CA2844084A1 - Apparatus and process for nickel plating and sealing - Google Patents

Abstract

A pre-treatment process surface preparation of workpieces for subsequent nickel coating is provided including a thermal process to remove residue, grit blasting for surface roughening, such as with garnet, and rust avoidance by submergence in hot ammonia. Use of expensive tank materials for electroless nickel coating are avoided through the provision of replaceable liners optionally protected from the subject workpieces using movable wall panels. Further, tank heating is effectively provided using industrial hot water heaters and an indirect water/nickel solution heat exchanger, obviating the need for risky and regulated heat sources. In a final option, a fluoropolymer sealer is provided over the nickel coating.

Description

1 "APPARATUS AND PROCESS FOR NICKEL PLATING AND

2 SEALING"

3

4 FIELD
This subject matter of this disclosure relates generally to apparatus, 6 process and systems for effectively corrosion-proofing metal workpieces, more 7 particularly cleaning and plating a steel workpieces with nickel and protective coatings.

Nickel plating is a process that deposits a thin layer of nickel onto an 11 underlying metal, often steel. Some of the benefits of nickel plating include increased 12 resistance to corrosion or rust, improved resistance to wear, strength and improved 13 ductility.
Workpieces are typically manufactured of vulnerable and inexpensive 14 materials.
Nickel plating is often used in the automotive and oil and gas industries to protect such materials from corrosion caused by CO2 and H25.
16 Two different methods may be used to add nickel plating to a workpiece.
17 The first is electrolytic, also called "galvanic". The second is purely a chemical process 18 known as electroless which relies upon a chemical reaction to apply the layer of nickel.
19 Before performing electroless nickel plating, the material to be plated is cleaned by a series of chemicals, known as the pre-treatment process. Chemical 21 cleaning is the standard cleaning process in the plating industry. Chemical cleaning is 22 only an effective as the cleaning solution. The success of the cleaning has a direct 23 impact on the adhesion of the nickel coatings. Workpieces that are not cleaned properly 24 will have poor adhesion. In particular, failure to remove unwanted residue or "soils"
from the workpiece's surface result in poor plating. Each pre-treatment chemical 26 treatment is therefore followed by several water rinses to remove chemicals that may 27 adhere to the surface. De-greasing removes oils from surfaces, whereas acid cleaning 1 removes scaling. The cleaning bath may require frequent replacement at a high cost.
2 Further, after the cleaning process is complete, the workpiece is exposed to a 3 Hydrochloric acid (HCI) pre-dip prior to plating. However HCI pre-dip tends to leave a 4 rust film or "Rose Bloom" on the surface of the workpiece which also negatively affects the adhesion of the nickel coating.
6 Once the workpiece has been prepared, it is placed in an alloy tank that 7 is passivated by nitric acid for plating. The tanks are manufactured of exotic metals 8 resistant to the chemicals and plating effects. The tanks are typically made of stainless 9 steel or alloys such as Inconel (International Nickel Company, Inc.).
The chemical bath or fluid in the tanks is typically heated by electric elements or boilers coupled with steam 11 heat exchangers. Electric elements are expensive, have a limited size of tank they can 12 heat and may take multiple hours to heat the fluid in the tank. If an electrical element 13 were to be damaged, it could cause a fire and pose a safety risk to the workers. Boilers 14 coupled with steam heat exchangers are far more efficient. However, they have specific operational and maintenance requirements which are often regulated and require 16 certified operators with special training. Steam heat exchangers are not readily 17 serviceable and are typically replaced when their life cycle ends.
Replacing steam heat 18 exchangers can be expensive and time consuming, with additional revenue lost during a 19 shut down and servicing.
The newly plated workpiece is sometimes further coated with a sealer to 21 enhance the corrosion resistance of the nickel coating. Unfortunately, most sealers 22 wipe or peel off over time and effectively offer little to no additional protection.
23 There is a need for an improved nickel plating process and equipment in 24 the coating industry.

