Ni-P Coatings as Hydrogen Permeation Barriers—A Review

Abstract

Hydrogen became one of the most studied energy carriers after the global energy crisis and can replace gas and oil as clean fuels. The main challenge is its safe storage and long-distance transportation: steel is among the materials most used for hydrogen storage and transportation. However, steel is susceptible to hydrogen embrittlement (HE). HE can be prevented by depositing hydrogen barrier coatings on the steel surface. This review provides an overview of the hydrogen permeation mechanism and the analytical methods employed to evaluate the performance of the hydrogen permeation barriers. The focus is on Ni and electroless Ni-P coatings deposited on steel as hydrogen barriers. These coatings have been used so far for their anti-corrosion and wear properties; they are currently of interest due to their low hydrogen permeability. The simplicity of production and the possibility of achieving a homogeneous coating, regardless of the geometry of the substrate, make the electroless deposition process of the Ni-P alloy a candidate for ‘in situ’ applications in existing pipelines. This process can be implemented by using and adapting the established pig batch technology.

1. Introduction

The reliability of energy storage technology is currently of utmost importance for the global energy transition and for stabilizing the supply from renewable energy sources [1]. Hydrogen became one of the most investigated energy carriers after the global energy crisis of 1974: “green hydrogen” can be easily generated by electrolysis from renewable sources, and it can replace gas and oil as a clean fuel, becoming the primary energy carrier in the future [2,3,4,5]. The main problem to be solved is the safe storage and long-distance transportation of hydrogen due to hydrogen embrittlement of high-strength steel [6,7]. This problem has become the driving force behind research into new materials for use as hydrogen permeation barriers [8,9]. Steel alloys, as structural materials, are characterized by high strength and durability, good performance, and reasonable cost, and for these reasons, they are widely used in different industrial applications [10,11,12]. In the gas pipeline and hydrogen technology field, for example, the predominant choice of materials involves using low alloy steel with different carbon contents and heat treatments [13]. However, it is well-known that hydrogen in steel reduces the ductility and formability of the steel [14,15,16,17,18]. Hydrogen in steel can originate from the manufacturing process, corrosion, or its interaction with the surrounding medium. The very small hydrogen atoms can easily be absorbed and pass through the crystal microstructure of the steel by diffusion, causing hydrogen embrittlement (HE) [14,19,20,21,22,23,24,25]. The diffusion mechanism of hydrogen entry into the material (e.g., steel) depends on several factors, such as temperature, pressure, the electrochemical potential at the electrolyte–metal interface, the pH, the composition of the electrolyte, composition of the material, the presence of promoters and inhibitors, and the current density of hydrogen evolution [26]. Diffusion constitutes a phase of a complex process called hydrogen permeation (see the following paragraph) [9].

Electroless Ni-P coatings are commonly used due to their anti-corrosion and anti-wear properties [27,28,29,30,31,32,33] and they are currently of interest as hydrogen permeation barriers (HPBs) [34,35]. This review aims to provide an overview of the mechanism of hydrogen permeation and of the analytical methods that can be applied to evaluate the performance of HPBs. It also describes the production and properties of electroless Ni-P coatings with a focus on their application as hydrogen permeation barriers.

2. Hydrogen Permeation Mechanism

Although the mechanism of hydrogen embrittlement is not completely clear, it certainly involves the diffusion of hydrogen atoms [9,36,37,38]. The hydrogen permeation mechanism can be depicted as a multistep process [39,40,41]: adsorption, dissociation, dissolution, diffusion, recombination, and desorption. First, the hydrogen molecule is adsorbed onto the surface and dissociated into hydrogen atoms. Second, hydrogen diffuses through the coating (hydrogen permeation barrier) and the substrate. Third, hydrogen atoms desorb after crossing the bi-layer and recombine to form hydrogen molecules (Figure 1a) or are oxidized to a proton (Figure 1b). Various factors influence hydrogen permeation, including vacancies, dislocations, and grain boundaries. These lattice defects can potentially serve as hydrogen traps. Hydrogen trapping occurs when negative trapping energies are measured. This indicates that the hydrogen atom in the potential well of the trap is more stable, resulting in a decrease in H-diffusivity [39,40,41,42].

