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Electrochimica Acta 48 (2003) 1115 /1121 www.elsevier.com/locate/electacta Precursor sites for localised corrosion on lacquered tinplates visualised by means of alternating current scanning electrochemical microscopy Bernardo Ballesteros Katemann a, Carlota González Inchauspe b, Pablo A. Castro c, Albert Schulte a, Ernesto J. Calvo b, Wolfgang Schuhmann a,* b a Analytische Chemie-Elektroanalytik & Sensorik, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany INQUIMAE */Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires Pabellon II, Ciudad Universitaria, AR-1428 Buenos Aires, Argentina c CINI */Centre for Industrial Research, FUDETEC, Dr. J. Simini 250 (2804), Campana, Buenos Aires, Argentina Dedicated to Professor Dr. Joachim Walter Schultze on the occasion of his 65th birthday. Abstract In solutions of low conductivity and at high frequencies the impedance of a SECM tip-auxiliary electrode cell is dominated by the solution resistance between the tip and counter electrode. Alternating current scanning electrochemical microscopy (AC-SECM) utilises the effect of an increasing (decreasing) solution resistance as the SECM tip approaches an insulator (conductor) for mapping domains of different conductivity/electrochemical activity on surfaces immersed into electrolytes. In the present study, we employed AC-SECM in aqueous solutions to evaluate the integrity of the solid/liquid interface of lacquered tinplates as commonly used in industry to manufacture, i.e. food cans. Significant differences were determined between the AC response and the phase shift measured with the SECM tip above the intact coating and above defects where the surface of the steel base is exposed. This allowed with high lateral resolution to detect and to visualise artificial micro cavities which we consider as an experimental model of microscopically small precursor sites for localised corrosion. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Scanning electrochemical microscopy; SECM; Electrochemical impedance spectroscopy; Localised corrosion; Lacquered tinplates; ACSECM 1. Introduction Various types of high-performance coatings have been developed and applied in industry to metallic bulk articles such as household appliances, food cans and automotive components in order to protect them * Corresponding author. Tel.: /49-234-322-6200; fax: /49-234321-4683; http://www.ruhr-uni-bochum.de/elan. E-mail address: [email protected] (W. Schuhmann). against corrosion, the environmental degradation of metals and alloys, and to improve their consumers appeal. Frequently used coatings include for instance organic paints, enamel, thin layers of corrosion-resistive metals like tin and chromium, films of passive oxides and combinations of these systems. Independent on the nature of the coating, it should be chemically inert, resistant to mechanical or thermal stress and free of any cracks and holes to ensure a well-operating protection to the underlying metal from any corrosive attack. Nonetheless, the coatings can become damaged during 0013-4686/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0013-4686(02)00822-8 1116 B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121 manufacturing procedures and/or continuous use, and failures in the protecting layer certainly can cause severe corrosion damage to the products over time. For this reason, corrosion monitoring is of major interest in industry not only because of safety concerns and because of the risk of lost production and/or product contamination but also on account of the development of preventive, predictive and corrective maintenance practices. Conventional electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and the measurement of steady-state polarisation curves [1,2] have been widely used to study corrosion and provided important informations about the global properties of the metal/coating interface and its large-scale corrosion activity. However, coated surfaces are typically not corroding uniformly but confined to very small surface areas (i.e. microscopic pits, cracks and pores) without any obvious superficial signs of deterioration. Moreover, the appearance of macroscopic (visible) symptoms of corrosion is only an expression of the final stages of a complex, dynamic sequence that starts at the microscopic level, making its detection and monitoring a challenging task. Recognition of the phenomena of localised corrosion has led to several attempts to spatially characterise the corrosion activity of microscopic