FUNCTIONALLY GRADED ALLOY COATING AND METHOD FOR PREPARATION

Abstract

A method, coated substrate, and coating wherein a metallic substrate is subject to an electrolytic deposition process including an electrolyte with an iron group element and a refractory group element. One or more electrolytic deposition waveform parameters are varied to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at an interface between the coating and the metallic substrate to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts. The coating may be diffusion bonded to the substrate.

Description

FIELD OF THE INVENTION

This subject invention relates to a coating, a coating deposition method, and post heat treatment methods for corrosion resilience in molten salt cooling systems in Molten Salt Reactors (MSRs), solar concentrator salt cooled cooling loops, and the like.

BACKGROUND OF THE INVENTION

The United States and other countries are developing clean, affordable energy sources to meet energy needs and to reduce greenhouse (GHG) emissions. Nuclear power is one of these clean sources that currently account for ˜70% of the low GHG emitting domestic electricity production. The use of molten fluoride salts to cool Molten Salt Reactors (MSR) for nuclear power generation or solar concentrators enables more economic operation due to higher temperatures of operation (>600° C.), thermal energy storage, and the ability to dissolve fuel in the coolant medium. In addition, MSR technology is safer and generates less nuclear waste compared to traditional nuclear reactors.

Implementation of these next generation systems requires an improvement in system sustainability by developing scalable processes to create new materials with corrosion resilience or to produce new overlay coatings that can be applied to the American Society of Mechanical Engineers (ASME) certified boiler and pressure vessels (made with, for example, 316H stainless steel). These new materials are required to improve the corrosion resistance of components within molten salt liquid-fuel and liquid cooled reactors/systems such as the conduits carrying the coolants through the reactor.

Researchers at the U.S. Department of Energy Oak Ridge National Laboratory (ORNL) have invented and demonstrated the potential of HASTELLOY® N (71 wt % Ni/16 wt % Mo) (See U.S. Pat. No. 9,540,714 B2 which is incorporated herein by this reference) as a Co free Ni alloy for enhanced corrosion resistance during molten fluoride salt operation.

However, a manufacturing base for this material has yet to be created and consequently limits the availability of components and shapes available in the open market, greatly increasing the cost of such materials. Additionally, these alloys have yet to be certified for use in boiler or pressure vessels and require significant resources to achieve implementation.

Published U.S. Patent Application No. 20150322588A1, Patent Application WO2009039282A1, and U.S. Pat. No. 8,361,288 B2 are incorporated herein by this reference.

BRIEF SUMMARY OF THE INVENTION

Aside from the preferred embodiments disclosed herein, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details set forth in the following description or illustrated in the drawings. Moreover, the claims are not to be read restrictively unless there is clear and convincing evidence manifesting in certain exclusion, restriction, or disclaimer.

Considering the cost of manufacturing the HASTELLOY® N product, it is desirable to create an overlay coating which can be applied to the surface of an already certified component and achieve an enhanced lifetime. The application of overlay coatings onto existing ASME certified materials that increase corrosion resistance and lifetime is desirable. The challenges associated with the application of any coating to a dissimilar material include: 1) the deposited coating and the substrate need to have similar coefficients of thermal expansion (CTE) to prevent delamination at varying temperatures; 2) the coating and the substrate must be well adherent and able to withstand typical operation environments; and 3) the coating surface must have an adequate material composition to provide the require corrosion and radiation resilience.

Featured is a functionally graded coating systems that enables a CTE match to a given substrate and a surface property to enhance the environmental resilience. The adhesion of electrodeposited coatings to substrate surfaces is improved to inhibit spalling, peeling or flaking. Also featured is the application of a functionally graded overlay coating that minimizes the CTE mismatch while allowing for the coatings surface to have the targeted corrosion resistance. The use of hot isostatic pressing (HIP) after application of the overlay coating can be utilized to diffusion bond the coating to the substrate and minimize porosity and thereby enhance the coating adherence and performance.

In one embodiment, a functionally graded coating system that has been diffusion bonded to the substrate (e.g., the interior and/or exterior of a conduit) to enhance adhesion and has an interfacial (substrate/coating) composition that has been tailored to improve the CTE match and a coatings surface tailored to enhance corrosion resistance when exposed to molten salt electrolytes (FLiNaK, FLiBe, MgCl—NaCl, NaCl—KCl, etc. . . . ) or other similarly corrosive environments is disclosed.

The substrate may be an ASME certified material for high temperature boiler and pressure vessel operation.

In one example, the coating material that enables CTE compatibility with the ASME certified material is an alloy of an iron group metal (Fe, Ni, Co) and a refractory metal (Mo, W, Re) in which the concentration of the refractory metal is less than 10 w/w %.

In one embodiment, the surface composition of the alloy coating which includes an iron group metal and a refractory metal is one in which the concentration of the refractory metal is greater than 25 w/w %.

Featured is an improved functionally-graded coating for corrosion resistance comprising a high molybdenum content corrosion resistant surface for molten fluoride, a coefficient of thermal expansion (CTE) close match between the ASTM certified material 316H Stainless Steel (SS) substrate for Molten Salt Reactors (MSRs) or solar concentrator cooling loops, and a well-adherent diffusion bonded coating having a composition adjacent said substrate surface consisting of high Ni or Co or Fe and a low composition of Mo, W, or Re that closely matches the CTE of the 316H SS while also providing protection of the 316H substrate from the molten fluoride cooling salt by having a surface composition of High Mo, W, or Re and a low composition of Ni or Co or Fe then hot isostatically pressing the overlay into the 316H SS substrate forming a diffusion bond between the two materials.

Also featured is a method of coating a metallic substrate. The metallic substrate is subject to an electrolytic deposition process including an electrolyte with an iron group element and a refractory group element. One or more electrolytic deposition waveform parameters are varied to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at an interface between the coating and the metallic substrate to better match the coefficient of thermal expansion of the coating and the metallic substrate. As the thickness of the coating increases, there is more of the refractory group element and less of the iron group element by varying the deposition waveform parameters for improving corrosion resistance to salts. The coating is then diffusion bonded (e.g., by hot isostatic pressing) to the substrate.

In one example, varying the electrolytic waveform parameters includes varying the current density of the waveform, the length of time the waveform is applied, or the time between successive waveforms and may further include switching between cathodic and anodic waveforms.

The method may further include increasing the roughness of the substrate before deposition and/or activating the substrate, for example, by nickel strike, acid etch, or plasma treatment.

The substrate can be a stainless steel material or tungsten in some examples. Typically, CTE of the functionally graded coating is lower than that of the substrate. The iron group can include Fe, Co and/or Ni and the refractory group can include Mo, W, and/or Re. In one example, the functionally graded coating adjacent the substrate interface comprises 90+% of the iron group and the functionally graded coating at its surface comprises between 25 and 60% of the refractory group.