2.1. Hydrogen Permeability

Hydrogen permeability (P_m), expressed as [mol H₂/(m s Pa0.5)], is the product of effective diffusion (D_eff) and solubility (S) [40]. It is calculated from the diffusion constant (D₀), the solubility constant (S₀), the activation energy diffusion (E_D), and the standard enthalpy of dissolution (ΔH_S) for hydrogen as well as the gas constant (R) and temperature (T) [8,43]:

$${\mathrm{P}}_{\mathrm{m}}={\mathrm{D}}_{\mathrm{eff}}·\mathrm{S}={\mathrm{D}}_0{\mathrm{S}}_0{\mathrm{e}}^{-\left(\frac{\Delta {\mathrm{H}}_{\mathrm{s}}+{\mathrm{E}}_{\mathrm{D}}}{\mathrm{RT}}\right)}$$

(1)

Thus, hydrogen permeability is governed by hydrogen diffusion and the thermodynamic equilibrium and is independent of the surface condition. However, experimental data on hydrogen permeability reported in the literature indicate that it is strongly influenced by various properties of the surface sample [44]. Equation (1) considers the hydrogen diffusion and its dissolution into the lattice but neglects the trapping effects and the influence of the defects within the material [44].

Hydrogen diffusion flux (J) (mol m⁻² s⁻¹) for an ideal homogenous single crystal, without any lattice defects, may be derived from Fick’s first (Equation (2)) and second (Equation (3)) laws [8,26,45]:

$$\mathrm{J}=-{\mathrm{D}}_{\mathrm{eff}}·{\displaystyle \frac{\delta \mathrm{C}}{\delta \mathrm{x}}}$$

(2)

$${\displaystyle \frac{\partial \mathrm{C}}{\mathrm{dt}}}={{\mathrm{D}}_{\mathrm{eff}}}^2·{\displaystyle \frac{{\delta }^2\mathrm{C}}{{\delta \mathrm{x}}^2}}$$

(3)

where D_eff is the effective diffusion constant, C the concentration of diffusible hydrogen and x the distance vertical to the surface; thus, dc/dx is the gradient of the hydrogen concentration in the permeation direction in the metal membrane [14], and t is the time.

The situation for a bi-layer coating/substrate has been solved by Song [46]. Equations (2) and (3) can be written for both layers, substituting D_eff with D₁ and D₂, respectively. As the flux through both layers must be the same, at the boundary between the two layers holds Equation (4), and the relation between the effective diffusion coefficient of the bi-layer D_eff and D₁ and D₂ is calculated [46].

$${\mathrm{D}}_1·{\displaystyle \frac{{\delta \mathrm{C}}_1}{\delta \mathrm{x}}}={\mathrm{D}}_2·{\displaystyle \frac{{\delta \mathrm{C}}_2}{\delta \mathrm{x}}}$$

(4)

2.2. Electrochemical Method for Evaluating the HP: Devanathan–Stachurski’s Cell

Several methods are reported in the literature, such as the gaseous hydrogen permeation method [47,48]; the dynamic method, which involves applying a quadrupole mass spectrometer (QMS) [49]; the static method, which involves recording the pressure rise in an accumulation volume by an inert pressure gauge [44] and can be used for evaluating hydrogen permeability (HP); and the electrochemical method. The electrochemical method provides information not only concerning diffusion constants, but also on the density of trap sites and their associated energies [8,50]. An instrument for measuring hydrogen permeability is called Devanathan–Stachurski’s (D-S) cell [50,51] (Figure 2). It is a dual electrochemical cell configuration, with reference electrodes (RE) (e.g., Ag/AgCl), counter electrodes (CE) (platinum grid) and a thin metal sample between the two cells that acts as the working electrode (WE). Each electrochemical cell is connected to a potentiostat. This set up accurately determines the hydrogen permeation rate [50,51].

In the cathode cell, H₂ is produced: for this reason, this cell is also known as a production cell. The atomic hydrogen passes through the tested sample (WE), and the hydrogen permeation current is recorded in the anode cell (detection cell), obtaining the hydrogen permeation curve (Figure 3).

The configuration of the cell and the standard procedure are reported in the ASTM 148-97(2018) [52] and in the ISO 17081:2014 [53]. The optimal geometric model and boundary conditions for accurately measuring the diffusion coefficient of hydrogen in metals are as follows:

• A membrane with a fixed thickness, x = L, is being considered where the in-plane dimensions y and z are significantly larger than L;
• The concentration of hydrogen atoms at the production side of a membrane remains constant over time, and the concentration is zero on the detection side, i.e., C = C₀ at x = 0 and C = 0 at x = L for any value of the time t.