defects in the metal/coating interface with high lateral resolution. In fact, localised electrochemical impedance spectroscopy (LEIS) [3], the scanning reference (SRET) and scanning vibrating (SVET) [4] electrode techniques, as well as the scanning Kelvin probe technique [5], the approach of the scanning droplet cell [6,7] and scanning electrochemical microscopy (SECM) [8,9] are amid the micro electrochemical methodologies currently subjected in a number of laboratories to investigate the process of localised corrosions. Based on an early work from Horrocks et al. [10], we recently developed alternating current scanning electrochemical microscopy (AC-SECM) as a novel tool for measuring local interfacial impedance properties with high lateral resolution [11]. AC-SECM combines electrochemical impedance measurements with SECM. In brief, a high-frequency alternating voltage is applied between the SECM tip and a counter electrode, the AC current response to the voltage perturbation is measured using a lock-in amplifier or frequency response analyser, and, using phase-sensitive demodulation, the impedance of the 2-electrode arrangement is determined. This impedance can be described by a simple RC equivalent circuitry consisting of a series combination of the solution resistance between the two electrodes (Rsol) and the double-layer capacitance of the tip electrode (Cd). In electrolytes of sufficiently low conductivity and at high frequencies the impedance of the circuitry is dominated by Rsol which was found to be strongly depending on the tip-to-sample distance and the nature of the sample material. In fact, the impedance increased (decreased) with the tip electrode approaching an insulator (conductor). Approach curves (plots of the AC response as a function of distance) were used to position the SECM tip at an appropriate working distance in a similar way as earlier described [11,12] and imaging in the AC mode of SECM was achieved by scanning the tip electrode across the sample surface (x , y -plane) and monitoring either the AC response or its phase shift as a function of tip location. This approach of local AC-response measurements allowed clearly to identify and to visualise microscopic domains of different conductivity/electrochemical activity present on an array of Pt-band microelectrodes immersed in a mediator-free electrolyte. In the present study, AC-SECM was used to examine in-situ the metal/coating interface of lacquered tinplates which are commonly used in industry to manufacture metal food containers. It is demonstrated, that ACSECM allowed to visualise microscopic cracks and holes in the coating of the lacquered tinplates with high lateral resolution. As a matter of fact, these local defects in the corrosion-resistive tin/polymer coating are leading to an exposure of the underlying steel and have to be considered as potential precursor sites for localised corrosion. 2. Experimental 2.1. Chemicals, samples and microelectrode preparation The electrolytes were prepared using double-distilled water and the impedance measurements performed in inert solutions of either 1 /10 mM sodium or potassium chloride (Sigma-Aldrich, Deisenhofen, Germany) or aqueous solutions of 1.5% sodium citrate (SigmaAldrich) and 1.0% sodium chloride with no added redox mediator. The tips for the AC-SECM studies were glassinsulated, disk-shaped Pt microelectrodes (12.5 mm disk radius) which were fabricated from 25 mm diameter Ptwires (Goodfellow, Bad Nauheim, Germany) following a procedure as previously described [13]. Lacquered tinplates were provided by the iron and steel industry (Siderar, Argentina). They consist of a low-carbon steel covered with a thin layer of electrodeposited tin and additionally lacquered with a film of an organic solventbased epoxyphenolic varnish (film thickness 9/15 mm). Artificial micro cavities such as holes and cracks (see insets in Figs. 2 and 4A) were introduced in some of the samples by mechanically puncturing or scraping their B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121 polymer coating using the sharp end of an electrochemically etched tungsten STM tip. 2.2. Instrumentation Conventional electrochemical impedance spectroscopy on lacquered tinplates was carried out in a 1compartment electrochemical cell, in 3-electrode configuration with a Pt-mesh counter and a Ag/AgCl reference electrode and using a frequency response analyser (S5720C, NF Corporation, Yokohama, Japan) and a potentiostat/galvanostat (Model IMP 88 PG, Jaissle Elektronik GmbH, Waiblingen, Germany) for the