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Title:
CORROSION-RESISTANT COATINGS FOR STEELS USED IN BROMIDE-BASED ABSORPTION CYCLES
Document Type and Number:
WIPO Patent Application WO/1999/014400
Kind Code:
A1
Abstract:
A method for coating steel and other metals in which a metallic material layer of at least one of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, and Zr is deposited onto a metal substrate. The metallic material layer is then annealed so as to form a diffusion layer between the metallic protective coating and the metal substrate. Thereafter, the metallic material layer is passivated, forming a stable composition of at least one of carbides, borides, nitrides, silicides, oxides, and mixtures thereof on the metallic protective coating. The protective coatings of this invention significantly reduce the corrosion rate of steels and other metals used in bromide-based absorption cycles.

Inventors:
Jayaweera, Palitha (34272 Lennox Court Fremont, CA, 94555, US)
Sanjurjo, Angel (15010 Penetencia Creek Road San Jose, CA, 95132, US)
Lau, Kai-hung (7716 Orogrande Place Cupertino, CA, 95014, US)
Jiang, Naixiong (391 Curtner Avenue #1 Palo Alto, CA, 94306, US)
Lowe, David M. (22310 City Center Drive #2124 Hayward, CA, 94541, US)
Application Number:
PCT/US1998/019372
Publication Date:
March 25, 1999
Filing Date:
September 17, 1998
Export Citation:
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Assignee:
GAS RESEARCH INSTITUTE (8600 West Bryn Mawr Avenue Chicago, IL, 60631, US)
International Classes:
C23C8/02; C23C26/00; F28F19/06; (IPC1-7): C23C26/00; F28F19/06
Foreign References:
US5543183A1996-08-06
DE3407293A11985-09-05
US5492727A1996-02-20
US5149514A1992-09-22
EP0077535A11983-04-27
DE2327250A11973-12-13
Other References:
PATENT ABSTRACTS OF JAPAN vol. 011, no. 141 (C - 421) 8 May 1987 (1987-05-08)
PATENT ABSTRACTS OF JAPAN vol. 011, no. 018 (C - 398) 17 January 1987 (1987-01-17)
Attorney, Agent or Firm:
Fejer, Mark E. (Speckman Pauley Petersen & Fejer Suite 365 2800 West Higgins Road Hoffman Estates, IL, 60195, US)
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Claims:
WE CLAIM:
1. A method for coating steel, copper, coppernickel alloys and other metals comprising: depositing a metallic material layer selected from the group consisting of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof onto a metal substrate; diffusing a portion of said metallic material layer so as to form a diffusion layer between said metallic material layer and said metal substrate; and passivating said metallic material layer, forming a stable compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides, and mixtures thereof on said metallic material layer.
2. A method in accordance with Claim 1, wherein said metallic material layer is deposited on said metal substrate by chemical vapor deposition.
3. A method in accordance with Claim 1, wherein said metallic material layer is deposited on said metal substrate in a fluidized bed reactor.
4. A method in accordance with Claim 1, wherein said metallic material layer is deposited on said metal substrate material by a pack cementation process.
5. A method in accordance with Claim 1, wherein said metallic material layer is applied at a temperature in a range of about 150°C and 1300°C.
6. A method in accordance with Claim 1, wherein said deposited metallic material layer is diffused into said metal substrate by annealing.
7. A method in accordance with Claim 1, wherein said metallic material layer is passivated by exposure to ammonia.
8. A method in accordance with Claim 1, wherein said metal substrate is selected from the group consisting of steel, copper and coppernickel alloys.
9. A corrosionresistant metallic material comprising: a substrate metal; a metallic protective coating selected from the group consisting of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof ; a diffusion layer formed by diffusion of said metallic protective coating disposed between said substrate metal and said metallic protective coating; and at least one compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides and mixtures thereof passivated onto said metallic protective coating.
10. A corrosionresistant metallic material in accordance with Claim 9, wherein said substrate metal is selected from the group consisting of steel, copper and copper nickel alloys.
11. A corrosionresistant metallic material in accordance with Claim 10, wherein said substrate metal is 409 SS.
12. A corrosionresistant metallic material in accordance with Claim 9, wherein said material is resistant to corrosion in a bromide environment up to a temperature of at least about 300°C.
13. A heat exchanger for use in a bromidebased absorption cycle comprising: a substrate metal in a shape of a heat exchanger; a metallic protective coating selected from the group consisting of Si, Ge, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof covering a surface of said substrate metal; a diffusion layer formed by diffusion of said metallic protective coating disposed between said substrate metal and said metallic protective coating; and at least one compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides and mixtures thereof passivated onto said metallic protective coating.
14. A heat exchanger in accordance with Claim 13, wherein said substrate metal is selected from the group consisting of steel, copper, and coppernickel alloys.
15. A heat exchanger in accordance with Claim 14, wherein said substrate metal is 409 SS.
16. A heat exchanger in accordance with Claim 13, wherein said material is resistant to corrosion in a bromide environment up to a temperature of at least about 300°C.
Description:
CORROSION-RESISTANT COATINGS FOR STEELS USED IN BROMIDE-BASED ABSORPTION CYCLES BACKGROUND OF THE INVENTION This invention relates to coatings for steels and other metals which are exposed to bromide environments, which coatings reduce the corrosion rate of said steels and other metals in the bromide environment. More particularly, this invention relates to coatings for steels and other metals used in bromide-based absorption cycles which reduce the rate of corrosion of said metals. The coatings of this invention allow use of low-cost steels as a construction material in place of expensive alloys, allow an increase in operating temperature of such bromide-absorption cycles, up to about 300°C, thereby enabling improvement in the coefficient of performance (COP), and result in extended service life, thereby reducing replacement and maintenance costs. Also disclosed is a method for coating such steels and other metals.

