Characterization of Boride Coatings on AISI 8620 Steels without and with Hydrogen Permeation

Te present work studied the characterization of boriding coatings without and with exposed hydrogen permeation on AISI 8620 through microstructure and mechanical behavior. A dehydrated paste pack process was carried out for the formation of boride coatings on AISI 8620 at 1173, 1223, and 1273K for 6 h of exposure time for each temperature. After boronizing, hydrogen was introduced into the borided steels by in situ cathodic charging using an H 2 SO 4 acid electrolyte. Specimens borided without and with hydrogen were characterized by scanning electron microscopy with energy dispersive spectroscopy and X-ray difraction. Te experimental techniques of the Vickers microhardness and three-point bending were used. As a result, the hydrogen difusion on steel AISI 8620 borided shows a signifcant increase in microhardness. Furthermore, three-point bend tests showed an increased strength on 8620 steel borided. However, the ductility and strength value on 8620 steel borided surfaces presented a drastic reduction due to hydrogen permeation. Te results revealed that the borided coatings provided strength and reduced the hydrogen embrittlement index on AISI 8620 borided.


Introduction
Hydrogen is currently the alternative energy source for the naval, petrochemical, space, geothermal, and nuclear industries. However, hydrogen in materials has been shown to damage and degrade materials [1,2]. Te AISI 8620 steel has an application for rods in an oilfeld in performance under several conditions of high corrosion and heavy wear in an acid environment containing CO 2 and H 2 S [3]. Te H 2 S damage on steel has been considered a problem in the oil and gas industry.
Hydrogen can be introduced into a material by various methods such as electroplating, during welding, and cathodic charging [4]. Hydrogen difusion using the electrochemical hydrogen permeation method developed by Devanathan and Stachurski [5]. In the case of cathodic charging the current density plays an important role in hydrogen absorption in steel. According to [6], the current density used must be between 0.8 mA/cm 2 to 62.5 mA/cm 2 .
Hydrogen difusion through Fe alloy results in several cracks, sulfde stress corrosion cracking, and susceptibility to hydrogen embrittlement. Previous works have studied the hydrogen embrittlement (HE) in iron bases alloys, and several mechanisms have been proposed [7,8]. However, the efect of hydrogen on the mechanical properties on behavior depends on diferent conditions such as loading, metallurgical structure, strength level, strain rate, type of charging, and charging current density [9].
One of the alternatives for preventing hydrogen embitterment is modifying the metal surface, producing a barrier for hydrogen difusion into a metal [10,11]. Tese barriers, such as carbides and nitrides, extend the service life of metallic materials [12]. Moreover, the prevention of HE in diferent materials can be considered by surface modifcation. Some surface modifcations, such as nitriding and carbonization are promising approaches for enhancing HE resistance on austenitic stainless steel [13]. At the same time, previous studies indicate that boronizing reduces HE on ASTM A-36 steel [14].
