201 (2007) 5822–5828www.elsevier.com/locate/surfcoat

Surface & Coatings Technology

Zirconia primers for corrosion resistant coatings

G. Gusmano a,b, G. Montesperelli c,⁎, M. Rapone a,b, G. Padeletti d,A. Cusmà d, S. Kaciulis d, A. Mezzi d, R. Di Maggio e

a Consorzio INSTM - Unità di Ricerca di Roma Tor Vergata, Italyb Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma - “Tor Vergata”, Rome, Italy

c Dipartimento di Fisica ed Ingegneria dei Materiali e del Territorio, Università Politecnica delle Marche, Ancona, Italyd ISMN - CNR, Monterotondo Stazione, Rome, Italy

e Dipartimento di Ingegneria dei Materiali, Università di Trento, Trento, Italy

Received 14 July 2006; accepted in revised form 19 October 2006Available online 8 December 2006

Abstract

The recent restriction in the use of chromium VI gave a new impulse in the field of anti-corrosion treatment for an alternative solution.A promising route seems to be the deposition of a thin ceramic layer on the metallic surface. In this study, four zirconia primers have been

deposited on 1050 aluminium sheets by sol–gel process. Sol preparation was obtained starting from two different precursors and two curingtemperatures were tested.

Surface microstructure and chemical composition were determined by AFM and XPS for all samples. Electrochemical Noise Analysis (ENA)was used in Harrison's solution to evaluate the corrosion resistant features of the samples. Tests have been also performed on samples protected bypolyester topcoat deposited on the aforesaid primers.

AFM analysis revealed that, for all the samples, covering and structure of the coating were uniform. XPS depth profiling allowed concludingthat film thickness was in the range from 18 to 30 nm and that chemical composition was constant through primer thickness.

Corrosion tests demonstrated that zirconia primers showed good performances in terms of corrosion resistance, comparable to chromate andfluotitanate layers industrially prepared.

Topcoated samples gave an optimal corrosion resistance.© 2006 Elsevier B.V. All rights reserved.

Keywords: Zirconia; Coating; Sol–gel; Corrosion; Aluminium alloy; AFM; XPS; Electrochemical Noise

1. Introduction

In recent years, the European Union promulgated directivesconcerning restrictions in the use of heavy metals such aschromium VI, lead, mercury and cadmium in vehicles [1] and inelectronic devices [2]. In particular the limitations in the use ofchromium VI, led a great demand in the corrosion protectionfield for new inhibitors and new coating formulations,characterized by low environmental impact, with the samecorrosion prevention and protection properties [3–5].

⁎ Corresponding author. Tel.: +39 071 220 4401.E-mail address: g.montesperelli@univpm.it (G. Montesperelli).

0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.surfcoat.2006.10.036

A suitable solution seems to be the use of inorganic primersbased on silica, zirconia or titania, topcoated with differentpolimeric layers [3,6,7]. The corrosion resistant features of sucha system depend on a number of characteristics such as: metal/primer and primer/organic film adhesion, number of defects andtheir dimension, lack of primer degradation phenomena, such ashydrolysis, when immersed in an aggressive environment,primer ability in inhibiting local corrosion phenomenonoccurring on metal. In particular, the presence of defects onthe primer, allows the onset of corrosion events in correspon-dence of such defects [3,4,6–8].

A very versatile technique for primer preparation is thedeposition by sol–gel route [9–11]. This method allows a goodreproducibility of coating performances even though someproblems have been evidenced due to complex shape substrate

Table 1Experimental conditions of samples preparation

Label Sample Precursors Temperature(°C)

Time(min)

Number oflayers

C Chromates – – – –F Fluotitanates – – – –ZO1 Zirconates Organic 250 4 3ZO2 Zirconates Organic 150 60 3ZO3 Zirconates Organic 250 4 2ZI Zirconates Inorganic 250 4 2

Polyesther topcoated samples have been labelled adding “TC” at the primer label(for example ZO2TC).

