Claims:I/We Claim:

1. A nickel based amorphous coating for a line pipe steel, comprising:
10-14% Phosphorus (P), 2-7 % Iron (Fe), rest being Nickel (Ni) and unavoidable impurities (all in wt.%).

2. The nickel based amorphous coating as claimed in claim 1, wherein the line pipe steel is API grade steel.

3. The nickel based amorphous coating as claimed in claim 2, wherein the line pipe steel comprises
C-0.024, Mn-1.6, S-0.003, P-0.016, Si-0.20, Al-0.04, B-0.0001, N-0.0068, Cr-0.26, Cu-0.01, Ni-0.15, Mo-0.15, V-0.034, Nb-0.056, Ti-0.014.

4. A system or a kit for coating nickel based amorphous coating over a line pipe steel, comprising:
a bath of
Nickel Sulphate in the concentration range of 35 – 45 gms/liter as the source of Ni;
Sodium hypophosphite in the concentration range of 20-30 gm/liter as a reducing agent and source of Phosphorus,
Succinic acid in the concentration range of 45-55 gm/liter as a complexing agent,
Sodium Dodecyl-sulphate (SDS) in the concentration range of 0.1-0.5 gm/liter as surfactant; and
pH of the bath being maintained at 4.0 to 7.0.

5. The system or kit as claimed in claim 4, wherein the bath temperature is 85-95 deg C.

6. The system or kit as claimed in claim 4, wherein residence time for coating is 60 + 10mins for deposition of 2-10 micron thickness of coating.

7. The system or kit as claimed in claim 4, wherein coating over the steel is done by dipping.

8. The system or kit as claimed in claim 4, wherein the bath pH is regulated by adding aqueous caustic soda.

9. The system or kit as claimed in claim 4, wherein the thickness of coating is 3.5 µm over the line pipe steel.
, Description:TECHNICAL FIELD
The present disclosure relates to a field of metallurgy and material science with particularly but not exclusively focusing on corrosion and its prevention. More particularly, the present invention relates a method of coating on line pipe steel.

BACKGROUND OF THE INVENTION
Line pipe grade of steels are used for the transportation of oil and natural gas, both for onshore and offshore applications. This grade of steels is hot rolled and then made into pipes which are laid both below and above the ground for the transportation of petroleum products. These pipes experiences atmospheric corrosion during service in open atmosphere. Furthermore, atomic hydrogen is adsorbed into these steel pipes which causes hydrogen embrittlement. The atomic hydrogen comes from the atmospheric corrosion or from the hydrogen sulphide (H2S) present in the petroleum products. It is known that a presence of few ppm level of hydrogen can cause the steel to fail even at one-tenth of it’s designed fracture toughness in an abrupt manner without any prior indication. In a typical line pipe application, a sudden and abrupt failure can cause huge losses; in financial, human lives or environment related damages. Therefore, a need arises to develop a suitable coating for the line pipe steels which is expected not only to provide a resistance to atmospheric corrosion but also to reduce the susceptibility to hydrogen embrittlement by providing a barrier to hydrogen ingress during service.

At present line pipe industry employs polymeric based coating and / or organic coating which provides resistance to atmospheric corrosion. However, these organic coatings are incapable in preventing hydrogen ingress into the steel as hydrogen is the smallest atom and capable of penetrating the porous and layered structure of organic coating easily.

Only Ni and Cd displays slow hydrogen diffusion or hydrogen barrier properties. Cd based coating has been previously applied as hydrogen barrier coating over aerospace components for the prevention of hydrogen embrittlement. However, Cd is an expensive metal and the application of Cd-plating involves Cd-salts which are reported to be carcinogenic. Hence, application of Cd as a coating is limited.

On the other hand, Ni based coating can be easily applied for its hydrogen barrier properties. Ni-electroplating is commercially done for its aesthetics. However, Ni-based coatings are applied using electroplating method, in which hydrogen is generated during the electroplating process and gets absorbed into the substrate and coating. This increases the risk of hydrogen embrittlement and therefore, Ni-electroplating is not a suitable method for applying hydrogen-barrier coating. Furthermore, resistance to atmospheric corrosion of a Ni-electroplated component is comparatively less.

