US 3778315 A
Description (OCR text may contain errors)
Dec. 11, 1973 P. p. 550mm ET AL 3,113,315
COATING PROCESS Filed May 24, 1971 2 Sheets-Sheet 1 16x 7] i a. I 22% 10 FIG. 1
INVEN'IURS LIP P. BOOKER HARD ZELIZNAK BY gnu-w Agent Dec. 11, 1973 P. P. BOOKER ETAL COATING PROCESS 2 Sheets-Sheet 2 Filed May 24; 1971 INVENIORS PHILIP F? BOOKER RICHARD ZELIZNAK Agent United States Patent 3,778,315 COATING PROCESS Philip P. Booker, Burlington, Ontario, and Richard Zeliznak, Thorold, Ontario, Canada, assignors to Cominco Ltd., Montreal, Quebec, Canada Filed May 24, 1971, Ser. No. 146,107 Claims priority, applicatgologanada, June 11, 1970,
3 Int. Cl. C23f 7/02 US. Cl. 148-63 8 Claims ABSTRACT OF THE DISCLOSURE A process for the production of coloured coatings on articles coated with a carrier metal of zinc, tin or lead-tin alloys having an oxygen-avid additive alloyed therewith, by oxidation of the said carrier metals to form an oxide film having discernible light interference colour effects. The oxide film thickness is controlled to produce desired colour effects such as by heating the coating during oxidation, by avoiding contact with preformed oxides and by pretreating articles to 'be coated to remove deleterious oxidizable elements, particularly in the operation of a continuous process.
BACKGROUND OF THE INVENTION This invention relates to the production of coloured coatings and more particularly relates to a process and apparatus for the continuous production of coloured coatings on the surfaces of articles by applying to such' a surface a carrier metal selected from the group consisting of zinc, tin and certain lead-tin alloys, having an oxygenavid additive as hereinafter defined alloyed therewith, and subsequently treating the coated surface of such an article under oxidizing conditions to form an oxide film of said oxygen-avid additive, which oxide film provides light interference colour effects.
In US. Pats. 3,530,013 and 3,630,792, there are described processes for producing oxide films on the surfaces of articles, which films have attractive and predictable light interference colour effects. In the broadest scope of the processes of the aforementioned patent applications, an article is coated with a carrier metal selected from the group consisting of zinc, tin and certain lead-tin alloys having an oxygen-avid additive alloyed therewith in an amount sutficient to form on the coating, upon reaction of the surface of the coating in molten form with free oxygen, an oxide film derived from the additive and having the desired discernible light interference colour effects.
The processes of the said patent applications are applicable to the formation of coatings on both non-metallic substrates such as graphite and on articles of various metals such as iron, steel, copper and nickel. In these processes, the zinc, tin or lead-tin alloy having the oxygen-avid additive alloyed therewith can be applied to the surface of the article to be coated by any suitable procedure but particularly effective results are obtained by applying it by dipping the article into a melt of the alloy or by spraying or flowing the molten alloy onto the surface of the article. It is to be noted that elaborate prepolishing of the article is not required prior to the coating operation.
The resulting coated surface is then contacted with a free oxygen-containing gas, such as air, with or without further heating, and then cooled to form 'a solid coating with a thin oxide film derived from the oxygen-avid additive and producing the desired light interference colour effect. The thickness of this oxide film, and hence the final colour, is dependent inter alia upon the alloy composition, the alloy temperature and the period of time for which the coating is permitted to react with oxygen at elevated temperature. A detailed consideration of the cf- 3,778,315 Patented Dec. 11, 1973 fects of these several factors is given in the aforementioned patents.
Typical oxygen-avid additives for use in the described process are titanium, manganese, vanadium, columbium, zirconium thorium and mischmetal. Using zinc as the carrier metal, coloured coatings can also be obtained using as the additive cadmium, arsenic, copper, lead and chromium. Especially attractive surface coatings are obtainable by the use of titanium, vanadium and manganese with zinc as the carrier metal. The lead-tin alloys which can be used as the carrier metal are those alloys which contain at least 5% by weight tin.