2 Generally, a pre-treatment process prepares workpieces, such as iron-3 bearing workpieces, for subsequent electroless nickel coating. The pre-treatment 4 process comprises thermal heat soaking and garnet grit blasting. Further, the workpiece can be submerged in hot ammonia, all of the above pre-treatment steps 6 resulting in minimal residue and actors adverse to subsequent nickel plating operations.
7 Further, use of expensive tank materials for electroless nickel coating can 8 be avoided through the use of mere carbon steel tanks protected by replaceable plastic 9 liners.
Somewhat fragile liners can be optionally protected from the subject workpieces using movable panels suspended strategically about the tank walls. Further, tank 11 heating is effectively provided using industrial hot water heaters and an indirect 12 water/nickel solution heat exchanger, obviating the need for risky and regulated heat 13 sources such as electrical or steam heating. In a final option, a fluoropolymer sealer 14 can be provided over the nickel coating.
In a broad aspect a pre-treatment method for preparing a workpiece for 16 plating comprises heat-soaking the workpiece for thermal removal of residue therefrom, 17 optionally at about 400 C for about 8 hours, and blasting the residue-free workpiece 18 with garnet grit. Further, the process can include submerging the workpiece in hot 19 ammonia.
In another aspect, the aforementioned pre-treatment methodology is 21 preparatory for a method of coating the workpiece with a protective coating including 22 using electroless nickel coating. The nickel coating can be conducted in a tank 23 containing a nickel solution, whilst heating the nickel solution to about 90 C. The nickel 24 solution can be heated by circulating the nickel solution through an indirect heat exchanger; and circulating hot water to the indirect heat exchanger for heating the nickel 26 solution.
Further, the nickel coated workpiece can be further protected with a polmer 27 sealer, such as a fluoropolymer sealer.

1 In another aspect, the electroless nickel coating can be conducted in 2 apparatus comprising a carbon steel tank; and a plastic liner for storing a nickel solution 3 separated from the carbon steel tank. The apparatus can also comprise one or more 4 bumpers for arrangement between the tank and the workpiece for protecting the liner from damage. The nickel solution can be heated using a heating system further 6 comprising: an indirect heat exchanger; a hot water heater; a hot water circulation loop 7 between the hot water heater and the indirect heat exchanger; and a nickel solution 8 circulation loop between the tank and the indirect heat exchanger.

BRIEF DESCRIPTION OF THE DRAWINGS
11 Figure 1A is a flow chart of various embodiments of workpiece surface 12 preparation including thermal cleaning, grit blasting, and hot ammonia pre-dip;
13 Figure 1 B is a flow chart of various embodiments of workpiece surface 14 plating and sealing including nickel coating and polymer sealing;
Figure 2 is an end cross-sectional view of an ENC tank having a liner, a 16 plurality of safety bumpers and a workpiece such as a valve body being lowered by rack 17 into the tank;
18 Figure 3A is an embodiment of an electroless nickel coating (ENC) or 19 plating embodiment illustrating hot water heaters, hot water loops, an indirect heat exchanger and ENC tank;
21 Figure 3B is a schematic overview of the heating system according to the 22 ENC embodiment of Fig. 3A including an optional ENC tank filter; and 23 Figure 4 illustrates scanning electron microscope or SEM images of a 24 first iron workpiece coated with nickel and a second iron workpiece coated with nickel and then coated with a subsequent fluoroploymer sealer according to embodiments 26 disclosed herein.

2 In accordance with one aspect of the disclosure, there is provided a 3 cleaning process, a nickel coating and sealing process, and methods of providing 4 protective coatings upon parts and components, referred to generically herein as workpieces, for improving corrosion resistance of said workpieces to toxic environments 6 that can cause premature wear or failure in industries including oil and gas and 7 automotive. The coating process includes preparation of the surface of the workpiece, 8 electroless nickel coating or plating in a chemical bath, and sealing the plated workpiece 9 with a sealant.
Generally, workpieces to be protected are prepared using a thermal 11 process to remove substantially all residues adverse to subsequent and relevant coating 12 processes referred to herein. Further, the workpiece surface is further treated to 13 improve coatings adhesion, more particularly using a grit blasting. Iron-bearing 14 workpieces can further be advantageously pre-treated using a hot ammonia bath.
Embodiments of nickel-plating tanks are provided herein having advantages in ease of 16 construction, scalability, and heating. Low cost tanks can be employed having 17 replaceable liners secured thereto. Heating is provided using hot water heaters, 18 obviating the need for troublesome and expensive electrical or steam boilers. In a final 19 option, a fluoropolymer sealer can be applied which is particularly effective in adhering to the nickel.
21 Herein, workpieces to be coated typical include valves and other 22 workpieces manufactured of base materials vulnerable to corrosion including steel.