measurements of impedance spectra. If not stated otherwise, AC-SECM measurements were performed in a two-electrode configuration, with the SECM tip as working and the sample (the tinplate) as counter electrode. The electrochemical cell with the tinplate fixed to its bottom was mounted on the threeaxis translation stage of a home-built SECM (for details of its design see Ref. [14]) driven by computer-controlled stepper motors having a nominal resolution of 0.6 mm per half step in each direction. A frequency response analyser (same as mentioned above) was used to apply a sinusoidal voltage perturbation (typically 10/100 mVpp at frequencies of 1/10 kHz) to the electrodes held at open circuit potential. The AC current response of the system was measured with a current amplifier providing a current sensitivity of 104 /1011 V A 1 (Model 427, Keithley Instruments, Cleveland, USA). The output of the current amplifier was fed into the input of the frequency response analyser which in turn provides the modulus Z (under the experimental conditions representing the solution resistance between the tip and counter electrode) and the phase shift (u ) of the AC signal of the electrode arrangement. Both, Z and u are parameters which are dependent on the tip-to-sample distance and the sample’s conductivity and in principle can be used for imaging with a SECM operated in the AC mode [11]. However, in this study, Z was chosen as a means to monitor the tip-to-sample distance for recording of approach curves and for tip positioning, and, in contrast to recent results by Wipf et al. [12], even as a function of the x - and y -tip position for AC-SECM imaging. A PC in combination with a Windows software programmed in Microsoft VISUAL BASIC 3.0 (Microsoft, Redmont, USA) was used to control all system parameters and for data acquisition. Alternatively, AC-SECM was performed using a lockin amplifier (PAR 5210, Perkin Elmer, Bad Wildbad, Germany) instead of the frequency response analyser in order to generate the sinusoidal voltage perturbation and to analyse the systems AC response. In these experiments, a highly sensitive potentiostat (PG100, 1117 Jaissle Elektronik GmbH, Waiblingen, Germany) was used to operate the electrochemical cell in 3-electrode configuration, with the SECM tip as working, a Pt-wire as counter and a Ag/AgCl as pseudo-reference electrode. The lock-in amplifier provided the current magnitude R of the AC current response with respect to the reference signal. Approach curves (R vs. d ) were recorded and used for tip positioning whereas monitoring R as a function of x and y tip position allowed imaging. 3. Results and discussion The lacquered tinplates were studied first using conventional (global) electrochemical impedance spectroscopy [15]. Impedance spectra were measured on samples immersed into an aqueous solution of 1.5% sodium citrate and 1.0% sodium chloride as a function of the immersion time. A significant decrease in the impedance modulus was observed with an increasing time of exposure, especially at lower frequencies (see Fig. 1). This drop in impedance is most likely due to a swelling of the organic film induced by an uptake of water [16]. In addition, impedance spectra were recorded on lacquered tinplates with an increasing number of artificial micro cavities in their polymer coating which were considered as experimental models for microscopically small precursor sites for localised corrosion. As expected, the impedance spectra displayed appreciable differences in that with an increasing number of defects the impedance modulus notably decreased (see Fig. 2). These two experiments are a good proof that measuring global impedance spectra offers valuable information Fig. 1. Impedance spectra (Bode plots) of a lacquered tin plate (exposed area: 12 cm2) immersed into an aqueous solution of 1.5% sodium citrate and 1.0% sodium chloride and recorded at different times of exposure to this electrolyte. The amplitude of the voltage sign wave used for perturbation was 50 mVpp with frequencies ranging from 0.1 Hz to 10 kHz. 1118 B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121 Fig. 2. Impedance spectra (Bode plots) of lacquered tin plates (exposed area was 12 cm2) with an intact coating and with a various number of artificial micro cavities (pin holes) introduced by mechanically puncturing the protecting polymer film. The EIS measurements were conducted in an aqueous solution of 1.5% sodium citrate and 1% sodium chloride. The amplitude of the voltage sign wave used for perturbation was 50 mVpp with frequencies