Hydrogen Permeation in Carbon Steels

Abstract

The atomic hydrogen generally comes from corrosion reactions that take place between the steel surface and the sour media. These reactions generate atomic hydrogen that due to its small size can be adsorbed and diffuse through interstitial sites of the crystalline lattice of steel, where it accumulates in nonmetallic discontinuities such as inclusions, dislocations and second phases, where atomic hydrogen begins to combine and form molecular hydrogen, which can no longer diffuse in the steel and over time produces high internal pressures in localized areas of the thickness of the material, leading to the nucleation and propagation of cracks and/or blisters in the absence of applied external stresses. This depends on the type of interaction that occurs between hydrogen and the metal, environmental conditions, heterogeneities of the metal, and the state of stress to which the components are subjected. Due to the increase in sour media in the petroleum industry, it was necessary to reconsider the application of electrochemical techniques and sensors to control and predict hydrogen-induced cracking (HIC).

1. Introduction

This chapter describes the main theoretical and experimental aspects of hydrogen permeation in carbon steels exposed to sour media, hydrogen-induced cracking (HIC) as well as electrochemical techniques and electrochemical sensors to monitor hydrogen permeation and predict the (HIC).

Atomic hydrogen is generated through corrosion reactions between the steel and the sour media that contain venoms substances such as H₂S and CO₂, mainly. This hydrogen is capable of diffusing through the crystalline lattice of the steel and accumulates (preferably) in steel heterogeneities such as inclusions, grain boundaries, second phases, cavities, and vacancies. This promotes the accumulation of atomic hydrogen in such sites, causing, among other phenomena, HIC.

It has been reported by various authors that atomic hydrogen causes a detriment to the mechanical properties of steel, promoting and propagating cracks even in components that are not subjected to stress.

Recently, progress has been made in electrochemical techniques for detecting and measuring the amount of hydrogen that diffuses into metallic components.

At the end of this chapter, an experimental work carried out by the authors is described, where the effect of the carbon content and the microstructure of four carbon steels used in the hydrocarbon transportation industry was analyzed regarding diffusion or hydrogen permeation and its susceptibility to (HIC).

2. Hydrogen permeation in carbon steels

2.1 Corrosion between steel and sour media

Electrochemically, the corrosion reactions of iron in an acidic medium can be represented by an anodic reaction, where the iron is oxidized to present its ionic state, an oxidation reaction, giving up two electrons to the system, and a cathodic reaction, where ionic hydrogen is reduced to form atomic hydrogen and subsequently gaseous hydrogen [1]. The oxidation and reduction reactions are carried out at the same speed, and the sum of both is the corrosion equation of iron in a medium without oxygen.

Figure 1 schematically shows some of the possible reduction stages of hydrogen ions on the metal surface. This indicates that ionic hydrogen must be adsorbed on the surface of the metal (stage 1), then, a transfer of electrons from the metal to the hydrogen ion is carried out, producing atomic hydrogen (stage 2), two atoms combine to form hydrogen gas (stage 3) leading to the formation of hydrogen bubbles. Stage 4 is the slowest and therefore the one that controls the kinetics of the reaction [1].

Figure 1.

Stages of hydrogen ion reduction [1].

When As, Se, S, and P ions are present in the solution, they further delay stage 3 in the cathodic reaction, causing the surface of the metal to become saturated with atoms H that are capable of diffusing into the core or interior of the material.

From the corrosion reactions of iron in a medium without oxygen, it is evident that hydrogen can be adsorbed on the surface of the material to form hydrogen gas (stage 4), or it can be adsorbed on the surface of the steel and diffuse through it.

In the presence of H₂S, the following cathodic reaction was postulated in Eq. (1) [2, 3].

$$H_2{S}_{\mathit{\text{solution}}}+e^{-}\to {\mathit{HS}}^{-}+H_{\mathit{\text{adsorbed}}}$$E1

The reduction of hydrogen ions involves the production of atomic hydrogen and the subsequent formation of hydrogen molecules. Therefore, the corrosion reactions shown above are sources of hydrogen in metals. Certain substances, such as H₂S ions, phosphorus, and arsenic compounds, reduce the rate of hydrogen gas formation. Apparently, most of these slow down the rate at which hydrogen combines to form molecules. In the presence of such substances, there is a large concentration of hydrogen on the surface of the metal [4].

2.2 Hydrogen-induced cracking and affecting factors

Hydrogen-induced cracking (HIC) is a phenomenon widely found in components made of low-carbon steel that are exposed to sour media [5, 6]. Fractures through HIC occur at room temperature, in steels in the presence and absence of stress. The fractures have a flat appearance with different facets interconnected in a stair-step way. If a crack forms near half the thickness of the steel component, the HIC will produce an arrangement of planar cracks that extend parallel to the tube wall; if the cavity forms near a free surface, the internal pressure of the cavity can cause plastic deformation in the remaining wall, forming a blister [7, 8, 9]. Hydrogen-induced cracking is frequently related to surface blistering [7, 10, 11].

The external manifestation of this deterioration can be presented in several ways depending on the variables involved in the system. Fractures identified as HIC are caused by the mechanism that involves the formation, growth, and interconnection of internal blisters, which frequently give a stepped appearance and are also known as step fracture [12]. HIC manifests itself as multiple cracks in the same plane or in different planes forming a staggered arrangement of cracks. HIC has the appearance of a brittle fracture, the most common mechanism is decohesion of the ferrite and pearlite bands of the susceptible steels [7, 8, 12].