The hot isostatic pressing step can be conducted for about 3½ hours at about 1250 C and at about 22,000 psi pressure.

Also featured is a metallic substrate coated with a functionally graded diffusion bonded coating with more of an iron group element and less of a refractory group element at an interface between the metallic substrate and the coating to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts.

Also featured is a coating for a metallic substrate comprising more of an iron group element and less of a refractory group element at a coating interface to better match the coefficient of thermal expansion of the coating and a metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts.

In one example, the functionally graded coating is diffusion bonded by hot isostatic pressing at a temperature, pressure, and time adequate to allow some of the coating constituents to diffuse into the substrate and some of the substrate constituents to diffuse into the coating.

Also featured is a method of coating a metallic substrate, the method comprising: subjecting the metallic substrate to an electrolytic deposition process including an electrolyte solution with an iron group element and a refractory group element; and instead of changing the electrolyte solution, varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at a first portion of the coating more of the refractory group element and less of the iron group element at a second portion of the coating.

In one example, the first portion of the coating is at an interface between the coating and the substrate and the second portion of the coating is at the surface of the coating. The waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate.

Also featured is a method of coating a metallic substrate, the method comprising: subjecting the metallic substrate to an electrolytic deposition process including an electrolyte solution with an iron group element and a refractory group element; and instead of changing the electrolyte solution, varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at a first portion of the coating more of the refractory group element and less of the iron group element at a second portion of the coating in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate.

The functionally graded coating can be applied to the external or internal surfaces of components prior to diffusion bonding.

In some examples, the coating deposition method includes a pulse current or pulse reverse current electrolytic deposition process using an electrolyte bath containing a specified concentration of iron group and refractory metal. The electrolyte may also contain a chelating agent to enable the alloy deposition of metals with a dissimilar reduction potential. The pulse current or pulse reverse current coating deposition method is tuned to control the composition and microstructure of the deposited material of the coating. The pulse current or pulse reverse current coating deposition method can be tuned to control the composition and microstructure of the deposited material based in part on the surface roughness or texture. The pulse current or pulse reverse current coating deposition method can be changed during the deposition process to locally control the composition of the coating throughout its thickness. The coating deposition process can be continued until a coating layer of at least 250 μm is achieved.

In one example, the problem of producing a high molybdenum content corrosion resistant surface for conduits carrying molten fluoride (i.e. fluorine-sodium-lithium-potassium (FLiNaK) or fluorine-lithium-beryllium (FLiBe)) cooling salts with a coefficient of thermal expansion (CTE) match between the ASTM certified material 316H Stainless Steel (SS) for Molten Salt Reactors (MSRs) or solar concentrator cooling loops and a well adherent diffusion bonded coating is solved by electrochemically depositing a compositionally graded layer of nickel-molybdenum (NiMo), or iron-molybdenum (FeMo), or cobalt-molybdenum (CoMo) overlay onto the surface of the ASTM certified 316H SS having a composition adjacent said substrate surface consisting of high Ni or Co or Fe and a low composition of Mo that closely matches the CTE of the 316H SS (15.9×10⁻⁶ K⁻¹) while also providing protection of the 316H substrate from the molten fluoride cooling salt by having a surface composition of High Mo and a low composition of Ni or Co or Fe and then hot isostatically pressing the overlay into the 316H SS substrate forming a diffusion bond between the overlay and substrate.

Metallic (e.g., 316 H Stainless Steel) substrates in the form of flat coupons and pipe segments have been coated with a functionally graded overlay coating of NiMo. Flat coupons and pipe segments were coated with a functionally graded NiMo overlay coating and hot isostatically pressed to form a diffusion bond between the substrate and the overlay. These were then tested in a relevant static fluorine-sodium-lithium-potassium (FLiNaK) molten salt environment at 700° C. These materials demonstrated an improved corrosion resistance over a conventional uncoated 316H SS substrate.

Depositing the coating can be accomplished using a single electrolytic bath rather than changing baths. Instead, the deposition waveform changes during deposition.

The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. For example, in some examples the bath is changed during the coating process.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 depicts an overlay coating with a functionally graded alloy illustrating metal group compositions suitable for use in the instant disclosure;

FIG. 2 is a schematic view of an example of a functionally graded coating system with a compositional change from iron-rich group metals near the substrate interface and refractory-rich group metals at the top surface of the coating;

FIG. 3 is a schematic view of a functionally graded coating system with a compositional change from iron-rich group near the interface and refractory-rich group at the surface after diffusion bonding between the coating and substrate is performed;

FIG. 4 is a schematic view of an electrolytic deposition apparatus;

FIGS. 5A-5D are graphs showing how alloys can be electrolytically deposited with different compositions;

FIG. 6 is a representation of a generalized pulse/pulse reverse waveform;

FIG. 7 is a representation of a duplex pulsating boundary layer;

FIGS. 8A-8B are representations of a macroprofile and a microprofile boundary layer under direct current and pulse current conditions;

FIG. 9 is a summary of guiding principles for the impact of pulse parameters on deposit distribution;

FIGS. 10A-10B depict exemplary process flow charts for the application of a functionally graded diffusion bonded coating;

FIG. 11 is a SEM image and EDX line scan of a NiMo coating with a single composition throughout its thickness;

FIG. 12 is a SEM image and EDX line scan of a functionally graded overlay coating prepared according to one embodiment of the instant disclosure;

FIG. 13 is a SEM image and EDX line scan of a functionally graded overlay coating prepared according to one embodiment of the instant disclosure;

FIG. 14 is a SEM and EDX of line scan of Ni concentration in the as-deposited overlay coating prepared according to one embodiment of the instant invention. It also shows a line scan of Ni concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer of the instant disclosure;

FIG. 15 is a SEM and EDX of line scan of Mo concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure also showing a line scan of Mo concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 16 is a SEM and EDX of line scan of Cr concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure also showing a line scan of Cr concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 17 is a SEM and EDX of line scan of Fe concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure also showing a line scan of Fe concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 18 is an optical image of the corrosive attack of a substrate without an overlay coating after exposure to a molten salt;

FIG. 19 is an optical image of the corrosive attack of a substrate with a single metal coating after exposure to a molten salt;

FIG. 20 is an image of the high temperature reactor vessel fused in the molten salt corrosion tests;

FIG. 21 is an SEM image of the corrosive attack that occurs on a substrate with a single metal coating that has been diffusion bonded after exposure to a molten salt;

FIG. 22 is a SEM and EDX of line scan of Cr concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 150 micron overlay also showing a line scan of Cr concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 23 is a SEM and EDX of line scan of Fe concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 150 micron overlay also showing a line scan of Fe concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 24 is a SEM and EDX of line scan of Ni concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 150 micron overlay also showing a line scan of Ni concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 25 is a SEM and EDX of line scan of Mo concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 150 micron overlay also showing a line scan of Mo concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 26 is a SEM and EDX of line scan of Cr concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 272 micron overlay also showing a line scan of Cr concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 27 is a SEM and EDX of line scan of Fe concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 272 micron overlay also showing a line scan of Fe concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 28 is a SEM and EDX of line scan of Ni concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 272 micron overlay also showing a line scan of Ni concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 29 is a SEM and EDX of line scan of Mo concentration in the as-deposited overlay coating prepared according to one embodiment of the instant disclosure for a 272 micron overlay also showing a line scan of Mo concentration after hot isostatic pressing (HIP) diffusion bonding form the diffusion bond layer;

FIG. 30 is a SEM image of a functionally graded sample generated using the three-step pulse waveform sequence; and

FIG. 31 is a SEM image of a functionally graded Fe/W alloy generated via a two-electrolyte manufacturing process.