Fig. 1. AFM image (5×5 μm) of ZO1 zirconia primer.

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and thick film deposition. Moreover, in some case, coatingcracking induced by the thermal treatment performed at the endof deposition, has been reported [8]. Usually, a thermaltreatment at a temperature around 600 °C provides a goodchemical homogeneity and good corrosion resistant features toprimers. Nevertheless, some promising results, in terms ofcorrosion resistant, have been obtained also at lower temper-ature (400 °C) [12,13].

This paper presents the results of corrosion tests carried outon 1050 aluminium sheets coated with zirconia primers,prepared by sol–gel route followed by a low temperature heattreatment. Primers obtained from both organic and inorganicprecursors were tested. As a comparison, some cromate andfluotitanate primers industrially prepared have been also tested.In a second stage, the same primers coated with a polyester toplayer have been tested.

Corrosion tests have been performed by means of currentand potential Electrochemical Noise Analysis (ENA) inHarrison's solution.

Surface characterizations by X-ray Photoelectron Spectros-copy (XPS) and Atomic Force Microscopy (AFM) have beenalso carried out for all samples.

2. Experimental

AA 1050 aluminium sheets were used as substrates. Beforedeposition, the sample surface was degreased with acetone. Solpreparation was obtained starting from two different precursors.A 0.1 M solution of zirconium tetrabutoxide (Zr(OBun)4),containing acetic acid as complexing agents was used fororganic (ZO) series [14] and a 0.4 M solution of zirconyl nitrate(ZrO(NO3)) was used for inorganic (ZI) series. The withdrawalrate was 1 mm/s for all samples. After film deposition, thesamples were treated in an oven at 150 °C for 1 h or at 250 °Cfor 4 min as reported in Table 1. The film thickness wasincreased by repeating deposition procedure as reported inTable 1.

Comparison tests have been carried out on AA 1050 sheetscoated with a chromate or a fluotitanate layers industriallyperformed by Chemetall. Chromate layer was obtained by usingGardobond C714. Fluotitanate samples were prepared byGardobond X4705. On fluotitanate primer, a further passivationstep was performed by Gardolene D6800/CC.

Tests have been also performed on samples protected bypolyester topcoat deposited on the already mentioned primers.

2.1. Test environment

Corrosion tests were carried out into two different Harrison'ssolutions. A diluted solution, in the case of samples only protectedby a primer [3.5 g/l (NH4)2SO4 + 0.5 g/l NaCl] and a concentratedone for topcoated samples [3.5 g/l (NH4)2SO4 + 5 g/l NaCl].

2.2. Electrochemical Noise Analysis (ENA)

Two working electrodes of 7.5 by 10 cm plate, 0.7 mm inthickness were used. The immersed surface was 9.6 cm2.Electrochemical current noise was measured as the galvaniccoupling current between two nominally identical workingelectrodes, while electrochemical potential noise was detectedas potential fluctuation between working electrodes and an Ag/AgCl reference electrode. Potential and current noises weresimultaneously recorded using a Solartron 1287 electrochem-ical interface. Data sets of 1024 readings were recorded at asampling interval of 250 ms (sampling frequency = 4 Hz). Thesampling interval was optimised by means of preliminary tests.After data collection the DC linear trend was removed by linearregression removal method. Data were transformed in thefrequency domain through the Maximum Entropy Method(MEM) and Fast Fourier Transform (FFT) algorithms per-formed by dedicated software. Some blank tests on “asreceived” electrodes have been also performed.

2.3. Atomic Force Microscopy Analysis (AFM)

The morphological characterization of the coatings has beencarried out in air by a Digital Instruments Dimension D3100AFM, operating in tapping mode with scanning areas from 1 × 1to 10 × 10 μm.

2.4. X-ray Photoelectron Spectroscopy (XPS)

The surface chemical characterization has been carried outby using an Escalab Mk II spectrometer (VG Scientific Ltd, U.