Te boriding process is a thermochemical surface treatment. In this process, boron atoms are introduced into the metal lattice at the surface of the substrate atoms [15]. Tis difusion occurs at temperatures of 1073 to 1323 K for a treatment time of 30 min to 10 h. In general, boriding produces a FeB and Fe 2 B coating. However, single or double intermetallic phases are obtained depending on process temperature, the chemical composition of substrate materials, boron potential, and boronizing time. As a result, the boriding processes modify the surfaces characteristics, enhancing the mechanical and tribological properties of the surface.
Reviewing the literature related to the boriding of AISI 8620 and some mechanical properties of boride coatings. Previous work on AISI 8620 steels borided has been reported, like López Perrusquia et al. [16] reported fracture toughness values of 2.64-3.55 MPa·m 1/2 . At the same time, Gunes et al. [17] studied the formation of borided coating using mixtures of B 2 O 3 paste by plasma and B 4 C/SiC by plasma paste boriding process. Tey reported that the lowest hardness of 1582 Hv 0.05 , and the highest hardness was 1992 Hv 0.05 . On the other hand, with regard to the mechanical properties of boride coatings, some steel borided subjected to bending tests, Kartal Sireli et al. [18] reported the improvement of the low carbon steel's hardness by cathodic reduction and thermal difusionbased borinding. Te bending strength of the steel was improved 2.5 times concerning no boriding sample. Similarly, Günen et al. [19] studied the bending strength on AISI 304 using nanoboron and ekabor III powder. Tey reported the value range of bending strength from 701.93 to 1504.28 MPa, the bending strength increasing parallel to the increment of thickness values of the boride coatings. However, in addition to increasing the surface mechanical properties with boride coatings, hydrogen permeation tests on borided AISI 8620 steels have not yet been performed.
Te dehydrated paste pack boriding (DPPB) brings new potential applications of borided substrate. Indeed, the boride coatings can be reduced hydrogen uptaken. A few studies are available regarding using boride coatings under hydrogen permeation. For example, Perrusquia et al. [14] reported that the borided coating on surface ASTM A-36 is less susceptible to formed cracks on the substrate by increasing the FeB and Fe 2 B coatings.
Tis work presents the characterization and exposure to hydrogen on AISI 8620 borided steel. Te DPPB formed boride coatings on 8620 steel at three diferent temperatures. Te 8620 steel borided and 8620 steel borided with hydrogen permeation surface were characterized using X-ray diffraction and scanning electron microscopy (SEM). In addition, the mechanical properties of 8620 steel were examined using microhardness Vickers and a three-point bending test. Furthermore, the mechanism of the fracture boride surface of 8620 steels with and without under hydrogen permeation was assessed using SEM. Before the boriding process, the specimens were cleaned with ethanol. Te boriding process was carried out with a DPPB by embedding the specimens in a cylindrical containing Durboride© dehydrated boron paste. Specimens were placed separately 13 mm from each other and at least 23 mm away from the cylindrical side of the container, as shown in Figure 1. Te DPPB was carried out in a conventional furnace without an inert atmosphere at 1173, 1223, and 1273 K with 6 h exposure time for each temperature. Once the treatment was complete, the container was removed from the furnace and slowly cooled to room temperature.

