IndianJournalof ChemicalTechnologyVol.5, July 1998,pp.251-262

Inhibition of corrosion of Al and aluminium alloys in basic solutions

W A Badawy,F M AI-Kharafi& A S EI AzabDepartmentof Chemistry,Facultyof Science,Universityof Kuwait,P.O.Box5969Safat,13060Kuwait

Received7 September1996;accepted20 December1996

The technological importance and wide range of application of aluminium and its alloys have led toan extensive investigations on these materials. The corrosion behaviour of AI, AI-6061 and Al-Cu in ba­sic aqueous solutions and the effect of three corrosion inhibitors were studied by EIS and polarizationtechniques. X-ray photoelectron spectroscopic (XPS) investigation of these materials revealed the pres­ence of Cu in the AI-Cu alloy surface just below the surface of a thin film of hydrated aluminium oxide.EIS studies indicated the effectiveness of dichromate and molybdate in inhibiting corrosion of alumin­ium and its alloys. XPS studies of these materials after immersion of each in basic solutions containingseparately 0.01 M dichromate, molybdate and sulphate ions respectively did not show the presence ofchromium, molybdenum or sulphur on the surface film of these metals. SEM investigation of the elec­trode surface before and after immersion in the basic solution showed that the corrosion process is oc­curring in the flawed regions of the metal surface.

Aluminium and aluminium alloys represent an im­portant category of materials of technological im­portance and wide range of industrial applications.The electrochemical, corrosion and passivationbehaviour of these materials is a major field of re­search. The metal and its allQYs have a selectivetendency to form a very stable passive film onanodic polarization I. The properties of the surfaceoxide formed on these materials and its improve­ment represent a wide range of interesting investi­gations2. It was shown that the barrier film formedon Al or AI-alloys is of duplex nature, consistingof an inner layer of adherent, compact, closed andvery stable oxide film covered with a less stableouter layer which is porous and more susceptibleto corrosion3-6. In the presence of aggressive ani­ons, corrosion takes place, even in the passive pHrange4-9, leading to pit formation and film break­down7-9-. Many investigations have been devotedto the effect of chloride ions on the corrosion be­

haviour of the metal and its alloys in the passivepH range7-12. The pit initiation process and mecha­nism of pitting were the main concern of differentgroupSI3·14.Few investigations have been carriedout in chloride free solutions5•15•

Electrochemical impedance investigations ofaluminium in acid and neutral solutions under po­larization conditions have shown that the corro-

sion/passivation processes occurring at the elec­trode/electrolyte interface possess more than onetime constant. The high frequency time constantwas related to the barrier film formation. The low

frequency time constant was attributed to the dis­solution of the formed film. The presence of aninductive loop was attributed to adsorbed specieslike Wad16.Investigations of Al and Al-Si alloys innitric acid solutions have shown that the material

surface passivates even in aggressive medium likenitric acid or nitric acid containing chloride solu­tionsS,6. The investigations of the corrosion proc­esses occurring outside the passitivity pH regionseems to be important in order to choose the effec­tive inhibitor for these processes. Chromates, mo­lybdates and sulphates have been reported to in­hibit the corrosion of Al or AI-alloys in a variety ofmediaI7-19.

Electrochemical impedance spectroscopy (EIS)is one of the most important techniques now usedto investigate corrosion and corrosion inhibitionprocesses2Q-22.It has been successfully used to ex­plain pitting processes and passivation phenomenaon AI and AI_alloys7.23.24.Beside specification of

the physical properties of the system, the techniqueleads to important mechanistic and kinetic infor­mations22-24. In the present paper, the corrosionbehaviour of Al and some of its alloys of industrial

INDIAN 1. CHEM. TECHNOL., JULY 1998

relevance was investigated in basic solutions. EISand polarization techniques were used to explainthe inhibition of the corrosion processes occurringat the electrode/electrolyte interface using somepassivators like chromates, molybdates and sul­phates. X-ray photoelectron spectroscopy (XPS)and scanning electron microscopy (SEM) wereused to investigate the role of the passivating anionin the corrosion inhibition process. A comparisonbetween the different anions as passivators for theelectrode surface and the effectiveness of each wasconsidered.

zation experiments were performed using EG & G(Princeton Applied Research) Model 273A poten­tiostatlgalvanostat interfaced to an IBM PS/3 com­puter. The presence of surface,contaminations afterelectrode immersion in the test solution was inves­

tigated by means of XPS using ESCA-Lab 200(VG instruments). The electrode surface wasetched as required by argon ion bombardment. Theetchin'g rate was calibrated by the etching rate ofoxide films of known thickness grown on thespecimen surface. The XPS peaks of CIS, 0 IS,Al2S & 2P, Cu 2PI & 2P3, Cr 2PI & 2P3, Mg IS,Mo 3d3, 3d5, 3Pl & 3P3 and S 2P were traced.The electrode surface was examined by SEM(scanning electron microscopy) before and afterimmersion in the test solution. All measurementswere carried out at a constant room temperature of25°C.