DESCRIPTION OF PRIOR ART Coatings are widely used for the corrosion protection of metals and other materials in a variety of environments. The protection of steel in a variety of environments is the subject of many articles and patents. Nevertheless, it is not obvious from the prior art what coating should be selected for the protection of steel in a high temperature bromide solution such as one which is used in bromide-absorption cycles. A good example is the failure of titanium coatings in high temperature bromide media. Although the resistance of titanium to halide induced corrosion is well documented, we have found that titanium is readily attacked in high temperature bromide solutions.

An improved corrosion resistant surface layer on a metal substrate formed by laser-induced remelting and solidifying under an aqueous solution of metal ion and reducing agent is taught by Japanese Patent 61281856. Here, the surface layer is formed by a treatment involving remelting and solidifying the surface of the base material and consists of a number of minutely thin layers. The improved surface layer has a different composition from the base material. The method is used to impart a corrosion resistant layer to a metal, especially to stainless steel, for use, for example, in a nuclear fuel reprocessing plant, a chemical plant, nuclear power plant, absorption refrigerator, or in a semiconductor package. European Patent Publication 0488165 teaches the plating non-electrolytically, electrically, or by any other means, of copper and nickel on the surface of a copper heat transfer tube followed by diffusion of the copper and nickel, which is then work hardened by means of rolling, swaging or other known means. The procedure is applicable to any absorbing refrigerator using solutions such as an aqueous solution of lithium bromide or any other salt solution.

The diffusion coating of a metal by simultaneous deposition of Cr and Si onto the metal is taught by U. S. Patent 5,492,727 and related U. S. Patent 5,589,220. The method utilizes a halide-activated cementation pack with a dual halide activator. Codeposition of chromium and silicon coatings on iron-based alloys by pack cementation using a mixed activator, that is a fused salt solution of NaF and NaCI. is taught by U. S. Patent 5,364,659.

A chemical vapor deposition (CVD) method for case hardening a ferrous metal interior tubular surface by exposure to diffusible boron with or without other diffusible elements such as silicon to enhance the wear, abrasion and corrosion resistance of the tubular surface is taught by U. S. Patent 5,455,068. The use of chemical vapor deposition for deposit of aluminum and a metal oxide on substrates for improved corrosion, oxidation, and erosion protection is taught by U. S. Patent 5,503,874.

A method for producing materials in the form of coatings or powders using a halogen-containing reactant which reacts with a second reactant to form one or more reactive intermediates from which the powder or coating may be formed by disproportionation, decomposition, or reaction is taught by U. S. Patent 5,149,514.

U. S. Patent 4,822,642 teaches a silicon diffusion coating formed in the surface of a metal article by exposing the metal article to a reducing atmosphere followed by treatment in an atmosphere of 1 ppm to 100% by volume silane, with the balance being hydrogen or hydrogen plus inert gas.