The hydrogen flow at the detection side can be derived as a function of the time as outlined in references [26,54,55] and a mathematical model for the anodic flux in a D-S cell over time is developed. From the hydrogen permeation curve (Figure 3), the effective hydrogen diffusion coefficient can be calculated for a single material or a bi-layer by the following equations [50,51]:

$${\mathrm{D}}_{\mathrm{eff}}={ \mathrm{L}}^2/15.3{ \mathrm{t}}_{\mathrm{b}}$$

(5)

$${\mathrm{D}}_{\mathrm{eff}}={ \mathrm{L}}^2/6{ \mathrm{t}}_{\mathrm{lag}}$$

(6)

where D_eff is the effective hydrogen diffusion coefficient in m²/s; L is the thickness of the sample in meters; t_b is the breakthrough time (corresponding to the intersection of the tangent line at the inflection point and the horizontal axis in the curve) in seconds; and t_lag is the retardation time: i.e., the time when the permeation current reaches 0.63 times the steady-state current value in seconds (Figure 3). To estimate the diffusion coefficient of the coating, Equation (9) in Section 3.5 has to be considered.

Moreover, it is possible to determine the subsurface concentration of hydrogen C₀ (in ppm) by using the following relationship [56,57]:

$${\mathrm{C}}_0={\displaystyle \frac{{\mathrm{i}}_{\mathrm{ss}}\mathrm{L}}{{\mathrm{FD}}_{\mathrm{eff}}}}{\displaystyle \frac{{\mathrm{M}}_{\mathrm{H}}}{{\rho }_{\mathrm{Fe}} }}{10}^6$$

(7)

where,

• i_ss: is the steady state permeation current density (A/m²),
• L: is the thickness of the sample (m),
• D_eff: is the effective diffusion coefficient (m²/s),
• F: is the Faraday constant (96,485 C/mol),
• M_H: is the molar mass of hydrogen (1 g/mol),
• ρ_Fe: is the iron density (7.87 × 10⁶ g/m³).

3. Hydrogen Permeation Barriers

3.1. General

Typically, hydrogen storage tanks or pipelines are made of steel that can suffer hydrogen embrittlement (HE). HE can be prevented by depositing a hydrogen barrier coating (HPB) on the steel substrate [9,38]. An efficient hydrogen permeation barrier should prevent or reduce the hydrogen adsorption and should have no structural defects, such as pinholes, pores or cracks. While D_eff and C₀ describe the hydrogen diffusion process, the HPB efficiency can be assessed by the permeation reduction factor (PRF) [44]. The PRF is the steady-state ratio of the permeation rate through the uncoated substrate j_uncoated divided by the permeation rate through the coated substrate j_coated (Equation (8)):

$$\mathrm{PRF}={\displaystyle \frac{{\mathrm{j}}_{\mathrm{uncoated}}}{{\mathrm{j}}_{\mathrm{coated}}}}$$

(8)

In the case of a multilayer system with both substrate and coating, the PRF value can be used to estimate the coating permeability P_coat according to Equation (9), taking into account the thickness and the permeability of the substrate, L_steel and P_steel, respectively, and the thickness of the coating, L_coat [58]. The permeability of the substrate is estimated from separate experiments as P_steel = D_steel × C_{0 steel} (Equations (6) and (7)).The higher the value of PRF, the better the HPB [44,58].

$$\mathrm{PRF}\cong {\displaystyle \frac{{\mathrm{L}}_{\mathrm{coat}}{\mathrm{P}}_{\mathrm{steel}}}{{\mathrm{L}}_{\mathrm{steel}}{\mathrm{P}}_{\mathrm{coat}}}}$$

(9)

Different coatings have been tested as hydrogen barriers [58]. Ceramic coatings are widely studied as HPBs due to their high-temperature suitability and corrosion resistance properties, which are important in the hydrogen environment [44,58,59,60,61]. The PRF value might vary from 236 for a Cr₂N coating to 4600 in the case of ZrN (Figure 4) [44,59,60,62,63]. Although the PRF value is an indicator of the coating’s efficiency and of the application potential of the barrier, it is not a sufficient value as it does not consider other aspects such as the adherence to and the suitability of the coating for the substrate [58].

