Characterization and corrosion behavior of electroless Ni-Mo-P/Ni-P composite coating in CO₂/H₂S/Cl⁻ brine: Effects of Mo addition and heat treatment

Highlights

• The corrosion resistance of as-plated Ni-Mo-P coating weakens with Mo addition.

• The added Mo causes the microstructure to change from amorphous to crystalline.

• Heat treatment enhances the anti-corrosion performance of Ni-Mo-P/Ni-P coating.

• The MoO₃-containing oxide film offers superb passivation and corrosion resistance.

Abstract

The electroless Ni-Mo-P/Ni-P composite coating was applied on N80 carbon steel, and the effects of Mo addition and heat treatment on the corrosion resistance enhancement in CO₂/H₂S/Cl⁻ brine were studied by electrochemical measurements and surface analysis techniques. The Mo addition in the as-deposited Ni-P coating causes the microstructural transformation from amorphous to crystalline due to the reduced P content, thereby suffering severe corrosion. The impaired corrosion performance of as-deposited Mo-incorporated coating is also originated from the absence of the oxide film on the coating surface. Nonetheless, the heat-treated Ni-Mo-P/Ni-P coating exhibits desirable corrosion resistance, which is reflected by the outstanding corrosion inhibition efficiency (η = 96.1%). Heat treatment facilitates the formation of Ni₄Mo phase and more importantly, the growth of an oxide film consisting of nickel and molybdenum oxides (H₂S-immuned MoO₃) with better passivation properties, which accounts for the remarkable corrosion resistance improvement.

Introduction

Growing energy consumption worldwide and dwindling light oil resources have prompted the recovery of heavy oil, and thus caused severe corrosion problems of carbon steels due to the extremely harsh environment containing acidic gases (e.g., CO₂ and H₂S). Electroless Ni-P coating, as an advantageous metallic coating with good mechanical properties, low cost and extraordinary corrosion resistance, has been adopted as a promising candidate for carbon steels in oil and gas industries [[1], [2], [3], [4]]. Although decent corrosion performance in CO₂-containing environments or other acidic media has been reported by researchers [[5], [6], [7]], H₂S still severely impedes the further application of Ni-P coating due to the high chemical affinity of S species to Ni, causing the acceleration of corrosion and electrolyte penetration [8,9]. Therefore, it is desirable to develop a Ni-P composite coating that displays superb anti-H₂S corrosion performance.

To date, several methods have been identified to improve the corrosion performance of Ni-P coating, for instance, introducing alloying elements [[10], [11], [12], [13]] and incorporating functional nanoparticles [3,[14], [15], [16], [17]]. The primary contribution of nanoparticles in the corrosion resistance enhancement is to fill in the intrinsic micropores formed due to the hydrogen evolution reaction during the coating deposition process, thereby hindering the penetration of corrosive electrolyte. Luo et al. [3] reported the increased microhardness and improved corrosion resistance of Ni-P/nano-WC composite coating in 3.5 wt% sodium chloride (NaCl) solution. The study on the incorporation of carbon nanotubes (CNTs) into Ni-P coating showed that added CNTs led to the formation of a denser and more homogeneous coating by filling in coating defects and accelerated the chemical passivity of the coating due to their high chemical stability [14]. However, very limited research has been devoted to the corrosion resistance improvement by nanoparticle incorporation in CO₂/H₂S environments. Recently, ZrO₂ nanoparticles were added into Ni-W-P/Ni-P coating and proved to reduce the corrosion rate in CO₂/H₂S/Cl⁻ brine [18], however the inhibition efficiency is relatively low. In this regard, the co-deposition of Ni-P coating and suitable alloying elements owing good anti-H₂S properties would be a better option.

Among all the alloying elements that are compatible with electroless Ni-P coating, molybdenum (Mo) element is believed to exhibit satisfactory anti-H₂S corrosion performance, which has been substantiated in corrosion-resistant alloys and stainless steels with alloyed Mo [[19], [20], [21]]. Several theories have been proposed on the roles of Mo in enhancing the corrosion resistance: (1) formation of insoluble Mo-containing compounds, e.g., molybdates, molybdenum oxides and molybdenum sulfide [[22], [23], [24]], (2) inhibition effect of molybdate ions [25,26], (3) cation selectivity of molybdenum sulfide, specifically in sour environments [19]. Pardo et al. [22] discovered the presence of molybdenum trioxide in the outer layer of the passive film of 304 and 316 stainless steel with alloyed Mo, proposing that the decreased dissolution rate was ascribed to this Mo-rich oxide film which acted as a barrier against the diffusion of dissolution species. Similar findings about molybdates and molybdenum sulfide were also reported [23,24]. Denpo et al. [25] studied the effect of MoO₄²⁻ ions on the passivity of Ni-Cr-Mo-Fe and reported that the formation of MoO₄²⁻ on the surface of passive films improved the passivity in H₂S/Cl⁻ environments. Moreover, MoO₄²⁻ ions facilitate the formation of a bipolar film together with CrO₄²⁻ ions, which provides ion selectivity and repels Cl⁻ ions penetrating through the film [26]. The cation selectivity of molybdenum sulfide was proposed by Tomio et al. [19], who claimed that the cation-selective stable molybdenum sulfide promoted the protectiveness of inner chromium-rich oxide film by retarding the dissolution reaction caused by Cl⁻ ions and attenuating the activity of dissolved H₂S. Therefore, Mo addition in the electroless Ni-P coating is expected to exhibit more desirable corrosion performance in CO₂/H₂S/Cl⁻ brine. Recently, ternary Ni-Mo-P coating has received growing attention in terms of its good mechanical properties and corrosion resistance in neutral solutions [[27], [28], [29]], but the effect of Mo has not yet been elucidated in H₂S-containing environments.

Moreover, heat treatment has been widely regarded as a favorable approach to improve the mechanical properties and corrosion resistance of Ni-P coating [[30], [31], [32], [33]]. The precipitation of Ni3P and Ni crystals because of heat treatment contributes to the enhanced mechanical properties [30,34], and the corrosion resistance of crystalline Ni-P coating can be improved via heat treatment by reducing grain boundaries because of the grain growth of crystalline Ni [33]. Furthermore, NiO tends to form on the coating surface upon air annealing at elevated temperatures and acts as a passivation layer to inhibit corrosion [34,35]. Moreover, as stated above, the superb protection by Mo element is mainly accredited to the formation of Mo-containing compounds or ions, which may be obtained by proper heat treatment procedures. In addition, since most underground oil and gas applications are operated at elevated temperatures, it is desirable to study the effect of heat treatment on as-deposited coatings in sweat/sour brines. However, limited studies have focused on the phase transformation and microstructural changes of heat-treated Ni-Mo-P coating and the corresponding variations in terms of corrosion resistance.

Herein, we fabricated a duplex outer Ni-Mo-P/inner Ni-P electroless coating in this work, evaluated the corrosion resistance by various electrochemical measurements, and characterized the chemical compositions of corrosion product via surface analysis techniques. This study aims at optimizing electroless Ni-P composite coating to achieve the outstanding anti-H₂S corrosion performance and elucidating the role of Mo addition and heat treatment in the corrosion resistance advancement.