1. Introduction
Hydrogen embrittlement (HE) arises when hydrogen atoms permeate into a metal, often during exposure to hydrogen-rich environments in processes such as operational exposure in scenarios like high-pressure hydrogen storage, acidic, or alkaline conditions. This infiltration of hydrogen can destabilize the metal through several mechanisms: it diminishes the cohesive forces between metal atoms (a phenomenon known as decohesion) [
1,
2], facilitating the initiation and propagation of cracks. Additionally, certain metals, notably titanium and zirconium, may react with hydrogen to form brittle metal hydrides [
3], enhancing their susceptibility to cracking. Additionally, diffused hydrogen can induce localized slipping within the metal’s crystal structure, triggering premature failure under stress.
Fatigue failure occurs when a material progressively weakens and ultimately fractures under repeated cycles of stress levels below the material’s ultimate tensile strength [
4,
5]. This failure initiates at stress concentrators such as notches, holes, or surface imperfections, where cracks start to form and subsequently extend with each load cycle. The propagation rate of these cracks hinges on the applied stress range [
6,
7] such as alternating tension and compression, and the properties of the material [
8,
9,
10] including chemical composition, microstructure (grains arrangement and size), fracture toughness, and hardness. Eventually, the crack reaches a critical size, rendering the remaining cross-sectional area inadequate to bear the load, which leads to a sudden material fracture with little to no forewarning. HE and fatigue failure, though distinct phenomena, are closely interlinked and can significantly compromise the structural integrity and lifespan of metallic components, particularly in high-stress engineering environments. The interaction between these two processes is crucial for understanding their combined impact on material failure.
The presence of hydrogen notably accelerates fatigue failure in metals [
11], primarily through reduction of the energy required to initiate cracks. Hydrogen intensifies the rate of crack propagation during each fatigue cycle as it promotes localized plasticity [
12] and reduces the fracture toughness of the material at the crack tip. This leads to accelerated crack growth under cyclic loading. HE also lowers the threshold stress intensity factor for crack growth [
13,
14], allowing cracks to extend at lower stress intensities than those observed in non-embrittled metals. Consequently, hydrogen presence significantly diminishes the fatigue life of a component, with cracks forming sooner and extending more rapidly.
In a study examining the impact of diffusible hydrogen on the fatigue life of spot welds in high-tensile-strength steel sheets [
15], researchers found that both the number of cycles to failure and the endurance limit decreased as the diffusible hydrogen content increased at a constant amplitude-loading frequency. This phenomenon was attributed to the accumulation of hydrogen and its influence on crack growth behavior, facilitated by two primary mechanisms: hydrogen-enhanced local plasticity (HELP) and hydrogen-enhanced strain-induced vacancies (HESIV).
The HELP mechanism reveals how hydrogen contributes to localized increase in plastic deformation around stress concentrators [
16,
17], which can lead to the early onset of material failure. On the other hand, the HESIV mechanism focuses on how hydrogen affects the metal's microstructural properties [
17]. Specifically, it emphasizes the interaction between hydrogen and the vacancies that arise from irregularities in dislocation movement during plastic deformation. This interaction leads to an increased concentration of vacancies, which significantly impacts the metal's mechanical properties. These vacancies may coalesce into microvoids or facilitate the nucleation of microcracks. Under cyclic loading, these microvoids and microcracks are prone to expansion, resulting in premature material failure.
Another comprehensive study conducted on the fatigue crack growth characteristics of electron beam melted Ti-6Al-4V alloy in a high-pressure hydrogen environment demonstrated a notably faster fatigue crack growth rate [
18] compared to that observed in air. This escalated rate of crack propagation was attributed to several hydrogen-related mechanisms that intensify the fatigue behavior of this titanium alloy. The primary mechanism cited by the researchers is HELP [
19], which suggests that the presence of hydrogen facilitates increased localized plastic deformation at the crack tip. This deformation accelerates the crack growth under cyclic loading conditions. Additionally, the study referenced the adsorption-induced dislocation emission (AIDE) mechanism [
20], which involves hydrogen atoms reducing the energy barrier for dislocation emission at the crack tip. This process enhances the mobility of dislocations, contributing to the rapid expansion of the crack.
