Hexafluorozirconic acid based surface pretreatments: Characterization and performance assessment

Electrochimica Acta 56 (2011) 1912–1924

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Hexafluorozirconic acid based surface pretreatments: Characterization and performance assessment Saikat Adhikari a,∗ , K.A. Unocic a , Y. Zhai a , G.S. Frankel a , John Zimmerman b , W. Fristad b a b

Fontana Corrosion Center, The Ohio State University, Columbus, OH 43210, United States Henkel Corp., Madison Heights, MI 48071, United States

a r t i c l e

i n f o

Article history: Received 2 April 2010 Received in revised form 12 July 2010 Accepted 15 July 2010 Available online 22 July 2010 Keywords: Surface pretreatment Phosphate conversion coating Cold rolled steel Corrosion protection Zirconium

a b s t r a c t A new phosphate-free pretreatment from Henkel Corp. named TecTalis® , was investigated. The treatment bath is composed of dilute hexafluorozirconic acid with small quantities of non-hazardous components containing Si and Cu. The corrosion resistance of treated steel was compared to samples treated in a phosphate conversion coating bath, in simple hexafluorozirconic acid and in TecTalis without the addition of the Cu-containing component. Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used to characterize the coating surface morphology, structure and composition. A Quartz Crystal Microbalance (QCM) was used for studying film growth kinetics on thin films of pure Fe, Al and Zn. Electrochemical Impedance Spectroscopy (EIS) was performed on treated and painted steel for studying long-term corrosion performance of the coatings. The phosphate-free coating provided longterm corrosion performance comparable to that of phosphate conversion coatings. The coatings uniformly cover the surface in the form of 10–20 nm sized nodules and clusters of these features up to 500 nm in size. The coatings are usually about 20–30 nm thick and are mostly composed of Zr and O with enrichment of copper at randomly distributed locations and clusters. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Surface pretreatments are used on metal surfaces before application of organic paints and finishes for better adhesion and improved corrosion protection. One such pretreatment process is the application of chemical conversion coatings, which are formed by precipitation onto the metal substrate from the pretreatment solution [1]. Phosphate conversion coatings have been the most commonly used surface pretreatments for ferrous and non-ferrous metals. The phosphating process finds widespread use in the automotive, agriculture and appliance industries since it is economical and forms a highly adherent surface film that is hard, continuous, insoluble and electrically non-conducting, providing excellent corrosion protection with the application of organic coatings [2]. However, phosphate conversion coatings are being increasingly replaced with various alternatives because of several drawbacks from environmental, energy and process standpoints. Phosphate discharges from the concentrated phosphate baths that are used during the surface treatment [2,3] have a detrimental effect on ground water sources because phosphorous is the most common cause of eutrophication in freshwater lakes and reservoirs

∗ Corresponding author. Tel.: +1 515 451 1497. E-mail addresses: [email protected], [email protected] (S. Adhikari). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.07.037

[4]. Owing to restrictions placed on phosphate discharges, costly waste treatment and disposal methods have to be employed by users. Furthermore, phosphating baths operate above room temperature, from 30 to 99 ◦ C (typically about 50 ◦ C), which requires energy input [2,5]. Another detrimental aspect is that phosphating baths usually form large amounts of sludge, which necessitates frequent desludging to maintain optimum bath operation [2,6]. Furthermore, phosphate coatings sometimes require an additional chromic acid sealing step to reduce the porosity and improve corrosion performance [2]. As a result of these growing concerns over phosphates, new environment-friendly surface conversion coatings need to be developed, without compromising on the corrosion protective performance. Organo-silanes have been used as phosphate replacements. These multi-functional molecules have an affinity for both polymer paints and metal substrate surfaces, thus acting as an adhesion promoter between the two. Although, silanes have shown reasonable corrosion protection and adhesion for aluminum, zinc, and hot dip galvanized (HDG) steel substrates, they perform well only with certain specific paints compared to phosphates, which are suited for a much wider variety of paints and resins [7,8]. Silane baths have shorter lives than phosphate baths and silane treated substrates are more susceptible to flash rusting [9]. In the last decade, another promising pretreatment technique that has emerged as a potential replacement for phosphating is the application of zirconium oxide on the surface by the sol–gel method

