METHOD OF COATING ARTICLES OF ALUMINUM AND AN ELECTROLYTIC BATH THEREFOR

Abstract: An electrolytic bath for coating articles of aluminum and its alloys consists essentially of an aqueous solution containing an alkali metal silicate, or hydrofluosilicic acid, a peroxide, a water-soluble carboxylic group containing organic acid and a water-soluble fluoride. A vanadium compound may also be included in the bath whenever the coated articles are intended to be used for decorative purposes. In the process, the aluminum article is immersed in the bath and a voltage shock is applied thereto by imposing a voltage potential between the aluminum metal serving as the anode and a cathode immersed in the bath. The voltage potential is quickly raised to about 300 volts within about 2 to about 10 seconds and thereafter, the voltage is increased gradually to about 450 volts within a few minutes until the desired coating thickness formed.

Description: METHOD OF COATING ARTICLES OF ALUMINUM AND ά fc fcCT-RόLV ie -r-ATH friήftftfoά BACKGROUND OF THE INVENTION Aluminum and its alloys have found a variety of industrial and household applications in the form of sheets, strips, bars, rods, tubes, structural members, household appliances and utensils, hardward and a host of other articles. See United States Patent No. 2,941,930, issued on June 21, 1960 to Mostovych et al. As mentioned in said patent, there is great outlet for aluminum articles, including decorative products of this metal and its alloys, for such uses as ornamental wall panels for inside or outside of various buildings, restaurant furnishings, art objects and a host of other applica¬ tions. Because of its light weight and tendency toward surface corrosion, it has been necessary to provide a suitable coating on the surface of the metal in order to impart structural strength thereto and to protect it against corrosion and/or environmental degradation. Some protection has been afforded the metal by painting or enameling its surface. However, the protection afforded the metal by painting or enameling has not been satisfac¬ tory because such organic coatings degrade at high temperatures and frequently they adhere poorly to the metal surfaces, particularly when subjected to tempera¬ ture variations. In order to provide a more suitable coating for improved protection of aluminum metal and its alloys, the metal has been anodized in a variety of electrolytic solutions. While anodization of aluminum affords the metal surface a more effective protective coating against corrosion or degradation than painting or enameling, still the resulting coated metal has not always been satisfactory that it is not entirely resistant against corrosion by many acids or alkalis. Moreover, the coat¬ ings imparted to the metal by the known electrodeposition methods often lack the desired degree of hardness, smoothness, durability, dherence and/or imperviousneβs required to meet the ever increasing industrial and household demands. Frequently, too, the coated aluminum articles have not been satisfactory for use as decorative articles because of the poor quality or appearance of the surface coating. There is a plethora of prior art patents which deal with anodizing aluminum metal and its alloys. The following is a list of patents which is representative of the efforts of the prior art workers in this field: United States Patent Noβ. 630,246; 1,735,286; 2,231,086; 2,260,278; 2,349,083; 2,363,339; 2,780,591; 2,791,553; 2,941,930; 3,003,933; 3,275,537; 3,355,368; 3,445,349; 3,532,607; 3,672,964; 3,899,400; 3,996,115; 4,113,579; 4,128,461; 4,170,525; 4,440,606; and 4,502,925. While this list is by no means exhaustive, a review of these patents illustrate the significant role which the electrolytic solution plays in the anodizing process and in providing aluminum and its alloys with the desired protective coating. Thus, in general, the nature and properties of the coating which is formed on aluminum and it alloys depend, to great extent, on the composition of the anodic bath (electrolytic solution) used in anodizing the metal. Other parameters such as the process condi¬ tions used during the electrodeposition also contribute to the nature and quality of the coating. Indeed, the present inventor recognized and discussed the signifi¬ cance of the electrolytic solution in the formation of suitable coatings on metals in his United States Patent No. 4,082,626 which issued on April 4, 1978. As mention¬ ed in that patent, a rectifier metal is anodized by a relatively low voltage electrodeposition process in an electrolytic solution consisting of a relatively pure potassium silicate at concentrations exceeding the potassium silicate concentrations theretofore employed. The process comprised immersing a rectifier metal (e.g., aluminum) in the electrolyte, the rectifier metal serving as the anode, immersing a second metal in