US 3812021 A
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United States Patent Oflice 3,812,021 Patented May 21, 1974 3,812,021 INORGANIC COATINGS FOR ALUMINOUS METALS H. Lee Craig, In, Miami, Fla., and Harold J. Coates, Richmond, Va., assignors to Reynolds Metals Company, Richmond, Va. No Drawing. Filed Dec. 11, 1972, Ser. No. 314,209 Int. Cl. C23b 9/02 U.S. Cl. 204-58 10 Claims ABSTRACT OF THE DISCLOSURE Siliceous coatings having good adhesion, hardness and smoothness and corrosion resistance are produced on the surface of an aluminous metal by immersing the metal in an aqueous bath of an alkali silicate, containing an alkali chromate, molybdate, or tungstate, and further containing an alkali carbonate, phosphate, or borate, and passing current through said bath at a voltage predominantly positive and sufficient to cause a uniform scintillation or flashing at the anodesurface. The coating thus obtained is decorative and scratch-resistant and also provides a good base for a cover coating such as a plastic.
BACKGROUND OF THE INVENTION The present invention relates to an improvement in the coating of aluminous metals.
The coating of aluminum and its alloys in order to obtain improved corrosion and abrasion resistance and to impart color to the metal surface is well known in the prior art. One important finish for aluminum is the electrolytic oxide coating produced by treatingthe metal as anode in various electrolytes to obtain a thin, inert and durable aluminum oxide film on the metal surface. This process is known as anodizing and the film formed is very hard and resistant to abrasion and corrosion, thus protecting the underlying metal. The anodic coatings may be clear and transparent, or they may be colored by the application of organic dyes or inorganic pigments to the coating, or by production of the colored oxide film directly from the electrolyte by varying the ingredients or by use of suitable additives.
More recently, proposals have been made to coat aluminum and other metals by deposition of a film thereon, using the metal as anode in a bath of sodium silicate, aluminate, or tungstate, employing an anodic spark re action wherein the voltage applied to the system is sufficiently high to cause sparking at the anode surface. The coatings thus produced are generally light in color, and quite hard. They have the disadvantage, however, of requiring rather high voltages for application, of the order of 400-600 volts D.C., and their field of usefulness is limited by their rough surfaces and their limited adhesion to the aluminous metal.
BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a novel alkali silicate system and method suitable for the coating of aluminous metals. The term aluminous metals as employed herein means aluminum and aluminum base alloys.
The invention concerns the treatment of an aluminous metal article as anode in an aqueous bath of an alkali silicate by immersing the article in said bath and passing an electric current through the bath accompanied by uniform scintillation or flashing at the anode surface, to deposit electrokinetically an adherent siliceous coating on the metal surface. This scintillation is to be distinguished from sparking or arcing which will be detrimental to the coating. It has been found, in accordance withthe invention, that a smoother, harder and more adherent coating of the type thus produced is obtained when there is incorporated in the silicate bath a suitable amount of an alkali chromate, molybdate, or tungstate so that the average voltage read as DC. volts required to obtain the scintillation at the anode surface is greatly reduced, only between about 150 and about 350 volts being thereby required to achieve the scintillation level.
It has been found further that by including in the silicate bath a suitable amount of an alkali carbonate, phosphate or borate, there is provided a fluxing action which fuses the coating to the metal surface.
Coatings thus produced on aluminous metal! surfaces are less expensive than conventional anodic oxide coatings by virtue of increased speed of operation and greater electrical efiiciency. The resulting articles are useful for the production of architectural and automotive trim, electrical products, packaging materials, and consumer appliances. They have a glass-like or vitreous finish, a matte, opaque appearance, and generally a light gray color. The coatings are chemically composed principally of silica with smaller amounts of alumina and of the metal components of the silicate bath.
DETAILED DECRIPTION OF THE INVENTION In accordance with the present invention, there is ap plied to the surface of an article of an aluminous metal, a siliceous coating, employing electrokinetic deposition from an aqueous bath of an alkali silicate, the article being made the anode.
It has long been known that an insulating film could be formed on the surface of aluminum or magnesium by anodic oxidation in an alkali silicate as an electrolyte.