ranging from 0.1 Hz to 10 kHz. Inset: Optical image presenting four of the pin holes in the coating of the tinplate. about the condition of a polymer-coated surface. However, these measurements are not capable of providing details about local properties of the surface of the sample. For instance, global impedance spectra do not allow to gain an insight in the number and size of defects possibly be present in the coating due to the fact that the signal is an averaged response of the entire surface of the sample exposed to the electrolyte. Thus, no difference between a low density of large pores and a large density of small pores in the coated metal can be observed. This limitation of global impedance spectroscopy gave good reason for applying the approach of AC-SECM to the lacquered tinplates with the aim to detect and visualise individual microscopic defects in their organic coating with high lateral resolution. In a typical AC-SECM experiment, a sinusoidal voltage perturbation with frequencies not smaller than 1 kHz was applied in a diluted solution of KCl (1 /10 mM) between the SECM tip and the counter electrode (sample), causing a small alternating current to flow. As already mentioned above, Z , the impedance modulus of such a two-electrode arrangement, is in solutions of low ionic strength and at high enough frequencies dominated by the solution resistance Rsol and was found to be strongly dependent on the tip-to-sample distance (d) and the chemical nature of the sample material. Approach curves (Z vs. d ) were recorded with a 12.5mm-radius Pt micro disk electrode approaching a lacquered tinplate or a Au surface which was used for simplicity reasons as a model of the conducting steel surface underneath the coating of the tinplates. As can be seen in Fig. 3A, at d values of a few times the tip diameter, the surface of the insulating lacquer starts to hinder the current flow leading to an increase of the modulus Z . In contrast, with the tip getting closer to the Au surface, Z decreased because the proximity to the conducting Au-surface enhances the current flow and lowers Rsol (see Fig. 3B). The nature of these approach curves is the principle prerequisite for imaging domains of different conductivity on the surface of the coated tinplates by means of AC-SECM. Obviously, the increase (decrease) in Z as the SECM tip is getting close to an insulator (conductor) follows a similar dependence as that of the tip current B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121 1119 sample surface (at distances of just about the radius of the SECM tip, see Fig. 3A). Imaging in the AC mode of SECM was achieved by scanning the SECM tip in constant height in the x, y-plane above the tinplates and monitoring Z (R ) as a function of tip location. Using this approach, we scanned lacquered tinplates with artificial scratches or pinholes of microscopic dimensions in its protecting polymer layer. As demonstrated in the Figs. 4 and 5, the lateral variations in the electrical conductivity between regions with an intact coating and regions with failures were clearly detected because of the remarkable differences in the observed AC signal and hence allowed to visualise the defects with a high spatial resolution. Single line scans (see Fig. 4A) were used to position the tip electrode either right above the centre of a scratch at an area where the steel base of the tinplates is Fig. 3. Comparison between approach curves (normalised modulus Z as a function of tip-substrate separation) obtained on (A) a lacquered tinplate and (B) a gold plate, both measured in 10 mM KCl with a 12.5 mm radius Pt micro disk electrode. The amplitude of applied sine voltage used for perturbation was 10 mVpp at a frequency of 10 kHz. obtained with SECM in the (amperometric) feedback mode in solutions containing an appropriate redox mediator [10]. AC-SECM offer the advantage over SECM in the feedback mode in that the measurements can be performed in solutions free of any redox mediator avoiding perturbations of the chemistry at the metal/coating interface otherwise at risk because of undesirable reactions between the electrochemically active species and the metal. Prior to imaging the surface of lacquered tinplates, approach curves (either Z or R , the current magnitude vs. d ) were measured and used for positioning the SECM tip at an appropriate working distance within the feedback range. Moving towards an area of intact coating, the approach was typically stopped when the values for Z (R ) changed by about 50% with respect to their values