said electro¬ lyte, said second metal being cathodiσ relative to the rectifier metal, imposing a voltage potential across the anode and the cathode and causing a current to flow therebetween until a visible spark is discharged at the surface of the rectifier metal, increasing the voltage potential to about 300 volts and maintaining the voltage substantially at this level until the desired coating thickness is deposited on the surface of the rectifier metal. While the coating produced by the method describ¬ ed in the aforementioned patent exhibits some improvement and more desirable features as compared to the coatings produced by the earlier methods, they still do not com¬ pletely fulfill the diverse and often stringent demands of various industrial and household requirements. More¬ over, the surface finish of the metal is not entirely satisfactory for decorative applications of the coated metallic articles. Accordingly, it is an object of this invention to provide an improved method of coating the surface of aluminum metal and its alloys. According to the invention therefore, there i provided a method of coating a product formed from alumi num or an aluminum alloy predominating in aluminum with hard, adherent, smooth, uniform and corrosion-resistan coating, which method comprises immersing the produc alloy in an aqueous electrolytic solution providing, second metal body in said solution and applying an elec tric potential between the product as an anode and th body as a cathode characterized in that the solutio comprises a peroxide, a water-soluble carboxylic grou containing organic acid, a water-soluble fluoride, and silicon compound from the group consisting of alkal metal silicate and hydrofluosilicic acid. The objects of this invention are achieved b providing a unique electrolytic solution comprisin certain specified ingredients designed to form a stabl anodic bath, improve the electrodeposition process an form a unique coating on aluminum or its alloys. Th coating formed on the metal is characterized, inter alia, by its highly adherent property, hardness, smooth texture uniformity, corrosion-resistant and decorative appear¬ ance. The anodic bath is an aqueous solution comprising a silicate, peroxide, water-soluble carboxylic group- containing acid and watersoluble fluoride. When it is intended to use the coated article for decorative purposes, a vanadium compound is included in the solu¬ tion. The bath ingredients react synergiatically to form a complex stable solution, particularly under the process conditions used herein. In addition, the ingredients of the bath form a unique complex coating on the metal sur ce. The electrolytic process comprises immersing the aluminum metal in the bath, in which aluminum serves as the anode. A second metal which is cathodic with respect to aluminum is also immersed in the bath. Alter¬ natively, the bath is placed in a container which itself is cathodic relative to the aluminum metal. A voltage "shock" is then applied to the aluminum metal by imposing a voltage potential between the two electrodes, which is quickly raised to about 300 volts within about 2 to about 10 seconds. Thereafter, the voltage is increased gradually to about 450 volts within a few minutes to form the desired coating thick¬ ness. With the foregoing in view, and other advan¬ tages as will become apparent to those skilled in the art to which this invention relates as this speci ication proceeds, the invention is herein described by reference to the accompanying drawings forming a part hereof, which includes a description of the best mode known to the applicant and of the preferred typical embodiment of the principles of the present invention, in which: DESCRIPTION OF THE DRAWINGS Figure 1 depicts a series of graphs of the voltage potential applied to the electrodes as a function of the time required for electrolytic coating o alumi¬ num. The significance of these graphs will become appar¬ ent from the ensuing discussion. Figure 2 is a photograph depicting a typical aluminum coated surface, with a degree of magnification of 500, produced according to the method described in the aforementioned Hradcovaky patent. Figure 3 is a photograph, magnified 1100 times, illustrating a coated aluminum surface produced by the method of this invention. In the drawings like characters of reference indicate corresponding parts in the different figures. DETAILED DESCRIPTION In accordance with the present invention, there is provided a unique electrolytic solution, sometimes referred to as an electrolytic bath or anodic bath, which is, inter alia, stable, particularly at the high voltages employed during the electrodeposition process, and which under the electrolytic process conditions of the present invention, imparts the desired coating to the surface of aluminum metal or alloys of aluminum which predominate in aluminum. Accordingly, the