Method of coating articles of aluminum and an electrolytic bath therefor

An electrolytic bath for coating articles of aluminum and its alloys consists essentially of an aqueous solution containing an alkali metal silicate, 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 is formed.

FIELD OF THE INVENTION 
This invention relates to a method of electrolytic coating of aluminum 
metal and its alloys. In one aspect, the present invention relates to an 
electrolytic method of coating aluminum and aluminum alloys to provide a 
hard, smooth, durable, impervious, adherent and corrosion-resistant film 
or coating thereon. In another aspect, the present invention is concerned 
with a method of providing a decorative electroplated film or finish on 
the surface of aluminum metal and its alloys, wherein the film is also 
hard, smooth, durable, impervious, adherent and corrosion-resistant. In 
still another aspect, this invention relates to an electrolytic bath which 
is uniquely suited for providing the aforementioned desired films or 
coatings on aluminum and its alloys. 
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 hardware and a host of other 
articles. See U.S. Pat. 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 
applications. 
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 
satisfactory because such organic coatings degrade at high temperatures 
and frequently they adhere poorly to the metal surfaces, particularly when 
subjected to temperature 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 in that it is not entirely resistant 
against corrosion by many acids or alkalis. Moreover, the coatings 
imparted to the metal by the known electrodeposition methods often lack 
the desired degree of hardness, smoothness, durability, adherence and/or 
imperviousness 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: U.S. 
Pat. Nos. 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 conditions used during the 
electrodeposition also contribute to the nature and quality of the 
coating. Indeed, the present inventor recognized and discussed the 
significance of the electrolytic solution in the formation of suitable 
coatings on metals in his U.S. Pat. No. 4,082,626 which issued on Apr. 4, 
1978. As mentioned 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 electrolyte, said second metal being 
cathodic 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 described 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 
completely fulfill the diverse and often stringent demands of various 
industrial and household requirements. Moreover, 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 protect the surface of 
aluminum metal and its alloys from corrosion and environmental attack and 
consequent degration. 
It is a further object of this invention to protect the surfaces of 
aluminum metal and its alloys with a hard, uniform, adherent, smooth, 
impervious and corrosion-resistant coating. 
It is yet another object of this invention to provide such coated articles 
of aluminum and its alloys which are useful for decorative applications. 
It is also an object of this invention to provide an improved method for 
anodic coating of the surfaces of aluminum metal and its alloys. 
It is still an object of this invention to provide the desired coating on 
the surfaces of aluminum metal and its alloy by a method which requires a 
relatively short time and relatively high voltage. 
It is yet another object of this invention to provide a uniquely 
electrolytic solution for anodic coating of aluminum metal and its alloys. 
It is still another object of this invention to provide an electrolytic 
solution which is a stable composition and which can withstand the 
relatively high voltage potential imposed during the electrodeposition 
process. 
The foregoing and other unique features of the electrolytic solution and 
the process of this invention will be further described in, and more 
readily appreciated from, the ensuring detailed description and the 
accompanying drawings. 
SUMMARY OF THE INVENTION 
The objects of this invention are achieved by providing a unique 
electrolytic solution comprising certain specified ingredients designed to 
form a stable anodic bath, improve the electrodeposition process and form 
a unique coating on aluminum or its alloys. The coating formed on the 
metal is characterized, inter alia, by its highly adherent property, 
hardness, smooth texture, uniformity, corrosion-resistant and decorative 
appearance. The anodic bath is an aqueous solution comprising a silicate, 
peroxide, water-soluble carboxylic group-containing acid and water-soluble 
fluoride. When it is intended to use the coated article for decorative 
purposes, a vanadium compound is included in the solution. The bath 
ingredients react synergistically 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 
surface. 
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. 
Alternatively, 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 thickness.

DETAILED DESCRIPTION OF THE INVENTION 
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. Nevertheless, and in view of the 
often unpredictable behavior of some chemicals, particularly when they are 
in admixture with other chemicals, the resulting electrolytic solutions 
exhibit marked differences in properties and abilities to impart coatings 
on metal surfaces. Frequently, too, the coatings imparted to the metal 
surfaces will exhibit significant differences in properties or 
constitution 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 organic acid, 
e.g., acetic acid, hydroflouric acid or a fluoride and a vanadate. It is 
believed that the synergistic interaction of these ingredients results in 
an electrolytic solution which, inter alia, (1) is a highly stable complex 
solution under the electrodeposition conditions of this invention and (2) 
imparts a unique coating on the surface of aluminum and renders the coated 
aluminum particularly useful for many industrial and household 
applications, including decorative applications. 