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[10–13] or immersion in a hexafluorozirconic acid (H2 ZrF6 ) solution [14–19]. Potentiodynamic tests on mild steel deposited with zirconia by the sol–gel method revealed good corrosion protection performance [10]. Gusmano et al. characterized zirconia primers deposited by the sol–gel process on aluminum 1050 sheets using AFM, XPS and Electrochemical Noise Analysis (ENA). The ZrO2 coatings were found to be amorphous, continuous, about 18–30 nm thick and to provide corrosion resistances comparable to those of industrial chromate coatings [11]. DiMaggio et al. reported corrosion performances better than phosphates for zirconia coatings on low carbon steel deposited via the sol–gel method with an acetic acid complexing agent [12,13]. Commercial H2 ZrF6 and zirconium salts with hydrofluoric acids have also been used as pretreatments for hot dip galvanized and Galfan® coated steels [14–16]. Zr adsorbed on the surface from these treatments was found to be mostly in the form of ZrO2 . The deposited zirconia layer was found to be 50 nm or less on both substrates and exhibited comparable corrosion performance to conventional chromate and zinc phosphate coatings. Verdier et al. studied coatings formed by modified aqueous baths of hexafluorozirconic acid and hexafluorotitanic acid on a Mg–6% Al alloy, AM60, using XPS, SEM and cyclic voltammetry [17,18]. The influence of fluoride concentration in the bath and its pH on coating performance was also investigated to obtain optimal parameters for zirconium or titanium rich films. Zirconium was found to be incorporated in the film in various complexes while titanium was only found in the form of TiO2 . Film formation was found to proceed by precipitation of zirconium or titanium complexes out of solution initiated by increase in interfacial pH resulting from cathodic water reduction reaction. Film formation on AM60 was found to be facilitated by increasing pH and inhibited by increasing fluoride concentrations. In this paper, a new surface pretreatment (commercially available as TecTalis® , Henkel Corp., Madison Heights, MI, USA) based on a hexafluorozirconic acid solution is studied as a replacement for the phosphating process. The treatment process is phosphate-free, can be applied by simple spray or immersion at room temperature, does not require chromic acid sealing and can be used for a variety of industrially-important metal surfaces like steel, aluminum and zinc. The bath is based on dilute H2 ZrF6 (Zr < 200 mg/l) with small quantities of non-hazardous components of Si and Cu added for better long-term performance. This study focuses on high resolution characterization of the physical structure and composition of the TecTalis film using Atomic Force Microscopy (AFM), Scanning Transmission Electron Microscopy (STEM) and X-ray Energy Dispersive Spectroscopy (XEDS) and evaluation of film formation kinetics using a Quartz Crystal Microbalance (QCM). The electrochemical reactivity of the TecTalis coating was investigated for unpainted cold rolled steel (CRS) samples by open-circuit potential (OCP) and polarization resistance measurements (Rp ). The long-term corrosion performance was also assessed and compared to commercial phosphate treatments using Electrochemical Impedance Spectroscopy (EIS) for fully painted CRS samples immersed in sodium chloride solutions. A General Motors Engineering Standards accelerated corrosion test, GM9540P [20] was also used to study the effect of TecTalis pretreatment on the corrosion resistance of painted steel panels. 2. Experimental 2.1. Materials and sample preparation Immersion treatment was performed on unpolished or polished cold rolled steel (ACT Laboratories, Hillsdale, MI) substrates with the following composition: C ≤ 0.1%, Mn ≤ 0.50%, P ≤ 0.030%, S ≤ 0.035%, Fe remainder. For polishing, samples with size about

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1 cm × 1 cm were cut from as received CRS panels and abraded with SiC polishing paper to 1200 grit and then with 1 ␮m diamond paste (in ethyl alcohol). The samples were then ultrasonicated in ethyl alcohol. Before immersion treatment, samples were cleaned with cold deionized (DI) water and then with an alkaline 8% Parco® Cleaner 1200, Henkel Corp., Madison Heights, MI (this step was skipped for polished samples) and then rinsed in warm and cold DI water sequentially. Three treatment solutions were used: hexafluorozirconic acid (FZA) with 60 mg/l Zr, commercially available TecTalis pretreatment solution or TecTalis without the Cu-containing component added to it. The temperature was 30 ◦ C for all treatments except during the QCM experiments, which were performed at room temperature (20–22 ◦ C). The pH of all treatment solutions was adjusted to 4.0 by addition of Parco® Neutralizer 700 (5–15% of ammonium bicarbonate, Henkel Corp., Madison Heights, MI). After treatment, samples were rinsed with cold DI water and air dried. 2.2. Experimental and instrumental approaches OCP and Rp values were monitored for 60 min in aerated 0.1 M Na2 SO4 solution. Each Rp measurement involved sweeping the potential from 10 mV below to 10 mV above the OCP value at a rate of 1 mV/s using a Gamry Reference 600 potentiostat. A three-electrode flat cell was used for the tests. The reference and counter electrodes were saturated calomel electrode (SCE) and Pt mesh, respectively. EIS measurements were made on unpolished, alkaline-cleaned CRS samples that were treated in the coating solution for various times between 0 and 360 s, and then coated with cathodically electrodeposited paint (e-coat), CathoGuard® 310B (available from BASF Corp., Southfield, MI). Plastic cylinders with inner diameter of 6 cm were glued onto the sample surfaces and about 75 ml of 0.5 M NaCl solution was poured into the cylinders. EIS measurements were made periodically using a Gamry Reference 600 potentiostat over a frequency range of 100 kHz to 10−2 Hz with an amplitude of 10 mV around the OCP. The reference and counter electrodes were SCE and a carbon rod, respectively. Fitting of the impedance data was done with Zview2.90 software from Scribner Associates Inc. For the GM9540P accelerated corrosion test, unpolished 4 × 6 CRS substrates were spray cleaned with an alkaline Parco® Cleaner 1533 (Henkel Corp., Madison Heights, MI) for 2 min at 120 F and then rinsed in warm water for 30 s and DI water for 30 s sequentially. The treatment solutions used were FZA, TecTalis and TecTalis without the Cu-containing component added to it, which were functionally equivalent to those used for EIS and polarization resistance measurements. The pH of all treatment solutions was adjusted to 4.0 by addition of Parco Neutralizer 700. After treatment for 90 s at room temperature the panels were rinsed with cold DI water then air dried. Cathodically electrodeposited paint (CathoGuard 310B) was applied to give a film thickness of about 20 ␮m. Panels were scribed for testing as detailed by General Motors Engineering Standard GM9540P [20]. In brief, this accelerated test involves cyclic exposure to a salt spray, then high temperature and relative humidity, then dry-off at intermediate temperature and relative humidity. Samples were exposed for 40 cycles and the width of the scribe creep was measured at the maximum point of creep. QCM measurements were made using 10 MHz AT-cut polished quartz crystals obtained from International Crystal Manufacturing Company Inc. The blank quartz crystal diameter was 1.37 cm and had a pre-deposited gold layer of 0.51 cm diameter and 100 nm thickness. Thin films of pure iron, aluminum and zinc were deposited on top of the gold electrode using a CHA Industries 4pocket electron beam evaporator. The thickness of the deposited films was 100 nm and the area around the gold electrode was cov-

