In-situ visualization of hydrogen atom distribution at micro-indentation in a carbon steel by scanning Kelvin probe force microscopy

2.1 Specimen preparation

The material used in this work was an AISI 1040 medium carbon steel with a chemical composition (wt%): C 0.39, Si 0.19, Mn 0.76, P 0.035, S 0.015, and Fe balance. The steel was annealed at 1200 °C for 1 h and cooled in a furnace to room temperature, eliminating possible deformation and internal stress induced from original fabrication processes to obtain a homogeneous microstructure. After etching in 2 % nital solution, the steel showed a microstructure containing distinctive polygonal ferrite (F, bright color) and pearlite (P, dark color) with a uniform grain size, as seen in Figure 1. The cementite precipitates in the pearlite result in an intra-grain lamellar microstructure, while the ferrite is free of impurities.

Figure 1:

Optical metallographic view of the annealed AISI 1040 carbon steel etched in 2 % nital solution, where ferrite (F) and pearlite (P) are marked.

The annealed steel was machined into 10 mm × 10 mm × 5 mm specimens and sealed in epoxy, exposing a work surface of 100 mm². The exposed face was wet ground with SiC paper up to 1200 grit, and mechanically polished using a 0.5 μm diamond paste to achieve a mirror-like appearance. A Buehler MicroMet 6000 micro-hardness tester was used to apply a Knoop indenter (Vander Voort and Lucas 1998) on the polished surface of the specimen with a load of 500 g for 18 s, generating a rhombohedral mark with the long diagonal of about 200 µm. The indented specimen was ultrasonically cleaned in anhydrous ethanol for 5 min, dried in compressed air flow, and kept in a desiccator until electrochemical hydrogen-charging tests were performed.

2.2 Electrochemical hydrogen-charging

The electrochemical hydrogen-charging was conducted using a conventional three-electrode cell setup, where the indented steel specimen was used as the working electrode, a platinum foil as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The reference electrode was connected to the cell through a double junction bridge to avoid chloride ion pollution to the charging electrolyte. The electrolyte was prepared by dissolving 600 g of borax (sodium tetraborate decahydrate, 99 %+, Fisher Chemical™) in 1 L glycerol (99 %, Thermo Scientific™) and 20 % (v/v) distilled water with a resistivity of 18 MΩ cm under a vortex stirring. The pH of the electrolyte was about 7. The used electrolyte could preserve integrity of the specimen surface at nanoscale during electrochemical hydrogen-charging (Hajilou et al. 2018), enabling successful in-situ SKPFM measurements. A GAMRY reference 600+ potentiostat controlled the hydrogen-charging under a galvanostatic mode with a constant cathodic current density of 1 mA/cm². All tests were conducted at an ambient temperature of 22 °C. After each time of hydrogen-charging, the steel specimen was removed from the electrolyte, rinsed by anhydrous ethanol, wiped to dry by Kimwipes^® low-lint wipers, and transferred to an SKPFM chamber. The time interval between the end of the hydrogen-charging and SKPFM measurements was controlled within 5 min.

2.3 In-situ SKPFM measurements

The SKPFM measurements were performed using an Asylum Research AFM (MFP-3D Origin, Oxford Instruments) in air at about 22 °C and a relative humidity of 30 %. The Kelvin probes were ASYELEC.01-R2 conductive Ti/Ir-coated silicon tips with a resonant frequency of 75 kHz and a spring constant of 2.8 N/m. The in-situ SKPFM measurements were conducted over an area of 50 μm × 50 μm on the steel specimen, with a resolution of 256 × 256 pixel and a scan rate of 0.3 Hz. The setup of the SKPFM probe and the target area on the indented steel specimen are illustrated in Figure 2. The cantilever of the probe was aligned parallel to the scanning X-direction and the longitudinal direction of the indentation. The vertex of the rhombohedral indentation marked with a red square was measured by SKPFM. The SKPFM was configured as two-pass mode, with a fixed probe lifting distance of 40 nm. The topographic image obtained by SKPFM was processed with Gwyddion 2.60 software to remove the first-order tilts, while the Volt potential image was present as it is without further processing. It is noted that, in many published work, the measured Volta potential signals were processed by “+”/“−” inversions (Guo et al. 2016; Örnek et al. 2017), while this work presented the measured raw data.

Figure 2:

Illustration of the experimental setup of the SKPFM probe and Knoop indentation on the steel specimen, where the dotted red square is the area (50 μm × 50 µm) for in-situ SKPFM measurements.

