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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
~ -2
-0'800
-0,900
-1,000>_ft --1-700UQl
~ +000III>
'" -1,300
-1-1.00
-7 -6 -5lIar.a • A/cm2
-4 -3
-3
....
, ",~ ;~:-:~~::~-.-... " .... )...." .,." '.... ',,"".' .• \
~ -1 ;("'\, \, 'i~ \ ~>-. ~ , :~J/L(''-CloE
62
o
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
"
.", ..
0123456789Real Part ,k.n..
2
3
a -2a..
>. -1'-Cl
= 001oE
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
(b)
(a)
200400
'!!oI
1000 800 600
Binding E nl'rgy I l'V
1200
Survey
o1400 1200 1000 800 600 400 200
Binding Enl'rgy l'V
l5f- SUrvl'Y
]0
25
)0
45 "JoII'
"0]0
(b) ,90
o 0 •.•.••
• -115
I
]0 -i75
~ •• u __ j
'll 2:;: -1601__ 1
o1 2 5.10 ]0 100 300 1K 3K 10K 30K lOOK
Frl'qul'nc y , H z
100m
)00
200
2K
<.. 1·5K
~ lKc.g 700..Q.! 500
(c)
(d)
200400
!!?oI
1000 800 600
Binding Energy, eV
1000 800 600
Binding Energy ,eV
1200
1200
5
o1400
o1400
25
)0, Survey
10
5
35f- Survey
30
25
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
lee:~N
I
(c)
!!!uI
Survey
o1400 1200 1000 800 600 400 200
Binding Energy, eV
10
30
40
1M
<II
-.: 20OJoU
lOOK
lOa
30L.
100m I 2 5 10 30 100 300 IK 3K 10K 30K lOOK
Frequency, Hz
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
"'" 20III
~ 151ju 10 i5
25
o I I_-L1400 1200 1000 800 600 400 200
Binding <::nE'rgy,E'V
Survey
10
15
70 ISurvey60
50
10
258
40
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11· 10c::>
o 20u
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C::>
<'3 20
II I III I
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.
References1 Young L, Anodic Oxide Films (Academic Press, New
York), 1961,4-9.2 Despic A & Parkhutik V, in Modern Aspects of Electro
chemistry (eds. J 0' M Bockris, R E White & B E Conway), Vol. 20, Chap. 6, Plenum, New York, 1989.
3 Diggle J W, Downie T C & Goulding C W, ElectrochimActa, 15 (1970) 1079.
4. Brett C M A, Gomes I A R & Martins J P S, Corros Sci,36 (1994) 915.
5 Badawy W A & AI-Kharafi F M, B Electrochem, 11(1995) 505..
6 AI-Kharafi F M & Badawy W A, Electrochim Acta, 40(1995) 1811.
7 Mazhar A A, Badawy W A & Abou-Romia M M, Surfaceand Coatings Technol, 29 (1986) 335.
8 Cabot P L, Centellas F A, Garrido J A, Perez E & Vidal H,Electrochim Acta, 36 (1991) 179.
9 Carroll W M ~ Breslin C B, Br Corros J, 26 (1991) 255.10 Dibart G A & Read H J, Corrosion, 27 (1971) 483.11 Feres S E, Stefenel M M, Mayer C & Chierclie T, J Appl
Electrochem, 20 (1990) 996.
I'
INDIAN J. CHEM. TECHNOL., JULY 1998262
12 Srinivasan H S & Mital C K, Electrochim Acta, 39 (1994)2633.
13 Baumgaertner M & Kaesche H, Corros Sci, 31 (1990)231.
14 Wiersma B J & Herbert K R, J Electrochem Soc, 138(1991) 48.
15 Badawy W A, EI-Basiouny M S & Ibrahim M M, IndianJ Techriol,24 (1986).
16 Lenderink H J, Linden M V D & Wit J H De, ElectrochimActa, 38 (1993 1989.
17 Moshier vi C & Davis G D, Corrosion, 46 (1990) 43.
18 Breslin C B, Treacy G & Carroll W M, Corros Sci, 36(1993) 1143.
19 Samuels B W,. Satoudeh K & Foley R T, Corrosion, 37(1981) 92.
20 Hitzig J, Juettner K, Lorenz W J & Paatsch, Corros Sci,24 (1984) 945.
21 Mansfeld F, Kendig M W & Lorenz W, J ElectrochemSoc, 132 (1985) 290.
22 MacDonald D D, Electrochim Acta, 35 (1990) 1509.23 Mansfeld F, Lin S, Kim S & Shih, Corros Sci, 27 (1987)
997.
24 Mansfeld F, Lin S, Kim S & Shih H, Werkst Korros, 39(1988) 487.
25 Al-Kharafi F M & Badawy W A, Indian J Chem Technol(in press).
26 Fasman G D, Practical Handbook of Biochemistry andMolecular Biology, (CRC Press Inc., Poca Raton, Florida)(1989) 544.
27 Badawy W A & Al-Kharafi F M, Corros Sci (Accepted forpublication).
28 MacDonald J R (ed.), Impedance Spectroscopy, (JohnWiley & Sons, New York) 1987, chap. 4.
29 Bretts C M A, J Appl Electrochem, 20 (1990) 1000.30 Bretts C M A, Corros Sci, 33 (1992) 203.
31 Hollingworth E H & Hunsicker H Y, Corrosio.n Resistallce of Aluminium Alloys, in Metals Handbook, 9th ed.(1990) (American Society for Metals, Metals Part, OH,USA) Vol. 2,204-236.
32 Adem E, VG Scientific XPS Handbook, 101 ed. (1989). VGScienitific Ltd (The Birches Industrial Estate, East Grinstead, West Sussex RH 19 1VB, England).
33 Abd Rabbo M J, RiChardson J A & Wood G C, CorrosSci, 15 (\ 975) 243.
34 McCaffeny E, Bernett M K & Murday J S, Corros Sci, 28(1988) 559.
35 Lee J D, Concise Inorganic Chemistry, 4th ed. (Chapman& Hall, Nf'W York) 1991,723-730.
36 Matsuda S & Uhlig H H, J Electrochem Soc, 11 (1964)156.
37 Uhlig H H & Gilman J R, Corrosion, 20 (1964) 289.
38 McCafferty E & Powers R A, J Electrochem Soc, 137(1990) 373.
39 Videm K, The Electrochemistry of Uniform Corrosion andPitting of Aluminium, Kjeller Report, 62 (1974).
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