The variables that affect HIC are the severity of the environment, that is, pH, chloride content, temperature, H₂S concentration, the presence of dissolved oxygen, and exposure time. The pH value is a factor that accelerates corrosion (pH ≤ 5) and therefore intensifies the interconnection of microcracks contained in the thickness of the steel. A high concentration of dissolved chlorides and sulfides increases the severity of the environment as well as the time of exposure to the sour environment [13, 14].

One of the fundamental factors in the HIC phenomenon is the microstructure, consisting of impurities and their segregation, nonmetallic inclusions as well as the alloying content and the microstructure, resulting from the manufacturing process and heat treatment.

The deoxidizing elements and the addition of sulfur promote the formation of nonmetallic inclusions, which can be spherical, elongated, and flat, and can be presented isolated or in groups. On the other hand, the controlled practice in rolling leads to the formation of flattened inclusions of manganese sulfide that facilitate the decohesion between the metal and the inclusion [15, 16].

Inclusions with sharp edges and long surfaces are the preferential sites for the initiation of hydrogen-induced cracking [5, 6, 10, 11], manganese sulfide inclusions as well as clusters of inclusions were found to be among the greatest hazards for hydrogen damage for steels exposed to sour environments [17]. Several modifications have been implemented in manufacturing practices to control these inclusions. Reducing the sulfur content was found to be beneficial in reducing the formation, as well as controlling the shape of the inclusions by adding calcium or rare-earth elements, which has helped to improve resistance to HIC [16, 17].

Microstructural bands of pearlite and martensite are also considered as a factor that increases crack propagation. This banding is caused by dendritic segregation of manganese and phosphorus. The homogenization of the microstructures through quenching and annealing can also simultaneously reduce this phenomenon and stress-corrosion-induced cracking (SCC) [18, 19].

Nonmetallic inclusions and anomalous microstructures are more important factors in determining susceptibility to HIC compared to the mechanical strength of steels [20]. These mechanisms involve the segregation of atomic hydrogen into the steel interfaces followed by a decohesion of the interface bonds, mainly due to the presence of high molecular hydrogen pressures at the plastic regions where the blisters are generated. The blisters generally fracture at their tips, where embrittlement and a concentration of stresses combine. The reported fractures are transverse and propagate through a brittle material, these then merge other parallel fractures giving the appearance of a step. This occurs not only in stress-free components but also in steels subjected to elastic stresses [17, 18, 19].

It has been found that the addition of copper is beneficial for resistance to this type of mechanical damage since it forms a dark protective film of mackinawite, iron sulfide enriched with copper (FeS 1-x), this reduces the corrosion rate at the same time. Time that reduces the amount of hydrogen absorbed in such a way that adding Cu, Cr, Ni, Co, and bismuth is useful to increase resistance to HIC [19, 21].

3. Hydrogen permeation

The diffusion or permeation of atomic hydrogen and the evolution of the hydrogen reaction take place during the cathodic reduction reaction of hydrogen ions in aqueous corrosion systems; this hydrogen can be absorbed due to the high pressures of hydrogen gas in the environment. Additionally, the material must be capable of retaining hydrogen or may have hydrogen retained as consequence of the mineral dissolution process, for example, during the smelting process.

The stages of hydrogen absorption are summarized as follows [5, 6, 9, 11]:

1. Hydrogen evolves in the metal and develops on its surface.

2. The absorption of hydrogen on the metal surface.

3. Transport phenomena of hydrogen absorption and diffusion toward microstructural sites where H accumulates and becomes sites with applied stress fields.

4. The accumulation of hydrogen at specific sites in the metal leads to the nucleation and propagation of cracks.

3.1 Hydrogen entrapment

Hydrogen H trapping can occur in two types of traps: reversible and irreversible, whose activity depends on the trapping potential, the diffusivity, the number of traps, and their orientation with respect to the orientation of the maximum principal stress. The most dangerous sites are the elongated inclusions, especially those of manganese sulfide (MnS), which promote the decohesion of the matrix with the inclusion due to the reduction in the interatomic strength of the interface of the metal with the inclusion [22, 23, 24].

4. Hydrogen embrittlement theories

4.1 Decohesion theory

This theory assumes that embrittlement occurs in places where atomic hydrogen accumulates. Troiano [25] divided the HIC into three stages:

1. Incubation.

2. Slow crack growth.

3. Catastrophic failure, that is, it occurs suddenly.

The last stage is caused by an overload of stress observed by fractographic analysis. The incubation stage is the time required by the atomic H to reach a maximum concentration. Slow crack growth is the period during which H diffuses toward the crack tip region, causing unstable extension [7, 25, 26, 27, 28].

4.2 Internal pressure theory

This theory establishes that for a crack to grow stably, the system must be subject to total saturation of its voids by hydrogen [20]. When that energy is released, the H₂ gas expands and propagates fractures. However, this theory requires that the material have a pre-existing crack in the material, the initiation of the crack must be one or several dislocations as well as an accumulation of external stresses where there is a relationship for the fugacity of hydrogen in microcracks and in the cavities as a function of the excess potential in the electrochemically charged hydrogen [8].

