| United States Patent | 3,762,883 |
| Shepard , et al. | October 2, 1973 |
COATED STEEL ARTICLE
Abstract
A steel substrate having an intermediate adherent coating of gold or silver or copper or brass, and an outer adherent coating of zinc, in which the iron content in the coatings, occasioned by alloying between the iron and the coatings thereon, is less than about 100 milligrams per square foot of surface.
| Inventors: | Shepard; George A. (Parma, OH), Brooker; Carl F. (Garfield Heights, OH) |
| Appl. No.: | 05/086,623 |
| Filed: | November 3, 1970 |
| Current U.S. Class: | 428/658 ; 428/671; 428/677 |
| Current International Class: | C23C 14/16 (20060101); C23C 28/02 (20060101); B23p 003/00 (); B32b 015/00 () |
| Field of Search: | 29/196.3 |
| 2918722 | December 1959 | Kenmore |
| 2490700 | December 1949 | Nachtman |
| 2547947 | April 1951 | Kleis |
| 3164448 | January 1965 | Pottberg |
| 3249409 | May 1966 | McLeod |
| 3555169 | January 1968 | Miller |
CROSS-REFERENCE TO RELATED APPLICATION
This application is a division of our co-pending application Ser. No. 680,607, filed Oct. 20, 1967 now abandoned, for COATING OF ZINC ON STEEL, which in turn is a continuation-in-part of our copending application Ser. No. 423,249 filed Jan. 4, 1965, now abandoned for "COATING OF ZINC ON STEEL."
What we claim is:
1. A coated steel article, comprising a steel substrate, an intermediate adherent coating of gold or silver or copper or brass, and an outer adherent coating of zinc, in which there is an iron content in the coatings, occasioned by alloying between the iron in the steel and the coatings thereon, said iron content being less than about 100 milligrams per square foot of surface.
This invention relates to vacuum metalizing of steel and is particularly directed to the provision of an adherent coating of zinc on steel surfaces by deposit from vapor of metallic zinc in a vacuum. The invention is primarily concerned with improved methods of providing such zinc coatings of steel by vapor deposition, and likewise to the resulting articles, such as steel sheet carrying an adherent, smooth zinc coating of the character stated.
Zinc-coated steel sheet, meaning strip or other sheet of steel, whether cold rolled or hot rolled, has distinct advantages for many fields of use, particularly in avoiding or reducing corrosion or like deterioration of the steel surface. While galvanized steel has been widely used for a long time, i.e., steel having a zinc coating applied by conventional hot-dip galvanizing methods, the nature of the surface of such coatings has prevented their employment in many circumstances, especially automotive work, e.g., as for automobile bodies, especially because of the relatively rough nature of the galvanized layer. The requirements of automobile paints or other finishes to be applied over such steel sheet, particularly the need for a highly smooth product, have prevented successful use of galvanized sheet in this and similar fields, in most cases. While electroplated zinc can provide a satisfactorily smooth finish, zinc electroplating operations are relatively costly and steel so coated has been unattractive to manufacturers in automotive and other industries for this reason.
Vacuum metalizing, involving the deposit of a metal from its vapor, is known to produce surfaces of good smoothness and can usually be performed more economically than procedures of electrolytic deposition, for coatings of significantly useful thickness. Moreover, such thickness can be readily controlled to any desired value in vapor deposition methods, as distinguished from hot-dip operations where it is difficult or impossible to apply anything but a relatively heavy layer, which may be needlessly excessive, and thus unduly costly for many purposes. Although satisfactory coating on steel has been achieved by vapor deposition of a number of metals, of which an outstanding example is aluminum, it has unfortunately been found that zinc layers applied in this manner tend to have poor adherence. Even with unusually thorough cleaning operation prior to the vacuum metalizing treatment, zinc layers thus applied to steel have been found to be subject to removal, i.e., by flaking or peeling off, on bending or otherwise flexing the surface. This difficulty is critical, especially for automotive and like uses, in that steel so employed must be bent or shaped to any of a wide variety of configurations, often involving a relatively deep draw in the forming process, to the extent of including some actual deformation or extension of the metal.