4.1 Characterization of hydrogen atom distribution at an indentation by SKPFM

When the hydrogen-charging on the steel specimen is completed, the charged hydrogen atoms tend to diffuse out of the steel, resulting in a progressive reduction of hydrogen content on the subsurface of the steel. The concentration of hydrogen atoms on the subsurface is examined by SKPFM (Evers et al. 2013). Since the time interval between the end of hydrogen-charging and the SKPFM measurement is relatively short, it is assumed that the SKPFM-measured hydrogen atom concentration is approximately the subsurface hydrogen concentration immediately after hydrogen-charging. Thus, the Volta potential difference measured by SKPFM in this work is proportional to the concentration of subsurface hydrogen atoms, which agrees with the results obtained from previous work (Hua et al. 2017; Oger et al. 2019). Principally, a quantification of hydrogen atoms in the steel by Volta potential measurements is possible, subject to calibrations of the Volta potential with results obtained by other methods such as atom probe tomography that can directly measure the concentration of hydrogen atoms. It is noted that the reported results by different researchers are usually not consistent. The main reason is that the Volta potential measurement is heavily dependent on the Kelvin probe, measurement parameters and the sample material. A qualitative comparison of the Volta potential difference due to hydrogen-charging is conducted in this work.

The micro-indentation applied on the steel surface generates a local plastic deformation, leaving a hollow feature with a confined geometry, while the residual stress field expands to the adjacent region. The bottom surface of the indentation maintains a compressive stress owing to an elastic rebound of the steel in the confined geometry after unloading the indenter (Okodi et al. 2021). Although a material extrusion is not obvious at the vertex of the Knoop indentation (Kobayashi et al. 1998), the tensile stress induced by an asynchronous contraction between the plastic and elastic regions is applicable to the rhombohedral indentation. In addition, the stress/strain field symmetrically distributes along the longitudinal direction of the indentation, and a tangential stress concentration is expected at the vertex.

It was reported (Li and Li 2004) that the ϕspecimen decreased when the specimen was under a tensile strain and increased under a compressive strain in elastic range. However, in the plastic range, the ϕspecimen always decreased with the plastic strain no matter of its tensile or compressive nature. The change of ϕspecimen in the elastic range is associated with an energy variation with a lattice distortion, while the decrease of the ϕspecimen in the plastic range is dominated by dislocation multiplication during plastic deformation (Li and Li 2002). This is the reason that the indentation vertex, where an elastic tensile stress is exerted, shows a more positive Volta potential than the undented area, as seen in Figure 3d. The Volta potential increases with the increased indentation depth due to a plastic compressive deformation.

Upon application of an indentation on the steel, the induced local stress condition is usually complex. A tensile stress, local plastic deformation and dislocation multiplication tend to facilitate hydrogen atom diffusion and accumulation (Kim and Kim 2012). A compressive stress usually decreases the hydrogen diffusivity (Bogkris et al. 1971), but can enhance the hydrogen absorption if a plastic deformation enables the dislocation multiplication. After hydrogen-charging starts, the indentation and the nearby area, which are subject to a high stress and plastic deformation, are preferential sites for hydrogen atom accumulation, especially at the vertex. The indentation side is associated with a plastic tensile stress and a high dislocation density, and the indentation bottom with a plastic compressive stress and a high dislocation density. Thus, the hydrogen atom enrichment occurs, as shown in the measured results after 0.5 h of hydrogen-charging in Figure 3. Specifically, the vertex is under an elastic tensile stress and a high dislocation density, both of which favor the hydrogen absorption (Kim et al. 2012). However, the indentation bottom is subject to a compressive stress, inhibiting the hydrogen diffusion. Thus, hydrogen atom enrichment occurs at the vertex with positive values of V_v/u and V_v/d after 0.5 h of hydrogen-charging.

As the hydrogen-charging time increases to 2 h, the area with high hydrogen atom concentration spreads around the vertex, indicating lateral diffusion of hydrogen atoms due to a concentration gradient between the vertex and its vicinity. The side of the indentation is under a tensile strain and serves as a preferential path for hydrogen atom diffusion, generating a wider area with a higher Volta potential. With further increase of the hydrogen-charging time, the diffusion of hydrogen atoms from the indentation vertex to the adjacent area is further enhanced. The hydrogen atom concentration gradient between the vertex and the undented area reduces. As a result, the Volta potential difference, V_v/u, decreases with the hydrogen-charging time, as shown in Figure 7. As the dislocations generated by plastic deformation can also serve as reversible traps for hydrogen atoms, the further hydrogen atom accumulation is inhibited when the traps are filled (Kim et al. 2012). The hydrogen concentration difference between the indentation bottom and vertex is affected by both dislocation density and the stress, which are not apparently affected by the hydrogen-charging time. Thus, the V_v/d remains constant with the charging time after the initial hydrogen-charging of 0.5 h.