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ered with a Teflon mask during the deposition to ensure that the actual electrode area for the thin film was the same as that of the initial gold electrode. The quartz crystal was then carefully mounted on a specially designed QCM cell obtained from CH Instruments. The quartz crystal was sandwiched between two rubber o-rings to secure the crystal. The deposited metal film (Fe, Al, or Zn) was exposed to the solution chamber of 5 ml capacity with a lid provided with holes for inserting reference and counter electrodes during the experiment. An Ag/AgCl electrode was used as the reference and a Pt wire was used as the counter electrode. A volume of 4 ml of the desired treatment solution (FZA/TecTalis with or without Cu) was introduced into the stagnant solution chamber and the mass change and open-circuit potential were monitored from the instant of introduction of the solution. All combinations of the three substrate metals and the three treatment solutions were studied and each experiment was repeated several times for reproducibility. Rp measurements were carried out separately in the respective treatment solutions for each electrode to calculate the corrosion rate in each solution and correct the mass gains obtained from QCM for corrosion mass loss. An Ag/AgCl reference and Pt wire counter electrode were used along with a Gamry Reference 600 potentiostat for these measurements. AFM analysis was performed on CRS samples that were either alkaline cleaned or polished prior to treatment. Topography and Volta potential mapping (Scanning Kelvin Probe Force Microscopy mode) were performed in air using a Veeco Nanoscope III AFM. Commercially available Pt–Ir coated Si tips were used for all imaging, which was performed in tapping mode. TEM foils were prepared from coatings deposited on unpolished CRS substrates that were alkaline cleaned and treated with either TecTalis or TecTalis without Cu at 30 ◦ C, pH 4.0 for 90 s. The samples were sputter-coated with a thin layer (∼30 nm) of gold to protect the conversion coating during TEM foil preparation with a dual beam FEI Helios 600 Focused Ion Beam (FIB) instrument. All FIB milling was performed using a Ga ion source. To further protect the surface coating from FIB damage, a protective layer of platinum was deposited on the top surface of the region of interest where the TEM foil was extracted. Rough trenches were milled in this location with an ion beam voltage of 30 keV and a current of 9.3 nA. To extract the bulk specimen from the area of interest, an in situ lift out method was performed using an OmniProbe® micromanipulator. The OmniProbe needle was attached to the specimen by depositing platinum on the OmniProbe and selected area on the sample. Afterwards the OmniProbe needle and foil were retracted then joined to a molybdenum TEM grid. The needle was then disconnected from the specimen with the FIB at 500 pA. The last step in specimen preparation was a final thinning operation using at lower ion beam current (100 pA). The final dimensions of the TEM foil were approximately 25 ␮m by 10 ␮m by 100 nm. Scanning TEM (STEM) was used for Z-contrast imaging to obtain compositional and structural information of the very thin conversion coatings. Most of the TEM micrographs in this report were dark-field images obtained with high-angle annular dark-field (HAADF) detector imaging in STEM mode. Some of the dark-field images were inverted, making pseudo bright field images. X-ray Energy Dispersive Spectroscopy (XEDS) was used to characterize the elements and their distribution within the coating. All TEM studies were carried out using an FEI Tecnai TF-20 TEM operating at 200 kV. 3. Results and discussion 3.1. OCP/Rp measurements of unpainted CRS The OCP and Rp of polished and pretreated (but not painted) CRS samples were monitored for 60 min in 0.1 M Na2 SO4 . Fig. 1(a) shows the time-dependence of OCP. The OCP of as-polished steel

Fig. 1. (a) OCP and (b) polarization resistance trends of CRS in 0.1 M Na2 SO4 after 90 s pretreatment in hexafluorozirconic acid (FZA), TecTalis without Cu, and TecTalis and an as-polished CRS sample.

decreased rapidly from an initial value of about −300 mV vs. SCE to a stable value of about −730 mV vs. SCE after 60 min. The OCP values of samples treated in FZA or TecTalis without the Cu were about 300 mV lower than the untreated sample, but decreased slower and stabilized at the same value of about −730 mV vs. SCE. The sample treated in TecTalis showed OCP values higher than those for the uncoated sample, starting near −200 mV vs. SCE but falling to the same value as the other samples by the end of the 1 h exposure time. Fig. 1(b) shows the time-dependence of Rp (testing area of 0.785 cm2 ) for the 4 samples corresponding to the OCP curves in Fig. 1(a). The Rp of the as-polished sample decreased to about 530  cm2 in the first 220 s and then increased gradually to almost 2400  cm2 by the end of the 1 h exposure. The decrease in Rp during the first minutes corresponded to the rapid decrease in OCP. This decrease in OCP during the 20 s measurement time probably resulted in under-prediction of Rp during this period, so these values can be considered to be an artifact. For the samples treated in FZA or TecTalis without the Cu component, the Rp values did not exhibit a decrease at early times, but started at around 1500 and 2100  cm2 , respectively, and then increased steadily reaching 3100  cm2 after 1 h exposure. The absence of the initial decrease in Rp values was probably because the rate of decrease in OCP for these coated samples was much slower than that of the as-polished samples. Since the Rp is inversely proportional to the corrosion rate [21], the slightly higher values of Rp for the pretreated samples in comparison to the as-polished samples indicate that there is a small