The present invention is therefore designed to provide for the application of zinc coatings, over steel articles, by vapor deposition in vacuum, and to achieve products consisting of articles so coated, wherein the zinc layer, of essentially any desired thickness, is firmly and permanently adhered, against removal even upon severe bonding, distortion or similar manipulation of the sheet or equivalent product. To these ends, the method of the invention involves treating the steel surface, after cleaning appropriate for vacuum metalizing, by applying a thin adherent coating of copper or other metal selected from a specific class of metals in the upper range of the electromotive force series, namely gold, silver and copper, brass being also deemed a member of the named group, in that the latter is an alloy consisting essentially of copper and zinc wherein copper predominates. A further, principal step of the method thereafter involves applying a coating of zinc over the thin copper or like film, by deposition from zinc vapor in vacuum, to achieve the desired thickness. By virtue, apparently, of some mutual penetration between the coatings, or at least of some close association or engagement of the molecules of zinc upon and around the molecules of the selected underlying metal, full and effective adherence is achieved, the underlying metal being such as to be inherently characterized by effective adherence, whether by inter-alloying or otherwise, to the actual steel surface.
While the first-applied film or coating, e.g., of copper or brass, can itself be achieved by vapor deposition, a further and particularly advantageous feature of the invention involves formation of this first coating by ionic deposition from an aqueous medium, whether by simple displacement or by electrolytic action. Thus for example the cleaned steel surface may be immersed in a solution of a copper salt, such as copper sulfate, whereupon a thin, effective film of copper is deposited pursuant to the well known reaction of displacement of the copper ions by iron. Alternatively, and with special advantage in many cases, the copper may be deposited by a brief electroplating operation, i.e., directly on the steel surface. Copper coatings applied by either of these methods afford highly safisfactory adherence of the vapor-deposited zinc layer subsequently applied thereon. They may be very readily controlled as to thickness, i.e., in the sense that only a very thin copper film is usually necessary. Since only a very brief chemical-type treatment is required, the cost is relatively small whether by immersion or by electroplating, and indeed this operation may simply be incorporated in the so-called cleaning line, e.g., as one of the final steps after the steel strip or other sheet has traversed the various baths and rinses required to remove oxide, grease, dirt and the like.
Following the application of the copper, brass or similar film (it being understood that brass may likewise be plated, by conventional methods) all that is necessary is that the surface be rinsed, as in plain water, and dried, before the vacuum metalizing step. Of course, the copper or similarly coated sheet should be maintained in a dust free condition, and under such circumstances, or for no more than a brief time, as to avoid appreciable oxidation. The vacuum coating with zinc is then performed in accordance with conventional metalizing procedure, preferably by directing a stream of zinc vapor into the coating chamber and upon the surface of the treated steel, for example as where such steel strip, wire or other article is continuously advanced. The zinc deposits in solidified, continuous form as a highly smooth coating and may be applied to any desired thickness, at least up to the point where it might tend to exist or remain in molten form. Thus for instance, zinc coatings of a thickness of about one mil are quite satisfactory, providing corrosion resistance equivalent to conventional hot-dip galvanized coatings. The resulting completed steel articles being sheet, strip, plate, wire, bar, rod or other shape, are characterized by excellent adherence of the zinc layer, as well as by complete smoothness of the outer surface, free of the so-called spangled or crystalline or otherwise relatively rough nature which is an attribute of galvanized products. The reason for the unusual result of the process and the effective adherence of the coating in the produced article, is not fully understood, especially in view of the fact that a number of other metals, such as aluminum, can be found to adhere very well, on direct application to steel. It is now believed that the problem of adherence of vapor-deposited zinc may occur because the molecules of zinc vapor travel at relatively low speeds, and because the zinc molecules, for that or other reasons, may tend to condense less readily. It is now understood that aluminum molecules travel at about twice the speed of zinc; hence it is conceivable that the correspondingly low momentum of the zinc vapor is a factor in its ordinarily poor adherence. In the same sense, there may be some tendency of the depositing zinc molecules or particles to bounce away from the surface, in contrast to the situation in the condensation of aluminum vapor. In any event, it is plain that with the first applied film of copper (or other metal of the specified class) over the steel surface, the zinc vapor condenses in a fully adherent state, apparently with some penetration of the zinc molecules into the copper as explained above. Indeed when very thin layers of zinc are applied over a copper film, the resulting surface has a yellow or brass-like color, evidencing such inter-penetration, and lending credence to the theory that the copper film permits immediate embedding, so to speak, of the slow-moving zinc molecules in a manner unattainable on a bare steel surface. It is believed that some alloying of the vapor deposited zinc with the copper or the base metal is necessary for good zinc adherence. The iron content in the alloy layer should not exceed about 100 mg. per square foot of surface, however.
