This series of articles is intended to highlight industry-wide engineering experiences, guidance, and focused advice to practising technologists. The series is written by ICorr Fellows who have made significant contributions to the field of corrosion management. This issue features articles on the use of existing pipelines for the transport of hydrogen, by Frank Cheng, Dept. of Mechanical & Manufacturing Engineering, University of Calgary, Canada, and on an interesting aspect of copper corrosion, by Peter Elliott, Corrosion & Materials Consultancy, Inc., Las Vegas, USA.
Suitability of existing pipelines for hydrogen service
It is believed that hydrogen will play a critical role in energy transition, and achievement of the 2050 net-zero emission goal. Hydrogen delivery is integral to the entire value chain of hydrogen economy. Compared with other transportation modes, such as tankers, pipelines provide an economic and efficient means to transport gaseous hydrogen with a high transportation capacity over long distances [1]. Particularly, repurposing existing natural gas pipelines is “a low-cost option for delivering large volumes of hydrogen” [2], contributing to accelerated realisation of a full-scale hydrogen economy, while saving high initial capital costs of constructing new hydrogen-dedicated pipelines.
However, hydrogen embrittlement (HE) can occur on pipelines transporting hydrogen in either blended or pure form, compromising the structural integrity to cause pipeline failures. Compared with newly constructed pipelines, the existing pipelines, after a long time of service in the field, include additional challenges to address in terms of the HE occurrence when converted for hydrogen service.
What is hydrogen embrittlement?
Hydrogen embrittlement is a general term describing the degradation of a material’s (mainly metal) properties and structural integrity, due to hydrogen-metal interactions. HE is a concept serving as an ‘umbrella’ under which many different modes of metallic degradation induced by hydrogen are referred to.
The first mode of HE phenomenon is hydrogen-induced cracking (HIC), which is also regarded as the most dangerous mode of HE-associated materials failure. The HIC occurs when the hydrogen atom concentration at a local site exceeds a threshold value under a given stress condition. Generally, the threshold H atom concentration required to initiate cracks in steels decreases with an increased stress. Many mechanisms or theories have been proposed to explain the HIC occurrence, such as hydrogen-enhanced localised plasticity (HELP), hydrogen-enhanced decohesion (HEDE), adsorption- induced dislocation emission (AIDE), and hydrogen-assisted microvoid coalescence (HAMC) [3].
Presently the dominant mechanisms for HIC of steels are the HEDE and HELP [4]. According to the HEDE mechanism, H atoms can weaken bonds between iron atoms in steels and when the external stress exceeds the atomic cohesive strength of Fe in the presence of H atoms, microcracks can be initiated. The HELP mechanism proposes that H atoms accumulate at dislocations to decrease the interfacial elastic energy between mobile dislocations, enhancing the mobility of the dislocations. As a result, the local deformation can occur at a lowered stress, facilitating plastic deformation and cracking occurrence. The HIC can initiate whether on the surface of a metal (i.e., external HIC,) or inside the metal (i.e., internal HIC).
The second mode of HE is hydrogen blistering, where the H atoms entering the metal become recombined to form hydrogen molecules (H2) at the trapping sites, such as voids. The elevated pressure due to accumulation of H2 molecules at a local site can cause formation of a blister (or bubble) on the metal surface. This hydrogen blistering usually occurs on low-strength and ductile metals where the bubbling process does not initiate cracks. Instead, the metals’ experience a remarkable local plastic deformation. If the metals have a high strength and limited ductility, the hydrogen-elevated pressure can cause cracking by the so-called hydrogen internal pressure (HIP) mechanism.
The most common mode of HE phenomenon is degradation of mechanical properties of the affected metals, which is usually shown as a reduction in fracture toughness and ductility. The direct evidence of this mode is the decrease in fracture elongation in the stress-strain curve measured on hydrogen- charged metals, compared with the tensile behaviour of hydrogen-free metals. Generally, a limited amount of hydrogen atoms may not be sufficient to initiate cracks, but can cause a reduction in ductility of the metals.
Uniqueness of pipeline hydrogen embrittlement in high-pressure gaseous environments
HE can occur on pipelines in high-pressure gaseous hydrogen environments. The entire HIC processes include six steps, i.e., generation of H atoms, adsorption of the H atoms on the steel surface, absorption of the H atoms by permeating into the steel, diffusion of H atoms in the crystalline lattice, trapping (accumulation) of H atoms at local sites, and the crack initiation, as schematically shown in Figure 1. The first three steps, i.e., H atom generation, adsorption, and absorption, depend heavily on the environment, whereas the other steps of diffusion, trapping and cracking, mainly rely on the metallurgical factors.