Cells of Hydrogen Permeation.
After the DPPB process, the specimens were removed from the container and the residual dehydrated paste left on the specimen surface was cleaned by brushing using a toothbrush. Hydrogen permeation was difusion by hydrogen ions through AISI 8620 borided surface. Figure 2 shows a schematic of the test setup for in situ hydrogen charging that was applied to the sample developed by Devanathan and Stachurski [5]. An electrode of platinum was used as an anode and the cathode was an 8620 steel-borided sample.
Te hydrogen cathodic charging was carried out in a 0.5 M H 2 SO 4 electrolytic solution at room temperature (22°C ± 1), applying a current density of 50 mA/cm 2 , the hydrogen was precharged for 24 h in specimens 8620 steel borided. Once the hydrogen charging was complete, the specimens borided were removed from the cells, and then the bending test was performed.

Microstructural Characterization.
Bend test specimens borided without and with hydrogen permeation were crosssectioned for metallographic preparation by progressively grinding with SiC papers. Te last stage was polished using 1 µm diamond paste, then etched with 3% nital solution. Te boride coating depth and morphology were studied by SEM and energy dispersive spectroscopy (EDS) using JEOL 6010 LV equipment. Furthermore, X-ray difraction was performed on the top surface on 8620 steel borided using a Bruker D8 Advance difractometer equipped with Cu-Kα radiation (wavelength of 1.5406Å) over a 2θ of range from 20°to 80°operating at 35 kV and 25 mA.