While it was initially thought that the plural coating structure might constitute a copper-zinc couple and would lead to corrosion of the zinc in an accelerated way, tests have shown that no such result appears to happen, i.e., under conditions of exposure that might give rise to it. The desireable avoidance of such results is also not fully explained, although it may be that the copper-zinc alloy serves as a buffer. Dealing further with the mechanical situation of the coating, a further possible comment is that the under-layer is constituted of a metal which forms alloys at low temperature, as contrasted to steel and iron, and whereas the condensing zinc vapor may not sufficiently heat up a bare steel surface to form an interfacial alloy, such alloying takes place much more readily with the copper, while the copper, in turn, is inherently able to engage the steel surface in a very close and thus suitably adherent manner.
As indicated, the coating of copper, brass, gold or silver, which is applied directly to the steel surface, is relatively thin, as for example from 0.25 .times. 10.sup..sup.-6 inch, and preferably from 0.5 .times. 10.sup..sup.-6 inch to any convenient and economically practical thickness, e.g., 100 .times. 10.sup..sup.-6 inch. Indeed, in some cases the coating can be even thinner, presumably no more than a few molecules in thickness (although a single molecular layer presently seems to be inadequate), especially in the case of gold and apparently also under some circumstances with silver and copper. While films thicker than the upper limit named above, being one ten thousandth of an inch, are presumably not deleterious, reasons of economy dictate the use of as thin a layer as possible. A film 5.0 .times. 10.sup..sup.-6 inch in thickness, or ordinarily much less, is believed to be fully adequate; a film thickness of about 4 .times. 10.sup..sup.-6 inch is preferred.
As stated, the copper, brass or other layer can be applied in various ways, but ionic deposition from solution seems unusually convenient and advantageous. Simple immersion plating is quite effective, although some adjustment of the solution used, as in pH, may be necessary to achieve optimum results, while avoiding waste of copper. That is to say, immersion plating depends on the activity of the steel surface, which may vary with different steels and with different surface characteristics. In contrast, electroplating ordinarily involves no such differences, and can be controlled in conventional ways to deposit a coating of any desired thickness. In the case of gold, silver and brass, electroplating is similarly effective, for applying corresponding films of selected character, by conventional techniques. Metals of less noble character than those named appear to be ineffective to constitute the underlying coating, i.e., in that no increase of adhesion of zinc layers, deposited from vapor, has been found with metals such as cadmium, iron or lead, nor any significantly satisfactory effect with nickel or tin. Aluminum similarly appeared to contribute no improvement in adherence of the zinc nor was it found of any advantage to treat the steel with a flash electroplating of zinc.
A further advantage of the process, in contrast to hot-dip galvanizing, is that the zinc coating can, if desired, be applied on only one surface of the metal sheet or like article, thus greatly economizing the operation where only a single face requires protection.