PRIOR ART
Japanese patent JPH0636277 discloses a method of Ni based coating comprising of Ni-Fe, Ni-Co, Ni-P, Ni-Zn, Ni-Cr, Ni-Mn, Ni-Cu and Ni-Mo type alloy electroplating. The coating weights are of 5 – 100 mg/m2 and are reported to prevent infiltration of hydrogen into the substrate. The coatings are intended to be applied over auto-bumpers, door guard, bar parts in automotive. Also, the patent discloses a post-plating dehydrogenation treatment at 150 to 200 oC which adds one more process after coating. Hence, the coating process is expensive and is not suitable for large pipelines. Also, the efficiency of the coating in terms of resistance to hydrogen embrittlement is expected to be low due to electroplating method of coating. Furthermore, other expensive alloying elements such as Co, Cr, Cu, Mo etc are used along with Ni limiting the applicability of such coatings.

US patent US3338740A discloses a method of electroless Ni-coating by gaseous reduction of Ni-based salts. A mixture of (H2 + N2) gas is used at 93 oC in a specially designed autoclave to reduce the Ni-based salts. After the reduction of Ni-salts, metallic Ni is released and deposited over the components. However, hydrogen gas is used in the coating process which itself may get absorbed in the substrate and can cause hydrogen embrittlement. In addition, specially designed reactors are required for the coating process which cannot be adopted for applying coating over large line pipes.

US patent US3513015A discloses a method of hydrogen barrier coating comprising of Cu or Ni or Cr. However, the coating process uses a method involving acid cleaning using Cr2O3 + H2SO4/H3PO4 mixture, surface sensitization using SnCl2, nucleating treatment using PdCl2 in H2O2, then reducing the Cu or Ni based salt using organic compound of formaldehyde or trioxymethylene. However, application of H2SO4/H3PO4 for acid pickling shall introduce hydrogen into the substrate causing hydrogen embrittlement. PdCl2 is a very expensive chemical. H2O2 is an explosive and a controlled substance. In addition, Cr2O3 is reported to be carcinogenic and its use is regulated. Therefore, the method of coating is not at all viable for coating line pipes.

OBJECTS OF THE INVENTION
An object of the present invention is to disclose a coating which can be coated over the line pipe steel and will provide resistance to corrosion and hydrogen embrittlement.

Another object is to disclose a coating that can be applied over the substrates with ease.

DISCLOSURE OF THE INVENTION
The present invention discloses a nickel based amorphous coating for a line pipe steel, comprising:
10-14% Phosphorus (P), 2 – 7 % Iron (Fe), rest being Nickel (Ni) and unavoidable impurities.

The deposition of the amorphous coating is required so as to provide the resistance to atmospheric corrosion and reduce hydrogen ingress during service. Ni-P based amorphous coating can provide very low hydrogen diffusivity and thereby act as hydrogen barrier coating.

Though the crystalline coating of Ni is well known in the prior arts, but such is not the case with amorphous coating.

Well known Hume-Rothery rules mandate the criteria for the formation of solid solution between two or more elements. For complete solid solubility, the criteria say:
1) the atomic radii between the elements should be less than 15%,
2) the elements should have similar electronegativity and
3) similar valency.

However, size of the Ni and P atoms are 163 and 195 pm (picometer), respectively. The size difference between them is 19.6%. Ni and P occupy group VIIIB and group VA of the periodic table. Therefore, they differ greatly in electronegativity. Additionally, Ni has a valency of 2 and P has a valency of 5. Therefore, it is not possible for Ni and P to show mutual solid solubility with a stable crystal structure.

Ni - 11 wt% P display a eutectic reaction in the Ni-P equilibrium phase diagram, which shows the ability to form amorphous structure in this composition range. Therefore, Ni with 10-14 % P and 2-7% Fe do not show formation of any crystalline solid solution, as the atoms do not adhere to the Hume-Rothery criteria. Rather, they form amorphous structure during deposition on the steel substrate.