It will be understood that the term oxygen-avid additive refers to an element that forms a stable oxide film of suitable thickness for the production of light interference colour effects. Elements such as sodium and potassium that will not form stable oxide films, and aluminum and magnesium that will not form a film of suitable thickness, do not provide discernible light interferenece colour effects and are not included in this term for the operation of the process of the invention.
It is known that the presence of aluminum in the coating alloy material in amounts in excess of from about 0.002% to about 0.005% by weight prevents the formation of titanium, manganese and vanadium oxide films having satisfactory light interference colour effects and it is believed that this results from the preferential oxidation of such aluminum to give a film of aluminum oxide which in turn prevents the formation of the required oxide film giving the desired colour effects. Preferably, the aluminum content of the alloy applied to the article should be below about 0.0005% by weight. Similarly, it is known that the production of titanium, manganese and vanadium oxide films having the desired light interference colour effects is prevented by the presence of small amounts (0.004 to 0.006% by weight) of magnesium in the alloy used for coating the surface of the article. In general, it is believed that the formation of the desired oxide film will be hindered, if not completely prevented, by the presence in the coating alloy of an element that oxidizes more readily than the oxygen-avid element, as defined, to form a stable oxide that inhibits the colouring process. In that it is absolutely essential to maintain the concentration of these impurity elements, such as aluminum and magnesium, in the coating alloy below very low limits, the use of the process hereinbefore described for the production of oxide films having colour effects on articles which include such impurity elements at least in their surfaces is not feasible.
For example, the described process could not heretofore be used for the treatment of articles which had been pregal'vanized using conventional galvanizing baths since use generally is made in such baths of zinc containing a minor but significant amount of aluminum to minimize the formation of iron zinc alloy in the coating in order to improve product formability. Articles pregalvanized using such baths generally contain from about 0.15 to about 0.2% by weight aluminum in their surface coatings. If a plurality of such pregalvanized articles is treated in a molten alloy bath of the carrier metal and an oxygen-avid additive, for the purpose of producing an oxide film showing light interference colour effects, the aluminum content of the molten alloy bath soon rises to a value at which the formation of the desired oxide film is completely in-- hibited. This problem is particularly serious in the application of the described process to the continuous coating of strip material in which case the impurity content of the alloy bath will progressively and continuously increase.
These limitations have heretofore resulted in the exclusion of the use of the described process of the treatment of articles containing significant proportions of impurity elements, such as aluminum and magnesium, and have also prevented the application of the described process to articles which have been pretreated so as to have protective coatings thereon where such protective coatings contain a significant amount of such an impurity element.
It is an object of the present invention therefore to provide a process whereby articles Which have been pregalvanized using a conventional galvanizing bath containing a significant proportion of an impurity element such as aluminum or magnesium can be treated with an alloy of the aforementioned type for the purpose of producing oxide films having light interference colour effects without risk of excessive build-up of the impurity element in the molten alloy used for forming such an oxide film.
It is another object of the present invention to provide a process especially adapted to the continuous production of oxide films showing light interference colour effects on strip material without risk of formation of such oxide films being hindered or prevented by the presence of impurity elements as hereinbefore defined at least in the surface of the strip material being treated.
Accordingly, the present invention provides in a process as described in the aforementioned patent applications for colour-coating article surfaces in which said article initially includes at least in its surface an oxidizable impurity element capable of forming an undesirable stable oxide under said oxidizing conditions, the step which comprises pretreating said article with an extractant to remove said oxidizable impurity element at least from the surface of said article, whereby subsequent production of said oxide film having light interference colour effects is not hindered or prevented by the presence or formation of said more readily formed oxide.
The selection of a suitable extractant for removing or at least reducing the content of the impurity element pres ent in the surface of the article to be treated Will depend on the nature of the impurity as well as on other process parameters. In general, the extractant will be a molten metal selected from the group consisting of zinc, tin and the aforementioned lead-tin alloys and will preferably be the same as the metal used as the carrier for the oxygenavid additive which forms the light interfering oxide film after the subsequent oxidation treatment. The pretreatment of the article in accordance with the invention can usefully be effected by immersing or dipping the article in a molten bath of the extractant such as by continuously passing said article in strip form through an extraction zone in which the surface of the strip material is contacted with an extractant to remove said oxidizable element at least from the surface thereof so that subsequent production of the oxide film having light interference colour effects is not hindered by the presence or formation of the more readily formed oxide.
