Electrochemical characterization of atomic layer deposited Al2O3 coatings on AISI 316L stainless steel
Introduction
Among the existing coating technologies, the interest in Atomic Layer Deposition (ALD) is increasing in the recent years since it can be used as an alternative method to produce corrosion resistant films [1], [2], [3]. Atomic layer deposited coatings have been recognized to be suitable for a wide range of applications such as microelectronics, optoelectronics, catalysts, electroluminescence as well as different areas of nanotechnology [4]. ALD can be exploited not only to apply coatings on materials surface, but also to precisely coat nanoparticles, nanowires and nanotubes [5]. However, only recently these coatings have been considered for corrosion protection purposes thanks to the capability of ALD technique to deposit thin and compact ceramic films onto different metallic substrates. From both scientific and technological point of view, ALD is an evolution of traditional chemical vapour deposition (CVD) technique: it consists in a series of self-terminating gas–surface reactions controlled by a separate pulsing of precursors [6]. Choosing the proper precursors, by means of ALD technique it is possible to control the growth of thin and conformal inorganic films onto many metals such as stainless steel [7], [8], [9], carbon steel [10], magnesium alloys [11], silver [12], [13] as well as a few polymeric substrates [14]. With respect to traditional vacuum deposition techniques such as CVD processes, ALD shows enhanced conformality, uniformity and a relatively simple control of the thickness (as it is determined by the number of pulse of the different gas reagents) [15]. Comprehensive and detailed overviews of the deposition mechanisms as well as the potential of ALD technique can be found in literature [4], [5], [6].
In this work, atomic layer deposited Al2O3 films have been studied for the protection against corrosion of AISI 316L stainless steel. ALD deposited alumina films, whose formation mechanisms have been object of a comprehensive review by Puurunen [16], are recognized to be effective to protect metals against corrosion [8], [17]. However, very often the corrosion protection properties of the coatings are investigated by means of polarization curves and Electrochemical Impedance Spectroscopy (EIS) collected only after very short immersion time, in the order of a few hours [2], [8], [10], [18], [19], [20]. This approach provides relevant and significant information related to the corrosion protection properties of the film as well as on its dielectric characteristics. However, to provide some insight into the fundamental mechanisms for the performance differences understanding of the corrosion resistance of the ALD coatings as well as to better understand their long term durability, it is necessary to monitor the electrochemical response of the coated substrates for prolonged immersion time. In fact it is believed that this approach would be helpful to obtain a more comprehensive knowledge to forecast the long term corrosion behavior. In addition, the failure mechanism of the coatings can be also better understood with time monitoring of the electrochemical properties of the coated substrates for prolonged immersion time. For this purpose, this work is devoted to the investigation of the long term electrochemical response of atomic layer deposited Al2O3 films immersed time in a NaCl containing solution. ALD coatings object of the experimental tests were applied onto mirror polished AISI 316L plates: the effect of a different number of self-terminating gas–surface reactions (i.e. different thickness) employed to produce layers Al2O3 was investigated for relatively long immersion time (about 1000 h of continuous exposure to the electrolyte).
The structure of the coatings was explored by means of Attenuated Total Reflectance (ATR) infra-red spectroscopy. Polarization curves and electrochemical impedance spectroscopy (EIS) measurements carried out in a 0.2 M NaCl solution were employed to investigate the initial corrosion protection properties of the different inorganic layers as well as their evolution during time. 0.2 M NaCl solution has been widely employed by other authors to investigate ALD coatings [2], [8], [9], [10], [11], [17], [20]. This fact is related to two main reasons: (1) as it contains chlorides, it is suitable to promote localized corrosion on passive metals and (2) it is close to the physiological solution (i.e. 0.9 wt%), which is commonly used as corrosive mean as many research study are devoted to the application of ALD coatings in the biomedical field. As the present work deals with ALD coatings on stainless steel, a 0.2 M NaCl solution was employed also to allow an easier comparison with previous work carried out on similar systems. After collection of the EIS spectra, analysis and discussion of the possible and suitable electrical equivalent circuits have been carried out. Based on the present literature and on the shape of the experimental spectra obtained, a possible model to interpret the EIS spectra has been proposed. The experimental results highlighted that relatively long term exposure (in the order of thousands of hours) can be exploited to better differentiate the corrosion protection properties of the investigated coatings compared short term electrochemical test (such as polarization curves or EIS in the very first hours of immersion).
Section snippets
Experimental
Mirror polished (i.e. grinding with 800, 1200, 4000 emery papers and then polished with 3 μm and 1 μm diamond paste) AISI 316L stainless steel plates (supplied by Ronda S.p.A, Italy) were used as substrate to coat. The composition is reported in Table 1. Prior to entering the deposition chamber, the substrate were cleaned with ethanol under sonication for 8 minutes and finally dried under nitrogen flux. Atomic layer depositions were carried out on a commercially-built Beneq TFS-500 equipment.
Results and discussion
Dynamic SIMS was employed to evaluate the thickness of the coatings. Fig. 1 (a,b) shows the concentration profiles for Al, Ni, Cr and Fe. Cr, Ni and Fe belongs to the substrate, while Al was exploited as trace element to define the thickness of the coatings. As one can see from Fig. 1, it is possible to extrapolate the thickness of Al2O3–500 and Al2O3–1000 samples which is in the order of 38 nm and 84 nm, respectively. Notice that due to the roughness of the substrate compared to the thickness of
Conclusions
In this work amorphous alumina films obtained by an ALD process at 350 °C were applied on AISI 316L stainless steel. Films obtained from 500 and 1000 deposition cycles were produced. According to dynamic SIMS investigations, the thickness of the different coatings was found to be around 38 nm and 84 nm, respectively. Similar values were obtained from EIS measurements
Electrochemical impedance spectroscopy measurements were carried out on the investigated samples for prolonged immersion time in 0.2 M
Acknowledgments
The authors gratefully acknowledge Fondazione Bruno Kessler, Center for Materials and Microsystems and in particular Dr. Giancarlo Pepponi and Dr. Mario Barozzi for the dynamic SIMS analyses
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