US 3278331 A
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Oct. 11, 1966 K. c. TAYLOR ET AL 3,278,331
PROCESS FOR COATING STEEL WITH ZINC Filed July 26, 1965 U2; 5.6K E nkw Qmbu QSL INVENTORS. /fE/VDR/CK C. TAYLOR DAV/0 E. .STO/(E5 ATTORNEYS.
United States Patent O 3,278,331 PROCESS FOR COATING STEEL WITH ZINC Kendrick C. Taylor, Oreland, and David E. Stokes, Jenkintown, Pa., assiguors to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed July 26, 1965, Ser. No. 478,020 5 Claims. (Cl. 117-107.1)
This application is a continuation-in-part of our copending application Serial No. 196,869 tiled on May 23,
1962, now abandoned.
The present invention relates to a coating process, and more particularly to a process for coating a metal with a layer of zinc.
It has become desirable to coat many metal objects, both ferrous and non-ferrous metals, with a layer of zinc. However, it has been found that a Imajor problem in coating a metal object with zinc is to obtain a strong adhesion 'between the zinc coating and the metal object. Heretofore attempts have been ymade to achieve a strong bond between a metal object and a zinc coating by thoroughly cleaning and degreasing the metal lobject prior to coating it. Such cleaning processes as pickling, cathodic cleaning, `and grit blasting have been tried. `However, it has been found that merely cleaning the metal article is not sufficient to provide a satisfactory adherence of a zinc coating to the metal object.
Heretofore, attempts to coat sheet steel with Zinc have been generally done by the dip method. Recent demands of the industry lrequire the sheet steel .to be coated on only one side. Attempts `to coat sheet steel on only one side by dipping two sheets back-to-back have not been successful since the process is cumbersome, dilicult to control, and is costly due to its slow rate of production.
The present invention is capable of providing a layer of Zinc on one or both sides of a substrate of metal. This is accomplished by first providing a thin alloy layer between the zinc and .the metal. Then, the alloy layer is coated With zinc. The temperature ranges and other relationships necessary to attain 4this result are set `forth in greater detail hereinafter. The process of the present invention may be performed with a metal substrate moving at a rate of 2000 feet per minute.
It is an object of the present invention to provide a novel process =for coating a 4metal substrate with a metalic coating.
It is another object of the present invention to provide a continuous process for coating a metal strip on one side with a strongly adherent layer of zinc.
It is a further obje-ct of the present invention to provide a process for coating istrip steel with a strongly adherent thin layer of zinc.
Other objects will appear hereinafter.
For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.
FIGURE 1 is a diagrammatic flow chart.
FIGURE 2 is a diagrammatic sectional view of apparatus which may be utilized in performing the method of the present invention.
In accordance with the process of the present invention, the metal article or substrate to be coated, which may be a ferrous or non-ferrous metal, is in a heated condition. Such heated condition may be attained before it enters the coating chamber which can be hermetically sealed and eva-cuated or it may be attained in the chamber. The metal article to be coated `may be cleaned and degreased prior to entering the coating chamber. The coating chamber contains a source of zinc coating mate- Mice rial. While the following description refers to zinc as the coating material, it will be obvious to those skilled in the art that other metals such as aluminum may be substituted.
The metal article to be coated is preferably in strip form such as a strip steel. The process of the present invention is preferably applied to the metal article immediately as it is made. Thus, the process of the present invention may be an integral part off the process for making strip steel. -In this manner, strip steel may be continuously coated with a layer of zinc on one side as it is being made. In order to accomplish this `desirable result, the present invention may be practiced without disrupting the lconventional process for making strip steel.
In FIGURE 2, there is illustrated lapparatus 10 which per se forms no part of the present invention but may be utilized to practice `the present invention. The apparatus 10 includes a housing 12 having a chamber 14 therein which is adapted to be evacuated by way of conduit 16. A substrate 18 adapted to be coated may first be fed through a chamber 20. The coated substrate 18 may be removed from the conduit 14 by way of an evacuated chamber 32.
The process of the present invention may include placing a hermetically sealed evacuated chamber 20 in the production line such as after the annealing and cleaning apparatus. The chambers 1'4, 20 and 30 will have roller `type inlet `and outlet seals at opposite ends of the chambers 14, 20 and 30 so that the strip steel continuously passes through the chambers 14, 20 and 30. Each roller type seal includes a plurality of spaced rollers sealingly engaging the strip steel so as not to disturb the vacuum in the chambers 14, 20 and 30.
While maintaining the vacuum within the coating chamber 14 at a pressure *between .l and 300 microns, the source 24 of lthe zinc coating material is heated to the temperature at which the coating material evaporates at the pressure within the coating chamber. The Zinc will be lheated above lthe vaporization temperature for the pressure of the evacuated chamber 14. For example, zinc vaporizes at a temperature of 343 C. to 405 C. over a pressure range olf .01 mm. of mercury to .1 mm. of mercury, respectively. At such pressure, the zinc would be superheated to temperatures `in excess of 450 C.
