Materials and Design 57 (2014) 325–329

Contents lists available at ScienceDirect

Materials and Design

journal homepage: www.elsevier .com/locate /matdes

Short Communication

Corrosion behaviour of Al 1050 severely deformed by rotary swaging

0261-3069/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2014.01.005

⇑ Corresponding author at: Institute of Materials Science and Engineering,Clausthal University of Engineering, Agicolastr. 6, 38678 Clausthal-Zellerfeld,Germany. Tel.: +49 5323 722760; fax: +49 5323 722766.

E-mail addresses: mansour.mhaede@yahoo.com, mansour.mhaede@tu-clausthal.de (M. Mhaede).

Mustafa Abdulstaar a, Mansour Mhaede a,b,⇑, Lothar Wagner a, Manfred Wollmann a

a Institute of Materials Science and Engineering, Clausthal University of Engineering, Agicolastr. 6, 38678 Clausthal-Zellerfeld, Germanyb Faculty of Engineering, Zagazig University, 44519 Zagazig, Egypt

a r t i c l e i n f o

Article history:Received 2 September 2013Accepted 3 January 2014Available online 11 January 2014

a b s t r a c t

In this study, corrosion behaviour of ultrafine-grained (UFG) commercial pure aluminium Al 1050processed by rotary swaging (RS) was examined using potentiodynamic polarization and weight lossimmersion test in 3.5% NaCl solution. Corrosion behaviour of UFG Al 1050 was compared with that ofcoarse grained (CG) as-received material. The results showed that ultrafine grain refinement by RS ledto marked improvement of the corrosion resistance. The improvement in corrosion resistance is profitedfrom the denser and stable passive film due to more grain boundaries, larger fraction of non-equilibriumgrain boundaries and residual stress of the UFG pure aluminium. The weight loss tests revealed low cor-rosion rate values of RS material compared to CG as-received material. Scanning electron microscopy(SEM) analysis revealed a higher number of rectangular shallow pits (more close to patches of generaldissolution); larger pits size was observed in the as-received compared to RS materials.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Commercially pure aluminium 1050 belongs to 1xxx alumin-ium alloys series and it is generally characterized by low strengthin comparison to other aluminium series, because of fewer soluteatoms and precipitated particles that offer a barrier against dislo-cation mobility. However, the strength of Al 1050 could be mark-edly improved by sever plastic deformation such as equalangular channel pressing (ECAP) [1–3], rotary swaging (RS) [4],friction stir welding (FSW) [5,6] and accumulative roll-bonding(ABR) [7], that exerting a beneficial effect by combining moderatestrength, light weight and low cost. These processes belong to thetop down approach and lead to substantial grain refinement sothat the grains can be reduced to the sub-micrometer or eventhe nanometer range.

Naturally, aluminium develops a thin and dense oxide layersthat protects the underlying material very well under normal atmo-spheric conditions. However, in some environment where chlorideions are present, the passive film could break down at specificpoints leading to the formation of pits on the aluminium surface.

The corrosion resistance of ultrafine-grained aluminium alloyshas been studied in relation to different severe plastic deformation(SPD) such as ECAP [8–10] and ABR [11]. It was found that the cor-rosion resistance of UFG aluminium is improved in comparison tothat of its counterpart material. Thereby it is promising to enhance

the corrosion resistance by processing the bulk materials throughSPD, meanwhile retaining better mechanical properties.

In this study, electrochemical properties of commercially purealuminium Al 1050 processed by rotary swaging (RS) were investi-gated. Corrosion resistance of ultrafine-grained (UFG) and coarsegrained (CG) as-received material was studied by potentiodynamicpolarization and weight loss immersion tests. Surface morphologywas investigated via scanning electron microscopy (SEM).