In the electrokinetic type of deposition employed in accordance with the present invention, the aluminous metal as the anode or positive pole is subjected to electrolytic treatment in an aqueous alkali silicate solution under conditions and at a voltage suflicient to cause a uniform scintillation or flashing at the anode surface.
This is achieved by control of the composition of thealkali silicate electrolyte, and by employing pulsating current having an average voltage read or D.C. volts ofthe order of about 150 to about 350 volts, the voltage being predominantly positive.
The alkali silicate employed may be, for example, potassium or sodium silicate. As available commercially, these substances are predominantly meta silicates, i.e. K SiO or Na SiO with ratios of S102 to K 0 or Na O varying widely. In accordance with this invention, there is employed an alkali silicate having a relatively high ratio of SiO to K 0 or Na O, ranging from about 4:1 to about 1.5 :1 in order to protect the bath against depletion of SiO through deposition. Preferred is potassium silicate K SiO having a molar ratio of SiO :K O of about 2.5 1. The practice of the invention will be described with regard to the use of potassium silicate, which provides a higher mole ratio of SiO to K 0, but it is to'be understood that this is for purpose of illustration only, and is not to be considered as limiting.
While potassium silicate may be prepared having any predetermined SiO /K 'O ratio, it is preferred to employ as a source thereof a commercially available aqueous solution sold under the designation Kasil No. 88 (Philadelphia Quartz Co., Philadelphia, Pa.) which has a SiO /K O mol ratio of 3.45, and contains 19.9% SiO' by weight. This product contains about 30 weight percent of solids, has a pH of about 12.14 and is as viscous as a light oil. The concentration of the Kasil No. 88 may be adjusted by dilution with water between practical limits of about 1 volume Kasil to 3 volumes (5.9% SiO by weight) to 1 Kasil: water (0.2% Si0 by weight). Too great a concentration of K SiO produces a nonuniform coating with loose particles about the surface. Too dilute a solution 3 produces a thin poorly distributed coating. However, the Kasil No. 88 may be used at its original SiO- /K O mol ratio of 3.45. To high a ratio results in an unstable solution which tends to precipitate silica.
Thus, the invention contemplates the use of an alkali silicate bath in which the concentration of alkali silicate is between about 0.2% and about 5% by weight of equivalent SiO (0.04 to 1.0 molar). The SiO /K O mol ratio of Kasil No. 88 may also be modified by adding K in the form of KOH solution (e.g. using a solution of 45% KOH by weight). In this way there may be achieved a range of SiO /K O mol ratio between 3.45:1 and 1.5 :1.
In accordance with the present invention, in order to obtain a smoother, harder and more adherent coating, and to reduce the voltage as explained above, there is incorporated in the alkali silicate bath a suitable amount of an alkali chromate, molybdate, or tungstate, or mixtures thereof. Preferably the alkali is the same as that of the silicate. Examples of suitable additives are the potassium or sodium salts, preferably the potassium salts such as potassium chromate K CrO potassium molybdate K MoO or potassium tungstate K WO These compounds are added in amounts sufiicient to achieve a total concentration in the silicate bath between about 0.005 and about 1.0 molar, preferably between about 0.2 and about 0.5 molar.
Further, in accordance with the invention there is also incorporated in the alkali silicate bath a suitable amount of an alkali carbonate, phosphate, or borate, the alkali again being the same as that of the silicate, namely potassium or sodium. The potassium salts are preferred, such as, for example, potassium carbonate K CO potassium orthophosphate K PO or potassium tetraborate K2B4O7. These salt additives serve to provide a fiuxing which fuses the silica to the metal substrate and are present in the general concentration range of about 0.01 to about 1.0 molar, preferably about 0.3 to about 0.45 molar.
It has been found, surprisingly and unexpectedly, that a siliceous coating having optimum properties can only be obtained when both types of additives are included in the alkali silicate bath. Thus, if it is attempted to apply a coating of the type contemplated by the present invention using potassium silicate solution with only potassium carbonate added, a partial, irregular coating is obtained. Conversely, where potassium chromate is substituted for potassium carbonate, only a barrier (anodic oxide) film is produced. However, when all three ingredients are present, a smooth, uniform, gray, hard adherent coating is formed within a few minutes.
