US3817844A - Method of electrolitic descaling activating and brightening and plating titanium and its alloys - Google Patents

Abstract

HONEYCOMB SANDWICH PANEL STRUCTURES FORMED OF TITANIUM AND ITS ALLOY ARE PREPARED FOR ELECTROPLATING AND SUBSEQUENT BRAZING-DIFFUSION BONDING BY SUBJECTING THE SAME TO A SERIES OF BATHS, ALL OPERATED AT AMBIENT ROOM TEMPERATURE. THE PANEL PARTS ARE FIRST CLEANED BY IMMERSION FOR 1-5 MINUTES IN A NON-AQUEOUS CHROMIC-SULFURE ACID BATH TO REMOVE MILL MARKS, FINGERPRINTS, AND OTHER SURFACE CONTAMINANTS. THE PAERTS ARE NEXT DESCALED TO REMOVE OXIDES BY IMMERSION FOR 5-10 MINUTES IN AN NONAQUEOUS ACETIC-SULFURIC-HYDROFLUORIC ACID BATH WHICH IS THE ELECTROLYTE IN AN ELECTROLYTIC SYSTEM IN WHICH IS TITANIUM PART IS THE ANODE AND THE CATHODE ELECTRODE IS FORMED OF CARBON OR COPPER. FROM 2-3 VOLTS ARE APPLIED ACROSS THE ELECTRODES IN DESCALING THE PANEL FACING SHEETS, AND FROM 4-5 VOLTS ARE USED IN DESCALING THE HONEYCOMB CORE WHICH IS ACTIVATED BY THIS TREATMENT AND THEREBY PREPARED FOR ELECTROPLATING TO RECEIVE ONE OR MORE PLATING LAYERS OF DIFFERENT METALS, THE ACTIVATED PARTS ARE BRIGHTENED BY IMMERSION FOR 1-5 MINUTES IN AN AQUEOUS NITRICBORIC-HYDROFLUORIC ACID BATH.

Description

June 18, 1974 E. w. KENDALL 3,317,844

METHOD OF mmmomrlc DESCALING-ACTIVATING AND BRIGH'I'ENING AND PLATING TITANIUM AND ITS ALLOYS Filed June 11. 1971 FIG. 2. FIG. 6.

PLATING ELECTROLYTE IO' FIG. 3. FIG. 7.

RIGHTENER FIG. 4 FIG. 8.

INVENTOR.

E .w. K ENDAL L ATTORNEY United States Patent US. Cl. 204-15 14 Claims ABSTRACT OF THE DISCLOSURE Honeycomb sandwich panel structures formed of titanium and its alloy are prepared for electroplating and subsequent brazing-diffusion bonding by subjecting the same to a series of baths, all operated at ambient room temperature. The panel parts are first cleaned by immersion for 1-5 minutes in a non-aqueous chromic-sulfuric acid bath to remove mill marks, fingerprints, and other surface contaminants. The parts are next descaled to remove oxides by immersion for -10 minutes in an nonaqueous acetic-sulfurichydrofluoric acid bath which is the electrolyte in an electrolytic system in which the titanium part is the anode and the cathode electrode is formed of carbon or copper. From 2-3 volts are applied across the electrodes in descaling the panel facing sheets, and from 4-5 volts are used in descaling the honeycomb core which is activated by this treatment and thereby prepared for electroplating to receive one or more plating layers of different metals. The activated parts are brightened by immersion for l-S minutes in an aqueous nitricboric-hydrofluoric acid bath.

Using the Dalic or Selectron process, the plating is applied in controlled amount to the edge faces only of the honeycomb core by rolling the same over a gauze covered roller partially immersed in the plating bath which supplies the plated metal, the core being the cathode in the electrolytic system and the anode being immersed in the electrolyte and insoluble therein.

Alternatively, the core before or after descaling activation is masked with a lacquer and edge faces thereof which have been exposed for plating are plated by immersion in the electrolyte. Removal of the lacquer from the core faces is accomplished by rolling the same over a gauze covered roller partially immersed in a solvent which is also used to remove all of the maskant after the plating is completed.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of patent application for Method of Plating Titanium and its Alloys" by Earl W. Kendall, Ser. No. 765,155, filed Oct. 4, 1968, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to the electroplating of the surfaces of titanium and its alloys and more particularly to new and improved methods for plating honeycomb core formed of such metals and for cleaning and activating such surfaces to render the same receptive to the placement of the plating thereon.