2. Experimental procedure

Commercially pure aluminium Al 1050 was received as hotextruded bars with chemical composition illustrated in Table 1.In Rotary swaging (RS), various deformation degrees were appliedat room temperature. Deformation degree (u) was calculated asfollowing: u = ln(Ao/A), where Ao is the initial cross section and Ais the finial cross section [4]. Corrosion tests were carried outusing VersaSTAT3 scanning potentiostat from the company PAR(Princeton Applied Research) and controlled by a computer. Elec-trochemical cell with three-electrode configuration was used forelectrochemical measurements, where Al specimens, a platinumfoil and saturated calomel electrode (SCE) were served as working,counter and reference electrodes, respectively. The Al specimenswere prepared by connecting a copper wire to one face of the spec-imen; the specimen with the attached wire were then coldmounted in resin and dried in air for 24 h at room temperature.The samples were consecutively wet grinded with 1200, 2400and 4000 SiC paper and then polished sequentially using 6, 3 and1 lm diamond paste. The samples were cleaned ultrasonically inan acetone bath and dried in a cool air stream before the electro-chemical measurements. The potentiodynamic polarization curves

Table 1Chemical composition of Al 1050 (wt.%).

Fe Si Mn Mg Ti Zn V Al

0.135 0.095 0.055 0.025 0.015 0.013 0.010 Bal.

Fig. 1. Microstructure of the as-received Al 1050.

326 M. Abdulstaar et al. / Materials and Design 57 (2014) 325–329

were obtained by scanning the potential in the forward directionfrom �1500 to 1500 mV against saturated calomel electrode(SCE) at a scan rate of 0.2 mV/s. The used electrolyte was 3.5% NaClaqueous solution. Scanning electron microscopy (SEM) was used toinvestigate the surface and pits morphology after potentiodynamicpolarization. The tests were carried in accordance with ASTM:G5-13e1.

Cylindrical samples of 10 mm in diameter and 10 mm length,with 2 mm diameter hole for mounting were prepared for weightloss immersion tests. The tests were carried out according toASTM: G31-12a. Before immersion in 3.5% NaCl solution, the sam-ples were consecutively wet grinded with 1200, 2400 and 4000 SiCpaper, ultrasonically cleaned in acetone bath and then dried in acool air stream. The immersed samples were recovered at differenttime intervals up to 70 days. After recovery the samples werewashed in distilled water and then ultrasonically cleaned in ace-tone bath for 60 s. The weight loss was recorded using a preciseelectronic balance isolated from ambient atmosphere. The corro-sion rate in mpy (mils per year) for weight loss was calculatedaccording to the following equation:

½ASTM : G31-12a� : CR ¼ K �WA� T � q

ð1Þ

where K = a constant (3.45 � 106), T = time of exposure in hours,A = total surface area in cm2, W = mass loss in g and q = density(2.71 g/cm3).

The total surface area of the cylindrical samples was calculatedby the following equation:

½ASTM : G31-12a� : A ¼ p2ðD2 � d2Þ þ lpDþ lpd ð2Þ

where l = length, D = diameter of the sample and d = diameter of themounting hole.

Corrosion rate in mpy, which was extracted from the extrapola-tion of the Tafel lines of each polarization curve, is calculated bythe following equation:

CR ¼ Icorr � K � EWq� A

ð3Þ

where Icorr = corrosion current density in Ampere (A), K = constantthat defines the units of corrosion rate (1.288 � 105 mils/A cm -year), EW = equivalent weight in (g/equivalent), q = density(2.71 g/cm3) and A = sample area in cm2. The equation indicatesthat, corrosion rate is directly proportional to corrosion currentdensity.

3. Results and discussion

3.1. Microstructure observations and mechanical properties

Fig. 1 showed the microstructure of the as-received conditionwith relatively coarse grains of about 150 lm. EBSD mapping re-sults for samples deformed with RS2 (u = 2) and RS3 (u = 3) arepresented in Figs. 2 and 3 respectively. Each figure constitutestwo maps; on the left the scan (a) is made at the plane normal tothe swaging direction, whereas on the right the scan (b) is at theplane parallel to swaging direction.

Taking the critical misorientation as 2�, the average grain sizewas found to be 0.76 lm and 0.75 lm for RS2 (u = 2) and RS3

(u = 3), respectively. The structure after both deformation degreeswas characterized with cell structure of low angle grain boundariesof an average misorientation angle 15�. The grains exhibited elon-gated shape parallel to the flow direction, this was quite obviousfor deformation degree RS3 (u = 3). For the plane parallel to theswaging direction, extremely fine dynamically recrystallized grainswere observed more frequently after RS3 than RS2. These grainswere heterogeneously nucleated at existing grain boundarieswhere dislocation density is expected to be high. Microstructureevolution during RS process was detailed in a previous work [4].