Corrosion in basic solutions

The corrosion behaviour of AI, AI-6061 and AI­Cu was investigated in chloride free borate buffersolution, prepared according to the Clark andLub's series2-6and adjusted by small additions ofsodium hydroxide to the required pH. The electro­chemical measurements were carried out after theelectrode had reached a steady-state in the test so­lution. The corrosion parameters of each materialwere measured in the different solutions and pre­sented in Table 1. It is clear from the results pre­sented in this table that for solutions of pH > 6 therate of corrosion of Al or its alloys increases as thepH of the solution increases. In this work, the inhi­bition of corrosion of AI, AI-6061 and AI-Cu inbasic solutions (pH ~ 10) has been considered. Fig.1presents the Tafel polarization curves of the threeelectrode materials after reaching the steady-state(i.e. 45 min of electrode immersion) in a buffersolution of pH 10. Unlike the behaviour in acidsolutions27,the rate of corrosion of the three inves­tigated materials increases with the increased timeof immersion in the basic solution. For compari­son, the values of ieom Rcorr and Ecorr of the threeelectrode materials after 225 min of electrode im­

mersion in the solution of pH 10 are presented inTable 2. From the corresponding values of the cor­rosion current, icorr , and corrosion resistance, Ream

Results and Discussion

'I I "1'1" ""1'1"1' "'r "I"'I ' III I

252

Experimental ProcedureCommercial grade aluminium and aluminium

alloys (AI-6061 and AI-Cu) were used as elec­trodes. The mass spectroscopic analysis of thesematerials was reported previously25. The investi­gated materials were cut as cylindrical rods andmoonted into glass tubes of appropriate diameterby an epoxy resin leaving an exposed surface areaof 0.31, 0.21 and 0.20 cm2 for AI, Al-eu and AI­6061, respectively to contact the test solution. De­tails of experimental procedures were as describedelsewhere5.25.The electrodes were pretreated bymechanical polishing with successive gl'ades em­ery papers down to 3/0, then rubbing with asmooth cloth and washing with triply distilled wa­ter. It should acquire reproducibly bright appear­ance before immersion in the test solution. Forcomparison, some experiments were carried outafter chemical etching of the electrode surface tobe sure that the mechanical polishing has no effecton the alloy structure. Chemical etching w.asdoneby dipping the electrode for 5 min in 80°C heatedphosphoric/acetic/nitric acids mixture25. The im­pedance data of the mechanically polished andchemically etched electrode show the same trendand no pronounced effect in the corrosion behav­iour was observed.

Electrochemical impedance spectroscopy wasperformed using the IM5d-AMOS system (ZahnerElektric GmbH & Co., Kronach, Germany). Allexperiments involved single frequency measure­ments in the frequency domain 0.1 to 105Hz. Theinput signal amplitude was usually 10 mV peak topeak. Low frequency experiments, down to 10-3Hz, were also carried out to check the presence ofanother time constant at lower frequencies. Polari-

BADA WY et at.: INHIBITION OF CORROSION OF AI AND ITS ALLOYS 253

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62

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Fig. I-Tafel plots of AI-Cu (I), AI-6061 (2) and AI (3) inborate buffer (pH 10) after reaching steady state (45 min ofelectrode immersion). 234 5

Real Part ) k...n..

AI-6061 is more corrosion resistant than the other Fig. 2-Nyquist plots of AI-Cu electrode after different time

investigated materials in the blank solutions. Its intervals of immersion in solutions of pH 10. (--) 60 min,corrosion rate is about one third the corrosion rate (.....) 105 min, (----) 165 min, (-.-.-) 225 min.

of aluminium after long immersion time.The polarization measurements are in good and gives direct read to the electrolyte resistance

agreement with the electrochemical impedance and charge transfer or corrosion resistance. Thespectroscopic investigations. Typical impedance Nyquist plots of AI-Cu presented in Fig. 2 ars;:verydata of AI-Cu taken after different time intervals of similar to those obtained with Al or AI-6061. Theelectrode immersion are presented in Fig. 2. The only difference is the size of the large semicircleNyquist plot format is more favourable for com- which is related to the value of the polarizationparison of corrosion behaviour of different elec- resistance28• At any time interval the Nyquist plottrode materials in the same corroding medium or a of each material consists of a high frequency semi­certain material after different corrosion intervais. circle which is related to the corrosion/passivationThe format emphasizes series circuit components processes occurring at the electrode/electrolyte

