WO2006101907A1

WO2006101907A1 - Grit blasting electrodes for surface preparation

Info

Publication number: WO2006101907A1
Application number: PCT/US2006/009355
Authority: WO
Inventors: Dino F. Difranco
Original assignee: Eltech Systems Corporation
Priority date: 2005-03-16
Filing date: 2006-03-14
Publication date: 2006-09-28
Also published as: TW200702495A; AR056942A1; WO2006101907B1

Abstract

The invention relates to a method for the preparation of a metal substrate surface which method prevents the deformation of the substrate surface and provides a surface profile of from about 100 to 180 microinches. The invention further relates to a metal substrate surface for use in an electrode structure, the metal substrate surface having a surface prepared by the method of the present invention.

Description

TITLE: GRIT BLASTING ELECTRODES FOR SURFACE PREPARATION

FIELD OF THE INVENTION

The present invention relates to a method for the preparation of a metal surface. More particularly, the invention relates to the surface preparation of a metal surface for subsequent use as an electrode, and an electrode prepared by such method.

BACKGROUND

Lifetimes of electrodes, composed essentially of an active electrocatalytic coating on a metal substrate are a function of both the amount of active material applied to the substrate and the current density. Failure of the electrode is generally attributed to loss of the active coating. As the strength and lifetime requirements for electrodes and the coatings thereon, especially those exposed to harsh chemical conditions, have increased, the processing complexity and the cost of these coated electrodes has greatly increased. It is, therefore, important to possess the ability to restore the coated metal surface of the electrode and materials dimensions and properties so that the electrode can be returned to service. This processing usually requires the removal of the overlying coating from the metal substrate.

Historically, electrodes have been refurbished by removal of the catalyst and substrate from the supporting structure followed by replacement of the substrate with new substrate and catalyst. This process is both costly and time consuming. In addition, tolerances and planarities present on the initial structure can be lost due to the mechanical refurbishing process. In another process, electrodes have been refurbished by removal of the old catalyst via chemical treatment. This process is also costly and time consuming, and often presents other difficulties such as chemical attack of other surrounding structural components.

Various mechanical techniques for coating removal and the preparation of metal substrate surfaces for refurbishment of an electrode or to receive a coating thereon for subsequent use as an electrode are known in the art. Such preparation may be by one or more of a mechanical operation such as machining, grinding and blasting, including one or more of sand, grit, and water blasting, intergranular etching of the metal, and sharp grit blasting of the metal surface, optionally followed by surface treatment to remove embedded grit and/or clean the surface, or combinations thereof. There can also be utilized sanding and buffing with abrasive brushes, papers and wheels. Preparation may also include a chemical procedure such as etching. The grit blasting manner of surface preparation of metal surfaces is analogous to sand blasting in that hard grains are hurled against the metal surface by a blasting apparatus utilizing a jet of compressed air or other suitable fluid, such as a liquid, gas or vapor, or utilizing centrifugal force for propulsion of the granular material.

Among the many materials which have been proposed for use in surface preparation processes involving grit blasting are microscopic glass beads, extra-fine grades of silica sand and spherical particles of zirconia (Zrθ2) or of mixed zirconia and silica, and aluminum oxide (AI₂O₃), among others. Determination of the particle type and size utilized may be largely based upon the surface to be treated.

Use of the foregoing particles, however, often causes deformation of the substrate due to the size of the particles, and difficulty in removal of the particles from the surface following blasting. It is preferred that the particular means employed for preparation of the metal surface be selected so as to minimize the contamination of the cleaned surface with, for example, loose particles of metal, coating, or the abrasive used for the cleaning operation.

SUMMARY

In one aspect, the invention relates a method for the preparation of a metal substrate surface for subsequent coating of the surface and use as an electrode, the method comprising mounting a metal substrate surface to expose the surface to blasting media; bombarding said surface with selectively-sized particles of aluminum oxide in the range of from about 80 microinches to about 180 microinches at a pressure of at least about 45 psi and a stand-off distance of about 1 to 5 feet; removing the particles from the metal surface; and wherein the method prevents the deformation of the substrate surface and provides a surface profile of from about 100 to 180 microinches.

In another aspect, the invention relates to a metal substrate surface for use in an electrode structure, wherein the substrate surface has been prepared by bombarding said surface with selectively-sized particles of aluminum oxide in the range of from about 80 to about 180 microinches at a blasting intensity so as to prevent deformation of said substrate surface and provide a surface profile measuring from about 100 to about 150 microinches.