Fig. 2. AFM image (1.75×1.75 μm) of chromates (C) samples.

Fig. 3. Comparison between the chemical compositions of the samples ZO1 andZO2.

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K.) with 5 channeltron detection system. Monochromatic Aland Mg X-ray sources (hν = 1486.6 eV, hν = 1253.6 eV,respectively) have been used for the excitation of photoelectronspectra. The binding energy scale was calibrated by fixing thepeak of adventitious carbon at 285.0 eV. For selected-area XPSdepth profiling, the samples were fixed on the standard Escalabholder stub by means of an Au foil mask with a window of3 mm in diameter. The Ar+ ion beam of 2.0 keVenergy, rasteredover a sample area of 3 × 3 mm2, has been used for the samplesputtering. More details on XPS depth profiling have beenpublished elsewhere [15].

3. Results and discussions

3.1. AFM results

AFM analysis showed a uniform covering for all samples.AFM image for ZO1 sample, shown in Fig. 1, pointed out adouble granular structure characterized by rounded small grains(40 ÷ 60 nm in diameter) upon which other larger grains (60 ÷100 nm) grew. Larger grains appear to be aligned along apreferential direction. ZO2 sample showed a microstructurequite similar to ZO1 with slightly larger grains than ZO1(diameter 60 ÷ 120 nm). ZO3 sample showed a granularstructure with grains in the range from 60 to 100 nm. ZI sampleshowed a significant increase of grain size that reached

Table 2AFM results

Label Grain size (nm) Ra (nm) Rq (nm)

C 50–100 60.20 75.67F 40–120 90.52 110.96ZO1 40–100 16.25 21.00ZO2 60–120 23.37 37.68ZO3 60–100 36.09 45.55ZI 120–250 26.45 33.17

dimension in the range 120 ÷ 250 nm, thus emphasizing aninfluence of the inorganic precursors on the grain growth.

On the contrary, as a general remark, it may be concludedthat the grain size was not affected by heating condition.

Chromates samples (C) showed rounded grains withdiameter from 50 to 100 nm. As already discussed in the caseof ZO1 sample, a double granular structure was noted, withouter grains arranged in annular structures (Fig. 2).

Fluotitanates samples (F) evidenced a very similar micro-structure with respect to C samples, with grains in the range 40–120 nm in diameter.

AFM examination did not evidence a granular structure fortopcoated samples but the polymeric layer was thin enough toreveal the granular structure of the substrate. AFM also permitsevaluation of the roughness of sample surface, taking into accountthe distribution curve of the relative highness between theacquired points within the scanning area and thus calculating theaverage roughness value (Ra) with regard to a central plane and itsstandard deviation (Rq). Fluotitanates and chromates show higherroughness parameters. As a general trend it may be pointed outthat passing from sample ZO1 to ZO2 (i.e. decreasing thetemperature and increasing the length of heat treatment) the

Fig. 4. XPS depth profile for ZO2 sample.

Fig. 5. XPS depth profile for ZO3 sample. Fig. 7. Current noise evolution for ZO1 samples after 14 and 15 days ofimmersion.

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average roughness increases. The results of the AFM analysis forall primers are summarized in Table 2. Topcoated samples showedvery low roughness in the range from 13 to 16 nm.

3.2. XPS results

The results of XPS measurements enabled us to establish thatall the samples have a constant composition through theirthickness without any significant alteration of the chemicalspecies. Fig. 3 shows the comparison of XPS quantification forZO1 and ZO2 samples. Both samples are characterized by thepresence of main constituents of the primer (zirconium, oxygen)and of the substrate (aluminium, silicon).

The peak-fitting analysis of C 1s spectra defined that this signalis very similar for all the samples: C–C and C–H component at285.0 eV, C–O at∼ 286.6 eV, C=O and –COOH at∼ 288.6 eV. Itis worth noting that no substantial differences were detected in C1s of the sample produced by using metallorganic and inorganicprecursors. Anyway, due to the complex shape of C 1s signal, thecontribution of organic residual of sol–gel process to the totalcarbon content can not be excluded and distinguished from theenvironmental contamination.