Although it is reported in the aforementioned patents that the processes described therein might be carried out either as a batch-type operation or in a continuous manner using a molten bath of the carrier metal having the oxygen-avid additive alloyed therewith, it has since been found that, particularly in continuous operations, difficulties may arise in practice due to the formation on the surface of the molten alloy bath of dross in the form of an oxide film of the oxygen-avid additive. As a strip of the material to be coated by the described proces is withdrawn from such a molten bath, the strip shows a pronounced tendency to pick up such a preformed film from the molten bath and this in turn causes the final oxide film on the strip material to have a varying thickness and consequently an unpredictable and irregular colour effect.
We have found that contact of the coated strip with preformed oxide of the oxygen-avid additive can be substantially obviated by the provision of a pair of partially submerged and pivotally mounted rollers between which the coated strip is withdrawn. Such rollers will, of course, be formed from a material, such as graphite, which is nonreactive with respect to the bath of molten alloy.
A useful alternative procedure for preventing contact of the coated strip with preformed oxide of the oxygenavid additive is to substantially inhibit the formation of such preformed oxide on the surface of the bath of molten alloy at least at that part of the surface through which the coated strip is withdrawn. Under normal circumstances, such formation of preformed oxide can be prevented by the exclusion of free oxygen, for example, as present in air, from the surface of the molten bath. Although the invention embraces the use of liquid and solid covers for the baths so as to exclude free oxygen therefrom, particularly effective results have been obtained using non-oxidizing gaseous covers in contact with the surface of such a bath or molten aloy.
A wide range of non-oxidizing gaseous atmospheres may be used for this purpose. It is, for instance, possible to utilize one of the inert gases such as argon or to use oxygen-free nitrogen. Hydrogen may also be used or be present in such a gaseous atmosphere.
Since the strip material leaving the molten bath of alloy carries on its surface a non-oxidized coating of the alloy of the bath and since this non-oxidized coating is to be subsequently oxidized, it is possible in accordance with a particularly beneficial feature of the present invention to utilize for forming the non-oxidizing gaseous cover for the bath a commercially available argon which contains minor but signficant amounts of both hydrogen and oxygen. To this end, the argon is first fed to a preliminary oxidation zone through which the strip material coated with the alloy first passes. In this zone, the relatively small amount of oxygen in the argon is substantially completely eliminated by reaction with the coating on the strip. The resulting non-oxidizing gaseous stream is then passed to an appropriate enclosure over the bath of molten alloy to form the required non-oxidizing gaseous atmosphere in contact with the surface of the bath.
It is another object of the present invention therefore to obviate the formation of dross on the melt surface, particularly in the surface area of the melt through which the strip is withdrawn, or to employ mechanical aids to remove said dross from said surface area, to prevent contact of the coated strip with preformed oxides.
As explained in the aforementioned Pat. 3,630,792, the oxide films causing the light interference colour effects are believed to consist of oxides of the oxygenavid additive which is used. These additives appear to react preferentially with oxygen. The oxide films obtained generally have thicknesses within the range of about A. to about 3100 A. and are formed in layers with an increasing oxygen to metal ratio towards the surface of the film. These surface oxide layers are underlaid by a layer of the carrier metal, for example, zinc, tin or the lead-tin alloy, which layer may be alloyed with the oxygen-avid additive or may be substantially completely depleted of such additive. The formation of the desired brilliant colour effect by an optical interference mechanism is believed to be possible only if this underlying layer of carrier material or carrier metal alloy is present to reflect that portion of the incident light which is transmitted inwardly through the surface oxide film. In cases where the process is used to form an oxide film having a light interference colour effect on a metallic article or substrate, it has been found that the aforementioned reflective surface of carrier metal or alloy is bonded to the metallic substrate through transition layers made up of alloys of the carrier metal and the metal of the substrate in varying relative proportions.