In a still further aspect, the invention relates to a method for the preparation of a metal substrate surface for subsequent coating of said surface and use as an electrode, the method comprising providing a metal substrate surface; bombarding said surface with selectively-sized particles of aluminum oxide in the range of from about 80 microinches to about 180 microinches at a blasting intensity so as to prevent deformation of said substrate surface; and removing the particles from the metal surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flow diagram of one embodiment of the method steps of the present invention. Fig. 2 is a schematic illustration of a metal surface of an electrode structure prepared by an embodiment of the method of the present invention.

DETAILED DESCRIPTION

The term "blasting intensity" as used herein is defined as the grit particle size, stand off distance, and air pressure at which no deformation of the metal surface being grit blasted occurs.

As used herein, the term "profile" refers to the average surface roughness (Ra) of the ridges and valleys created during the grit-blasting, as is commonly understood in the art. The metal surface for preparation in the present invention generally comprises a conductive metal base having an electrocatalytic coating on its surface for subsequent use as an electrode structure. For cathodes, the conductive base may be a metal such as nickel or stainless steel. For anodes, the conductive base may be titanium or any film-forming metal such as, tantalum, zirconium, niobium, tungsten and, and alloys containing one or more of these metals with titanium being preferred for cost reasons. By "film-forming metal" it is meant a metal or alloy which has the property that when connected as an anode in the electrolyte in which the coated anode is subsequently to operate, there rapidly forms a passivating oxide film which protects the underlying metal from corrosion by electrolyte, i.e., those metals and alloys which are frequently referred to as "valve metals", as well as alloys containing valve metal (e.g., Ti-Ni, Ti-Co, Ti-Fe and Ti-Cu), but which in the same conditions form a non-passivating anodic surface oxide film.

Of particular interest as an anode substrate for its ruggedness, corrosion resistance and availability is titanium. As well as the normally available elemental metals themselves, the suitable metals of the substrate include metal alloys and intermetallic mixtures, as well as ceramics and cermets such as contain one or more valve metals. For example, titanium may be alloyed with nickel, cobalt, iron, manganese or copper. More specifically, grade 5 titanium may include up to 6.75 weight percent aluminum and 4.5 weight percent vanadium, grade 6 up to 6 percent aluminum and 3 percent tin, grade 7 up to 0.25 weight percent palladium, grade 10, from 10 to 13 weight percent plus 4.5 to 7.5 weight percent zirconium and so on. Where the metal surface will be utilized as a cathode structure, nickel or an oxide thereof is preferred due to its electrically conductive nature and resistance to corrosion in a caustic environment.

By use of elemental metals, it is most particularly meant the metals in their normally available condition, i.e., having minor amounts of impurities. Thus, for the metal of particular interest, i.e., titanium, various grades of the metal are available including those in which other constituents may be alloys or alloys plus impurities. Grades of titanium have been more specifically set forth in the standard specifications for titanium detailed in ASTM B 265-79. The method of surface preparation of the present invention may be serviceable for metal surfaces to be utilized in both anode and cathode structures and, in particular, anode and cathode structures in membrane cells. The metal surfaces can be mesh surfaces, louvered surfaces, punched plate and sheet surfaces.

One embodiment of the method of the present invention is illustrated in reference to the attached drawings. Fig. 1 is a flow diagram of the method of the present invention. The metal surface 1 (Fig. 2) to be treated is mounted in a manner so as to expose the metal surface 1 to the blasting media. The portions of the metal surface 1 are grit-blasted to remove coating and impurities or deposits from the metal surface which might contaminate a newly-applied coating. In addition, the blasting is used to remove corrosion products such as nickel oxide or titanium oxide which can prevent the passage of current on the anode.

Removal of the coating is achieved using blasting media comprised of selectively-sized particles of aluminum oxide grit 2 in the range, in one embodiment, of from about 80-180 microinches, and, in one embodiment, from about 120-150 microinches. Such particles 2 are selected so as to create a surface profile measuring about 100 to about 180 microinches, in one embodiment, and, in one embodiment from about 110 microinches to about 150 microinches. All of such foregoing surface characteristics are as measured by a profilometer. In one embodiment, a 120 grit aluminum oxide is used to create a profile measuring about 120 to about 140 microinches.