Bath temperatures may lie between slightly above the freezing point of the solution and about 50 0., preferably about room temperature. Too high a temperature tends to reduce film thickness. Coating time required may range from as little as one minute to about one hour, typically between about two and about 5 minutes.
The siliceous coating thus obtained analyzes about 75% to 80% silica, with about 5% to 7% of aluminum oxide present, possibly as a result of concurrent anodic oxidation. There is, however, a relatively high metal ion content present in the coating, ranging from about 10% to about 12% which differentiates the siliceous coatings of the present invention from those of the prior art, in which the alkali metal content is of the order of 12%.
Coating thicknesses of from about 0.10 up to 3.5 mils can be obtained, with thickness of 0.15 to 0.5 being typical. The coating thicknesses is dependent upon the SiO content of the bath. The siliceous coatings produced according to the invention are useful in many novel applications. One of these is as a dense, thin coating on aluminum conductor foil. Another is as a base coat for a polytetrafluoroethylene (Teflon) overcoat on cooking or baking utensils, including pie plates. These applications are illustrated in the examples below.
In accordance with another aspect of the invention, it was found that if direct current from a DC. generator was employed during the siliceous coating deposition process, it was difficult to prevent isolated burning or arcing at the anode, which results in formation of a thick crusty mass of fused silicate glass having poor adhesion. The use of alternating current alone, and without modification, tends to cause anodic oxidation of the aluminous metal during the positive phase, no deposition of silicate during the zero voltage interval, and formation of hydrogen at the maximum negative phase, resulting in gaseous displacement of the coating. 7
Hence, for purposes of the present invention, the power source employed is preferably a pulsed D.C. current, such as unfiltered, full-wave rectified alternating current. The wave form of the alternating current may be modified by the introduction of suitable rectifying means on the supply side of the circuit, such as a silicon rectifier, diode, or mercury arc rectifier. In this way there are supplied to the alkali silicate electrolyte bath and the electrodes, pulses of current at the desired predominantly positive voltage. This wave form inhibits the arcing and breakdown of the coating. It also avoids harmful negative voltages and the danger of removal of portions of the coating through generation of hydrogen.
The use of controlled composition of the electrolyte and of rectified alternating current, as described, results in uniform scintillation and the deposition of uniform reyishwhite, opaque coatings varying from matte to glossy in finish having good adherence to the aluminum substrate.
The choice of a suitable power supply will depend upon the maximum area of the article or workpiece being coat ed, as well as on the bath characteristics. There appears to be a minimum current density below which coating formation does not take place, but above this minimum, current density is not critical, and may rangebetween about 15 and about 100, preferably 35 to 60 amperes per square foot. Similarly, there is a minimum scintillation voltage which must be exceeded to form the coating. The average voltage range is advantageously between about and about 350 volts D.C. One type of suitable electrical arrangement is to employ a variable transformer in series with a GE. Diode IN R, with a maximum AC voltage of about 400 volts and 8 ampere current.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples illustrate the practice of the lnvention but are not to be regarded as limiting:
Example l.Potassium silicate-carbonate-chromate bath A 60 liter bath containing'Kasil (potassium silicate) in a dilution of 1:60 by volume and 0.33 mols potassium carbonate and 0.01 mols potassium chromate was prepared. 2 x 2" panels were cut from 0.016" gage 1100- H14 aluminum. Small holes were punched in the tops of each panel through which the electrical connections were made by inserting an insulated aluminum wire which was then crimped over the hole. The samples were mounted well below the solution level by the insulated aluminum wire. The samples were coated in a stainless steel 2 liter beaker which was placed inside a 4 liter plastic wrapped stainless steel beaker through which cooling water was circulated and agitation was maintained. The electrical system consisted of a partially rectified alternating current system as previously described, including a step-up transformer (220440 volts). Current output was controlled by a 9 ampere rheostat. The power .was turned on and increased to two amperes which was maintained for 5 minutes. The current density was maintained at 36 amperes per square foot. The samples were completely coated in 10 minutes and the coating had good adhesion and was hard and smooth to the touch. The coating thickness was 0.10 mils.
