US3063918A - Method of making electric conduit with alloyed coating - Google Patents

Description

Nov. 13, 1962 F. J. KENNEDY v 3,063,913

METHOD OF MAKING ELECTRIC CONDUIT WITH ALLOYED COATING Original Filed Oct. 29. 1957 OPTIONAL CONVERSION I COATING ALKALI ELECTROPLATING ACID ELECTROPLATING FER ROUS METAL ALLOYED INTERFACES ELECTROT'LATING ACID ELECTROLYTE OUTER ELECTROPLATING ALKALI ELECTROLYTE PROTECTIVE comma CONVERSION SOLUTION (OPTIONAL I RAPID HEATING TO CRITICAL TEMPERATURE RANGE HOLDING WITHIN CRITICAL RANGE FOR ALLOYING PERIOD ENAIIELLING INVENTOR.

AT TORAIE Y3 3,063,918 Patented Nov. 13, 1962 3,053,918 METHOD F MAKENG ELETRIC CONDUIT WITH ALLOYED CGATHNG Frank J. Kennedy, Mount Lebanon, Pittsburgh, Pa., assignor, by mesne assignments, to H. K. Porter Company, Inc., Pittsburgh, Pa, a corporation of Delaware Original application Oct. 29, 1957, Ser. No. 693,154. Dr-

vided and this application Sept. 11, 1958, Ser. No. 760,313

' 7 Claims. (Cl. 204-67) This invention relates to methods for the application of protective zinc coatings to ferrous bodies, particularly conduits, and more especially to electric conduits which are often required to be bent around short radii when installed in buildings.

The most effective of the commercially practical processes for applying zinc coatings to conduits, prior to the present invention, has been the process of sherardizing. In that process pipes or conduits are tumbled in large retorts containing zinc dust and at temperatures which cause alloying of the ferrous base metal with the zinc dust. The assumption is made that the tumbling action allows a uniform distribution of the dust throughout the process.

In actual practice, the unbalanced distribution of the zinc dust is so serious that there is a 3 to 1 coating ratio at different areas along the length of the conduit; and a coating ratio of 5 to l is not unusual. With this nonuniformity of the coating, even when great care is exercised, some of the product has weak spots, where the minimum protection is not provided, whereas other areas carry wasteful quantities of the coating metal.

Another weakness of the sherardizing process is the unavoidable contaminating of the pure Zinc dust by iron dust through the initial phases of the tumbling operation. This iron dust is reabsorbed by the coating in the later stage of the coating process, making it impossible to provide a pure zinc outer layer which would be the most effective for corrosion protection.

More uniform distribution of a protective zinc coating has been applied to ferrous strip material by electro-depositing of the zinc on the ferrous metal, as disclosed in the Nachtman Patent No. 2,357,126 dated August 29, 1944, but the process of that patent is apparently obsolete for coating strip, and it is entirely unsuited for making conduit because of the manner of heat treating and the smooth surface which is unsatisfactory for holding paint or lacquer.

Many other processes have been devised for obtaining uniform protective coating but prior to the present invention, the electro-deposited coatings did not obtain the adherence required for sharp bends, or had other unaccepta able defects in the quality of the surface of the zinc, and sherardizing remained the most effective process in spite of its non-uniform coating.

One of the serious disadvantages of sherardizing has been the large amount of labor and equipment required, and the resulting high cost.

It is an object of this invention to provide an improved, and at the same time more economical, process for alloying zinc to the base metal as a tenaciously adhering coating capable of sharp bends without flaking off.

Another object is to provide an improved method for making electric conduit with the zinc that contacts with the ferrous metal deposited from an acid electrolyte.

An initial layer of zinc deposited from an acid electrolyte of high efiiciency prevents the absorption of hydrogen by the ferrous base metal. Subsequent zinc can then be deposited in an alkaline electrolyte, and the underlying Zinc protects the ferrous metal from absorption of hydrogen generated in the alkaline electrolyte.

Another advantage of this invention is that it applies a purer coating of zinc. With sherardizing, the zinc coating applied to the ferrous metal is not as pure as with the present invention. It is necessary, in sherardizing, to deposit a much thicker coating in order to meet the same corrosion-resistant specification, for example: four oneminute immersions in copper sulphate. With the present invention, 99.9% pure zinc is deposited electrolytically, then heat treated, and experience has shown that the coating of this invention, when only one half the thickness of those applied by the sherardizing process, will give the same resistance to corrosion. This results in a large savings of materials, labor, and maintenance of equipment, and the thinner coating bends without flaking.

Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.

In the drawing, forming a part hereof, FIGURE 1 is a perspective view of a ferrous conduit coated in accordance with this invention;

FIGURE 2 is a greatly enlarged, fragmentary sectional view through a portion of the conduit shown in FIG- URE l; and

FIGURE 3 is a flow sheet showing the order of steps in carrying out applicants invention.

The invention will be described as applied to the coating of conduits such as are used for electrical wiring systems, but it will be understood that the invention can be applied also to any ferrous metal articles for the purpose of obtaining a corrosion-resistant surface.

Where conduits are to be treated in accordance with this invention, the first step is a thorough cleaning of the surfaces. This may be done by pickling in sulphuric acid to remove any rust and scale, this being a conventional method of cleaning metal parts.

After pickling, the conduits are cleaned electrolytically in order to obtain better adhesion of the zinc coating. This electrolytic cleaning is done in an alkaline solution with the work pieces as the anode. Such cleaning is known in the art as anodic cleaning.

After having their surfaces thoroughly cleaned, the conduits are electroplated with zinc for a thickness of at least 0.00001 inch. A thicker coating can be applied but the coating should be at least this much. This first step of plating with zinc is done in an acid electrolyte.

The advantage of using the acid electrolyte is the substantial elimination of hydrogen. The acid electrolyte isv from 99% to 100% eificient in its use of the current to deposit metal and there is, therefore, very little or no hydrogen generated at the zinc-ferrous metal interface. If hydrogen is present, it is absorbed by the ferrous metal and when the work piece is heated quickly for subsequent heat treatment, the sudden expansion of the hydrogen forces the zinc away from the ferrous metal surface and destroys adhesion.

It is possible to drive absorbed hydrogen out of the ferrous metal by slow heating, as is done for eliminating hydrogen embrittlement of metal parts, but such heating takes hours of time and would greatly increase the cost.

The initial electroplating step can be done in a standard zinc sulphate electrolyte containing 24 to 48 ounces of (ZnSO (7H O)) per gallon and 4 ounces of ammonium chloride per gallon. Better results are obtained if from three to five ounces of sodium fluoride are added to each gallon of the electrolyte. The pH of the electrolyte should be between 3.2 and 4.5.

Current densities between 30 and 100 amperes per square foot have been used successfully. The current density does not appear to be critical. The full thickness of the zinc coating required by the conduit specifications can be applied in this initial plating step; but the porosity of the coating is reduced and better results are obtained 3 with a thin coating if only part of the zinc is applied in the first step and the remainder applied in a subsequent electroplating step which is done in an alkali zinc cyanide type of plating solution. The percentage of the total coating which is put on in each electroplating step can be varied, but it is necessary to put on the first 0.00001 inch of the zinc in the acid electroplating solution.

The outstanding advantage of the alkali zinc cyanide electrolyte is that the zinc can be deposited three or four times as fast as with the acid electrolyte because of the greater current densities that can be used. The efiiciency of the cyanide bath is comparatively low and may run from 90% to 95%, leaving a substantial part of the current generating hydrogen; but the hydrogen is not absorbed to any substantial extent by the acid-deposited zinc, as previously explained. Instead of a zinc cyanide solution, the second plating step can be carried out with a pyrophosphate electrolyte; or with a zincate bath of caustic soda, zinc hydroxide and water. These plating electrolytes are well known in the art.

It will be understood that the thickness of the zinc coating employed is a matter which is determined in some cases by the specifications of the conduits as ordered by the customer. Where conduits are to be used under conditions which are particularly severe for corrosion, heavier coatings of zinc may be specified; but the application of such zinc coatings would be uneconomical and unnecessary for ordinary uses. With this invention, zinc coatings between 0.00025 and 0.003 have been found to be sufficient for the services in which electrical conduits are ordinarily used. Other metal parts subject to similar conditions of exposure and atmosphere are protected by similar thicknesses of coating on the base metal.

