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
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.
Section snippets
Electroless coating preparation
N80 carbon steel substrates with a size of 10 mm × 10 mm × 10 mm were used for electroless coatings, and the chemical composition of N80 steel is as follows (wt%): 0.29 C, 1.38 Mn, 0.25 Si, 0.037 Cr, 0.009 Cu, 0.002 Ni, 0.002 P, 0.002 S and Fe balance. The substrates were first sequentially ground up to 1200 grit, cleaned with deionized water and ethanol, and dried with flowing air. The electroless composite coating process contains two steps: 1 h of Ni-P coating and 1 h of Ni-Mo-P coating. The
Characterizations of as-deposited and heat-treated coatings prior to tests
Fig. 1 shows the surface characterization results of as-deposited coatings. As seen in Fig. 1a, the as-deposited Ni-P coating exhibits typical a nodular microstructure with apparent nanopores observed on the coating surface. However, a crystalline microstructure with a much smaller crystal size is identified on the as-deposited Ni-Mo-P/Ni-P coating (Fig. 1c), as evidenced by the XRD pattern showing a sharp Ni (111) peak (Fig. 1h), as compared to the broad Ni peak of amorphous Ni-P coating (Fig.
Conclusions
In summary, the corrosion behavior of electroless Mo-incorporated Ni-P coating was investigated in CO2/H2S/Cl− brine, and the effect of heat treatment on the corrosion resistance improvement was further revealed. The results show that Mo addition adversely affects the corrosion resistance of as-deposited Ni-P coating, while heat treatment profoundly advances the anti-corrosion performance of Ni-Mo-P/Ni-P composite coating.
The Mo incorporation into the as-deposited Ni-P coating results in the
CRediT authorship contribution statement
Jiankuan Li: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Software, Validation, Visualization, Writing - original draft. Chong Sun: Investigation, Writing - review & editing. Morteza Roostaei: Writing - review & editing. Mahdi Mahmoudi: Writing - review & editing. Vahidoddin Fattahpour: Writing - review & editing. Hongbo Zeng: Supervision, Writing - review & editing. Jing-Li Luo: Conceptualization, Supervision, Resources, Project administration, Funding
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Natural Sciences and Engineering Research Council of Canada/RGL (RGL Reservoir Management Inc.) Collaborative Research and Development Grants (No. CRDPJ 488361) and the Natural Sciences and Engineering Research Council of Canada-Discovery Grant (No. GRPIN-2016-05494).
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