Mechanical Properties of the Boride Coatings.
Te mechanical behavior was performed by Vickers microindentation and three-point bending tests on specimens borided without and with hydrogen charging. Vickers microindentation tests were carried out with a Hardness model TUKON 1102 microhardness tester employing ASTM E384 standard [20]. Te indentation load is 2.45 N applied at 15, 25, 35, 45, 55, and 65 µm from the boride steel surface, 15 measurements were taken for each distance.
Tree-point bending test (TPBT) was employed in a universal testing machine (Instron UNITED STM1 model 1125), as shown in Figure 3(a). Te loading was applied perpendicular to borided coatings with a cross-head speed of 1 mm/min, the schematic diagram of TPBT is shown in Figure 3(b), and a representative cross section at midspan on specimen borided (Figure 3(c)). Seven sets of identical 8620 steel specimens in each experimental condition were tested to obtain the average bending stress and bending strain.
Te parameters of bending stress (MPa) and bending strain (%) are obtained by ASTM E 855-08 [21] and calculated from the test data using the following equations:1 where P is the load (N), L is the length of the sample (mm), W is the width of the sample (mm), D is the depth or thickness of the sample (mm), and d is the displacement/ midspan defection (mm). Te elongation and reduction area is often used as the evaluation criteria due to the establishment of the efect of hydrogen on ductile material [22]. Te area reduction is 15  Advances in Materials Science and Engineering associated with the change in the ductility of bending strain that was modifed by hydrogen embrittlement. Te degree of hydrogen on iron steels infuence is related to the decrease in ductility [23]. Hence, the hydrogen embrittlement index HEI was evaluated as follows: where ε o is the maximum bending strain value of the sample without hydrogen charging, and ε H is the maximum bending strain subjected to hydrogen. Te increment of the HEI index indicates an efect of hydrogen in the mechanical property of ductility. Furthermore, the bending specimens before being fractured under three-point bending test were examined by SEM to reveal the mechanism behind the boride coatings fracture. Figure 4 shows SEM micrographs of the cross section of boride coatings on the surface of AISI 8620 at 1173 and 1273 K for 6 h. Tree diferent regions were observed on the surface of AISI 8620: (i) at the top FeB coating, (ii) between FeB coating and steel matrix is observed Fe 2 B coating, and (iii) steel  An et al. [3] found that AISI 8620 steel boride shows a saw-toothed morphology with a two-borided coating, whereas Tabur [24], presented that the total boride coatings thickness (FeB + Fe 2 B) had a ranged from 80 to 250 μm on the surface of AISI 8620 steel. Although these studies were using the solid-state pack boronizing method, the results of thickness borided are similar to values obtained to the DPPB process [25][26][27].

Characterization of Boride Coatings.
On the other hand, Figure 5 shows a similar FeB and Fe 2 B region coatings on AISI 8620 steel after hydrogen charging.
After the hydrogen charging process, the thickness of FeB and Fe 2 B coatings in each condition are kept uniform. However, FeB/Fe 2 B/substrate regions are observed cracks. Tese cracks in AISI 8620 steel boride are marked in Figure 5 with black arrows. Tis type of crack is characteristic of the material with hydrogen induction. As shown in Figure 5, the formation of cracks in the 8620 steel is lower in the smaller FeB and Fe 2 B thicknesses of the 1173 K sample than in the 1273 K sample ( Figure 5(b)). However, at high thicknesses ( Figure 5(b)), the number of small cracks increases, also found in perpendicular cracks. Tese cracks may be due to increased hydrogen absorption quantity. Tis efect means that the FeB and Fe 2 B coatings with small thickness prevent the crack formation in steel exposed to hydrogen charging.
Te EDS analysis exhibits the presence of element in the boride coating, Figure 6 shows the plot of counts versus KeV obtained during SEM microanalyses carried out on three regions on cross section on AISI 8620 steel borided.
Te presence of Fe and B in EDS spectra show the formation of the FeB and Fe 2 B coatings in the surfaces of AISI 8620 steel. Te FeB coating contains 16.23 wt% of B (Figure 6(a)) and Fe 2 B coating of 8.99 wt% of B (Figure 6(b)), these wt% B are according to the Fe-B phase diagram [28]. Te levels of chromium and molybdenum appear to be lower than iron in the boride coatings due to their lower solubility, and carbon does not dissolve in the FeB phase [29] and Fe 2 B phase [30]. Te substrate zone revealed the alloying elements present in AISI 8620 without the presence of the boron atom, as shown in Figure 6(c).
X-ray difraction (XRD) patterns on AISI 8620 borided steel are present in Figure 7. Tese reveal the FeB (JCPDS 00-002-0869) and Fe 2 B (JCPDS 00-003-1053) peaks in all conditions. Te results exhibit FeB and Fe 2 B difraction peaks at 2θ of 45.068°with (200) and (211), respectively. Te XRD patterns obtained on the boride surfaces show the increment of the intensity of the peaks that represent the Fe 2 B phase with the increment of the temperature, specially the peaks at 25.5 and 36.5 in 2 theta.
In addition, boron atoms reduce the lattice spacing generating compressive stress at the surface on AISI 8620 steels. Due to the compressive stress states generated by boriding treatment, the AISI 8620 borided exhibit high resistance to HE. Furthermore, the HE resistance of the AISI 8620 steel borided increases when the borided layer thickness is small. As well, a saw-tooth morphology at the FeB/ Fe 2 B and Fe 2 B/steel matrix interface is an indicator of a good adhesion; in this context, if proper adhesion then HE possibilities are reduced [31], as reported in [14,32,33]. Te saw morphology in the borided layer are efective to reduce the crack propagation, although they can not purely eliminate hydrogen embrittlement.