The formation of blisters is due to the high internal stresses and pressures that lead to plastic deformation in areas close to the surface.

Crack propagation occurs at very low hydrogen pressures, which contradicts this internal pressure mechanism. This mechanism may be effective for environmental conditions with high hydrogen fugacity [7, 9, 11].

5. Fick’s law for diffusion of materials

Mass transport by diffusion is a complex mechanism that influences the nature of the species involved and their concentration. Diffusion can be defined as the movement of chemical species from a region of high concentration to a region of low concentration. In general, the rate of diffusion is proportional to the concentration gradient; if a steel plate is considered exposed to the same pressure of hydrogen on both sides of it, the concentration of hydrogen dissolved in the steel is considered uniform across the plate. As seen in Figure 2, at the instant, t = 0, the upper surface is subjected to a higher pressure, which promotes the diffusion of hydrogen toward the central regions of the plate, until reaching a constant stationary state, to maintain the concentration difference on the plate [29, 30].

Figure 2.

Establishment of the concentration gradient in steady state: Fick’s first law.

If the concentration of a species A is given in units of mass, then the diffusion rate can be expressed as:

$$W_{\mathit{Ax}}=-D_A\left(\frac{{\partial \rho }_A}{\partial x}\right)$$E2

where W_Ax is the A mass flux in the x direction, g (of A)-cm⁻²·s⁻¹, ρ_A is the concentration of A, g (de A)-cm⁻³ (of bulk material), and D_A diffusion coefficient of A, cm²-s⁻¹.

For the diffusion rate is expressed in terms of molar concentration:

$$j_{\mathit{Ax}}=-D_A\left(\frac{{\partial C}_A}{\partial x}\right)$$E3

where j_Ax is the mass flux A in the x direction x, g (de A)-cm⁻²·s⁻¹, C_A is the concentration of A, and g (de A)-cm⁻³ (of bulk material).

These equations correspond to Fick’s first law, which states that A species A diffuses in the direction of decreasing concentration of A, similar conductivity of heat that flows from areas of high temperature to areas of low or lower temperature.

When the density of the species involved is not uniform, an equation, such as Eq. (2), can be applied:

$$j_{\mathit{Ax}}=-{\mathit{CD}}_A\left(\frac{{\partial X}_A}{\partial x}\right)$$E4

This equation is the local molar concentration of the solution at the point where the gradient is measured, mol (of all components)-cm⁻³ (of the entire solution), and X_A is the fraction of A (C_A/A) in the solution [30].

Using Fick’s first law, the hydrogen flux through the metal membrane J_ss can be calculated in terms of the permeation current density i_max in its steady state using Eq. (5):

$$J_{\mathit{ss}}=-D_{\mathit{eff}}{\left(\frac{\partial C}{\partial x}\right)}_{x=L}=\frac{i_{\mathit{max}}}{\mathit{nF}}$$E5

where J_ss is the steady-state hydrogen permeation flux (atoms/cm²s), D_eff is the effective hydrogen diffusion coefficient (cm²/s), i_max is the steady-state current density (μA/cm²), n is the number of electrons transferred, F is Faraday’s constant (96485.33 Coulomb/mol), L is the thickness of the sample (cm), and (∂C/∂x)_x = L is the concentration gradient.

Fick’s second law describes the concentration of diffusing species as a function of time in a nonsteady state. The equation that describes one-way diffusion through the interstitial network is as follows:

$$\frac{\partial C\left(x,t\right)}{\partial t}=\frac{\partial }{\partial x}{D}_{\mathit{eff}}\frac{\partial }{\partial x}\left[C\left(x,t\right)\right]$$E6

where C(x,t) is the concentration of hydrogen in the crystalline lattice of the metal as a function of its position and time (mol/cm³); when solving Eq. (6), the following Eq. (7) is obtained [31]:

$$D_{\mathit{eff}}=\frac{L^2}{15.3\left(t_b\right)}$$E7

Then, the value for D_eff can be obtained from the measured value t_b (s), time it takes for H to cross the metallic membrane, for a (L) cm = thickness of the metallic membrane. Therefore, the concentration of H inside the metallic surface ${\mathit{C}}_{\mathit{H}}^{\mathbf{0}}$ (mol/cm³) can be obtained by solving the first Fick law [29, 30, 31], resulting Eq. (8)

$$C_H^0=\frac{i_{\mathit{max}}L}{D_{\mathit{eff}}\mathit{nF}}$$E8

6. Effect of carbon content and microstructure on hydrogen permeation in steel

Carbon produces significant changes depending on the accommodation that occurs in both ferrite and austenite as well as in the formation of Fe₃C iron carbides. According to the Fe-C diagram, ferrite has a lower capacity to dissolve carbon than austenite. At room temperature, the solubility of carbon in ferrite is negligible. When the solubility limit of carbon in austenite is exceeded, Fe or cementite carbides are formed [32].

The effect of carbon content on H diffusion is indirect, and the best-known fact is that carbon steels are sensitive to hydrogen embrittlement and HIC [6], which implies that carbon content can affect the diffusivity of H in steel.

Hydrogen diffusion in steels is affected by the microstructure of the metal due to the presence of phases, grain boundaries, vacancies and dislocations, grain shape and size, interactions with nonmetallic inclusions, precipitates, and cavities [10].

