an adsorption layerand charge transfer across the metal-electrolyte interface. Finally,the dimensions of the localized and pitting corrosion increaseswith the aluminium content of the AlxCrFe1.5MnNi0.5 alloys in anacid that contains chloride ions.

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Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environmentsIntroductionExperimentalTest materialsElectrochemical measurements and surface morphology

Results and discussionPotentiodynamic polarizationCyclic potentiodynamic polarizationElectrochemical impedance spectroscopySEM photomicrographs of corroded surfaces

ConclusionsReferences