In order to ensure that the required light interference colour effects are obtained, it is necessary to avoid the growth or extension of the aforementioned transition layers made up of alloys of the carrier metal and the metal of the substrate outwardly into the optically reflective layer comprising the carrier metal possibly together with unoxidized oxygen-avid additive.
Also, thick growths of the said transition layers have been found to reduce the ductility of coated strip and sheet articles relative to articles having thin transition layers.
Since outward growth or extension of these transition layers occur in part during the heating of the article for the purpose of oxidizing the coating thereon, it will be understood that, if the article is heated excessively for such oxidation, the transition layers may grow sufficiently to reach the reflective layer and, therefore, prevent the desired colour effects being obtained. Formation and growth of the transition layers also occur during the application of the coating, such as when the article is immersed in a bath of the alloy when the latter is applied as a molten alloy. For these reasons, the process described when applied to a metallic substrate such as steel is somewhat restricted in the extent to which the oxide films can be developed.
It is accordingly the principal objcet of the present invention to provide a process of the type described in which rapid development of the oxide film is promoted but in which growth of the aforementioned transition layers is slow thereby permitting a wide range of colour effects to be obtained and providing ductile strip and sheet having good forming properties. This result is achieved in accordance with the invention by carrying out the process in such a way that the time for and the temperature of application of the coating are minimized and heat is applied to the article so as to selectively control oxide film development and maintain the optically reflective layer essentially free of the metal of the substrate.
SUMMARY OF THE INVENTION In general, the present invention provides a process for the production of a coloured surface on an article of a base or supporting metal by applying to the surface of said article a molten coating of a carrier metal selected from the group consisting of zinc, tin and leadtin alloys, having a selected oxygen-avid additive alloyed therewith, and maintaining said surface under oxidizing conditions to oxidize said coating to form a surface oxide film having light interference colour effects, an underlying transition layer comprising an alloy of said base metal and said carrier metal and, intermediate said surface oxide film and said underlying layer, an optically reflective surface comprising said carrier metal, in which process said article is heated so as to selectively supply heat to said coating so that the development of said surface oxide film is effected before growth of the transition layer causes base metal to enter said optically reflective layer and said article is cooled to form thereon a solidified coating that has an optically reflective layer that is essentially free of said base metal and that has good forming properties. Also, the present invention provides for removing deleterious elements if present, from article surfaces to be coated and for precluding the pick-up of dross on article surfaces exiting from a melt surface to provide colour control in the continuous production of articles having discernible light interference colour effects.
BRIEF DESCRIPTION OF THE DRAWINGS The process of the present invention will now be de-' scribed with reference to the accompanying drawings, in which:
FIG. 1 is a sectional view of a metal coated according to the process of the invention;
FIG. 2 is a sectional view of an embodiment of the process of the invention showing immersion of strip in a coating bath and post-heating of the coated strip;
FIG. 3 is a schematic illustration of an embodiment of the process of the invention showing immersion of strip in a heated pretreatment bath and immersion in a coating bath with dross control by means of exit rolls; and
FIG. 4 is a schematic illustration of another embodiment of apparatus for control of bath dross.
DESCRIPTION OF THE PREFERRED EMBODIMENTS With particular reference to FIG. 1, a section of base metal such as steel is shown coated, by way of illustration, with a zinc-titanium alloy applied according to the process of the present invention in the sequence of a ferrous base metal 10, Fe-Zn transition layer 12, Zn-Ti alloy 14 (with Ti partially depleted) optically reflective surface 18 formed on the Zn-Ti alloy, and titanium oxide film 16 in the form of TiO and Ti O having light interference characteristics. For uniform and controlled colour production it is essential the surface oxide film 16 producing light interference characteristics have a uniform thickness and thus be free of irregularities due to the pick-up of preformed oxide films in the form of dross. Also, it is imperative that the transition layer 12, Fe-Zn in the illustration shown, not extend into the optically reflective surface 18 to interrupt the plane of the surface. Also, to permit sharp bends in the coated strip or sheet without cracking of the coating, transition layer 12 should be as thin as possible. The presence in the zinc, tin or lead-tin alloys of elements in amounts which preferentially form unstable oxide films or oxide films not having discernible light interference effects, such as aluminum or magnesium as has been discussed, precludes the formation of the oxide film shown in FIG. 1.