Fig. 6. Potential noise evolution for ZO1 samples after 14 and 15 days ofimmersion.

XPS depth profiling allowed calculation of the thickness ofall the samples. The results of the depth profiling of the sampleZO2 are shown in Fig. 4. Considering that the ion sputteringrate at 2.0 keV energy is about 0.1 nm/min, total analyzedthickness amounts to nearly 30 nm, while the coating is about23 nm thick. Fig. 5 reports XPS depth profile for the sample

Fig. 8. σv and σI for ZO3 sample as a function of immersion time.

Table 3ENA results

Label Rn (Ohm) Onset time (days) Comments

Blank 3.58·104 18 Uniform corrosionC 3.89·104 54 Small pitsF 8.06 ·104 119 No corrosionZO1 4.75·104 15 Several pitsZO2 5.83·104 44 One deep pitZO3 5.65·104 76 Several pitsZI 3.35 ·104 63 Several pitsZO1TC 6.83·106 NA No corrosionZO2TC 4.57·106 NA No corrosionCTC 4.54·106 NA No corrosionFTC 5.49·106 NA No corrosion

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ZO3, with the coating thickness of about 32 nm. The coatingthickness in the sample ZI was estimated to be about 18 nm,thus evidencing a negative influence of the use of inorganicprecursors on the film thickness.

3.3. Electrochemical results

Fig. 6 shows the potential noise evolution for the ZO1samples after 14 and 15 days of immersion. Given theexperimental configuration, potential noise coincides with theopen circuit potential (OCP) fluctuation. The analysis of noisesignals in the time domain, allowed us to find out the initiationtime of corrosion attack. Data acquired up to 14 days ofimmersion are always characterized by small fluctuations. Thefast decreasing and the irregular trend of the OCP after 15 days,suggests the onset of a localised attack. Current noiseacquisitions gave consistent indications with small fluctuationsup to 14 days and spikes starting from 15th day (Fig. 7).

According to the literature data, for oxide-coated metal sys-tems, the corrosion processes should be controlled by the pene-tration of the electrolyte through the coating defects, causing theonset of localised attack [3,4,6,8]. More clear and interestinginformation were obtained by the study of the standard deviationsas a function of time. In Fig. 8 potential and current standarddeviations (σv and σI respectively) as a function of immersion

Fig. 9. σv and σI for ZI sample as a function of immersion time.

time for ZO3 samples are shown. In the potential signal, it can beobserved the occurrence of some spikes during the first days ofimmersion. These spikes are followed by a flat trend until a strongincrease occurred after 80 days. As a consequence of this be-haviour, the current signal does not show any appreciable varia-tion during the first 60 days of immersion. This behaviour can beexplained in terms of a metastable pitting taking place in thefirst part of tests until a stable pitting propagation occurred after80 days.

In the case of sample ZI, although the potential standard de-viation varied in a very narrow range, a clear trend may be detec-ted, as shown in Fig. 9A.During the first 60 days of immersion, analmost constant value of σv was detected and thereafter a slightincrease was observed. σI evidenced a more clear trend (Fig. 9B)consistent with σv variation and very similar with that showedby ZO3, confirming the occurrence of a localised attack after80 days.

The analysis of σv and σI allowed the determination of thepitting onset time for all samples and they are summarized inTable 3.

Fluotitanate samples do not show stable corrosion phenom-ena, although potential fluctuations of higher amplitude weresometimes observed, as in the case of the three acquisitionsshown in Fig. 10, recorded during the 116th day after 30 minfrom each other.

Fig. 10. Potential noise acquisitions performed during the 116th day for Fsample.

Fig. 11. σI for F samples as a function of immersion time.