DETAILED DESCRIPTION OF THE INVENTION

Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.

Provided is a functionally graded diffusion bonded coating deposition and diffusion bonding method for enhanced molten fluoride salt reactor corrosion for cooling systems in Molten Salt Nuclear Reactors, solar concentrator, and other systems. The preferred functionally graded diffusion bonded coating comprises an alloy combination including an iron group metal (e.g., Ni, Co, or Fe) and a refractory metal (e.g., Mo, W, or Re) to a provide coefficient of thermal expansion match between a ASME certified substrate such as 316H stainless steel and a coating surface that is corrosion resistant in molten fluoride salt environment.

The composition of the alloy is graded to achieve each objective where the coating/substrate interface must have a CTE approaching SS while the surface of the coating exposure to the molten fluoride salt should have a high composition of the refractory. The rate of grading from the interface to the surface controls the local CTE improving the interfacial adhesion of the coating and enabling corrosion resistance at the refractory rich surface. The addition of a hot isostatic treatment allows for constituents in the alloy coating to diffuse into the substrate and constituents in the substrate to diffuse into the substrate creating a bond layer.

FIG. 1 shows a range of functionally graded coatings suitable for use with the instant invention. More specifically, the iron group metal may vary from approximately 90 to 100% at the substrate to 40 to 75% at the surface for a 150 μm overlay coating. The refractory group metal may vary from approximately 0 to 10% at the substrate to 25 to 60% at the surface for a 150 μm overlay coating.

FIG. 2 is an illustration of a functionally graded overlay coating (200) of the instant invention deposited on substrate (100). The substrate (100) may be any ASME certified boiler or pressure vessel material designated for MSR or solar concentrator cooling systems, such as stainless-steel alloy S31609 (Unified Numbering System) or tungsten. The functionally graded coating (200) comprises an alloy with a high iron group metal composition and low refractory metal composition near the substrate and high refractory group metal composition and low iron group metal composition near the surface. The iron group metal material is one or more materials selected from the group consisting of iron, nickel, or cobalt. The refractory metal material is one or more materials selected from the group consisting of molybdenum, rhenium, or tungsten. By adjusting the coating deposition method as described herein, the alloy composition applied can be controlled throughout the thickness of the functionally graded overlay coating (200). As shown in Table I, the CTE profile is graded by controlling the coating composition and influenced through the choice of materials, in this case illustrated for Ni and Mo. The CTE of the functionally graded coating is lower than the substrate material, resulting in compressive thermally induced stress in the coating under high temperature conditions. One skilled in the art understands that acceptable CTE value is dependent on the materials selected to keep stress limits tolerable. Provided that the material stress limits are not exceeded by the delta in thermal expansion, a lower CTE coating is desirable as compressive stress has a beneficial effect on the fatigue life, crack propagation, coating adhesion and on the durability of the top coat during service. Furthermore, the targeted presence of a refractory rich surface enabled the desired corrosion resistance needed for use in high temperature molten salt systems.

TABLE I
The coefficient of thermal expansion for the base materials
and as a function of the graded coating.
                         Coefficient of Thermal Expansion
Composition              [m/mK]
Stainless 316H Substrate 15.9 × 10−6
100% Ni                  13.0 × 10−6
90% Ni/10% Mo            ~12.5 × 10−6
80% Ni/20% Mo            ~11.7 × 10−6
70% Ni/30% Mo            ~10.9 × 10−6
60% Ni/40% Mo            ~10.1 × 10−6

FIG. 3 shows an example functionally graded overlay coating (200) deposited on substrate (100) after hot isostatic pressure (HIP) diffusion bonding according to one embodiment of the instant invention. The substrate (100) may be any ASME certified boiler or pressure vessel material for MSR or solar concentrator cooling systems, such as stainless-steel alloy S31609 (Unified Numbering System) or tungsten. The functionally graded coating (200) comprises an alloy with a high iron group metal composition and low refractory metal composition near the substrate and high refractory group metal composition and low iron group metal composition near the surface. The iron group metal material is one or more materials selected from the group consisting of iron, nickel, or cobalt. The refractory metal material is one or more materials selected from the group consisting of molybdenum, rhenium, or tungsten. By performing a hot isostatic press operation according to the instant invention constituents from functionally graded coating system (200) can diffuse into the substrate (100) creating a diffusion bonded region (220) between the substrate (100) and the functionally graded overlay coating (200). Although the diffusion bonded region (220) is illustrated as a distinct layer in FIG. 3, one of ordinary skill in the art understands that the composition of constituents varies across the region and into the substrate (100) and overlay coating (200).

FIG. 4 is a schematic representation of an electrolytic deposition apparatus (300). Included is a power supply (320) with an anode lead (340) and a cathode lead (360) capable of delivering a direct current (DC), pulse current (PC) or pulse reverse current (PRC) to an electrolytic cell (400). The electrolytic cell includes a cell container (410) with an anode (420), a cathode (440) (e.g., the workpiece), and an electrolyte solution (430) with one or more metal elements dissolved in the electrolyte solution (430). Anode 420 may be titanium with a metal oxide coating. Power supply can be configured to electronically deliver the waveforms described herein.

Electrolytic deposition may be practiced using direct current, pulse current or pulse reverse current. In direct current deposition processes, the current is applied to the electrolytic cell and generally held constant for a period of time, after which the deposit is formed on the oppositely charged electrode substrate. In pulse current/pulse reverse current electrolytic deposition, the current is interrupted and or reversed in predetermined ways. By properly selecting the pulse current/pulse reverse current waveform parameters, the deposit thickness, uniformity of deposition, localization of deposition, and properties are tuned for the specific application. Numerous embodiments of pulse current/pulse reverse current deposition are described by the common assignee of the instant invention in U.S. Pat. Nos. 6,080,504; 6,203,684; 6,210,555; 6,303,014; 6,309,528; 6,319,384; 6,524,461; 6,551,484; 6,652,727; 6,750,144; 6,827,833; 6,863,793; 6,878,259; 8,603,315; 10,100,423; 10,684,522; and 11,411,258 which are incorporated herein by reference.