These metal heterogeneities reduce the mobility of hydrogen by acting as traps and can be classified as reversible and irreversible depending on the strength of their bond with the hydrogen atoms and their binding energy, those with less than 60-70 kJ/mol are considered reversible (6). Grain boundaries increase H diffusion by providing faster pathways for diffusion and are considered reversible traps (5). These grain boundaries are also considered diffusion retardants by acting as sites where atomic hydrogen accumulates. Ishimura and co-workers suggested that the maximum permeation coefficient is a function of optimal grain size [5, 6, 33, 34].

Irreversible traps are those where hydrogen is permanently contained at temperatures close to ambient with high binding energies. These traps are nonmetallic inclusions and precipitates. Several types of inclusions have been identified in pipe steels such as Al₂O₃ and sulfides and oxides of Mn and Fe. Precipitated particles, such as Nb, V, and Ti, are also considered irreversible traps [6, 33].

To increase the resistance to H permeation or reduce the susceptibility of a steel to hydrogen-induced cracking, a refined microstructure of equiaxed ferrite grains is more recommended since these steels have a higher diffusion coefficient D_eff and a lower ${\mathit{C}}_{\mathit{H}}^0$. The acicular ferrite and the micro constituents of a steel are considered reversible traps [5, 6, 33].

Regarding the effect of the second phase on diffusion, pearlite, and martensite/austenite, which are hard constituents, have high trapping efficiency, while ferrite and acicular ferrite are softer and tend to resist HIC better [6, 35, 36]. Steels with ferrite + bainite microstructure have greater resistance to HIC than steels with ferrite + martensite, and polygonal ferrite and granular bainite + acicular ferrite microstructures have greater resistance to HIC [37, 38].

The effect of hydrogen depends on two factors: The main one is the atomic concentration of hydrogen on the surface of the metal, ${\mathit{C}}_{\mathit{H}}^0$. The second factor is the microstructure due to the aforementioned defects. Hydrogen diffusion in steel (α-Fe ferrite) is 10^-6cm²s⁻¹ [33, 34].

The influence of carbon content on hydrogen permeation has been reported in the literature since H is an element provided with high diffusivity inside the crystalline lattice of metallic materials. Diffusivity is a function of the reticular parameter; therefore, the amount of hydrogen that is trapped in the metallic membrane is different for each steel, depending on the phases present and the structure, whether ferritic pearlitic or with the presence of bainite or martensite [32, 37, 38, 39, 40].

6.1 HIC prevention in low C steels

Preventive measurements that can be taken to prevent the occurrence of HIC in steel components exposed to bitter media are:

During the service of metallic components, injection of inhibitors and reducing the H₂S content to <10 ppm. Reducing inclusions and other impurities in steel phosphorus <100 ppm, oxygen <10, and CaO-Al₂O₃-SiO₂ inclusion groups [2.61-63] as well as reducing the hydration of gases passing through the thickness of metallic components since a clean steel has less H trapping sites. Several metallurgical parameters can also be considered and controlled, which help resistance to hydrogen-induced cracking, these are:

• Reducing the amount of hydrogen that enters steels through the formation of protective films.

• Reduction of manganese sulfide and control of the shape of inclusions.

• Control of microstructure, especially in segregation areas.

The first of these methods is often insufficient since the passive films formed are dependent on the pH and the aggressiveness of the medium, at lower pH the protective films frequently do not form. It was previously mentioned that the addition of copper helps to enrich the formation of protective films through the formation of mackinawite. The most effective way to prevent HIC is to modify the inclusion’s shape and quantity as well as control the microstructure. The complete elimination of inclusions significantly increases the resistance to hydrogen-induced cracking as well as fracture under stress and corrosion. Modifications in steel manufacturing must control the formation of sulfides through the addition of rare-earth elements. In addition, rare earth sulfides tend to produce ionization effects that interfere with the stability of the arc in welding procedures. Adding calcium has also been found to increase resistance to cracking, which must be delicately controlled so that manganese sulfides and calcium sulfides do not form clusters [12, 13, 15, 16, 17, 18, 19, 21].

Grain size refinement helps to reduce stress concentration as grain boundaries block dislocation motion and therefore prevent dislocation-induced hydrogen transport; they also help mitigate impurity segregation and act as a barrier for hydrogen permeation as it mitigates mobility and makes the distribution of H uniform within the crystalline lattice of the steel [6, 34]. Specifically, the refinement stops the free movement of the dislocations preventing the hydrogen transport induced by them [33, 34, 41, 42, 43].

7. Electrochemical methods to measure H permeation in metals

7.1 Types of sensors

Currently, there are two main types of sensors for monitoring infiltrated hydrogen, such as amperometric and potentiometric [44, 45]. Amperometric sensors measure the equivalent flow of hydrogen through steel, for which the concentration inside ($C_H^0$) can be estimated. While the potentiometric sensor measures the equivalent pressure of hydrogen ($P_{H_2\mathit{eq}}$) in the steel, this can be very useful to estimate corrosion inside, in addition to providing information with which the concentration of hydrogen inside the steel pipelines and containers used for the transportation and handling of hydrocarbons can be estimated [2, 4, 46].