The tensile properties and hardness values of the various condi-tions were summarized in Table 2. As expected, the RS materialsexhibited high tensile strength and hardness in comparison toas-received materials. After RS process, yield (YS) and ultimate ten-sile strength (UTS) of the as-received material increased sharply bya factor of 8 and 2.3, respectively. However, the tensile elongationwas markedly decreased after RS process.

3.2. Corrosion behavior

The potentiodynamic polarization curves of pure Al 1050 in CGas-received and RS conditions are presented in Fig. 4. The corrosionrate and corrosion current density of UFG material were signifi-cantly lower than those of CG as-received material. After RS2 andRS3, the corrosion current density dropped to 65% and 75% in com-parison to CG as-received. Corrosion rate dropped to 65% after bothRS2 and RS3. The electrochemical properties are listed in Table 3.

It is well known that the oxidation of metal occurs at crystallinelattice defects such as grain boundaries and dislocations [12].Rotary swaging breaks down the CG as received microstructure[13] that led to marked increase of the number of grain boundariesand the dislocation density, therefore the formation of surfaceoxide films is easier to form in UFG material. Previous studiesreported that the corrosion behavior of Al-alloys was improvedthrough SPD using equal angular channel pressing (ECAP) andaccumulative roll bonding (ABR) [8–11,14]. Other studies referredto the improvement of corrosion behavior of UFG Ti and Cu [15,16].Therefore UFG microstructure with non-equilibrium grain bound-aries and high residual stresses inside the grains are preferentialto come into being denser passive films than ordinary CG counter-parts. It was also reported that residual stresses play an importantrole in diminishing the fatigue degradation caused by pitting corro-sion of Al-alloys [17,18].

Fig. 5 presented the surface morphology of the investigatedconditions after the potentiodynamic polarization in 3.5% NaClsolution. The micrographs revealed the formation of higher

Fig. 2. Color coded orientation map for RS2; (a) normal to the swaging direction, and (b) parallel to the swaging direction. (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 3. Color coded orientation map for RS3; (a) normal to the swaging direction, and (b) parallel to the swaging direction. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Table 2Tensile properties and hardness of Al 1050 in various conditions.

Condition YS (MPa) UTS (MPa) Elongation (%) HV 10

As-received 20 72 120 24RS2 (u = 2) 137 139 12.9 46RS3 (u = 3) 158 163 11.6 49

M. Abdulstaar et al. / Materials and Design 57 (2014) 325–329 327

number of rectangular shallow deep micro-size metastable pitsthat have a frequently grow and repassivation (more close topatches of general dissolution), which cover the entire exposedsurface. The shape and distribution of the micro-pits were quitesimilar for all the investigated conditions. Interesting to mention,that pits size of RS material was smaller than that of CG counter-

10-9

10-6

10-3

10 0

Cur

rent

den

sity

,mA

/cm

2

-1.5 -1.0 -0.5 0.0 0.5 1.0

Potential,V vs SCE

Fig. 4. Potentiodynamic polarization curves of the a

part, this is quite obvious for the samples swaged with RS3(u = 3). This suggests that internal residual stresses play an impor-tant role in inhibiting dissolution of aluminum. Large fraction ofnon-equilibrium grain boundaries and high residual stress furnishthe UFG material with more nucleus to form denser nature oxidefilm than ordinary CG material. This denser oxide film can be keptrelative integral in the aggressive environment [11].

Fig. 6 refers to the variation of the open circuit potential (OCP)as a function of the exposure time. The figure represents that thefrequency of occurrence of oscillations is increased with the time.These oscillations are related to activation and repassivation pro-cesses i.e., initiation and healing of metastable pits [19]. Whenthe metastable pits occur first the current increases as the pitsnucleate and begin to grow, and then decreases after a short time.

1.5

s-received, RS2 and RS3 in 3.5% NaCl solution.

