US3149987A

US3149987A - Method of coating metals

Info

Publication number: US3149987A
Application number: US160691A
Authority: US
Inventors: Roger L Crandall
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1961-12-11
Filing date: 1961-12-11
Publication date: 1964-09-22
Anticipated expiration: 1981-09-22

Description

Sept. 22, 1964 R. L. CRANDALLv METHOD OF COATING METALS Filed Dec. 11, 1961 INVENTOR Royer L. Granda ATTORNEY United States Patent() 3,149,987 METHOD GF CGATNG METALS Roger L. Crandall, Bethlehem, Pa., assignor to Bethlehem Steel Company, a corporation of Pennsylvania Filed Dec. 11, 1961, Ser. No. 160,691 18 Claims. (Cl. 117-55) This invention relates to a method of coating a base metal with a molten metal coating, and more particularly to a method of galvanizing ferrous metal strip or sheet material on one side only.

This application is a continuation-in-part of application Serial No. 8,725, filed February 15, 1960, now abandoned.

Galvanized sheet metal is normally used where the metal is necessarily exposed to the corrosive effects of the elements. This type of material has been considered for use in automobile structures to improve corrosion resistance, particularly on the interior surfaces of such structures. However, galvanized sheets which are coated on both sides are not adaptable to this particular use, as the zinc coating prevents effective Welding. It is there fore desirable to use for such structures, galvanized steel members which have been coated with zinc on one side only. With this type of coating, the galvanized side of a sheet may be used as the interior surface of a structure, while the outer, exposed ungalvanized surface lends itself to welding operations. The outer surface can ultimately be treated with a protective coating such as paint.

The principal object of this invention is to provide a stop-off material for masking one side of a strip or sheet, so that only one side of the strip or sheet will be galvanized during its passage through a galvanizing bath.

Another object is to provide a stop-off material which is not affected by the annealing temperature of the annealing furnace in a continuous galvanizing line, so that the unaffected stop-off material will effectively prevent oxidation of the ungalvanized side of the strip as the strip leaves the galvanizing bath.

A further object is to provide a stop-off material which not only prevents galvanizing of one side of the strip, but which will itself remain free of zinc so that there is no excessive drag-out of zinc from the coating bath.

An additional object is to provide a stop-off material which is readily removed from the metal strip after it has passed through the galvanizing bath.

I have discovered that bentonites, having certain characteristics, make an eiiicient stop-off material for masking one side of a metal strip where it is desired to galvanize one side of the strip only, and where the side not galvanized must be protected against oxidation at least until the galvanized strip has cooled to a non-oxidizing temperature. The bentonite coatings of this invention are readily removed from the strip, after the strip has been galvanized and cooled to a non-oxidizing temperature, and after such removal, the ungalvanized side of the strip presents a bright unoxidized surface.

The bentonite type of clays are found in extensive deposits in the United States, and certain of these, known as western bentonites, are particularly useful in this invention. The western bentonites may be classified further as sodium bentonites and sodium-calcium bentonites. The sodium bentonites have a greater preponderance of exchangeable sodium ions in relation to the exchangeable calcium ions than have the sodium-calcium bentonites. The exchangeable sodium and calcium ions in the bentonite give the material its gelation property. I have found that both sodium bentonites and sodium-calcium bentonites produce coatings which are exceptionally Well suited for one-side galvanizing.

By the term sodium bentonite, I refer to bentonites lCC which have, in their natural state, a ratio of milli-equivalent exchangeable sodium ions to milli-equivalent eX- changeable calcium ions or 4:1 or greater. In like manner, the term sodium-calcium bentonite, as used herein, refers to bentonites which have, in their natural state, a ratio of milli-equivalent exchangeable sodium ions to mll-equivalent exchangeable calcium ions from 1:1 up to 4: 1. The exchangeable NazCa ion ratio is determined from the ratio of milli-equivalents of exchangeable sodium ions to mini-equivalents of exchangeable calcium ions in 10() grams of clay. A milli-equivalent ion is equal to 1/1000 of a gram equivalent weight of the ion.

An example of a sodium bentonite, which has proved quite useful in preparing a stop-off coating, is given below. This bentonite had the following chemical analysis:

Wt. percent Si02 59.7 A1203 19.9 Fe2G3 3.4 CaO 0.99

Ignition loss 11.4

This example is a typical sodium bentonite. The exchangeable cation ratio (Na1Ca) of the example equals 7.2:1. In the bentonite of the foregoing chemical composition, it was found by analysis that the milli-equivalent exchangeable sodium ions equaled 94, While the milliequivalent exchangeable calcium ions equaled 13, giving a ratio of 94: 13, or 7.211.

In another sample of a sodium bentonite which produced excellent results as a stop-off coating material, the exchangeable cation (Na:Ca) ratio was found to be 8.1:1.