The authors have carried out a set of experiments with the three sensors to evaluate their operation and compare the results obtained. Two amperometric sensor and one potentiometric were used to monitor the hydrogen in the steel in contact with corrosive solutions saturated with H₂S at room temperature, and the results were compared. The results showed that the apparent diffusion coefficient for hydrogen in steel determined by experiments using the amperometric sensors is independent of the hydrogen concentration in the steel. Experiments with the potentiometric sensor indicated that the apparent diffusion coefficient increases when the hydrogen concentration increases. The hydrogen concentration on the steel surface obtained with the amperometric sensors showed a poor relationship with pH changes. The hydrogen partial pressure calculated from the voltage developed in the potentiometric sensor was strongly dependent on the pH of the corrosive solution (2). The potentiometric sensor for hydrogen monitoring has also been used to relate the measured hydrogen pressure with the hydrogen concentration and the appearance of cracks in steels, in order to establish hydrogen pressures at which steels could crack [47]. In this work, five commercial API steels were charged with hydrogen through corrosion reactions. During charging stage, the potentiometric sensor responded to changes in the medium, the partial pressure of hydrogen measured was found to be strongly dependent on the pH and the presence of H₂S. The apparent diffusion coefficient increases with increasing hydrogen concentration, the severity of the cracking in steels is highly influenced by the pH of the corrosive medium and the determination of a critical hydrogen partial pressure at which cracking occurs was not determined [47].

Elboujdaini et al. developed and used a potentiometric sensor to determine the hydrogen intake into the steel under controlled conditions, the results of such trials are very similar to the behavior of the steel components in service exposed to sour environments [48]. The concentration of hydrogen on the interior surface was determined for different media and compared with values ​​obtained in the field. The results of these experiments reflect that the conditions that could be closest to the service conditions are those of the media at pH 4.1 – 4.7. These measurements to calculate hydrogen concentration that can be performed in laboratory, but at the sacrifice of accuracy, in addition to the fact that the sensor responds to changes in the severity of the medium [48]. In another study, an amperometric sensor was used to relate its measurements with the ultrasound results used to detect cracks [47]. In that study, they used steel of the same grade as that used in pipes in service, showing that the steel is resistant to cracking even at pH 1.1 which was evident by ultrasound before and after exposure to acid solutions. The pH of the medium has a strong influence on the hydrogen concentration since as the pH of the medium decreases, the hydrogen concentration increases.

7.2 Electrochemical techniques

Among the electrochemical techniques used to measure H influx, the most relevant for this work is the electrochemical hydrogen permeation test developed by Devanthan [49]. This test allows the calculation of the hydrogen concentration as a function of the time that takes for the H atoms to pass through the thickness of the sample (t_b), the effective diffusion coefficient that represents the speed with which the H atoms diffuse through the sample (D_eff), the maximum current density, which is the faradic equivalent of H atoms that are constantly oxidized in the oxidation cell (i_max), and finally the surface concentration ($C_H^0$), which is the concentration of H at the surface of interaction between H and steel due to corrosion reactions in the charging cell [31]. It has been established that when the ($C_H^0$) is below a limiting value ${\mathit{C}}_{\mathit{H}}^{\mathit{lim}}$, the interstitial sites of the lattice are the most common trajectories for the diffusion of H, but when the ($C_H^0$) exceeds the ${\mathit{C}}_{\mathit{H}}^{\mathit{lim}}$, the HIC begins to spread by consuming part of the diffused H [49, 50]. When the concentration of H is high enough to cause embrittlement, the flux of H passing through the metal increases with time, and after reaching a steady state, the current decays due to the consumption of H ions from the solution, as shown in Figure 3.

Figure 3.

Double cell permeation of hydrogen curve.

As mentioned, in a sour environment, the reactions that take place in the charging cell during the corrosion process are the following:

$$\mathrm{Fe}\left(\mathrm{S}\right)\to \mathrm{Fe}\left(\mathrm{ac}\right)+{\text{2e}}^{-}\text{anodic reaction}$$E9

$${\text{2H}}^{+}+{\text{2e}}^{-}\to \text{2H}\left(\mathrm{ad}\right)\text{cathodic reaction}$$

Both reactions take place at the same rate, so the total reaction is given by:

$$\text{2Fe}+{\text{2H}}^{+}\to {\text{2Fe}}^{2+}+{\mathrm{H}}_2$$E10

If “poison” species, such as arsenic, selenium, sulfur, or phosphorus, are present in the solution, the formation of molecular hydrogen is delayed or inhibited [9]. Therefore, the metal surface will be saturated with atomic H, which is capable of diffusing into the metal according to the following reaction:

$${\mathrm{H}}^0-{\text{1e}}^{-}\to {\mathrm{H}}^{+}$$E11

7.3 Double oxidation cell

In the double oxidation cell shown in Figure 4, the equivalent flow of hydrogen through the metal membrane can be measured, and the concentration ($C_H^0$) can be calculated. The double cell, proposed by Devanathan and Starchusky, is composed of two chambers that are connected through a metal membrane. One chamber contains a slightly basic solution while the other cell has the study solution, this causes corrosion reactions in the loading chamber; these reactions are what generate atomic hydrogen, which flows through the thickness of the metal toward the oxidation chamber where this hydrogen is detected as it oxidizes, which produces an oxidation current.

Figure 4.