5 2 With reference to Fig. 1A, a pre-treatment surface preparation process is 3 useful prior to plating and maximizes the effectiveness of nickel coating.
4 A first step in an embodiment of the surface preparation process is to clean a workpiece 10 as shown in Blocks 100,110 and 120.

6

7 Thermal Cleaning

8 At Block 100, the workpiece 10 is thermally cleaned in an oven 20.

9 Applicant has found that, a majority of the time, thermal cleaning is most effective for removing foreign substances from the workpiece 10 for ensuring sufficient adhesion of 11 materials during subsequent plating processes. The oven 20 is heated to about 400 C
12 and the workpiece 10 is heat-soaked for a duration of about eight hours.
Thermal 13 cleaning leaves no chemical residue on the surface of the workpiece, maximizing 14 adhesion of subsequent nickel coating in the plating process.
16 Grit Blasting 17 At Block 110, a next step in preparing the surface of the workpiece

10 for 18 plating is grit blasting. Most conventional abrasive grit such as aluminum oxide, steel 19 shot, or silica grits provide minimal surface roughening and grit residue is damaging to subsequent nickel coating chemicals and processing. Applicant has determined that a 21 garnet grit is suitable for pre-nickel plating preparation without the aforementioned 22 disadvantages. In one embodiment, garnet abrasive grit is used in the blasting process 23 for creating a uniform surface profile virtually free of grit embedment, and providing an 24 excellent roughened surface for coating adhesion.
A source 30 of grit 32 is provided for delivery to blasting apparatus 34 for 26 delivery under pressure to the workpiece 10. An effective size for the garnet grit or 27 abrasive particles is about 120 mesh (Tyler Equivalent). The pressure ranges of the 1 blasting apparatus 34 can range from about 60 psi to about 80 psi with a preferred 2 pressure of about 75 psi. The grit blasting is done to a National Association of 3 Corrosion Engineers (NACE) specification #1 standard.

Hot Ammonia Bath 6 At Block 120, once the grit blasting is complete, the workpiece 10 is 7 placed into a hot ammonia bath 40 for the final stage of the surface preparation. The 8 ammonia bath has a concentration ranging from about 0.06% to about 0.10%
and is 9 heated to a temperature range of about 60 C to about 75 C. The workpiece 10 is submerged in the bath 40 for about 0.5 hours to about 1.5 hours as appropriate

11 depending on the thickness and volume of the workpiece 10. The hot ammonia bath 40

12 is used to lessen or eliminate any potential rust film or "rose bloom"
that could otherwise

13 occur on the workpiece's surface.