Electrolytic alloy deposit composition is determined by numerous factors including the concentration of the alloying components in the electrolyte, the local current density, and the deposition overpotential for each given species in the electrolyte. FIG. 5 shows general considerations of alloy deposition for two metals, M₁ and M₂, with equilibrium potentials V_0,1 and V_0,2, respectively, where V_0,1 is more positive than V_0,2. At the deposition potential, V_dep, the alloy composition is determined by the ratio of the currents for the two metals, provided V_dep is negative of V_0,1 and V_0,2. The Tafel line is the trace of the voltage versus logarithmic current and the relationship of the Tafel lines for each alloying component determine the alloy composition of the deposit.

In case (a), the Tafel lines are parallel and the alloy deposit is always rich in M₁ (FIG. 5A). In case (b), the Tafel lines diverge, so the alloy is always rich in M₁ (FIG. 5B). In case (c), the Tafel lines converge, and the alloy may be rich in M₁ or M₂, depending on V_dep (FIG. 5C). In Region (I) (above the intersection), the alloy will be rich in M₁; in Region (II) (below the intersection), the alloy will be rich in M₂. In case (d), the deposition of M₁ becomes mass transport limited (FIG. 5D). This case is similar to the converging Tafel line case and the alloy may be rich in either M₁ or M₂, depending on V_dep.

The selection of electrolytic deposition parameters affects the mass transport and kinetics of the individual electrodeposition processes for each element in the electrolyte. By properly adjusting the electrolyte and electrolytic deposition parameters in conjunction with the concentration of the alloying components in the electrolyte, the mass transfer and kinetics for M₁ and M₂ can be modified to obtain the “Converging Tafel Lines” in Case (c) or “Diffusion Control for M₁” in Case (d). In these cases, the ratio of M₁ to M₂ in the overlay can be modified by controlling deposition potential, V_dep.

FIG. 6 represents a generalized pulse current/pulse reverse current waveform. The generalized waveform parameters are characterized by a cathodic pulse followed by an off-time and followed by an anodic pulse and followed by an off-time. One or both off-times may be eliminated and either the cathodic pulse or the anodic pulse may be eliminated. The waveform parameters are: 1) anodic pulse current density, i_anodic, 2) anodic on-time, ton, anodic, 3) cathodic pulse current density, i_cathodic, 4) cathodic on-time, ton, cathodic, 5) cathodic off-time, toff, cathodic and 6) anodic off-time, toff, anodic. The sum of the anodic and cathodic on-times and the off-time is the pulse period, T. The inverse of the pulse period is the frequency, f, of the pulse. The anodic, γ_a, and cathodic, γ_c, duty cycles are the ratios of the respective on-times to the pulse period. The average current density (i_aver) or net deposition rate is given by:

$\begin{array}{cc}\mathrm{iaver}=ic\gamma c-ia\gamma a& \left(1\right)\end{array}$

Just as there are infinite combinations of height, width, and length to obtain a given volume, in pulse processing there are unlimited combinations of peak voltages/current densities, duty cycles, and frequencies to obtain a given deposition rate in electrolytic deposition processes. These parameters provide the potential for much greater process/product control compared to conventional DC deposition processes.

Mass transport in pulse current/pulse reverse current electrolytic and electrophoretic deposition processes is a combination of steady state and non-steady state diffusion processes. The mass transfer limited current density (i_l) is related to the reactant concentration gradient (C_b-C_s) and to the diffusion layer thickness (δ) by the following equation:

$\begin{array}{cc}{i}_l=-nF{D\left(\delta C/\delta x\right)}_{x=0}=-nFD\left[\left(C_b-C_s\right)/\delta \right]& \left(2\right)\end{array}$

where n, F, and D are the number of equivalents, Faraday's constant, and diffusivity of the reacting species, respectively. In DC electrolysis, δ is a time-invariant quantity for a given electrode geometry and hydrodynamic condition. In pulse/pulse reverse electrolysis, however, δ varies from zero at the beginning of the pulse to its steady state value when the Nernst diffusion layer is fully established. The corresponding mass transport limiting current density would then be equal to an infinite value at t=0 and decreases to a steady state value of the DC limiting current density. The advantage of pulse/pulse reverse electrolysis is that the current can be interrupted before δ has a chance to reach steady state. This allows the reacting ions to diffuse back to the electrode surface and replenish the surface concentration to its original value before the next current interruption. Therefore, the concentration of reacting species in the vicinity of the electrode pulsates with the frequency of the modulation.

FIG. 7 illustrates a “duplex diffusion layer” including a pulsating layer, δ_p, and a stationary layer, δ_s during pulse electrolysis. Since the thickness of the pulsating diffusion layer is determined by the waveform parameters, this layer may be thought of as an “electrodynamic diffusion layer”. By assuming a linear concentration gradient across the pulsating diffusion layer and conducting a mass balance, the pulsating diffusion layer thickness (δ_p) as:

$\begin{array}{cc}{\delta }_p={\left(2D{t}_{on}\right)}^{1/2}& \left(3\right)\end{array}$

where t_on is the pulse on time. When the pulse on time is equal to the transition time, the concentration of reacting species at the interface drops to zero at the end of the pulse. An expression for the transition time, τ, is:

$\begin{array}{cc}\tau =\left({\left(nF\right)}^2{C}_b^{2}D\right)/2i_c^2& \left(4\right)\end{array}$

More exact solutions are given by integrating Fick's diffusion equation:

$\begin{array}{cc}{\delta }_p=2{\left(\left(D{t}_{on}\right)/\pi \right)}^{1/2}& \left(5\right)\end{array}$ $\begin{array}{cc}\tau =\pi \left({\left(nF\right)}^2{C}_b^{2}D\right)/4i_c^2& \left(6\right)\end{array}$

The same equation for the pulsating diffusion layer is also relevant to pulse-reverse deposition. The key points in the development of pulse current/pulse reverse current deposition processes are: (1) the electrodynamic diffusion layer thickness is proportional to the pulse on time and (2) transition time is inversely proportional to the current.

In electrolytic deposition processes, deposit distribution is determined by the current distribution. The current distribution is controlled by primary (geometrical), secondary (kinetic) or tertiary (mass transport) effects. The addition of secondary or tertiary effects tends to make the current distribution more uniform, as compared to primary effects alone. If the applied waveform is designed such that the pulse on-time is much longer than the transition time, the tertiary current distribution will play an important role in the deposition. With the addition of tertiary control, the concept of macro- and micro-profiles influence the current distribution. FIG. 8A illustrates a macroprofile wherein the roughness of the surface is large compared with the thickness of the diffusion layer, and the diffusion layer tends to follow the surface contour. Under mass transport or diffusion control, a macroprofile results in a uniform current distribution and tends to follow the surface contour producing a conformal deposit during deposition. FIG. 8B illustrates a microprofile wherein the roughness of the surface is small compared with the thickness of the diffusion layer. Under mass transport control, a microprofile results in a non-uniform current distribution and a non-conformal deposition of material, beneficial for creating a more uniform coating surface devoid of pores. By applying the appropriate waveform, one can effectively focus or defocus the current distribution to create non-uniform or uniform deposition respectively. The effect of surface roughness on the distribution of metal deposited on the surface and the corresponding use of modulated fields, i.e., pulse reverse current, to produce a uniform filling of recesses in the surface is discussed in the patents incorporated by reference.

