AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15 (2022) 067–072

Contents lists available at http://qu.edu.iq

Al-Qadisiyah Journal for Engineering Sciences

Journal homepage: http://qu.edu.iq/journaleng/index.php/JQES

* Corresponding author.
E-mail address: pme.19.38@grad.uotechnology.edu.iq (Zainab Ali)

https://doi.org/10.30772/qjes.v15i1.815

Studying the effect of multicomponent nano-coating alloy on hydrogen

embrittlement of AISI 1018 steel by gas-phase process

Zainab Z. Ali*, Baha S. Mahdi and Ameen D. Althamer

Production and Metallurgy Department, University of Technology, Baghdad, Iraq.

A R T I C L E I N F O

Article history:

Received 26 March 2022

Received in revised form 25 May 2022

Accepted 18 June 2022

Keywords:

Hydrogen embrittlement

Tensile test

DC sputtering

BNi-2

SEM.

A B S T R A C T

The present study is attempted to investigate the influence of multicomponent Ni-alloy (BNi-2) on

the hydrogen embrittlement (HE) behaviour of AISI 1018 steel by using cathodic protection and

tensile test. The results show that the HE indexes (HEI) decrease notably when AISI 1018 steel is

coated with BNi-2 alloy by DC sputtering process. This coating processes leads to decrease in HE

susceptibility of the AISI 1018 steel, which can be rationalized to the enhancement in corrosion

resistance and the decrease in hydrogen absorption of the AISI 1018 steel after coating. The tensile

strengths of bared samples were decreased with increasing charging time until 24 hours when stable

at values 350 MPa. while the coated samples showed an increasement in the tensile strength from

570 MPa to 750 MPa. stabilization in strength at value of 600 MPa was observed after exceeding 48

hours. Also, the tensile test for uncoated specimens indicated a clear reduction on the modulus of

elasticity compared with other coated ones.

1. Introduction

The deterioration effect of hydrogen penetration on steel properties have

been observed since 1870s, Johnson noted the changes in the toughness and

breaking strain of iron, when it was temporarily immersed in acid for just a

few minutes. The acids which produce the hydrogen during the reaction

with iron lead to a serious deterioration in properties. [1]. Rates of hydrogen

diffusion in metals depend on the hydrogen concentration gradient,

temperature, stress, susceptible materials, hydrogen-diffusion coefficients

(D) and the crystal structure [2]. For example, high-strength martensitic

steels are so susceptible to hydrogen penetration, since cracks are observed

at solute hydrogen concentrations less than 1 ppm (wt.). For ferritic steels

with strengths less than 750 MPa, relatively high hydrogen concentrations

(~10 ppm) are often necessary for the hydrogen penetration to be

significant. Nickel alloys, alumina, stainless steels, and copper alloys show

little susceptibility to hydrogen penetration; Fig. 1, [2-3]. The main sources

of hydrogen can be summarized as cathodic protection, pickling,

electroplating, special case of arc welding, galvanic corrosion, and acid or

gasses chemical reactions [4]. Hydrogen penetration weakens the

interatomic bonds between the grains of steel casing and causes premature

failures. This phenomenon could pose a risk to the sustainability of the oil

and gas projects. Therefore, nano-coatings were used to treat this problem.

Nanocoating term is used if the coating thickness is referred to as thin films

or the structures are in nanostructure scales. There are various methods of

producing nano-coatings, thin films, and nanostructure [5], which are

chemical vapor deposition (CVD) and physical vapor deposition (PVD).

The Phase structure, morphology, and mechanical properties of coatings

deposited using DC.

http://qu.edu.iq/
https://doi.org/10.30772/qjes.v15i1.815
http://creativecommons.org/licenses/by/4.0/

68 ZAINAB Z. ALI ET AL. /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15 (2022) 067–072

Figure 1. Hydrogen diffusion coefficients for Fe and Nb (bcc), Ni (fcc)

and Ti (hcp) as a function of temperature [2] [3]

Such as type of inert gas, working pressure, voltage, deposition time,

substrate temperature, the distance between target and substrate, the surface

finish of substrate, ion current, etc. [6] [7] [8]. They are directly affecting

phase structure, preferred orientation, chemical composition, and

deposition rate of the coatings [9] [10] Several researchers revealed the

effect of nanocoating on hydrogen penetration and corrosion resistance for

steel. S. Marikkannu and others reported that Indium oxide In 2O3 nano

coating deposited films are successfully deposited on different substrates

by using the gas phase process, the results are encouraging for the use of

thin films in low-cost industry applications [11].