The post-heating apparatus of the present invention may be used to alter and to provide controlled development of colour effects on, for example, strip emerging from a coating bath, as hereinafter explained.
It will be understood that if the coating of the applied metal having the oxygen-avid additive alloyed therewith, i.e. carrier metal, is applied to the article of the base metal using a molten alloy of the said carrier metal, transition layers comprising alloys of the base and carrier metals will be formed during the coating operation and it will generally, therefore, be necessary to restrict the temperature of the bath of molten alloy or the residence time of the article in such a bath so as to prevent excessive growth of the transition layers during the coating operation. Such restriction of the bath temperature or residence time, by failing to provide elevated molten coating temperatures required for higher order colours, will restrict the range of colour effects which can be obtained. In accordance with the invention, the range of colour effects which can be obtained is broadened by post-heating the coated article so as to selectively upply heat to the surface of the coating to promote the desired development of the surface oxide film while restricting the growth of the transition layers so that such transition layers do not 1nterfere with the aforementioned optically reflective surface and do not decrease ductility of the strip by becoming unnecessarily thick.
Although the selective heating of the surface of the coating can be effected in any appropriate manner, it has presently been found that the use of radiant heating for this purpose is somewhat impractical. The highly reflective layer underlying the surface oxide film reflects a large proportion of the incident heat and, in order to obtain sufficient heating of the surface film, it is consequently necessary to use a very high intensity heat output radiant energy heating source. More effective and sufliciently selective heating operations have been carried out using a high temperature flame impinging on the surface of the coated article and by the use of electrical induction heating of such a frequency that a major portion of the heat generated in the article is provided selectively in the surface coating.
In continuous processes of this type, the coating alloy may conveniently be applied to the strip material by continuously passing the latter through a molten bath of the said coating alloy metal having the oxygen-avid additive alloyed therewith, as has been described, or alternatively be applied to the strip material in other ways if so desired such as by flowing or spraying. With the use of a molten alloy bath, the present invention permits shorter residence times for the strip in the molten alloy bath and the use of lower bath temperatures.
In the arrangement shown in FIG. 2, strip material such as steel strip 11, that ha been chemically cleaned or that has been coated with zinc of such a grade that metals contained therein do not interfere with the production of desired coloured coatings, is fed in the direction of the arrows A over steel guide roll 82 into a heated vessel 84 containing a molten alloy bath 86 of zinc and an oxygenavid additive such as titanium, manganese or vanadium. Guided by submerged graphite guide rolls 88, the strip 11 leaves the bath 86 and passes through a heating furnace 90 to a steel guide roll 94 and then to a take-up coil 96.
In the construction illustrated, the furnace 90 is provided with a side inlet 92 through which air may be introduced to ensure an adequate supply of free oxygen in contact with the surface of the coated strip material to effect desired oxidation of the coating. Air flow is shown by arrows B. After leaving the furnace 90, the oxidized coated strip material cools and the coating hardens before the strip reaches the guide roll 94.
Previously colour-coated strip may in some instances be post-heated to alter the colour of the strip. However, postheating normally is done immediately after coating of the strip while the coating remains in a molten state.
With reference now to FIG. 3, an embodiment of the process of the present invention will be discussed in which steel strip 13, which has been pregalvanized with zinc containing from about 0.15% to about 0.20% by weight aluminum, is colour coated. Strip 13 is fed in the direction of the arrow A through a heated pre-treatment vessel 17 having a bath of molten zinc 19 at about 450 C., said strip being guided through this bath by steel rolls 20 and 21 and by submerged graphite rolls 22 and 24 for about to seconds immersion. From the pretreatment bath 17, the strip 13 passes over steel guide rolls 21 and 26 into a heated vessel 28 containing a bath of a molten alloy 30 of zinc and an oxygen-avid additive such as titanium, manganese or vanadium. Strip 13 is guided by submerged graphite guide roll 32. The molten zinc 19 in vessel 17 protects bath 30 and the surface of strip leaving bath 30 against contamination by excess aluminum in the zinc used for pregalvanizing. After leaving bath 30, the coating on strip 13 will be oxidized to the extent required to give the desired colour effect before the strip material reaches the coil 96. The strip may be cooled to control the development of colours that immersion time and temperature permit, as explained in the aforementioned patent application, or may be post-heated to control the development of higher order colour effects by passage through furnace 90 or by flame impingement as hereinbefore described.