Table I--Mass spectrometric analysis of the different electrode materials in mass %

Alloy

AICuMgSiFeMnNiZnPbSnTiCr

AI

99.230.0430.2170.0380.1640.0010.0100.0270.0010.0030.0060.001

AI-6061

97.090.2011.400.6010.1930.0120.0100.0290.0000.0000.0160.248

AI-Cu

93.434.800.2290.0470.4990.0240.0120.0250.7210.0060.0150.001

Table 2-Corrosion resistance, Room corrosion current, irorrand corrosion potential, Eoom for AI, AI-6061 and AI-Cu electrodes insolutions of different pH (I'll 45 min from electrode immersion)

pH

Roomkncm2 icorr>I1A cm-~EoommVAI

AI-6061AI-CuAlAI-6061AI-CuAIAI-6061AI-Cu

4

28.04117.1161.71.380.650.188-576-465-391

6

39.06156.8200.81.420.390.161-602-542-531

7

26.6157.28164.42.90U50.303-720-554-490

8

31.0047.74101.42.481.460.748-731-670-555

9

13.8013.3012.024.564.845.68-778-751-622

10

1.092.371.6440.313.7119.7-1238-1225-1107

11

0.1700.5200.280142.669.2298.52-1362-1425-129112

0.0890.1010.066340.0322.0457.8-1210-1464-1295

Table 4-Rcom ieorr and ECOfT after 45 min from electrode immersion of AI, AI-6061 and AI-Cu electrodes in solution of pH= I0containing 0.01 Mpassivator

Alloy

SO/- MoO/-Cr2O/-

Rcom

icorrEcorrRcorr,icorrEeorrReorr>icorrEcorr

kn cm2IlA cm-2mVkncm2IlA cm-2mVkncm2IlA cm-2mV

AI

3.1014.3-10615.892.03-85928.741.00-1110AI-6061

2.9217.8-10029.381.31-79975.600.45-1275AI-Cu

2.7217.6-92935.21.53-59161.780.16-1016

INDIAN J. CHEM. TECHNOL., JULY 1998

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The presence of Mg in AI-6061 improves thecorrosion characteristics of the aloy whereas Cu inAI-Cu decreases its corrosion resistance. AI-6061contains 1.4% Mg and 0.60% Si, which is a com­bination that leads to the formation of Mg2Si phasein a heat treatable alloy. The formation of suchphase is the base of precipitation hardening31•

Mg2Si phase, either in solid solution or as submi­croscopic precipitate, has no pronounced effect on

the electrode potential, which explains the closerelation between Ecorr of Al and AI-6061 (cf. Tables1 and 2 and Fig. 1). There is no detrimental effectsderive from the major alloying elements or fromminor components like Cr and/or Zn which areusually added to control the grain structure, sincethe alloy is a heat treated one. Copper additions areknown to increase the alloy strength. These addi­tions are limited to a very small amount (0.2%) inAI-6061, since the increase of eu content de-

-5

-6

Fig. J..-Nyquist plots of A ( .... ), AI-Cu (----) and AI-6061(-. -), after 225 min of electrode' immersion in solutions of

pH 10.

...-----------'-

Electrode Rcom kn cm2iCOfT•IlA cm-2Eeorr> mV

AI

0.7463.95-1183AI-6061

2.0617.50-1214AI-Cu

1.1921.08-1037

Table J..-RCOfT' ieorr and ECOfT of AI, AI-6061 and AI-Cu elec­trodes in solutions of pH= I0 after 225 min from electrode

immersion

interface, the electrode impedance in this case isdetermined by the metal oxide interface, the oxidefilm and the oxide/solution interface16, and a lowfrequency inductive loop, which is due to the re­laxation processes in the oxide film7•29,3o. The di­ameter of the high frequency semicircle changeswith the immersion time in the corrosive mediumand hence it specifies the corrosion resistance ofthe investigated electrode under the working con­ditions. The diameter of the low frequency semi­circle, on the other hand, is independent of thetime of immersion of the electrode in the test solu­

tion. The data presented in Fig. 2 show that thediameter of the high frequency semicircle de­creases with increasing the time of electrode im­mersion in the basic solution which means that thecorrosion resistance of the electrode decreases andan increased rate of corrosion at the interface oc­

curs. For AI-606l in a solution of pH 10 a decreasein Reorr from 2.37 kn cm2 after reaching the steady­state (45 min from electrode immersion) to a valueof 2.06 kn cm2 after three hours (225 min fromelectrode immersion) was recorded. As can be seenfrom Tables 3 and 4, Al-6061 is the more corrosionresistant of the three investigated materials. Theimpedance bel1aviour of these materials after 225min of electrode immersion in solution of pH 10 ispresented in Fig. 3. The results of this figure showthat the corrosion resistance is different for thedifferent materials. This means that the corrosion

behaviour of the material is affected by the alloy­ing elements.