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In this stage the system was characterized by the increase ofthe potential mean value from − 562 mV to − 553 mV and thecontemporary attenuation of the potential fluctuations with theincreasing of immersion time. This behaviour can be explainedwith the formation of metastable pits. The current standarddeviation plot evidences the days in which metastable pits tookplace (Fig. 11).

A very useful parameter from a practical point of view is theNoise Resistance (Rn) defined as the ratio of σv and σI andformally equivalent to the Polarization Resistance (Rp) [16]. Rn

was calculated for all samples. In particular, Fig. 12 shows the Rn

plot of the ZO1 samples topcoated with organic coating. Themultilayer coatings showed very high Rn, that means very goodcorrosion resistance from the initial immersion time, whichimproved gradually. Similar trend was recorded for all topcoatedprimer samples, and no corrosion attack was observed duringmore than 80 days immersion tests.

Noise data were transposed in the frequency domain bymeans of FFT and MEM algorithms. Both the elaborations weresignificant even though MEM was preferred to FFT, because itproduces smoother plots and the parameter determination ismore accurate.

Fig. 12. Rn for ZO1TC samples as a function of immersion time.

Fig. 13 reports a selection of current PSD plots by MEMalgorithm.

It is known that, the power level is proportional to the kineticrate of the involved reactions [17]. The analysis in the frequencydomain confirms the findings in the time domain. The power levelincreased during initiation and propagation of the attack.Moreover, the slope of PSD plot, estimated after linear trendremoval of measured signals, was recognized in the main lite-rature as the most significant parameter to distinguish betweendifferent form of corrosion [18,19]. From Fig. 13 is clear that,when the corrosive attack occurred, the current PSD slopedramatically increased and, as a consequence, the power level atthe low frequency limit, increased. In particular for the ZO1sample the low frequency plateau increased from 3 ·10−16 A2/Hzto 2·10−15 A2/Hz and the slope increased more than 3 dB/dec inthe period between 14 and 15 days of test.

The increase was also greater for the samples ZO3. In fact, inthe period between 75 and 76 days of immersion, the low fre-quency plateau raised from 2.5·10−17 A2/Hz to 2·10− 15 A2/Hzwith an increasing of the PSD slope more than 8 dB/dec.

The results of the corrosion tests are summarized in Table 3.Amongst zirconia primers, ZO3 sample showed the best corrosionresistance properties, even better than chromate reference samples.ZI samples also gave good results, comparable to the chromate

Fig. 13. MEM power spectra of corrosive attack a) ZO1 sample, b) ZO3 sample.

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primers. Due to the additional passivation treatment performedafter conversion, fluotitanate primers exhibited the best behaviourwith no corrosion at all. As expected, blank tests showed the worstbehaviour, with a uniform attack after 18 days of immersion.Topcoated specimens had an optimal corrosion resistance and noblisters were observed even for long term immersion.

4. Conclusions

The AFM analysis implies that, for all the samples, coveringand structure of the coating were uniform. The grain size rangedbetween 40 and 120 nm for the organic precursors and between120 and 250 nm for the inorganic one. No clear correlation wasfound between roughness and deposition parameters.

From the XPS depth profiles, was determined the thicknessof the coatings: 23 nm in the samples ZO1 and ZO2, 32 nm inthe sample ZO3 and only 18 nm in the sample ZI. The chemicalcomposition was always constant through the coating thickness.

Sol–gel primers increased the corrosion resistance of the alumi-nium AA 1050. Amongst zirconia primers prepared by organicprecursors, sample with two layers deposited and treated at 250 °Cfor 4 min had the best corrosion resistance. Zirconia primersprepared by inorganic precursors showed lower corrosionresistance feature. Both exhibited corrosion resistances comparableto chromates, but inferior to fluotinates. Corrosion resistancefeature of fluotitanate primers was enhanced by a passivation stageat the end of preparation. The topcoated specimens gave an optimalcorrosion resistance.

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