Corrosion Science 52 (2010) 1948–1957

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Corrosion Science

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The adhesion properties and corrosion performance of differently pretreatedepoxy coatings on an aluminium alloy

M. Niknahad a, S. Moradian a, S.M. Mirabedini b,*

a Polymer Engineering Department, Amirkabir University of Technology, P.O. Box 15875-413, Tehran, Iranb Colour, Resin & Surface Coatings Department, Iran Polymer and Petrochemical Institute, P.O. Box 14965-115, Tehran, Iran

a r t i c l e i n f o

Article history:Received 11 August 2009Accepted 6 February 2010Available online 14 February 2010

Keywords:A. AluminiumB. EISB. IR spectroscopyB. SEMC. Polymer coatings

0010-938X/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.corsci.2010.02.014

* Corresponding author. Tel.: +98 21 4458 0040; faE-mail address: m.mirabedini@ippi.ac.ir (S.M. Mira

a b s t r a c t

The influence of various blends of hexafluorozirconic-acid (Zr), polyacrylic-acid (PAA) and polyacryl-amide (PAM) pretreatment on the performance of an epoxy coated aluminium substrate was investigatedand compared to that of a so-called chromate/phosphate conversion coating (CPCC).

Adhesive-strength of epoxy coated substrates was evaluated using pull-off and tape tests. Salt spray,humidity chambers and EIS were employed to characterize corrosion performance of coated substrateswith different initial surface pretreatments. Among the Zr-based formulations, PAA/Zr and PAA/PAM/Zrshowed the best adhesion strength, while the later revealed a good corrosion performance as well. How-ever, CPCC pretreated sample was still superior in these aspects.

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1. Introduction

Aluminium and its alloys are widely used because of their somewhat unique properties, such as lightweight, relative low toxicityand a fair corrosion resistance [1,2] and the surface pretreatmentsplay an important role in the protection of aluminium and alumin-ium alloys [3]. Although the aluminium surface naturally coverswith an air-formed oxide film, about 2–3 nm, it is an insufficientbarrier for moderately long-term corrosion prevention of theunderlying substrate. The mentioned oxide-film covered alumin-ium is attacked in certain environments, even after being furthercoated by an organic protective coating [4].

Consequently, chemical conversion coatings are applied to alu-minium to improve corrosion resistance and/or to establish a basefor subsequent application of organic coatings. Conversion coatingson aluminium and its alloys are generally performed in solutionscontaining chromate and fluoride ions [1]. The coating usuallydevelops in the presence of fluoride ions, which involves a cathodicreduction of Cr(VI) to Cr(III) together with the evolution of hydro-gen. Chromium-based treatments have been used widely withgood anti-corrosive performance, but the toxicity and carcinogenicnature of hexavalent chromium are well documented [5]. Thus,several alternative treatments, including zirconium- or titanium-containing processes, or thin layers of polymeric materials, havebeen introduced [1,6–13].

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x: +98 21 4458 0023.bedini).

Chibowski [14,15] has studied the influence of ionic strength,pH and molecular weight of PAA on adsorption onto an aluminiumoxide surface. Adsorption of PAA on metal oxides having surpluspositive charges increases with increasing degrees of dissociatedcarboxylic groups in a PAA chain. The ratio of COOH/COO� de-creases when pH values of the solution changes from 3 to 9. Thedissociated carboxylic groups COO� in the polymer chain repeleach other and hence are responsible for stretching and conforma-tion of the polymer chain.

van den Brand and co-workers [16] have studied the effect ofdifferently functionalised interfacial polymer layers on adhesionstrength of an epoxy coated aluminium substrate. A good adhesionstrength and durability has been found for polyethylene maleicanhydride based systems, as a result of formation of cured interfa-cial region. PAA based system showed the presence of carboxylateion throughout the interfacial region [15,16]. This indicates that acuring reaction between the epoxy coating and PAA had occurredbut had not been fully completed. Moreover, a hydrophilic, rela-tively cured region can absorb a considerable amount of water.Such a system would not reveal a good adhesion strength anddurability in the presence of water [16,17].

The role of hexafluorozirconate in the formation of conversioncoating indicates that the fluoride ion is involved in the formationof an Al–Zr–O–F based layer that increases the hydrophilicity of thesurface with increasing surface activity. These, in turn enhance theinteraction between polymers and the aluminium alloy surfaceresulting in the formation of a polymeric conversion coating [18].