254

'I I III I " • I" 'I' !,' I '~"'I'III II" "'"II Ilfltlll ,-I "I I I

BADA WY et a/.: INHIBITION OF CORROSION OF Al AND ITS ALLOYS 255

'Fig. 4--{a) Computer fitted values of Ro=0.16 kn, Rc:orr=3.55

kn and C=7.5 ~F (--) to experimental impedance data ofAI-Cu alloy (000) after 225 min of electrode immersion insolution ofpH=lO.(b) Equivalent circuit model for the electrode/electrolyte inter­face of Al or AI-alloys in solutions of pH 10.

(I) Roomcorrosion or polarization resistance(2) C, electrode capacitance(3) Ro, ohmic drops in the solution. o

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1000 800 600

Binding Energy ,eV

1200

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5

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Fig, 5-X-ray photoelectron survey spectra of naturally pas­sivated Al (Fig. Sa), AI-6061 (Fig. 5b), Al-Cu (Fig. 5c) andAI-Cu after 20 min etching by argon ion bombardment (Fig.5d),

creases the corrosion resistance of the alloy, whichis reflected clearly in both the polarization and im­pedance behaviour of the alloy (cf..Figs 1 and 3).

The impedance data of the alloy electrodes can 20

be fitted to a parallel capacitor/resistor combina- .•.~

tion in series to a small resistance equivalent to ~'5solution resistance and any ohmic drop. For data u 10

fitting, Bode plots are recommended as standardplotsI6•28• As an example of data fitting, the ex­perimental data of Al-eu after 225 min of elec­trode immersion in the buffer solution of pH 10were fitted to computer generated data accordingto the proposed model (cf. Fig. 4b) and presentedin Fig. 4a. The computer generated data were 7.5p,F for the capacitance C, 3.55 kO for the 'polariza­tion resistance (corrosion resistance, Reo,,) and 0.16kO for the ohmic drop in the solution, Ro. As canbe seen from Fig. 4a, the data fitting is very good,the low percentage error in the absolute impedance(1.2%) and the small deviation in the phase angle(1.10) indicate that the barrier layer on the elec­trodes surface fits welI to the parallel capaci­tor/resistor modelS.

The nature of the passive film formed on eachelectrode and its constituents was investigated byXPS. The survey XPS spectra of the three investi­gated electrodes after 225 min of electrode immer-

INDIAN J. CHEM. TECHNOL., JULY 1998

Inhibition of the corrosion processThe acceptable mechanism of the corrosion of

Al and its alloys in basic solutions is based on thedissolution of Al atoms from the active sites or

flawed regions of the naturally occurring barrierfilm and the gradual removal of these atomsthrough the formation of hydroxide with increasedcoordination from 1 to 3 to form independent mo­lecular species of AI(OH)3 which react in a purechemical manner to form a soluble aluminate ionthat goes in solution leaving a bare surface siteready for another dissolution process22. Such,

sion in solutions of pH 10 are presented in Fig. 5.In all spectra the characteristic peaks of aluminium(Al2P at 75.5 eV and Al 2S at 120.0 eV), oxygen(0 IS at 532.5 eV) and carbon, as residual fromthe oil vapours of the diffusion pump, (C 1S at285.5 eV) were recorded32.The XPS spectra of AI­6061 (cf. Fig. 5b) did not show pronounced XPSpeaks of magnesium. The XPS spectrum of AI­6061 after the long immersion in the test solutionsupports the conclusion that the passive film onAI-6061 like that of Al consists of Al203 and hy­drated aluminium oxide. Mg is present as Mg2Siphase in the bulk of the alloy. Unlike the behaviourof AI-Cu in acid solutions27,the XPS spectra ofthis material in solutions of pH 10 did not showclear copper peaks (Cu 2P3 at 932.5 eV and Cu2Pl at 952.5 eV or the small peaks Cu 3S and Cu3P peaks at 120.0 and 75.1 eV, respectively). Thismeans that the surface is covered with hydratedaluminium oxide like Al or AI-6061. Etching ofthe AI-Cu surface by argon ion bombardment evenfor few minutes leads to the appearance of the Cupeaks. Fig. 5d shows the XPS spectrum of thesame electrode of Fig. 5c after etching by argonion bombardment for 5.0 min only which isequivalent to the etching of about 1.0 nm thicknessof the surface. The appearance of the Cu peaks at934.4 eV and 954.0 eV, respectively indicatesclearly that the Cu-free surface film is very thinand consists of hydrated aluminium oxide in thebasic solution. The appearance of copper in thebarrier film initiates flawed regions which decreasethe corrosion resistance of the alloy and hence theobserved high corrosion rates of AI-Cu, especiallyat higher pH (cf. Table 1). This was confirmed bySEM investigated as will be discussed later.