Heretofore it has been difficult to electroplate titanium and its alloys because the surfaces of these metals are in a passive state due to the presence of oxides and contaminants which render the surfaces non-receptive to ice the placement of metallic platings thereon. These oxides are not readily removed by acid attack and, accordingly, the plating of titanium has heretofore been considered to be difficult to accomplish, it at all.

In the brazing-diffusion bonding process for bonding titanium honeycomb sandwich panel structures disclosed and claimed in the copending application of J. R. Woodward for Combined Brazing-Diffusion Method of Bonding Metallic Honeycomb Sandwich Panel Structure, Ser. No. 765,156, filed Oct. 4, 1968, a small amount of brazing alloy, in laminar form as one or more layers of different metals, is placed on the faces of the honeycomb core to form at the brazing temperature a liquid interface and resultant metal to metal contact between the faying surfaces of the core and its facing sheets whereby atomic diffusion of the basis titanium materials is accelerated across the bonded joint by the diffusion bridge which is thus established by the liquid interface. The placement of the laminar braze materials on the titanium core may be accomplished by various means including electroplating which is preferred and is accomplished by the methods and baths as hereindisclosed and claimed.

SUMMARY OF THE INVENTION In accordance with the present invention, a process is provided for plating a series of metals on the edge faces of titanium honeycomb core to form a eutectic melt for joining the facing plates to the core by the aforementioned brazing-diffusion bonding process. The plating is limited by placement and by known weight to the edge face areas of the core so that flow of the brazing metal by capillary action up and along the sidewalls and nodes of the core is restricted.

The plating is applied in the manner of the well known Dalic or Selectron process in which the electrolyte is carried by a brush-type electrode to thus control the placement and amount of the plating. In the instant case, the plating is restricted to the edge faces of the core by making the core the cathode and passing the same over a gauze covered roller which is partially immersed in the electrolyte which provides the plating metal, the anode being an insoluble electrode in the plating system. The electrolyte tends to fall by gravity away from the superimposed core to thus minimize any tendency for the electrolyte to move up the cell side walls by capillary action or otherwise. The plating is thus limited to the desired brazing areas at the edge faces of the core.

The titanium honeycomb sandwich panel components are prepared for electroplating, and thereafter for the aforesaid brazing-diffusion bonding process by subjecting the same to a series of baths operative at ambient room temperature of the order of 70 to F. Surface contaminants such as finger prints and mill marks, soils, oils, and the like, are first removed by immersion for 1-5 minutes in a chromic-sulfuric acid bath such as disclosed and claimed in Pat. No. 3,379,645, of Earl W. Kendall for Process and Composition for Removing Protective Paint Films."

The panel components are next descaled electrolytically for 5-10 minutes in a bath and by a method substantially as disclosed and claimed in the copending application of Earl W. Kendall for Electrolytic Descaling of Titanium and Its Alloys," Ser. No. 600,362, filed Dec. 9, 1966, now Pat. No. 3,468,774. In accordance with this method and bath, the panel part is immersed in the bath which is a non-aqueous acetic-sulfurichydrofluoric acid solution and the part is made the anode and a carbon or copper electrode is made the cathode in the electrolytic system. From 2-3 volts are applied across the electrodes to effectively descale and clean the panel facing sheets for the subsequent brazing-diffusion bonding operation,

and from 4-5 volts are used in the preparation of the honeycomb core both to activate its edge faces for plating and to clean the surfaces for the brazing-diffusion bonding cycle.

After the cleaning and activation has been completed, the honeycomb core and panel facing sheets are subjected by immersion for 1-5 minutes in a nitric-boric-hydrofiuoric acid brightener bath such as disclosed and claimed in Pat. No. 3,228,816 of Earl W. Kendall for Process and Composition for Cleaning and Polishing Aluminum and its Alloys.