Furthermore, hydrogen-enhanced decohesion (HEDE) [
2] was identified as a critical factor influencing the alloy's fatigue performance. This mechanism proposes that hydrogen atoms weaken the metallic bonds within the material's lattice structure, particularly around the crack tip, thereby promoting the separation of the lattice and facilitating crack extension. These findings were thoroughly contextualized in a review by Lynch [
20], which provides a detailed analysis of these mechanisms.
Given the synergistic effects between hydrogen embrittlement and fatigue failure, it is imperative for engineers to consider both factors when designing and selecting materials for applications likely to encounter hydrogen exposure or where high-cycle fatigue is anticipated. This dual consideration is essential to enhance safety and reliability in such applications.
In the transition toward sustainable energy systems and the reduction of carbon emissions, the co-firing of hydrogen (H
2) with natural gas emerges as a critical technology for cleaner power generation [
21,
22,
23,
24]. Hydrogen can significantly contribute to this transition by displacing gaseous fossil fuels, as its combustion produces fewer greenhouse gases (GHGs). The blending of hydrogen with natural gas, however, may lead to HE under static and cyclic stresses, which necessitates the need to investigate this phenomenon.
Numerous studies have examined the HE behaviors of various materials under hydrogenated conditions. However, there is a need for further research on other mechanical properties of these materials, as information on some aspects remains scarce. Particularly, there is limited data on the fatigue behavior of materials concerning hydrogen concentration. Given the critical areas of application of low-carbon steel, which may experience cyclic loading during service in the presence of hydrogen environment, such as, oil and gas pipelines and pressure vessels due to pressure changes, it is imperative to study how low-carbon steel responds to hydrogen permeation and diffusion under fatigue loading.
This study aims to assess the impact of varying hydrogen concentrations on the fatigue life of cold-finished mild steel, establish the hydrogen concentration threshold above which fatigue performance significantly deteriorates, investigate the microstructural damage in mild steel subjected to different hydrogen concentrations and their correlation with fatigue properties, explore the fundamental mechanisms of hydrogen embrittlement under cyclic loading conditions and their contribution to fatigue failure.
2. Experimental Protocols
2.1. Material
Standardized Cold-finished mild steel (AISI 1018) coupons with dimensions as shown in
Figure 1 were used. Samples were carefully machined to ensure smooth, flat surfaces and uniform dimensions.
To verify the uniqueness of the properties of the acquired samples presumed to be cold-finished mild steel for this study, various characterization techniques were employed to provide an all-inclusive information on the chemical composition, microstructure, and crystal structure.
Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was utilized to determine the chemical composition of the test samples. This technique operates by dissolving and ionizing the samples, followed by measuring the mass-to-charge ratio of the ions present. The resulting data provided precise elemental composition details of the samples as given in
Table 1 below.
Scanning Electron Microscopy (SEM) was employed to analyze the microstructure, surface morphology, and metallographic features of the samples as given in standards [
25,
26]. The sample preparation involved sectioning and mounting in bakelite resin, followed by grinding with progressively finer grit papers (240, 320, 400, 600, and 800) to remove sectioning scratches and achieve a smooth surface. To obtain a mirror-like, oxidation-free surface essential for effective microstructural examination, the sample was further polished using 1-micron and 0.3-micron alumina suspensions on an emery cloth. The polished samples were then etched with Nital to reveal the microstructure before being examined under SEM.
While steel is generally known to have a body-centered cubic (BCC) structure, X-Ray Diffraction (XRD) analysis was performed to verify the crystalline structure of the test samples. The sample was prepared to meet the specifications required by the Bruker D8 Advanced X-ray diffraction system (Bruker, USA). The Bragg angle range of scanning was set from 20 to 100 degrees with 0.0490 step size. The diffraction pattern obtained was analyzed using the EVA software equipped with the XRD system to identify the crystalline phases present in the samples.