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Fig. 2. Trends of impedance magnitude |Z| for pretreated and painted (e-coat) CRS samples immersed in 0.5 mol/l with testing area of 28.3 cm2 . (a) |Z|0.001 Hz () for iron phosphate, zinc phosphate and TecTalis without Cu for 60 s, (b) |Z|0.01 Hz () for treatment times between 0 and 360 s for TecTalis without Cu, (c) |Z|0.01 Hz () for treatment times between 0 and 360 s for TecTalis. Note that 0 s treatment time indicates a clean-only control sample.

protection provided by the zirconia coatings at least up to 1 h of exposure to the mildly aggressive testing solution. The Rp values for the sample pretreated in the complete TecTalis (with the Cu additive) bath exhibited a behavior somewhat similar to that of the as-polished sample, starting at around 2500  cm2 , decreasing to about 1500  cm2 at 500 s and then increasing gradually to values close to 3000  cm2 after 1 h. Again the initial decrease in Rp was probably because of the rapidly decreasing OCP values. In spite of the early decrease, the Rp values for this sample were always higher than those for the as-polished sample, indicating lower corrosion rates and hence slightly better corrosion protection. 3.2. Corrosion resistance of fully painted CRS EIS studies were conducted to compare the corrosion protection behavior of the TecTalis coating with zinc and iron phosphate conversion coatings after the application of paint. Fig. 2(a) shows impedance magnitude values at 0.001 Hz for the e-coated CRS sample with pretreatments of TecTalis (without the Cu component), zinc phosphate or iron phosphate, tested for about 70 days in stagnant 0.5 M NaCl solution. Although there is some scatter in the low-frequency impedance values at early exposure times, the impedance magnitudes for all three pretreatments are approximately 109  or 3 × 1010  cm2 , so the long-term corrosion protection performance of the TecTalis-pretreated samples with a paint top coat is similar to that of Zn and Fe phosphate pretreated steel. CRS samples treated for various times between 0 and 360 s in the TecTalis bath with or without copper and then e-coat

painted were also exposed to a 0.5 M NaCl solution and periodically tested for about 4 months. Fig. 2(b) and (c) shows impedance magnitude values at 0.01 Hz for the two variants of the TecTalis pretreatment. Note that the data in Fig. 2(a) plots impedance values at a lower frequency of 1 MHz and hence shows higher impedance than in Fig. 2(a) for the same 60 s TecTalis (no Cu) treatment. Both coatings, with or without copper, provide very good corrosion protection even up to 4 months of exposure to the NaCl solution as the impedance values were up to 5 × 108 , or about 1.4 × 1010  cm2 . Also, the low-frequency impedance values for TecTalis treated samples were much higher than for the clean-only CRS sample. The sample treated in TecTalis without Cu for 90 s showed much lower impedance around 50 days but it recovered later indicating that the lower impedance value measured was probably an artifact. Similarly, the 30 s TecTalis treated sample shows a drop in impedance values after about 20 days. For treatment times greater than 30 s in the case of TecTalis and 90 s in the TecTalis without Cu, the painted sample shows consistently high impedance values around 5 × 108  or about 1.4 × 1010  cm2 even up to 120 days in the NaCl solution, indicating a high resistance to corrosion. Thus, painted samples for both coatings show very good long-term performance especially for treatment times 90 s or higher. Pretreated and painted steel samples were tested by the GM9540P accelerated corrosion test. Clean-only CRS samples and samples pretreated in FZA, TecTalis without Cu and TecTalis were all deposited with the same CathoGuard 310B paint, scribed and tested after 40 cycles. Table 1 shows the maximum scribe creep width for the 4 test samples. The maximum creep for the clean-only sam-

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Fig. 3. (a) Polarization resistance (Rp ) and corrosion current density (icorr ) for Fe electrode in TecTalis bath. (b) Mass loss due to corrosion for Fe electrode in hexafluorozirconic acid (FZA), TecTalis without Cu and TecTalis. (c) Uncorrected (solid) and corrected (dashed) mass gains for Fe in FZA, TecTalis without Cu and TecTalis.

ple was the highest at 15.8 mm which reduced significantly with a simple FZA treatment to 7.7 mm. The sample treated in TecTalis without the Cu component performed similar to the FZA treated sample showing a maximum scribe creep value of 6.4 mm while the sample treated in TecTalis showed the lowest creep of 3.8 mm. Thus, the TecTalis pretreatment provided a significant improvement in resistance to delamination in the GM9540P accelerated corrosion test, on painted CRS samples. 3.3. Kinetics of film formation A direct comparison of film formation kinetics was performed for thin film pure Fe, Al and Zn substrates using QCM. Baths containing either hexafluorozirconic acid only, TecTalis or TecTalis without the Cu component were used in combination with each of the metal substrates. The mass change with QCM for each solution–electrode combination was recorded for treatment times up to about 30 min which is much longer than usual pretreatment times used in indus-

Table 1 GM9540P accelerated corrosion test results for cleaned, pretreated and then painted CRS samples after 40 cycles. Surface pretreatment

Width of scribe creep (mm)