In another embodiment, the present invention discloses a system or kit for coating nickel based amorphous coating over a line pipe steel, comprising:
a bath of
Nickel Sulphate in the concentration range of 35 – 45 gms/liter as the source of Ni;
Sodium hypophosphite in the concentration range of 20-30 gm/liter as a reducing agent and source of Phosphorus,
Succinic acid in the concentration range of 45-55 gm/liter as a complexing agent,
Sodium Dodecyl-sulphate (SDS) in the concentration range of 0.1-0.5 gm/liter as surfactant; and
pH of the bath being maintained at 4.0 to 7.0.

In an embodiment, the bath temperature is 85-95 deg C.

In an embodiment, residence time for coating is 60 + 10mins for deposition of 2-10 micron thickness of coating.

In yet another embodiment, coating over the steel is done by dipping.

In still another an embodiment, the bath pH is regulated by adding aqueous caustic soda.

In still another an embodiment, the thickness of coating is 3.5 µm over the line pipe steel.

BRIEF DESCRIPTION OF THE DRAWINGS
In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, where:

FIG. 1 shows a Scanning electron microscope (SEM) image of the (a) Ni-based amorphous coating (claimed) (b) Ni-based crystalline coating (conventional) as per the experimental analysis.

FIG. 2 shows a Grazing Angle X-ray Diffraction (XRD) data of the (a) Ni-based amorphous coating (claimed) (b) Ni-based crystalline coating (conventional) as per the experimental analysis.

FIG. 3 shows Energy Dispersive X-ray Spectroscopy (EDS) spectra of the (a) Ni-based amorphous coating (claimed) (b) Ni-based crystalline coating (conventional) as per the experimental analysis.

FIG. 4 shows Transmission electron microscopy (TEM) images of the Ni-based amorphous coating: (a) Bright field image, (a) Selected area diffraction pattern (SADP) of the Ni-based amorphous coating.

FIG. 5 shows comparative Potentiodynamic polarization curves for the uncoated, Ni-electroplated and Ni-based amorphous coating as per the experimental analysis.

FIG. 6 shows electrochemical hydrogen permeation graph for the uncoated, Ni-electroplated and Ni-based amorphous coating using Devanathan-Stachursky electrochemical permeation cell.

FIG. 7 shows red rust formation in salt spray test (SST) chamber for the uncoated, Ni-electroplated and Ni-based amorphous coated samples. (a) Uncoated X70 steel - 0 hr, (b) Uncoated X70 steel – 24 hrs, (c) Ni-electroplated X70 steel – 0 hr, (d) Ni-electroplated X70 steel – 24 hrs, (e) Ni-amorphous coated X70 steel– 24 hrs, (f) Ni-amorphous coated X70 steel – 696 hrs, (g) Ni-amorphous coated X70 steel – 1416 hrs in SST chamber as per experimental analysis.

FIG. 8 shows comparative slow strain rate tensile test (SSRT) images for the uncoated, Ni-electroplated and Ni-based amorphous coated samples showing Hydrogen Embrittlement Susceptibility (HES) at 10-6 per second strain rate as per experimental analysis.

DESCRIPTION OF THE INVENTION
In accordance with an embodiment of the invention a nickel based amorphous coating for a line pipe steel grades. The coating comprises 10-14% Phosphorus (P), 2 – 7 % Iron (Fe), rest being Nickel (Ni) and unavoidable impurities (all in wt%).

Phosphorus: Presence of 10-14% P along with balance Ni ensures amorphous structure formation in the coating. The amorphous structure is required for good corrosion resistance of the coating along with hydrogen barrier properties. P differs with Ni in-terms of atomic radii (difference >16%), differs in terms of electronegativity and valency. Further, P and Ni shares group VA and VIII respectively in the periodic table. Therefore, a mutual solid solubility between P and Ni is not expected. Upon deposition P and Ni forms amorphous structure.

Iron (Fe): Fe co-deposits along with P in the present method of coating. Fe shares the same group VIII with Ni in the periodic table. Furthermore, electronegativity and valency differs between Fe and P. Therefore, presence of a small quantity of Fe with P ensures amorphous structure.

Nickel (Ni): The present coating is Ni based. Presence of Ni ensures hydrogen diffusion barrier properties. Ni along with Fe and P forms amorphous structure which is required for resistance to corrosion and hydrogen ingress during service.