The two examples given above were natural bentonites, and each one proved highly adequate when appplied as a stop-off coating to a steel strip, which was subsequently galvanized. In each case, the strip was annealed after the bentonite coating had been applied, and the annealing temperature (1200 F.-1400 F.) had no apparent adverse effect on the coating.

The sodium-calcium bentonites can also be used in preparing a stop-olf coating, although this type of coating will not prove serviceable, generally, if the bentonitecoated strip is subjected to an anneal prior to being galvanized. This type of bentonite, is, however, applicable as a coating material at the ordinary temperature of a galvanizing bath, say around 850 F., and up to 900 F.

An example of a natural sodium-calcium bentonite is shown in the following chemical analysis:

Wt. percent y SiOZ 65.6

A1203 16.3 F6203 Ca() Ignition loss 1n this sample, the milli-equivalent sodium ions equaled 68 and the milli-equivalent calcium ions equaled 18.5, resulting in an exchangeable sodium to calcium ion ratio of 3.6: 1. Y

Another sample of a sodium-calcium bentonite was found to have an exchangeable sodium to calcium ion ratio of 2.1:1.

Each of the sodium-calcium bentonites referred to above `was successfully applied as stop-off material when subjected to a temperature not above 900 F. At an annealing temperature of from 1200o F. to 1400o F., however, the sodium-calcium bentonites could not be removed readily by water washing, but these bentonites are entirely satisfactory where the bentonite-coated strip is not sub- `jected to an anneal prior to its entry into the zinc coating bath.

The three essential qualities of a coating of the type used in this invention are the ability to completely stop-off any metallic coating from the portion of the strip or sheet surface which is to be masked, ability to prevent oxidation of the surface of the strip during cooling after the metal coating has been applied, and washability, or the ease of removal of the bentonite coating by washing with water after the coating has served its dual function as a stop-off and as a shield against oxidation.

The sodium bentonites will wash readily under all conditions to which a stop-olf coating will be subjected in a galvanizing operation, including in-line annealing. The sodium-calcium bentonite coatings will wash readily if the coatings are not subjected to a temperature above 900 F.

Although some natural bentonites, for example calcium bentonite or calcium-sodium bentonite, will not produce a satisfactory coating in their natural state, such bentonites can be modified by increasing the sodium to calcium exchangeable ion ratio in an amount sufficient to render them usable at temperatures in the annealing range. This modification can be performed by treating the natural bentonite with a water solution of a sodium salt such as sodium chloride.

In order to apply the bentonite to a metal surface, the clay is made into a Water slurry by mixing Water and clay in proportions of about 1000 parts water to 65 or more parts of clay. To the slurry is added 1/2 weight percent of a water solution of ammonia hydroxide (28% NH3). While satisfactory coatings can be made without the addition of ammonia, the ammonia acts as a dispersing agent, with the result that a more uniform coating can be obtained. With ammonia added to the slurry in the amount suggested, the slurry will develop a pH of about 12. In any event, the slurry should be kept well on the alkaline side to provide a satisfactory dispersion.

To obtain maximum eciency when applying the coating, the coating should be applied to strip while the strip is wet. While the proportions given here, for the cornposition of the slurry, have been found suitable, depending on the speed of travel of the strip and the amount of water carried thereon, it may be desirable in some instances to alter the viscosity of the slurry. This can be accomplished readily by an obvious change in the proportions of water and clay in the mix.

The actual application of the slurry to the strip is performed, preferably, by spraying. In the case of a continuous moving strip, the coating is applied by spraying one side f the strip as the strip moves upwardly in a vertical direction. In this manner, a uniform coating is obtained.

The accompanying drawing contains a schematic view of one form of apparatus suitable for carrying out the process of this invention.

The following detailed description presents one mode by which my invention .can be performed.

Referring to the accompanying drawing, the strip to be galvanized on one side only was rst cleaned by passing it from the reel 11 into the cleaning tank 12 containing a hot alkaline solution, through the scrubbers 13 and then through the rinse tank 14, where the strip was rinsed by water contained therein. While the strip was still wet from the rinse the stop-off material, which comprised a suspension of bentonite, prepared by placing 85 parts by weight of sodium bentonite in 1000 parts of water, was applied to one side of the strip by the sprays 15. After being coated with the bentonite, the

strip was dried by the drier 1'7, after which it was passed in-to the furnace 16 where it was heated to a temperature of approximately l300 F. for annealing the strip. After leaving the furnace 16, the strip was passed through the holding furnace 18 containing a non-oxidizing atmosphere where it was cooled to approximately 900 F. and then was passed into the galvanizing bath 19, which was maintained at a temperature of approximately 850 F., where the side of the strip not coated with the bentonite was galvanized. The bentonite coating on the other side, being resistant to wetting by the molten zinc, prevented that side of the strip from being coated by the zinc while the strip was in the galvanizing bath. On leaving the galvanizing bath 19 the strip passed through atmospheric air, and the bentonite coating, which was effective to prevent air from coming into contact with the ungalvanized side of the strip, prevented that side of the strip from becoming oxidized while cooling in the atmosphere from the bath temperature of approximately 850 F. to room temperature. The strip was then passed through the washing tank 2t) containing water, the scrubber 21 and the water rinse tank 22 where the bentonite coating was completely removed from the strip, leaving one side ungalvanized. The ungalvanized side was bright and free of oxide, clean of any residue of the bentonite, and free of even minor adherent particles of zinc. The strip was then wound by suitable means onto the reel 23.