The vapor depositing step for applying the zinc layer, e.g., over the copper or similar film, can be carried out in any satisfactory manner, as in conventional vacuum metalizing equipment. Preferably, the zinc vapor is generated in a separate chamber and directed as a stream toward the passing steel strip or the like in the main evacuated region, e.g., under pressures from 20 to 30 microns down to one micron or less; pressures of 20 microns or less are preferred. The rate of vapor flow and the time of exposure, i.e., speed of strip travel, are appropriately correlated to the desired thickness of zinc coating as will be readily understood in the vacuum metalizing art. It should be noted that higher zinc condensation rates are preferred over lower zinc condensation rates during zinc vaporization. The coppercoated steel surface should be maintained at a temperature within the range 300.degree. to 600.degree. F. during vaporization of the zinc coating in order to ensure good adherence of the coating. The zinc vapor may raise the substrate to the desired temperature; in the case of thin zinc coatings, however, it may be necessary to preheat the substrate to ensure the desired temperature. Preheating the copper-coated steel surface may expand the substrate temperature range, e.g., so that the range may be from 250.degree. F. to 660.degree. F. Post heating the zinc coated article may enhance adherence of the coating. As has been indicated, the zinc layer may have any desired thickness, although the invention is particularly suitable for such coatings in the range of 25 .times. 10.sup..sup.-6 to 2000 .times. 10.sup..sup.-6 inch and most preferably 100 .times. 10.sup..sup.-6 to 1000 .times. 10.sup..sup.-6 inch (0.1 to 1 mil). Present understanding is that a zinc coating of about one mil thickness affords adequate protective function for ordinary automotive uses.
The invention is applicable to a wide variety of steels, including particularly those most commonly employed in automobile body work and the like. Examples of these and other steels are as follows: low carbon aluminum killed steel; low carbon (mild) steel; and low carbon silicone killed steel. The process is also appropriate for various special ferrous alloys.
If desired, one of the zinc coatings may be omitted, as by simply directing vapor to one face of the sheet during its travel through the vacuum chamber. The copper film may also be omitted from such face, although immersion techniques may make it simpler to coat both sides.
By way of specific example, steel sheet material has been treated in accordance with the described process, using various coatings of the named metals directly on the steel surface, followed by vacuum deposition of zinc from a stream of zinc vapor. In general, the steel used was low carbon, cold rolled steel strip having a gauge or thickness of 0.030 inch, but tests have indicated that the process is equally applicable to steels generally, as explained above, with essentially no modification in detail, except for instance in the situation where a copper coating is applied by simple immersion plating. In the latter instance, the time of immersion and the acidity of the plating solution may require adjustment for best efficiency, as will be readily understood or as can be easily determined in any given case.
Preliminary to the treatment, the steel surface or surfaces should be appropriately cleaned, for example as by any series of operations generally suitable as preparation for vacuum metalizing. One effective cleaning sequence is described in U.S. Pat. No. 2,959,494, issued Nov. 8, 1960 (G. A. Shepard), such operations being abundantly suitable to clean the steel surface appropriately for the coatings of the present invention, in lieu of the aluminum coating primarily contemplated by the patent. Indeed the sequence of soak-cleaning, electro-cleaning (cathodic) and acid pickling described in the patent may usually be sufficient for present purposes, without the final alkaline soak step there described. That is to say, in such case, the pickled strip or sheet is then rinsed and immediately passed through the immersion plating or electroplating tank, for deposition of the first metal coating. After such treatment, the surface or surfaces are rinsed with water and dried, and can be immediately subjected to the step of vapor deposition of zinc.
Continuing the description of examples of the process, immersion platings of copper were achieved by exposing the surface or surfaces to a water solution of suitable copper salt, e.g. copper sulfate, in a concentration of 7.5 grams/liter, adjusted to a pH of about 2.0. With the selected steel, an adequate copper coating of about 0.5 .times. 10.sup..sup.-6 inch was achieved on immersion for 2 seconds, and even better results were obtained in some cases with somewhat thicker coatings, up to 1.2 .times. 10.sup..sup.-6 inch, by leaving the steel in solution for periods up to 4 seconds. Immersions for times substantially shorter, yielding copper films having a thickness believed to be of the order of several molecules, were also employed, with significantly useful results as to adherence of the later-applied zinc coating, from zinc vapor.
