X-ray photoelectron spectroscopy investigation of X60 steel

API 5L grade X60 steel (standardized by American Petroleum Institute) has been used since long time as steel for natural gas and petroleum pipelines and in oil and gas applications. Recently, the material has gained interest for hydrogen storage and transportation (Ref. 1). The surface chemistry of X60 steel plays an important role when performing electrochemical hydrogen permeation tests (Ref. 2) because it might affect the hydrogen production and detection kinetics. In this work, x-ray photoelectron spectroscopy was used for characterizing the surface of X60 steel after a thorough cleaning procedure and following mechanical polishing. No significant changes in the chemical state of the elements are detected between the samples upon the different sample preparation procedures. The mechanical polishing reduces the thickness of the natural oxide layer present on the steel surface.

Specimen: X60 steel, thickness 2 mm, diameter 5 cm; cleaned (# 01976) and mechanically polished (# 01977)

CAS Registry #: Unknown

Specimen Characteristics: Homogeneous; solid; polycrystalline; conductor; metal;

Chemical Name: API 5L grade steel X60

Source: RINA Consulting—CSM S.p.A.

Composition: Fe, Mn < 1.4%, Cr, Mo, Ni, Cu, C < 0.28%, and Si < 0.45% (Ref. 3)

Form: X60 steel disk 50 × 2 mm² (diameter × thickness)

Structure: XRD results performed by Rina consulting CSM S.p.A showed that X60 steel has body centered cubic (BCC) structure. SEM cross-sectional analysis of X60 steel (etched with 2% Nital solution for 5 s) conducted in the Metallography Laboratory of Rina consulting CSM S.p.A, showed that X60 steel has tempered martensite and bainite microstructures with the distribution of precipitated carbides.

History and Significance: The X60 steel was cut from a pipeline tube to obtain disks of 2 mm thickness and a diameter of 50 mm. The surface roughness determined by a SV-3200 surface roughness tester was 0.3–0.4 μm.

As Received Condition: The “as received” X60 steel after cutting is covered with an oil/grease film.

Analyzed Region: Center and at least two other analysis points of the sample surface

Ex Situ Preparation/Mounting: Two different procedures were applied:

Five solvents’ cleaning (# 01976): The as received samples were cleaned by following the described procedure in ultrasonic bath at 40 °C: butan-2-one (5 min), acetone (5 min), ethanol (5 min), petroleum ether (5 min), and double distilled water [specific conductivity = 1.5(0.1) μS/cm] (10 min). After the cleaning procedure with five solvents, the sample was dried under an argon stream and analyzed by XPS. The time between the drying step and the sample insertion into the fast entry air lock of the spectrometer is less than 1 min.

Five solvents’ cleaning and mechanical polishing (# 01977): XPS characterization was also carried out on the X60 sample following cleaning with five solvents and mechanical polishing using SiC 4000-mesh paper in the presence of ethanol as lubricant for 3 min and 30 s. After grinding, the samples were immersed in ultrasonic bath at 40 °C in ethanol for 5 min and in double distilled water for 10 min. Also, in this case, the samples were dried under an argon stream. The time between the drying step and the sample insertion into the fast entry air lock of the spectrometer was less than 1 min.

In Situ Preparation: None

Charge Control: No charge control

Temp. During Analysis: 300 K

Pressure During Analysis: <1 × 10⁻⁷ Pa

Preanalysis Beam Exposure: 0 s

Manufacturer and Model: Thermo Scientific Theta Probe

Analyzer Type: Spherical sector

Detector: Other, multichannel plate detector

Number of Detector Elements: 128

Analyzer Mode: Constant pass energy

Throughput (T = E^N): The energy dependence can be determined by the following equation: $A E p= ( a 2 ( a 2 + R 2 ) ) b$⁠, where a and b are constants, E_p is the pass energy, A is the peak area, and R is the retard ratio equal to E/E_p, where E is the kinetic energy. Three spectral regions [Cu 2p (925–940 eV), Cu 3p (68–82 eV), and Cu L₃M_4,5M_4,5 (561–577 eV)] are recorded on a sputter cleaned copper sample at different pass energies (10, 20, 50, 100, 125, 150, 200, 300, and 400 eV) The values of a and b are then determined to be 26.66 and 1.11, respectively, by a linear least square fit of the data applying the equation described above (Ref. 4).

Excitation Source Window: None

Excitation Source: Al K_α monochromatic

Source Energy: 1486.6 eV

Source Strength: 100 W

Source Beam Size: Nominal spot size: 400 × 400 μm²spot size measured by analyzing Au/Si fresh cleaved sample using line scan measurement resulted to be equal to 355.8 × 219.2 μm².

Signal Mode: Pulse single channel

Incident Angle: 30°

Source-to-Analyzer Angle: 67.38°

Emission Angle: 53°

Specimen Azimuthal Angle: 70°

Acceptance Angle from Analyzer Axis: 60°

Analyzer Angular Acceptance Width: 30° × 30°

Manufacturer and Model: Thermo Scientific EX050

Energy: 3000 eV

Current: 0.002 mA

Current Measurement Method: Biased stage

Sputtering Species and Charge: Ar⁺

Spot Size (unrastered): 200 μm

Raster Size: 3000 × 3000 μm²

Incident Angle: 45.00°

Polar Angle: 58.43°

Azimuthal Angle: 24.20°

Comment: Sputtering of Ag, Au, and Cu samples was performed before checking the linearity of the binding energy scale.

Energy Scale Correction: Linearity of the energy scale was performed recording the binding energy of Cu 2p_3/2, Ag 3d_5/2, and Au 4f_7/2 signals on sputter cleaned high purity copper, silver, and gold foils (Goodfellow, Cambridge, UK). The binding energy values were compared with those reported in ISO 15472:2010. The C 1s of aliphatic carbon at 285.0 eV was taken as internal reference.

Recommended Energy Scale Shift: −0.16 eV for the five solvents’ cleaned sample (# 01976) and −0.1 eV for the mechanically polished sample (# 01977).

Peak Shape and Background Method: Spectra were processed using CasaXPS software (V.2.3.25PR1.0) (Ref. 5). Shirley background subtraction was applied before curve fitting; the curve-fitting was carried out using mixed Gaussian - Lorentzian product function as line shape. In the case of Fe 2p_3/2, the metallic component was fitted adding the tail for taking into account the asymmetry of the signal (Ref. 6).

Quantitation Method: N/A

This study was carried out under the project “e.INS, Ecosystem of Innovation for Next Generation Sardinia,” funded by the National Recovery and Resilience Plan (PNRR)—Mission 4, Component 2, “From research to business” Investment 1.5, “Creation and strengthening of Ecosystems of innovation” and construction of “Territorial R&D Leaders” (No. CUP F53C22000430001). The Ph.D. fellowship of Raouaa Hannachi is cofunded by Ministero dell’Università e della ricerca (MUR)—Italy (No. CUP F22B22000560005) and by RINA Consulting—CSM S.p.A.

The authors have no conflicts to disclose.

Raouaa Hannachi: Data curation (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Deborah Biggio: Data curation (equal); Investigation (equal); Writing – review & editing (equal). Bernhard Elsener: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). Marzia Fantauzzi: Conceptualization (equal); Data curation (equal); Investigation (equal); Methodology (equal); Writing – review & editing (equal). Antonella Rossi: Conceptualization (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Writing – review & editing (equal).

The data that support the findings of this study are available within the article and its supplementary material.