14 At Block 130, the workpiece 10 is removed from the ammonia bath 40 and ready for plating.

18 Once the workpiece 10 has been prepared, electroless nickel plating 19 (ENC) can be effectively implemented. ENC is known in the art and comprises deposition of a nickel-phosphorous or nickel-boron alloy onto the workpiece. A
known 21 reducing agent reacts with metal ions in a metal workpiece to deposit the coating.
22 Herein, the plating process is known however the tank and heating process are 23 improved over the known apparatus and methodologies.
24 A nickel super alloy coating result that imparts superior corrosion resistance and added wear resistance to the workpiece. The coating is a grain-free 26 amorphous structure with excellent barrier corrosion protection. The morphology of the 27 coating is similar to that of a metallic glass coating, the absence of a well-defined crystal 1 structure avoiding intergranular corrosion. Typically, in a nickel-phosphorous system, 2 the process results in coating having a nickel metal content (wt%) of about 87.5% -3 89.5% and a phosphorus content (wt%) of about 10.5% to 12.0% with traces of other 4 elements.
Turning to Block 140, the workpiece 10 is lowered into an ENC tank. The 6 tank includes a liner 52 and a heated nickel coating solution 54. The coating solution 54 7 is heated through an indirect heat exchanger 56. An industrial hot water heater 58 8 provides the heat source for the heat exchanger 56. The workpiece is plated with nickel 9 forming a nickel plated workpiece 10N.
At Block 150, the nickel-plated workpiece 10N is introduced to a sealer 11 tank 60 for submergence of the workpiece IONS or a spray booth for larger workpieces 12 10N. A sealer 62 is applied forming a sealed, nickel-plated workpiece IONS.
13 In an embodiment, a fluoropolymer sealer 62 is used to enhance 14 corrosion resistance of the nickel coating. The advantage of the fluoropolymer sealer is it does not wipe off or peel off as prior art sealers or dyes do. Applicant believes this is 16 accomplished by the nature of the chemical bonding between the sealer and the nickel 17 coating.
18 With reference to Figs. 1A to 3, various arrangements and configurations 19 of the plating equipment, and operation thereof, are described in greater detail.
21 Coating Equipment 22 With reference to Fig. 2, upon completion of the surface preparation, the 23 workpieces 10 are treated in the nickel plating tank 50. The workpieces can be 24 arranged and supported in a rack or basket for processing in tank 50. In the present embodiment, ENC treating tanks 50 can up to 35 feet long for long tubular workpieces.
26 Custom tanks can be made that can accommodate a larger workpieces, Applicant 1 regularly provides tanks 50 having dimensions in the order of 6 feet wide by 10 feet long 2 by 6 feet deep.
3 In the prior art, stainless steel tanks are typically passivated by nitric acid 4 to maximize the natural anti-corrosion properties of the stainless steel.
Continuous operation requires alternating operation of dual tanks sitting side-by-side for every 6 operational set up. Typically one tank is in operation and the other is being passivated 7 with nitric acid. The time required to passivate one tank is several days.
8 Instead, in embodiments disclosed herein, mere carbon steel tanks can 9 be used, implementing a replaceable plastic liner just slightly smaller than the tank itself.
Accordingly a tank 50 has walls 70, made from a less expensive material such as 11 carbon steel, can be used in combination with the inexpensive liner 52.
Accordingly, 12 tanks of virtually any dimensions can be readily manufactured or modified for various 13 configurations at a fraction of the cost of tanks manufactured of more expensive, exotic 14 corrosion resistant materials. Thus, the nickel solution can be stored in the carbon steel tank, the carbon steel tank 50 incorporating the plastic liner 52 for storing the nickel 16 solution separated from the carbon steel tank.
17 In an embodiment, dual side-by-side tanks can still be employed for 18 alternating use; one tank being used for plating while the alternate tank's polypropylene 19 liners are changed out, the time and cost to replace liners 52 being substantially less than that for prior stainless steel tanks.
21 The carbon steel tank 50 comprises an open top 72 having a top edge 22 about a perimeter of the enclosing walls 70. The liner 52 is positioned inside the tank 23 50 for forming a liquid containment and is stretched out over the top edge 74, protecting 24 the material of the tank walls 70. The liner can be disposable or cleaned.
Workpieces 10 are lowered into the tank 70 in a basket, from a rack 76 or 26 other apparatus. The liner 52 is at risk of damage from the suspended workpieces.
27 Accordingly, as shown, one or more wall panels or safety bumpers 77 formed of sheet 1 materials are provided, such as those made from a durable material such.
as 2 Polytetrafluoroethylene (PTFE) or Teflon (registered trademark of DuPont) are 3 movable and are installed at various points along the perimeter of the tank 50 to protect 4 the liner 52 from damaging contact with workpieces. One means for retaining the safety bumpers 77 is to incorporate a "U" shaped hook or holder 78 at an upper end for 6 engaging the top edge 74 of the tank, a downwardly depending panel portion 79 7 sandwiching the liner against the wall 70. The panel portion 79 of the bumper extends 8 toward the bottom of the tank adjacent the interior of the perimeter wall 70 for protecting 9 the liner. A plurality of safety bumpers 77 can be placed and moved anywhere about the perimeter of the tank as necessary. for protecting the liner from damage.
11 The bumpers 77 can be strategically arranged placed between the tank 12 and the workpiece dependent upon workpiece ingress and egress locations or be 13 implemented along substantially the entirety of the tank walls 70.
Overhead fluid lines 14 can strategically access the tanks fluid contents spaced inwardly from the liner 52 or aligned with bumpers 77 to minimize risk of liner damage.
16 In one embodiment, the liner 52 is a disposable polypropylene material.
17 When the bath needs to be emptied the polypropylene liner 52 can be reused or simply 18 discarded. The liner can be continuous into and out of which fluids are passed from 19 overhead fluid lines. Alternatively, the liner can be manufactured with an integrated outlet, and optionally an integrated fluid inlet for in-tank fluid access.
21 In an embodiment, an advantage of using a disposable liner 52 is that 22 only one operational tank could be required as the change out time is minimal, being 23 four hours or less, minimally impacting operations. Also, capital costs for using a carbon 24 steel material for the tank 50 with a liner 52 are typically less than one third the cost of using a stainless steel tank. The polypropylene liners 52 can be custom made to fit any 26 size tank 50. Typically, one polypropylene liner 52 can be used for multiple cycles, 27 typically two bath cycles.