The metal substrate 1&8 may be heated to a temperature between ambient temperature and `the vaporization temperature of zinc, such as 300 F. to drive off impurities such as gases and surface clean the substrate. Such heating of the substrate may be accomplished with conventional equipment in the chamber 20 such as by coil 22W or prior entry into the chamber 20. The former is preferred. The vapors from the zinc coa-ting material evaporate in a line-of-sight orientation, oriented toward the metal substrate 18. The vapors of the coating material which contact the lheated continuously moving metal substrate 1'8, such as strip steel, will condense on the cooler metal substrate to provide a thin layer of the zinc between .000001 and .0001 inch thick on the steel strip.
At this point, the metal substrate 18 is cooler than the zinc and the exposure time is short so that only a small amount of Zinc condenses on the metal substrate 18. Thereafter, the metal substrate 1'8 is heated to an elevated temperature by coils 26 to cause the thin layer of `zinc to form an alloy layer of zinc and the substrate. It has been found that -an alloy layer of zinc and steel results when such steel strip has a temperature of approximately 450 C. or slightly above. This step of forming an alloy layer is considered to be an important feature of the ypresent invention.
After the lthin alloy layer of Zinc and steel has been attained, the steel strip 18 is cooled lto a temperature which is below the temperature of the zinc coating material by plates 28. The steel strip 18 continues to -be exposed to vaporized zinc from source 30. However, the alloying process has reduced sharply and vaporized zinc from source 30 is now coated on the zinc-steel alloy layer. By controlling the rate of movement of the steel strip 18 and the rate of vaporization of the zinc from source 30, the zinc coating on lthe alloy layer may be extremely thin and have a thickness between .0000011 to .001 inch.
The process of the present invention permits ease of control of the amount of the coating material applied to the metal article so that thin films of the zinc coating material can be obtained. By the process of the present invent-ion, strongly adherent layers of zinc of a thickness from 1 to 1000 microinches have been obtained on metal articles.
Thus, it will be seen that the present invention includes condensing a metallic vapor on a metal substrate in two stages with intermediate `steps of heating the substrate to obtain an alloy Ilayer and then cooling the substrate prior to the second condensing phase.
The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.
1. A process for coating a strip of steel with a layer of zinc comprising the steps of feeding said strip through a chamber, evacuating said chamber, heating said strip to a temperature between about ambient temperature and the vaporization temperature of zinc at lthe pressure in said chamber but below the alloying ltemperature of zinc and steel, exposing said strip to zinc vapors while the strip is in said chamber and at said temperature to condense a `film of zinc between 1 and 100 micro-inches thick on said strip while maintaining the vacuum in said chamber, then heating 4the strip and `film to an alloying temperature of approximately 460 C. to form an alloy of said zinc film and strip, then cooling the thusly-coated strip, then coat-ing the alloy film with a thin layer of zinc while the alloy film is at a temperature below the vaporization temperature of zinc.
y2. A process in accordance with claim 1 wherein the speed of movement of the strip through said chamber is accomplished so that the combined thickness of the zinc film and zinc layer is from 1 to `1,000 microinches.
3. A process -in accordance with claim 1 wherein said step of coa-ting the alloy `film with a thin layer of zinc comprises exposing the strip to the vapors of zinc to condense a film of zinc thereon.
4. A process of coating a strip of steel with a layer of zinc comprising continuously feeding said strip through a chamber, evacuat-ing said chamber to a pressure of between 0.1 to 300 microns, heating said strip to a temperature being between about ambient temperature and the vaporization temperature of zinc at the pressure in said chamber but below the alloying temperature of zinc and steel, exposing Ithe strip while it is at said temperature to the vapors of zinc to condense a film of zinc between 1 and 100 microinches thick on the stri-p while maintaining the vacuum in the chamber, then heating the strip to an alloying temperature of approximately 460 C. to form an alloy of said zinc film and strip, then cooling the thusly-coated strip of steel, and then coating the alloy film Iwith a thin layer of zinc while the alloy film is at a temperature below the vaporization temperature of zinc.
5. A process in accordance with claim 4 wherein said step of coating the a-lloy `film with a thin layer of zinc comprises exposing the strip to the vapors of zinc to condense a lm of zinc thereon.
References Cited by the Examiner UNITED STATES PATENTS 2,384,500 9/1945 Stoll 117-107.1 2,405,662 8/1946 McManus et al. 117-l07.1 2,442,485 6/1948 Cook 117-107.1 X 2,812,270 11/1957 Alexander 117-1011 ALFRED L. LEAVITT, Primary Examiner.
RALPH S. KENDALL, Examiner.