BNi-2 alloy has been used in applications in environments withstanding

temperatures up to 1,000 °C, with properties that include abrasion

resistance to guard against erosion, kinetics that makes it impervious to

atomic hydrogen penetration and ingress can cause steel to become brittle

and crack. This work focuses on reducing the hydrogen embrittlement in

AISI 1018 steel by coating processes. Multicomponent nano-coating alloy

BNi-2 as protective films will be deposited by using DC sputtering

technique.

2. Material and experimental setup

2.1. Materials used

Steel (AISI 1018), which is used in oil industry storage tanks, is used as a

substrate material exposed to hydrogen charging with and without coating.

The chemical composition analysis of this workpiece was done by using

spectrometer analysis instrument available in Baghdad in the General

Company for Examination and Rehabilitation Engineering, as shown in

Table 1.

Table 1. Chemical composition of test specimen (in %).

C Si Mn Cr Mo Ni Co Cu Nb Fe

Thin film deposition has been obtained via DC or Direct Current Sputtering.

The target material used was the foil of nickel alloy called BNi-2 or (EN

ISO 17642). The thickness of this foil was 35 mm and its chemical

composition are explained in table (2) [12].

Table 2. Chemical composition of target foil (BNi-2).

Cr B P Si Fe C Ni

14 3 0.02 4.2 3 0.04

2.2. Specimen preparation

Surface preparation of substrate samples was carried out by emery papers

of silicon carbides with different grit sizes from 220, 400, 800, and 1000.

The samples also were ultrasonically cleaned in ethanol for 15 min prior to

the deposition process.

2.3 Coating process

Coating process was performed by DC sputtering, where the nickel alloy

foil was prepared as a circular target with a diameter of 50 mm and fixed

2.5 centimeters above the substrate of specimens. The chamber was

evacuated partially to a pressure of 10-1 mbar of pure Argon gas. The target

was bombarded with ionized Argon gas molecules causing atoms to be

“Sputtered” off into the plasma. These took off atoms then deposited when

they condensed as a thin film on the substrate to be coated. Adjusted DC

sputtering for coating thin-film current was 16 mA and the sputtering time

was fixed for all specimens to 50 minutes.

2.4 Effect of Hydrogen Embrittlement (HE):

Firstly, the dog bone-shaped (tensile) specimens of mild steel were welded

to one end of a copper wire by discharge welding, and then the other end of

the wire was connected to the cathode wire of the power supply. The tensile

specimens were covered with an acid-resistant insulating material (epoxy)

except the gauge length remained uncoated, and the uncoated area

calculated for specimens was about 250 mm2, as shown in figure -3-. The

anode was a stainless-steel bar.

Figure 2. Coated and uncoated work substrate before covering

ZAINAB Z. ALI ET AL. /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15(2022) 067–072 69

Sulfuric acid (H2SO4) dissolved in distilled water with a concentration of

0.5 M was used to create hydrogen-releasing environments. The test

specimens were immersed in H2SO4 solutions in a beaker. For better

understanding and statistical analysis of hydrogen damage, ten specimens

were immersed in each of H2SO4 solutions, a half of them were coated and

the other not; they were immersed for different intervals of time (2, 4, 6,

24, and 48 hours). The power supply was adjusted with 0.3 v and the

average current was 0.4 A with a current density of 120 mA/cm2.

Figure 3. Partially covered tensile test specimens by epoxy resin

Figure 4. Specimen failed after tensile test.