Figure 1. Entire processes for HIC occurrence on metals (steels).
Previous work demonstrated that H atoms can generate and become adsorbed on steel surfaces through a so-called dissociative adsorption mechanism in high-pressure gaseous environments [5]. The Gibbs free energy changes for production of H atoms from gaseous H2 molecules are negative, indicating a thermodynamic feasibility, under pipeline operating conditions with typical temperatures, pressures and H2 gas/natural gas blending ratios. Moreover, the generated H atoms can adsorb stably at On-Top (OT) and 2-fold (2F) Cross-Bridge sites of the Fe (100) crystalline plane, while the hydrogen adsorption at 2F sites is more stable due to a higher electron density and a stronger electronic hybridisation between Fe and H. Upon entering the steels, the H atoms predominantly stay at tetrahedral void sites due to a low energy path and exothermic feature. It has been noted that a uniform distribution of H atoms at the tetrahedral voids in a crystalline lattice would not cause HE or HIC, if the amount of H atoms is within the limit of H solubility. However, the local solubility can be exceeded if H atoms diffuse towards high stress zones, or become trapped at metallurgical features such as grain boundaries, dislocations, and non-metallic inclusions.
Nowadays, substantial studies have been conducted to investigate HE of metals, including pipeline steels, in aqueous environments, where most investigations focused on H atom generation during steel corrosion or cathodic over-protection. Four major differences exist in H atom generation and permeation in steel between a gaseous environment (“gaseous” hydrogen) and the aqueous environment (“cathodic” hydrogen), as listed in Table 1, making it infeasible to directly use data obtained in aqueous environments for HE investigations on pipelines in high-pressure gaseous environments. First, the amount of “cathodic” hydrogen generating during electrochemical cathodic charging, either potentiostatically or galvanostatically, is always substantial. A significant H atom concentration gradient exists between the outside and inside subsurface of the steel. However, the amount of H atoms generated in a gaseous environment is usually limited. After several months of exposure of a pipe steel to high-pressure (10 MPa) H2 gas, the measured H concentration is less than 1 ppm [6]. Secondly, due to the high concentration gradient of H atoms across the steel surface, the permeation of “cathodic” hydrogen is always one way, entering the steel from the adsorption to absorption state. As a comparison, the “gaseous” H atoms, even entering the steel and becoming absorbed inside, can still leave the absorption site and change to adsorption state [5]. Thus, the permeation of “gaseous” hydrogen is reversible. Third, the electrochemical hydrogen-charging usually generates reproducible results, making the testing method (i.e., the Devanathan-Stachurski cell) a standard method for hydrogen atom permeation tests [7]. Although there has been limited work to measure the H atom permeation in gaseous environments, the obtained data are usually scattered, and sometimes, even conflict each other. Finally, both constant concentration model and constant flux model have been developed to fit the electrochemical cathodic H-charging results to derive the H permeation parameters such as H diffusivity, subsurface H concentration and trapping density [8]. To date, a model applied for “gaseous” hydrogen permeation has not been established. Due to the differences listed, the testing results and modeling methods for aqueous “cathodic” hydrogen testing cannot be used for gaseous hydrogen permeation testing.
‘Gaseous’
hydrogen ‘Cathodic’
hydrogen
Amount generated to adsorb on steel surface Limited Substantial
Permeation pathway Reversible between adsorption and absorption One way from adsorption to absorption
Testing results Scattering, and sometimes, controversial Reproducible
Numerical model to derive hydrogen permeating parameters None Constant concentration model and constant flux model
Table 1. Comparison of the H atom generation and permeation in steels in a gaseous environment and in an aqueous environment.
Additional challenges when repurposing existing pipelines for hydrogen service
After a long time of service in the field, existing pipelines become aged and contain various surface defects such as dents, corrosion, scratches, winkles and microcracks. These defects, if passing the assessment criteria [9], would not be required to be repaired, and the pipelines can continue to operate. However, they serve as effective traps to accumulate H atoms. Particularly, dents, a common type of mechanical damage present on pipelines, is a permanent inward deformation on the pipe body, greatly changing the local stress and strain distributions. As a result, the H atoms, once entering the pipe steels, tend to diffuse towards the dent and accumulating locally. Modeling results showed that there were maximum H atom concentrations at both sides of the dent along the circumferential direction, as shown in Figure 2. The H atom distribution coincided with the distribution of hydrostatic stress on the pipe. Moreover, as the dent depth increases, the H atom concentration increases at the local area.