In our previous study, the surface free energy of differently pre-treated 1050 Al alloy was investigated [19]. The results showed

Table 1Pretreated samples with different combinations of PAA, PAM and Zr.

Treatment no. PAA PAM Zr

1 0.0 0.0 0.02 0.0 0.5 0.053 0.0 1.0 0.14 0.0 2.0 0.25 0.5 0.0 0.56 0.5 0.5 0.07 0.5 1.0 0.28 0.5 2.0 0.19 1.0 0.0 0.1

10 1.0 0.5 0.211 1.0 1.0 0.012 1.0 2.0 0.0513 2.0 0.0 0.214 2.0 0.5 0.115 2.0 1.0 0.0516 2.0 2.0 0.0

The number of pretreatment solutions were used are emphasized (bold) in Table 1.

M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957 1949

that hot water treated, PAA and CPCC pretreated samples providethe highest surface free energies. Comparison of these results withadhesion strength measurements showed that good wettability isan essential factor in achieving relatively good dry adhesive joints,however; this cannot be considered to be a sufficient reason forhigh bond strengths achieved in practice.

In a further study [20], Zr and PAA were selected to provide achrome-equivalent corrosion protection as well as to facilitatethe adhesion of a polyester/epoxy powder coating. Experimentalresults revealed that adhesion of an aluminium substrate in thepresence of a polyester/epoxy coating had indeed improved by aPAA/Zr based pretreatment and a chromate-based equivalent cor-rosion performance was achieved [20].

The main purpose of the present study was to evaluate theinteraction between PAA, PAM and Zr on an 1050 aluminium alloysurface. Additionally, the effect of different composition of PAA/PAM/Zr pretreatments on the adhesion strength and corrosion pro-tection performance of a clear epoxy coating on the aluminium al-loy were also studied.

Table 2Mixture compounds for FTIR analysis.

No. Combination

1 PAA + Al2O3

2 PAM + Al2O3

3 PAM + Zr4 PAA + Zr + Al2O3

5 PAA + PAM

2. Experimental

2.1. Materials

Aluminium alloy (1050) was supplied by Arak Al Company as0.5 mm thick sheets in H18 temper. PAA (average molecularweight: 104,000) as a 63% solution in distilled water, and carboxylmodified PAM (average molecular weight: 200,000) were bothsupplied by ACROC Organic (Fisher Chemicals). Hexafluorozirconicacid was obtained from Aldrich Chemicals Company as a 45% solu-tion in distilled water.

Aluminium oxide powder with an average particle diameter of0.1 lm was purchased from Merck Company. Epikote 1001, epoxyclear coat, and its hardener, Ardur115 polyamide were supplied byShell Company. All chemicals used in this study, were of analyticalgrade having high percentages of purity.

2.2. Sample preparation

Preliminary surface cleaning was carried out in acetone using asoft brush. For chemical etching, the specimens were immersed ina 5 wt.% solution of NaOH for 3 min at 50 �C followed by washingwith distilled water. For desmutting of etched aluminium surfaces,the specimens were then placed for 1 min in a 50% v/v solution ofnitric acid in water were then subsequently washed with distilledwater, and dried for 1 h at 50 �C. The samples were then treated byimmersion in a solution of different combined weight percentagesof PAA, PAM and Zr at 20 ± 1 �C for 3 min.

Taguchi statistical analysis method was employed to optimizethe number of pretreatment solutions shown in Table 1. The spec-imens were then allowed to dry under ambient temperature. Post-heating treatment was carried out in an oven at 145 �C for 20 min.For comparison purposes, a chromate/phosphate conversion coat-ing (CPCC) solution was also used; the treatment involved the Alo-dine procedure [1], with a specific immersion time of 5 min.

2.3. FTIR spectroscopy

In order to study possible interaction between aluminium oxidepowder and pretreatment solutions, 1 wt.% of various mixtures ofthe materials and 1 wt.% of alumina powder in distilled water (Ta-ble 2), were prepared. The solutions were then agitated using amagnetic stirrer with a rate of 300 rpm at 20 ± 1 �C. After 24 h, alu-minium oxide powder was filtered from the solutions, dried at

30 �C for 1 h, subsequently heated for 20 min at 145 �C, rinsed withdistilled water, and finally dried at 30 �C for 1 h.