Thus, and by way of illustration, a suitable electrolytic bath will contain 
potasium silicate (K.sub.2 SiO.sub.3), sodium peroxide (Na.sub.2 O.sub.2), 
acetic acid (CH.sub.3 COOH), hydrofluoric acid (HF.H.sub.2 O), sodium 
vanadate (Na.sub.3 VO.sub.4) 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 (Na.sub.2 SiO.sub.3), lithium silicate (Li.sub.2 
SiO.sub.3), potassium tetrasilicate (K.sub.2 SiO.sub.4), potassium 
fluosilicate (K.sub.2 SiF.sub.6). Also, hydrofluosilicic acid may be used 
alone or in conjunction 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 invention. While hydrofluoric acid is the preferred 
fluoride, other water-soluble fluorides such as, for example, fluosilicic 
acid, sodium fluoride, potassium fluoride or lithium fluoride may be 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 (C.sub.8 H.sub.17 COOH), propionic acid 
(C.sub.2 H.sub.5 COOH), tartaric acid (CHOH COOH CHOH COOH) and other 
water-soluble organic acids. 
Sodium vanadate is the bath ingredient responsible for imparting color to 
the resulting coating. Other vanadium compounds may also be efficaciously 
used for this purpose. These include hypovanadate M.sub.2 (V.sub.4 
O.sub.9).H.sub.2 O, e.g., sodium pyrovanadate (Na.sub.2 V.sub.2 O.sub.7) 
and potassium metavanadate (KVO.sub.3). Even some of the vanadium 
fluorides may be employed for imparting color to the coated aluminum 
surface Such fluorides include vanadium trifluoride (VF.sub.3.H.sub.2 O), 
vanadium tetrafluoride (VF.sub.4) and vanadium pentafluoride (VF.sub.5). 
In addition to the aforementioned ingredients, one could use sodium 
molybdate (Na.sub.2 WO.sub.4) or some of the other molybdates. 
B. Preparation of the Electrolytic Solution 
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 agitating 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, hydroflouric 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 solution. 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 
requirements. However, for laboratory and experimental operations, the 
anodic bath may 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 aforedescribed 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 liter; and the organic acid is usually added 
in sufficient quantity to adjust the pH to the desired level as aforesaid. 
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 sufficient 
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: 
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Example 1 
K.sub.2 SiO.sub.3.sup.(1) 
10 cm.sup.3 
Na.sub.2 O.sub.2 3 grams 
CH.sub.3 COOH.sup.(2) 
3 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
2 cm.sup.3 
Na.sub.3 VO.sub.4 1 gram 
H.sub.2 O 1000 cm.sup.3 
Example 2 
K.sub.2 SiO.sub.3.sup.(1) 
20 cm.sup.3 
Na.sub.2 O.sub.2 3 grams 
CH.sub.3 COOH.sup.(2) 
3 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
2 cm.sup.3 
Na.sub.3 VO.sub.4 0.5 grams 
H.sub.2 O 1000 cm.sup.3 
Example 3 
K.sub.2 SiO.sub.3.sup.(1) 
25 cm.sup.3 
Na.sub.2 O.sub.2 5 grams 
CH.sub.3 COOH.sup.(2) 
5 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
0.2 cm.sup.3 
Na.sub.3 VO.sub.4 0.1 grams 
H.sub.2 O 1000 cm.sup.3 
Example 4 
K.sub.2 SiO.sub.3.sup.(1) 
5 cm.sup.3 
Na.sub.2 O.sub.2 2 grams 
CH.sub.3 COOH.sup.(2) 
10 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
5 cm.sup.3 
Na.sub.3 VO.sub.4 0.2 grams 
H.sub.2 O 1000 cm.sup.3 
Example 5 
K.sub.2 SiO.sub.3.sup.(1) 
100 cm.sup.3 
Na.sub.2 O.sub.2 3 grams 
CH.sub.3 COOH.sup.(2) 
10 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
10 cm.sup.3 
Na.sub.3 VO.sub.4 0 grams 
H.sub.2 O 1000 cm.sup.3 
Example 6 
K.sub.2 SiO.sub.3.sup.(1) 
50 cm.sup.3 
Na.sub.2 O.sub.2 10 grams 
CH.sub.3 COOH.sup.(2) 