Referring further to the bath 30 of molten zinc alloy, it will be noted that reaction of the molten material of the bath with the free oxygen in the air above the bath results in the formation of a surface film 38 comprising an oxide of the oxygen-avid additive present in the zinc alloy.
In accordance with an important feature of this particular embodiment of the invention, contact of the surface oxide film 38 with the strip material 13 as the latter is withdrawn from the molten alloy is substantially complete obviated by the provision of a pair of partially submerged rolls 40 and 42 which are pivotally mounted for free rotation in the directions of the arrows C and between which the strip material 13 passes as it is withdrawn from the alloy bath 30. The rolls 40 and 42 may as previously indicated be formed from any suitable non-reactive material such as graphite.
It will further be understood that in a continuous operation with this arrangement a small pool 44 of molten alloy will be maintained between the rolls 40 and 42 and that this pool will be automatically and continuously replenished with the molten zinc alloy. As a result, the strip 13, as it is withdrawn from the alloy bath 30, is continuously in contact with substantially non-oxidized molten alloy. There may be slight surface oxidation of the material of the pool 44 but this will generally be insufiicient to adversely affect the formation of the desired uniform coating on the strip 13. The rolls 40 and 42 may be mounted at a fixed separation or they may be spring mounted so as to be able to accommodate strips 13 of different thicknesses. Control of the development of colour effects may be effected by post-heating by passage through furnace or by flame impingement as hereinbefore described. In a typical operation using %-inch diameter graphite rolls with steel strip having a width of 3 inches and a thickness of 0.024 inch, effective results were obtained with bath 17 maintained at a temperature of 455 C. and bath 30 containing zinc including 0.1% by weight titanium maintained at a temperature within the range of from 530 to 550 C. for strip speeds within the range of from 5 to 30 feet per minute. After cooling to room temperature, the resulting oxide film showed a gold colour interference effect of highly uniform quality. The aluminum and magnesium contents of bath 30 were maintained well below the maximum limits of 0.002 to 0.005% and 0.004 to 0.006% by weight respectively.
Reference will now be made to the alternative procedure in accordance with the present invention as illustrated in FIG. 4. It will be seen from FIG. 4 that the vessel 50 is provided with a hood 52 which contains a gaseous atmosphere or cover designated generally at 54 and which is in contact with the surface of the molten alloy 56. This gaseous cover 54 is essentially completely free of any oxidizing components, particularly free oxygen, so as to substantially completely present the formation of a film of oxide on the surface of the molten alloy bath 56. An opening 58 through the hood 52 permits passage of the strip material 15, that has been chemically cleaned or preglavanized like strip 11 in FIG. 2 or has been pretreated like strip 13 in FIG. 3, into the bath 56.
The arrangement shown in FIG. 4 can utilize a gaseous stream of, for example, commercial grade argon which contains a minor but significant proportion of oxygen. To prevent oxidation of the surface of the molten alloy 56 by this oxygen, it is essential that such oxygen be removed from the argon before the latter comes into contact with the surface of the molten alloy bath 56. For this purpose, the hood 52 is provided with an elongated camber 60 through which the strip material passes after its withdrawal from the molten alloy bath 56, the strip entering the chamber 60 through an inlet opening 62 and leaving this chamber through an outlet opening 64 and held in alignment with the said openings by guide rolls 51, 53 and 94. The argon or other suitable gaseous stream is fed in the direction of the arrow D into the chamber 60' through a gas feed inlet 66 and, to ensure adequate contact of the hot alloy coating on the strip 15 and the gaseous atmosphere within the chamber 60, a number of baffles 68 are provided within the chamber 60.