3.2. Ex Situ Surface Analytical Techniques for Evaluating HPBs’ Performance

The development of suitable coatings as hydrogen permeation barriers requires controlling the microstructure of the films to optimize the properties of technological interest. The combination of in situ electrochemical tests (mentioned above) and ex situ surface analytical techniques is fundamental for understanding the performance of the HB [9]. A list of analytical techniques commonly use for evaluating HPBs’ performance is reported in Table 1.

As the coating’s morphology and homogeneity is of utmost importance for its effectiveness as a barrier, substrate surface preparation is essential for a good coating deposition and in order to avoid the presence of microscopic defects that are one of the main issues in gas transport [64,65].

Generally, three substrate pretreatments are performed: mechanical pretreatment (e.g., grinding, polishing), wet chemical cleaning in an ultrasonic bath, and ion etching in a vacuum chamber [64]. The main problem caused by the substrate’s pretreatments is the possible formation of small pits (or cavities) in the substrate that can lead to the formation of a pinholes in the coating [66,67].

Metallographic microscopes and scanning electron microscopes (SEMs) are generally used to analyze the surface morphology of the coating, allowing the observation of cracks, pores, pinholes and other different microstructure defects, and they can give information about the grain’s fine structure, allowing the determination of the particle size and the thickness of the coating [68,69,70,71,72]; in addition, atomic force microscopy (AFM) can be used to obtain information about the roughness of the coating, providing guidance on the development of suitable surface coating deposition methods [68,69,70,71]. Another fundamental surface analytical technique for evaluating the performance of HPBs is X-ray photoelectron spectroscopy (XPS). This technique provides information on the chemical composition and elemental distribution of the sample surface both before and after deposition of the coating, giving further insight into the quality of the deposition method [73,74,75].

Angle-resolved X-ray photoelectron spectroscopy (ARXPS) is a non-destructive method that can be applied for determining in-depth composition profiles of nano-sized thin films [76,77]. This method gives essential information about the mechanism of surface film formation and about the properties of the coating in terms of corrosion and wear resistance [76,77]. Scorciapino et al. [77], based on a combination of electrochemical and surface analytical studies, proposed a model for the protective properties of 20 μm Ni-P coated mild steel samples when immersed in a sulfate solution at pH 6.3. The surface of the Ni-P coating showed a multilayer structure with different thicknesses and compositions (Figure 5a): an outermost contamination layer made of organic carbon and oxygen, a second thin layer (about 1 nm thick) made of nickel polyphosphate, a highly phosphorus-enriched interface zone of about 2 nm thick with a marked concentration gradient, and the bulk of the alloy. Figure 5b shows the reconstructed depth profile obtained by combining the maximum entropy method (MEM) with the ARXPS data of a Ni18P alloy after 1 h polarization at +0.1 V SCE in 0.1 M Na₂SO₄ [75]. The nickel–polyphosphate layer detected at the surface might act as a barrier at the surface of the alloy and hinders the dissolution of nickel ions, explaining the excellent tribological properties of Ni-P alloys and their excellent corrosion resistance.

Another interesting and increasingly used analytical technique for characterizing multilayer systems is hard X-ray photoelectron spectroscopy (HAXPES) [78,79]. A recent work [80] showed that the combination of XPS/HAXPES with Elastic Recoil Detection Analysis (ERDA) and Rutherford Backscattering Spectroscopy (RBS) allowed for studying the effect of hydrogen incorporation on alumina films (grown on silicon substrates by atomic layer deposition) and on the local chemical binding state of Al, O and H.

Table 1. List of analytical techniques for evaluating HPBs performance.

	Technique	Information	References
In situ	Gas permeation test	Diffusion coefficient; permeability coefficient; hydrogen solubility coefficient; permeation reduction factor	[44,47,48,49,71]
	Devanathan–Stachurski’s (D-S) cell Electrochemical test	Diffusion coefficient; trap density; subsurface concentration of hydrogen; permeation reduction factor	[50,51,52,53]
Ex situ	OM	Morphology; particle size	[64,70]
	SEM	Information about the topography, chemical composition, and film thickness (combined with EDS); 2D detailed images	[64,66,67,68,69,70,71,72]
	AFM	Topography; surface roughness	[64,68,71]
	XPS/ARXPS/HAXPES	Information about chemical composition, chemical state identification and film thickness; qualitative and quantitative analysis; imaging/mapping.	[65,71,73,74,75,76,77,78,79,80]
	ERDA/RBS	H-concentration in films	[80]

Table 1. List of analytical techniques for evaluating HPBs performance.