FTIR–ATR spectra of treated specimens and filtered powderswere recorded on a FTIR–ATR spectrometer (Equinox 55, Bruker,Germany). The measurement conditions being: collecting 8 scansin the 400–4000 cm�1 range with 4 cm�1 resolution. The mixturecompounds were mixed with KBr powder in a ratio 1:100 for FTIRanalysis.

2.4. Application of coating layer

The clear coating was sprayed on the prepared samples at ambi-ent temperature and 50% relative humidity. Panels were allowed tocure for at least 10 days prior to examination. The thicknesses ofthe cured coating films were measured using an Elcometer 345instrument to an accuracy of 0.1 lm, with measurements carriedout on a set of five replicate samples. The coating thicknesses var-ied from 37.5 to 41.3 lm.

2.5. Accelerated corrosion tests

Two accelerated test methods, namely: salt spray and humidityexposure were used for each treated aluminium specimen. Two setsof samples were then exposed to a standard salt spray chamber for1000 h at 35 ± 1 �C according to ASTM B 117 and exposed to a humid-ity cabinet for 1000 h at 38 ± 2 �C according to ASTM D 2247, respec-tively. Visual assessments of the macroscopic surfaces were carriedout at various time intervals during the exposure time.

2.6. Electrochemical impedance spectroscopy (EIS)

A three-electrode arrangement, including an Ag/AgCl referenceelectrode, a platinum counter electrode, and the coated aluminiumspecimen (exposed area 40 � 40 mm2) immersed in a 3.5 wt.%NaCl solution was used. EIS was carried out at the open circuit po-tential, using an Auto Lab G12 instrument. The data were obtained

1950 M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957

as a function of frequency, using a sine wave of 10 mV amplitudepeak to peak; a frequency range of 30 kHz to 10 mHz was selected.

2.7. Adhesion measurements

Pull-off adhesion testing of the coating was performed accord-ing to the procedure described in ASTM D 4541. The dollies of20 mm diameter were degreased by acetone and then glued tothe surface of the coated panels with two components epoxy-based or a cyano-acrylate adhesive. After adhesive curing, a testingapparatus was attached to the loading fixture and was strained at5 mm/min in an Instron machine (Universal Testing Instrument)until the coating material had detached from the substrate. Foreach test, seven replicate samples were employed, and the averagevalue is quoted. The adhesion strength of the coated specimens inthe presence of water (wet adhesion) was measured after oneweek exposure of the specimens in a humidity cabinet.

Tape adhesion measurements were undertaken immediatelyafter the humidity test according to ASTM D 3359. Six, approxi-mately parallel, score lines were made with a separation of1.0 mm; a further six score lines were scribed perpendicular tothe original score lines. For individual specimens, 25 grids weregenerated. Adhesive tape was placed on the grids, using a soft era-ser; the tape was then removed with a firm, steady pulling action.

2.8. Scanning electron microscopy (SEM)/energy dispersivespectroscopy (EDS)

A Philips XL 40 SEM was used to examine the morphology of thetreated aluminium specimens. In this analysis, the samples, mea-suring 10 � 10 mm2, were placed in vacuum chamber of theinstrument. Samples were fixed onto an aluminium stud using adouble-sided cello-tape. The samples were examined at variousmagnifications and the images were recorded photographically.

3. Results and discussion

3.1. FTIR analysis

The FTIR spectra of different mixture compounds as describedin Table 2, within the spectral range of 400–4000 cm�1 are shownin Fig. 1. The most predominant and characteristic IR reflectancebands are listed in Table 3. The presence of a peak at 1568 cm�1

in the spectrum treated with PAM and PAA mixture indicates theformation of carboxylate groups (Fig. 1A).