terms "aluminum" or "aluminum metal" as used throughout the present specification and claims are intended to denote not only aluminum but such alloys as well. As it was' previously noted, there is a plethora of electrolytic solutions or anodic baths which have heretofore been employed for anodic coating of aluminum. The different baths frequently differ from one another with respect to only one or two ingredients. Neverthe¬ less, and in view of the often unpredictable behavior of some chemicals, particularly when they are in admixture with other chemicals, the resulting electrolytic solu¬ tions exhibit marked differences in properties and abili¬ ties to impart coatings on metal surfaces. Frequently, too, the coatings imparted to the metal surfaces will exhibit significant differences in properties or consti¬ tution which reflect the differences in composition of the electrolytic solution. Therefore, the selection of the ingredients used to form the electrolytic solution is of paramount significance in the anodic treatment of metals . A. The Electrolytic Solution: In order to protect the aluminum surface with a coating having the unique features and properties which were mentioned previously, and after extensive experimentations it has been found that the most effective electrolytic solution for the purposes of this invention is an aqueous solution containing a silicate, a peroxide, a water-soluble organ¬ ic acid, e.g., acetic acid hydrofluoric acid or a fluoride and a vanadate. It is believed that the βynβr- gistic interaction of these ingredients results in an electrolytic solution which, inter alia, I) is a highly stable complex solution under the electrodeposition con¬ ditions of this invention and 2) imparts a unique coating on the surface of aluminum and renders the coated alumi¬ num particularly useful for many industrial and household applications, including decorative applications. Thus, and by way of illustration, a suit¬ able electrolytic bath will contain potasium silicate (K2Siθ3), sodium peroxide ( 2θ2), acetic acid (CH3C OH), hydrofluoric acid (HF.H2O), sodium vanadate ( 3V0 ) and water. As it can be appreciated certain other compounds may be used instead of, or together with any of the aforementioned components. While potassium silicate is the silicate of choice for forming the electrolytic bath, other alkali metal silicates can be used, including sodium silicate ( a2Siθ3), lithium silicate ( i2Siθ3), potassium tetra- silicate (-^SiO.^), potassium fluosilicate (K^Si β). Also, hydrofluosilicic acid may be used alone or in con¬ junction with any of the aforementioned silicates. In lieu of sodium peroxide, or in admixture therewith, one could use other peroxides such as, for example, potassium peroxide, lithium peroxide or cesium peroxide. The inclusion of the fluoride in the bath constitutes an essential feature of the present inven¬ tion. While hydrofluoric acid is the preferred fluoride, other water-soluable fluorides such as, for example, fluosilicic acid, sodium fluoride, potassium fluoride or lithium fluoride maybe used instead of, or in conjunction with, hydrofluoric acid. Another essential ingredient of the bath is acetic acid. The use of this acid not only permits adjusting the pH of the bath but also promotes formation of a complex with and among the other ingredients, thus resulting in a stable complex solution. In lieu of acetic acid, or in admixture therewith, one can use other organic carboxylic group-containing acids including pergonic acid (CgHi COOH), propionic acid (C2H5COOH), tartaric acid (CHOH COOH CHOH COOH) and other water- soluable organic acids. Sodium vanadate is the bath ingredient responsible for imparting color to the resulting coating. Other vanadium compounds may also be e ficaciously used for this purpose. These include hypovanadate 2( θg) .H2O, e.g., sodium pyrovanadate (N 2V " 2θ7) and potassium metavanadate (KVO3). Even some of the vanadium fluorides may be employed for imparting color to the coated aluminum surface. Such fluorides include vanadium trifluoride (VF3.H2O), vanadium tetra luoride (VF4) and vanadium pβntafluoride (VF5). In addition to the afore¬ mentioned ingredients, one could use sodium olybdate (N 2 04) or some of the other molybdates. B. Preparation of the Electrolytic Solution 1 The preparation of the electrolytic solution or the anodic bath basically comprises, first, the addition of the silicate to water at about room temperature, or preferably lower. The silicate usually constitutes the dominant ingredient of the bath and the resulting coating as well. The silicate is added as a 30 Be' and various industrial grades silicates are available in this strength. For example, potassium silicate may be used as 30 Be' KASIL 88 solution available from Philadelphia Quartz Co., Philadelphia, PA. Next, the peroxide is added while agitat¬ ing the solution, followed