Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Agitating or moving electrolyte during coating
Reexamination Certificate
1999-03-10
2001-05-22
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Agitating or moving electrolyte during coating
C205S234000, C205S322000, C205S324000, C205S332000
Reexamination Certificate
active
06235181
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method of preparing anode oxide films.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 5,837,121, and patent application Ser. No. 09/090,164, filed Jun. 4, 1998, now U.S. Pat. No. 6,149,793, each describe a method and electrolyte for anodizing the “valve” metals (metals, alloys, inter-metallic compounds and metallic glasses which form non-porous and adherent anodic oxide films having high electrical resistance, e.g., aluminum, tantalum, titanium, niobium, etc.).
Both the patent and the application are directed to electrolytic solutions for anodizing a metal comprising forming a film on the metal with an electrolytic solution. The film is formed at a temperature of 150° C. or higher. The patent is directed to an electrolytic solution comprising glycerine and dibasic potassium phosphate and having a water content of less than 1000 ppm (0.1 wt %) and prepared by mixing the glycerine and the dibasic potassium phosphate and then heating to about 150 to 180° C. for about 1 to 12 hours. The application is directed to an electrolytic solution having a pH less than about 7 and comprising glycerine and an organic salt, an inorganic salt, a mixture thereof and having a water content of less than 0.1 wt % and prepared by mixing the glycerine and the salts or their acidic and basic ionogens and then heating the solution to above 150° C.
Under constant voltage conditions, the anodic oxide grows thicker indefinitely and yet is found to be non-porous and very electrically resistive when removed from the unusual anodizing conditions and electrolyte described in these documents.
This method of anodizing, which has been labeled: “non-thickness-limited” anodizing, is further described in the paper, entitled: “The Non-Thickness-Limited Growth of Anodic Oxide Films on Valve Metals”, by Brian Melody, Tony Kinard, and Philip Lessner, which appears in
Electrochemical and Solid State Letters
, Vol. 1, No. 3, September, 1998, published by the Electrochemical Society.
It is known that there is an induction period between the application of voltage across an anodic oxide film and the build-up of the anodizing current to a peak value after the application of voltage under conditions otherwise known to give rise to non-thickness-limited anodic oxide growth (i.e., appropriate electrolyte composition and temperature as described in the patent documents cited above). The length of the induction period has been found to be, generally, inversely proportional to the field through the oxide; the thinner the oxide, the higher the field and the shorter the induction period for a given applied voltage. Thus, a pulsed voltage may be utilized to achieve a more uniform oxide thickness on irregularly shaped objects or within porous anode bodies than is readily obtained with d.c. voltage.
By using the process and electrolytes described in the above-cited documents, it is possible to prepare non-porous, highly electrically resistive anodic oxide films, also known as “barrier” films, on valve metals which greatly exceed in thickness barrier anodic oxide films prepared by other known electrolyte process. Utilizing this non-thickness-limited anodizng technology, anodic oxide films have been grown upon aluminum to a thickness in excess of 10 microns, and upon tantalum to a thickness in excess of 20 microns.
While the non-thickness-limited anodizing technology described in the above-cited documents facilitate the growth of barrier anodic oxide films on valve metals which may be 10 times or more the thickness possible with any other known anodizing method, the thickness of the films is difficult to predict. The reason it is difficult to predict the thickness of these films is that, while the anodic oxide film growth is a Faradaic process, i.e., the film thickness is a function of the total current passed per unit area of valve metal surface (though the process is not 100% efficient; a portion of the current does not produce anodic oxide), the current density varies sharply with electrolyte temperature at constant voltage. Then, under constant voltage conditions, small variations in the electrolyte temperature tend to result in large variations in current density, which, in turn, result in large variations in anodic oxide growth rate.
Furthermore, at the temperatures required for non-thickness-limited film growth, (i.e., above about 150° C.), it is very difficult to control temperature fluctuations of only a few degrees throughout the total mass of electrolyte present in production-scale anodizing equipment. Temperature fluctuations can be controlled by careful placement of stirring impellers, etc., and, especially through the use of ultrasonic agitation at relatively high power densities of ultrasonic sound (e.g., 100+ watts/liter at 40 kHz). Note that the high room temperature viscosity of glycerine and of the glycerine-based electrolyte solutions would normally preclude the use of ultrasonic energy to produce effective agitation. At about 150° C. and above, however, the electrolytes described in the above-cited patent documents are sufficiently fluid to be effectively agitated ultrasonically.
By the use of impellers or ultrasonic agitation, the temperature fluctuations within the bulk is drastically reduced or eliminated. However, the average temperature of the bulk of the electrolyte will drift over time. Further, there is a small change in the oxide resistivity due to increasing thickness over the course of an anodizing run. The changes in average temperature and the oxide resistivity will cause a variation in the current density and, hence, in the anodizing current efficiency.
Additionally, any glycerine lost through evaporation during the anodizing must be replaced periodically by glycerine additions. Commercially available glycerine generally contains at least 0.3 wt % water. Unless the glycerine is dried before use (by heating to 180° C. for 1 hour, for example), the water in the glycerine will cause a reduction in current density when the glycerine is added to the anodizing electrolyte. Furthermore, the current density will tend to rise (under constant voltage conditions) as the water content of the electrolyte is reduced through evaporation. Thus, the constant voltage current density tends to fall initially then rise after each evaporation make-up glycerine addition.
The magnitude of the variability of the current density at constant voltage which may occur during non-thickness-limited anodizing with the electrolytes described in the above-cited documents is shown in Table 1. A tantalum coupon was anodized in an electrolyte consisting of 10 wt % sodium tetraborate decahydrate in glycerine which was previously heat-treated to reduce the water content to below 0.1 wt %. The voltage was fixed at 30 volts and the nominal temperature was 180° C. The current was observed to surge to 3.89 milliamperes/cm
2
initially and to vary between 0.28 and 1.47 milliamperes/cm
2
over a period of only 15 minutes. This demonstrates the difficulty in predicting the amount of material produced by a Faradaic process having such high current variability.
TABLE 1
Nominal Temperature = 180° C.
Time
Voltage
Current
(min.)
(volts)
(mA/cm
2
)
(on)
(rising)
3.89
1
30
1.11
2
30
0.37
3
30
0.29
4
30
0.28
5
30
0.29
6
30
0.31
7
30
0.35
8
30
0.39
9
30
0.44
10
30
0.51
11
30
0.61
12
30
0.74
13
30
0.93
14
30
1.17
15
30
1.47
SUMMARY OF THE INVENTION
The present invention is directed to a method of anodizing a metal comprising immersing a metal substrate into an a glycerine-based electrolytic solution and applying a constant current to produce a uniform film.
The present invention is further directed to an anodizing process which produces uniform and predictable oxide thickness using an electrolyte composition and valve metal substrate.
The present invention is further directed to an anodizing process conducted at a temperature above about 150° C. which provides predictable results in uniform oxide thickness produced per unit time.
The present invention is further directed a process wherei
Kinard John T.
Melody Brian J.
Wheeler David A.
Banner & Witcoff , Ltd.
Gorgos Kathryn
Kemet Electronics Corporation
Leader William T.
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