Table 3Electrochemical characteristics of various tested conditions.

Condition Corrosion potential (mV) Current density (lA/cm2) Corrosion rate (mpy)

As-received �716 1.055 0.451RS2 (u = 2) �708 0.379 0.162RS3 (u = 3) �720 0.283 0.154

Fig. 5. SEM micrographs of surface morphology after potentiodynamic polarization; (a) as-received, (b) RS2 and (c) RS3.

Fig. 6. Variation of the open circuit potential with time. Fig. 7. Variation of corrosion rate with immersion time.

328 M. Abdulstaar et al. / Materials and Design 57 (2014) 325–329

This is illustrated by the higher magnification of the oscillationsoccurs during the anodic polarization as shown in Fig. 4. It wasreported that the number of metastable pits on pure aluminiumincrease with an increase of the anodic potential (until the pittingpotential) of a constant concentration of chlorides [20]. It is wellestablished that metastable pits form sometimes several hundredmillivolts below the pitting potential and during the inductiontime for stable pit formation [21]. Frankel et al. [22] found thatmetastable pits on stainless steel are covered by a remnant of pas-sive film.

The corrosion behavior was also investigated at room tempera-ture using weight loss method. The corrosion rate of Al 1050 ex-pressed by mpy was recorded after 10, 27, 45 and 67 days ofimmersion in 3.5% NaCl solution. The results are presented inFig. 7 for the CG as-received and RS2 (u = 2) conditions. The result-ing corrosion rates were in consistent with those obtained bypotentiodynamic polarization, whereas the RS material revealedlower corrosion rates than the CG as-received material. The corro-sion rates after 10 days were comparable with those obtained frompotentiodynamic polarization, as the immersion time increased thedifference in corrosion rate between the two conditions decreasedand the corrosion rate dropped to less than 0.04 mpy after 67 days.The decrease of corrosion rate with time is attributed to the forma-tion of protective passive film on the surface that inhibits furthercorrosion reactions.

Similar results have been reported on the corrosion behavior ofAl 1100 and Al 5083 after 7 days of immersion in seawater [23].However the values of corrosion rate were higher than the valuespresented in the current study because of the nature of intermetal-lic particles present in this type of alloys that led to high dissolu-tion rate. As the immersion time increased the corrosion ratedecreased [23]. The decrease in corrosion rate with exposure timewas associated with the buildup of protective layers of aluminumhydroxide. The formation of hydroxide layers diminished the rateof aluminum dissolution with time [24]. The results presented inFig. 7, suggested that the passive film formed during immersiontests was sufficiently stable which contribute in the reduction ofthe corrosion rate, indicating the beneficial use of Al 1050 in mar-ine environment.

4. Conclusions

Potentiodynamic polarization curves revealed that UFG Al 1050processed by rotary swaging (RS) exhibited higher corrosion resis-tance in comparison to CG as-received material. The high densityof grain boundaries and dislocation offers good sites for easier for-mation of passive films, while internal residual stresses keep thepassive film stable and integral compared with CG as-receivedmaterial. SEM analysis after potentiodynamic polarization showed

M. Abdulstaar et al. / Materials and Design 57 (2014) 325–329 329

rectangular shallow pits with larger pit size in as-receivedcondition compared to RS condition. Weight loss tests confirmedthe better corrosion resistance of the RS material; however bothRS and CG as-received conditions exhibited low corrosion rates.

Acknowledgment

One of the authors (M. Abdulstaar) would like to thank theGerman academic exchange service (DAAD) for financial supportduring his studies at TU Clausthal.

References

[1] El-Danaf EA. Mechanical properties and microstructure evolution of 1050aluminum severely deformed by ECAP to 16 passes. Mater Sci Eng, A2008;487:189–200.

[2] El-Danaf EA. Mechanical properties, microstructure and micro-textureevolution for 1050AA deformed by equal channel angular pressing (ECAP)and post ECAP plane strain compression using two loading schemes. Mater Des2012;34:793–807.

[3] El-Danaf EA. Mechanical properties, microstructure and texture of single passequal channel angular pressed 1050, 5083, 6082 and 7010 aluminium alloyswith different dies. Mater Des 2011;32:3838–53.