In an embodiment, the line pipe steel is of API grade X-70.

The elemental composition of the line pipe steel is C-0.024, Mn-1.6, S-0.003, P-0.016, Si-0.20, Al-0.04, B-0.0001, N-0.0068, Cr-0.26, Cu-0.01, Ni-0.15, Mo-0.15, V-0.034, Nb-0.056, Ti-0.014.

To develop nickel based amorphous coating over the API grade steel a system/kit has been designed. The system or kit comprises a bath of Nickel Sulphate in the concentration range of 35 – 45 gms/liter as the source of Ni, Sodium hypophosphite in the concentration range of 20-30 gm/liter as a reducing agent and source of Phosphorus, Succinic acid in the concentration range of 45-55 gm/liter as a complexing agent, Sodium Dodecyl-sulphate (SDS) in the concentration range of 0.1-0.5 gm/liter. SDS acts as a wetting agent for the Ni and P on the steel substrate and reduces surface tension. The bath pH is maintained at 4.0 to 7.0. The bath temperature is maintained at 85-95 deg C depending upon the deposition rate.

It should be appreciated that the ingredients of the coating bath are all water soluble and can be easily prepared in any container or tank. Such a convenient process of coating can be adopted for larger components such as line pipes without the need of any special reactor such as autoclave.

The coating is applied by dipping the steel substrate. A residence time 60 ± 10 mins is sufficient for deposition of 2-10 µm of defect free compact coating.

Ni++ ions gets attached with the complexing agent and gets carried to the substrate steel surface where it is finally gets reduced and deposited. SDS is used as a surfactant in the bath solution. The bath pH is maintained and regulated using adding aqueous caustic soda.

Nickel sulphate is reduced to generate Ni++ ions in the bath which is then cathodically reduced on top of the steel substrate and gets deposited. Simultaneously, P is also reduced from Sodium Hypo-phosphite and gets deposited on top of the steel substrate.

Ni++ ions gets attached with the complexing agent and gets carried to the substrate steel surface where it is finally gets reduced and deposited.

The reaction sequence is mentioned below:

It should be appreciated that deposition of the amorphous coating is required so as to provide the resistance to atmospheric corrosion and reduce hydrogen ingress during service. The hydrogen barrier properties of Ni-P based amorphous ribbons (composition Ni81P19) have been reported to be five orders of magnitude lower than that of the steel [Ref: Y. Sakamoto, K. Takao, K. Baba, Diffusivity of hydrogen in amorphous Ni81P19 and Ni70Cr6.7Fe2.5Si8.0B12.8 alloys, Materials Science and Engineering, Vol. 97 (1988) 437-440.]. Therefore, it is expected that Ni-P based amorphous coating can provide very low hydrogen diffusivity and thereby act as hydrogen barrier coating.

Though the crystalline coating of Ni is well known in the prior arts, but such is not the case with amorphous coating.

Well known Hume-Rothery rules mandate the criteria for the formation of solid solution between two or more elements. For complete solid solubility, the criteria say:
1) the atomic radii between the elements should be less than 15%,
2) the elements should have similar electronegativity and
3) similar valency.

However, size of the Ni and P atoms are 163 and 195 pm (picometer), respectively. The size difference between them is 19.6%. Ni and P occupy group VIIIB and group VA of the periodic table. Therefore, they differ greatly in electronegativity. Additionally, Ni has a valency of 2 and P has a valency of 5. Therefore, it is not possible for Ni and P to show mutual solid solubility with a stable crystal structure.

Furthermore, Ni- 11 wt% P display a eutectic reaction in the Ni-P equilibrium phase diagram, which shows the ability to form amorphous structure in this composition range. Therefore, in the accordance with an embodiment of the invention Ni with 10-14 % P and 2-7% Fe do not show formation of any crystalline solid solution, as the atoms do not adhere to the Hume-Rothery criteria. Rather, they form amorphous structure during deposition on the steel substrate. The amorphous structure of the coating has been identified with the help of Grazing angle XRD and TEM, as shown in FIGs 2(a) and 4, respectively.

BRIEF DESCRIPTION OF THE TABLES
Table 1: Composition of the API X70 steel substrate (in wt%).