Aluminum oxide particles 2 have a durable, blocky crystal structure with irregular and sharp edges, making the particles ideal for blasting applications. Aluminum oxide is an extremely sharp, long-lasting blasting abrasive that can be recycled many times. It is the most widely used abrasive in blast finishing and surface preparation because of its cost, longevity and hardness. Harder than other commonly used blasting materials, aluminum oxide grains penetrate and cut even the hardest metals and sintered carbide. Approximately 50% lighter than metallic media, aluminum oxide has twice as many particles per pound. The fast cutting action minimizes damage to thin materials by eliminating surface stresses caused by heavier, slower cutting media. The chemical composition, by weight, of the aluminum oxide particles 2 is set forth in Table 1 below:

The abrasive aluminum oxide particles 2 are mixed with dry, compressed air and forced through a nozzle 3 and bombarded against the metal substrate surface 1. Air pressure is kept at a constant, in one embodiment, of from at least about 45 pounds per square inch (psi), and, in one embodiment of from about 60 to about 65 psi. Nozzle size and shape is determined by the surface being blasted. Where a tight blast pattern is desired for blasting small areas, a straight bore nozzle 3 will be utilized. Blasting of larger surfaces is accomplished by use of a Venturi bore nozzle which creates a wide blast pattern. However, the method of the present invention is not limited to the foregoing nozzles. For maximum productivity, the largest nozzle bore size that will provide the required pressure will be utilized.

The stand-off distance from which the blasting of the metal surface 1 will occur is an important aspect of the invention. Too close of a distance will result in deformation of the metal surface 1. Too far of a distance from the metal surface 1 will result in poor removal of coating and impurities from a structure, or poor preparation of the metal surface 1 for a new structure. Stand-off distance is also a factor in blasting intensity. In one embodiment, the stand-off distance will be from about 1 foot about 5 feet, and in one embodiment, from about 1 foot to about 3 feet.

The angle of impact at which the particles are projected against the metal substrate surface 1 will depend on the type of metal surface being blasted. Where the metal surface is a curved surface, e.g., a louver, the angle of impact will be varied in order to prevent areas of the surface from not being blasted due to shadowing and blocking of the grit. For substrate surfaces such as a mesh, the angle of impact is less critical. Generally, the angle of impact for louvered and mesh surfaces will be from about 30° to about 90°. Where the substrate surface 1 is a flat surface, e.g., a sheet, the angle of impact is less than 90° so that particles 2 rebounding from the surface 1 do not interfere with particles 2 being projected onto the surface 1. It was unexpected that the method of the present invention would provide a sufficiently cleaned and prepared surface 1 utilizing a small grit size aluminum oxide and without deforming the substrate surface 1. It was appreciated that larger grit sizes had been previously utilized. Thus, it was not expected to achieve a desirable method for treatment of a metal surface and subsequent application of a coating to the surface for use as an electrode with a finer grit size as disclosed in the present invention.

Following blasting of the metal surface 1 , loose particles 2 and debris are removed from the surface by vibration of the surface on a vibratory table. For smaller metal surfaces 1 , it is contemplated that the surface 1 can be manually hammered or a small welding vibrating device attached to the surface to aid in removal of the particles 2. Pressurized air or water can also be used to aid in removing the grit or debris. Clean pressurized water and/or cleaning agents can also be used to remove grit and debris. Pressurized air or water will be applied at from about 20 psi to about 150 psi in one embodiment, and in one embodiment from about 50 psi to about 100 psi.

Once the final cleaning step is complete, care must be exercised to avoid the return of contaminants to the metal electrode surface prior to application of a coating. The metal surfaces are interleafed with paper and/or cardboard and covered with the same to avoid contamination.

The following examples illustrate the metal surface preparation method in accordance with the present invention.

EXAMPLE 1 A metal substrate surface of an electrode structure to be refurbished is held vertically and scanned with high pressure Dl water to remove dust from the surface. The structure was then placed on a vibratory table manufactured by Cleveland Vibrator Company.

Parts are measured for flatness on a table designed for this purpose. An indicator (dial indicator, micrometer, etc) is positioned at various points on the metal surface and the distance of the surface above the table is recorded. The table components are machined for accuracy and planarity. Table 1 : Flatness Measurements for Anode to be Recoated

Anode Flatness Statistics: Summary

Louver Surface (in.) Flange Surface (in.)