[4] Abdulstaar MA, El-Danaf EA, Waluyo NS, Wagner L. Severe plastic deformationof commercial purity aluminium by rotary swaging: microstructure evolutionand mechanical properties. Mater Sci Eng, A 2013;565:351–8.

[5] Kwon YJ, Shigematsu I, Saito N. Mechanical properties of fine-grainedaluminum alloy produced by friction stir process. Scr Mater 2003;49:785–9.

[6] Kwon YJ, Shigematsu I, Saito N. Production of ultra-fine grained aluminumalloy using friction stir process. Mater Trans 2003;7:1343–50.

[7] Topic I, Höppel HW, Göken M. Influence of rolling direction on strength andductility of aluminium and aluminium alloys produced by accumulative rollbonding. J Mater Sci 2008;43:7320–5.

[8] Chung MK, Choi YS, Kim JG, Kim YM, Lee JC. Effect of the number of ECAP passtime on electrochemical properties of 1050 Al alloys. Mater Sci Eng, A2004;366:282–91.

[9] Song D, MA AB, Jiang JH, Lin PH, Yang DH. Corrosion behavior of ultra-finegrained industrial pure Al fabricated by ECAP. Trans Nonferrous Met Soc China2009;19:1065–70.

[10] Sherif EM, Soliman MS, El-Danaf EA, Almajid AA. Effect of equal-channelangular pressing passes on the corrosion behavior of 1050 aluminum alloy innatural seawater. Int J Electrochem Sci 2013;8:1103–16.

[11] Wei W, Wei KX, Du QB. Corrosion and tensile behaviors of ultra-fine grainedAl–Mn alloy produced by accumulative roll bonding. Mater Sci Eng, A2007;454–455:536–41.

[12] Uhlig HH. Corrosion and corrosion control. Tokyo: Sangyo Tosho; 1968.[13] Mhaede M, Wagner L, Ibrahim Kh. Thermomechanical treatments and surface

treatments to enhance the mechanical properties and fatigue performance ofrecycled cp-Ti. Int J Mater Res 2013;08:713–20.

[14] Son I, Nakano H, Oue S, Kobayashi S, Fukushima H, Horita Z. Effect of equal-channel angular pressing on pitting corrosion of pure aluminum. Int J Corr2012;2012:1–9.

[15] Balyanov A, Kutnyakova J, Amirkhanova NA, Stolyarov VV, Valiev RZ, Liab XZ.Corrosion resistance of ultra fine-grained Ti. Scr Mater 2004;51:225–9.

[16] Vinogradov A, Mimaki T, Hashimoto S, Valiev R. On the corrosion behavior ofultra-fine grain copper. Scr Mater 1999;41:319–26.

[17] Abdulstaar M, Mhaede M, Wagner L. Pre-corrosion and surface treatments onthe fatigue life of AA6082-T6. Adv Eng Mater 2013;15:1002–6.

[18] Mhaede M. Influence of surface treatments on the surface layer properties,fatigue and corrosion fatigue performance of AA7075 T73. Mater Des2012;41:61–6.

[19] El Shayeb H, Abd El Wahab FM, Zein El Abedien S. Electrochemical behaviourof Al, Al–Sn, Al–Zn and Al–Zn–Sn alloys in chloride solutions containingindium ions. J Appl Electrochem 1999;29:473–80.

[20] Pride ST, Scully JR, Hudson JL. Metastable pitting of aluminum and criteria forthe transition to stable pit growth. J Electrochem Soc 1994;141:3028–40.

[21] Szklarska-Smialowska Z. Pitting corrosion of aluminum. Corro Sci1999;41:1743–67.

[22] Frankel GS, Stockert L, Humkeler F, Bohni H. Metastable pitting of stainlesssteel. Corrosion 1987;43:429–36.

[23] Ezuber H, El-Houd A, El-Shawesh F. A study on the corrosion behavior ofaluminum alloys in seawater. Mater Des 2008;29:801–5.

[24] Carlson K. Mechanism of aqueous corrosion of aluminum at 100 �C. Int JElectrochem Soc 1987;104(1):147–54.