Electroplated copper coatings were likewise achieved by subjecting the steel sheet to a conventional plating operation, utilizing pure copper anodes and an aqueous solution having the following composition: copper cyanide 3.0 oz./gal., sodium cyanide 4.5 oz./gal., sodium carbonate 2.0 oz./gal. (room temperature). Plating was carried out with a D.C. current density of about 15 amperes per square foot of steel surface, and satisfactory copper coatings were achieved, having thicknesses ranging from 0.5 to 50 .times. 10.sup..sup.-6 inch, with plating times ranging from 2 seconds to 2 minutes. Again, somewhat thinner coatings down to several molecules were achieved with a still shorter duration of electroplating.
Brass coatings, having a composition of approximately 80% copper and 20% zinc, were also applied to surfaces of the steel strip by conventional electroplating procedure, utilizing anodes of brass having the stated composition. A solution used had the following composition: copper cyanide 4 oz., zinc cyanide 1.25 oz., sodium cyanide 7.5 oz., sodium carbonate 4 oz., water one gallon. With plating current of 5 amperes per square foot of the steel surface, through the aqueous solution, suitable brass films were adherently applied, having thicknesses from 0.5 to 100 .times. 10.sup..sup.-6 inch, with plating times of 5 seconds to 10 minutes.
Silver and gold films were also respectively applied to further specimens of steel, utilizing conventional plating solutions. In the case of gold, the solution contained 4 oz./gal. of potassium gold cyanide, and for silver plating, the composition of the solution was 4 g./liter of silver cyanide. With current densities of about 20 amperes per square foot of the steel surface, highly effective coatings of gold and silver were respectively achieved, having thicknesses from 0.25 to 100 .times. 10.sup..sup.-6 inch. In these cases, some extremely thin films were also produced and found to afford satisfactory results, especially in the case of gold where a film believed to be no more than a few molecules thick was achieved with a flash plating and subsequently found to afford adherence of the vapor-deposited zinc.
Sections of steel strip plated in all of the various ways described above were rinsed, dried and subjected to vapor deposition of zinc under vacuum conditions, e.g. usually a pressure of not more than 20 to 30 microns. It was found that the conditions of zinc coating could be the same for the various precoated specimens, the chief adjustment in this step being with respect to the intensity and duration of the vapor treatment, for the attainment of different thicknesses of zinc. Thus quite satisfactory results were obtained by exposing the pre-coated steel surface to a flow of zinc vapor, evaporated at about 1350.degree. F. and having a delivery rate of about 0.07 to 0.12 pounds of zinc per minute. The actual deposit of zinc on the surface of the sheet or strip was at the rate of 2.5 to 26.0 pounds per square foot per minute, for the achievement of zinc coatings having thicknesses ranging from 0.1 mil to 1.0 mil over exposure times of 1.5 to 8 seconds.
Within the ranges specified by example above, all of the steel strip first coated respectively with copper (by immersion or electroplating), brass, silver and gold, and thereafter subjected to vapor deposition of zinc to thicknesses of the range indicated, were found to afford excellently adherent zinc coatings. Adherence was tested by flexing the strip back and forth several times, or indeed even many times, there being no evidence of removal of any part of the zinc layer. In contrast, steel strip directly subjected to the same zinc vapor deposition, yielded zinc coatings which quite often commenced to flake or peel away, at the zones of flexure, even after no more than a few bending operations. Another suitable test for adherence of vacuum-deposited coatings, such as the zinc coatings of the present invention, is set forth in the above-mentioned U.S. Pat. No. 2,959,494 (column 5, lines 13 and following), such test involving applying a lacquer over the coating, then pushing a ball into the steel sheet, while the latter is drawn over the ball to the point of rupture. If a pressure-sensitive adhesive tape applied over the drawn area, and thereafter removed, shows that it has carried away pieces of the zinc coating, adherence is generally inadequate. This test, which has shown good adherence with the various examples of the present invention, is significant in demonstrating the ability of the coated product to withstand forming operations in use.