Mechanical Properties of Coatings Uncharged and Exposed
to Hydrogen. Microhardness measurement was carried out from the surface to the substrate to obtain microhardness behaviour on the: FeB, FeB + Fe 2 B, Fe 2 B, transition zone, and substrate. Figure 8(a) shows the hardness variation in boride coatings formed on the surface AISI 8620, before and after the hydrogen cathodic charging process (H).
Te microhardness profle on the surface AISI 8620 borided without hydrogen permeation is shown in Figure 8(b) (black circle). Te microhardness improved when the boriding process temperature increased from 1173 K to 1273 K. Te microhardness of the FeB coating enhanced from 1505 to 1789 Hv. While the microhardness of the Fe 2 B coating improved from 1342 to 1757 Hv, the microhardness in the transition zone was in the range from 810 Hv to 1129 Hv, and in the steel matrix, it was of 253 Hv.

Advances in Materials Science and Engineering
Te hardness of the boride coatings is higher than that of the AISI 8620 substrate, which is a consequence of the presence of the FeB and Fe 2 B phases, also the dissolution of Cr into FeB/Fe 2 B coatings. In the work of Tabur et al. [24], solidstate pack boronizing on AISI 8620 steel was found that the surface hardness of boriding steels was 1850 and 1750 HV for 6 h at 1223 K with 6 h and 1123 K with 6 h, respectively. Te hardness of the borided coatings obtained from the DPPB process agrees with the hardness values obtained by traditional powder pack boriding [5,23]. For the case of AISI 8620 borided with hydrogen permeation, the microhardness profle is shown in Figure 8(c) (black circle), the FeB coating is a range from 1626 to 1861 Hv and zone of Fe 2 B coating from 1543 to 1884 Hv, the transition zone from 968 to 1389 Hv, and the steel matrix is 287 Hv. Te microhardness values for each zone increase due to the interaction of hydrogen. Tus, the hydrogen afects the mechanical properties that show the same behaviour on ASTM A-36 with FeB/Fe 2 B coatings reported by Perrusquia et al. [14].
In addition, Figure 9 shows the stress-bending strain curve obtained from Tree-point bending tests and applying the equations (1) and (2) and Table 2 shows the average values for borided and unboride samples without and with the hydrogen permeation (H) as well as the HEI on each 8620 steel borided condition.
Te AISI 8620 steel as received was observed to have a maximum stress peak of 1403.42 ± 5.05 MPa at a corresponding strain with 5.90 ± 1.10% (Figure 9(a)). Furthermore, Figure 9(a) for AISI 8620 borided without permeation hydrogen were seen to have a maximum stress peak with 1809. 35 Figure 9(b)). While, the maximum stress peak of the boride surfaces at 1173,1223, and 1273 K with hydrogen permeation decreased to 1530.42 ± 15.15, 1600.98 ± 14.5, and 1655.30 ± 6.05 MPa at a corresponding strain of 1.63 ± 0.65, 1.94 ± 0.5, and 1.90 ± 0.45%, respectively. According to equation (3), the values of HEI on AISI 8620 as received was 50.38%, and 8620 borided steels were 9.28, 13.00 and 17.03% for 1173 K, 1223 K, and 1273 K, respectively. Te HEI is the reduction of ductility and strength due to the entry of hydrogen into the metal lattice. For that, while the unboride surfaces of AISI 8620 reduced its HEI by 50.58%, the formation of FeB and Fe 2 B coatings on surfaces 8620 steel at 1173, 1223, and 1273 K reduced their HEI by 9.44, 13.00, and 17.03%, respectively. A previous work by Yadav et al. [35], showed that the HEI on AISI 8620 is close to 54% after the glycerin method. Whereas, Perrusquia et al. [14] found that in the formation of high FeB and Fe 2 B thicknesses on ASTM A-36 with hydrogen permeation, the boride coatings act as a difusion barrier of hydrogen atoms in the substrate without causing damage by cracking, and reducing the loss of ductility of the surface.
Hydrogen atoms can be absorbed in borided 8620 steel, the absorbed hydrogen may be present either as atomic or molecular form. During the hydrogen permeation time, the hydrogen difuses in FeB/Fe 2 B coatings, then in the 8620 steel grain boundaries and forms bubbles at grain boundaries. Te accumulation of molecular hydrogen at the local site tends to increase the pressure, then embrittle the 8620 steel. Te decohesion and hydrogen-enhance localized plasticity theories are the most plausible [36].
Hydrogen embrittlement can be prevented by adding a metal alloy into the substrate and applying coating above it. Some author suggests that the hard coating such as TiC, TiN, TiO 2 , BN, Cr 2 O 3, and WC have used a protective barrier  Advances in Materials Science and Engineering against the permeation of hydrogen [37]. So the formation of FeB/Fe 2 B coatings on surface AISI 8620 steels can reduce the hydrogen embrittlement.