I 1;11'1111"~liI; ~llillllllq 1llll'lll.liHlilil IIIII111 1"11

Fig. 6-Bode-impedance plots of AI-6061 after 225 min ofelectrode immersion in solutions of pH 10 containing 0.01 M

of (----) 80/-, ( ) MoO/-, (--) Crp/- and for com-parison the blank solution (.-.-.-.-).

100m 1 2 510 30 100 300 1K 3K 10K 30K lOOk

F reoqueon cy , Hz

lOOK

mechanism represent an irreversible coupled ano­dic metal dissolution/cathodic reduction reaction

and can be represented by the following scheme:

AI(surface)+OH-~AI(OH)ads. +e (1)

AI(OH)ads.+OH- ~AI(OH)2ads. +e- (2)

AI(OH)2ads.+OH- ~AI(OH)3ads. +e- (3)

AI(OH)3ads.+OH- ~AI(OH)4 ads (4)

AI(OH)4ads.~AI(OH)4solution (5)The coupled cathodic reaction takes place throughthe reduction of most likely, the water moleculesaccording to:

H20(surface)+ e- ~H +OH- (6)

H+H20(surface)+e- ~H2 +OH- (7)In oxygen rich alkaline solutions the cathodic partoccurs through oxygen reduction according to:

~02 +H20(SurfaCe)+e- ~OHads. +OH- .. (6)'

OHads.+e- ~OH- ... (7)'

300K

To inhibit the corrosion process, it is essential topassivate the active sites of the surface or to repairthe flawed regions of the barrier film. In this re­spect many inhibitors and passivators have beenused. We shall concentrate on the effect of threewell-known passivating anions, namely, dichro­mates, molybdates and sulphates and their functionin passive film repair. Typical example of the im-

'I 'I" '! ' I" "~IIIIIIII 11" "I''I ' "I I

256

BADA WY et a/.: INHIBITION OF CORROSION OF Al AND ITS ALLOYS 257

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30

40

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<II

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30L.

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Fig. 7--Bode-impedance plots of AI-6061 electrodes in solu­tions of pH 10 containing different concentrations of CrzO/-·(__ ) 0.1 M CrzO/-, ( ) 0.01 M CrzO/-, (-----) 0.001 MCrzO/- and (-.-.-.-) blank.

30K

< 10K

~ 3Kc:

.g lK••Q.

E 300

Fig. 8---X-ray photoelectron survey spectra of naturally pas­sivated AI-eu alloy in solutions of pH 10 containing 0.01 Mof 80/- (Fig. 8a), 0.01 M MoO/- (Fig. 8b) and 0.01 MCr20/- (Fig. 8c).

pedance results of the different inhibitors is theimpedance spectra of Al-606l taken after 225 minof electrode immersion in solution of pH 10 con­taining the same concentration (0.01 M) ofCr20/-,MoO/- or SO/-. These .spectra are presented asBode (impedance) plots in Fig. 6. Bode plots arerecommended as standard. impedance plots, sinceall experimental data are equally represented onthe ploe6• Analysis of the data of Fig. 6 shows thatthe SO/- ion has no pronounced effect on the cor­rosion behaviour of the alloy. The corrosion resis­tance of the electrode in the blank solution (pH 10)and that containing 0.01 M Na2S04 is in the rangeof ~2.0 kn cm-2. The presence of the same con­centration of molybdate increases the corrosionresistance to a value of 5.1 kn cm-2(i.e. 2.5 timesincrease), whereas the dichromate anion gives avalue of 79.0 kn cm-2 (~ 40 times increase).Similar results have been obtained with Al or Al­

Cu electrodes. The most interesting feature is that,unlike the behaviour in acid solutions27,.Al-Cuelectrodes show better corrosion inhibition char­acteristics in the presence of MoO/- and Cr20/-.The value of the corrosion resistance in the blankand sulphate containing solutions is less than thatof Al-6061 (~ 1.0 kn cm-2), the values in theMoO/- and Cr20/- containing soluti<?nsare muchhigher. Reorr in the molybdate solutions amounts to10.1 kn cm-2 (i.e. H) times that of the blank anddouble that of Al-6061) and in the dichromate so-