FIG. 9 summarizes four pulse current waveform types, independent of cathodic pulse or anodic pulse, to influence the current distribution and hence the deposition distribution in an electrolytic deposition process. Additionally, the inventors have observed that proper selection of the pulse current/pulse reverse current parameters the Tafel line for alloying components under direct current deposition that would be diverging are converted to converging Tafel lines as illustrated in FIG. 5 Case (c) and Case (d).

In one embodiment of the instant invention, a direct current is employed to deposit a functionally graded coating system. In another embodiment of the instant invention, a pulse reverse current is employed to deposit a functionally graded coating system. In another embodiment of the instant invention, a functionally graded coating system is uniformly deposited across a surface. In another embodiment of the instant invention, a functionally graded coating system is locally deposited across a surface.

FIG. 10 illustrates the process steps in a functionally graded alloy coating electrolytic deposition method. FIG. 10A shows the general process while FIG. 10B lists all the steps utilized in preparation of the working examples provided. The process can include a substrate pre-treatment step 501 to clean/degrease the surface of the substrate freeing it from any oils or particulates, this is commonly done with a solvent like acetone or isopropanol. After a water rinse (520) the surface oxide is generally removed and the surface roughened via a physical, plasma, chemical etching, roughening, or blasting step (502), in this case we utilized either a grit blast technique (for external surfaces) or a hone technique (for internal surfaces). Roughening of the surface is known in the art to improve coating adhesion, which is desirable, but can also affect final coating surface conditions in a negative manner within a corrosive environment, where a smoother finish devoid of pores, etc. is desirable. After an optional water rinse the surface is ready to be activated which one skilled in the art recognizes that an electrolytic Ni strike (503) is an ideal way to remove an oxide from the surface and apply a thin (<0.1 μm) film of Ni over the plating area (as discussed in U.S. Pat. No. 9,540,735 B2). Other techniques exist to activate the surface such as plasma or chemical treatments. Some of these treatments could achieve the desired surface roughness and surface activation in one step or be easier to perform on a given size/shape substrate. This step is often followed by a water rinse (520) before the functionally graded alloy deposit (510) is applied to the surface. Thus, a properly prepared substrate (500) attributes include: clean, free of oxidation, roughened to a desired and known roughness, and having an activated surface ready to receive the functionally graded alloy deposition. The coated component is often then rinsed (520) and air dried (530). Finally, the coated component is hot isostatically pressed (540) by applying a temperature (within 20% of the melting point) and pressure (in the form of Ar gas) to the coated substrate sufficient to create a strong diffusion bond and also relieve residual stress in the coating. The coating process parameters define the rate of deposition and the percentage of metal groups being deposited, and deposition time determines the coating thickness, while deposition process parameters can be varied to affect the filling of roughened areas in the substrate to achieve a smooth surface finish. Following hot isostatic pressing (H-I-P), components from the substrate such as Cr and Fe for S31609 diffuse into the coating. One skilled in the art recognizes that the coating thickness and H-I-P conditions need to be tuned to prevent Cr and Fe diffusion to the surface of the coating. This is important because Cr dissolution during molten salt operations is considered the key corrosion failure mechanism of the S31609 substrate. The inventors have found that coating thicknesses of 150 μm are effective in resisting corrosion, and thicknesses beyond −200 μm have no additional corrosion benefit. One of ordinary skill in the art recognizes that additional coating thicknesses could be a beneficial wear layer in a potentially abrasive environment such as that of molten salts. Refractory metal concentration at the surface of 25-60% offer sufficient corrosion resistance provided they are selected to yield a suitable CTE profile.

The efficacy of a functional grading and diffusion bonding is determined by cross-sectional scanning electron microscope (SEM) characterization of the overlay coating (200), substrate (100) and diffusion bonded region (220). While the corrosion resistance is demonstrated by exposing the coated of the functionally graded diffusion bonded coating system to molten salt electrolytes at 700° C. for 500 hours and cross-sectioning the coating and substrate to estimate the amount of attack on the substrate.

The following examples illustrate various embodiments of the instant invention.

Working Example I (without Functional Grading)

NiMo coating with constant composition coatings were prepared. The substrate was stainless steel alloy S31609. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from [0.4 μm Sa] prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 110 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface was rinsed and the NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfate hexahydrate (NiSO₄·6H₂O), 0.18 M sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9. The NiMo was electrolytically deposited using a pulse-reverse waveform with a cathodic current of 33 mA/cm² for 400 ms followed by an open circuit time off of 2.4 ms; then an anodic current of 772 mA/cm² was applied for 0.1 ms this was also followed by an open circuit time off of 2.4 ms repeat consistently for 20 h. This operation was completed at approximately 25° C. After application of the NiMo coating, the samples were air dried and held for post characterization. The cross-sectional SEM images and EDX line scan compositional data for this sample are presented in FIG. 11. The image shows the 2.5 mm thick stainless-steel substrate coated with a 0.4 mm thick NiMo coating with an approximate composition of 32 w/w % Mo and 68 w/w % Ni.

Working Example II (Functional Grading)

NiMo coatings with functional grading were prepared according to an embodiment of the invention. The substrate was stainless steel alloy S31609. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from [0.4 μm Sa] prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 110 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface rinsed and the functionally grading NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfate hexahydrate (NiSO₄·6H₂O), 0.18 M sodium citrate dihydrate (Na₃C₆HsO₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9. The NiMo was electrolytically deposited using a sequence of pulse-reverse waveform conditions as shown in Table II. This operation was completed at approximately 25° C. After application of the functionally graded NiMo coating, the samples were air dried and held for post characterization. The cross-sectional SEM images and EDX line scan compositional data for this sample are presented in FIG. 12. The image shows the 2.5 mm thick stainless-steel substrate coated with a 0.25 mm thick NiMo coating with a functional grading going from approximately 15 w/w % Mo and 85 w/w % Ni at the coating interface to 33 w/w % Mo and 67 w/w % Ni at the surface of the material.