The present illustrate that there is an ionic interaction betweenCOO� and Al3+ with an absorption located near 1557 cm�1 [21].Spectra of mixtures that have been shown in Fig. 1A and B revealedpossible chemical interactions of PAA and PAM molecules with alu-mina powder. The presence of COO� functional group in the case ofa relatively thin deposited layer of PAA results from a partial ioni-zation of COOH groups in the presence of water molecules asfollows:

ACOOHþH2O $ ACOO� þH3Oþ

In the FTIR spectrum of mixture of PAM and H2ZrF6, Fig. 1C, dueto the dipole moment in the carbonyl group and the resonance ef-fect of nitrogen, an absorption band in the region 1600–1700 cm�1

was observed. Additionally the C–O stretch band at 1270 cm�1 hasdisappeared from the spectrum of mixture compounds withH2ZrF6. The disappearance of this band and the presence of anadsorption band at 830 cm�1 occur when acrylic acid reacts witha flourozirconate compound. Applying polymers in combinationwith a zirconium compound has been claimed to cause cross-link-ing at the aluminium surface [1].

Additionally, the shifts of the absorption of NH2 groups from1417 and 1463 cm�1 to 1408 and 1454 cm�1 most likely indicatethe formation of amine salts and provide evidence for electrostaticinteraction between COO� and NHþ3 . Despite weak absorption ofPAM onto Al2O3, it could form strong bonds with PAA. Formationof a relatively thin and insoluble layer on aluminium surface wasrevealed previously by FTIR and EDXA techniques [20].

3.2. Accelerated corrosion tests

Visual assessments of the pretreated aluminium alloy samplesduring humidity and salt spray tests are summarized in Tables 4and 5. Appearance of the samples after 1000 h exposure in humid-ity conditions and salt spray tests are illustrated in Figs. 2 and 3,respectively. The data does reveal consistently that PAA/PAM gavethe lowest scribe ratings. CPCC treatment clearly provided out-standing performance. Equivalent performance was achieved withPAA/PAM/Zr treatment.

3.3. Electrochemical impedance spectroscopy (EIS)

It has been shown that EIS plays an important role to monitorand predict degradation of organic coatings [22–25]. Fig. 4 showsNyquist plots for the impedance of the epoxy coatings on alumin-ium substrate with different pretreatments over various immer-sion times in 3.5% NaCl electrolyte. The plots usually show oneor two semi-circles, with two-time constants at frequencies ofabout 200–100 and 3–0.2 Hz, respectively.

From Fig. 4, the first measurement (1-day) of the degreasedsubstrates clearly shows capacitive behaviour with a parallel resis-tive component in excess of 106 X cm2, most likely due to the poly-mer film [26–28]. With increasing immersion time (10 days), theradius of the high frequency semi-circle decreased. The coatingresistance values decrease during first few days of immersion, indi-cating the entry of electrolyte into the epoxy coating [29]. This isthis step of electrolyte penetration through an the coating, due towater uptake, when molecules of pure water diffuse into themicropores of the polymer net according to Flick’s law.

It is supposed that decreasing in the coating resistance valuemay be due to the penetration of water and movement of ionicspecies among the coating layer, increasing the coating conductiv-ity. Initially, the electrolyte penetrates through the coating layer,and sets up conducting paths at different depths within the coating[21].

Further, a second semi-circle in the impedance spectrum is ob-served at reduced frequencies, suggesting [27,28] that electro-chemical reactions at the interface between the coating and themetal surface are making progress. At this stage, penetration iscompleted and the electrolyte phase meets the metal/oxide inter-face and a corrosion cell is activated. With increased immersiontime (20 days), the barrier properties of the coating decreased fur-ther; however, the radius of the second semi-circle also decreased,suggesting an increase in the corrosion rate, possibly through thepresence of further pores in the coating or an increase in the areaexposed at the base of the existing pores or flaws [27].

It can be seen from Fig. 4 that epoxy coating on aluminium pre-treated by CPCC has greater resistance than both epoxy coating ondegreased aluminium and other treatments, indicating the benefi-cial role of chromium on the overall coating resistance. The spectrademonstrate essentially capacitive behaviour. Not much changewith exposure time was observed in the capacitive region of thespectra. Cr(VI) may exist included in the conversion coating andis reduced at flaws [4] to Cr(III) to re-passivate any damagedsurface.

Plots (C and F) in Fig. 4 show relative poor barrier properties ofPAM/Zr and PAA/PAM treated samples. This is perhaps due to the

Table 3FTIR assignments.

No. Wave number (cm�1) Functionality

1 3300–2200 OH (acid)2 1725–1700 COO (acid)3 1557 Acid salt. Ionized form4 1456 CH2

5 1417 CHCO6 1275 OC–OH (dimmer)7 1106 OC–OH (dimmer)8 3500–2500 NH2 (two peaks)9 1600–1700 –C–NH2

10 1416–1457 NH2

11 1262 C–N

Fig. 1. FTIR spectra of different mixture compounds.

