Corrosion and mechanical performance of HVOF WC-based coatings with alloyed nickel binder for use in marine hydraulic applications

Highlights

• Corrosion/mechanical properties of HVOF WC-based Ni binder coatings were studied.

• HVOF coatings had excellent corrosion performance in seawater field exposure tests.

• HVOF coatings have high impact strength than Monel or Al₂O₃.40TiO₂ coating.

• HVOF coatings have high adhesion strength compared to Al₂O₃.40TiO₂ coating.

• HVOF WC-based Ni binder coatings are suitable for use in maritime industry.

Abstract

Hydraulic components used in the maritime environment suffer damage due to the effects of corrosion and marine biofouling accumulation. The application of engineered coatings can overcome these problems. This study investigated the corrosion and mechanical performance of novel high velocity oxygen fuel (HVOF) sprayed ceramic-metal composite coatings; i.e., WC-18 wt% Hastelloy C and WC-10 wt% Ni-5 wt% Cr, designed for the protection of marine hydraulic components. A conventional atmospheric plasma sprayed (APS) ceramic coating (i.e., Al₂O₃-40 wt% TiO₂) and uncoated Monel K500 substrate were tested for benchmarking purposes. The corrosion performance of the samples was assessed using a combination of laboratory-based tests (i.e., electrochemical polarization, neutral salt spray, hot water immersion) and field exposure tests by immersion in seawater. The mechanical properties of the samples were assessed via a drop-weight impact test and the tensile adhesion test. The results showed that the HVOF coatings exhibited better corrosion resistance and mechanical performance compared to the baseline APS ceramic coating and uncoated Monel K500 substrate.

Introduction

Corrosion and biofouling cause functionality problems for a wide range of structures and components used in marine environments. Hydraulic components, such as shafts, valves and splines, are examples of critical parts that are affected by these issues. Corrosion and biofouling growth can lead to the damage of seals that impacts the integrity and service performance of a hydraulic system. This, in turn, may lead to water ingress into the hydraulic oil reservoir, or oil leaks, and eventual equipment failure [1,2]. Failure of these critical components can severely compromise a marine platform and preventative maintenance and costly repairs are essential to mitigate interruption to service.

Monel K500 is an alloy that exhibits excellent corrosion resistance and high strength, which renders it a material of choice for hydraulic shafts, fasteners, and external hardware in naval vessels [3]. However, in stagnant and slow-moving seawater, this alloy may suffer from biofouling accumulation and increased corrosion attack [4]. One solution to overcome this problem is to apply a protective coating for improved performance characteristics and service life.

Thermal spray coating is a method that extends the lifespan of underlying surfaces against severe corrosive environments [5]. As an example, atmospheric plasma spray (APS) is a standard coating technique to protect hydraulic piston rods in naval vessels with ceramic-based materials such as Al₂O₃-40 wt% TiO₂. APS coatings are suitable for applications where a barrier layer is needed to protect underlying surfaces against wear and corrosion; however, these ceramic coatings are relatively brittle, exhibit poor adhesion strength and can delaminate due to physical abrasion: for example, during mechanical scrapping that is employed during routine maintenance to remove hard accretions of calcareous biofouling [6]. These coatings also generally require an additional post-application sealant to minimize their inherent porosity, which can lead to premature coating failure if improperly applied [7].

Electrolytic hard chrome plating (EHC) is another protective coating method that has been used extensively in engineering applications, as it exhibits excellent wear and corrosion resistance properties. Common use cases for EHC include piston rods, valves and turbine blades. EHC coatings, however, are produced using carcinogenic chromic acid and, thus, there is an environmental requirement to employ alternative coatings [8,9]. In order to be commercially and operationally viable, any alternative coatings to EHC need to display similar performance characteristics, including high corrosion and wear resistance, impact resistance, high hardness and a low coefficient of friction.

High velocity oxygen fuel (HVOF) thermal spraying is an alternative to EHC plating and/or replacement for conventional APS coatings in a range of applications [[10], [11], [12], [13], [14]]. HVOF coatings are formed by the impact of particles sprayed under conditions of a high velocity at a low temperature; which confers a dense coatings with reduced oxidation and/or decomposition of the feedstock materials. HVOF coatings exhibit lower porosity, high hardness, high bond strengths and lower roughness compared to APS coatings; and is an environmentally friendly alternative to EHC [15]. The current industry standard to replace EHC is HVOF WC-Co and/or WC-CoCr coatings. Whilst these coatings have demonstrated desirable mechanical properties, their susceptibility to corrosion remains an issue [16,17]. As an alternative, new nickel-chromium based HVOF feedstock materials, such as Hastelloy® and Inconel®, have shown promise for WC-based metal matrix composite coatings with superior corrosion resistance when used in marine applications [15,18]. However, there is little information/data available on the corrosion and mechanical properties of these coatings which is required before they can be used with confidence in real-world applications.

The aim of this work was to assess the corrosion and mechanical performance of two candidate novel HVOF coatings, WC-18 wt% Hastelloy C and WC-10 wt% Ni-5 wt% Cr, designed for marine hydraulic applications. An APS ceramic coating i.e. Al₂O₃-40 wt% TiO₂ currently used in marine applications and Monel K500 were used as controls. The corrosion performance of the samples was assessed via standard laboratory tests, including electrochemical polarization, salt spray, hot water immersion, and field immersion studies. The mechanical performance of the samples was evaluated via a drop-weight impact and tensile adhesion tests.