Coating processes – Solid particles or fibers applied – Uniting particles to form continuous coating with...
Reexamination Certificate
1999-12-23
2001-12-25
Zimmerman, John J. (Department: 1775)
Coating processes
Solid particles or fibers applied
Uniting particles to form continuous coating with...
C427S191000, C427S201000, C427S331000
Reexamination Certificate
active
06333072
ABSTRACT:
A. FIELD OF THE INVENTION
The invention relates to a process for producing adherent metal oxide coatings on metallic surfaces. More particularly, the invention relates to the production of an adherent, porous, metal oxide coating on metallic surfaces for the purpose of (1 ) testing, such as for heat transfer or corrosion of the underlying metallic material, (2) improving the surface characteristics to perform a specific function, such as changing the heat transfer characteristics or optical properties, or (3) protecting the metallic surface from environmental degradation.
B. BACKGROUND OF THE INVENTION
Processes for simulating corrosion product deposits (sludge) are now well known. In one known method, alumina silicate binders are combined with dry powder mixtures of metal oxides, mineral species and metals, such as copper or zinc, in order to form coatings of synthetic deposits. The synthetic deposits based on alumino silicate binders have been previously used for chemical and mechanical cleaning qualification programs. The known process may be used to simulate corrosion product depositions on both flat and curved surfaces. However, these alumino silicate based deposits lack sufficient consolidation, strength and water resistance for many heat transfer test purposes. Consequently, there is a need for a process of forming an adherent metal oxide coating that can withstand severe flow boiling conditions up to about 200,000 BTU/hr·ft
2
heat flux. One such process described herein is based on the use of an orthophosphoric acid binder with mixed metallic oxide and metal powders.
The use of orthophosphoric acid for consolidating some oxides is briefly discussed in ASM Handbook Volume 5, Surface Engineering (page 472, published by ASM international, copyright 1994). In the ASM Handbook, the oxides used are limited to aluminum oxide, zinc oxide, hafnium oxide, and zirconium oxide.
U.S. Pat. No. 5,620,645 discloses mixing metal or metal compounds with orthophosphoric acid and then adding a moldable filler material, usually refractory in nature. Preferred materials include aluminum hydroxide, chromium trioxide, magnesium oxide, and zinc oxide, The mixture is placed in a mold and dried at a temperature of less than 100° C. to remove water and harden the material, The material is then oven dried at 300° C. where additional water is driven off and the mixture is pressed into its final configuration and desired porosity. The liberation of H
2
gas and the resulting increase in porosity caused by the reaction of orthophosphoric acid and metal species is either not desired or useless to the method described in this patent. This patent also makes no direct or indirect reference to simulation of heat-exchanger deposits.
U.S. Pat. No. 1,761,186 discloses dipping, repeatedly, if desired a ferrous material heated to 500° F. (260° C.) in an orthophosphoric acid solution to apply a relatively thin, rust inhibitive coating of ferrous-ferric oxide directly on the ferrous material. The coating is then oven dried. It is contemplated that dissolving manganese, zinc or iron phosphate in the acid could result in these rust inhibiting compounds forming part of the coating. Porous coatings are not desired for such an application. The use of a phosphoric acid solution to rust-proof steel surfaces is, according to the description in this patent, an established part of the prior art. However, the stated purpose of the described process is to create a phosphatic, rust-inhibiting, coating.
U.S. Pat. No. 4,643,778 is similar to U.S. Pat. No. 1,761,186 in that a thin coating is chemically formed directly on the base metal by dipping the material in a solution of phosphoric acid In this case, the film or coating formed allows adherence of paint. The coating formed is either zinc phosphate or iron phosphate. The relevant properties of the zinc phosphate or iron phosphate coatings that result from the process described in the patent concern only the corrosion protection provided.
SUMMARY OF THE INVENTION
The principle objective of the invention is to provide a process for forming an adherent metal oxide coating that can withstand severe flow boiling conditions of about 200,000 BTU/hr·ft
2
heat flux for the purpose of conducting heat transfer or corrosion testing. Other objectives include modifying the optical and heat transfer properties or corrosion resistance of the metallic surface.
Another objective of the invention is to provide an adherent metal oxide coating that can withstand severe flow boiling conditions of about 200,000 BTU/hr·ft
2
heat flux.
The present invention provides a novel process for forming an adherent metal oxide coating on a metallic surface comprising the steps of:
providing at least one reactive metal oxide selected from the group consisting of iron oxides, nickel oxides, ferrite compounds, and zinc oxides typically having an average particle diameter of from about 1 to about 10 &mgr;m and an average surface area of from about 0.5 to about 5 m
2
/gram;
combining phosphoric acid with at least one said reactive metal oxide to form a coating mixture where phosphoric acid is added in an amount to provide a porosity of about 20 to about 80% in the adherent metal oxide coating; and
applying the coating mixture to a metallic surface and heating said coating to form an adherent metal oxide coating on the metallic surface.
The present invention also provides an adherent metal oxide coating on a metallic surface, said adherent metal oxide coating comprising;
at least one reactive metal oxide selected from the group consisting of iron oxides, nickel oxides, ferrite compounds, and zinc oxides, having an average particle diameter of from about 1 to about 10 &mgr;m and an average surface area of from about 0.5 to about 5 m
2
/gram; and
at least one metal phosphate selected from the group consisting of iron phosphates, nickel phosphates and zinc phosphates, binding said reactive metal oxide together in a coating having a porosity of about 20 to about 80%.
The advantages of the present process includes the creation of an adherent metal oxide coating that can withstand severe flow boiling conditions of about 200,000 BTU/hr·ft
2
heat flux, the formation of a porous deposit, and the ability to create deposits up to about 50+ mils thick on either a flat or curved surface.
DETAILED DESCRIPTION
The present invention provides a process of producing an adherent synthetic corrosion product coating on metallic surfaces. The method includes a reaction between a dry solid powder mixture of at least one reactive metal oxide and deposit precursor with orthophosphoric acid (hereinafter referred to as “phosphoric acid”). Examples of reactive metal oxides are iron oxides, nickel oxides, ferrite compounds (e.g., nickel ferrites) and zinc oxides. Examples of deposit precursors are metal oxides, mineral species and pure metals. The phosphoric acid reacts with the reactive metal oxide of the mixture to form an inter-particle binder species of metal phosphates. If iron oxide is used as the reactive metal oxide, Fe(H
2
PO
4
)
2
and other iron phosphate species will be formed by the reaction with phosphoric acid. Hydrogen gas may also be liberated by the reaction which, upon release from the mixture prior to drying, causes pores to form in the final coating matrix. It has been found that successful consolidation, binding and adherence of the coating to the metallic surface requires precise control of the ratio of phosphoric acid to specific surface area of the dry deposit powders prior to reaction.
Use of too little phosphoric acid results in incomplete consolidation and a loosely adherent coating. Too much phosphoric acid results in a mixture which reacts too quickly to be useful for deposition and a coating which has little or no porosity. The phosphoric acid can be varied to provide a porosity between about 20% to about 50% in the adherent metal oxide coating. Porosities of up to 80% in the adherent metal oxide coating can be achieved when a preconsolidation process is used. The formed metal oxide coating can be heat
Lane Michael H.
Varrin, Jr. Robert D.
Caress Virginia B.
Gottlieb Paul A.
Piziali Andrew T.
The United States of America as represented by the Department of
Zimmerman John J.
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