TABLE II
denotes the sequenced operating conditions to apply a functionally
graded NiMo coating.
	Cathodic/	Current Density	ton	toff	Step Duration
	Anodic	Applied (mA/cm2)	(ms)	(ms)	(min)
Waveform 1	Cathodic	110	0.5	0.2	20
	Anodic	—	—	—
Waveform 2	Cathodic	89	0.5	0.2	40
	Anodic	—
Waveform 3	Cathodic	67	0.5	0.2	180
	Anodic	—	—	—
Waveform 4	Cathodic	44	0.5	0.2	300
	Anodic	—	—	—
Waveform 5	Cathodic	33	0.5	0.2	480
	Anodic	—	—	—

Working Example III (Functional Grading)

NiMo coatings with functional grading were prepared according to an embodiment of the invention. The substrate was stainless steel alloy S31609. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from (0.4 μm Sa) prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 110 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface rinsed and the functionally grading NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfate hexahydrate (NiSO₄·6H₂O), 0.18 M sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9. The NiMo was electrolytically deposited using a sequence of pulse-reverse waveform conditions as shown in Table III for sample 100. This operation was completed at approximately 25° C. After application of the functionally graded NiMo coating, the samples were air dried and held for post characterization. The cross-sectional SEM images and EDX line scan compositional data for this sample are presented in FIG. 13. The image shows the 2.5 mm thick stainless-steel substrate coated with a 0.38 mm thick NiMo coating with a functional grading going from approximately 10% Mo and 90% Ni at the coating interface to 33% Mo and 67% Ni at the surface of the material.

TABLE III
denotes the separate sequenced electrolyte operating conditions to apply a
functionally graded NiMo coating.
		Current Density
	Cathodic/	Applied	ton	toff	Step Duration
	Anodic	(mA/cm2)	(ms)	(ms)	(min)
Waveform 1	Cathodic	110	0.5	0.2	15
	Anodic	—	—	—
Waveform 2	Cathodic	83	0.5	0.2	37.8
	Anodic	—
Waveform 3	Cathodic	56	0.5	0.2	153
	Anodic	—	—	—
Waveform 4	Cathodic	33	0.5	0.2	315.6
	Anodic	—	—	—
Waveform 5	Cathodic	33	0.5	—	1008
	Anodic	56	0.1	—

Working Example IV (Diffusion Bonding)

NiMo coatings without functional gradings were diffusion bonded to the stainless-steel alloy S31609 substrate prepared according to an embodiment of the invention. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from [0.4 μm Sa] prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 110 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface was rinsed and the NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfate hexahydrate (NiSO₄·6H₂O), 0.18 M sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9. The NiMo was electrolytically deposited using a pulse-reverse waveform with a cathodic current of 33 mA/cm² for 400 ms followed by an open circuit time off of 2.4 ms; then an anodic current of 772 mA/cm² was applied for 0.1 ms this was also followed by an open circuit time off of 2.4 ms repeat consistently for 300 min. This operation was completed at approximately 25° C. After application of the NiMo coating, the samples were air dried and held for post diffusion bonding by hot isostatic pressing operations and characterization.

Table IV highlights the hot isostatic press operating condition and the effect of Ni and Mo diffusion into the substrate and Fe and Cr out of the substrate. FIG. 14 shows the Ni diffusion bonded (220) formed as a result of the hot isostatic press process, where Ni diffused out of the NiMo overlay into the stainless-steel substrate (Table IV). FIG. 15 shows the Mo diffusion bonded (220) formed as a result of the hot isostatic press process, where Mo diffused out of the NiMo overlay into the stainless-steel substrate (Table IV). FIG. 16 shows the Cr diffusion bonded (220) formed as a result of the hot isostatic press process, where Cr diffused out of the stainless-steel substrate into the NiMo overlay (Table IV). FIG. 17 shows the Fe diffusion bonded (220) formed as a result of the hot isostatic press process, where Fe diffused out of the stainless-steel substrate into the NiMo overlay (Table IV).

Table IV denotes the amount of diffusion that occur during the diffusion bonding operation per the conditions. Negative numbers represent diffusion into the 316H SS substrate, while positive numbers represent Fe and Cr diffusion from the substrate into the overlay.

	HIP	HIP	HIP	Element Penetration/Diffusion
Sample	Temp	Pressure	Duration	Ni *	Mo *	Fe	Cr
ID	(° C.)	(psi)	(hrs)	(μm)	(μm)	(μm)	(μm)
HIP	1185	22,000	7.00	−32	−24	80	80
NO HIP	NA	NA	NA	0	0	0	0

Working Example IV (Prior Art Corrosion Resistance without Coating)

In FIG. 18 the corrosion data for stainless steel alloy S31609 without a coating system are presented. The S31609 substrate was welded into a test vessel containing NaF, LiF, KF salt commonly known as FLiNaK. Next, this component was placed into a furnace and heated to 700° C. and let sit for 500 hours. Following the 500 hour corrosion trial the S31609 substrate was cross-sectioned and examined as shown in FIG. 18. Specifically, FIG. 18, shows the cross-section image that indicates attack of the S31609 substrate with over 200 microns of grain etch depth into the substrate.

Working Example V (Corrosion Resistance without Diffusion Bond)

The corrosion resistance to molten fluoride salts for NiMo coatings with a single metal NiMo coating was prepared according to an embodiment of the invention. The substrate was stainless steel alloy S31609. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from (0.4 μm Sa) prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 110 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface was rinsed and the NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfate hexahydrate (NiSO₄·6H₂O), 0.18 M sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9. The NiMo was electrolytically deposited using a pulse-reverse waveform with a cathodic current of 33 mA/cm² for 400 ms followed by an open circuit time off of 2.4 ms; then an anodic current of 772 mA/cm² was applied for 0.1 ms this was also followed by an open circuit time off of 2.4 ms repeat consistently for 300 min. This operation was completed at approximately 25° C. After application of the NiMo coating, the samples were air dried and held for post corrosion testing.

Next, corrosion testing was done at ORNL using a NaF, LiF, KF salt commonly known as FLiNaK. The stainless-steel substrate with the NiMo without a diffusion bonded coating was welded into a sealed container filled with the FLiNaK material. Next, this component was placed into a furnace and heated to 700° C. and let sit for 500 h. FIG. 19 shows cross-sectional data post the corrosion trial for stainless steel alloy S31609 with a NiMo coating system without diffusion bonding. The data indicates delamination of the NiMo and attack of the S31609 substrate with over 200 microns of grain etch depth into the substrate.

Working Example VI (Corrosion Resistance)

The corrosion resistance to molten fluoride salts for diffusion bonded NiMo coatings with a single metal NiMo diffusion bonded coating were prepared according to an embodiment of the invention. The substrate was stainless steel alloy S31609. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from (0.4 μm Sa) prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 91 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface was rinsed and the NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfamate (Ni(SO₃NH₂)₂), 0.18 M sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9.5. The NiMo was electrolytically deposited using a pulse-waveform with a cathodic current of 81 mA/cm² for 0.5 ms followed by an open circuit time off of 0.5 ms repeating constantly for 6 h. This operation was completed at approximately 25° C. After application of the NiMo coating, the samples were air dried and shipped for diffusion bonding by hot isostatic pressing. The hot isostatic press conditions used to diffusion bond the coating to the substrate were 1185° C.; 22,000 psi; 3.5 hour duration.