Electrochemical double cell diagram.

Amperometric tests for hydrogen permeation, using a device proposed by Devanathan and Stachurski, are presented in ASTM G-148. The typical graph expected with this device is illustrated in Figure 3, where it is observed that the maximum current values ​​and the transfer time tb can be obtained with these data the diffusion coefficient and the hydrogen concentration in the steel [31].

8. Experimental work

8.1 Introduction

To analyze the role of the effect of carbon and microstructure on the characteristics of H diffusion in steels with low-carbon content, a group of steels with a bainite + ferrite microstructure was studied, also, an additional commercial steel with pearlite + ferrite microstructure was tested [51]. All these steels had a variation in carbon content and second phases.

Four steels used in the hydrocarbon pipeline transportation industry were selected. The samples were obtained from sections of pipelines removed from service, samples of two plates of steel with extra low-carbon content, and bainitic-ferritic microstructure with API 5 L specification X-52 and X-70, and two others plates of steel with carbon greater than 0.1% with pearlite-ferrite microstructure with API 5 L X-60 and X-65 specification were obtained. The main objective was to relate the metallurgical variables (microstructure, grain size, inclusions, and chemical composition), especially the carbon content and the phases present in steels with i_max, t_b, D_eff, ($C_H^0$).

8.2 Methodology

The medium used to analyze the effect of C and the microstructure was the one suggested by the ASTM G148 standard. A sodium sulfide poison solution was added to this medium to prevent atomic hydrogen from recombining into its molecular form, obtaining a value of pH =0. Hydrogen permeation into the samples was performed without coating the coupons exposed to the acid solution. In these tests, also, no cathodic charging current was applied to reproduce the real operating conditions of the components exposed to sour environments.

In the solutions suggested by the ASTM G-148 standard, 500 ml of a 0.1 M NaOH solution, prepared with high-purity reagents, was used for the oxidation cell. For the loading cell, 500 ml of 2 M H₂SO₄ + 0.1 M Na₂SO₄ was used, and 30 ml of 0.1 M Na2S, pH = 0, were added.

To perform the hydrogen permeation test, a metal plate was used; it was roughed out with SiC sandpaper until a uniform grade 600 finish was achieved for both sides of the plate. The sample was cleaned with acetone, alcohol, and distilled water to remove any residual dirt or grease. Then, it was placed between the chambers of a double electrochemical cell and was firmly adjusted with the help of screws and clamping joints to prevent any leakage of the solutions used, as seen in Figures 4 and 5.

Figure 5.

Double cell oxidation assembly used for this work.

The entry of atomic hydrogen was carried out by a cathodic reaction, where the aqueous electrolyte of H₂SO₄ transported the H⁺ ions. An oxidizing agent composed of 1.0 M sodium hydroxide (NaOH) was placed in the second electrochemical cell known as the oxidation or detection cell. Once the ions pass through the metal membrane, they are oxidized producing a current that can be measured. This current is the faradic equivalent of the flow of H⁺ through the membrane toward the surface from the charging cell to the oxidation cell [31, 33, 49, 50].

The experiments were carried out under normal conditions of temperature (21°) and atmospheric pressure, which were maintained throughout the test. Then, the oxidation chamber was filled with 500 ml of a 0.1 M NaOH solution prepared with high-purity reagents, and nitrogen was bubbled for 10 minutes to purge the solution and displace the oxygen present. A calomel SCE +0.2415 V electrode was used in front of the standard hydrogen electrode (Corning hi-Stab Reference) as a reference electrode (KCl saturated calomel electrode)(A) using a Luggin capillary (B) as well as the auxiliary electrode made of a compact graphite bar (C) and the metallic sample with an area exposed to the solutions of 1.2666cm² (D). The total arrangement can be seen in Figure 5.

Once the double cell was assembled and connected to the potentiostat, an open circuit potential test was performed for 1 hour to determine E_corr. After the stabilization time of the open circuit potential, the permeation test was started, applying 300 mV against the E_corr (previously obtained). The permeation test was stabilized for 3600 s and then 500 ml of a solution of 2 M H₂SO₄ + 0.1 M Na₂SO₄ was added to the charging chamber. At the same time, 30 ml of 0.1 M Na₂S was added, which acts as a poison in the solution and prevents the recombination of atomic hydrogen into molecular hydrogen. This solution in the loading chamber was maintained, such as that in the oxidation chamber, with a constant bubbling of N₂ gas.

8.3 Results

The results obtained from the hydrogen permeation tests are shown in Table 1 and correspond to the permeation graphs presented in Figure 6.

Steel	% C	L; (cm)	tb; (s)	Deff *E^−06; (cm^2/s)	Background current density (μA/cm^2)	imax; (μA/cm^2)	$C_H^O$; (μmol/cm^3)
X-70 B	0.023	0.315	580	11.2	1.11	10.81	3.15
X-52 B	0.052	0.32	650	10.3	0.48	9.15	2.94
X-60 P	0.187	0.377	1850	5.02	1.39	5.98	4.65
X-65 P	0.24	0.382	1400	6.81	0.54	7.51	4.36

Table 1.

Parameters obtained from hydrogen permeation tests.

Figure 6.

Hydrogen permeation curves of API-X70, API-X52, API-X65 and API-X60 steels.