Table 2: Composition of the coating as measured in Energy Dispersive X-ray Spectroscopy (EDS) in SEM.

Table 3: Corrosion resistance evaluation using the Open circuit potential (OCP) and potentiodynamic polarization test results.

Table 4: Time required for failure in SSRT for the uncoated, Ni-electroplated and Ni-amorphous coated X70 steel.

EXAMPLES
The disclosure herein provides for examples illustrating the above described embodiments. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the disclosure/embodiments herein may be practiced and to further enable those of skill in the art to practice the invention. Accordingly, the following examples should not be construed as limiting the scope of the embodiments described herein.

Example 1: Experimental Analysis
For comparison of the properties vis-à-vis claimed coating as per disclosure with conventional coating, the following experiments were conducted.

Claimed Coating (Ni based amorphous coating) as per disclosure:
The nickel based amorphous coating is applied over API X70 line pipe grade of steels (substrate 1) as per the disclosed system/kit. The coated samples were successively evaluated for detailed characterization and for the evaluation of protection against atmospheric corrosion and hydrogen embrittlement using electrochemical potentiodynamic polarization tests, hydrogen permeation test and slow strain rate tensile tests (SSRT).

Thin coupon sample of 1.5 mm thickness with 20 x 40 mm area on both sides are prepared from the X70 plates. The sample is mechanically polished to make it free from the rust or debris on the surface. The sample was metallographically polished up to 600 grit emery paper. Then the sample was ultrasonically cleaned in acetone for 5 mins.

Table 1: Composition of the API X70 steel substrate (in wt%)
C Mn S P Si Al B N Cr Cu Ni Mo V Nb Ti
0.024 1.6 0.003 0.016 0.20 0.04 0.0001 0.0068 0.26 0.01 0.15 0.15 0.034 0.056 0.014

The concentration of Nickel Sulphate (NiSO4) is maintained at 40 gms/liter. Sodium Hypo-phosphite (Na2H2PO2) is added in the bath at a concentration of 25 gm/liter.

Succinic acid is added in the bath at a concentration of 50 gm/liter which acts as the complexing agent. SDS is used as a surfactant in the bath solution. The concentration of the SDS compound is maintained at 0.10 gm/liter concentration. The bath pH is maintained at 4.0 and is regulated using adding aqueous caustic soda. The bath temperature is maintained at 90 oC for a faster deposition.

Conventional Coating (Ni based crystalline coating):
Substrate 2 is similar to the substrate 1 with similar dimension as that for Experiment 1. Similar cleaning and processing was done as that for substrate 1 before applying electroplating. Below is the conventional coating method:

The bath consists of 300 gms/liter Nickel Sulphate with 60 gms/liter Nickel Chloride and Boric Acid at a concentration of 40 gms/liter. The Ni-electroplating bath was maintained at pH of 4.0 at a temperature of 50 oC. The electroplating was carried out using 99.99% pure Ni-anode at a current density of 430 A/m2 for 7 minutes.

The detailed characterization of the coating is done using Scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Transmission Electron Microscopy (TEM), Grazing angle X-ray diffraction (XRD).

A comparative hydrogen permeation test is done between the Ni-electroplated and Ni-amorphous coated steels using Devanathan-Stachursky electrochemical permeation cell following ASTM G148-97 standard.

The resistance to atmospheric corrosion of the samples are evaluated using potentiodyanmic polarization test and Tafel’s extrapolation according to the ASTM G59-97 (2014) standard.

Further red rust formation has been monitored in the SST chamber in 5% NaCl solution following ASTM D117 standard.

The resistance to Hydrogen embrittlement has been compared using SSRT following ASTM G129 – 00(2013) standards at a slow strain rate of 10-6 mm per second. The sample was evaluated both before and after the electrochemical hydrogen charging. Hydrogen charging was done before SSRT in a solution of 0.5M/ liter H2SO4 with a few drops of 100mg/l of NaAsO2 , introduced during charging. A constant hydrogen charging current of -0.1 A/cm2 is maintained for 12 hrs.

