Comparative study on the corrosion and wear behavior of plasma-sprayed vs. high velocity oxygen fuel-sprayed Al8Si20BN ceramic coatings

Abstract

Corrosion and wear are common problems encountered in the oil and gas industry. These entail the gradual destruction of materials by mechanical action on the opposite surface, and the chemical and/or electrochemical reaction with their environment. In this research, Al8Si20BN ceramic powder with specific properties against corrosion and wear was selected, and it was sprayed with high velocity oxygen fuel (HVOF) and plasma spray methods onto carbon steel substrates. The coatings were characterized with respect to phase composition, microstructure, microhardness and adhesion strength. Their wear behavior was inspected by applying 5, 10, 15 and 20 N loads by pin-on-disc machine, after which the results of both methods were compared. According to the results, the HVOF-coated models were more durable than the plasma-coated models under different loads in the same condition. In addition, the corrosion deterioration of the coated specimens in both brine (3.5% NaCl) and fossil oil were tested for one month (30 days). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) assessment in 3.5% NaCl solution indicated that the HVOF-sprayed specimens had better corrosion protection than the plasma-sprayed specimens. Generally, the HVOF technique facilitated more durable coats with greater corrosion and tribological resistance compared to the plasma coating technique.

Introduction

In the petrochemical and petroleum industries, both onshore and offshore, apparatuses performing in aggressive environments are subject to wear, corrosion and thermal cycling [1]. Although due to component degradation in oil and gas external and internal pipeline parts and facilities are very costly, they must be continually maintained and well-inspected. Therefore, wear and corrosion observations must be highly promoted in crucial plant regions to secure facilities and pipes against crude oil and seawater. Hence, investigators are seeking superior components and techniques for operating with oil and gas accessories and pipelines in order to enhance their lifespan [2].

Aluminum silicon (AlSi) matrix is a chemical composite material that can be used in many abradable coating systems due to its good combination of erosion resistance and abradability against wear. The silicon present in the material is virtually pure, acting to increase the hardness of coatings produced from hard materials and improving abrasion resistance. These properties have led to vast application in different industries. Aluminum with silicon is a simple eutectic system with a low melting temperature [3], [4]. Dry sliding conditions are used to investigate the wear properties of these ceramics. Previous research works have investigated the effect of silicon content in ceramics on mild wear [5], mild wear mechanics in hypoeutectic alloys [6], as well as the mechanism and wear maps of Al–Si ceramics [2], [3], [7]. The output has shown that adding silicon can reduce the melting temperature to 577 °C (1071 °F) while increasing the fluidity, specific gravity and coefficient of thermal expansion. It also decreases the contraction associated with solidification. AlSi coatings have a lower melting temperature (577 °C/1071 °F) than pure aluminum coatings (660 °C/1220 °F); therefore, AlSi is more suitable for co-spraying with temperature-sensitive materials [8], [9]. Toptan et al. [4] demonstrated that aluminum-silicon materials produce coatings that are harder and slightly denser than coatings of pure aluminum, as these components can achieve dimensional restoration of worn or mis-machined components. In addition, under certain constant load and sliding speed conditions, corrosion and wear rate decrease as well [9], [10]. Such materials are thus suitable for critical applications, such as aircraft landing gear, engines, turbine blades and valves to enhance the wear and abrasion resistance of these types of components [11].

The wear property of AlSi is very good, but its corrosion behavior is not as good. Thus, adding hexagonal boron nitride powder to this chemical composition enhances the corrosion resistance, particularly in marine environments, and can reduce the temperature to 450 °C (842 °F). Hexagonal boron nitride (hBN) is a very inert lubricant that improves abradability by reducing frictional heating on contact, especially at high translational speeds. It also helps weaken the interparticle bond strength within the aluminum silicon matrix for better friability [12], [13]. Generally, the corrosion behavior of such abradable materials has not been studied extensively and their corrosion mechanism is not yet well understood. Therefore, it is necessary to study the corrosion behavior of Al–BN abradable coatings [17].

The thermal spray coating technique is one of the most acceptable techniques in the field of corrosion and wears protection. It can facilitate thick coatings of about 20 µm to several millimeters depending on the feedstock and procedure. Compared to other coating procedures like chemical, physical and electroplating vapor deposition, thermal spraying has massive region at elevated deposition ratio [16]. Coating elements available for thermal spraying contain alloys, metals, ceramics, composites and plastics. These coating elements are used in powder or wire form, heated to molten or semi-molten condition and thereafter atomized to micrometer-size particles and accelerated towards the substrate. Generally, energy sources for thermal spraying are electrical discharge and combustion. The accumulation of countless sprayed particles thus results in new coatings [3], [18], [19].

The most flexible coating method amongst all thermal spray mechanisms is plasma coating [2], [18]. An arc forms among two electrodes in a plasma-forming gas that often contains either argon/helium or argon/hydrogen as part of the plasma-spraying tool. Expanding and accelerating among a shaped nozzle and generating velocities up to Mach (Ma) 2 are followed by heating the arc plasma gas. While the plasma jet temperature is still 18,000 °F (10,000 K) a few centimeters from the nozzle exit, the temperature in the arc zone proceeds towards 36,000 °F (20,000 K) [3], [18]. Fig. 1 represents the cross-section of a plasma gun. By using different nozzle designs, the elasticity of injection systems, combined with capability of generating high processed temperatures, which empower the usage of various coatings in plasma spraying. These coatings range from low melting point polymers like nylon to very high melting point components like refractory materials including tantalum, ceramic oxides, tungsten and other refractory components. By using thermal protection, materials will last longer and operate better at higher temperatures, which lead to advanced general system productivity [18], [19].

Furthermore, High Velocity Oxy-Fuel (HVOF) flame spraying is known as a crucial thermal spray coating procedure in surface science. HVOF is becoming significant, as it can generate dense coatings easily based on remarkably high particle velocities attained by injecting powdered components into supersonically expanding jets of hot gas. Since the particles are accelerated to high kinetic energy and cause sufficient deformation on impact at the surface being coated to produce dense and well-bonded coatings, it is not necessary for the particles to be melted. HVOF coatings offer another potential advantage over plasma coatings owing to the lower process temperatures of the flames used as heat source. Fig. 2 shows a cross section of an HVOF gun [19], [20].

HVOF is used widely to generate wear-resistant types of coatings like ceramic and metal mix composites such as tungsten-carbide cobalt. Furthermore, highly dense coatings (with porosity below 0.5%) can be generated by HVOF, which are suitable for use as corrosion-resistant coatings containing for instance ceramics, stellite, stainless steel and Inconel. HVOF coatings can also be incorporated in complex materials design. For example, they can be used for high-tech medical tools utilized in intricate surgeries or plain materials in agricultural combine bolts [19], [20].

In this research, two thermal spray-coating techniques, namely plasma and HVOF (using propylene gas as fuel), were used to coat ceramic Al8Si20BN onto S45 carbon steel substrate. The coating thickness with both methods was around 250 µm. Preheating between 75 and 85 °C was applied prior to coating to reduce oxidation. The main focus of this work was on comparing these two different thermal spray methods for ceramic coatings. The coating quality was investigated in terms of adhesion strength, microstructure, density and hardness. The wear behavior with the two coating techniques was analyzed by applying various loads with a pin-on-disc. The corrosion characteristic of AlSiBN ceramic was calculated in seawater (3.5%NaCl solution) at 30 °C and crude oil at 60 °C. It was determined that the thermal spray coating techniques can remarkably increase the lifespan of components and decrease the amount of unscheduled and scheduled shutdowns.