2 Heating the Coating Tanks 3 With reference to Figs. 1B, 3A and 3B, a heating system for the nickel 4 plating process is provided. A nickel coating solution used in plating processes is maintained, within the ENC tank, at a temperature of about 90 C. Industrial hot water 6 heaters 58 with large capacity heat exchangers 56 are utilized for providing the heat 7 source required for the nickel plating process. Such hot water heaters are easy to 8 operate and maintain and do not require specially certified workers to operate as do 9 steam heat sources.
The industrial hot water heaters indirectly heat the solution. Large 11 capacity plate-type heat exchanges 56 are serviceable and easy to maintain compared 12 to electric elements and boilers. The cost for heating the system is less than that 13 associated with electric elements and steam boilers commonly used in the industry.
14 The hot water heater, such as a boiler, heats water and circulates the hot water through a main loop using a main lop pump. When heat is required for electroless 16 nickel coating tank, a primary pump pulls water from the main loop and circulates the 17 hot water to a plate heat exchanger. Similarly, a recirculation pump circulates nickel 18 solution through the same heat exchanger for heating the solution. The heat demand 19 by one or more heat exchangers and the resulting heat loss from the main loop is reheated by the hot water heater.
21 In more detail, and having reference to Figs. 3A and 3B, a main heated 22 water loop 80 acts as an intermediary stage between the hot water heater 58 and heat 23 exchanger 56. The heat exchanger 56 has a heat-providing side and a heat-receiving 24 side, for heating and maintaining the nickel coating solution bath at a temperature of about 90 C. Thus, the heating system comprises the indirect heat exchanger 56, the 26 hot water heater 58; hot water circulation loop 80 between the hot water heater and the 1 indirect heat exchanger; and a nickel solution circulation loop 91 between the tank 50 2 and the indirect heat exchanger 56.
3 Hot water is provided to the main loop 80 by a hot water boiler pump 82.
4 A main hot water loop pump 84 circulates the hot water through the main loop 80. The main loop can provide heat for one or more of the process needs such as a heat 6 exchanger serving the hot ammonia bath.
7 A temperature sensing device, such as a thermocouple 86, monitors the 8 temperature of the nickel coating solution bath in tank 50. When the bath temperature 9 falls below a predetermined threshold temperature, the main heated water is routed through the heat exchanger 56 by a primary pump 88.
11 With regards to the heat receiving side, a recirculation pump 90 is 12 provided for circulating the nickel coating solution along the nickel solution circulation 13 loop 91 to and from the treating tank 50. The solution receives heat from the heated 14 water at the heat exchanger 56 and is circulated back into the treating tank 50. The solution is circulated through a filter 92 with a filter pump 94 for maintaining a desired 16 quality of the nickel coating solution for coating the workpiece, and maximizing the 17 solution life.
18 Upon completion of the nickel coating process the workpiece 10 is 19 inspected for quality assurance purposes and then sent to the sealer tank.
21 Sealer 22 Returning to Fig. 1B, a fluoropolymer sealer is used to enhance corrosion 23 resistance of the nickel coating, the effectiveness of which is believed effective due to a 24 chemical bonding between the PTFE and the nickel. A particularly effective fluoropolymer sealer for application over nickel coatings, such as for excellent chemical 26 corrosion resistance for CO2, H2S, brine and chlorides, is a polytetrafluoroethylene 27 (PTFE). A suitable PTFE is Teflon (a registered trademark of DuPont) such as an 1 acqueous dispersion of PTFE TE-3893 fluoropolymer resin made by DuPontTM.
As set 2 forth in DuPont's product information for PTFE TE-3893 fluoropolymer resin, the product 3 is a milky white liquid containing typically 60% (total wt%) of 0.05 to 0.5 um of PTFE
4 particles suspended in water and may contain 6% (wt% of PTFE) of a non-ionic wetting agent and stabilizer. The sealer is suitable for conventional dip or flow techniques.
6 The sealer tank 60, which is typically about 6 feet long by 35 inches wide 7 by 4 feet deep is used to submerse the nickel-plated workpiece 10N in the PTFE resin 8 for about 5 minutes at room temperature (approximately 20 C to about 25 C). If the 9 workpiece 10N is too large to be submerged in the sealer, the sealer 62 can also be applied by spraying the nickel surface. After spraying, excess sealer 26 can be rinsed 11 off of the workpiece 10NP. For fluoropolymer resins, excess sealer 62 can be rinsed off 12 after about 5 minutes.
13 The sealer can be applied in a single coat application or multiple coats if 14 required and is self-lubricating and non-stick. The sealer coating results in a low coefficient of friction, providing a high level of release and non-stick properties.
16 The advantage of the fluoropolymer sealer is it does not wipe off or peel 17 off as prior art sealers or dyes do. Applicant believes this is accomplished by the nature 18 of the chemical bonding between the sealer and the nickel coating.
19 As shown in Fig. 4, a scanning electron microscope or SEM of a cross-section of two iron (Fe) workpiece surfaces are illustrated, both having nickel (Ni) 21 coating, and one workpiece IONS having a PTFE sealer and the other workpiece 10N
22 without.