A method for depositing a hard metal alloy in which a volatile halide of titanium is reduced off the surface of a substrate and then reacted with a volatile halide of boron, carbon or silicon to effect the deposition on a substrate of an intermediate compound of titanium in a liquid phase is taught by U. S. Patent 4,040,870.

SUMMARY OF THE INVENTION Advanced absorption cycles fueled by natural gas may offer significant advantages over conventional heating, cooling, and refrigeration systems. These advanced cycles include double effect, triple effect, and generator absorber heat exchange cycles. The advanced absorption cycles reduce energy consumption, thus improving the economics of natural gas consumption. These advanced absorption cycles reduce the gas consumption considerably. The reduced gas consumption per unit results in a lower operating cost to the consumer and lower emissions to the environment.

These advanced cycles transfer heat from both the absorber and condenser from a higher temperature absorption cycle using one refrigerant/absorbent pair to a second (or third) refrigerant/absorbent pair operating at a lower generator temperature. The triple effect chiller is 30% to 60% higher in coefficient of performance (COP) than a double effect cycle using equivalent heat exchangers. The triple effect chiller has the potential of being less expensive than double effect chillers because the triple effect chillers can use several existing absorption fluids. In addition, it uses only conventional heat exchangers and it requires less total heat exchanger per unit of capacity than single or double effect cycles. Water/LiBr, water/LiBr with additional absorbents, for example, ZnBrr, LiCl, ammonia/water, and ammonia/water with other salts, for example LiBr, are some of the widely used absorbents and <BR> <BR> refrigerants. Conventional absorbent cycles operate in the range of 200° to 350°F, but triple effect and other advanced cycles operate at much higher temperatures in order to transfer <BR> <BR> absorber heat at temperatures up to 300 °F. Because the condenser is also operating at 200 °F as well as being under high pressure, the generator temperatures must be very high, typically <BR> <BR> greater than 450°F. These very high temperatures result in very low concentrations of refrigerant in the liquid leaving the generator, which is extremely corrosive to the metals used in the construction of these heat exchangers.

The major barrier towards commercialization of advanced absorbent cycles is the prohibitive cost of the construction material. The alloys that survive in salt mixtures at the extreme temperatures required for high COP are very expensive and difficult to machine. As a result, the use of efficient advanced absorption cycles is not financially attractive.

Accordingly, it is an object of this invention to provide materials which are relatively low cost with respect to the expensive alloys currently required by advanced absorption cycles, thereby eliminating the major barrier to the commercialization of such advanced absorption cycles.

It is another object of this invention to provide coated, low cost steel and other metals suitable for use in such advanced absorption cycles having the same or better corrosion resistance than the expensive alloys currently used.

It is yet another object of this invention to provide a method for producing such corrosion-resistant coated low cost steels and other metals.

These and other objects are achieved by a method for coating steels and other metals in which a metallic protective coating selected from the group consisting of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof is deposited onto a metal substrate. The metallic protective coating is then diffused so as to form a diffusion layer between the metallic protective coating and the metal substrate. The metallic protective coating is then passivated, forming a stable compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides and mixtures thereof on the metallic protective coating. Suitable deposition methods for deposition of the metallic protective coating on the metal substrate include chemical vapor deposition, pack cementation, and a fluidized bed reactor.

The corrosion-resistant metallic material produced by this process comprises a substrate material, a metallic protective coating selected from the group consisting of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof, a diffusion layer disposed between the substrate metal and the metallic protective coating, and at least one compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides, and mixtures thereof passivated onto the surface of the metallic protective coating.

One of the benefits of the method for coating steel and other metals in accordance with the process of this invention is that it can be applied to a variety of convoluted geometries such as heat exchangers or bundles of tubes producing, for example, a heat exchanger suitable for use in a bromide-based absorption cycle wherein the heat exchanger comprises a metallic protective coating selected from the group consisting of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof, a diffusion layer formed by diffusion of the metallic protective coating disposed between the substrate metal and the metallic protective coating, and at least one of a carbide, boride, nitride, silicide or oxide passivated onto the metallic protective coating.

BRIEF DESCRIPTION OF THE DRAWINGS These and other object and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein: Fig. 1 is a micrograph of a chromium diffusion coating on 409 stainless steel; Fig. 2 is a micrograph of a chromium diffusion coating on 409 stainless steel at higher magnification than shown in Fig. 1; and Fig. 3 is a graphic depiction showing the depth profile of chromium diffusion on a 409 stainless steel substrate.

