**1. Introduction**

Pipeline steels are manufactured from high-strength low-alloy steels (HSLA) to meet the strict requirements of the oil and gas industry with respect to its increasing demand for weight reduction and enhanced productivity in the transportation of their products [1,2]. The intrinsically high mechanical strength of HSLA steels accounts for their reliable operation in oilfield production [3]. Notwithstanding, the harsh environments to which they are subject pose challenging design issues related to both wear and corrosion control [4,5]. In view of the critical role played by HSLA steels in the safe operation of transmission pipelines, it is of prime importance to properly manage surface properties and to guarantee long-term operation without failure [6].

Corrosion is particularly pointed as a major cause of degradation of the load bearing capacity of HSLA transmission pipelines [7]. Coatings have been traditionally employed to protect the internal tubing walls from corrosion in the petroleum industry [8,9]. Epoxybased organic coatings are often employed with this purpose due to their chemical inertness and strong adhesion to metallic substrates [10]. It is well-known, though, that these materials lose their barrier properties with time, allowing electrolyte penetration through pores and flaws and, ultimately, to adhesion failure [11].

Electroless nickel coatings have emerged as a viable alternative to overcome the above-mentioned limitations [12]. These coatings are based on conventional binary Ni– P films and have consolidated engineering applications in the automotive, aerospace, and food industries [13]. They owe their outstanding performance to a combination of

**Citation:** de Oliveira, M.C.L.; Correa, O.V.; da Silva, R.M.P.; de Lima, N.B.; de Oliveira, J.T.D.; de Oliveira, L.A.; Antunes, R.A. Structural Characterization, Global and Local Electrochemical Activity of Electroless Ni–P-Multiwalled Carbon Nanotube Composite Coatings on Pipeline Steel. *Metals* **2021**, *11*, 982. https://doi.org/10.3390/met11060982

Academic Editor: Aleksander Lisiecki

Received: 27 May 2021 Accepted: 17 June 2021 Published: 20 June 2021

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strong adhesion in a variety of substrates, shapes and sizes, and high hardness in the annealed state, due to the precipitation of nickel phosphides promoting good wear and corrosion resistance [14,15]. In recent decades, further developments in the electroless deposition of nickel-based coatings have been driven by growing needs for optimized surface properties of metallic materials to expand their applicability to more aggressive environments. Ternary, quaternary, and composite coatings have been developed with this purpose [16–19]. Co-deposition of a variety of inorganic particles has been tested for the electroless plating of Ni–P composite coating, such as SiO2, SiC, TiO2, ZrO2 and B4C [20–24].

Carbon nanotubes (CNT) have also been employed to reinforce particles for electroless Ni–P coatings. Several authors [25,26] reported that multiwalled carbon nanotubes (MWCNT) improved the friction and wear properties of conventional electroless Ni–P. In addition to improved wear behavior, corrosion resistance is often reported as being favorably affected by incorporating CNT particles into the Ni–P matrix. The intrinsic chemical inertness and high length-to-diameter ratio of the CNTs would account for the optimized corrosion protection ability of the Ni–P–CNT coatings [27,28]. The ability of the CNT particles to block pores and cavities in the coating layer is associated with the improved barrier properties of the composite coatings [29,30].

In the present work, we expand the current knowledge related to the development of electroless Ni–P/MWCNT coatings by evaluating the local electrochemical activity using scanning electrochemical microscopy (SECM). Its use as an analytical tool to investigate local corrosion processes of a variety of metallic alloys has been reported in the literature. Both uncoated and coated alloys have been probed [31,32]. SECM can detect the local electrochemical activity associated with nucleation of pits, dissolution of metallic inclusions, and defects through coatings [33–35]. In this respect, for the first time we report the assessment of the local electrochemical activity of Ni–P/MWCNT composite coatings by SECM. Furthermore, Ni–P/MWCNT on HSLA pipeline steels is innovative. In order to support discussion on the electrochemical results, film structure and adhesion properties of the deposited layers were also evaluated.

#### **2. Materials and Methods**

#### *2.1. Substrate and Coating Preparation*

API 5L X80 pipeline steel (Usiminas, Ipatinga, MG, Brazil) was employed as substrate. Its chemical composition is shown in Table 1. Specimens were cut from the as-received plate into rectangular pieces with the final dimensions of 30 mm × 30 mm × 5 mm. Before deposition, the specimens were ground with silicon carbide waterproof paper up to grit 1200. Next, the surface was cleaned with alcohol, rinsed with deionized water, and dried with a heat gun.