In an alternative plating method, the titanium honeycomb core following the cleaning bath to remove surface contaminants, is coated, that is, is masked by immersion in a so-called brazing stop off lacquer followed by oven baking to resist attack by the activator bath. This coating and subsequent baking may be repeated several times. The maskant is next removed from the edge faces of the core by rolling the same over and above a gauze covered roller which is partially immersed in a suitable solvent such as MEK (methylethyl ketone). The masked and partially exposed core is then ready for activation in the aforementioned electrolytic descaling bath followed by electroplating, which in this alternative process, is by immersion in the electrolyte because the maskant restricts the plating action to the exposed core faces.

After the plating is completed in one or more layers of different metals, the maskant is removed by immersion in one or more solvent baths consisting of the aforementioned MEK, acetone, or other ketones.

OBJECTS OF THE INVENTION An object of the invention is to provide new and improved methods for electroplating the surfaces of titanium and its alloys, particularly the surfaces of honeycomb panel structures formed of said metals which are to be joined by a brazing-diffusion bonding process.

Another object is to provide new and improved methods and baths for cleaning and activating the surfaces of titanium and its alloys for electroplating and joining by brazing-diffusion bonding.

Another object is to provide new and improved means for plating the edge faces of honeycomb core formed of titanium and its alloys.

Still another object is to activate the core faces to be plated to enable the same to accept and hold a metal plating thereon.

Yet another object in plating titanium honeycomb core is to control the composition, placement, and amount of the plating.

A further object is to plate the titanium core as aforesaid in laminar form of one or more layers of different metals to form a eutectic melt at brazing temperature.

Still a further object is to restrict the plating to the edge face areas of the core to form a micro melt thereon in a brazing cycle.

Still other objects, feaures and advantages of the present invention will become more fully apparent as the description proceeds, reference being had to the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a cleaning bath;

FIGS. 2 and 6 are schematic illustrations of the electrolytic descaling and activator bath;

FIG. 3 is a schematic illustration of the electroplating system;

FIG. 4 depicts the brightener bath;

FIG. 5 is a schematic representation of a system for exposing the edge faces of honeycomb core which has been covered with a maskant;

FIG. 7 depicts an immersion type electroplating system; and

FIG. 8 depicts a solvent bath.

4 SPECIFICATION Reference is now made to the drawings for a more complete understanding of the invention and of the plating methods as hereindisclosed and claimed. It will be unnderstood that although not limited thereto, these plating methods are applicable to the plating of titanium and its alloys with such metals, and combinations in laminar form, of such metals as Ag, Au, Cu, Ni, Sn, In, Zr, Zn, Cr, Co, Cd, Fe, Rh, Sb, and other metals.

In the aforesaid copending application of I. R. Woodward, Ser. No. 765,156, for example, a dual laminar plating of tin and silver, plated by the methods of this invention, is described, and a micrograph is disclosed of the resulting bonded structure using that particular laminar plating produce a micromelt and eutectic in the brazing and diffusion bonding cycle which is also described. Accordingly, reference may be had to that application for further details regarding the brazing-diffusion bonding process and the utility of the laminar plating as a subsystem of that process. It suffices herein to state that in the tin-silver example, aforesaid, the honeycomb core and its facing plates were formed of commercially pure (C.P) titanium and the edge faces only of the core received the laminar plating. The tin plating was applied first and constituted about 3.1% by weight of the laminar plating; the silver being plated next, constituted about 96.9% of the total laminar Weight which increased the weight of the panel structure by 0.027 gram per square inch, or 3.89 grams per square foot. Other laminar combinations provided by these brazing methods, in practice, have been found to add weight ranging from 1-5 grams/ft}.

The titanium panel components to be joined by the aforementioned combined brazing-diffusion bonding process which, as aforedescribed, involves the plating of the edge faces of the core as herein described, are first cleaned by immersion for 1-5 minutes in the cleaner bath of FIG. 1 which is operated at ambient room temperature of the order of 70 to F. and wherein the core 10 is shown immersed. This cleaner bath is used to remove oils, soils, mill marks, fingerprints, and the like, and any suitable cleaner composition may be used for the purpose, Brantner 1315 being commercially available and is preferred. Brantner 1315 has substantially the same composition as disclosed and claimed in the aforesaid Pat. No. 3,379,645 of Earl W. Kendall, which patent is assigned to the assignee of the instant application.

The preferred cleaner bath is non-aqueous and consists of from 15-25 grams of chromic acid (CrO for each 175-225 grams of sulfuric acid (sp. gr. 1.84). The preferred composition of this bath consists of 10 grams of powdered chromic acid (CrO for each mls. of concentrated sulfuric acid (sp. gr. 1.84).