5 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
10 cm.sup.3 
Na.sub.3 VO.sub.4 10 grams 
H.sub.2 O 1000 cm.sup.3 
Example 7 
K.sub.2 SiO.sub.3.sup.(1) 
20 cm.sup.3 
Na.sub.2 O.sub.2 5 grams 
CH.sub.3 COOH.sup.(2) 
3 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
5 cm.sup.3 
Na.sub.3 VO.sub.4 0.5-10 grams 
H.sub.2 O 1000 cm.sup.3 
Example 8 
K.sub.2 SiO.sub.3.sup.(1) 
50 cm.sup.3 
Na.sub.2 O.sub.2 10 grams 
CH.sub.3 COOH.sup.(2) 
15 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
10 cm.sup.3 
Na.sub.3 VO.sub.4 0.5-10 grams 
H.sub.2 O 1000 cm.sup.3 
Example 9 
K.sub.2 SiO.sub.3.sup.(1) 
150 cm.sup.3 
Na.sub.2 O.sub.2 15 grams 
CH.sub.3 COOH.sup.(2) 
20 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
10 cm.sup.3 
Naf 10 grams 
Na.sub.3 VO.sub.4 1 grams 
H.sub.2 O 1000 cm.sup.3 
Example 10 
K.sub.2 SiO.sub.3.sup.(1) 
60 cm.sup.3 
Na.sub.2 O.sub.2 7 grams 
CH.sub.3 COOH.sup.(2) 
7 cm.sup.3 
KF 5 grams 
Na.sub.4 V.sub.2 O.sub.7 
3 grams 
H.sub.2 O 1000 cm.sup.3 
Example 11 
Na.sub.2 SiO.sub.3 50 cm.sup.3 
Na.sub.2 O.sub.2 7 grams 
CH.sub.3 COOH 7 cm.sup.3 
NaF 5 grams 
Na.sub.3 VO.sub.4 3 grams 
H.sub.2 O 1000 cm.sup.3 
Example 12 
Li.sub.2 SiO.sub.3.sup.(1) 
40 cm.sup.3 
Na.sub.2 O.sub.2 7 grams 
CH.sub.3 COOH.sup.(2) 
7 cm.sup.3 
LiF 5 grams 
Na.sub.4 V.sub.2 O.sub.7 
3 grams 
H.sub.2 O 1000 cm.sup.3 
Example 13 
K.sub.2 SiO.sub.3.sup.(1) 
65 cm.sup.3 
Na.sub.2 O.sub.2 8 grams 
CH.sub.3 COOH.sup.(2) 
7 cm.sup.3 
NaF 5 grams 
Na.sub.3 VO.sub.4 1 gram 
H.sub.2 O 1000 cm.sup.3 
Example 14 
H.sub.2 SiF.sub.6 40 cm.sup.3 
Na.sub.2 O.sub.2 15 grams 
CH.sub.3 COOH.sup.(2) 
15 cm.sup.3 
HF.H.sub.2 O.sup.(3) 
15 cm.sup.3 
Na.sub.3 VO.sub.4 0.7 grams 
H.sub.2 O 1000 cm.sup.3 
______________________________________ 
.sup.(1) 30 Be'. 
.sup.(2) 99.9% glacial reagent diluted with water in a ratio of 6 volumes 
of water to one volume of the acid. 
.sup.(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 invention 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 be immersed in a 
container containing 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, preferably 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 characteristics hereinbefore 
described. The application of voltage shock as aforesaid also reduces the 
overall time and even the energy required to form the desired coating 
thickness. 
Referring now to FIG. 1, the voltage-time graph for the process of this 
invention is designated as D. But for this graph, FIG. 1 is the same as 
FIG. 1 of the aforementioned Hradcovsky patent and, therefore, the 
disclosure of that patent is incorporated herein by reference. Thus, graph 
V.sub.1 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 FIG. 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 considerably longer time 
required for the voltage potential to reach a similar level by the process 
of the aforementioned 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 particularly suitable for 
decorative applications. Such applications 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 FIGS. 2 and 3. As it is noted 
from a comparison of these two photographs, the coating produced by the 
method of 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 or 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. 
While the invention has heretofore been described and illustrated with 
certain degree of specificity, it is apparent to those skilled in the art 
that some changes and modifications may be made therein, either in the 
bath or in the electrodeposition process. Such changes and modifications 
are suggested by the present disclosure and are, therefore, within the 
scope and contemplation of this invention.