2.5. machining testing

Tensile test specimens were prepared according to ASTM E8 with a gage

length of 25 mm, and the total length for each specimen was 100 mm. The

machining process for all specimens was done using a CNC machine, as

shown in Fig. 2.

2.6. Morphology analysis

Coated and charged samples with dimension (10×10) mm were prepared to

test using SEM. Figure 5 shows the coated AISI 1018 steel with BNi-2

alloy, where the scratched surface belongs to the prior grinding process.

From the image, the coating layer completely covers the steel surface.

3. Results and discussions

3.1 Hydrogen Penetration:

Standard tensile specimens of AISI 1018 steel were charged with hydrogen,

the specimens were tested by a tensile test at the room temperature, and the

results are shown in Fig. 5.

Figure 5. A graph illustrating the results of all tensile tested

specimens

The first impression of the tested specimens is that the uncharged sample

will give the maximum tensile strength as it is known, in this experiment it

reached to 490 MPa, where the uncharged specimen is free of side effects

of hydrogen penetration defects. Also, all the charged specimens will suffer

from defects according to charging time and this is also applied to the

coated specimens, where the coat has not blocked all the hydrogen atoms

but it can reduce the penetration. In Fig. 5, the result is different from what

was expected. Charging the uncoated specimens give the expected results

as mentioned above by decreasing the tensile strength for all the uncoated

specimens but at different values when compared to the charging time.

Fitting curve for uncoated specimens as depicted in Fig. 6 has been done to

understand the relation between tensile strength as mechanical properties

and hydrogen charging. As it is expected, the uncoated specimens evince a

decrease in tensile strength with longer charging time until reaching a

steady state value about 30% of the original uncharged metal after 24 hours

of immersion in dilute sulfuric acid. In the actual embedded pipelines in

sites, the same happened but for a longer time depending on the cathodic

current density, soil type, and humidity. Nearest curve fitting is the second

order polynomial formula as stated in Eq. 1, where σ u is the tensile strength

in MPa, and T is the charging time in hours. Note that this equation is

applied for a specific environment involved in this work.

By curve fitting, the sputtered tensile charged specimens illustrated Fig. 7

showed the unexpected behavior, where the tensile strength increased

above the uncharged steel for just nano coating thickness. And, Eq. 2

represents the second-order polynomial curve fitting, the increasing

percentage value can be 24% which is very high, and it is noted that this

happened without heat treatment.

𝜎𝑢 = 503 − 9.299𝑇 − 0.124𝑇
2
(1)

𝜎𝑢 = 405 − 29.3𝑇 − 0.556𝑇
2
(2)

70 ZAINAB Z. ALI ET AL. /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15 (2022) 067–072

Figure 6. Tensile strength for uncoated specimens with different

charging times.

Figure 7. Tensile strength for coated specimens with different

charging times.

By curve fitting, the sputtered tensile charged specimens illustrated in Fiq.

7 showed the unexpected behavior, where the tensile strength increased

above the uncharged steel for just nano-coating thickness. Eq. 2 denotes the

fitting of a second-order polynomial curve. the increasing percentage value

can be 24% which is very high, and it is noted that this happened without

heat treatment.

Increasing tensile strength in coated and charged hydrogen specimens

necessitates additional tests such as TEM, which are not included in this

study. However, there are several explanations for this phenomenon, the

first of which is the entrance of the coating elements into the base metal via

a diffusion process aided by the high energy of hydrogen. These elements

react with iron and form strain hardening inside metal, finally the

occurrence of phase transformations. But this explanation is weak because

the coating elements as nickel, chromium and others are hard to diffuse

substitutionally at room temperature.

The second explanation is that the BNi-2 nano-coating layer on steel could

heal most nano and micro cracks in the substrate surface and reduce the

stress concentrations, causing improved mechanical properties [13].