The zinc is fused during its heating to the alloying temperature, but because of the thin coating of zinc on the base metal, the fusion causes no running of the zinc and the highly uniform distribution of zinc, resulting from the electro-depositing, is not disturbed by the fusion. One of the reasons for the improved results obtained with this invention is the uniform thickness of the zinc coating; the variation being not over as compared with the variations of at least 3 to 1 and sometimes 5 to 1 in the zinc coatings applied by the best sherardized coatings of the prior art.

With the thinner coatings used for this invention, there is not only a substantial saving in the cost of the coating material, but the coating is more adherent when subjected to bending, and the corrosion resistance of work pieces coated by this invention is twice as good as the sherardized coatings.

Another advantage if this invention is that if the Underwriters Specification should be changed to the basis of weight of zinc coating instead of the present basis of the Preece test (which change seems to be more or less expected in the not-too-distant future), the new specification can be more easily met by the new coating than by the sherardized coating.

After the electroplating has been completed, the zinc can be given a treatment to reduce oxidation during the heating step. When this step is to be included, the conduits can be washed in a solution containing from 3 to ounces per gallon of sodium di-chromate (Na Cr O with sufficient nitric acid to bring the pH to a value of from 0.5 to 1.5. The Wash can be carried out at room temperature and it forms a coating of zinc chromate on the outside surfaces of the conduits. After this treatment, the conduits are rinsed in water and allowed to dry.

This zinc chromate coating is merely representative of an inert coating for preventing oxidation of the underlying zinc during the heat treatment. Other washes can be used in place of the sodium di-chromate to produce other inert coatings, such as a Zinc'phosphate coating which is also resistant to oxidation. These treatments for zinc are well understood in the art, but have not been used in the same combination as in the process of this invention.

The zinc'chromate and zinc phosphate coatings are known as conversion coatings";"but other protective boatfor preventing oxidation of the surface of the zincis suitable so long as it does not adversely affectthe zinc beneath it; but the process of this invention can be carried out without any coating to protect the zinc, if the initial deposit of zinc on the conduits is sufficient to compensate for some loss by oxidation during the heating step.

In the preferred and commercial operation of this invention, no conversion coating is used. It is desirable to have a slight oxidation of the surface of the zinc because the zinc is fused during the heat treatment and after solidification of the zinc coating, a matte surface is obtained if there has been some oxidation of the surface.

This matte surface is much more desirable for subsequent enameling of the pipe. Fused Zinc with no oxidation hardens to a smooth shiny surface less suitable for enameling. It is important, however, to control the oxidation accurately to avoid formation of substantial quantities of zinc oxide in the form of loose powder on the surface. This powder represents a waste of zinc.

Excessive oxidation of the zinc coating is prevented with this invention by using a two step heating method. Several considerations are involved. One is that oxidation depends to a large extent upon the time that the surface remains at an elevated temperature. Others are that the temperature must be kept within a relatively narrow critical temperature range during the heat treating; and the time during which the metal is held within the critical range must be accurately controlled.

In order to reduce the heating time and to avoid overheating of the metal, the conduits are first exposed to a high temperature, preferably in a furnace with radiant gas burners, to bring them into or near the critical temperature range quickly. The conduits are then moved into a second heating zone where they are held within the critical range for a definite length of time. The second heating zone is preferably also a furnace with radiant gas burners, but at a lower temperature than the first furnace since the puipose of the second heating zone is to hold the conduits within the critical temperature range, rather than to add heat. In the practical operation of the invention there is usually some increase in temperature in the second or holding furnace but always within the limits of the critical range.

Furnace heating has the advantage that it can be conveniently used with a conveyor for moving the conduits. Various heating methods can be used, such as induction heating. The conduits are held in the critical temperature range for the length of time necessary to obtain the amount of alloying desired. The critical temperature range is between 800 and 950 F. and best results are obtained between 850 and 900 F.

Alloying begins below 800 but is too slow below this temperature for practical work. A- zinc coating which shows an initial alloying in ten minutes at 800 holding temperature, and an intermediate alloying in 15 minutes, and which is alloyed completely through the full zinc coating in 20 minutes, will alloy completely in 7 minutes at 900 holding temperature. In the preferred heating step of this invention, the conduits are heat-treated at a holding temperature of between 850 and 900 for a period of from 4 to 9 minutes, the time being somewhat longer when the temperature is toward the lower end of this holding temperature range. These time periods provide a substantial amount of alloying of the zinc with the base metal without permitting the alloying to extend completely through the zinc coating. These examples permit the alloying to extend through the acid plated layer of the zinc, and it is preferred to have the alloying extend through this inner layer in all cases. The alloying is prevented from extending through the full thicknessof themetal by cooling the metal below the alloying temperature range as soon as the alloying has proceeded to the intended depth.