**Table 1.** Chemical composition of the API 5L X80 steel plate (wt.%).

Electroless deposition was accomplished by preparing a Ni–P plating bath consisting of nickel sulfate (30 g.L−1), nickel hypophosphite (40 g.L−1), sodium citrate (10 g.L−1), acetic acid (10 mL.L−1), lactic acid (10 mL.L−1), and sodium hydroxide (40 g.L−1). Sodium dodecyl sulfate (2 g.L−1) was added to facilitate dispersion of the multiwalled carbon nanotubes in the bath. The bath was operated at 88 ◦C and was magnetically stirred during deposition. The pH was 4.5, adjusted with ammonium hydroxide. Multiwalled carbon nanotubes (MWCNT) were purchased from the Federal University of Minas Gerais (Brazil). Three different CNT concentrations were added to the plating bath: 0.25 g.L−1, 0.50 g.L−1, and 1.0 g.L−1. These samples are designated as CNT-0.25, CNT-0.50, and CNT-1.0 throughout the text.

Before deposition, the specimens were cleaned in an alkaline solution consisting of 10 wt.% NaOH at 50 ◦C and activated in a 50% vol. H2SO4 solution at room temperature. After washing with deionized water, the specimens were immersed in the plating bath. The total deposition time was 2 h. After deposition, the specimens were annealed at 400 ◦C for 1 h in a tubular furnace under argon atmosphere, followed by cooling inside the furnace.

#### *2.2. Structural, Morphological and Adhesion Characterization*

The crystalline character of the Ni–P/MWCNT composite coatings was assessed by X-ray diffractometry (Rigaku DMAX-2000) in the θ-2θ configuration, employing Cu-kα radiation. The 2θ range was from 20◦ to 70◦. The surface morphology and cross-sections of the different coatings were examined by scanning electron microscopy (SEM) coupled to an X-ray energy dispersive spectrometer (EDS) to study the elemental composition at the coating/substrate interface.

The adhesion strength of the Ni–P/MWCNT layers was evaluated through scratch tests by means of a Ducom T101 apparatus equipped with a Rockwell C-type diamond tip. The normal load was continuously increased from 1 N to 38 N at a rate of 2 N.min−1, a scratch velocity of 0.5 mm.s<sup>−</sup>1, and a total scratch length of 10 mm. Confocal laser scanning microscopy (Olympus, LEXT OLS4100) was employed to evaluate the penetration depth of the indenter and topographic features at the interface between the unscratched and scratched regions.

#### *2.3. Global and Local Electrochemical Tests*

Conventional electrochemical tests were carried out using an Autolab M101 potentiostat/galvanostat. A classical three-electrode cell set up was employed with a platinum wire as the auxiliary electrode, Ag/AgCl as the reference, and the coated API 5L X80 specimens as the working electrodes. The tests were performed in 3.5 wt.% NaCl solution at room temperature. Initially, the open circuit potential was monitored for 1 h. Right after, electrochemical impedance spectroscopy measurements were made at the OCP in the frequency range from 100 kHz to 10 mHz. The amplitude of the perturbation signal was ±10 mV and the acquisition rate was 10 points per decade. Next, the samples were subject to potentiodymamic polarization by sweeping the potential between −300 mV versus the OCP up to +1, 0 VAg/AgClat 1 mV.s−1.

SECM current maps were acquired using a commercial Sensolytics system, operating in the substrate generation-tip collection mode (SG-TC). In this operation mode, electroactive species generated on the corroding surfaces are reduced or oxidized at the tip, providing the values of the current related to electrochemically active sites on the material [36,37]. The SG-TC operation mode was reported in the investigation of localized corrosion sites on the stainless steel [33,38], whereby the selective monitoring of reacting species provided from the specific reactions required the operation of the corroding system without the need to insert a redox mediator to reach electrochemical responses. In this present work, the SG-TC operation mode was used to investigate the electrochemical activity of the studied material associated with the localized production of Fe2+ ions above the surfaces. Thus, Fe2+ ions produced from the corrosion of the material is sensed in an oxidation reaction at the Pt tip, according to Equation (1).

SECM current maps were acquired using a commercial Sensolytics system, operating in the substrate generation-tip collection mode (SG-TC). The reaction shown in Equation (1), which is typical of ferrous alloys corrosion in aqueous media [33,37], was probed by biasing the tip at +600 VAg/AgCl.

$$\text{Fe}^{2+} \leftrightarrow \text{Fe}^{3+} + \text{e} \tag{1}$$

A glass-insulated 10 μm diameter Pt microelectrode was used as the tip, Ag/AgCl as the reference electrode, and a Pt wire as the auxiliary electrode. The specimens were mounted horizontally facing upwards. The tip was at a height of 30 μm above the substrate surface. The potentials were controlled with a bipotentiostat coupled to the SECM system. The specimens were at the open circuit potential. The measurements were carried out in a 0.1 M NaCl solution at room temperature. The electrolyte was less concentrated than that used for the global electrochemical tests, as the results obtained in 3.5 wt.% NaCl did not allow to distinguish the local electrochemical response of each sample by SECM due to excessively high current densities measured throughout the probed area.