In all cases of the exemplified procedure, the resulting new product of steel carrying a vapor-deposited coating of zinc had effectively useful adherence of such zinc coating, and in its structure, consisted essentially of the underlying steel, carrying the defined film of selected first-coating metal, and thereover the protective coating of zinc applied by vapor deposition.
In tests conducted employing coatings of copper over steel plate, followed by a coating of zinc, it was found to be advantageous to have some alloying of zinc with the copper or underlying steel. For relatively thick copper coatings, the zinc will alloy with the copper only and will not penetrate the copper sufficiently to alloy with the underlying steel. In the case of thinner copper coatings, however, the zinc will alloy with the copper to form brass and will also alloy directly with the steel; also, the brass so formed will alloy with the steel. It has been found that although alloying is desirable, generally the iron content in the alloy should not exceed 100 milligrams per square foot of surface. Excessive alloying (greater than 100 mg.lft..sup.2) has generally resulted in poor adherence of the zinc coating.
To provide proper alloying of the zinc with the copper or steel, it has been found that the substrate should reach a temperature during the zinc vaporization step that is generally within the range 300.degree. to 600.degree. F. Examples of steel plate coated with copper followed by vapor deposition of zinc in vacuum have produced good adherent coatings when the substrate reached a maximum temperature within this range. If a relatively thin coating of zinc is being applied, the zinc vapor may not raise the temperature of the substrate sufficiently so that the temperature comes within the desired temperature range of 300.degree. to 600.degree. F. In such a case it may be necessary to heat the substrate independently, e.g., by electrical resistance heating in order to ensure that the substrate reaches a temperature within the desired range.
Samples of steel plate coated with copper of a thickness of approximately 4 .times. 10.sup..sup.-6 inch, followed by glow discharge cleaning for 30 seconds and then plating by zinc vapor in vacuum, produced zinc coatings of excellent adherence when the maximum temperature of the substrate during evaporation was within the range 300.degree. to 600.degree. F., starting with a substrate at ambient or room temperature. Different samples were produced, each with a different maximum substrate temperature during vaporization in the range 300.degree. to 750.degree. F., with maximum temperature varying in increments of 25.degree. F. from 400.degree. to 675.degree. F. The samples in the range 300.degree. to 625.degree. F. showing an iron content in the alloy layer of 100 milligrams per square foot or less all produced zinc coatings of excellent or good to excellent adherence. It is believed that the glow discharge cleaning is not necessary for adherent coatings.
It has also been found that pre-heating the substrate prior to the deposition of zinc on the copper coating extends the temperature range just referred to by 50.degree. or 60.degree. F. on either end of the range so that the range for good plating results is from 250.degree. to 660.degree. F. It is believed that pre-heating the substrate extends the range at the lower end, particularly, i.e., to 250.degree. F., because alloying of the zinc vapor with the copper or underlying steel may take place at temperatures lower than 300.degree. F. due to the relatively high energy of the zinc vapor. If the substrate is not pre-heated, alloying will not take place between the zinc and the substrate when the vapor initially deposits until the substrate is increased in temperature by the action of the zinc vapor depositing on the substrate. In this regard, the deposited zinc is of a lower energy than the vapor prior to deposit, and hence must be subjected to a higher substrate temperature in order to produce the desired alloying between the zinc and substrate. It is believed, then, that by pre-heating the substrate, proper alloying during the zinc coating step may take place at a lower temperature with the higher energy zinc vapor.