The oxidation of the surface uniform in actual operations because different portions of the conduit absorb heat more rapidly, the color of the electroplated surfaces not being completely uniform. Darker areas absorb heat faster than lighter ones. Although it is possible to control the process so as to have uniform heating and uniform oxidation of the zinc surface, it is more economical to heat the conduits rapidly and with some lack of uniformity, and to include in the process a brushing step for wiping any loose zinc oxide from the conduits so as to obtain a uniform appearance over the entire surface. When the process of this invention is operated with some variations in temperature at different parts of the conduit surface, the oxidation is controlled so that there is some oxidation of the entire surface; and if there is excessi e oxidation on any part of the surface, resulting in loose white Zinc oxide powder, this excess is brushed off, where uniform color is desired prior to the coating of the conduit with clear lacquer. The slightly oxidized surface is a matte gray, but the excessively oxidized areas turn white because of the formation of loose powder consisting of Zinc oxide. The degree of oxidation depends upon temperature, time, and atmosphere. The control of furnace atmospheres to control oxidation is well understood in the heat treating art.

The final step in the manufacture of conduits is the coating of the metal with lacquer. If a gray, black, or other colored lacquer is to be used, brushing of the conduits is unnecessary, because the lacquer hides any variations in the color of different parts of the surface.

This application is a continuation-in-part of my copending patent application, now abandoned, Serial No. 464,651, filed October 25, 1954, for Method of Producing Alloyed Coating on Fibrous Conduits, and this application is also a division of my co-pending application, Serial No. 693,154, filed October 29, 1957.

The preferred embodiment of this invention has been illustrated and described, but changes and modifications can be made and some features can be used in different combinations without departing from the invention as dufined in the claims.

What is claimed is:

1. In the application of protective zinc coating to the surface of a ferrous metal body, the improvement which comprises electroplating the ferrous metal with the zinc in an acid electrolyte and for a thickness of between 0.00001 inch and 0.003 inch, alloying the zinc to the base metal by heating the metals to a temperature between 800 and 950 F., producing a roughened surface on the zinc by subjecting it to contact with an atmosphere containing a reactive percentage of oxygen, and cooling the metal body as soon as the alloying has progressed through the desired thickness of the Zinc.

2. The application of a protective zinc coating as do scribed in claim 1 and in which the zinc is fused during the heat treatment.

3. The method of applying a protective coating of zinc to a ferrous metal body including electroplating a layer of zinc, from an acid electrolyte, on the surface of the body for a thickness of between 0.00001 inch and 0.003 inch, limiting the thickness to a at about 900 F., heating the coated body quickly to a temperature range between 800 and 900 F. to fuse the zinc and cause alloying of some of the zinc with the terrous base metal, subjecting the zinc during the heating step to an atmosphere containing a reactive percentage of oxygen to produce a rough surface on the zinc when it hardens after fusion, and cooling the metal body below 800 F. after an alloying period of from four to nine minutes.

of the zinc is not usually 4. In the application of a protective coating of Zinc to ferrous base metal, the method which comprises electroplating the surface to be protected with a coating of zinc applied in an acid plating solution having a pH between 3.2 and 4.5, continuing the plating until the zinc has reached a thickness of between 0.00001 inch and 0.003 inch, and thereafter heat-treating the zinc coating by raising it to a temperature of about 850 to 900 F, holding the temperature while the Zinc diffuses into and alloys with the base metal, roughening the surface for better adherence of enamel by subjecting the surface to contact with an atmosphere containing a reactive percentage of oxygen, and cooling the zinc and base metal before the alloying proceeds through the full thickness of the zinc coating.