In operation, the oxygen present in the gaseous stream entering through the gas inlet opening 66 is removed from the gaseous stream by its reaction with the oxygen-avid additive present in the alloy coating on the strip material as this strip material passes through the chamber 60. The gaseous stream leaving the chamber 60 through the opening 62 is consequently substantially completely free of oxygen and its presence in the hood 52 excludes oxidizing gases from the surface of the molten alloy bath 56.
It will be understood that in operation there will be some loss of gas through the strip inlet opening 58 and through the strip outlet opening 64 and that such loss will be made up by further gas supplied through the gas inlet opening 66.
After leaving the elongated chamber 60, the oxidation of the coating on the strip material 15 will be developed to the extent required to give the desired colour effect before the strip material reaches the roll 94. The strip may be cooled to control the development of colours that immersion time and temperature permit, as explained in the aforementioned patent application, or may be post-heated to control the development of higher order colour effects by passage through furnace 90 or by flame impingement as hereinbefore described.
In a typical operation using the arrangement illustrated in FIG. 4, steel strip 15 having a thickness of 0.024 inch and a width of three inches was fed through the equipment at a linear speed of 30 feet per minute. The molten alloy 56 was maintained at a temperature within the range of from 530 to 550 C. and comprised an alloy of zinc including 0.1% by weight of titanium.
In experiments using this technique, argon which was fed into a hood at a rate of about three cubic feet per minute to maintain a slight positive pressure within the hood had the following composition:
Hydrogen Less than 1 p.p.m. Nitrogen Less than 10 p.p.m. Oxygen Less than p.p.m. H O Do. Hydrocarbons Less than 1 p.p.m. Carbon dioxide Do.
The effectiveness of this procedure in preventing the formation of an oxide film on the surface of the molten alloy 56 was apparent on visual inspection. After cooling to room temperature, the oxidized film on the strip material 15 showed a gold coloured light interference effect of exceptionally uniform quality.
In a typical operation in which immersion was followed by post-heating, a three inch wide steel strip having a thickness of 0.024 inch was fed through the apparatus and bath at a linear speed of 20 feet per minute. The molten alloy 56 comprising zinc containing 0.1% by weight vanadium was maintained at a temperature of 530 C. The furnace 90 had a 20 kw. inductor therein having a length of 21 inches spaced about one-quarter inch from the strip and was operated using a supply current having a frequency of 20 kilocycles per second.
With the furnace inoperative, the oxidized strip material showed a first order pale gold colour effect when cooled to room temperature whereas, when the furnace was operated, a range of colours including golds, bronzes, reds and blues was obtained by varying the power input to the inductor. It was also possible to regulate the final colour effect of the oxide film by varying the speed of the strip as it passed through a constant field strength.
It was found that suffiient heat could be induced into the zinc alloy coating to provide rapid surface oxidation before the growth of iron-zinc alloys layers damaged the highly optically reflective zinc interface.
Flame impingement on cooled strip for selective heating also was found effective in providing colour control. A three inch wide strip having a thickness of 0.024 inch was fed through the apparatus and a molten bath of zinc with 0.1% by weight titanium maintained at a temperature of 550 C. The strip produced upon normal cooling in air a pale gold colouration. A repeat of this procedure with an oxyacetylene flame played on the surface of the emerging strip produced all the titanium-type colours as desired, thereby providing an effective colour development control.
In addition to the colour control provided by postheating of coated strip, the shortened immersion time in the molten alloy bath of about three to five seconds, compared to 30 to 100 seconds without post-heating, resulted in the formation of a relatively thin iron-zinc alloy transition layer which gave the strip excellent forming proper ties. For example, strip coloured according to the process of this invention could be flattened on itself through a 180 bend without cracking the coating. The arms of strip that was immersed for more than 30 seconds could .be understood that similar considerations apply to the use of other oxygen-avid additives. The different alloying additives produce different colour shades and intensities, for example, soft pastel shades from the titanium alloy systems and more intense colours from the vanadium systems. It will further be understood that the invention also includes the use of ternary, quaternary and higher alloys for the coatings.