Claims (20)

EMBODIMENTS OF THE INVENTION FOR WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for preparing a workpiece for plating comprising:
heat-soaking the workpiece for thermal removal of residue therefrom; and blasting the residue-free workpiece with garnet grit.

2. The method of claim 1 wherein the heat-soaking further comprises heating the workpiece at about 400°C for about 8 hours.

3. The method of claim 1 or 2, wherein the garnet grit is about 120 mesh.

4. The method of claim 1, 2 or 3, further comprising submerging the workpiece in hot ammonia.

5. The method of claim 4 wherein the hot ammonia has a concentration of about 0.06 to 0.10 % in water.

6. The method of claim 5 wherein the workpiece is submerged in the hot ammonia at temperatures of about 60°C to about 75°C for about 0.5 to about 1.5 hours.

7. The method of any one of claims 1 to 6, wherein the workpiece is iron bearing.

8. A method of protective coating a workpiece comprising:
preparing the workpiece using the method of any one of claims 1 to 7;
and nickel coating the workpiece using electroless nickel coating.

9. The method of claim 8 wherein the nickel coating is performed in a tank containing a nickel solution, further comprising heating the nickel solution to about 90°C.

10. The method of claim 9 further comprising:
circulating the nickel solution through an indirect heat exchanger; and circulating hot water to the indirect heat exchanger for heating the nickel solution.

11. The method of claim 9 or 10 further comprising recirculating the nickel solution through a filter.

12. The method of any one of claims 8 to 11, further comprising applying a polymer sealer over the nickel coating.

13. The method of any one of claims 8 to 11, further comprising applying a fluoropolymer sealer over the nickel coating.

14. The method of any one of claims 8 to 11, further comprising applying a sealer comprising a dispersion of a PTFE fluoropolymer.

15. The method of any one of claims 9 to 14, further comprising:
storing the nickel solution in a carbon steel tank.
lining the carbon steel tank with a plastic liner for storing the nickel solution separated from the carbon steel tank.

16. The method of claim 15, further comprising:
extending the liner over a top edge of the tank; and arranging one or more bumpers between the tank and the workpiece for protecting the liner from damage.

17. The method of claim 16 further comprising:
providing a plurality of the bumpers; and arranging one or more of the plurality of bumpers between the tank and the workpiece.

18. Apparatus for electroless nickel plating of a workpiece comprising:
a carbon steel tank; and a plastic liner for storing a nickel solution separated from the carbon steel tank.

19. The apparatus of claim 18, further comprising one or more bumpers for arrangement between the tank and the workpiece for protecting the liner from damage.

20. The apparatus of claim 18 or 19, further comprising:
an indirect heat exchanger;
a hot water heater;
a hot water circulation loop between the hot water heater and the indirect heat exchanger; and a nickel solution circulation loop between the tank and the indirect heat exchanger.