Fractography of Coatings Uncharged and Exposed to
Hydrogen. Figures 10 and 11 show the fracture surface on AISI 8620 steel after three-point bend tests of sample borided and sample borided with hydrogen permeation. A general view of the boride surfaces at 1223 K for 6 h is shown in Figure 10(a); FeB + Fe 2 B coatings formed near the notch acted as crack initiation sites. Tis performance produced two diferent areas: one in the area close to the notch, where there were cracks and dimples caused by tension stress (Figure 10(b)), and the second was the lower area of the specimen, where observed cracks and dimples produced by the compression stress (Figure 10(c)). Figure 10(b) shows the interface (dash line) between boride coatings and substrate, where the FeB/Fe 2 B coatings and Fe 2 B/substrate transition area were fractured in brittle form. Te FeB + Fe 2 B areas were not deformed, so more applied strength than 2009.88 MPa of stress is required to crack the coating. Te fracture type obtained is cleavage type. Figure 10(c) shows that the FeB/Fe 2 B coating did not fracture; nevertheless, a brittle fracture is observed between the interface of the boride coatings and substrate.
Due to the applied load in 8620 steel borided surface with the notch, under boride coatings there was a ductile tearing and subsequently a transgranular fracture, which in the center shows mixed ductile-brittle fracture mode.   Furthermore, cracks in the notch are due to the formation of the brittle FeB coatings under static bending load. Also, the presence of Fe 2 B coatings increases the static load capacity due to the high compressive stresses [38]. Although the notch is a stress concentrator that can induce the failure of a component, the other cause was the presence of the brittle boron compounds themselves, for example, Ruiz et al. [39] reported that FeB has a fracture toughness (K IC ) ranging between 2.18 and 3.58 MPa·m 1/2 and Fe 2 B has a K IC value ranging from 2.58 to 3.88 MPa·m 1/2 .
Te fracture surface of 8620 steel borided at 1223 K with hydrogen charging is shown in Figure 11.
In the same way as the sample without hydrogen, the same two areas are presented: one is close to the notch and the lower area of the specimen. Figure 11(a) shows a general view of brittle fracture mode with transgranular fracture of the surface. Te presence of a secondary crack on the notch was because the notch tip leads to defect concentration. Terefore hydrogen accumulates in these lattice defects, and as the stress increases continuously, the crack initiates at a critical combination of stress and hydrogen concentration [40]; in the area close to the notch observed, cracks arrest ( Figure 11(b)). Tis zone did not present deformation in the boride coatings. Although the strength required for fracture was 1600.98 MPa, this load decreased compared with the condition of AISI 8620 steel borided (2009.88 MPa) due to hydrogen interaction. Furthermore, Figure 11(c) shows the lower area of the specimen in borided coatings. Tese exhibited fat facets with dimples that indicate a quasicleavage type of failure.
Te fracture surface in the center of AISI 8620 steel borided + H has a relief appearance, as shown in Figure 11(a), and a few hydrogen-induced cracks initiation. Figures 11(b) and 11(c) exhibit fracture of boride specimens tested with hydrogen charging, revealing the transgranular cleavage mode fracture in boride coating and quasicleavage mode in the substrate. In addition, there is evidence of the protective action of boride coatings against hydrogen diffusion into the AISI 8620 steel substrate.
Tese results show that hydrogen permeation afects the mechanical properties of the AISI 8620 steel without and with the DPPB process. Te DPPB process is an alternative for the borided steels, and it can be carried out by pack boriding in a solid medium without a controlled atmosphere with relative simplicity. In high alloy steels, borided coatings increase the service life on engineered components, and the optimal layer thickness ranges between 25 and 76 µm [41]. In this work, the formation of boride coatings can be applied to improve mechanical properties and reduce hydrogen uptake.

Conclusions
Te FeB and Fe 2 B phases were obtained by dehydrating paste pack boriding on surface AISI 8620 steel that prevents hydrogen embrittlement on these steels. Te coatings thicknesses were between 16.02 to 37.26 μm and 78.66 to 200.87 μm for FeB and Fe 2 B, respectively. Tese coatings showed saw morphology at each interface FeB/Fe 2 B and Fe 2 B/8620 steel.
Te accumulation of the hydrogen in boride coatings produced at 1173 K for 6 h prevents the hydrogen penetration into the AISI 8620 substrate. In contrast, microcracks were observed in boride coatings at 1273 K for 6 h after hydrogen charging, thus promoting hydrogen permeation into the AISI 8620 substrate.
Te reduction performance of the hardness from the FeB coating to the Fe 2 B coating and transition zone until reaching the steel matrix was similar to the surfaces with and without hydrogen penetration.
Te HEI showed a high value of 50.38% on AISI 8620 steel. However, the ranges of HEI of AISI 8620 boride with hydrogen charging decreased from 17.03 to 9.28% due to the formation of FeB + Fe 2 B coatings that prevented hydrogen difusion. Furthermore, the HEI was characterized by a change in fracture surface type from mixed fracture surface to quasicleavage type to brittle fracture. Te FeB + Fe 2 B coatings may prevent hydrogen charging of vehicles part and oilfelds in a hydrogen generation environment, decreasing the hydrogen embrittlement.

Data Availability
Te data used to support the fndings of this study are included in the article.

Conflicts of Interest
Te authors declarethat there are no conficts of interest regarding the publication of this paper.

Authors' Contributions
Marco Antonio Doñu Ruiz carried out Conceptualization, Formal analysis, Methodology, and Writing-original draft. David Sanchez Huitron and Victor Jorge Cortes Suarez carried out Methodology. Tomas de la Mora Ramirez carried out Supervision. Ernesto David Garcia Bustos carried out Visualization. Noé López Perrusquia carried out Conceptualization, Supervision, Writing-review and editing.