I

lutions increases to 103.9 kn cm-2 (i.e. over 100times that of the blank solution). In either case thedichromate anion is the most effective in the corro­

sion inhibition process. The corrosion resistance isdependent on the concentration of the inhibitor orpassivator anion in the ambient electrolyte. TypicalBode-impedance plots of Al-6061 in solutionscontaining different concentrations of Cr20/- arepresented in Fig. 7. The corrosion resistance in­creases from ~ 1.50 kn cm-2for the blank solutionto 71.0 ill cm-2 for that containing 0.001 MCr20/-. In presence of 0.01 M Cr20/- a value of79.0 kn cm-2 was measured. For solutions having0.1 M Cr20/-, Reorr increases to 264.0 kn cm-2.The same trend was observed for Al and Al-Cu

electrodes. The role of the passivating ion and itseffect on the barrier film formed on Al or its alloyswas investigated by XPS-experiments. XPS-surveyspectra of AI-Cu after 225 min of electrode immer-

Fig. 9--XPS spectra of Fig. 8 after etching by argon ion bom­bardment for 30 min. (etching of"" 6.0 nm of the surface)(a) Sulphate containing solutions (8a), (b) Molybdate con­taining solutions (8b) and (c) Dichromate containing solutions(8c).

sion in solutions of pH 10 containing 0.01 MSO/-,0.01 M MoO/- and 0.01 M Cr20/- are presentedin Figs 8 a, band c, respectively.

Unlike, the behaviour in acid solutions27, thesurvey spectra did not show the characteristicpeaks of S (S 2S at 229 eVor S 2P at 164 eV), Mo(3Pl at 412, 3P3 at 317, 3d3 at 230 and 3d5 at 227eV) or Cr (Cr 2Pl at 583.4 or 2P3 at 574.1 eV). Inthe sulphate containing solutions, the spectrumcontains an addition<tl.sodium XPS peak (Na 1S at

1073.8 eV) which com~s essentially from the con­stituents of the solution (borax, NaOH andNa2S04)' This means that the barrier film occur­ring at the electrode surface accommodates sodiummay be in the form of aluminate adsorbed on theoxide film. The Na peak declines on etching theelectrode by argon ion bombardment, e.g., etching

y

the surface for 20 min which is equivalent to about4 nm gives the spectrum of Fig. 9a with a smallerNa peak. The XPS-spectra of the same electrode inthe molybdate containing solutions did not showany additional peaks in the survey spectra. Thecharacteristic Mo peak (Mo 3P3 at 294.0 eV) andalso the Cu peaks (Cu 2Pl and Cu 2P3) appear af­ter surface etching by argon ion bombardment. Fig.9b shows the XPS-spectra of the same electrode ofFig. 8b after 30 min of surface etching (:::::6 nmthickness). In the case of Cr20/- containing solu­tions, the characteristics peaks for Cr (the Cr 2Pland Cr 2P3 peaks) could not be recorded even­after surface etching by argon ion bombardmentfor 30 min. The general features of the spectrumremain the same except that the Cu 2Pl and Cu2P3 peaks appear clearly and that the Na peak isdiminished (cf. Fig. 9c). The XPS-data of Figs 5,8& 9 support the conclusion that the barrier filmformed on AI, AI-6061 or AI-Cu in alkaline solu­tions consists also of two layers. The inner layer isthe compact one in which the passivating anionsmay be incorporated and in which other alloyingelements like Cu may appear (cf. Figs 5d and 9a, b& c). The outer layer consists mainly or' corrosionproducts i.e. aluminium hydroxide and aluminateas described in the scheme of the anodic dissolu­tion process (step 1 to 4). The corrosion inhibitionprocess in this case is based on the oxidizing prop­erties of the passivator and its stability in the me­dium.