	Technique	Information	References
In situ	Gas permeation test	Diffusion coefficient; permeability coefficient; hydrogen solubility coefficient; permeation reduction factor	[44,47,48,49,71]
	Devanathan–Stachurski’s (D-S) cell Electrochemical test	Diffusion coefficient; trap density; subsurface concentration of hydrogen; permeation reduction factor	[50,51,52,53]
Ex situ	OM	Morphology; particle size	[64,70]
	SEM	Information about the topography, chemical composition, and film thickness (combined with EDS); 2D detailed images	[64,66,67,68,69,70,71,72]
	AFM	Topography; surface roughness	[64,68,71]
	XPS/ARXPS/HAXPES	Information about chemical composition, chemical state identification and film thickness; qualitative and quantitative analysis; imaging/mapping.	[65,71,73,74,75,76,77,78,79,80]
	ERDA/RBS	H-concentration in films	[80]

3.3. Production of Electroless Ni-P Coatings

To deposit a good coating, substrate pretreatment (cleaning, polishing) is important (see Section 3.2). Electroless Ni-P (ENP) plating is commonly applied as an anti-corrosion coating (e.g., for steel) [30,32], but it is currently of interest due to its low hydrogen permeability [35,81,82]. The advantage of Ni-P electroless coatings is not only the anti-corrosion properties but also the lower cost of the synthesis [30,32,35,77,81,83] compared to other coatings. The process of electrolytic nickel plating, for example, requires an external electric current, is time-consuming, and results in a less uniform coating. The great advantage of electroless coating is that it can be applied uniformly to any surface, regardless of the complexity of the surface geometry of the item to be coated, so it could also be applied to existing pipelines [28,84,85]. In addition, electroless Ni-P coating is reported to be harder and to possess higher corrosion resistance than those obtained through electrodeposition [86,87].

Various conditions are reported in the literature that are commonly adopted for electroless coatings [30,35,82,84]. In general, the main requirements for electroless Ni-P deposition are [83]:

• Source of Ni²⁺ ions: e.g., nickel sulfate or nickel chloride;
• Reducing agent: usually sodium hypo-phosphite (NaH₂PO₂);
• Complexing agents: as organic acids or their salts (acetic, malic, succinic, or citric). They prevent the formation of an excessive concentration of free metal ions, and they act as buffers and to delay the precipitation of nickel phosphite. These complexing agents exert a significant influence on the deposits’ quality and porosity levels;
• Stabilizers or accelerators: they are added in small amounts (ppm) in order to raise the deposition rate. The most common stabilizers used are Pb, As, Mo, Cd ions, malic, and thioureas;
• Temperature: it influences the kinetics and speed of deposition and it must be controlled to obtain a high-quality coating. The optimum operating temperature of an acid hypophosphite plating solution ranges from 85 °C to 90 °C. High temperatures beyond 90 °C might lead to solution “plate-out” or bath decomposition [28,88];
• pH regulator: pH is an important parameter since it affects the phosphorus content: the higher the pH value, the lower the phosphorus content obtained. The common pH regulators used are sodium hydroxide and/or sulfuric acid.

The properties of the electroless Ni-P coating are mainly dependent on the content of phosphorus that controls their microstructures [89]. Based on the phosphorus content, Ni-P alloys can be divided into three groups: low (1–5 wt.%), medium (5–9 wt.%) and high (above 9 wt.%) phosphorus content [90]. The low phosphorus deposits are either crystalline or consist of a microcrystalline nickel phase [91,92,93]; the medium phosphorus coatings are either fully amorphous or contain mixtures of microcrystalline nickel and amorphous phases [92,94]; and at a high phosphorus concentration of about 10 wt.% (c.a. 20 at.%), the structure is X-ray amorphous [86,89,90,91]. The control of the plating parameters, namely the phosphorous acid concentration, temperature and time, allow for obtaining different Ni-P coatings containing various concentrations of phosphorous, and thus, different crystal structures [27,33,95,96,97,98].