Clean-only FZA TecTalis without Cu TecTalis

15.8 7.7 6.4 3.8

try. All QCM experiments were conducted at room temperature, which was mostly in the range of 20–22 ◦ C. The pH for each solution was adjusted to 4.0. The mass change data obtained from the QCM experiments are a combination of mass increase on the metal electrode due to coating deposition and mass decrease from metal dissolution in the acidic coating bath. If the mass loss due to dissolution of the metal is larger than the mass gain due to coating deposition, then the QCM will show an overall mass loss during the exposure to the coating bath. Hence it was important to de-convolute the dissolution mass loss from the QCM data, so that mass changes from film formation could be compared. Dissolution mass loss values were estimated from the corrosion current (icorr ), which was calculated from measurements of the polarization resistance (Rp ) for each electrode in the treatment bath. This assumes that the film deposition process is non-faradaic so that the electrochemical measurement of polarization resistance only reflects the electrochemical corrosion processes. Tafel slopes of ±100 mV/dec were used in the SternGeary equation [21] for the determination of corrosion rate from polarization resistance. The values for the polarization resistance and corrosion current in the case of a Fe thin film electrode in a TecTalis bath are shown in Fig. 3(a). The open-circuit potential of Fe changed very rapidly in the treatment bath during the first 4 min of exposure, after which it became stable. Each Rp measurement involved sweeping the potential from 10 mV below to 10 mV above the OCP value at a sweep rate of 1 mV/s. OCP changes within the 20 s time span of the Rp measurement led to inaccurate Rp values, so only values for treatment

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Fig. 4. QCM mass gain for (a) Fe, (b) Al and (c) Zn in hexafluorozirconic acid (FZA), TecTalis without Cu and TecTalis (22 ◦ C and pH 4.0).

times of more than 4 min were used. The values for icorr calculated from Rp were around 2.5–16.5 ␮A/cm2 in all cases. The mass losses from corrosion were calculated by integrating the icorr –t data and are shown for the Fe substrate in Fig. 3(b). Note that this assumes a constant corrosion rate during the first 4 min, which is a source of error given the rapidly changing OCP during that period. Dissolution mass loss for Fe was lowest in FZA and highest in TecTalis, although the differences are not very substantial. The mass loss of 4.6 ␮g/cm2 at the end of 30 min dissolution for Fe in TecTalis (which is the highest among the three solutions) corresponds to a total removal of 6 nm of a pure Fe film. Typical treatment times used in the industry are less than 5 min, in which case the mass loss values go only up to 0.8 ␮g/cm2 . The mass gains during coating deposition for Fe in the 3 treatment baths before and after correction for corrosion mass loss are shown in Fig. 3(c). Since the mass loss due to corrosion was very small at treatment times less than 5 min, there is little difference between the curves representing total mass change and corrected mass change for any of the coating baths. At times greater than 5 min, differences in mass gain due to correction for dissolution loss start becoming substantial. QCM mass gains for Fe, Al and Zn are shown in Fig. 4(a)–(c), respectively. The mass gain directly obtained from the QCM was corrected for corrosion loss (as described for the case of Fe substrate above), although the error was found to be small for all substrates. Coating mass gains for the Fe and Zn substrates were highest for TecTalis and lowest for simple hexafluorozirconic acid at almost all times. For Al, TecTalis mass gains were always highest. For Al in simple hexafluorozirconic acid, the Al film consistently detached from the gold underlayer after about 5–7 min, as seen by the drastic

mass decrease in Fig. 4(b). The reason for this delamination in FZA is not clear, but it was consistently observed. In TecTalis, the coating deposition rate was highest for Al followed by Fe and then Zn. However, it should be noted here that the substrates used for the QCM study were pure metal samples and hence their behavior relative to each other in the treatment baths could be different from that displayed by commercial alloys. A mass change of 10 ␮g/cm2 , corresponding to a 5 min treatment time for Fe in TecTalis, corresponds to a film of thickness 17 nm, assuming that the coating is composed of pure ZrO2 and is uniform in thickness. The data also indicate that the coating mass gain rates (coating deposition rates) decrease after about 10 min as indicated by a change in slope of the mass gain curves. It is possible that after 10 min there is a depletion of reactive metal sites or reactants in the stagnant solution. Note that commercial applications would involve actively agitated and replenished baths. The open-circuit potential values for Fe, Al and Zn in the three pretreatment baths are shown in Fig. 5(a)–(c), respectively. These OCP values were measured simultaneously with the mass change in the same QCM cell and hence the temperature was again 22 ◦ C and pH was fixed at 4.0. For all substrates the OCP initially decreased rapidly corresponding to oxide film dissolution or activation of the surface (note that the decrease in OCP for Zn is so rapid that it is not clearly evident for the time scale shown in Fig. 5(c)). Subsequent increases in values of OCP correspond to precipitation and growth of the coating. It can also be seen that the OCP values for each substrate in TecTalis were always more noble than in hexafluorozirconic acid or TecTalis without the Cu. In short, it can be concluded that coating growth rates are higher for TecTalis com-

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Fig. 5. Open-circuit potentials (vs. SCE) for (a) Fe, (b) Al and (c) Zn in hexafluorozirconic acid (FZA), TecTalis without Cu and TecTalis (22 ◦ C and pH 4.0).