DESCRIPTION OF PREFERRED EMBODIMENTS Coatings are widely used for corrosion protection of metals and other materials in a variety of environments. The coatings and methods for producing such coatings in accordance with this invention significantly reduce the corrosion rate of steels and other metals used in bromide-based absorption cycles. These coatings allow the use of low cost steels as the construction material in place of expensive alloys, increase the operating temperature of such cycles (up to 300°C), thereby improving the coefficiency of performance, and extend service life, thereby reducing replacement and maintenance costs. We have found that the corrosion-resistant coatings for low cost steels in accordance with this invention produce coated, low cost steel having the same or better corrosion resistance than expensive alloys.

In addition, the coatings, as well as the coating method, in accordance with this invention have a substantial economic advantage over the use of the more expensive alloys.

We have found that the coatings in accordance with this invention significantly reduce the rate of corrosion of, for example, 409 stainless steel in high temperature bromide solutions. The metallic protective coatings of this invention comprise an outer metallic material layer selected from the group consisting of Si, Ge, Cr, W, Mo, Ta, Nb, Ni, Zr and mixtures thereof which is deposited onto a metal substrate and a diffusion layer disposed between said outer metallic material layer and said metal substrate. The outer metallic material layer is treated so as to diffuse a portion of the outer metallic material layer into the metal substrate, forming a diffusion layer between the outer metallic material layer and the metal substrate. Coatings with some degree of diffusion, as in the coatings of this invention, do not have a well defined interface between the substrate and the coating and, thus, do not delaminate upon temperature cycling or exposure to aggressive environments. The coating metal and the substrate metal are interdiffused to some extent, thereby forming an alloy at the interface. By controlling parameters associated with the coating method of this invention, we can obtain a surface composition similar to a corrosion-resistant alloy such as DURIRON'for Si coatings, and AL6XN'for Cr coatings. After formation of the diffusion layer, the outer metallic material layer is passivated, thereby forming a stable compound selected from the group consisting of carbides, borides, nitrides, silicides, oxides, and mixtures thereof on the metal protective coating. Once the coating process in accordance with this invention is complete, the coated specimen shows corrosion resistance similar to that of an expensive alloy.

For example, Figs. 1 and 2 show micrographs of a chromium diffusion coating on 409 stainless steel. On average, the grains are over 100-200 microns compared with the 30- 40 micron grains of as-received 409 stainless steel, showing grain growth during the coating process.

Fig. 3 shows the depth profile of the chromium diffusion. The Cr concentration on the surface is very high (about 44% by weight of the coated metal) and decreases to about 10% by weight (bulk concentration of Cr in uncoated 409 SS) at a depth of about 400 microns.

Even at a depth of 100 microns, the Cr concentration is greater than about 23 weight percent, more than that of AL6XN@, the currently used alloy for prototype triple effect generators. The metallic protective coating of this invention may be deposited onto the metal substrate by any suitable deposition process. In accordance with one preferred embodiment of this invention, the protective metal coating is deposited onto the metal substrate by chemical vapor deposition. In accordance with another embodiment of this invention, the metallic protective coating is deposited onto the metal substrate material by a pack cementation process. In accordance with yet another embodiment of this invention, the metallic protective coating is deposited on the metal substrate in a fluidized bed reactor.

We have coated 409 SS coupons, one square inch, with Si, Cr, W and Mo by chemical vapor deposition. We used halide chemistry in a fluidized bed reactor or pack cementation process for Si and Cr deposition. The coating temperatures for Si and Cr were 550°C and 1,000°C, respectively. The chemical vapor deposition coating technique can be carried out in a fluidized bed reactor and is particularly suitable for coating convoluted geometries, such as heat exchangers or bundles of tubes. Fine powders of the metal to be deposited are pneumatically injecte in and through the heat exchanger system in, for example, an argon carrier containing 1-5% of an H2-HCl mixture. In the case of Si, the reaction with HCl produces a mixture of SiH4, SiHCl3, SiH2C12, and SiCl2 vapors which transport the Si from the particles to the surface of the metallic wall where they are reduced. The metallic walls of the heat exchanger act as a sink because the activity of Si in them is typically very low (less than about 1 %) while the gas is saturated. In the initial stages, deposition and diffusion are very fast for steel above 550°C.