It is well-known that the dichromate ion is a

powerful oxidizing agent which is capable of oxi­dizing Al or AI(OH)3to the passive AI20/3.34.Theoxidation of the corrosion products or Al appearingat the surface from any flawed regions or activesites leads to the formation of the stable Al203which decreases the rate of corrosion of thespeci­men. The molybdate ions are less oxidizing ascompared to the dichromate ions and hence theycannot passivate the surface of the alloy as the di­chromates do. The rate of corrosion of the materialin molybdate solutions is more than ten times thatin solutions containing dichromates. The calcu­lated corrosion resistance, corrosion current andcorrosion potential of AI, AI-6061 and AI-Cu afterreaching the steady-state (i.e. 45 min. of electrodeimmersion) in solutions of pH 10 containing thesame concentration of sulphate, molybdate and

(a)

INDIAN 1. CHEM. TECHNOL., JULY 1998

I Survey10

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~ 151ju 10 i5

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BADA WY et a/.: INHIBITION OF CORROSION OF Al AND ITS ALLOYS 259

(8)

(9)

... (10)

dichromate are summarized in Table 3. The data of

Table 3 were confirmed with both polarization andimpedance measurements. It is clear from thesedata that the dichromate is the most efficient pas­sivator for the investigated materials in basic solu­tions (the same trend was obtained in solutions of

pH 11 and 12). Sulphates did not show any pas­sivating effects, which can be attributed to the fact

that sulphates are similar as passivating ions asborates, which are the main constituent of thebuffer solution used.

In solutions containing dichromates of the sameconcentration (0.01 M) and after the same time ofelectrode immersion (225 min) the value of Reorr

for AI-6061 was 79.0 ill cm2 whereas that of AI..Cu was 103.1 kn cm2 and that of Al was 36.27 kncm2• The measured corrosion resistance of the

same electrodes after reaching the steady state (45min from electrode immersion) were 28.7, 75.6and 61.8 kn cm2 for AI, AI-6061 and AI-Cu, re­spectively, as presented in Table (3). This meansthat, although the corrosion resistance of AI-6061remains in the same range reached after the steady­state, the corrosion resistance of AI-Cu increases to

more th,an double its original value. The increasedcorrosion resistance of AI-Cu may be attributed tothe disappearance of Cu from the electrode surfacedue to the absorption of corrosion products whichare then oxidized to the passive A1203. In basicsolutions the dichromates are reduced accordingto:

CrzO.;- +20H- ~ 2CrO;- +HzO

crO~- +4H20+3e~Cr<OH)3 +50H­

2Al+ 2CrO;- +5HzO~AI203

+ 2Cr(OH) 3 +40H-

2Al(OH)3 + 2CrO;- + 2HzO ~ Al203 + CrZ03

+3HzOz+40H-

... (11)

CrZ03 +3H20+60H- ~2Cr(OH)~- ... (12)

CrZ03 +5HzO+40H- ~2Cr[(OH)s·HzO]z­

(13)

Cr(OH)3 + 30H- ~ Cr(OH)~- ... (14)

2Cr(OH)3 +HzO+20H- ~Cr[(OH)5.HzO]2­

... (15)

In basic solutions, both Cr(OH)3 and CrZ03 arepresent either as Cr(OH)63- or [Cr(OH)6 HzO]2­according to equationslZ-15.3.5and thus explain whyCr does not appear in the XPS spectra of the in­vestigated materials as was observed in acid solu­tions27. Inct:easing the pH of the solution leads toan increase in the rate of corrosion and the sametrend was obeyed i.e. in blank solutions the Al­6061 is the most corrosion resistant (cf. Table 1),whereas in the presence of strongerpassivator likeCrzO/-, Al-eu gives higher corrosion resistance.The presence of Cu after etching the electrode sur­face by argon ion bombardment may explain theincreased corrosion resistance of the AI-Cu alloy.The Cu which is insoluble in basic solutions de­

creases the possibility of the uptake of Al from theelectrode surface by decreasing the accessible ac­tive sites that could be involved in the corrosion

process according to the suggested corrosion reac­tion scheme Eq. (1-5).

The dependence of the corrosion resistance ofthe investigated materials on the concentration ofthe passivator anion e.g. the dichromate anion asshown in Fig. 6 indicates that the inhibition proc­ess depends on the uptake of passivator ion fromthe solution. The anion uptake process was alwaystreated as a competitive adsorption process at theelectrode/electrolyte interface36. The formation of acomplete adsorption layer of anions like CrzO/-,MoO/- or even SO/- is difficult. The passivationaction is better explained by the involvement ofthese anions in the redox reactions occurring at theelectrode surface and the oxidation power of each.The ability of the passivator to oxidize the surface,either the metal atoms or the corrosion products(cf. reactions 10, 11) and its oxidation power playthe main role in this process. The redox reaction isalways preceded by an absorption step in whichthe passivator ion adsorbs on the active sites18 andthen oxidation of these sites takes place. The r,e­duced forms of the passivator may be incorporatedin the formed passive film or may go in solution ifthey form soluble·complexes. In acid solutions Moand Cr were found to be incorporated into the bar­rier layer on the electrode surfacez7. Depth profil­

ing experiments of the electrodes immersed for225 min in basic solutions containing CrzOl- did

not show any peaks of Cr, which indicates that Cris not present in theoarrier layer up to an etched

f'

