Investigation of tantalum oxynitride for hard and anti-corrosive coating application in diluted hydrochloric acid solutions

Highlights

• Tantalum oxynitride coating was synthesized through optimization of reactive sputtering deposition parameters.

• Deposited coatings were having law roughness, good uniformity and without defects like pin holes or cracks.

• The coating was studied for mechanical behaviour and tantalum oxynitride has shown high hardness and mechanical strength.

• Coating reduces the corrosion current and offers high corrosion resistance compared to a native oxide film on steel.

• Low porosity and immune behaviour of tantalum enhances the anticorrosive behaviour of tantalum oxynitride for hard coatings.

Abstract

Tantalum oxynitride coating was investigated for mechanical properties and corrosive behavior for the anti-corrosive applications. The material phases and surface morphology were investigated by XRD, XPS, FE-SEM and AFM. The hardness and corrosion tests were carried out using nanoindentation and potentiodynamic polarization technique in dilute HCl solutions at room temperature respectively. The results reveal that the coating is uniform and with low roughness (<10 nm). The tantalum oxynitride film has high mechanical strength in comparison to the other tantalum oxide coatings and 304-grade stainless steel substrate. In the presence of diluted HCl, it was observed that the severe attack of chloride ion destroyed the native passive film of the stainless steel and increases the corrosion rate (1.87 mmpy). The tantalum oxynitride coated specimen shows good resistance to such attack and reduce the corrosion current density significantly (60–80 %). The electrochemical impedance spectroscopy was performed to get an insight into coating behavior in the acidic media.

Introduction

A wide spectrum of industrial applications relies on the stainless-steel family due to their versatile properties and long cycle life. One of the stainless-steel i.e. grade 304 (18/8) has excellent forming and joining characteristics. Particularly, this austenitic steel does not require any intermediate annealing in machine parts manufacturing processes. Although the steel family is exhibiting high resistance to corrosion in a variety of environments, they are suffering from acute galvanic corrosion in presence of acidic electrolytes. Which upheaves the maintenance cost of the steel mechanical systems [[1], [2], [3]].

An effective solution for the corrosion of the steel equipment is the constant endeavor of engineers and researchers. To overcome this problem, various methods are adopted, for example alloying of other metals with steel i.e. especially Cr, Mn, V etc. [[4], [5], [6]], applying organic paint coating [7,8], adding the corrosion inhibitors in coatings or electrolytes [9,10], depositing thin-film/ barrier coating [11,12] or hybrid coatings [13,14]. The summary of some observations made from a relevant literature studies are as follows:

• Diffusion of Cr in the ferritic martensitic steel forms Cr-rich carbide layer at the surface, which intern hampers the corrosion of the steel.

• Alloying of Cr and Mn leads to refinement of adherent rust grains and reduces the weight loss effect due to corrosion.

• Alloying of V leads to retardation of salt film dissolution and metastable pit formation in the corroding electrolytes, which protects the surface from damages.

• Organic coatings with high cross-links offer resistance against the percolation of corroding elements and reduces corrosion effect. Additionally, organic coatings are ease of applying and have good adhesion to the variety of surfaces.

• Inhibitors slow down the corrosion process by various means such as by removal of reducing species from the electrolyte, forming a bond with surface atoms of the metals, decreasing the surface energy of the materials, impeding the corrosion current etc.

For example, coating of zirconium oxide matrix with inorganic beads or Lignin, carbon nanotubes/ graphene oxide with PMMA, cerium as inhibitor in various metal alloys etc.

Among these available solutions, organic coating and corrosion inhibitors are suffering from various issues like high toxicity, detrimental to environment, high economical cost etc. The barrier/ protective film coating on steel is the most promising method due to its number of advantages like simple operation to apply, low cost and extensive application scope.

Over the last two decades, transition metal oxide and nitride are used for wear-resistant and corrosion-resistant hard coating applications respectively. They are chemically stable and mechanically hard as they are forming covalent bonds in the atomic structure. From the group of transition metals, Hf, Zr, Cr, Ta and their compounds, mainly oxide and nitride, were much explored for a wide range of applications i.e. electronics [[15], [16], [17], [18]], optics [19,20], protective coatings [21,22]. Over and above, the materials are under explored for other remarkable properties i.e. anticorrosive and anti-wear abilities in the form of metal oxynitrides. B. Rahmati and the group have studied the mechanical properties of the Ta-O films on Al alloy for hard and biocompatible coating application. They have observed the film has good adhesion due to anchoring of ions at the substrate surface and enhancement in the film density [11]. Yan li et al. (2008) used Ta plasma implantation to enhance corrosion resistance in TiNi alloy and observed that the stable tantalum oxide film formed at the upper surface, which effectively reduces the corrosion events [22]. Hence, the integration of good mechanical and anticorrosive properties of the metal oxynitrides can serve better for hard coating applications.

In the present work, synthesis and characterization of tantalum oxynitride films for mechanically hard and anticorrosive coating for the protection of stainless steel was carried out. For the coating purpose, the reactive magnetron sputtering technique was used with optimized Ar: O₂: N₂ ratio and other deposition parameters. Afterwards, the nanoindentation tests were performed on the deposited samples to derive the mechanical behavior of the coating and potentiodynamic polarization tests using 2.0 M, 2.5 M and 3.0 M HCl electrolytes were conducted for the measurement of corrosion resistance at the room temperature.