4.2 Effect of metallurgical microphase on distribution of hydrogen atoms

It was reported that the differences of ferrite and pearlite in microstructure and content affect the hydrogen atom diffusion and solubility (Cheng 2013). Due to the plastic deformation induced by micro-indentation, the effect of the microphases on Volta potential difference, which is associated with the hydrogen atom accumulation, is not observed until 6 h of hydrogen-charging. Moreover, the resulting Volta potential difference between ferrite and pearlite is about 4 times smaller than the Volta potential at the vertex. It is thus seen that the role of metallurgical microphase in hydrogen atom accumulation and distribution is not as important as the plastic stress and strain incurred by indentation.

The interaction between hydrogen atoms and various traps is determined by hydrogen-binding energy, which is summarized in Table 1 (Sun and Cheng 2021b). After annealing, the steel experiences grain recrystallization, grain boundary migration, stress releasing, and annihilation of dislocations, leaving two main types of hydrogen traps, i.e., grain boundaries and ferrite/cementite phase interfaces. Since recrystallization tends to correct lattice mismatch between grains, the total energy of grain boundaries is lowered to enable reduced hydrogen trapping in the indented steel. Thus, there is little hydrogen atom accumulation observed at the grain boundaries, but hydrogen atoms preferentially accumulate at the indented area and its vicinity in this work. The ferrite/cementite interfaces possess a hydrogen-binding energy that is lower than dislocations but greater than ferrite lattice. Therefore, the pearlite consisting of numerous ferrite/cementite interfaces is expected to host more hydrogen atoms and possess a more positive Volta potential than ferrite. However, this work finds a opposite result that the pearlite has a lower Volta potential than the ferrite after hydrogen-charging for 6 h, as seen in Figure 8. It is noted that the maximum diffusion distance (L) of hydrogen atom depends on hydrogen diffusion coefficient D and time t by (Koyama et al. 2017):

where D is about 1.05 × 10⁻⁹ m²/s according to (Luu and Wu 1996). It is estimated that L is 4.76 mm after 6 h of hydrogen-charging, which is almost same as the specimen thickness. Thus, the steel subsurface is saturated with hydrogen atoms and the pearlite is not capable of containing as much hydrogen atoms as ferrite due to the high content of lamellar cementite in pearlite, which has an extremely low solubility of hydrogen atoms (McEniry et al. 2018).

4.3 Implications of the current findings on HIC of pipelines

After many years of service in the field, pipelines inevitably contain various surface defects such as dents, scratches, corrosion defects and micro-cracks (Qin and Cheng 2021). These defects, once passing the assessment for fitness-for-service of the pipelines, are not required to repair and the pipelines can continue to operate. However, this work confirms that the defect such as a micro-dent, can serve as an effective trap to accumulate hydrogen atoms due to the local stress concentration when the hydrogen atoms, which are generated by dissociative adsorption of gaseous hydrogen molecules, penetrate the steels. The experimental finding is meaningful to the existing natural gas pipelines converted for hydrogen transport as the pipelines always contain dents on the pipe body. The dents will become hydrogen traps to cause initiation of HIC once a threshold hydrogen concentration is reached under certain stressing and metallurgical conditions. Thus, the defect assessment standards and codes used today must be modified to include diffusion and accumulation of hydrogen atoms at the defects for improved integrity management of pipelines transporting hydrogen in either pure or blended form. To the authors’ best knowledge; none of available defect assessment methods considers the defect effect on hydrogen, which constitutes a major gap.

This work further finds that, for typical low- and medium-grade pipeline steels containing ferrite and pearlite microphases, hydrogen atoms tend to accumulate in ferrite than pearlite. Although the ferrite is softer and more ductile than the pearlite, upon hydrogen atom accumulation, cracks can still be initiated in the ferrite once a threshold concentration is exceeded. This finding corrects a common misunderstanding that, when pipeline steels with a sufficient grade are used for hydrogen service, the HIC problem could be avoided. Actually, the low-grade steels can also experience HIC, causing pipeline failure. It is noted that the finding is obtained on the steel where metallurgical defects such as non-metallic inclusions, precipitates, and high-angle grain boundaries are not contained.