5. in the application of a protective coating of Zinc to a ferrous metal work piece, the method which comprises initially pickling the work piece to clean it, thereafter subjecting it to anodic electrolytic cleaning of the surfaces to be coated, applying a coating of Zinc to the cleaned surfaces by electroplating in an acid electrolyte until the thickness of the zinc plating reaches 0.00001 inch, thereafter adding additional zinc plating to the work piece, over the initial Zinc plating, by electroplating in an alkali electrolyte to a total thickness within a maximum limit of 0.003 inch, and subsequently heat-treating the work piece at a temperature between 800 and 950 F. which alloys the base metal with at least a portion of the thickness of the zinc which was applied in the acid solution and roughening the surface for better adherence of enamel by subjecting the surface to contact with an atmosphere containing a reactive percentage of oxygen.

6. In the application of a protective layer of zinc to a ferrous metal work piece, the method which comprises electroplating the work piece W'th Zinc in a zinc sulphate electrolyte containing approximately four ounces of sodium fluoride per gallon of electrolyte and some free acid to produce a pH of about 3.2 to 4.5, carrying out the electroplating with the temperature of the electrolyte less than about F., and continuing the electroplating to a coating having a thickness of between 0.00001 inch and 0.003 inch, and thereafter heat-treating the Zinc coating at an alloying temperature within the range of 850950 F. to alloy the zinc to the base metal for a portion, and only a portion, of the thickness of the coating and roughening the surface for better adherence of enamel by subjeeting the surface to contact with an atmosphere containing a reactive percentage of oxygen.

7. The method of applying a zinc coating to a ferrous metal work piece, as described in claim 6 and in which a second coating of zinc is applied over the first coating with the second coating electroplated in a zinc cyanide plating solution and to a total depth of plating of between 0.0005 to 0.003 inch, and in which the heat-treating of the zinc coating is carried out after the second electroplating step and at a temperature between 850 and 900 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,307,853 Dimm June 24, 1919 1,567,625 Smith Dec. 29, 1925 2,056,399 Hochwalt et a1. Oct. 6, 1936 2,357,126 Nachtman Aug. 29, 1944 2,665,232 Neish J an. 5, 1954 2,719,820 Allen Oct. 4, 1955 OTHER REFERENCES Gray, Modern Electroplating, 1953, pp. 444-452. Transactions Electrochemical Society, vol. 78 (1940), pp. 317327.

Diggin, Metal Industry, June 25, 1943, p. 408.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pa tent No. 3,063,918 November 13, 1962 Frank J, Kennedy It is hereby certified that error appears in the above numbered patant requiring correction and that the said Letters Patent should read as @orrected below.

Column 5, line 59, after "treatment," insert claims 8 and n1 as follows:

3, The application of a protective zinc coating as described in claim 1 and in which the alloying of the zinc and base metal is performed by maintaining the zinc and ferrous metal at a holding temperature between 850 and 900 F.

4. The application of a protective zinc coating as described in claim 1 and in which the metals are held at a temperature between 800 and 950 F0 in an atmosphere containing oxygen, and the metal is cooled below 800 F. before the alloying of the ferrous metal proceeds outwardly far enough to meet the oxidized layer that develops on the outside of the zinc,

tolumns 5 and 6, claims numbered 3 to 7 should be renumbered to appear as claims 5 to 9; in the heading to the printed Specification, line 10, for "7 Claims," read 9 Claims.

Signed and sealed this 21st day of May 1963.

IjIEIEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD attesting Officer Commissioner of Patents

Claims (1)

1. IN THE APPLICATION OF PROTECTIVE ZINC COATING TO THE SURFACE OF A FERROUS METAL BODY, THE IMPROVEMENT WHICH COMPRISES ELECTROPLATING THE FERROUS METAL WITH THE ZINC IN AN ACID ELECTROLYTE ANS FOR A THICKENESS OF BETWEEN 0.00001 INCH AND 0.003 INCH, ALLOYING THE ZINC TO THE BASE METAL BY HEATING THE METALS TO A TEMPERATURE BETWEEN 800* AND 950*F., PRODUCING A ROUGHENED SURFACE ON THE ZINC BY SUBJECTING IT TO CONTACT WITH AN ATMOSPHERE CONTAINING A REACTIVE PERCENTAGE OF OXYGEN, AND COOLING THE METAL BODY AS SOON AS THE ALLOYING HAS PROGESSED THROUGH THE DESIRED THICKNESS OF THE ZINC.

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