Example 2.--Potassium silicate-carbon-sodium molybdate bath Example 3.Potassium silicate-carbonate-tungstate bath Proceeding as in Example 1 there was employed a bath containing Kasil in a dilution of 1:60, together with 0.30 M potassium carbonate and 0.01 M sodium tungstate. Using the same conditions as in Example 1 the resulting coating was very hard, had good adhesion and the same.-
thickness was produced uniformly in minutes.
Example 4.Potassium silicate-carbonate-chromate and sodium molybdate bath Proceeding as in Example 1 a bath was prepared containing Kasil in a volume ratio of 1:6 with the addition of 1 M potassium carbonate, 0.01 M potassium chromate and 0.05 M sodium molybdate. A coating was obtained which had good adhesion and hardness.
Example 5.Potassium silicate-chromate-borate bath A bath was prepared containing Kasil in a volume ratio of 1:60, 0.3 M potassium borate, and 0.01 M potassium chromate. The resulting coating was smooth and hard and had good adhesion. Sodium molybdate in a concentration of 0.01 M can be substituted for the potassium chromate.
Example 6.Conductor foil coating A strip of 1100 aluminum conductor foil was treated in a silicate bath of the character described in Example 1 to produce thereon a coating having excellent uniformity and adhesion.
Example 7.-Pie plate coating A piece of 1100 pie plate metal, previously cleaned with methyl ethyl ketone was immersed in the bath of Example 1 and treated in the manner described in that example using a voltage of 235 volts and a current density of 12 amperes per square foot, at a temperature of 25 C. The uniform coating having a thickness of 0.05 mils was obtained.
What is claimed is:
1. Method of electrokinetically producing a siliceous coating on an aluminous metal surface, comprising the steps of:
(a) immersing the metal as anode in an aqueous bath consisting essentially of an alkali silicate, a salt selected from the group consisting of an alkali chromate, an alkali molybdate and an alkali tungstate,
and a salt selected from the group consisting of an alkali carbonate, an alkali phosphate and an alkali borate, and
(b) passing current through said metal and bath at a voltage predominantly positive and sufiicient to cause a uniform scintillation or flashing at the anode surface for a period of time sufficient to elfect formation of said siliceous coating.
2. The method of claim 1 in which the alkali silicate is potassium silicate.
3. The method of claim 2 in which the potassium silicate has a mol ratio of SiO /K O between about 4:1 and about 1.5 :1.
4. The method of claim 1 in which the concentration of alkali silicate in said bath is between about 0.2% and about 5% by weight of equivalent SiO;.
5. The method of claim 1 in which a concentration of alkali chromate, molybdate, or tungstate is between about 0.005 and about 1.0 molar.
6. The method of claim 1 in which the concentration of alkali carbonate, phosphate, or borate, is between about 0.01 and about 1.0 molar.
7. The method of claim 1 in which the coating is carried on at a temperature between slightly above the freezing temperature of the bath and about 50 C.
8. The method of claim 1 in which the average voltage is between about and about 350 volts D.C.
9. A bath for electrokinetically producing a siliceous coating on an aluminous metal surface, consisting essentially of an aqueous solution of:
(a) an alkali silicate having a concentration between about 0.2% and about 5% by weight of equivalent SiO- (b) a salt selected from the group consisting of an alkali chromate, an alkali molybdate, and an alkali tungstate having a concentration between about 0.005 and about 1.0 molar; and
(c) a salt selected from the group consisting of an alkali carbonate, an alkali phosphate and an alkali borate having a concentration between about 0.01 and about 0.1 molar.
10. The composition of claim 9 in which said alkali silicate is potassium silicate having a mol ratio of Si0=l K 0 between about 4:1 and about 1.5:1.
References Cited UNITED STATES PATENTS 3,658,662 4/ 1972 Casson et a1 204-58 3,445,356 5/1969 Harendza-Harinxma 204-58 3,293,158 12/1966 McNeill et al. 204-58 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 106-1