Corrosion behavior of electroless Ni–P alloy coatings containing tungsten or nano-scattered alumina composite in 3.5% NaCl solution

https://doi.org/10.1016/j.surfcoat.2007.05.030Get rights and content

Abstract

The corrosion protection performance of electroless deposited nickel–phosphorus (Ni–P) alloy coatings containing tungsten (Ni–P–W) or nano-scattered alumina (Ni–P–Al2O3) composite coatings on low carbon steel was studied. The effect of heat treatment on the coating performance was also studied. The optimum conditions under which such coatings can provide good corrosion protection to the substrate were determined after two weeks of immersion in 3.5% NaCl solution. Electrochemical impedance spectroscopy (EIS) and polarization measurements have been used to evaluate the coating performance before and after heat treatment. The Ni–P–W coatings showed the highest surface resistance compared with Ni–P–Al2O3 and Ni–P. The surface resistance of Ni–P–W coatings was 12.0 × 104 Ω cm2 which is about the double of the resistance showed by Ni–P–Al2O3 (7.00 × 104 Ω cm2) and twenty times greater than the surface resistance of Ni–P (0.78 × 104 Ω cm2). XRD analysis of non-heat-treated samples revealed formation of a protective tungsten phosphide phase. Heat treatment has an adverse effect on the corrosion protection performance of tungsten and alumina composite coatings. The surface resistance decreased sharply after heat treatment.

Introduction

Electroless deposited Ni–P alloy coatings have been widely used in many industrial applications for their unique properties such as corrosion resistance, wear resistance, non-magnetism, improved micro-hardness and coating thickness uniformity [1], [2], [3], [4]. The combination of uniform thickness coating with the high hardness values makes autocatalytic nickel as the preferred coating for a wide range of industrial applications, in spite of its greater cost. These properties in addition to the good wear and corrosion resistance have been responsible for the development of autocatalytic nickel deposition over the last years [5], [6], [7], [8], [9]. There are a lot of articles dealing with Ni–P as a high performance coating for corrosion protection of steel [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. However, most of these articles are focusing on the effect of phosphorous ratio in the binary alloy coating [5], [7], [8], [9], [12], [13], [14].

The functional composite coatings with specific characteristics such as wear, corrosion resistances and/or high hardness can be produced easily by choosing sui particulate starting materials. The traditional electroless composite coatings have been prepared by adding micro-meter particles to an electroless bath such as SiC, BN, PTFE, diamond [15], [16], [17], [18], [19]. However, most of the current micro-meter composite coatings cannot meet the needs of manufacturers in industry.

Advances in the chemical tailorability of composite coatings using nano-meter technology have lead to create long-lived, corrosion-resistant, composite coatings. Electroless nano-meter composite coatings where nano-meter particles such as alumina are used as a reinforcing phase have found increasing research interest owing to their excellent mechanical properties [13], [14]. But the published information about the corrosion behavior and the effect of heat treatment of such new coatings is very scarce.

Newly developed Ni–P alloy coatings have been recently reported by the introduction of a third element such as tungsten to form ternary Ni–P–W [20], [21], [22]. Published data showed that that inclusion of tungsten in binary Ni–P deposit positively affects the deposition rate, composition and deposit properties such as hardness, thermal stability, wear, corrosion resistance and melting point [23], [24], [25], [26]. However, the effect of heat treatment on the characteristics of Ni–P coating containing different rations of tungsten and alumina has not been studied widely. Moreover, the literature available about the effect of different tungsten and alumina ratios in the Ni–P alloy coatings on the corrosion resistance and coating performance is very scarce.

In previous works [27], [28], [29], [30], the effect of experimental parameters on the electroless deposition of Ni–P alloy coatings onto steel substrates was studied. The optimum experimental parameters under which such coatings can provide the highest micro-hardness, wear resistance and smooth coating appearance were investigated. Results showed that the composition of coated alloy strongly influences its properties and is controlled by the pH, bath chemical compositions, bath temperature, coating time and heat treatment, which are important factors that affect the thickness, hardness, structure and morphology of the coating.

The objective of this work is to provide extensive study about the electrochemical corrosion behavior of such new ternary coatings in 3.5% NaCl solution. Electrochemical impedance spectroscopy and cyclic voltammetry was used to evaluate the coatings performance in after two weeks of immersion 3.5% NaCl solution. The effect of tungsten or alumina ratios in the Ni–P alloy coatings on the impedance behavior was also measured. The influence of heat treatment on the corrosion protection performance of the ternary coatings was investigated.

Section snippets

Materials

Specimens of low carbon steel, in the form 30 × 30 mm taken from sheet of 1 mm thick were used. The nominal composition was as follows (wt.%): 0.025 C; 0.087 Si; 0.44 Mn; 0.002 Cr; 0.0009 Mo; 0.0005 Al; 0.0005 Ni; 0.029 Co; 0.0003 Cu; 0.0006 Nb; 0.013 Ti; 0.026 V; 0.011 W; 0.011 Pb; remainder Fe.

Surface preparation

Each specimen was abraded to a 800 grit finish with SiC paper, degreased in acetone, washed with distilled water, and dried in dry air. The specimens are then subjected to ultrasonic cleaning in deionized

Ni–P alloy coatings group

In this group, the effects of electroless deposited Ni–P alloy coatings on the corrosion resistance of low carbon steel were studied. Generally, Ni–P electroless alloy coatings improve the surface resistance due to formation of protective layer of metallic nickel and nickel phosphide phase, as confirmed by XRD, that act as barriers to oxygen diffusion to the metal surface. According to the EIS results (Fig. 1) and visual inspection, the Ni–P alloy coatings samples showed the best corrosion

Conclusions

The optimum experimental parameters under which tungsten and alumina containing nickel–phosphorus ternary coatings can provide superior corrosion resistance in chloride containing solutions were determined.

The presence of alumina and tungsten improves the corrosion resistance of steel in 3.5% NaCl solution. The highest surface resistance was obtained from the coatings that contain 75 g/l alumina (Al3).

Increasing the tungsten ratio up to 5 g/l, improves the surface resistance of as-polished

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