3.4. Properties of Electroless Ni-P Coatings

Ni-P as protective coatings against corrosion have been studied since 1946 [99,100]. These coatings can be applied on different substrates, and they can provide excellent chemical and mechanical properties, such as adhesion, good corrosion resistance, high hardness and wear resistance [101,102,103,104].

The ISO 4527:2003 [105] lists the different requirements and test methods for analyzing specific properties of the electroless nickel–phosphorus alloy depending on the application. In addition to the uniformity, morphology and chemical composition, other parameters such as the layer thickness, adhesion and porosity, should be evaluated for optimizing the properties of Ni-P coatings [103]. The minimum coating thickness required for corrosion resistance in service, applied on a ferrous substrate, ranges from 0.1 μm, for very mild service conditions (e.g., for thin film resistors), to 125 μm, for high severe conditions (e.g., for oil field applications) [103,105].

The upgrading of the adhesion, hardness and wear resistance of electroless Ni-P coatings can be achieved by applying a heat treatment [105]. For improving the adhesion onto steel, for example, a temperature range of 180 °C–200 °C for 2–4 h is required. In general, after the heating procedure, the hardness increases with a decrease of the phosphorus content. Heat treatment below 200 °C performed to minimize the risk of hydrogen embrittlement should not substantially increase the wear and corrosion resistance and/or the hardness of the coating. However, a temperature above 260 °C should lead to a magnetic coating.

One of the most important parameters to examine is the porosity: nickel–phosphorus coatings should be free of porosity as the corrosive processes can occur at through-thickness pores or microstructure defects [103,105]. ISO 4527:2003 [105] recommends a roughness value R_a < 0.2 μm to prevent corrosion. A low porosity is even more relevant when using these alloys as HPB, as the porosity affects hydrogen diffusion [106].

3.5. Nickel and Ni-P Coatings as Hydrogen Permeation Barriers

Permeability and diffusion of hydrogen through iron and iron-based alloys is high, even at room temperature, and the risk of hydrogen embrittlement is critical. Thus, the steel parts have to be protected by suitable coatings that reduce or avoid hydrogen permeation into the steel. Various hydrogen barrier coatings have been studied, including metals and alloys, oxides, ceramics, etc. [8,9,41,81]. Among these different coating materials, electrodeposited nickel and electroless nickel–phosphorus alloys are promising candidates due to their low hydrogen permeability [35,81,82,107,108,109,110,111,112,113,114,115,116].

3.5.1. Pure Nickel

Hydrogen diffusion of pure nickel was studied by Tanabe et al. [109]. Specimens of 2.1 cm diameter and thickness from 0.275 to 1.02 mm were annealed in vacuum at about 650 °C for 25 h before the tests that were performed in a temperature range from 350 to 600 °C. The diffusion coefficient of hydrogen D_eff was found as 7.11 × 10⁻³ exp(−10,700/RT) cm²s⁻¹, and for room temperature, a value of 9.5 × 10⁻⁵ cm²s⁻¹ can be extrapolated. This value is in good agreement with results of a review of Robertson where D_eff is reported to be 1.33 × 10⁻⁴ cm²s⁻¹ [110].

3.5.2. Electrodeposited Nickel (ED)

Electrodeposition of nickel is performed by application of a galvanostatic current in a plating bath [86]. The effect of nickel coatings on the permeation of hydrogen through iron membranes showed a reduction of the permeation current between a factor of 10 for 3.2 µm electroless and a factor of 20 for 12.4 µm electrodeposited nickel coatings [111]. The hydrogen permeability of electrodeposited nickel on 50 µm thick AISI 1017 steel samples was studied by Paatsch [112] to correlate the coating morphology with the hydrogen diffusion coefficient. The diffusion coefficient was in the range of 5 × 10⁻⁸ to 8 × 10⁻⁸ cm²s⁻¹ for a plating current density from 0.05 to 1 mA/m². Kim et al. [113] studied hydrogen permeation through AISI 4340 steel covered with an electroplated nickel layer. The diffusion coefficient of the Ni layer was found in the range of 3.9 ± 0.6 × 10⁻⁹ cm²s⁻¹ for 5 µm to 1.4 ± 0.4 × 10⁻⁹ cm²s⁻¹ for a 15 µm layer (Figure 6). Results of other studies [107] are included in

Table 2. Effective diffusion coefficients of coated samples (D_eff), diffusion coefficients of the coating (D_coat) and subsurface concentration of hydrogen (C₀) reported in the literature for pure nickel and for electrodeposited nickel (ED) and electroless Ni-P coated steel (EN).