Next, corrosion testing was done at ORNL using a NaF, LiF, KF salt commonly known as FLiNaK. The stainless-steel substrate with the NiMo diffusion bonded coating was welded into a sealed container filled with the FLiNaK material (FIG. 20 shows an image of the welded test vessel). Next, this component was placed into a furnace and heated to 700° C. and let sit for 500 h. Following the 500 hour corrosion trial the NiMo diffusion bonded coated substrate was cross-sectioned and examined as shown in FIG. 21. Specifically, FIG. 21 shows the cross-section SEM image of the instant invention after 500-hour exposure at 700° C. Based on the cross sections the substrate was not attacked. The NiMo coating was shown to be protective in these conditions.

Working Example VII (Coating Thickness Diffusion Bonding)

NiMo coatings of varying thicknesses were diffusion bonded to the stainless-steel alloy S31609 substrate prepared according to an embodiment of the invention. Prior to application of the electrodeposited NiMo coating, the stainless-steel substrate was grit blasted with 80 grit alumina particles resulting in an increase in roughness from [0.4 μm Sa] prior to grit blasting to approximately 5.7 μm Sa after grit blasting. Sa is arithmetic mean height of the asperities and is a measure of surface roughness known to those of ordinary skill in the art.

Next a Ni Strike surface activation approach was used to activate the surface and electrolytically deposited a thin film of Ni onto the stainless-steel alloy S31609 substrate. The strike electrolytic cell consisted of a stainless-steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The Ni Strike electrolyte consisted of 240 g/L NiCl₂ and 125 mL/L HCl (35° Baume). The Ni was electrolytically deposited using a direct current condition at a current density of 110 mA/cm² for 1 min. This operation was completed at approximately 25° C.

Next the surface was rinsed and the NiMo deposition process commenced. The NiMo electrolytic cell consisted of a Ni coated stainless steel alloy S31609 substrate separated from the mixed metal oxide anode by approximately 2 cm. The NiMo electrolyte consisted of 0.20 M Ni from a nickel sulfate hexahydrate (NiSO₄·6H₂O), 0.18 M sodium citrate dihydrate (Na₃C₆H₅O₇·2H₂O), 0.01 M Mo from a sodium molybdate dihydrate (Na₂MoO₄·2H₂O) with addition of ammonium hydroxide to a pH of 9. The NiMo was electrolytically deposited using a pulse-reverse waveform with a cathodic current of 33 mA/cm² for 400 ms followed by an open circuit time off of 2.4 ms; then an anodic current of 772 mA/cm² was applied for 0.1 ms this was also followed by an open circuit time off of 2.4 ms repeat consistently for 10 h (150 μm), or 20 h (272 μm). This operation was completed at approximately 25° C. After application of the NiMo coating, the samples were air dried and held for post diffusion bonding by hot isostatic pressing operations and characterization. The hot isostatic press conditions used to diffusion bond the coating to the substrate were 1250° C.; 22,000 psi; 3.5 h duration.

The resulting diffusion bonding that occurred of is shown in FIGS. 22-28 for each element (Cr, Fe, Ni, and Mo) at starting overlay thicknesses of 150 μm (FIGS. 22-25) and 272 μm (FIGS. 26-29). Utilizing cross-sectional SEM images and EDX line scans we were able to assess the amount of Cr, Fe, Ni, and Mo diffusion (220) as a function of the thickness of the NiMo alloy coating. The resulting elemental diffusion as a result of the HIP conditions are highlighted in Table V. As the data suggests, generally, 100 microns can be effective and more preferably 150 microns is sufficient to prevent Cr and Fe diffusion to the surface of the overlay. This is important because Cr dissolution during molten salt operations is considered the key corrosion failure mechanism of the S31609 substrate.

TABLE V
contains the numerical summary of the diffusion effects of
hot isostatic pressing for various coating thicknesses.
	Element Penetration/Diffusionβ
Thickness (μm)	Ni * (μm)	Mo* (μm)	Fe (μm)	Cr (μm)
272	−66	−57	56	94
150	−47	−24	89	94

Working Example VIII: Electrolytic Deposition of a Functionally

Graded Fe—W Alloy from a Single Electrolyte A functionally-graded Fe—W alloy coating was prepared in a single electrolyte by varying the waveform parameters. The substrate was a 99.95% tungsten sheet. Prior to application of the electrodeposited Fe—W alloy, the tungsten substrate was grit blasted with 80 grit alumina particles to increase surface roughness. The tungsten surface was rinsed with water and the Fe—W alloy deposition process commenced. The electrolytic cell consisted of a tungsten substrate separated from the mixed metal oxide anode by approximately 5 cm. The composition of Electrolyte A is summarized in Table IV. The Fe—W alloy was electrolytically deposited at 40° C. using the three-step pulse waveform sequence shown in Table VII with a solution flow rate of 1 gallon per minute (GPM). After application, the Fe—W alloy coatings were rinsed with water and air-dried. After drying, the samples were cross-sectioned and characterized via scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). SEM/EDS of the Fe/W alloy manufactured using this waveform (FIG. 30) exhibits three distinct layers with tungsten content of 33%, 23%, and 11%, respectively. Direct current (DC) deposition from Electrolyte A resulted in a higher porosity coating compared to pulse current (PC) deposited coatings.

TABLE VI
Fe—W alloy electrodeposition electrolyte for Working Example VIII.
Components	Role	Electrolyte A
Glycolic Acid	Complexing Agent	1.43M
Citric Acid	Complexing Agent	0.3M
[Fe2(SO4)3•xH2O]	Iron Source	0.1M
[Na2WO4•2H2O]	Tungsten Source	0.2M
		pH = 6.5-6.7

TABLE VIII
Pulse waveform sequence to generate the functionally graded Fe—W
alloy of Working Example VIII. All steps were performed at 40° C.
using a flow rate of 1 GPM.
	duration	cathodic current	on-time	off-time
Step	(hr)	density (mA/cm2)	(ms)	(ms)
1	36	15.5	30	50
2	20	51.7	150	250
3	10	200	0.5	0.5

SEM/EDS data for a functionally graded sample generated using the three-step pulse waveform sequence detailed in Table VII (FIG. 30) with a plot of the signal intensity of tungsten 301 overlaid with SEM image. Location of the EDS line scan denoted by line 303. Three distinct layers can be identified with average tungsten content of 33%, 23%, and 11% W w/w, respectively.

