Effect of galvanic coupling with TiN, TiAlN, and CrN coatings, and titanium to the corrosion of steels

Abstract

Physical vapor deposition is a surface engineering technique, applied in low-pressure atmospheres. The deposition of the desired film takes place in several consecutive steps. A metal or alloy solid target is vaporized and ionized; and this metallic vapor is deposited onto the substrate, forming a dense, compact, and well-adhered film. Depending on the use of inert or reactive process gases, films with metallic or ceramic character (metals, alloys, mixtures, as well as transition metal oxides, nitrides, carbides, carbonitrides, oxynitrides, etc.) may be produced on metallic, ceramic or polymeric substrates. The technique was first commercialized in the 1970s and is now applied industrially for many applications, such as prolonging the life performance of cutting and drilling tools, and surgical instruments by improving wear and tribo-corrosion resistance, solar collectors and decorative applications by improving optical and visual properties. The transition metal nitrides produced with these methods are very hard and wear-resistant. Furthermore, their corrosion resistance is also high, especially in atmospheric and aqueous environments where structural materials are used frequently. Besides, they're also resistant to a wide range of acidic and alkaline aqueous electrolytes. Accordingly, PVD coatings were promising candidates for their corrosion protective ability, leading to many scientific and industrial studies. However, these investigations haven't yielded the expected protection due to the pores or defects reaching the substrate. Unless the substrate is corrosion-resistant, coatings produced with conventional methods can't fulfill the desired corrosion protection. Among transition metal nitrides, the most commonly used are single-element nitrides such as titanium nitride (TiN) and chromium nitride (CrN) or multi-element nitrides such as titanium aluminum nitride (TiAlN). Current efforts to increase the corrosion protective ability of PVD-coated substrates focus on the deposition of multi-layered coatings, deposition of a corrosion-resistant interlayer, and methods to eliminate porosity in coatings. According to the current state-of-art, the main factor limiting the corrosion protection ability of these coatings is these pores and substrate corrosion within these pores. The cathodic or anodic character of the coating compared to the substrate is expected to accelerate or inhibit the substrate's corrosion, due to the galvanic interactions occurring beneath the pores (similar to the zinc or tin coatings on steel). Noble and electrically conductive transition metal nitrides are generally considered to accelerate substrate corrosion due to the galvanic effects. However, there is a lack of comprehensive research in the literature regarding these acceleration effects on substrate corrosion. Furthermore, in the case of a noble coating on a substrate, the acceleration of the corrosion rate due to the galvanic effect doesn't depend solely on the potential difference between the corrosion potentials but also on the cathodic reaction rate of the cathode material. This thesis aims to investigate the extent of galvanic interaction between the three common transition metal nitride coatings (TiN, CrN, and TiAlN) and different substrates (steel and stainless steel). These three coatings were selected to study the effect of nitride type on these interactions. Moreover, the effect of galvanic interaction between the substrate and metallic interlayer which is used to improve adhesion to the substrate, was also considered by the metallic titanium-steel galvanic couple that is common for titanium-based coatings. The selected coatings were deposited on inert substrates by the cathodic arc PVD method. They were used to determine the inherent anodic and cathodic behaviors of the coatings and the galvanic interactions with the substrates without being affected by an additional substrate effect. TiN and CrN were deposited on alumina substrate at 80 A cathode current with 150 V bias using a Ti and Cr cathode, respectively. TiAlN coatings were deposited using a TiAl (75:25) cathode at 60 A cathode current with 50 V bias. In the first experimental section (Chapter 5.1), the interaction between the nitride-based coating and steel substrate is investigated. For the first two chapters, two different electrolytes were selected to represent the solution properties during corrosion in localized defects: an aerated (fixed oxygen-concentration), neutral and chloride-containing electrolyte, and a deaerated, acidic and chloride-containing electrolyte. The uncoupled electrochemical behavior of the electrodes was determined by OCP measurements and potentiodynamic polarization. EIS measurements at OCP and Mott-Schottky analysis on the coatings were also conducted. Galvanic behavior may be determined by the combination of polarization diagrams or by zero-resistance-ammeter (ZRA) and both techniques were used in this study. Moreover, EIS measurements at a potential corresponding to galvanic couple potential were obtained from the coatings. Results showed that the galvanic interactions with the nitride coatings significantly accelerate the steel substrate's corrosion rate in aerated neutral electrolyte but don't cause a significant contribution in deaerated acidic electrolyte. This may be explained by the cathodic reaction kinetics on the coatings: in acidic solutions where the cathodic reaction is hydrogen ion reduction, the high binding energy between the adsorbed hydrogen to the nitride surface caused slow reactions that were supported by the DFT calculations from the literature. The effect of nitride type and their respective charge carrier concentrations are significant for their performance during galvanic interactions. The acceleration of self-corrosion in the substrate adjacent to the coatings was verified by the immersion of cross-section samples. Accordingly, the low charge carriers in CrN led to slower reaction kinetics and caused less dissolution at the CrN coating-steel interface than the interface of TiN coated-steel or TiAlN coated-steel. In the second experimental section (Chapter 5.2), the interaction between the nitride-based coating and stainless steel substrate is studied. The common corrosion type for stainless steel is pitting corrosion in many of the corrosion testing environments. Therefore, firstly the pitting potential of the stainless steel was determined by the polarization curves. Then, the possibility of the stainless steel's potential being shifted to this pitting potential under the effect of galvanic coupling with nitride coatings is explored. Experiments showed that the couple potentials lie below the pitting potential of the stainless steel in both electrolytes and that galvanic interaction with the nitride coatings would not cause significant damage under the testing environments used in the study. The steady-state galvanic currents are low (in the range of 1±0.5 μA/cm2) for TiN, TiAlN, and CrN coatings. These results highlighted the importance of the substrate's inherent corrosion resistance along with the cathodic reaction kinetics on the coatings. Under these experimental conditions, galvanic coupling with the nitride coatings didn't cause pitting of the stainless steel. In the third experimental section (Chapter 5.3), the interaction between the titanium interlayer and steel substrate is investigated. For this purpose, galvanic corrosion experiments were conducted between the metallic titanium and steel. It was observed that titanium can readily self-activate in acidic electrolytes where pH <2 and act as the anode of the titanium-steel galvanic couple. The degradation of the titanium's native oxide is a function of pH: the activation time at pH 0 is very fast (11 min) whereas it increased to 115 min at pH 0.5 and to 400 min at pH 1. Interestingly, the activation is significantly accelerated under cathodic polarization impressed by galvanic coupling. These were 22 min at pH 0.5, 32 min at pH 1, and 60 min for pH 2 which didn't exhibit reversal at OCP for 10 hours. Consequently, potentiostatic polarization experiments were conducted to study the effect of cathodic potential on the oxide degradation times, causing both chemical and electrochemical dissolution. During galvanic coupling experiments in electrolytes with higher pH, the titanium acted as the cathode of the titanium-steel galvanic couple, but the slow cathodic reactions on the titanium cathode produced very limited currents, thus, the effect on the steel's corrosion was not pronounced. This chapter determined that the behavior of the titanium-steel couple was pH-dependent; the steel substrate may act as the cathode of the galvanic couple; and the titanium's protective oxide degradation was accelerated by the induced cathodic polarization during the galvanic coupling.