pared to the other two treatment baths used on pure Fe, Zn, and Al substrates. 3.4. Surface morphology AFM topography images of unpolished CRS samples treated in TecTalis with and without Cu are shown in Fig. 6(a) and (b), respectively. Both images are 5 ␮m by 5 ␮m and have a Z-range of 500 nm. The treatment in each solution was 90 s at 30 ◦ C and pH 4.0. The surface in both cases was covered with a large number of particles (nodules) and clusters of these nodules. The number of particles on the surface after TecTalis treatment clearly exceeded the number of particles on the sample after treatment in the bath without copper. The clusters in the case of TecTalis treatment were also larger in size. Fig. 7(a) and (b) shows AFM images of a polished CRS sample treated in TecTalis for 90 s at 30 ◦ C and pH 4.0. As was seen in the unpolished sample, there is a distribution of particles and clusters formed by the coating deposition densely covering the surface. Fig. 7(a) shows 2 ␮m by 2 ␮m region with a typical distribution of the large clusters found on the surface. The clusters seen with TecTalis were 100–500 nm in diameter and around 50–200 nm in height. Fig. 7(b) presents a 1 ␮m by 1 ␮m 3-D image of another region that had no large clusters so it represents the background film between the large clusters. The particles in this region are as small as 20 nm in diameter and about 10 nm in height. With increasing treatment time, the number of particles and clusters and the size of the clusters increased. The Volta potential values exhibited by the surface were also monitored in the SKPFM. To avoid errors in Volta potential mea-

surements arising from tip variations, all potential measurements were calibrated by comparison to potential measured for pure Ni, which has been found to have a stable potential [22]. Since the Volta potential measurements done in air are linearly related to open-circuit potentials in solution, they give a measure of the local nobility of the surface [22]. Fig. 8 shows Volta potential values measured in air for polished CRS treated in TecTalis for various times between 0 and 360 s. The potential values increased from about +50 mV for clean-only CRS to +475 mV for a 360 s TecTalis treated CRS sample, indicating the surface became more noble with increasing treatment times in TecTalis. Potential maps (not shown here) indicate that the Volta potential values for the clusters were about 20–30 mV higher than the remainder of the coated surfaces. Note that the Volta potential for the entire surface increased with treatment time indicating that a uniform layer of coating was present on the entire surface with the clusters on top of it. 3.5. Coating structure and composition Analytical TEM of TecTalis coating with and without copper on CRS substrate was performed to study the coating structure and composition. All TEM analysis was performed on unpolished CRS substrates, cleaned and treated with TecTalis (with or without copper) for 90 s at 30 ◦ C with the pH adjusted to 4.0. TEM HAADF micrographs of TecTalis-coated CRS with and without the copper in the solution are shown in Fig. 9. After the deposition of the coating, a thin layer of Au (∼30 nm) and a thicker layer of Pt (more than a micron) were deposited on the top to protect the coating during FIB milling. Low camera length and the HAADF detector were used, which results in reduction of the

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Fig. 7. Topography images of polished CRS sample treated in TecTalis. (a) 2-D topography image, image size: 2 ␮m × 2 ␮m. Z-range: 200 nm. (b) 3-D topography image of another region without large clusters, image size: 1 ␮m × 1 ␮m. Z-range: 50 nm. Fig. 6. Topography images of unpolished CRS samples. Image size: 5 ␮m × 5 ␮m. Z-range: 500 nm. (a) TecTalis without Cu and (b) TecTalis.

diffraction scattering effect and enhancement of the Z-contrast effect. Therefore, the high atomic number (Z) elements such as Au and Pt appear to be brighter than the elements with lower atomic number, like Fe. The coating appears to be darker than the substrate, which implies a lower density or a lower average Z, probably because of the presence of Zr oxide and C in the coating. The coating has a few brighter spots about 5–10 nm in size, indicating higher average Z areas, probably richer in Zr. The coating on the sample treated in TecTalis without the copper was about 20 nm thick and the TecTalis coating was 30 nm thick, even though the treatment conditions and time were the same for both samples. This small difference in the coating thicknesses was reproducible and was seen in all STEM micrographs and also in the EDS line scans shown later. Also, with both coatings, the coating was detached from the substrate at various randomly spread locations. These gaps were most likely formed during the FIB milling and were not present in the deposited coatings.

Fig. 8. Volta potentials for CRS (vs. Ni reference surface) treated in TecTalis for various times (at pH 4.0, 30 ◦ C).

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Fig. 10. EDS line scan through the TecTalis without Cu coating on CRS. (a) STEM HAADF micrograph showing the position of the scanned line. (b) Profile of the wt.% for all detected elements. Line scanned from the top (inside the Pt layer) to the bottom (inside the substrate).

Fig. 9. STEM HAADF micrographs of coated CRS (a) TecTalis without copper and (b) TecTalis.

EDS line scans were carried out across both coatings and the respective counts for the detected elements were converted to composition by wt.% along the EDS scan lines. A STEM micrograph and the corresponding EDS line scan for a layer of TecTalis without Cu on a CRS sample are shown in Fig. 10(a) and (b), respectively. The location of the line scan is indicated in Fig. 10(a). The square in Fig. 10(a) was used as a drift correction image during the EDS data acquisition to minimize drift and to maintain the exact location of EDS analysis. As shown in Fig. 9(a), the coating was about 20 nm thick and had gaps within the coating produced by the FIB milling. The EDS line scan in Fig. 10(b) passed through a region of coating detachment. The point at which the Au concentration decreases sharply and the Zr concentration increases represents the interface between the gold and the oxide coating. The coating/CRS interface is the position where Zr level decreases to a value less than 1 wt.% and Fe level reaches its maximum constant value. It can be seen that the coating mostly is composed of Zr (up to 80 wt.% at the center of the coating). Fe is also present in the coating, although it is