The third explanation which needs good attention and is the nearest to be

approved is that the nano coating layer acts as a sieve for the hydrogen

atoms, which means the coating allows the hydrogen atoms to pass into the

structure in a small amount. Therefore, they are distributed within the

crystal lattice in a uniform and homogeneous manner. Hydrogen atoms

occupy interstitial sites within the crystal lattice, leading to a distortion in

it (Because the space inside the lattice is not enough to accommodate a

solute atom without the displacement of neighboring atoms). This

distortion is the tension and compression energy between the iron and

hydrogen atoms inside the metal, where the crystal structure is stable and

the distance between the iron atoms are fixed. So, when the hydrogen atoms

enter, the iron atoms move away from each other; thus, the tensile and

compressive forces are generated, and it looks like cold work inside the

metal [14].

3.2 SEM analysis and finding the thickness:

Figure 8 seems that it is not completely uniform as there are some

scratches like a terrace, this is due to surface finish, and this is because of

the nature of the sputtering process which requires some roughness to

increase the surface area and mechanical interlocking between the coating

atoms and substrate. It is clear that these terraces are formed by the stacking

of layered crystals. On the other hand, roughness is required to reduce the

hydrogen blistering as concluded by several researchers. The grinded

surface presents a lower sensitivity to the hydrogen-induced blisters in

comparison with the polished surface, which is attributed to the suppression

of hydrogen invasion caused by higher residual compressive stress and

higher dislocation density in the grinded surface [15].

Figure 8. SEM image for coated surface with low magnification

To identify the thickness of the nanolayer, the best method is by scratching

the coated surface using a sharp razer smoothly. By increasing the

magnification, the scratch becomes clearer, and the white marked region

represents the accumulated coating layer of BNi-2, as shown in Fig. 9. To

recognize the layer thickness, the magnification must be higher. With

higher magnification and lower beam intensity (4 mA) for the electron

beam, the segmented separated coating layers from the surface can be seen,

as portrayed in Fig. 10. Thin fragmented layer parts are shown in Fig. 10 as

ZAINAB Z. ALI ET AL. /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15(2022) 067–072 71

semi-transparent to electron beam as marked some of them by dotted

yellow lines to make better vision.

Figure 9. SEM image for scratched surface using sharp razer

Figure 10. SEM image for the scratched surface in figure (10) with

higher magnification.

To confirm the thickness result, another image with a higher magnification

was taken as shown in Fig. 11, where the measured thickness was 114.56

nm. According to the above, one can say that the coat layer thickness lies

between (90 to 110) nm, and it can be classified as nano or thin layer. Fig.

12 displays the SEM image with low magnification for coated charged

samples, and it is observed that there are different areas of the

inhomogeneous samples. In this figure, the microcracks, and ripping of

coating from small areas, as well as some spots, can be seen; this means

that the hydrogen atoms began to slightly and regularly penetrate the

coating layer. Coating microstructure plays an important role in affecting

mechanical and tribological properties as well as hydrogen permeation,

since the nickel structure (FCC), is a better metallic coating type for

resisting the hydrogen penetration than (BCC) elements because the

packing factor of (FCC) is more than the (BCC) structure related to fewer

voids between atoms in (FCC) compared to the (BCC) crystal lattice.

Therefore, the hydrogen atoms cannot easily penetrate throughout the

structure, and that is happened in this sample. The surface roughness values

for coated, uncoated, and charged specimens are measured by using a

roughness device, and the results were as follows. The substrate roughness

before coating and before the charging process was found to be equal to

0.1466 µm, coated uncharged steel equal to 0.093 µm, and Coated glass

was 0.04 µm. The roughness that appeared in the steel-coated layer is

related to the steel surface itself and is not due to the sputtering process as

in Fig. 13 showing the glass coated with the same amount (time and other

parameters) for steel. The smoothness of the coated glass is higher in orders

compared to the coated steel surface. When comparing Figs. 8-13, one can

notice clearly the homogeneity of lass rather than metal substrate, because

the tops and valleys in metal substrate act as nuclei which collect the

sputtered atoms, but that does not exist in a glass. This is the reason for

different images in metal and glass substrates.

Figure 11. High magnification SEM images show measurement of

nano sputtering layer.

Figure 12. SEM images of BNi-2 deposited on metal with HE.