by the addition of glacial acetic acid (99.9% reagent which has been diluted with water in a ratio of 6:1 volumes of water to the acid). While the mixture is being agitated, hydrofluoric acid (35% concentration diluted with water in a ratio of 6:1 volumes of water to the acid) is added to the mixture, followed by the addition of the vanadate. For commercial operations, and as a practical matter, it is recommended that the resulting bath be diluted with sufficient quantity of water to produce from about 0.5 to about 2 Be' anodic bath solu¬ tion. For commercial production purposes, if the anodic bath significantly exceeds 2 Be', the electrodes may be damaged or burn out due to large current density require¬ ments. However, for laboratory and experimental opera¬ tions, the anodic bath nay be as high as 30 Be' without severe adverse impact on the electrodes. It is also important to maintain the pH of the anodic bath at from about 10.5 to about 13, preferably at from about 11 to about 12. Accordingly, the amount of the acetic acid in the bath may be varied to adjust the pH to the optimum level. In the afore¬ described method of preparing the electrolytic solution, the ingredients have been referred to generically for the sake of simplicity. It must be emphasized, however, that regardless of which silicate, peroxide, organic acid, etc., are used, the order of addition of the ingredients and preparation of the bath remains essentially the same. The amounts of the various ingredients used to form the anodic bath can vary widely. Thus the amount of silicate (30 Be') can vary from about 1 to about 200 cubic centimeters per liter; the peroxide quantity is between about 1 to about 20 grams per litrer and the organic acid is usually added in sufficient quantity to adjust the pH to the desired level as afore¬ said. Also, the quantity of hydrofluoric acid can vary from about 0.1 to about 30 cubic centimeters per liter and the vanadate is added in suf icient amounts to obtain the desired color depth in the coating. This amount is usually about 0.1 grams per liter or more depending on the desired color depth. It has been noticed that the resulting coating is generally gray at the lower vanadate concentrations, tending to be black and deeper in color as the amount of vanadate is progressively increased. The following examples are typical anodic baths which are suitable in the practice of this invention: Example 1 KaSiOst 1 ) 10 cm 3 Na2θ2 3 grams CH3COOH( 2 ) 3 cm 3 HF.H 0( 3 ) 2 cm 3 Na3 ©4 1 gram tt 2 0 1000 cm 3 Example 3 K 2 Si0 3 <l) 25 cm 3 Na 2 0 2 5 grams CH 3 COOH( 2 ) 5 cm 3 HF.H 2 θ( 3 ) 0.2 cm 3 Na3Vθ 0.1 grams H 2 0 1000 cm 3 Example 4 K 2 Si0 3 ( i ) 5 cm 3 N 2θ2 2 grams CH 3 COOH(2) 10 cm 3 HF.H 2 0( 3 ) 5 cm 3 3 V0 0.2 grams H 2 0 1000 cm 3 Example 5 K 2 Si0 3 ( l ) 100 cm 3 2 02 3 grams CH 3 C00H(2) 10 cm 3 HF.H2θ( 3 ) 10 cm 3 N 3 V0 0 grams H 2 0 1000 cm 3 K 2 Si0 3 ( 1 ) 50 cm 3 a2θ2 10 grams CH3C00H( 2 ) 5 cm 3 HF.H2θ( 3 ) 10 cm 3 N 3V0 4 10 grams H 2 0 1000 cm 3 Example 7 K2Siθ3( 1 ) 20 cm 3 N 2θ2 5 grams CH3C00H( 2 ) 3 cm 3 HF.H2θ( 3 ) 5 cm 3 3 04 0.5-10 grams H2O 1000 cm 3 Example 8 K2Si0 3 ( 1 ) 50 cm 3 Na 2 0 2 ° grams CH3COOH( 2 ) 15 cm 3 HF.H2θ( 3 ) 10 cm 3 asVθ4 0.5-10 grams H2O 1000 cm 3 Example 9 K Si0 3 ( 1 ) 150 cm 3 Na 0 15 grams CH3C00H( 2 ) 20 cm 3 HF.H 2 0( 3 ) 10 cm 3 NaF 10 grams 3Vθ .1 grams H 2 0 1000 cm 3 Example 10 N θ2 7 grams KF 5 grams Na3V2θ7 3 grams H 2 0 1000 cm 3 Example 11 K 2 Si0 3 (i) 50 cm 3 N 2θ2 7 grams CH 3 COOH( 2 ) 7 cm 3 NaF 5 grams Na3V0 4 3 grams H 2 0 1000 cm 3 Example 12 Li 2 Si0 3 ( 1 ) 40 cm 3 a2θ 2 7 grams CH3C00H( 2 ) 7 cm 3 Li 5 grams Na V 2 θ7 3 grams H 2 0 1000 cm 3 Example 13 K2Siθ3( L ) 65 cm 3 Na2θ2 8 grams CH3COOH( 2 ) 7 cm 3 NaF 5 grams Na-}V0.a. 1 grams H2O 1000 cm 3 Example 14 H2Si g 40 cm 3 Na 2 0 2 15 grams CH 3 COOH( 2 ) 15 cm 3 HF.H 2 0( 3 ) 15 cm 3 3V04 0.7 grams H 2 fi 1000 cm 3 (1) 30 Be' . (2) 99.9% glacial reagent diluted with water in a ratio of 6 volumes of water to one volume of the acid. (3) 35% concentration diluted with water in a ratio of 6 volumes of water to one volume of the acid. C. The Coating Process: The process of coating the surfaces of aluminum in the present invention is somewhat similar to the process described in the aforementioned Hradcovsky patent with several basic differences. In addition to the differences in the nature of the anodic bath, in the process of this inven¬ tion the voltage applied to the electrodes is raised quickly, i.e., the metal is "shocked" to about 300 volts within about 2 to about 10 seconds, and thereafter, the voltage is increased gradually to about 450 volts over a period of about 5 to about 10 minutes to obtain the desired coating thickness. Thus, the present coating process comprises immersing the aluminum article to be coated in the anodic bath in which the aluminum is made anodic with respect