Working Example IX: Electrolytic Deposition of a Functionally

Graded Fe—W Alloy from Multiple Electrolytes A functionally-graded Fe—W alloy coating was prepared using a single waveform by varying the electrolyte composition. The substrate was 99.95% tungsten sheet. Prior to application of the electrodeposited Fe—W alloy, the tungsten substrate was grit blasted with 80 grit alumina particles to increase surface roughness. The tungsten surface was rinsed in water and the Fe—W alloy deposition process commenced. The electrolytic cell consisted of a tungsten substrate separated from the mixed metal oxide anode by approximately 5 cm. The two electrolytes used during electrolytic deposition are summarized in Table VIII. A polarized-off pulse waveform was used throughout electrolytic deposition with a first cathodic current density of 62 mA/cm2 with an on-time length of 0.1 ms, and a second cathodic current density of 6.2 mA/cm2 with an on-time of 0.6 ms. First, electrolytic deposition in Electrolyte B (60° C., 2 GPM, 24 hours) applied a tungsten-rich (>50% W w/w) Fe—W alloy to the tungsten substrate. Next, electrolytic deposition in Electrolyte A (50° C., 2 GPM, 24 hours) applied an iron-rich (<50% W w/w) Fe—W alloy to the tungsten-rich layer. After application, the Fe—W alloy coatings were rinsed with water and air-dried. After drying, the samples were cross-sectioned and characterized via SEM/EDS. SEM/EDS (FIG. 31) clearly shows a three-layer stack consisting of the tungsten substrate, a tungsten-rich Fe/W alloy (60% W w/w), and an iron-rich Fe/W alloy (40% W w/w). DC electrodeposition from Electrolyte B resulted in an unacceptable blistered and flakey coating.

TABLE VIIIII
Fe—W alloy electrodeposition electrolytes for Working Example IX.
Components	Role	Electrolyte A	Electrolyte B
Glycolic Acid	Complexing Agent	1.43M	—
Citric Acid	Complexing Agent	0.3M	0.3M
[Fe2(SO4)3•xH2O]	Iron Source	0.1M	0.01M
[Na2WO4•2H2O]	Tungsten Source	0.2M	0.3M
(NH4)2SO4	Electrolyte Stabilizer	—	0.13M
Sodium Dodecyl Sulfate	Wetting Agent	—	0.1 mM
		pH = 6.5-6.7	pH = 7

SEM/EDS data for a functionally graded Fe/W alloy generated via the two-electrolyte manufacturing is shown in FIG. 31. A plot of the signal intensity of tungsten (305) and iron (307) are overlaid with the cross-sectional image. Line scan location is denoted by line 309 across the thickness of the film.

Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims.

In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for any claim element amended.

Claims

1. A method of coating a metallic substrate, the method comprising: subjecting the metallic substrate to an electrolytic deposition process including an electrolyte with an iron group element and a refractory group element;varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at an interface between the coating and the metallic substrate to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts; anddiffusion bonding the coating to the substrate.

2. The method of claim 1 in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate.

3. The method of claim 2 in which varying the electrolytic waveform parameters includes varying the current density of the waveform, the length of time the waveform is applied, and the time between successive waveforms.

4. The method of claim 3 in which varying the electrolytic waveform parameters further includes switching between cathodic and anodic waveforms.

5. The method of claim 1 further including increasing the roughness of the substrate before deposition.

6. The method of claim 1 further including activating the substrate.

7. The method of claim 6 wherein the activation is accomplished by nickel strike, acid etch, or plasma treatment.

8. The method of claim 1 wherein the substrate is a stainless steel material or tungsten.

9. The method of claim 1 wherein the CTE of the functionally graded coating is lower than that of the substrate.

10. The method of claim 1 wherein the iron group includes Fe, Co and/or Ni and the refractory group includes Mo, W, and/or Re.

11. The method of claim 1 wherein the functionally graded coating adjacent the substrate interface comprises 90+% of the iron group and the functionally graded coating at its surface comprises between 25 and 60% of the refractory group.

12. The method of claim 1 wherein the diffusion bonding is accomplished by hot isostatic pressing.

13. The method of claim 12 wherein the hot isostatic pressing is conducted for about 3½ hours at about 1250 C and at about 22,000 psi pressure.

14. A metallic substrate coated with a functionally graded diffusion bonded coating with more of an iron group element and less of a refractory group element at an interface between the metallic substrate and the coating to better match the coefficient of thermal expansion of the coating and the metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts.

15. A coating for a metallic substrate comprising: more of an iron group element and less of a refractory group element at a coating interface to better match the coefficient of thermal expansion of the coating and a metallic substrate and more of the refractory group element and less of the iron group element as the thickness of the coating increases for improving corrosion resistance to salts.

16. A method of coating a metallic substrate, the method comprising: subjecting the metallic substrate to an electrolytic deposition process including an electrolyte solution with an iron group element and a refractory group element; andinstead of changing the electrolyte solution, varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at a first portion of the coating more of the refractory group element and less of the iron group element at a second portion of the coating.

17. The method of claim 16 in which the first portion of the coating is at an interface between the coating and the substrate and the second portion of the coating is at the surface of the coating.

18. The method of claim 17 in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate.

19. The method of claim 18 in which varying the electrolytic waveform parameters includes varying the current density of the waveform, the length of time the waveform is applied, and the time between successive waveforms.

20. The method of claim 19 in which varying the electrolytic waveform parameters further includes switching between cathodic and anodic waveforms.

21. The method of claim 16 further including increasing the roughness of the substrate before deposition.

22. The method of claim 16 further including activating the substrate.

23. The method of claim 22 wherein the activation is accomplished by nickel strike, acid etch, or plasma treatment.

24. The method of claim 16 wherein the substrate is a stainless steel material or tungsten.

25. The method of claim 16 wherein the CTE of the functionally graded coating is lower than that of the substrate.

26. The method of claim 16 wherein the iron group includes Fe, Co and/or Ni and the refractory group includes Mo, W, and/or Re.

27. The method of claim 17 wherein the functionally graded coating adjacent the substrate interface comprises 90+% of the iron group and the functionally graded coating at its surface comprises between 25 and 60% of the refractory group.

28. The method of claim 16 further including diffusion bonding the coating to the substrate.

29. The method of claim 28 wherein diffusion bonding includes hot isostatic pressing for about 3½ hours at about 1250 C and at about 22,000 psi pressure.

30. A method of coating a metallic substrate, the method comprising: subjecting the metallic substrate to an electrolytic deposition process including an electrolyte solution with an iron group element and a refractory group element; andinstead of changing the electrolyte solution, varying one or more electrolytic deposition waveform parameters to deposit on the substrate a functionally graded coating with more of the iron group element and less of the refractory group element at a first portion of the coating more of the refractory group element and less of the iron group element at a second portion of the coating in which the waveform parameters vary from a waveform that preferentially influences the iron group element to transport to the substrate and to deposit on the substrate to a waveform that preferentially influences the refractory group element to transport to the substrate and to deposit on the substrate.