The bentonite used in the foregoing coating operation was a sodium bentonite having a sodium to calcium exchangeable ion ratio of 7.2: l.

This process can also be practiced without an annealing rstep prior to galvanizing. In such case, the strip will normally be preheated to a temperature approximately that of the galvanizing bath, i.e., around 850 F. When following this procedure, a sodium-calcium bentonite will function satisfactorily.

The foregoing description of a specific example of my invention relates to its use in a continuous galvanizing line incorporating an annealing furnace. However, my invention is equally beneficial in other galvanizing operations. It can be used in continuous galvanizing operations which do not include an annealing step, and also in discontinuous or pot galvanizing operations. The essential steps are, the precoating with a liquid suspension of bentonite, drying the coating, the passage of the metal through a molten galvanizing bath, and retention of the bentonite coating on the metal strip until it has cooled to a non-oxidizing temperature.

While I have described my invention as used with a zinc galvanizing bath, it is also capable of being used with coating baths of other metals such as tin or aluminum.

For the purposes of this invention, the term strip, as used in the appended claims, refers to either strip or sheet material.

I claim:

1. A process for galvanizing one side only of a ferrous metal strip or sheet, comprising applying a coating consisting essentially of sodium bentonite and water to the side of the Istrip not to be galvanized, subjecting the strip to an annealing temperature, cooling the strip while surrounded by a non-oxidizing atmosphere to approximately the temperature of the galvanizing bath, passing the strip while still surrounded by said non-oxidizing atmosphere into a galvanizing bath and galvanizing the uncoated side of the strip, then cooling the strip in the atmosphere to a non-oxidizing temperature and passing the strip through a water wash and rinse to completely remove the bentonite coating from the ungalvanized side of the strip thus leaving the ungalvanized side of the strip with a bright non-oxidized surface.

2. In a process of galvanizing one side of a ferrous metal strip, the steps comprising applying a coating consisting essentially of bentonite of the class consisting of sodium bentonite and sodium-calcium bentonite and water to the side of the strip which is not to be galvanized, subjecting the strip with said coating to heat within a range wherein said coating remains in an unfused condition, passing the said strip into a galvanizing bath and galvanizing the uncoated side of the strip, cooling the strip to a non-oxidizing temperature and then passing the strip through a Water wash and rinse to completely remove the bentonite coating from said strip and leave the ungalvanized side of the strip bright and free from oxide.

3. The process of galvanizing one side only of a ferrous metal strip, comprising cleaning the strip, applying a coating consisting essentially of sodium bentonite and Water to the side of the strip Which is not to be galvanized, subjecting the strip with said coating to an annealing temperature, cooling the strip to a temperature approximately equal to the temperature of the galvanizing bath, passing the said strip into the galvanizing bath and galvanizing the uncoated side of the strip, cooling the strip after it is galvanized to a non-oxidizing temperature with the bentonite coating on the ungalvanized side of the strip and passing the strip through a water Wash and rinse to completely remove the bentonite coating from the strip.

4. A process of coating one side of a ferrous metal strip with a metallic coating `by passing it through a metallic coating bath, comprising applying a coating consisting essentially of sodium bentonite and Water to the side of the strip which is not to be coated With the metallic coating, drying said bentonite coating, subjecting the strip coated on one side with bentonite to an annealing temperature, cooling the strip While surrounded by a non-oxidizing atmosphere to approximately the temperature of the metallic coating bath, passing the strip while still surrounded by said non-oxidizingr atmosphere into the metallic coating bath and applying a metallic coating to the uncoated side of the strip, cooling the strip to a non-oxidizing temperature and passing the strip through a Water bath and rinse to remove the bentonite coating from the strip.

5. In a process for galvanizing one side only of a ferrous metal strip or sheet, the steps comprising cleaning the strip, applying a coating consisting essentially of bentonite of the class consisting of sodium bentonite and sodium-calcium bentonite and Water Which is readily removable by a water wash after the strip is galvanized and cooled to a non-oxidizing temperature to the side of the strip not to be galvanized, passing the strip through a galvanizing bath and galvanizing the side of the strip not coated With bentonite and cooling the strip to a non-oxidizing temperature with the bentonite coating thereon.