The titanium panel components are next subjected to the action of the electrolytic bath disclosed in FIG. 2 wherein the surface-cleaned core 10, for example, is disclosed as immersed therein. This bath is basically a descaling solution effective to remove pyrolytic refractory oxides from the surfaces of titanium and ferrous alloys including such metals as nickel, chromium, cobalt, tungsten, vanadium, and molybdenum. It is available commercially as Brantner 2226 and is substantially the same composition as disclosed and claimed in copending patent application Ser. No. 816,460, filed Feb. 28, 1969 as a division of the aforementioned copending application of Earl W. Kendall for Electrolytic Descaling of Titanium and Its Alloys, Ser. No. 600,362, now Pat. No. 3,468,774, which applications are assigned to the assignee of the instant application.

This descaling solution is non-aqueous and is operated at ambient temperature of the order of 70 to 90 F. and

consists of the following constituents and proportions thereof by volume of the bath:

Percent Acetic acid (glacial) 55-70 Sulfuric acid (concentrated) 20-30 Hydrofluoric acid (48-50%) 10-15 Inhibitor (amide-acetylenic alcohol mixture) 0.3-0.5 Wetting agent (anionic surfactant) 0.2-0.3

The sulfuric acid serves as a buffer to limit the metal attack of the hydrofluoric acid on the metallic articles in the bath, particularly in the event of an inadvertent increase in the current flow. The inhibitor, on the other hand, limits the hydrofluoric acid attack in the event the current is inadvertently discontinued. The inhibitor for this purpose is a mixture of amides and acetylenic alcohol in which the amides consist of alkylformamides having the general structure which is also often expressed as o O-t J-NH:

of which the alkyl group approximates the C range. Such amides are available commercially as Primid F-12 of Rohm & Haas Company of Philadelphia, Pa., the formula of which is defined principally as This inhibitor mixture preferably is composed of 65% by volume of the amides and 35% by volume of propargyl alcohol.

A preferred wetting agent is Benax 2A1 of Dow Chemical Company, of Midland, Mich. This is an anionic surfactant completely effective in a high acidic medium and identified as dodecylated oxydibenzene disulfonate sodium salt.

As disclosed in FIG. 2, the titanium part, for example, core 10, is the anode in the electrolytic system, the positive lead 11 being connected thereto. The cathode 12, which is immersed in the descaling bath, is connected to the negative lead 13. The cathode electrode 12 may be formed either of carbon or copper.

The descaling bath is used to prepare the titanium panel components for the brazing-diffusion bonding process aforesaid, and provides the requisite stringent surface clean conditions required for these brazing and solid state difl'usion operations. The descaling bath also serves to activate the core 10 to render the same receptive to the plating operation, subsequently to be described. Thus, the panel facing skins are subjected to the electrolytic action of the bath of FIG. 2 by immersion therein for 5 to minutes with 2-3 volts applied across the input leads 11*" and 13 The core 10, which must additionally be sufliciently activated to receive and hold the plating to be applied to its edge faces, is immersed in the activator bath of FIG. 2 for 5 to 10 minutes with 4-5 volts applied across the input lead 11" and 13 Prior to plating the activated core 10, it is preferred to brighten the surfaces thereof by immersion in the bath of FIG. 4 wherein the core 10 is shown immersed. This bath is effective to remove any smut or other surface conditions which may result from the action of the activator bath. Any brightener suitable for the purpose may be used. Brantner 2226B is commercially available and is preferred. Brantner 22263 is substantailly the same composition as disclosed and claimed in the aforementioned Pat. No. 3,228,816 of Earl W. Kendall, which patent is assigned to the assignee of the instant application. This brightener solution is an aqueous bath and operates for purposes herein at ambient room temperature 6 of the order of 70 to F. The bath consists of the following constitutents and proportions thereof per mls. of aqueous solution:

bifiuoride 1% by weight of the aqueous solution.

The core 10 is now ready for plating of the edge facings thereof by the electroplating system of FIG. 3 wherein it will be understood that the conventional commercially available plating electrolyte has the requisite composition to supply at the core faces the particular metal to be plated thereon. The plating method disclosed is similar to the well known Dalic or Selectron brush type plating process, and the current and voltage supply and controls afforded by that process conveniently may be employed to energize the leads 14 and 15 of FIG. 3 as required herein to practice the plating process of this invention.