Material	Coating	Thickness µm	Deff cm2s−1	DNi cm2s−1	C0	Reference
Nickel	-	-	9.5 × 10−5	-	-	[109]
	-	-	1.33 × 10−4			[110]
AISI 1017	ED nickel	50		5.0 × 10−8		[112]
	ED nickel	50		8.0 × 10−8
AISI 430	-	-	1.2 × 10−6	-	6.9 × 10−6	[113]
	ED nickel	5	5.0 × 10−7	3.9 × 10−9	4.8 × 10−6
	ED nickel	10	2.2 × 10−7	2.0 × 10−9	4.8 × 10−6
	ED nickel	15	8.4 × 10−8	1.4 × 10−9	4.8 × 10−6
Mild steel	-	-	1.04 × 10−6	-		[107]
	ED nickel	8	6.78 × 10−7	1.9 × 10−8
	EN nickel EN nickel	6 16	1.38 × 10−7 1.10 × 10−7	1.2–2.4 × 10−9
Ni81P19	Ribbon	38	-	3 × 10−10		[115]
Ni81P19	Ribbon			9 × 10−11	1823 × 10−6	[116]
X70 steel	-	-	0.75 × 10−6	-		[108]
	ED nickel	3.8	0.54 × 10−6	* 4.7 × 10−9
	EN nickel	3.5	0.39 × 10−6	* 2 × 10−9
	EN nickel	8.5	0.042 × 10−6	* 2.7 × 10−10

* calculated with Equation (9).

. The hydrogen permeation behavior of polycrystalline nickel implanted with He, Ar, Ni, Yt and Pt was also investigated [117] to elucidate the effect of surface modification. It was found that the effective diffusion coefficient of hydrogen was lower, and the effective solubility was larger in Ni, Yt and Pt-implanted nickel with a dose of more than 1 1015 ions cm⁻² compared to un-implanted nickel.

3.5.3. Amorphous or Electroless Nickel–Phosphorus

Amorphous Ni81P19 alloys prepared by rapid quenching were tested early with respect to hydrogen permeation [115,116] and the effective hydrogen diffusion coefficient D_eff determined with the break-through method from the permeation curves was found to be in the order of 10⁻¹⁰ cm²s⁻¹ (Table 1) and thus very promising for a hydrogen permeation barrier. Electroless nickel–phosphorous coatings on X70 steel were studied by Samantha et al. [35,81,108] mainly for their wear resistance. A thorough characterization of the coatings was performed [81,108] and the resistance to hydrogen embrittlement of the steel/coating systems was evaluated. The effective diffusion coefficient of the steel/coating systems was calculated from the breakthrough times of the hydrogen permeation curves reported earlier [35,81]. The resulting effective diffusion coefficients D_eff [108] for the uncoated steel, the Ni-electroplated steel and the electroless nickel EN Ni-P coated steel are all reported in the range from 0.75 to 0.18 × 10⁻⁶ cm²s⁻¹ (Table 2); no attempts were made by the authors to estimate the true diffusion coefficient D_coat of the electroless Ni-P coating, and the values given in Table 2 for ref [108] were calculated in this work. In Figure 7, a comparison of the diffusion coefficient values of EN nickel layers (D_Ni) is reported.

For a multilayer system, in addition to the PRF value calculated by Equation (9), considering the permeability values, the diffusion coefficient of the coating can be estimated taking into account the effective diffusion coefficients of a steel/coating system and the thickness of the substrate and the coating (Table 1), with the equation proposed in [107]:

$${\displaystyle \frac{{\mathrm{L}}_{\mathrm{coat}}}{{\mathrm{D}}_{\mathrm{coat}}}}+{\displaystyle \frac{{\mathrm{L}}_{\mathrm{steel}}}{{\mathrm{D}}_{\mathrm{steel}}}}={\displaystyle \frac{{\mathrm{L}}_{\mathrm{tot}}}{{\mathrm{D}}_{\mathrm{eff}}}}$$

(10)

where,

• L_coat: is the thickness of the coating (μm)
• L_steel: is the thickness of the steel substrate (μm)
• L_tot: is the thickness of the coated sample (μm)
• D_coat: is the diffusion coefficient of the coating (cm²s⁻¹)
• D_steel: is the diffusion coefficient of the steel substrate (cm²s⁻¹)
• D_eff: is the effective diffusion coefficient of the coated sample (cm²s⁻¹)