The SEM micrograph of the electroless Ni-coated sample (claimed) and Ni-electroplated sample (conventional) are shown in the FIG. 1 (a) and (b), respectively.

A thin, uniform and compact coating can be observed in the FIG. 1a. Such thin, uniform and compact cannot be witnessed in coating of FIG. 1b.

The structure of both the claimed and conventional examples are investigated using grazing angle XRD as can be seen from the FIG. 2a & 2b. The XRD image of the claimed sample in FIG 2(a) is showing diffused peak of the amorphous coating achieved in the Ni-electroless coating. Whereas, the XRD image of the conventional sample in FIG. 2(b) is showing clear peaks of crystalline Ni-coating. The FCC (face centered cubic) reflections of the crystalline peaks were identified with peak position (angle 2?) and marked in the figure.

The chemical composition of the coatings is estimated from EDS spectra. The EDS spectra, both for the claimed sample and the conventional sample, are shown in the FIG. 3(a) and (b) respectively. The corresponding composition (from five readings) are tabulated in the Table 2.

Table 2: Composition of the coating as measured by using Energy Dispersive X-ray Spectroscopy (EDS) in SEM
Element Ni-amorphous coating (claimed sample, in wt%) Ni-electroplating (conventional sample, in wt%)
Ni 84.408 ± 1.07 94.72 ± 1.09
Fe 2.586 ± 0.17 5.28 ± 1.08
P 11.288 ± 0.275 Nil

The Transmission Electron Microscopy (TEM) of the coating confirms the present of amorphous structure in the coating. The bright field image of the coating along with SAD pattern of the inventive example are shown in the FIGs 4(a) and (b), respectively. The respective coating area is marked in the FIG. 4(a) along with the spot from which the SAD pattern is taken. It is evident that the SAD pattern is completely a diffused ring type pattern proving that the coating is amorphous.

Shown in FIG. 5 is an estimate of the corrosion resistance of the NiP-amorphous coating in comparison to the Ni-electroless coating and uncoated X70 steel, potentiodynamic polarization tests are done in 3.5% (w/v) NaCl solution following ASTM G59-97(2014) standard. Saturated calomel electrode (SCE) was used as the reference. The potential scan was varied from -1.5 V to +1.5 V (w.r.t SCE) from the open circuit potential (OCP) at a scan rate of 0.166 mV/s. The corresponding corrosion potential (Ecorr) and current (icorr) was determined from the polarization graph and the corrosion rate was calculated following the standard. The comparative vales are mentioned in the Table 3.

It is evident from the Table 3 that the corrosion rate of the Ni-amorphous coating is much less that of the Ni-electroplated component and uncoated bare steel. Additionally, the pitting potential of the Ni-amorphous coating is much higher than that of the Ni-electroplating. This indicates that a much higher resistance to pitting corrosion is offered by the Ni-based amorphous coating than that offered by the Ni-electroplated coating. Furthermore, the passivation potential range is higher for Ni-amorphous coating indicating a good resistance to atmospheric corrosion.

Table 3: Open circuit potential (OCP) and potentiodynamic polarization test results. Ecorr: Corrosion potential; icorr: Corrosion current density; Epit: Pitting potential; ipass: Average passive current density
Comparison results
Sample Uncoated Ni electroplating (conventional sample) Electroless Ni-P
Coating (claimed sample)
Ecorr
(mV vs. SCE) -640 -365 -388
icorr
(µA/cm2) 28.71 2.24 0.59
Epit
(mV vs. SCE) - -30.81 232
Passivation range
(mV) - 334.19 620
Corrosion rate
(mm/year) 0.333248 0.048336 0.012769

The comparative hydrogen permeability tests were done with the uncoated, Ni-electroplated (conventional sample), and the Ni-amorphous coated (claimed sample) X70 steel following the ASTM G148-97 (2014) standard in a Devanathan-Stachursky electrochemical hydrogen permeation cell. The conventional hydrogen permeabilities between the samples are plotted in FIG. 6. The permeation tests were conducted following galvanostatic-potentiostatic condition, in which a constant cathodic current density of -1mA/cm2 was maintained in the hydrogen generation half-cell and the permeating hydrogen was oxidized in a potentiostatic condition at +0.150 mV (with respect to Hg/HgO reference electrode). It can be followed from the permeation test that a huge quantity of hydrogen can permeate through the uncoated X70 steel.