M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957 1951

hydration of aluminium oxide at the water-rich interface [21], withthe water being replaced by aluminium hydroxide and polymer-oxide bonds. In the presence of under-film corrosion, the coatingresistance is low due to the existence of anodic and cathodic sites,ionic migration within the film (electrolyte), local film changes andelectro-osmotic movement of water [30].

Davis and co-workers [31] suggested that hydrated aluminiumoxide decreases the stability of adhesive joints in the presence ofwater. The coating layer may detach from metal substrates at theinterface due to time dependant diffusion of both water and oxy-gen through the coating to the metal/coating interface increas-ingly. Water may solvate un-reacted PAA molecules, resulting ina loss of adhesion [20]. The high hydroxyl ion concentration dis-solves the aluminium oxide and the aluminium hydroxide possibly

Table 4Visual observations of samples during 1000 h humidity test.

Treatmentno.

Samples conditions during humidity test

1 Very small blisters were observed from the second day oftesting. These expanded as time elapsed

4 Very small blisters were observed after 15 days of exposure.These expanded with increased exposure time

10 and 13 After 15 days of testing only slight changes in samples’ colourswere observed. After 30 days slight brown stains were observed.With further increase exposure time, light brown stainsdeveloped over 5% of the samples. Blisters were not observed forsamples 10 and 13

16 Some dark gray stains were observed over the sample surfacearea after the second day of testing. With further exposure timegray and dark areas covered the exposed sample surface. After15 days very small blisters were observed over the samplessurfaces

CPCC No visible changes observed

Table 5Visual observation of samples during 1000 h salt spray test.

Treatmentno.

Samples conditions during salt spray test

1 Corrosion products and very small blisters were observed after aweek and blisters expanded fast

4 Corrosion products and pitting corrosion was observed after aweek. After 10 days white corrosion covered the entire sample

10 and 13 After 10 days slight changes in the appearance were evident.Beyond 20 days isolated black stains were observed with a grayback ground over 10% of the sample area. After 25 days,corrosion products were occasionally observed and black stainsdeveloped on the surface. These changes occurred at a slowerrate for sample No. 10

16 From the second day of exposure black and gray stainsdeveloped over 25% of sample surface area. After a week whitecorrosion products were observed with a gray background.Pitting corrosion with white products was observed after10 days. After 20 days white corrosion products covered thesample, with small pits evident. The number of pits increasedwith time

CPCC The sample appearance did not change with exposure time

1952 M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957

attacks the polymer at the interface between the polymer and thesubstrate.

With increased immersion time, the radius of the high fre-quency semi-circle (Fig. 4C and F) is rapidly decreased, suggestingthat the barrier properties of the coating is progressively de-creased, and the corrosion rate continues to increase to the endof the test.

Fig. 4 reveals that PAA-based treated samples exhibit more ten-dencies to negative direction. When an aluminium surface treatedwith PAA is immersed in the test solution, as mentioned previ-ously, un-reacted functional groups on the PAA polymeric chain

Fig. 2. Visual appearance of sampl

may dissociate in water. Dissociation in water might change thepH of the solution to lower values. Such increased H+ concentrationmay increase the aggressive behaviour of the solution and, as a re-sult, the corrosion rate may increase.

From Fig. 4D for PAA/PAM/Zr treated Al samples, the spectra aredominated by capacitance of the coating, which remained constanteven after more than 50 days exposure to 3.5% NaCl solution. It isclear that the impedance values recorded for PAA/PAM/Zr treatedsamples are higher than the impedance values obtained for de-

es after 1000 h humidity test.

Fig. 3. Visual appearance of samples after 1000 h salt spray test.

M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957 1953

greased and PAA treated samples. PAA/PAM/Zr pretreated samplesreveal relatively good corrosion performance over the first 50 daysof immersion in the electrolyte.

It is suggested [12] that zirconium-based treatments may pro-tect the surface against the corrosive environment, by inhibitionof anodic reactions over the Al surfaces; consequently the passivityof the metal increases [32].