mostly enriched closer to the coating/CRS interface. At the position on the scan corresponding to the gap in the coating, there is a dip in the Zr wt.% indicating that the coating was not present in this region. Another rise in the Zr concentration is evident after the dip and closer to the CRS substrate, indicating that this gap was present in the coating and not at the coating–CRS interface. The presence of the coating underneath the gap is further indication that it was formed by damage to the coating during FIB and not during the coating deposition. The O content in the coating was about 6–10 wt.%. Inside the coating, the average Zr:O atomic ratio was 1:0.53. Therefore, the coating does not correspond well to a composition of ZrO2 . However, the standardless analysis of composition is only approximate for the determination of O concentration [23]. The Si and F content in the coating were found to be less than 2 wt.% throughout the coating thickness. Some of the samples treated in this bath also showed an enrichment of F near the coating–CRS interface. The STEM micrograph and corresponding EDS line scan for a CRS sample treated in TecTalis are shown in Fig. 11(a) and (b), respectively. The location of the line scan is shown in Fig. 11(a). It can be seen that the TecTalis coating was about 30 nm thick and mostly composed of Zr (70–80 wt.%). One of the major differences relative to the TecTalis without copper coating is the distribution of Zr

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Fig. 11. EDS line scan through the TecTalis on CRS. (a) STEM HAADF micrograph showing the position of the scanned line. (b) Profile of the wt.% for all detected elements. Line scanned from the top (inside the Pt layer) to the bottom (inside the substrate).

and O. For example, the Zr content in TecTalis coating increased up to about 80 wt.% close to the Au–coating interface and stayed high throughout the coating. In the coating formed in the bath without the copper additive, the Zr content increased slowly to a sharp peak at the middle of the coating thickness and then dropped back slowly to low values at the coating–CRS interface. The TecTalis coating has a higher Zr content inside the coating. Similarly, the oxygen content in the TecTalis coating also increased sharply at the Au/TecTalis interface, stayed at about 16–20 wt.% within the coating and then decreased to values close to zero at the TecTalis/CRS interface. Thus, Zr and O are more uniformly distributed in the TecTalis coating. Also, the Zr:O atomic ratio was 1:1.73. Hence, for TecTalis, the coating composition closely corresponds to the composition of ZrO2 . Fe was also present inside the TecTalis coating and was mostly enriched close to the TecTalis/CRS interface. The Si and F content again stayed very low (between 1 and 3 wt.%) and F was again enriched close to the metal substrate. The Cu content usually stayed lower than 2% throughout the coating thickness but increased to values of 8–10 wt.% at some locations in the coating (25 and 49 nm positions in Fig. 11(b)). Interestingly, these locations did not correspond to any visual features in the TEM image of Fig. 11(a).

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Fig. 12. EDS line scan through the TecTalis on CRS. (a) STEM HAADF micrograph showing the position of the scanned line. (b) Profile of the wt.% for all detected elements. Line scanned from the top (inside the Pt layer) to the bottom (inside the substrate).

The only other compositional feature seen is the slight decrease in the Zr and O content at the positions where an increase in Cu is noticed. Broad interpretation of TEM analyses can be dangerous because they represent only an extremely small part of a sample. It is essential to study multiple locations on multiple samples to be sure of trends. Fig. 12(a) is an image of a different region of the TecTalis coating on the same CRS sample. Although in Fig. 11 the enrichment of Cu was seen close to the top and bottom interfaces of the coating, Fig. 12 shows that Cu enrichment can also be in the interior of the coating. Fig. 12(b) corresponds to the line indicated in Fig. 12(a). For this region of the TecTalis coating, the Zr:O atomic ratio was found to be 1:1.99. A 10 nm region in the interior of the coating where the Cu content increased up to 18 wt.% can be seen. Again, the Zr and O content decreased slightly from their constant values within the coating. Thus, this increase in Cu content probably corresponds to a 10 nm size particle or region rich in Cu. There was no visual feature in the TEM image (Fig. 12(a)) corresponding to this increase in Cu, nor is there a change in any other elemental composition at this location other than Cu, Zr and O. Hence it is possible that the particle is actually a Cu deposit in its ground state

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Fig. 13. EDS line scan through the TecTalis on CRS. (a) STEM HAADF micrograph showing the position of the scanned line. (b) Profile of the wt.% for all detected elements. Line scanned from the top (inside the Pt layer) to the bottom (inside the substrate).

(Cu0 ) rather than a Cu compound. Also, because the Cu enrichment does not correspond to any visual feature or compositional changes in the coating (other than reduction in ZrO2 content), it seems that the Cu deposits are randomly distributed and not restricted to any specific locations within the coating. As shown in Fig. 7(a), AFM imaging of pretreated surfaces revealed clusters of particles about 100–500 nm in diameter and 50–200 nm in height above the coated surface. The Volta potential values of these clusters were also found to 20–30 mV higher than the rest of the coating. Fig. 13(a) shows an STEM micrograph of a region on TecTalis-coated CRS where the coating thickness was as much as 150 nm at some distinct locations. Fig. 13(b) shows the EDS line scan through one of the mounds as shown in Fig. 13(a). The Pt and Au contents were intentionally omitted in Fig. 13(b) to make the Cu, Zr and O lines easier to identify. The EDS line scan intersected a mound that was about 150 nm high above the otherwise thin coating with diameter at the base of about 130 nm. Cu was the most dominant element present in this region with concentration of 40–60 wt.%. Zr (10–20 wt.%) and O (5–10 wt.%) were the other major components in this region. Very little Si, F or Fe

Fig. 14. (a) FE-SEM image of iron fine and (b) STEM HAADF micrograph showing a FIB section through the iron fine.