The Deff and imax values ​​for bainitic steels are higher than those obtained for pearlitic steels, as shown in Figure 7(B) and (C) as well as in Table 1.

Figure 7.

Main parameters of the tested steels vs. % carbon content.

The background current density from the hydrogen permeation tests is low and relatively stable compared to the current densities measured when hydrogen entered the sample.

8.4 Results discussion

Regarding the tests of the influence of the carbon content, it was considered that the presence of the other alloying elements does not exert a greater effect than the carbon content on the hydrogen permeation in this work due to the very small quantities (< 1% by weight) that the steels presented.

From Table 1, as well as Figure 7(B) and (C), it is observed that steels with lower carbon content (bainitic steels) showed higher D_eff and i_max compared to those with higher carbon content (perlitic steels). It is interesting to note that among the bainitic steels, the highest values ​​of D_eff and i_max were found for a lower carbon content (8.03 and 15.35% variation, respectively), but, in the pearlitic steels, the opposite effect was observed, that is, the higher the carbon content, the higher D_eff and lower i_max, this implies that the effect of carbon cannot be separated from the effect of microstructure.

The effect of the carbon content on the diffusion parameters t_b and $C_H^0$ is greater for the pearlitic steels and the highest value corresponds to the X-60 steel because the atomic hydrogen is retained at the ferrite/pearlite interphase and in nonmetallic inclusions that act as hydrogen sinks. This behavior is also attributed to the segregation of pearlite and the chemical composition, as well as the lower ferrite content as shown in the image in Figure 8, while the bainitic steels showed lower values ​​for these parameters mainly due to having a greater amount of ferrite phase and fewer inclusions.

Figure 8.

Microstructure of steel samples. Optical microscope.

These results indicate that the effect of carbon content on hydrogen diffusion tends to be more significant for higher carbon contents. This can be attributed to the fact that a large number of interstitial sites are occupied by carbon atoms in the ferrite crystal lattice. In contrast, the depletion of lattice carbon to form carbides in bainite microstructures increases diffusivity. While in ferrite-pearlite steels, only the carbon content in excess of the equilibrium solubility in the ferrite forms cementite sheets that constitute part of the pearlite, trapping a greater amount of hydrogen.

In Figure 7(D), it was observed that the surface concentration of hydrogen $C_H^0$ is greater for steels with higher carbon content that have pearlitic microstructure. This behavior is attributed to the amount of hydrogen retained at the ferrite/cementite interphase [52]. In summary, it can be said that steels with higher carbon content tend to retain greater amounts of hydrogen on their surface. Therefore, these steels are expected to be more susceptible to HIC.

8.5 Comparison with other works

The results of this work were compared with those obtained in similar studies reported in the literature, in order to validate our results and to determine if similar effects of carbon content and microstructure are observed. The results of Araujo et al. [53] were obtained for ferrite-pearlite steels, similar to the P steels tested in this work. The API 5 L-X80 steel with carbon content of 0.084% wt showed a D_eff = 3.37 E⁻⁰⁶ cm²s⁻¹, while an API 5 L-X60 steel with carbon content of 0.12% wt showed a D_eff = 2.11 E⁻⁰⁶ cm²s⁻¹, These values are within the same range of the D_eff values obtained here.

The results obtained here also coincide with those reported by Luu and Wu [54], who analyzed the effect of carbon content and microstructure on the value of D_eff, obtaining the results listed in Table 2.

Steel	%C	Deff E^−06 cm^2/s	${\mathit{C}}_{\mathit{H}}^{\mathit{O}}$ E^−06 moles/cm^3
Tempered	0.05	10.5	0.44
S45 C	0.45	2.96	1.18

Table 2.

Results of Deff obtained by Luu and Wu [54].

These results show that at higher carbon content, the D_eff is lower and ${\mathit{C}}_{\mathit{H}}^{\mathit{O}}$ increases. This behavior is consistent with the tendency observed here.

Also, the effect of microstructure observed here is similar to that reported by Ramunni [55], where a pearlitic microstructure with a large grain size had higher D_app = 6.43 ± 0.40 x 10⁻¹⁰m²s⁻¹ than a steel of ferritic matrix with fine carbide particles and finer grain size D_app = 2.19 ± 0.11 (x10⁻¹⁰m²s⁻¹).

Regarding the hydrogen concentration in the steel, M. Liu et al. [39] showed that ${\mathit{C}}_{\mathit{H}}^{\mathit{O}}$ increases with carbon content and time. The AISI 1018 with 0.13-0.2% C had lower values of ${\mathit{C}}_{\mathit{H}}^0$ than the AISI 4340 with 0.38-0.43% C. These data are very similar to those obtained.

Another research done by Chan [56] showed that the hydrogen concentration increased as the amount of ferrite/pearlite increased just as observed in the tests described here.

Finally, Gye-Won Hong [52] showed that the steady state of H permeation flux decreases as the pearlite area increases, and the apparent diffusivity of hydrogen in carbon steel decreases as the α-Fe-Cementite increases (Table 3).

Steel	%C	Dapp; $x{10}^{-3}xexp\left(-\frac{26.79\mathit{kJ}{\mathit{mol}}^{-1}}{\mathit{RT}}\right)\left({\mathit{cm}}^2{s}^{-1}\right)$
0.12C Steel	0.12	7.201
0.49C Steel	0.49	6.163

Table 3.