Tungsten and molybdenum were deposited on 409 SS by chemical vapor deposition at 450°C using the respective carbonyl as a precursor. We believe that the reaction may be a decomposition reaction such as that from a carbonyl: M (CO) n-M + nCO where typical cases may include Nb (CO) 6 = Nb + 6CO W (CO) 6 = W + 6CO Mo (CO) 6 = Mo + 6CO Cr (CO) 6 = Cr + 6CO or mixtures of metal carbonyls such as W (CO) 6 + Cr (CO) 4 = W-Cr (on steel) + 10CO where it forms a W-Cr coating on the substrate. However, the reaction may also be a decomposition of a hydride or other substitute that disproportionates on steel. The gas may be injected as a vapor or as a condensed powder or absorbed liquid. For example, in the case of hydrides, the deposition reactions are as follows: SiH4-Si (on steel) + 2H2 GeH4-Ge (on steel) + 2H2 B2H6-2B (on steel) + 3H2 SiHBr3+ 2H2-Si (on steel) + 6HBr Diffusion of the outer metallic material layer into the metal substrate to form a diffusion layer, in accordance with one embodiment of this invention, is carried out by annealing the metal surface coated with said outer metallic material layer. The resulting coated surface of the substrate can then be further protected by application of surface passivation as the final step in the diffusion coating process. Although any suitable surface passivation technique may be used, we routinely performed this procedure in a fluidized bed reactor, most commonly for surface nitridation. As an example, after the protective metal, for example Si or Ti, is deposited on the metal substrate, the coated surface is exposed to 2% NH3 as the final step in the coating process. Ammonia reacts with silicon and/or titanium to form extremely protective thin silicon or titanium nitride films. We have also used this process to passivate coated surfaces resulting in the formation of corrosion-protective surface compounds selected from the group consisting of carbides, borides, nitrides, silicides, oxides, and mixtures thereof.

To establish the corrosion resistance of the coatings of this invention, we exposed coated 409 SS specimens to 90% lithium bromide and zinc bromide solution at 260°C in a laboratory test setup. This salt mixture was chosen due to its favorable thermodynamic properties for advanced absorption cycles. It is also known to be more corrosive than other commonly used bromide-based salt combinations. Numerous metals and alloys were also tested for comparison and identification of coating metals. The corrosion rates were measured using weight loss technique, DC corrosion experiments, and electrochemical impedance analysis. In the DC corrosion experiment, the metal specimen was polarized anodically and cathodically 100 mV from the natural corrosion potential. The resulting current was plotted in a log I versus E graph and fitted to the Stern-Geary equation using a nonlinear least squares technique to obtain anodic and cathodic Tafel slopes (ba and bj, and the corrosion rate. In the AC impedance analysis, a small sinusoidal waveform (5 mV) was applied on the electrode at the natural corrosion potential of the metal. The frequency of the sine wave was swept from about 10 kHz to 1 MHz and the resulting current information was collected along with its <BR> <BR> phase relationship to the original waveform and presented in Nyquist plots (Zimaginary versus<BR> Zreal) Polarization resistance, which is inversely proportional to the corrosion current, was calculated from the X-axis intercepts of the semicircle fit. The proportionality constant is a function of anodic and cathodic Tafel slopes. Therefore, the corrosion rate can be calculated using polarization resistance from AC impedance analysis and Tafel slopes from a DC potential scan. In some cases, AC impedance itself is used as a quantitative measure of corrosion protection by comparing polarization resistance of coated and uncoated specimens.

Table I summarizes the corrosion rates of some pure metals, expensive alloys, and coated 409 SS in 90% lithium bromide and zinc bromide solution at 260°C.

TABLE I Corrosion Rate Corrosion Rate Pure Metals (mpy) A ! toys and Coatings (mpy) Mo 0.04 409 SS (Reference) 96 W <0.01 AL6XNs (Reference-Goal) | 2.4 Ni 0.11 DURIRONO 1. 1 Ti 28.5 W-coated 409 SS 74.7 Si <0.01 Si-coated 409 SS 24.0 Zr <0.01 Mo-coated 409 SS 15.7 Ta 0.01 Mo/Si-coated 409 SS 14.4 Cr-coated 409 SS 0. 01 When each of the coatings of this invention are applied to a substrate of 409 SS, it can be seen from TABLE I that each said coating significantly reduces the corrosion rate of the 409 SS compared to uncoated 409 SS and that Cr-coated 409 SS provides the best protection against corrosion, having a corrosion rate substantially below that of AL6XN@.

While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.