were observed and the Fe and F were mostly enriched closer to the coating–CRS interface. The higher Cu content of the mound is in line with the higher Volta potential for the clusters in the SKPFM images. The agreement in the physical dimensions and the expected Volta potential trends for the mound observed in TEM and the clusters observed by AFM suggests that they are probably the same and that the clusters are regions in the TecTalis coating with very high copper content. Fig. 6 shows that the cluster size and density were larger for samples treated in a bath containing Cu, which also supports the notion that the clusters are Cu-rich. It has been suggested that having clean surfaces free of contaminants is very important for coating deposition processes on steel since the properties for deposited coatings are directly affected by residual compounds resulting from industrial preparation processes [24–27]. In spite of all the rigorous cleaning procedures used, amorphous carbon (formed by cracking of rolling-lubricants during annealing) and embedded or protruding small particles of Fe called iron fines (originating from the rolling process), are usually found on the surface of cold rolled steel [25]. Fig. 14(a) shows an FE-SEM

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Fig. 15. EDS line scan through a portion of the iron fine after TecTalis pretreatment (a) STEM HAADF micrograph showing the position of the scanned line. (b) Profile of the wt.% for all detected elements. Line scanned from the top to the bottom.

image of a protruding particle on TecTalis-coated CRS that is about 2 ␮m in the largest dimension. The small bumps on the surface in Fig. 14(a) are associated with the thin layer of Au (∼30 nm) that was deposited on the entire surface. Fig. 14(b) shows an HAADF STEM micrograph of a section through the same particle. Before the FIB sectioning of the sample, Pt was also deposited on top of the gold to further protect the TecTalis coating from ion beam damage. This particle is apparently associated with an iron fine, indicated by the Fe-containing structure embedded in the particle as will be shown below. A variety of structures was created by exposure of the iron fine and CRS surface to the TecTalis treatment bath. The CRS substrate region directly beneath the iron fine was darker than the rest of the CRS to a depth of about 250 nm. Darker contrast in this HAADF image indicates lower density or lower Z number. EDS line scans were performed through various parts of the iron fine to identify the compositional distribution. Fig. 15(a) and (b) shows the HAADF STEM micrograph and the corresponding EDS profiles through one part of the fine and through the dark region. The end of the scan (at positions greater than 380 nm on the x-axis) represents the CRS substrate, which is composed primarily of Fe. The dark region below the particle (at positions between 210 and 380 nm) is a layer com-

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Fig. 16. EDS line scan through a portion of the iron fine after TecTalis pretreatment (a) STEM HAADF micrograph showing the position of the scanned line. (b) Profile of the wt.% for all detected elements. Line scanned from left to right.

posed of entirely of Fe and O. Above this iron oxide region (positions from 170 to 210 nm) is a layer containing Zr and O. The iron fine was apparently not well connected to the surface, which allowed the TecTalis film to coat both the underside of the particle and the surface of the metal under the particle. It is possible that depletion of Zr in this crevice region limited the amount of film formation, but that reaction with the acidic solution allowed continued attack of the CRS substrate to form the dark oxide structure under the particle. Above the Zr oxide layer (brighter region from 80 to 160 nm) is a region rich in Fe and also some O. This is likely the main part of the iron fine. A carbon rich region (0–30 nm) exists near the top surface of this Fe-enriched region. Fig. 16(a) and (b) shows the HAADF STEM micrograph and the corresponding line scan through the top part of the fine. This region shows a high Cu content (20–40 wt.%) with Zr (15–40 wt.%) and O (5–15 wt.%) being the other two major components, which is more typical of Cu-enriched clusters as seen in Fig. 13. This part of the coating is probably formed directly by exposure of the top surface of the iron fine to the coating solution. Unreacted parts (brighter looking regions) of the iron fine are also seen clearly in Fig. 16(a).

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4. Conclusions

References

Zr oxide films formed on metal surfaces from immersion in fluorozirconic acid baths as pretreatments for painting were studied by a variety of approaches. The following was observed:

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1. TecTalis-pretreated and painted CRS exhibited good long-term corrosion protection, comparable to that of phosphate conversion coatings, as indicated by EIS measurements. 2. QCM measurements indicated that the Zr oxide coating growth rate is highest for TecTalis followed by TecTalis without Cu and lowest for the hexafluorozirconic acid only bath for pure Fe, Al and Zn thin film substrates. 3. TecTalis treated CRS surfaces are covered with small nodules ∼20 nm in size and clusters of these features that were 100–500 nm in diameter. Clusters exhibited 20–30 mV higher surface potentials than other parts of the coating. 4. The TecTalis coating is about 30 nm thick, while the TecTalis coating without the Cu is slightly thinner at about 20 nm. The TecTalis coatings are mostly composed of Zr and O with Fe and F enriched closer to the coating–CRS interface. Cu is enriched, even up to 50–60 wt.% at some locations within the TecTalis coatings and also inside the clusters of particles seen on the surface by AFM. 5. A particle associated with an iron fine resulted in a similar surface morphology as the clusters, but exhibited a complex structure. Acknowledgements The authors gratefully acknowledge Henkel Corp. (Madison Heights, MI, USA) for financial support. The authors also wish to thank Bruce Goodreau, Brian Bammel, Kirsten Lill and Kevin Meagher at Henkel Corp. for numerous interesting discussions. TecTalis® and Parco® are registered trademarks of Henkel Corporation and its affiliates in the US and globally. Galfan, Cathoguard and Omniprobe are trademarks and/or registered trademarks of their respective owners.