In the case of baths containing titanium, vanadium and manganese in zinc, the lower practical limit for the additive content in the molten coating bath is of the order of 0.1% by weight. The upper practical limits of additives in the molten carrier metal are determined by the solubilities of the additives in the said carrier metal at the operating temperatures. Amounts of the additives in excess of the solubility limits and present in elemental or intermetallic compound form do not prevent the formation of coloured coatings.
Coating alloy compositions of zinc and titanium, manganese, vanadium, columbium, zirconium, thorium or mischmetal provide coloured coatings on steel and pregalvanized materials at temperatures within the range of from about 419 C., i.e. the melting point of the alloy composition, to about 600 C. and above.
The embodiments of the process of the present invention provide a number of important advantages. Deleterious oxygen-avid agents commonly present in galvanizing coatings can be removed from said coatings during or prior to the colour coating operation of the present invention. Molten coating baths can be controlled to obviate or minimize dross formation and thus substantially preclude the pick-up by coated articles of oxide films which might impair colour uniformity. The coating metal alloy can be applied in relatively thin layers to provide excellent formability of the finished coloured product and the oxide film development can be controlled to attain the desired colour by selective post-heating of articles that have been coated at the lower temperatures that make the excellent formability possible.
It will be understood, of course, that modifications can be made in the preferred embodiments of the invention described herein without departing from the scope of the invention as defined by the appended claims.
1. A continuous process for the production of a coloured surface on an article comprising strip of a base metal which comprises applying to the surface of said article a coating of a carrier metal selected from the group consisting of zinc, tin and lead-tin alloys having an oxygen-avid additive alloyed therewith effective to produce a surface oxide film having discernible light interference colour effects and having less than about 0.005 percent by Weight aluminum and less than about 0.006 percent by weight magnesium, immersing said strip of base metal in a molten bath of a carrier metal alloy for from about three to five seconds to form a thin coating of carrier metal alloy thereon bonded to said base metal by a transition layer comprising an alloy of said base metal and said carrier metal; withdrawing coated strip from said bath; maintaining an atmosphere of a nonoxidizing gas in contact with the surface of said bath by first passing a gaseous stream containing a major proportion of a non-oxidizing gas and a minor proportion of free oxygen in countercurrent contact with withdrawn coated strip whereby free oxygen is completely removed from said gaseous stream by reaction with the oxygenavi-d additive in the coating of carrier metal alloy to form a partially oxidized coating and then passing said stream into contact with the surface of said bath to form said atmosphere; passing said strip with partially oxidized 1 1 coating from contact with said gaseous stream into contact with an oxidizing gas to continue oxidation of the surface of said coating in the molten state to form said surface oxide film and an optically reflective layer; selectively heating the surface of the coating on said article during said continuing oxidation, said immersion period and said selective heating ensuring that the development of said surface oxide film is completed to produce the desired colour before growth of said transition layer extends into and damages said optically reflective layer; and solidifying said molten coating provided with said oxide film.
2. A process as claimed in claim 1 in which said oxygen-avid additive is selected from the group consisting of titanium, manganese, vanadium, columbium, zirconium, thorium and mischmetal.
3. A process as claimed in claim 1 in which said oxygen-avid additive is selected from the group consisting of cadmium, arsenic, copper, lead and chromium.
4. A process as claimed in claim 2 in which said base metal is selected from the group consisting of iron, steel, copper and nickel.
5. A process as claimed in claim 1 in which said article having said coating thereon is heated by electrical induction heating.
6. A process as claimed in claim 1 in which said ar 12 ticle having said coating thereon is heated by a high temperature flame means.
7. A process as claimed in claim 4, said base metal being prepared from steel strip pregalvanized with Zinc containing aluminum, in which said article is pretreated for removal of aluminum by immersion in a bath of the molten carrier metal prior to application of the carrier metal with the oxygen-avid additive alloyed therewith.
8. A continuous process as claimed in claim 1 in which said free oxygen is excluded from the surface of said bath of molten carrier metal alloy by maintaining a nonoxidizing atmosphere comprising a substantial portion of argon in contact with the surface of said bath.
References Cited UNITED STATES PATENTS RALPH S. KENDALL, Primary Examiner US. Cl. X.R.