For purposes of this invention, as disclosed in FIG. 3, lead 14" is connected to the anode electrode 16 which is immersed in the plating electrolyte and is insoluble therein. Lead 15 is connected directly to core 10 which is mounted over and above a roller 17 for reciprocal rolling movement of its edge faces therealong parallel to the surface of the bath as depicted by the arrows in FIG. 3. Roller 17 is immersed in the bath to a depth therein not to exceed the the order of 75% of its diameter.

Roller 17 comprises a tube formed of polyvinyl chloride (PVC) which is wrapped with a single layer 18 of gauze such as tube gauze manufactured by the Dalic Plating Process Company, the ends of the tubing gauze being tucked within the PVC tube. A glass tube 19 which is about /2 the diameter of the PVC tube is inserted therein and provides the roller bearing support for the roller, the ends of glass tube 19 being journalled in holes or notches (not shown) provided in the sidewalls of the plating tank 20.

Rolling movement of core 10 on roller 17 causes the roller to rotate and move the electrolyte picked up thereby to the core face in contact therewith, thereby to cornplete the current path from the electrolyte to the core. Since the core is mounted for rolling movement above the roller, the electrolyte tends to move by gravity away from the core to thus minimize any tendency of the electrolyte to move by capillary action or otherwise up and along the cell Walls where plating is not desired. The plating is thus limited or restricted to the edges of the core where it is desired and where the thickness of the plating is determined by the time and current density employed.

An alternative plating system and process is disclosed in FIGS. 5 to 8 wherein the core 10' is assumed to have been covered with a maskant such as Micro-Stop 01f lacquer produced by Michigan Chrome and Chemical Co. of Detroit, Mich. The core is coated by immersion of the same in this maskant followed by baking in an oven at 220 F. for 2-5 minutes. Dipping and baking is repeated to provide three such operations, the maskant being thinned down about 50% with solvent, as required. Any suitable solvent may be used with the maskant, a ketone such as methyl ethyl ketone (MEK), acetone, or methyl isobutyl ketone (MIBK) being preferred.

FIG. 5 discloses a method of removing the maskant from the edge faces of core 10' to expose the same for plating, it being understood, as before, that core 10 had been cleaned, as in the bath of FIG. 1, prior to application of the maskant.

The roller 17 of FIG. is constructed and mounted in substantially the same manner as roller 17 of FIG. 3. Tank 21, however, is filled with solvent such as the aforementioned MEK. Movement of core over roller 17' removes the maskant from the face of the core as the solvent is picked up by the roller and moved to contact the core. Both faces of the core are thus exposed, and the solvent is permitted to evaporate to seal off the exposed edges of the core. The baking of the maskant enables the same to resist the attack of the activator thereon, the core 10' with its exposed faces now being subjected to the action of the activator bath of FIG. 6. In this instance, only the exposed edges of core 10' are activated so that the core may be plated conventionally by immersion, as disclosed in FIG. 7.

It will be understood that the operation of the activator bath of FIG. 6 is identical to that of FIG. 2, as aforedescribed. It will also be understood that when additional plating layers of different metals are to be added to the faces of core 10 and 10, that is simply becomes necessary only to change the electrolytes in the tanks of FIGS. 3 and 7, as required, to provide the particular metallic platings desired, such, for example, as the aforementioned second layer of silver which was applied to a first layer of tin.

FIG. 8 represents the bath by which the maskant is removed following completion of the plating operation, the solvent being one of the aforementioned ketones. Alternatively, the maskant may be removed in a series of such solvent baths which may utilize one or more different ones of the ketones mentioned.

From the foregoing, it should now be apparent that methods and baths have been provided for the activation and plating of the surfaces of objects formed of titanium and its alloys and, whereas the invention has been described with reference to certain examples thereof, the same is not to be considered as being limited thereto. For example, the surfaces may be activated before the maskant is applied, the roller-plating method may be used even though the maskant and roller-solvent exposure method is used, and metallic alloys including titanium as well as titanium itself may be plated to provide the micro melt materials and ultimate diffusion bridge. The laminar plating may be used exclusively in a brazing process, honeycomb structures formed of metals other than titanium may be plated by the hereinvolved methods, and other honeycomb panel parts such as edge members as well as other non-panel parts may be plated by these methods.