with the tip in infinite distance from the Fig. 4. (A) A single line-scan displaying the modulus Z (normalised to values Zo measured above the intact coating) as a function of the tip position in x -direction at the same time as the tip moves in constant height across a microscopic scratch in the coating of a lacquered tinplate. The measurement was performed using a 12.5 mm radius Pt micro disk electrode as the SECM tip, 10 mM KCl as the electrolyte, and a perturbation of 20 mV at 5 kHz frequency. Inset: optical image of the scratch. (B) 3D-image of the scratch shown in (A) obtained with AC-SECM in a constant-height mode and using multiple line scans (same parameters as described in A). 1120 B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121 Fig. 6. Local electrochemical impedance spectra with the SECM tip electrode kept in fixed position either above the intact region of the coating of a lacquered tinplate (%) or above the centre of a scratch (m) as for example shown in Fig. 4. The spectra (100 Hz to 50 kHz) were measured in 10 mM KCl with a disk-shaped 12.5-mm-radius Pt microelectrode and a 20 mV perturbation. Fig. 5. AC-SECM images of microscopically small pin holes in the coating of lacquered tinplates. (A) 2-D image of an array of pin holes (overview). (B) 3-D image of one pin hole at higher magnification. Measurements were performed in 1 mM NaCl and in a 3-electrode configuration with a Pt counter and a Ag/AgCl pseudo reference electrode. The images were obtained with AC-SECM in a constantheight mode operating a disk-shaped, 12.5 mm radius Pt microelectrode (working electrode) with a perturbation of 100 mV at 1 kHz frequency. exposed to the electrolyte or above the intact coating just next to the defect. Microelectrode impedance spectra with frequencies ranging from about 100 Hz to 50 kHz were then recorded at these discrete locations with the tip held in fixed position. Although the impedance values in spectra as shown in Fig. 6 actually are a measure of the solution resistance, they still indirectly reflect the specific nature of the surface beneath the sensing surface of the SECM tip since obviously the solution impedance is modulated by the properties of the sample material. In fact, the impedance moduli were found to be significantly higher with the tip above the intact coating as compared with the ones obtained with the tip above the micro cavities. These differences were largest in the higher frequency domain of the impedance spectra. For this reason, frequencies not smaller than 1 kHz have been used for imaging tinplate samples in the AC mode of SECM. In conclusion, AC-SECM was used to characterise the solid/liquid interface of lacquered tinplates in contact with aqueous solutions. We provided evidence that ACSECM has the capability to distinguish between regions on the samples surface with an intact coating and with failures in the protecting polymer film. Furthermore, microscopic domains of a varying conductivity/electrochemical activity on surface artificial micro cavities such as cracks and pinholes used as a representative model of precursor sites for localised corrosion were clearly identified and properly visualised at a high spatial resolution. Acknowledgements We gratefully acknowledge the financial support of this work by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the SFB 459, project A5. The exchange of scientists between Germany and Argentina was supported by PROALAR grant jointly funded by the DAAD (Germany) and ANPCyT (Argentina). CGI gratefully acknowledges a postdoctoral fellowship under the industrial scheme of CONICET (Argentina) and FUDETEC (Technit. Corp.). References [1] P.R. Roberge, Handbook of Corrosion Engineering, McGrawHill, New York, USA, 1999. [2] F. Mansfeld, M. Kendig, Werkstoffe Korrosion 36 (1985) 473. [3] F. Zhou, D. Thierry, H.S. Isaacs, J. Electrochem. Soc. 144 (1997) 1957. [4] J.W.H. de Wit, D.H van der Weijde, A. de Jong, F. Blekkenhorst, S.D. Meijers, Electrochemical Methods in Corrosion Research VI, Pt. 1 and 2, 289-2, 1998, pp. 69. B.B. Katemann et al. / Electrochimica Acta 48 (2003) 1115 /1121 [5] M. Stratmann, R. Feser, A. Leng, Electrochim. Acta 39 (1994) 1207. [6] M.M. Lohrengel, A. Moehrig, M. Pilaski, Fresenius J. Anal. Chem. 367 (2000) 334. [7] T. Suter, H. Böhni, Electrochim. Acta 42 (1997) 3275. [8] J.V. MacPherson, P.R. Unwin, in: A.J. Bard, M.V. Mirkin (Eds.), Scanning Electrochemical Microscopy, Marcell Dekker, New York, USA, 2001, p. 521. [9] A. Schulte, S. Belger, W. 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