The diffusion coefficients of the electrodeposited nickel coatings vary between 4.7 × 10⁻⁹ cm²s⁻¹ for 3.8 µm and 1.4 × 10⁻⁹ cm²s⁻¹ for 15 µm coating thicknesses (Table 2). The diffusion coefficients of the electroless deposited nickel–phosphorous coatings are in the range from 2 × 10⁻⁹ to 2.7 × 10⁻¹⁰ cm²s⁻¹, reaching the values of the melt spun Ni₈₁P₁₉ ribbons (0.9–3 × 10⁻¹⁰ cm²s⁻¹), acting as an efficient hydrogen permeation barrier.

3.6. Application for Hydrogen Distribution Pipelines

Hydrogen pipelines have been operating worldwide for many decades with a good safety record. These pipelines are typically used for transporting hydrogen or syn gas to petrochemical plants over short distances and at low pressures [118]. However, this experience cannot be extrapolated to long distances and high pressure for the future energy demand. A large and distributed network of already existing gas pipeline infrastructure, especially in Europe [119], can serve as a possible transportation method that can be repurposed for hydrogen transportation service, whether for pure or blended gaseous hydrogen. The cost for this repurposing is estimated at 10–35% of new construction costs [120]. A summary of possible liners and coatings is presented in [118].

Among the different technological solutions, electroless nickel–phosphorus coatings are mentioned. Electroless plated nickel coating is a commercial product that results in a uniform nickel–phosphorus layer that has been successfully used as an internal coating on pipelines and fittings to improve its corrosion resistance and anti-wear properties [27]. The advantage compared to electrodeposited commercial nickel coatings is that the autocatalytic nature of the deposition does not require passing an electric current through the bath and the substrate. Electroless plating creates a uniform metal layer regardless of surface geometry. In a review it is stated [118]: “The flexibility in plating volume and thickness, ability to achieve a bright finish with minimal surface roughness, and the mature application methods make electroless plating a candidate for hydrogen pipeline coatings”. It can be imagined that the process of electroless plating could be applied “in situ” in existing pipelines, using and adapting the mature technology of the “pig batch method”. This is a procedure used for coating existing pipelines or for new pipelines, post-construction, that require a continuous and consistent coating [121].

4. Summary, Perspectives, and Prospects

In this review, an overview of the mechanism of hydrogen permeation, the in situ and ex situ analytical techniques, and the analytical methods that can be applied for evaluating the performance of HPBs are reported. The focus is on the properties of the electroless Ni-P coating as a HPB by reporting on the findings so far in the literature.

A literature review is provided on combining in situ and ex situ techniques to evaluate the performance of different materials such as HPB. Different methods can be used to obtain information on hydrogen permeability, solubility, and diffusion from gaseous and electrochemical permeation tests. For example, ex situ techniques can provide information on the morphology, topography, thickness, and chemical composition of the material before and after permeation tests, which helps in understanding the possible mechanisms involved in hydrogen diffusion.

Electroless Ni-P coating has been studied since 1946 as a protective coating against corrosion. It can be applied uniformly to different substrates (regardless of the geometry), and it can provide excellent chemical and mechanical properties, such as strong adhesion, good corrosion resistance, high hardness and good wear resistance. In addition, a free-pores morphology is expected for these coatings: the porosity, indeed, is an important parameter that can affect the diffusion coefficient.

Although few works have been reported in the literature regarding the use of this coating as a HPB, recent investigations suggest that they might act as an efficient hydrogen permeation barrier: the diffusion coefficient of electroless deposited nickel–phosphorous coatings is in the range from 2 × 10⁻⁹ to 2.7 × 10⁻¹⁰ cm²s⁻¹, reaching the values of melt spun Ni₈₁P₁₉ ribbons (0.9–3 × 10⁻¹⁰ cm²s⁻¹).

Given the excellent properties of these coatings and their ease of production, one can imagine that the electrodeposition process could be applied “in situ” in existing pipelines, using and adapting the mature technology of the “pig batch method”. For these reasons, further investigations would be necessary and interesting in order to optimize the performance of electroless Ni-P coating as a hydrogen permeation barrier, with a view to future applications in hydrogen distribution lines.