A further possibility is that when a polymer is present in theconversion coating solution, it may act as a surfactant and modifyuniformity of the conversion coating. PAA molecules may assistproduction of a more uniform conversion-coating layer on thesurface.

It is clear that the impedance values recorded for PAA/PAM/Zrtreated aluminium alloy samples are higher than the impedancevalues obtained for PAA/Zr treated samples. This is attributed tothe interactions between PAA and PAM molecules (as it is evidentfrom FTIR spectra analysis) which leaves only a few un-reactedPAA molecules in the interface, and also the presence of zirconiumcompound that increases the inhibition of anodic reactions on theAl surfaces and in so doing improves the corrosion performance.

An equivalent circuit model, proposed by many authors[26,33,34], based on the Nyquist plots for epoxy coated samplesis also shown in Fig. 5, either chromate or zirconium-based conver-sion treatments. This model reveals the electrolyte resistance, Rs,

the coating capacitance, Cc, the coating resistance, Rpf, the chargetransfer resistance, Rct, and the double layer capacitance, Cdl.

3.4. Adhesion strength measurements

The adhesion of coatings may be influenced by and are manyfactors surface pretreatments play an important role in the corro-sion protection of aluminium alloys [35]. The adhesion strength ofdifferently treated Al samples is shown in Fig. 6. As shown in Fig. 6the pull-off adhesion test revealed that samples Nos. 10 and 13 hadhigher dry adhesion values compared with samples Nos. 4 and 16.The adhesive strength of CPCC treated samples had an average va-lue of 140 kg/cm2. This performance generally correlated with theformation of an anchor pattern through which mechanical inter-locking takes place.

There are indications [19,20] that good adhesion between ahigh energy surface and the coating layer can occur through inti-mate contact between the substrate and the molecules of the coat-ing. CPCC, PAA/PAM/Zr and PAA/Zr treated substrates have highersurface energy than other treatments. Furthermore, as a result ofchemical interactions between PAA molecules and aluminiumoxide layer on the surface (as it is evident from FTIR spectra anal-ysis), dry adhesion strength of epoxy coating increasedadditionally.

Fig. 4. Impedance spectra of epoxy clear coated aluminium substrates during 20 days immersion in 3% NaCl electrolyte.

1954 M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957

In the dry state, all coated specimens indicated no failed re-gions, proving good adhesive strength. However, it is understoodthat the adhesive strength of the coatings in the dry state does

not reflect durability and performance [20]; thus, the adhesivestrength in the presence of water and possible corrosion products,i.e., wet adhesion, was determined. Initial, dry adhesion has been

Fig. 5. Equivalent circuit for the epoxy coated aluminium alloy substrates.

Fig. 7. EDS spectra of A: sample No. 10 pretreated with PAA/PAM/Zr and B: CPCCtreated sample.

M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957 1955

recognized for a long time as a poor predictor of coating perfor-mance in the presence of water or electrolyte [36].

Fig. 6 reveals that in the wet stage (immediately after one weekexposure in humidity condition), the adhesion strength of all spec-imens are decreased in comparison with dry stage. The main rea-son for failure of most organic coatings is the loss of bonding (ifany) between the coating and the substrate after exposure to thehumid atmosphere. When a coated aluminium surface is exposedto an aqueous environment, water molecules penetrate throughthe coating layer, after a certain initial period, water moleculesare accumulated in the coating/substrate interface, within the alu-minium oxide layer. Mechanical interlocking or/and chemicalinteractions throughout the interface may affected by this phe-nomenon and as a result, adhesion strength decreasing will bedetected.

For PAA-based treated samples, adhesion strength decreasingmay arise from the nature of the chemical interaction betweenthe treatments and aluminium surface. FTIR analysis has con-firmed the conversion of carboxyl acid functional groups to carbox-ylate ions at the PAA/aluminium oxide interface. However, onlyabout 40% of the acidic functional groups on the polymeric chainsmay convert to the salt form. It is proposed that steric and/or con-formational hindrance, associated with the polymer backbone, pre-vent reaction of all carboxylic acid groups in PAA with surfacehydroxyl functionalities to form carboxylate complexes [12].