260 INDIAN 1. CHEM. TECHNOL., .RJLY 1998

-)00-f

y

surface of ~ 6.0 nm. Depth profiling experimentsof the electrodes after the same immersion time in

molybdate containing solutions have shown thepresence of Mo incorporated in the barrier film.This result is illustrated in Fig. 10. The oxidation­reduction processes occurring at the elec­trode/electrolyte interface account for the passiva­tion of the electrode in the solutions containing

Fig. II-Scannmg electron mIcrographs of mechanically pol­ished (a) AI, (b) AI-6061, (c) AI-Cu, (d) AI-Cu surface after 3

h immersion in borate buffer of pH 10, (e) AI-Cu electrode

after 3h immersion in borate buffer of pH 10 containing 0.0 IM Crp/-, and (f) the same electrode after immersion in bo­rate buffer of pH 10 containing 0.0 I M M0042-.

425 420 415 410 405 400 395 390 385

Binding En •.•rgy, •.•V

55

801 Mo 3pl 3p375

60

"'. 70l'lc:

6 65u

Fig. 1G-Depth profiling experiments of the molybdenumpeaks of naturally passivated AI-Cu alloy in solutions ofpH=10 containing 0.01 MMoO/-.

111

BADAWY et al.: INHIBITION OF CORROSION OF Al AND ITS ALLOYS 261

barrier layer up to an etched surface of::::l 6.0

run. Depth profiling experiments of the elec­trodes after the same immersion time in mo­

lybdate containing solutions have shown the

presence of Mo incorporated in. the barrierfilm. This result is illustrated in Fig. 10. Theoxidation-reduction processes occurring at theelectrode/electrolyte interface account for thepassivation of the electrode in the solutionscontaining Cr20/- and MoO/-. The dichro­mate anion is powerful in repairing the passivefilm by oxidizing the active sites and hence astable passive film which consists of Al203

covered with a porous layer of 9ydrated alu­minium oxide and alumiI1.ates incorporatingsodium is formed on the electrode surface.

SEM investigationsThe morphology of the surface of the three in­

vestigated materials was examined by scanningelectron microscopy. The results of these investi­gations are summarized in Fig. 11. Comparison ofthe scanning electron micrographs of the mechani­cally polished electrodes without any treatmentshow that the AI-Cu alloy has flawed regions on itspolished surface (Fig. 11c) which do not exist onpure Al (Fig. lla) or AI-6061 alloy (Fig. lIb). Thepresence of such regions cap be attributed to thepresence of copper on the AI-Cu surface as indi­cated by the XPS spectra of the polished elec­trodes. Upon the immersion of the AI-Cu elec­trodes in the basic solution corrosion takes placeaccording to the suggested scheme (Eqs 1-4) andthe electrode surface is covered with the corrosion

products, especially at the flawed areas, as can beseen clearly in Fig. 11d. The adsorbed corrosionproducts cannot protect the electrode surface com­pletely and hence appreciable rates of corrosioncan be measured. Immersion of the electrode insolutions of the same pH containing 0.01 MCr20/- leads to the repair of the flawed regionsand oxidizes the active sites and corrosion prod­ucts as described in section 3.2 (Eqs 10 and 11).This process leads, in turn, to the coverage of thesurface by a protecting film and hence a more ho­mogeneous surface is formed which appears in themicrograph of Fig. lIe. Accordingly, the copperdisappears from the electrode surface and no Cu-

XPS peaks can be recorded as was shown previ­ously. Similar scanning electron micrographs havebeen obtained with Al and AI-6061 specimens. Themicrographs obtained with AI-Cu electrodes im­mersed in solutions containing molybdates showan improvement in the surface morphology (cf.Fig. 11e). Such improvement can be attributed,also to the repair of the corrosion areas and oxida­tion of the corrosion products as in the case of di­chromate, but to a lesser extent [compare Fig. lIeand 11f]. This explains why the corrosion rate inmolybdate containing solutions is higher than thatmeasured in dichromate containing solutions.

ConclusionsThe barrier film formed on AI, AI-6061 or AI­

Cu in basic solutions consists of aluminium oxideand hydrated oxide with adsorbed aluminate.Cr20/- is more effective as passivator anion forthese materials due to its powerful oxidizing prop­erties. The inhibition properties of the anion de­pends essentially on its concentration in the corro­sion medium.

AcknowledgementThe authors are grateful to the research admini­

stration-Kuwait University for the financial sup­port of this work under the research grant SC060.Thanks are also due to Mr K. Jose for carrying outthe XPS experiments.

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