However, the hydrogen permeation reduces when the steel is electroplated with Ni (conventional example). The hydrogen permeation further reduces when the steel is coated with the Ni-amorphous coating (inventive example). This results clearly suggests that Ni-amorphous coating can reduce the hydrogen ingress in the substrate and thus reduces the risk of hydrogen related failures. The Ni-amorphous coating can very well serve better as a hydrogen-barrier coating than the conventional Ni-electroplating.

The resistance to the atmospheric corrosion of the Ni-amorphous coating (claimed), Ni-electroplating (conventional) and uncoated X70 steel was compared using salt-spray test (SST) in 5% Sodium Chloride solution according to the ASTM B117 standard. The resultant images of the samples after the SST chamber is shown in the FIG. 7. The uncoated steel showed formation severe red rust just after 24 hrs in SST chamber, as can be seen in FIG. 7(a) and (b). Whereas, the Ni-electroplated X70 steel showed formation mild red rusts after 24 hrs in SST chamber, FIG. 7(c) and (d). However, Ni-amorphous coated sample have not shown any red rust even after 1416 hrs of SST chamber, as can be seen in FIG. 7(e), (f) and (g). In contrast, both the uncoated and Ni-electroplated sample developed completely red rust after few days. The images of the Ni-amorphous coated samples indicate a much better resistance to atmospheric corrosion than the Ni-electroplated sample.

The resistance to hydrogen embrittlement was evaluated by the slow strain rate tensile test (SSRT) at the strain rate of 10-6 per second following ASTM G129 – 00(2013) standards as shown in FIG. 8. To evaluate the effect of hydrogen in the coated and uncoated steels, samples were tested before and after the hydrogen charging. The hydrogen charging was done electrochemically in a solution 0.5M / liter H2SO4 with a few drops of 100mg/l of NaAsO2. The cathodic current density was maintained constant at -0.1 A/cm2 for steady hydrogen flux onto the sample. The charging was done for 12 hrs. The uncoated X70 steel shows a lower elongation and higher strength when charged with H, as expected, due to hydrogen embrittlement. However, when the steel was coated with 3.5 µm Ni-amorphous coating (as shown in FIG 1(a)) as per the present disclosure, the elongation improved after the hydrogen charging than that of the uncoated sample. Also, the strength did not rise in the hydrogen charged coated sample indicating less hydrogen embrittlement. In addition, the time-to-failure in SSRT for the uncoated, Ni-electroplated and Ni-amorphous coated steel is tabulated Table 4. It can be clearly seen that the time required for failure after the electrochemical hydrogen charging is the highest for the Ni-amorphous coated steel. This indicates a less hydrogen ingress during the hydrogen charging (which was same for the other two types of samples) in Ni-amorphous coated sample, resulting in less hydrogen embrittlement and more time-to-failure in SSRT. This proves the effectiveness of the Ni-amorphous coating in preventing the hydrogen embrittlement.

Table 4 shown below depicts time required for failure in SSRT for the uncoated, Ni-electroplated and Ni-amorphous coated X70 steel.

Table 4
Uncoated X-70 Ni (crystalline) Electroplated Ni Amorphous
Without H charging 79 Hrs 79 Hrs 91 Hrs
After H charging 66 Hrs 68 Hrs 70 Hrs

ADVANTAGES
In the present disclosure, a thin Ni-based amorphous coating is disclosed which can be synthesized easily without the need of a specialized reactor. The amorphous nature of the coating reduces the hydrogen migration greatly than that would be possible through a crystalline Ni-based coating (Ni-electroplating). Additionally, the coating provides an excellent resistance to atmospheric corrosion.

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.

The foregoing description of the specific embodiments fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this disclosure have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

All references, patents, articles, publications, general disclosures etc. cited herein are incorporated by reference in their entireties for all purposes. However, mention of any reference, patent, article, publication etc. cited herein is not, and should not be taken as, an acknowledgment or any form of suggestion that they constitute valid prior art or form part of the common general knowledge in any country in the world.