to a second metal immersed in said bath which serves as the cathode. Alternatively, the aluminum article may bβ immersed in a container contain¬ ing the bath and the container itself serves as the cathode. After the aluminum article and the second metal have been immersed in the electrolytic solution, an electric voltage potential is applied between the two electrodes and this voltage is quickly raised to about 300 volts within about 2 to 10 seconds,pre e ably within about 3 to about 5 seconds. Following this shock, the voltage is gradually increased to about 450 volts over a period of about 5 minutes to about 10 minutes to form the desired coating thickness. During the shock period, a high current density of about 100 amperes/sq.ft. is passed through the electrode. Subsequently, however, the current density is reduced to as low as about 10 to about 50 amperes/ sq.ft. In general, however, the current density can vary depending on the composition of the electrolytic bath and the aluminum alloy where an alloy is employed. At such high voltage levels, a visible spark is discharged across the aluminum surface which creates a thermal environment in which the constituents of the anodic bath unite chemically with the aluminum, as well as with other ingredients of the bath to form a highly adherent complexed coating having the unique characteris¬ tics hereinbefore described. The application of voltage shock as aforesaid also reduces the overall time and even the energy required to form the desired coating thick¬ ness. Referring now to Figure 1, the voltage-time graph for the process of this invention is designated as D. But for this graph, Figure 1 is the same as Figure 1 of the aforementioned Hradcovsky patent. Thus, graph VI represents a voltage-time relationship for coatings produced at low prior art silicate concentrations, and V is a voltage-time relationship for the method described in the aforementioned Hradcovsky patent. As seen from graph D in Figure 1, the voltage applied across the electrodes in the present process rises rapidly and reaches about 300 volts within few seconds. This is to be contrasted with the con¬ siderably longer time required for the voltage potential to reach a similar level by the process of the afore¬ mentioned Hradcovsky patent, and even the longer times required by the other methods referred to in said patent. D. The Coating: As it was mentioned earlier, a principal object of the present invention is to produce coated aluminum articles which are particular¬ ly suitable for decorative applications. Such applica¬ tions mandate that the coating on the aluminum surface not only be hard, adherent, durable and corrosion- resistant, but must also be smooth, homogeneous and even- textured, with luster and color depth as required for many decorative purposes. With this objective in mind, the composition of the bath and the process conditions are carefully selected as aforesaid in order to obtain the desired coating. The superior appearance of the coatings produced by the practice of this invention can be appreciated by reference to Figures 2 and 3. As it is noted from a comparison of these two photographs, the coating produced by the .method .o_f the present invention, using an anodic bath having the constitution of any of the baths described in Examples 1-14, supra, are more uniform, homogeneous and less pervious than the coating produced in accordance with the method described in the aforementioned Hradcovsky patent. Such differences in properties are of paramount significance in customer appeal and eventual saleability of the coated aluminum articles * While not wishing to be bound by any structural theory or mechanism, it is believed that the coating produced by the present invention is a complex formed by the union of the different ingredients with each other as well as with aluminum oxide on the surface of aluminum. In all instances, however, the silicate usually constitutes the dominant component. Also, while vanadates of vanadium fluoride is used for imparting color to the coated surface, the use of these components is not strictly necessary. Anodic bath compositions of the types hereinbefore described, and illustrated in the foregoing examples, can be employed except that the vanadium compound may be omitted therefrom (see Example 5). Such baths nevertheless produce coatings which are superior in appearance, i.e., homogeneity, surface uniformity, adherence to the metal and smoothness, than the prior art coatings. However, they may have more limited use for decorative purposes. Since various modifications can be made in my invention as hereinabove described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without departing from such spirit and scope, it is intended that ail matter contained in the accompanying specification shall bβ interpreted as illustrative only and not in a limiting sense.