The hardness of the samples was determined using the Rockwell hardness testing technique according to standards [
27,
28] with scale B. The sample surface was prepared to be smooth and free of contaminants that could affect the test results. The sample was then securely positioned on the testing platform of the Rockwell hardness tester. A 1/16 inch steel ball indenter was brought into contact with the sample surface, and a minor load of 10 kgf was applied to create an initial indentation, allowing the indenter to seat properly on the surface. Once the minor load was stabilized, a major load of 100 kgf was applied, resulting in deeper penetration of the indenter into the sample. The depth of the indentation was automatically measured by the equipment after holding the major load for a specified dwell time.
In a preceding study on the tensile behavior [
29] of similar samples, mechanical properties such as yield strength, ultimate tensile strength, and toughness were determined by subjecting the samples to uniaxial tensile testing. Plots of stress against strain were generated, with a 0.2% offset calculated to determine the yield strength which was found to be ~650MPa.
2.2. Electrochemical/Cathodic Charging and Electrolyte
During the electrochemical hydrogen charging process conducted on 11 samples at varying hydrogenating conditions (0.00, 0.05, 0.20, 0.40, 0.60, 0.80, 1.00, 1.40, 1.60, 1.80, and 2.00 wppm), a solution of sodium hydroxide (NaOH) and ammonium thiocynate (NH4SCN) with a constant pH of 12.5 was used as the electrolyte with the later serving as a recombination poison. Each sample underwent a standardized charging time of 180 minutes. To ensure consistent and controlled hydrogen charging, a 1-ampere potentiostat was employed to maintain a constant current throughout the charging duration per sample. The potentiostat's working electrode was connected to the sample, while the counter electrode was connected to the charging cell.
The samples were immersed in the electrolyte such that the narrow (gauge) area was fully in contact with the electrolyte. This setup ensured uniform exposure and facilitated effective hydrogen diffusion into the samples characterized by bubbling during the process. The amount of hydrogen diffused into each sample during the charging process was subtly estimated based on the charging current, as detailed in a previous study [
30].
2.3. Fatigue Testing
Immediately after cathodic charging, each sample was tested under cyclic loading with a constant but alternating tensile and compressive loads (
Figure 2) on top and bottom surfaces of samples respectively. The load, which was applied at the free end was calculated based on 50% of material’s yield strength known from preceding experiment with respect to the standard bending equations for specimen as a circular cross-section cantilever given in equation 1 below. Cyclic loading on samples was done using TecQuiment’s Rotating fatigue machine (SM1090V-Nottingham, UK). This equipment functions based on Wohler’s test [
31,
32]. Samples were subjected to cantilever loading as shown in
Figure 3. It has a main unit, and a separate control and instrumentation unit (
Figure 4 and
Figure 5).
Where l is the distance from the midpoint of the sample to the end where load is applied, F is the applied load, D is the minimum neck area diameter, and σ is the applied stress.
The main unit rotates the sample under applied constant load as its motor turns a coupling and driveshaft which turns a collect chuck. The chuck grips the end of the test sample with uniform pressure around its circumference. The gimbal assembly houses a self-aligning bearing which rotates together with the free end of the test specimen (
Figure 4a, b). Counting of the number of cycles (rotations) and the measurement of the applied load are respectively done by an embedded sensor and a load cell. It is also equipped with a safeguard cover which stops the test automatically when opened.
The instrumentation unit allows the control of the rotating speed of the specimen (
Figure 5a, b). It also displays the applied testing conditions; frequency and load as well as the instantaneous number of cycles.
A constant frequency of 60Hz was applied for all samples hence, the exposure of samples to cyclic loading remained unchanged implying that any changes in fatigue behavior were primarily due to changes in material properties rather than external loading conditions.