Samples of steel plate coated with copper were produced in which the steel base was first cleaned and then approximately 4 .times. 10.sup..sup.-6 or 12 .times. 10.sup..sup.-6 inch of copper was deposited upon the cleaned steel base. Different samples of both copper thicknesses were subjected to pre-heating within the temperature range 300.degree. to 750.degree. F. (in increments of 100.degree. F.). Following this, and while the substrate was still hot, the zinc was vapor deposited upon the pre-heated copper coated steel to produce relatively thick zinc coatings (over 0.5 .times. 10.sup..sup.-3 inch in thickness). For each sample, the substrate temperature during vaporization of zinc varied over a range that encompassed the pre-heat temperature. Typically, the substrate temperature during zinc vaporization for each sample varied from 150.degree. below the pre-heat temperature to 200.degree. above the pre-heat temperature. For example, in the case of a sample coated with 4 .times. 10.sup..sup.-6 inch of copper and pre-heated to approximately 500.degree. F., vaporization of zinc took place within a range of 275.degree. to 600.degree. F. Zinc coatings of excellent or good adherence were generally produced for samples pre-heated to between 150.degree. and 600.degree. F.
Samples similar to those just described were pre-heated to temperatures within the range 175.degree. to 660.degree. F. (in increments of 25.degree. to 100.degree. F. for the different samples). Each copper plated sample was cleaned by glow discharge for 30 seconds (glow discharge cleaning is not believed necessary for adherent coatings) and was zinc vapor plated within a range having as a lower limit the pre-heat temperature and an upper limit approximately 50.degree. higher than the pre-heat temperature. The samples pre-heated to between 225.degree. and 660.degree. F. all provided zinc coatings having excellent adherence (thin coatings, less than 0.5 .times. 10.sup..sup.-3 inch thick).
It has also been found that higher condensation rates in the order of 608 .times. 10.sup..sup.-6 inches/min. compared with lower condensation rates in the order of 64 .times. 10.sup..sup.-6 inches/min. provide better coatings of zinc on copper over steel. A number of steel bases were cleaned and plated with copper and then coated with zinc by vapor deposition in vacuum in which the rate of vaporization varied. None of the samples was pre-heated; each sample was brought to a temperature within the range approximately 300.degree. to 600.degree. F. (in increments of roughly 50.degree. F. for the different samples). All the samples coated with zinc at a relatively high condensation rate (600 .times. 10.sup..sup.-6 inches per minute) provided zinc coatings having excellent adherence. Samples coated with zinc at a relatively medium condensation rate (250 .times. 10.sup..sup.-6 inches per minute) provided coatings of excellent adherence except those deposited at temperatures of 300.degree. to 350.degree. F. Samples coated with zinc vapor at a relatively low condensation rate (60 .times. 10.sup..sup.-6 inches per minute) provided coatings of excellent adherence only within the range 400.degree. to 500.degree. F. As a result, it is believed desirable to vaporize at a relatively high condensation rate, in which case vapor condensation can take place over a substrate temperature range from 300.degree. to 600.degree. F. or 250.degree. to 660.degree. F., as noted above.
The time during which vaporization of the zinc onto the copper takes place is not critical except in its effect upon the raising of the temperature of the steel substrate. As the vapor is deposited on the substrate, the temperature of the substrate is raised. The time of vaporization thus should not be so long as to raise the temperature of the substrate above the 600.degree. or 660.degree. F. upper limiting temperatures referred to above.
It has been found that strongly adherent coatings are produced when the vacuum level is maintained at 20 microns of mercury or less. Samples of steel base were cleaned and copper plated and then coated with zinc vapor under vacuum; for each sample the substrate temperature varied from room temperature to approximately 450.degree. F. during vaporization. For those samples of which the vacuum level was maintained under 20 microns, zinc coatings were produced having excellent adherence.
It has been found that post heating of the zinc and copper coated steel plate improves the adherence of the zinc coating. Samples of steel plate were prepared with approximately 4 .times. 10.sup..sup.-6 inch of copper. The zinc was vapor deposited in vacuum at a substrate temperature of approximately 250.degree. F. In all cases the initial adherence of the zinc coating was poor. The samples were then post heated in air for 5 minutes to temperatures in the range 300.degree. to 750.degree. F. (in increments of generally 50.degree. or 100.degree. for the different samples). The poor adherence was improved by post heating within the range 400.degree. to 500.degree. F.
It is to be understood that the invention is not limited to the specific operations and compositions herein described but may be carried out in other ways without departure from its spirit.
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