US2994126A - Ferrous metal body with alloyed zinc coating - Google Patents

Description

Augv 1, 1961 F. J. KENNEDY 299949326 FERROUS METAL BODY WITH ALLOYED ZINC COATING Filed on. 29, 1957 OPTIONAL CONVERSION COATING ALKALI ELECTROPLATING ACID ELECTROPLAT ING FER ROUS METAL ALLOYED INTERFACES ELECTROF'LATING ACID ELECTROLYTE OUTER ELECTROPLATING IN ALKALI ELECTROLYTE PROTECTIVE COATING CONVERSION SOLUTION (OPTIONAL I RAPID HEATING T0 CRITICAL TEMPERATURE RANGE HOLDING WITHIN CRITICAL RANGE FOR ALLOYING PERIOD l COOLING i BRUSHING I ENAMELLING I INVENTOR.

n7- TaRA/E Y5 United States Patent 2,994,126 FERROUS METAL BODY WITH ALLOYED ZINC COATING Frank J. Kennedy, Mount Lebanon, Pa., assignor, by

mesne assignments, to H. K. Porter Company, Inc., Pittsburgh, Pa., a corporation of Delaware filed Oct. 29, 1957, Ser. No. 693,154

3'Claims. (Cl. 29'196.5)

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 dilferent areas along the length of the conduit; and a coating ratio of S-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 electrodepositing of the zinc on the ferrous metal, as disclosed in the Nachtman Patent #2,357,l26, 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 unacceptable defects in the quality of the surface of the Zinc, and sherardizing remained the most effective process in spite of its non-uniform coating.

The most serious disadvantage 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 product with electro-deposited zinc alloyed to the base metal as a tenaciously adhering coating capable of sharp bends without flaking off.

Another object is to provide a conduit with a zinc coating alloyed to a ferrous base metal and with at least a semi-matte outside surface to which paint or lacquer is adherent.

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.

In the description hereof, the expression acid electrolyte zinc is used to describe the zinc which is deposited on the base metal by electroplating in an acid electrolyte; and the expression alkali electrolyte zinc is used to describe the zinc which is deposited on the base metal by electroplating in an alkali 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 specifications, for example: four one-minute immersions in copper sulphate. With the present invention, 99.9% pure zinc is deposited electrolytically, 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 drawings, 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 FIGURE 1; 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 obtianing 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. a

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 is from 99% to 100% eflicient 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 embrittlernent 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 lying zinc during the heat treatment.

3 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 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 10%, 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 of this invention is thatif 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 dichromate (Na Cr O with sufficient nitric acid to bring the pH to a value offrom 0.5 to 1.5. The wash can be carried out at room temperature and it forms acoating of zinc chromate on-the outsidesurfaces of the conduits. After this treatment, the conduits are rinsed'in water and allower to dry.

This'zinc chromate coating is merely representative of an inert coating for preventing oxidation of the under- 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 coatings can be used, such as a sodium silicate water solution, or a permanganate coating, the latter obtained by dipping the conduits in potassium permanganate. Any coating for preventing oxidation of the surface of the zinc is suitable so long as it does not adversely affect the 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 sufiicient 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 dependsto 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 purpose 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 ofbetween 850 and 900 for aperiod 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 thickness of the metal 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 of the zinc is not usually 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 conduit 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 diflerent parts of the conduit surface, the oxidation is controlled so that there is some oxidation of the entire surface; and if there is excessive 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 in 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 difierent parts of the surface.

This application is a continuation-in-part of my copending patent application, Serial No. 464,651, filed October 25, 1954, now abandoned, for Method of Producing Alloyed Coating on Fibrous Conduits.

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 6 i l combinations without departing from the invention as defined in the claims.

What is claimed is:

1. An article of manufacture including a body composed of ferrous metal that is to be exposed to corrosive atmosphere, the body including a corrosion resistant surface formed by a coating of zinc which has been applied by electro-depositing an initial layer of the zinc on the ferrous metal from an acid electrolyte and for a thickness of at least 0.00001 inch, and an outer layer of zinc which has been electro-deposited on the initial layer from an alkaline electrolyte to a total thickness of zinc at least 0.0005 inch, and in which the zinc is bonded to the ferrous metal by heat treating of the body after the two layers of zinc have been deposited thereon, the bonding being obtained by heating the body to a temperature between 800 and 950 F. to alloy the zinc and ferrous metal but the time of heat treatment being less than that required for the ferrous metal to alloy with the full thickness of the combined layers of the zinc coating.

2. The article of manufacture described in claim 1, and in which the body is a conduit and the portion of the zinc coating which is alloyed to the body extends radially at least as far as the thickness of the acid electrolyte zinc.

3. The article of manufacture described in claim 1, and in which the body is a conduit and the outer layer of zinc is thicker than the inner layer.

References Cited in the file of this patent UNITED STATES PATENTS 1,104,842 Smith July 28, 1914 1,552,040 Fowle Sept. 1, 1925 1,795,081 Davis Mar. 3, 1931 2,059,053 Stareck Oct. 27, 1936 2,126,244 Cook Aug. 9, 1938 2,192,901 Elder Mar. 12, 1940 2,233,500 Westbrook Mar. 4, 1941 2,277,668 Rubensaal Mar. 3 1, 1942 2,572,219 Valentine Oct. 23, 1951 2,689,399 Gray Sept. 21, 1954 OTHER REFERENCES Protective Coatings for Metals, by Burns and Bradley, published 1955, Reinhold Publishing Corp., 430 Park Avenue, New York, N.Y., pp. -108.

Claims (1)

1. AN ARTICLE OF MANUFACTURE INCLUDING A BODY COMPOSED OF FERROUS METAL THAT IS TO BE EXPOSED TO CORROSIVE ATMOSPHERE, THE BODY INCLUDING A CORROSION RESISTANT SURFACE FORMED BY A COATING OF ZINC WHICH HAS BEEN APPLIED BY ELECTRO-DEPOSTING AN INITIAL LAYER OF THE ZINC ON THE FERROUS METAL FROM AN ACID ELECTROLYTE AND FOR A THICKNESS OF AT LEAST 0.00001 INCH, AND AN OUTER LAYER OF ZINC WHICH HAS BEEN ELECTRO-DEPOSITED ON THE INITAL LAYER FROM AN ALKALINE ELECTROLYTE TO A TOTAL THICKNESS OF ZINC AT LEAST 0.0005 INCH, AND IN WHICH THE ZINC IS BONDED TO THE FERROUS METAL BY HEAT TREATING OF THE BODY AFTER THE TWO LAYERS OF ZINC HAVE BEEN DEPOSITED THEREON, THE BONDING BEING OBTAINED BY HEATING THE BODY TO A TEMPERATURE BETWEEN 800 AND 950*F. TO ALLOY THE ZINC AND FERROUS METAL BUT THE TIME OF HEAT TREATMENT BEING LESS THAN THAT REQUIRED FOR THE FERROUS METAL TO ALLOY WITH THE FULL THICKNESS OF THE COMBINED LAYERS OF THE ZINC COATING.

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