It is believed that un-reacted groups may act as weak sites atthe interface, therefore, when aluminium surface, on which PAAbased treatment was deposited, is exposed to aqueous environ-ment, dissolution may proceed leaving little ionic or complexinteraction with the metal surface. The final poor wet adhesionat the local site is the result of the presence of un-reacted PAA mol-ecules on the metal surface. These molecules may dissolve inwater, creating considerable osmotic pressures, with subsequentincrease in the water volume at the interface. This concludes to

0

40

80

120

160

200

No. 1 No. 4 No. 10Treatm

Adh

esio

n St

reng

th (k

g/cm

2 )

Dry Adhesion Strength (kg/cm2)Wet Adhesion Strength (kg/cm2)Adhesive Remaining (%)

Fig. 6. Pull off adhesion strength of various pretreated epoxy coats on al

mechanical stress on the adhesive bonds. Once the stress passesa critical value, the bonds between reacted PAA molecules andthe aluminium oxide break, and the detached surface area in-creases [20].

The results revealed that PAM/Zr treatment has little effect onthe adhesion of coating to the aluminium substrate in wet or drycondition. This is perhaps due to weaker chemical interactionand lower surface energy of PAM/Zr treatment comparison withPAA based treatments. It is believed that in treatment No. 10, thereare two forms of chemical interaction, between PAA moleculesand aluminium oxide, and PAA molecules and PAM molecules.

No. 13 No. 16 CPCCent

0

20

40

60

80

100

Adh

esio

n R

emai

ning

(%)

uminium and percentage of adhesive remaining after humidity test.

1956 M. Niknahad et al. / Corrosion Science 52 (2010) 1948–1957

Therefore, un-reacted PAA molecules are very limited in thecoating/substrate interface, as a result, decreasing water sensitivityand increasing adhesion strength in dry and wet conditions.

Tape test were performed under dry and wet conditions. In thedry state all coated specimens indicated no failed regions, 5B, and

Fig. 8. SEM micrographs of variously pretreated aluminium alloy samples,magnification: 1000�, 20 kV.

implying good adhesive strength. In the wet state CPCC treatedsamples reveal improved performance compared with other sam-ples. For comparison, the percentage of remaining adhesion wasdetermined using the expression: AR% ¼ n

25� 100, where AR repre-sents adhesion remaining and n is the average number of squaresof undetached coatings. The results of five individual tape adhesionmeasurements, in wet condition, are shown in Fig. 6. The percent-age adhesion remaining varied widely, with CPCC, PAA/PAM/Zr andPAA/Zr treatments giving the highest levels.

Typical scanning electron micrographs and EDS spectra havebeen shown previously [20,21] to contain zirconium, oxygen andaluminium for sample treated with PAA and H2ZrF6. This is alsogained in Fig. 7. EDS spectrum (Fig. 7A) and SEM micrograph(Fig. 8C) of sample treated with PAA/PAM/Zr tend to suggest thata conversion-coating layer had developed over the aluminium sur-face. The EDS spectrum in particular illustrates the presence of zir-conium, oxygen and aluminium indicating a thin film over themacroscopic alloy surface.

Figs. 7B and 8A, B and D show the respective EDS spectra ofCPCC as well as SEM micrograph of Zr, PAA/Zr and CPCC for com-parison purposes. EDS analysis of PZr treated specimens revealedthe presence of zirconium, oxygen and aluminium, suggesting thata thin film had formed over the macroscopic alloy surface.

4. Conclusion

FTIR analysis showed that there are two possible chemicalinteractions, one between PAA molecules and aluminium oxideand the other between PAA and PAM. The first is a purely ionicinteraction between COO� and Al3þ giving an absorption bandaround 1557 cm�1, and the second is related to the interactions be-tween COO� and NHþ3 groups which appears as a doublet in the re-gion 1408–1454 cm�1. FTIR and EDS spectra indicate the formationof a relatively thin and insoluble layer of the pretreatment on thealuminium surface. EIS tests showed that the combination ofPAA/PAM/Zr as the pretreatment demonstrated a much better cor-rosion performance than other formulations even after the acceler-ated tests, ranked second after CPCC pretreatment.

Although the polyacrylamide component does not noticeablyincrease dry adhesion strength, however, it markedly improvesthe corrosion protection performance of the PAA/PAM/Zr pretreat-ment compared to the PAA/Zr treatment.

Acknowledgements

The authors wish to acknowledge support for the research workreports in this paper from the Amirkabir University of Technologyand Iran Polymer and Petrochemical Institute.

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