Results of Dapp obtained by Hong and Lee [52].

8.6 Experimental work conclusions

Although no electrochemical charging was applied in the charging cell, the results and the data obtained here match well with those reported in the literature, especially D_eff and ${\mathit{C}}_{\mathit{H}}^{\mathit{O}}$ for the same type of steel. Therefore, it is expected that bainitic steels with fine grain size and less nonmetallic inclusion content have lower ${\mathit{C}}_{\mathit{H}}^{\mathit{O}}$ than pearlitic steels.

The steels with low-carbon content had higher effective diffusion coefficients D_eff, and higher maximum oxidation currents, i_max in comparison to the steels with higher carbon content.

Despite the non-clear tendency of the diffusion parameters t_b and ${\mathit{C}}_{\mathit{H}}^{\mathit{O}}$, the effect of the carbon content was observed. The highest values correspond to carbon content of 0.187% wt, while the lowest values were obtained for the steel with carbon content of 0.023% wt.

The steels with a bainitic microstructure showed the highest D_eff and i_max, in comparison to ferrite-pearlite steels. This effect was concomitant with the effect of carbon content and is attributed to free paths for hydrogen passage and the number and efficiency of hydrogen trapping sites. Bainitic steels appear to provide easier paths for hydrogen passage, while their spherical carbides are not efficient hydrogen trapping sites, in opposition to ferritic-pearlitic steels, where the pearlite bands may act as physical barriers for hydrogen diffusion, in combination with a high efficiency of pearlite as hydrogen trapping site.

The results of D_eff obtained in this work are consistent with published data, both in their magnitudes as well as on the effect of carbon content and microstructure.

9. Chapter conclusions

Hydrogen-induced cracking (HIC) is a phenomenon widely found in components made of low-carbon steel that are exposed to sour media. Fractures through HIC occur at room temperature, in steels in the presence and absence of stress.

HIC manifests itself as multiple cracks in the same plane or in different planes forming a staggered arrangement of cracks. HIC has the appearance of a brittle fracture; the most common mechanism is decohesion of the ferrite and pearlite bands of the susceptible steels.

The variables that affect HIC are the severity of the environment, that is, pH, chloride content, temperature, H₂S concentration, the presence of dissolved oxygen, and exposure time.

One of the fundamental factors in the HIC phenomenon is the microstructure, consisting of impurities and their segregation, nonmetallic inclusions as well as the alloying content and the microstructure resulting from the manufacturing process and heat treatment.

The diffusion or permeation of atomic hydrogen and the evolution of the hydrogen reaction take place during the cathodic reduction reaction of hydrogen ions in aqueous corrosion systems, and this hydrogen can be absorbed due to the high pressures of hydrogen gas in the environment.

Currently, there are two main types of sensors for monitoring infiltrated hydrogen, such as amperometric and potentiometric. Amperometric sensors measure the equivalent flow of hydrogen through steel, for which the concentration inside ($C_H^0$) can be estimated. While the potentiometric sensor measures the equivalent pressure of hydrogen ($P_{H_2\mathit{eq}}$) in the steel, this can be very useful to estimate corrosion inside, in addition to providing information with which the concentration of hydrogen inside the steel pipelines and containers used for the transportation and handling of hydrocarbons can be estimated.

Among the electrochemical techniques used to measure H influx, the most relevant for this work is the electrochemical hydrogen permeation test developed by Devanthan. This test allows the calculation of the hydrogen concentration as a function of the time that takes for the H atoms to pass through the thickness of the sample (t_b), the effective diffusion coefficient that represents the speed with which the H atoms diffuse through the sample (D_eff), the maximum current density, which is the faradic equivalent of H atoms that are constantly oxidized in the oxidation cell (i_max), and finally the surface concentration ($C_H^0$), which is the concentration of H at the surface of interaction between H and steel due to corrosion reactions in the charging cell.

It has been established that when the ($C_H^0$) is below a limiting value ${\mathit{C}}_{\mathit{H}}^{\mathit{lim}}$, the interstitial sites of the lattice are the most common trajectories for the diffusion of H, but when the ($C_H^0$) exceeds the ${\mathit{C}}_{\mathit{H}}^{\mathit{lim}}$, the HIC begins to spread by consuming part of the diffused H. When the concentration of H is high enough to cause embrittlement, the flux of H passing through the metal increases with time, and after reaching a steady state, the current decays due to the consumption of H ions from the solution.

Finally, regarding the research and experiments that we carried out, there is the design and development of potentiometric and amperometry sensors to detect the hydrogen permeation in hydrocarbon transport pipelines, also the characterization of carbon steels and their susceptibility to hydrogen permeation and hydrogen-induced cracking phenomena. These sensors can be applied in the study and design of electrochemical cells to measure the diffusion of hydrogen in metallic components and can be used in the study and design of cells for hydrogen fuel systems and the characterization of steels used in storage, transportation, and distribution of green hydrogen and the relationship with its permeation and deterioration by hydrogen in renewable energies.

Acknowledgments

The authors express their gratitude for the use of the Corrosion Laboratory of the Metallurgical and Materials Engineering Department of the Nacional Polytechnic Institute (DIMM-ESIQIE-IPN).

Conflict of interest

“The authors declare no conflict of interest.”