6. In a process for galvanizing one side only of a ferrous metal strip or sheet, the steps comprising applying to the side of the strip not to be galvanized a coating consisting essentially of bentonite of the class consisting of sodium bentonite and sodium-calcium bentonite and Water which is readily removable by a Water Wash after the strip is galvanized and cooled to a non-oxidizing temperature, passing the strip through a galvanizing bath and galvanizing the side of the strip not coated with bentonite and cooling the strip to a non-oxidizing temperature with the bentonite coating thereon.

7. The process of coating one side of a metal strip with a metallic coating in a coating line containing a coating bath of molten metal, comprising the steps of cleaning the strip, coating one side thereof with a coating consisting essentially of bentonite of the class consisting of sodium bentonite and sodium-calcium bentonite and water and having a pH of not less than 12, passd ing the strip through the metallic coating bath and coating the side of the strip, not coated with bentonite, with a metallic coating, the bentonite coating acting as a stopoff material which prevents adherence of the metallic coating to that side of the strip during the passage of the strip through said coating bath whereby only one side of the strip is coated with the metallic coating.

8. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of a bentonite of the class consisting of sodium bentonite and sodium-calcium bentonite and water, drying the coating, passing the: strip through a. galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing ternperature.

9. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of sodium bentonite and water, drying the coating, passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing temperature.

l0. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of sodium bentonite and water, heating the strip, passing the strip through a molten galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing temperature.

11. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of sodium bentonite and Water, annealing the strip, passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing temperature.

12. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of sodium bentonite and Water, annealing the strip at a temperature of 1200o F. or above, passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, cooling the strip to a non-oxidizing temperature and Washing and rinsing the strip and thereby removing said coating.

13. A process for galvanizing one side only of a ferrous metal strip Which comprises applying to one side of the strip a coating of sodium-calcium bentonite and water, drying the coating, passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing temperature.

14. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of sodium-calcium bentonite and Water, heating the strip to a temperature below the fusion point of the bentonite coating, passing the strip through a molten galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing temperature.

15. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of sodium-calcium bentonite and water, heating the strip to a temperature not greater than about 900 F., passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, cooling the strip to a non-oxidizing temperature and Washing and rinsing the strip and thereby removing said coating.

16. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of a bentonite having a sodium to calcium exchangeable ion ratio of not less than 4:1 and Water, drying the coating, passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, cooling the strip to a non-oxidizing temperature and Washing and rinsing the strip and thereby removing said coating.

l7. A process for galvanizing one side only of a ferrous metal strip Which comprises applying to one side of the strip a coating of a bentonite having a sodium to calcium exchangeable ion ratio of not less than 7.2:1 and Water, drying the coating, passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, cooling the strip to a non-oxidizing temperature, and Washing and rinsing the strip and thereby removing said coating.

18. A process for galvanizing one side only of a ferrous metal strip which comprises applying to one side of the strip a coating of a bentonite having a sodium to calcium exchangeable ion ratio of not les-s than 2.1:1 and water, heating the strip to a temperature not exceeding 900 F., passing the strip through a galvanizing bath and galvanizing the uncoated side of the strip, and cooling the strip to a non-oxidizing temperature and washg ing and rinsing the strip and thereby removing said coating.

References Cited in the le of this patent UNITED STATES PATENTS 2,894,850 Greene et al. July 14, 1959 2,900,272 Schneider Aug. 18, 1959 FOREIGN PATENTS 15,566 Great Britain 1914 112,220 Great Britain Ian. 3, 1918

Claims

1. A PROCESS FOR GALVANIZING ONE SIDE ONLY OF A FERROUS METAL STRIP OR SHEET, COMPRISING APPLYING A COATING CONSISTING ESSENTIALLY OF SODIUM BENTONITE AND WATER TO THE SIDE OF THE STRIP NOT TO BE GALVANIZED, SUBJECTING THE STRIP TO AN ANNEALING TEMPERATURE, COOLING THE STRIP WHILE SURROUNDED BY A NON-OXIDIZING ATMOSPHERE TO APPROXIMATELY THE TEMPERATURE OF THE GALVANIZING BATH, PASSING THE STRIP WHILE STILL SURROUNDED BY SAID NON-OXIDIZING ATMOSPHERE INTO A GALVANIZING BATH AND GALVANIZING THE UNCOATED SIDE OF THE STRIP, THEN COOLING THE STRIP IN THE ATMOSPHERE TO A NON-OXIDIZING TEMPERATURE AND PASSING THE STRIP THROUGH A WATER WASH AND RINSE TO COMPLETELY REMOVE THE BENTONITE COATING FROM THE UNGALVANIZED SIDE OF THE STRIP THUS LEAVING THE UNGALVANIZED SIDE OF THE STRIP WITH A BRIGHT NON-OXIDIZED SURFACE.