The novel principles of this invention transcend the scope of the invention as suggested or implied by the several embodiments hereinbefore described, and the invention may be embodied in other forms or carried out in other ways which have been conceived and reduced to practice during the course of this development, without departing from the spirit or essential characteristics of the invention. The embodiments disclosed herein therefore are to be considered as in all respect illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Having thus described my invention, what I claim as new and useful and desire to secure by US. Letters Patent is:

1. The electrolyte method of activating the surfaces of a titanium object preparatory to electroplating the same which consists of immersing the titanium object in a non-aqueous electrolytic bath with 4-5 volts applied for 5-10 minutes to the titanium object as the anode and to a cathode electrode immersed in the electrolyte, said electrolytic bath consisting of the following constituents and proportions thereof by volume of the bath:

Percent Glacial acetic acid 55-70 Concentrated sulfuric acid 20-30 Hydrofluoric acid (48%-50%) 10-15 Amide-acetylenic alcohol inhibitor mixture 3.0-0.5 Anionic surfactant wetting agent 0.2-0.3

solution:

Urea grams 0.1-1.0 Nitric acid (42 B.) (1.42 sp. gr.)

mls-- 8-12 Boric acid grams 4-6 Ammonium bifluoride --do--- 1-1.5

2. The method as in claim 1 wherein the cathode electrode is formed of carbon.

3. The method as in claim 1 wherein the cathode electrode is formed of copper.

4. The method as in claim 1 and including the initial step prior to activation of the titanium surfaces of cleaning the same by immersion of the titanium object for 1-5 minutes in a non-aqueous chromic acid sulfuric acid cleaner consisting of from 15-25 grams of chromic acid (C10 for each 175-225 grams of sulfuric acid (sp. gr. 1.84).

5. The method as in claim 4 wherein the preferred cleaner composition consists of 10 grams of powdered chromic acid (CrO for each mls. of concentrated sulfuric acid (sp. gr. 1.84).

6. The method as in claim 1 and including the steps of electroplating the activated brightened titanium surfaces.

7. The method as in claim 1 in which the brightener composition consists of:

aqueous solution.

8. The method as in claim 7 in which the plating electrolyte composition includes the metal to be plated on the titanium object, the object is the cathode in the electroplating system, and the anode is immersed in the plating electrolyte and is insoluble therein.

9. The method as in claim 8 in which the plating is applied to the object by a roller partially immersed in the electrolyte.

10. The method as in claim 9 in which the object to be plated is titanium honeycomb core of which a face thereof is moved over that portion of the roller which extends above the surface of the electrolyte.

11. The method as in claim 8 and including the initial steps prior to activation of the surfaces to be plated of masking the entire surface area of the titanium object, and removing the maskant from the surface area to be plated, and including the step of removing the remaining maskant subsequent to plating the titanium object.

12. The method as in claim 11 in which the maskant is baked to resist attack by the activator bath.

13. The method as in claim 11 in which the maskant is removed by application of a solvent.

References Cited UNITED STATES PATENTS Missel et a1. 204-32 Missel 204-32 Boam et a1. 204-38 Thomas 252-793 Tao 204-141 10 Covington 204-141 Kendall 252-793 Kendall 252-792 Kendall 204-141 Kendall 252-792 Kendall 204-141 DANIEL E. WYMAN, Primary Examiner P. E. KONOPKA, Assistant Examiner US. Cl. X.R.

UNITED S'IA'IES PATENT 0mm CERTIFICATE OF CORRECTION Patent No. 3,817,844 Dated June 18, 1974 Inventor(s) Earl W. Kendall It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, in the Abstract, line 2, "alloy" should read --alloys-- Column 8, line 4 (claim 1) under the title "Percent", "3.0-0.5" should read --0.3-0.5--.

Column 8, line @(claim 6), "steps" should read --step--.

Signed and sealed this 8th day of October 1976,

(SEAL) Attest:

MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner of Patents powso USCOMM-DC 60376-F'69 1* US GOVERNMENT PRINTING OFFICE "I! D8l6-3Jl.

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