Claims: - 24 - (1) A method of coating a product formed from aluminum or an aluminum alloy predominating in aluminum with a hard, adherent, smooth, uniform and corrosion- resistant coating, which method comprises immersing the product allow in an aqueous electrolytic solution provid¬ ing, a second metal body in said solution and applying an electric potential between the product as an anode and the body as a cathode characterized in that the solution comprises a peroxide, a water-soluble carboxylic group- containing organic acid, a water-soluble fluoride, and a silicon compound from the group consisting of alkali metal silicate and hydrofluosilicic acid * (2) A method according to claim 1 wherein the electric potential is applied such that the voltage is raised rapidly to a voltage of the order of 300 volts in a time period less than a time to the order of ten seconds, and thereafter is gradually raised to a voltage of the order of 450 volts until the desired coating thickness is formed. (3) A method as in claim 1 or 2 wherein said alkali metal silicate is selected from the group consist¬ ing of potassium silicate, sodium silicate, lithium sili¬ cate, potassium tetrasilicate, potassium fluosilicate and mixtures thereof. (4) A method as in claim 1, 2 or 3 wherein said peroxide is selected from the group consisting of potassium peroxide, sodium peroxide, lithium peroxide, cesium peroxide and mixtures thereof. (5) A method as in any preceding claim wherein said carboxylic group-containing organic acid is selected from the group consisting of acetic acid, pergonic acid, propionic acid, tartaric acid and mixtures thereof. (6) A method as in any preceding claim wherein said fluoride compound is selected from the group con¬ sisting of hydrofluoric acid, fluosilicic acid, sodium fluoride, potassium fluoride, lithium fluoride and mixtures thereof. (7) A method as in any preceding claim wherein said bath is maintained at from about 0.5 to about 30 Be'. (8) A method as in any preceding claim wherein said aqueous electrolytic solution further includes a vanadium compound for imparting color to the coating. (9) A method as in any preceding claim where¬ in said solution consists essentially of potassium sili¬ cate, sodium peroxide, acetic acid, hydrofluoric acid, sodium vanadate and water. (10) A product formed from aluminum or an aluminum alloy predominating in aluminum when provided with a coating by a method according to any preceding claim. (11) An electrolytic bath for forming a coat¬ ing on the surface of aluminum and aluminum alloys _pre-- dominating in aluminum, characterized in that said electrolytic bath consists essentially of an aqueous solution containing from about 1 to about 20 grams per liter of a peroxide, from about 1 to 30 cm3 per liter of a wter soluble carboxylic group-containing organic acid, from about 1 to about 30 cm3per liter of a water-soluble fluoride and from about 1 to about 200 cm3 per liter of a β-l- -fsn fΛ prtnnΛ sβ.«*-.* ■ «**. fmm the group consisting of the alkali metal silicates and hydrofluosilicic acid.

Inventors: HRADCOVSKY RUDOLF, HRADCOVSKY, RUDOLF
Applicants: HRADCOVSKY RUDOLF, HRADCOVSKY, RUDOLF
Application year: 1987
Main classification: C25D11
IPC codes: C25D3, C25D5, C25D11