72 ZAINAB Z. ALI ET AL. /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15 (2022) 067–072

Figure 13. SEM image of the BNi-2 deposited on glass.

4. Conclusions

1) Uncoated AISI 1018 steel and charged by hydrogen manifested

a decrease in tensile strength until a steady state about 30% of

its original tensile strength after a charging time of 24 hours with

a current the density of 120 mA/cm2.

2) Increasing the tensile strength by 24% to the original metal by

hydrogen charging for BNi-2 nanolayer on the AISI 1018 steel.

3) Most suggested cause of increasing the tensile strength by

hydrogen charging for the coated metal is the action of the nano-

coating layer as a sieve for the diffused hydrogen atoms

distributed interstitially between the iron atoms, leading to stress

in the lattice as occurring in the cold work strengthening.

4) Average coat thickness of BNi-2 on AISI 1018 Steel was 90 to

100 nm and the maximum coat layer was 114.5 nm.

References

[1] W. H. Johnson, Proceedings of the Royal Society of London, p. 168–179., 23

(1875).

[2] J Völkl and G. Alefeld, "Hydrogen diffusion in metals," in Diffusion in Solids, p.

231–302, 1975.

[3] M. D. a. S. Z. H.-J. Christ, "Hydrogen diffusion coefficients in the titanium alloys

IMI 834, Ti 10–2–3, Ti 21 S, and alloy C, Metal," pp. 1507–1517, 2000.

[4] s. lynch, "Hydrogen embrittlement (HE) phenomena and mechanisms," in Stress

Corrosion Cracking, Australia, 2011, pp. 90-130.

[5] a. L. B. Injeti G., "“Electrodeposition of nanostructured coatings and their

characterization”," J. of Science and Technology of Advanced materials, pp. 12 1-

129, 2008.

[6] B. Warcholinski and A. Gilewicz, " Effect of substrate bias voltage on the

properties of CrCN and CrN coatings deposited by cathodic arc evaporation.,"

Vols. 90, p. 145–150., 2013.

[7] A. Obrosov, M. Naveed, A. Volinsky and S. Weiß, " Substrate frequency effects

on CrxN coatings deposited by DC magnetron sputtering.," Vols. 26, p. 366 –373.,

2017.

[8] A. Krella and A. Czyzniewski, "Cavitation resistance of Cr-N coatings deposited

on austenitic stainless steel at various temperatures.," Vols. 266, p. 800–809,

2009.

[9] X. Wan, S. Zhao, Y. Yang, J. Gong and C. Sun, " Effects of nitrogen pressure

and pulse bias voltage on the properties of Cr-N coatings deposited by arc ion

plating.," Surf. Coat. Technol., Vols. 204, p. 1800–1810, 2010.

[10] H. Shah, R. Jayaganthan and D. Kaur, " Effect of sputtering pressure and

temperature on dc magnetron sputtered CrN films," Vols. 26, p. 629–637, 2010.

[11] M. K. A. A. S. MARIKKANNU, "STUDIES ON JET NEBULISER

PYROLYSED INDIUM OXIDE THIN FILMS," Journal of Ovonic Research,

vol. 10, pp. pp. 115 - 125, August 2014.

[12] Metals Handbook, " Properties and Selection nonferrous," in ASM, 2015, pp.

p1832.

[13] K. N., J.-M. Y., J. K. Tamaki Naganuma, "The effect of a compliant polyimide

nanocoating on the tensile properties of a high strength PAN-based carbon

fiber," composites science and technology, 2009.

[14] Underneath the Bragg Peaks – Structural Analysis of Complex Materials, vol.

16, pp.2 – 481. 2012.

[15] Y. W. 1. W. H. 1. J. Z. 2. a. X. W. 1. Xinfeng Li 1, "Effect of Surface Roughness

on Hydrogen-Induced Blister Behavior in Pure Iron," Metals, vol. 10, no. 745,

2020.

ZAINAB Z. ALI ET AL. /AL-QADISIYAH JOURNAL FOR ENGINEERING SCIENCES 15(2022) 067–072 73