Minimum 1.639 Minimum 1.635

Maximum 1.746 Maximum 1.747

Average 1.695 Average 1.692

Std Dev 0.018 Std Dev 0.030

Range 0.107 Range 0.112

After grit blasting the surface of the substrate according the method of the present invention, the structure was then placed back on the vibratory table and measure for flatness as above.

Table 2: Flatness Measurements for Anode After Grit Blasting and Coating

Anode Flatness Statistics: Summary

Louver Surface (in.) Flange Surface (in.)

Minimum 1.645 Minimum 1.642

Maximum 1.755 Maximum 1.742

Average 1.701 Average 1.698

Std Dev 0.019 Std Dev 0.024

Range 0.110 Range 0.100

As can be seen from Table 2, the method of the present invention provides a cleaned and prepared metal substrate surface without deforming the substrate surface.

While the invention has been explained in relation to its preferred embodiments, it is to be understood that various modifications thereof will become apparent to those skilled in the art upon reading the specification. Therefore it is to be understood that the invention disclosed herein is intended to cover such modifications as fall within the scope of the appended claims.

Claims

What is claimed is:

1. A method for the preparation of a metal substrate surface for subsequent coating of the surface and use as an electrode, the method comprising: mounting a metal substrate surface to expose the surface to blasting media; bombarding said surface with selectively-sized particles of aluminum oxide in the range of from about 80 microinches to about 180 microinches at a pressure of at least about 45 psi and a stand-off distance of about 1 to 5 feet; removing the particles from the metal surface; and wherein the method prevents the deformation of the substrate surface and provides a surface profile of from about 100 to 180 microinches.

2. The method of claim 1 , wherein the metal substrate surface is bombarded with selectively-sized particles of aluminum oxide in the range of from about 120 to about 150 microinches.

3. The method of claim 2, wherein the particles are projected at a pressure of from about 60 psi to about 65 psi and at a stand-off distance of about 1-3 feet.

4. The method of claim 1 , wherein the particles are removed from the metal surface by vibration of the metal surface or pressurized air or water.

5. The method of claim 4, wherein the pressurized air or water is applied at a pressure of from about 20 psi to about 150 psi.

6. The method of claim 3, wherein the method provides a surface profile of from about 110 to about 150 microinches.

7. The method of claim 6, wherein the method provides a surface profile of from about 120 to about 150 microinches.

8. The method of claim 1 , wherein the metal substrate comprises a conductive metal base having an electrocatalytic coating thereon.

9. A metal substrate surface for use in an electrode structure, wherein said substrate surface has been prepared by bombarding said surface with selectively-sized particles of aluminum oxide in the range of from about 80 to about 180 microinches at a blasting intensity so as to prevent deformation of said substrate surface and provide a surface profile measuring from about 100 to about 150 microinches.

10. The metal substrate of claim 9, wherein the substrate comprises a conductive metal base having an electrocatalytic coating thereon.

11. The metal substrate of claim 10, wherein the conductive metal base comprises one or more of titanium, tantalum, zirconium, niobium, tungsten and/or alloys thereof.

12. The metal substrate of claim 9, wherein the metal surface comprises one or more of a mesh, a louver, a punched plate and/or a sheet.

13. The metal substrate of claim 9, wherein the particles of aluminum oxide are in the range of from about 120 to about 150 microinches.

14. The metal substrate of claim 9, wherein the metal substrate surface comprises an anode or a cathode in an electrolytic cell.

15. The metal substrate of claim 14, wherein said electrolytic cell is a membrane cell.

16. The metal substrate of claim 9, wherein the blasting intensity provides a surface profile of from about 120 microinches to about 150 microinches.

17. The metal substrate of claim 9, wherein the blasting intensity comprises a particle size of about 80 to about 180 microinches, a stand-off distance of from about 1 foot to about 5 feet, and applied pressure of at least about 45 psi.

18. The metal substrate of claim 12, wherein the substrate is a mesh or louvered surface and the particles are projected at an angle of impact of from about 30° to about 90°.

19. The metal substrate of claim 12, wherein the substrate is a flat sheet surface and the particles are projected at an angle of impact of less than 90°.

20. A method for the preparation of a metal substrate surface for subsequent coating of said surface and use as an electrode, the method comprising: providing a metal substrate surface; bombarding said surface with selectively-sized particles of aluminum oxide in the range of from about 80 microinches to about 180 microinches at a blasting intensity so as to prevent deformation of said substrate surface; and removing the particles from the metal surface.