Figure 2. Distributions of the H atom concentration (unit: mol/m3) on an X52 steel pipe segment containing a constrained dent and the cross-sectional view. The dent depth is 3% of pipe outer diameter and the internal pressure is 10 MPa [10].
Different from a dent, a corrosion defect causes metal loss on the pipe wall. Moreover, the corrosion defect is dynamic in nature, and will grow with time in the service environment. It is acknowledged that corrosion is one of the primary mechanisms causing pipeline failures. Generally, the presence of a corrosion defect on the pipe body can decrease the pressure-bearing capability of the pipelines.
Thus, many standards and codes have been developed for corrosion defect assessment. Similar to a dent, if a corrosion defect passes the assessment criteria, the corroded pipelines can continue to operate, without a need to repair the corrosion defect, but a cautious monitoring of the defect growth should be followed. It was confirmed that the H atoms which were originally distributed in the crystalline lattice of a steel would diffuse towards the corrosion defect. Moreover, as the corrosion defect length reduces and the depth increases, the H atom concentration becomes more apparent, as seen in Figure 3.
Figure 3. Distributions of the H atom concentration (unit: mol/m3) on an X52 steel pipe segment containing a corrosion defect with varied lengths and depths [10].
A technical assessment programme for the suitability of aged pipelines in hydrogen service
Obviously, a technical programme must be developed to assess the suitability of the existing aged pipelines to transport hydrogen in either pure or blended form. The assessment should consider the synergism of steel metallurgy, stress, and hydrogen, on HE or HIC occurrence. Particularly, the surface defects present on the pipelines should be paid much attention, in addition to the metallurgical features such as grain boundaries, dislocations and non-metallic inclusions, which are effective hydrogen traps. The specific service history of the pipelines which were made of various grades of steel should be considered, and the technical assessment should be conducted case by case. Once successfully developed, the technical assessment programme will be able to: evaluate the possibility of H atoms generation and the amount of H atoms adsorbed on the steel surface under pipeline operating conditions, quantify the accumulated H atom concentration at the surface defects and metallurgical features under given conditions, estimate the threshold H atom concentration at a local defect to initiate cracks under a certain stressing condition, rank the HE susceptibility of the aged pipelines while considering the metallurgical and stress factors, and recommend proper operating conditions (e.g., temperature, pressure and blending ratio) to minimise and eliminate pipeline HE in high-pressure gaseous environments.
References
[1]
A.S. Hawkins, Technological Characterization of Hydrogen Storage and Distribution Technologies, UKSHEC Social Science Working paper no. 21, Policy Studies Institute, London, UK, 2006.
[2]
U.S. Department of Energy, Hydrogen Pipelines, Washington DC, US, 2020.
[3]
Yinghao Sun, Y. Frank Cheng, Hydrogen-induced degradation of high-strength steel pipeline welds: A critical review, Eng. Fail. Anal, 133 (2022) 105985.
[4]
M.B. Djukic, G.M. Bakic, V. Sijacki Zeravcic, A. Sedmak, B. Rajicic, The synergistic action and interplay of hydrogen embrittlement mechanisms in steels and iron: Localized plasticity and decohesion, Eng. Frac. Mech., 216 (2019) 106528–106561.
[5]
Yinghao Sun, Y. Frank Cheng, Thermodynamics of spontaneous dissociation and dissociative adsorption of hydrogen molecules and hydrogen atom adsorption and absorption on steel under pipelining conditions, Int. J. Hydrogen Energy, 46 (2021) 34469-34486.
[6]
G. Golisch, G. Genchev, E. Wanzenberg, J. Mentz, H. Brauer, E. Muth
mann, D. Ratke, Application of line pipe and hot induction bends in hydrogen gas, J. Pipeline Sci. Eng., 2 (2022) 100067.
[7]
M.A.V. Devanathan, Z. Stachurski, The adsorption and diffusion of electrolytic hydrogen in palladium, Proc. Royal Soc., A270 (1962) 90-102.
[8]
Y.F. Cheng, Analysis of electrochemical hydrogen permeation through X-65 pipeline steel and its implications on pipeline stress corrosion cracking, Int. J. Hydrogen Energy, 32 (2007) 1269–1276.
[9]
Guojin Qin, Y. Frank Cheng, A review on defect assessment of pipelines: Principles, numerical solutions, and applications, Int. J. Press. Vessel Pip., 191 (2021) 104329.
[10]
Frank Cheng, Hydrogen transport in aged pipelines II. Technical assessment of the susceptibility to hydrogen embrittlement, AMPP / ASM International Calgary Chapter Luncheon, Calgary, Canada, Oct. 19, 2022.
Frank Cheng
