Electrochemical characteristics of steel coated with TiN and TiAlN coatings
Introduction
Hard material coatings such as nitride coatings, mainly based on titanium, chromium and aluminum, have found increasing application in wear- and corrosion-resistant technologies [1], [2], [3], [4]. In relation to their corrosion performance, the protection characteristics of a number of hard PVD nitride coatings have been recently reviewed [5]. Titanium nitride (TiN), in particular, shows very good corrosion properties in most environments. TiN is the most extensively investigated hard coating. TiN coatings are stable in aqueous solutions at anodic potentials substantially higher than its thermodynamic oxidation potential [6], [7] due to the formation of a nitrogen enriched layer on the surface [8], [9], [10]. However, hard coatings such as TiN are often porous and therefore not always corrosion resistant. Furthermore, it has been demonstrated that corrosion performance of TiN-coated materials varies with the nature of the substrate the coating is attached to [10], [11], [12], [13], [14], [15], and passivable metal substrates are paramount if corrosion resistance enhancement is seeked through the coating process [14], [16], [17], [18].
Other binary and ternary nitrides are being investigated as a result of the wide range of properties that they present, which entitle them for use in many different applications that impose harsh requirements to the performance of the materials. Amongst these various nitride formulations, aluminum nitride (AIN) coatings are attracting much interest due to their chemical inertness and thermal stability in a wide temperature range [19], [20], [21]. As a result of the outstanding characteristics of binary TiN and AIN hard coatings, research is also directed to the study of ternary titanium–aluminum nitride (TiAlN) films, as they may be considered very promising materials for tribollogical and corrosion-resistant applications.
This paper concentrates on the evaluation of the corrosion resistance of TiN and TiAlN coatings deposited by cathodic arc plasma on MB40-P40 steel. The corrosion characteristics of these materials were first investigated by a standard potentiodynamic polarization technique, and next by electrochemical impedance spectroscopy (EIS), in 0.5 M NaCl aqueous solution at ambient temperature. Using EIS, and with an appropriate physical model of the electrochemical reactions occurring on the electrodes, information on the performance of coatings on metallic substrates could be achieved. We report the most significant results in which the corrosion characteristics of untreated steel specimens and of those coated are compared.
Section snippets
Material selection and procedures
Cemented carbide steel grade SB40-P40 was used as the substrate material. The TiN and TiAlN coatings were deposited at the University of Kocaeli in Izmit (Turkey) using an ion-arc PVD coating technique with a Metaplas MR303. Each coating was about 400 μm thick.
Corrosion testing
The investigation of the corrosion characteristics of the materials was made by electrochemical methods at the Department of Physical Chemistry in La Laguna (Spain). Testing was carried out in 0.5 M (3.0% weight) NaCl aqueous solution
DC electrochemical studies
Fig. 2 and Table 1 show the results of the electrochemical polarization measurements on coated and untreated samples in 0.5 M NaCl aqueous solution at ambient temperature. It is evident from Fig. 2 that there is a decrease of the anodic current of the coated samples with respect to the bare material. This demonstrates that the coated samples had a better corrosion resistance than that of the substrate, and this results in greater values for the polarization resistance Rp (Table 2). The current
Conclusions
- 1.
The electrochemical data obtained in this work indicate that the ternary TiAlN coatings exhibit corrosion properties very similar to those exhibited by binary TiN coatings. Both hard materials effectively increase the corrosion resistance of the underlying steel substrate.
- 2.
Over the frequency range applied, the equivalent circuit employed for the description of the EIS spectra for the coated samples provides the best fit of the experimental data. The electrochemical behavior of the materials can
Acknowledgments
Financial support by the North Atlantic Treaty Organization (NATO; Scientific Affairs Division, Brussels) under Project No. CRG.951299 is gratefully acknowledged.
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