Method of anodizing valve metals

A method of non-thickness-limited anodizing for valve metals and alloys which are resistant to the non-thickness-limited growth of anodic oxide, such as niobium and high niobium content alloys. Non-thickness-limited anodic oxide film growth is produced on such valve metals by employing a first glycerine-based electrolyte containing about 1 to about 3 wt % water for the initial production of anodic oxide. After the substrate is anodized using the first electrolyte, it is immersed in a second glycerine-based electrolyte having less than about 0.1 wt % water. The second electrolyte may be produced by allowing water to evaporate from the first electrolyte solution until the solution contains less than about 0.1 wt. % water.

FIELD OF THE INVENTION
 The invention is directed to non-thickness-limited anodizing of valve
 metals and alloys, particularly niobium and its alloys.
 BACKGROUND OF THE INVENTION
 Anodic oxide films have been employed commercially for over 100 years.
 These films find use in a variety of industrial applications, including
 electrolytic capacitors, rectifiers for converting alternating current to
 direct current, lightning arrestors, insulation on aluminum and aluminum
 alloy motor and transformer windings, as decorative coatings on furniture
 and appliances, as decorative coatings on niobium and titanium jewelry,
 and as a hard wear surface on aluminum or titanium machine and aircraft
 parts.
 Anodic oxide films have traditionally been categorized as belonging to one
 of two basic types of film. The first type is the non-barrier or
 decorative type of film. These oxide films are usually grown on aluminum,
 titanium, or alloys thereof in electrolyte solutions which partially
 dissolve the oxide film.
 Anodic aluminum films grown in cold sulfate or phosphoric acid solutions
 are porous, having a very large number of pores, generally of hexagonal
 shape, through which the electrolyte is in contact with the base metal
 (through a relatively thin oxide layer at the bottom of each pore) and
 supplies oxygen for continued anodic oxide growth so long as current is
 supplied. These films are usually grown with less than 50 volts applied
 across the anodizing cell. The pores in these films readily accept a wide
 variety of dyes, and they may be exposed to dye during or after the
 anodizing process. The pores for both decorative and wear-resistant anodic
 films on aluminum or its alloys are usually sealed by exposure to
 solutions which cause the pores to fill with a bulky aluminum oxide
 hydration product. Nickel acetate solutions have frequently been used to
 seal decorative and wear surfaces on aluminum.
 Decorative anodic films on titanium are usually produced in cold sulfuric
 acid electrolyte solutions. Although these films are less porous than
 decorative films on aluminum and tend to be more uniform in thickness,
 they tend to be of a lamellar structure and are sometimes present as a
 series of very thin layers connected at many points and appearing uniform
 and continuous to the naked eye. The uniformity of thickness and
 transparency of anodic films on titanium produced in cold sulfuric acid
 solutions results in a vivid series of interference colors, similar to
 those characteristic of the so-called barrier anodic films on tantalum, so
 that no dyes are required to produce decorative results. The lamellar
 structure of these films, mentioned above, probably accounts for the
 observation that they tend to not be as effective as thermally produced
 films for the purposes of wear or corrosion resistance.
 The second basic type of anodic oxide film is the barrier film. This type
 of anodic oxide is generally produced in electrolyte solutions which are
 relatively non-corrosive toward the substrate metals upon which the films
 are grown although barrier films may be produced on aluminum in
 electrolyte solutions which have significant solvent action on the
 hydrated forms of the oxide, such as borate solutions. Barrier anodic
 oxide films tend to be very uniform in thickness with the thickness being
 directly proportional to the applied voltage and the absolute (Kelvin)
 temperature of the electrolyte solution as described by Torissi (Relation
 of Color to Certain Characteristics of Anodic Tantalum Films, Journal of
 the Electrochemical Society, Vol. 102, No. 4, April 1955, pp. 176-180).
 Barrier anodic oxide films age down to very low current values when held at
 constant voltage in barrier film forming electrolytes, in contrast to
 non-barrier films which grow thicker as long as voltage is applied.
 Barrier anodic oxide films also exhibit the property of rectification;
 they are highly insulating with the base metal positive relative to the
 electrolyte solution and readily pass electric current with the base metal
 biased negative relative to the electrolyte solution. The rectification or
 electronic valve action has led to the name valve metals, for the group of
 metals upon which anodic films can be grown which exhibit this property.
 Barrier anodic oxide films have traditionally been limited to relatively
 thin layers, generally well under a micron in thickness. This is due to
 the extremely small amount of barrier oxide produced per volt applied,
 10-25 angstroms per volt depending upon the valve metal. This results in
 electric fields of up to 10,000,000 volts/cm across the thickness of the
 oxide. In order to prevent electron avalanche failure of barrier anodic
 oxide films at these high field levels, it has been found necessary to
 employ higher resistivity electrolytes to produce higher voltage films.
 The breakdown voltage of these films has been found to be proportional to
 the logarithm of the electrolyte resistivity. Electron avalanche failure
 of barrier films generally limits the maximum voltage to well under 1,000
 volts or less than one micron in thickness. The maximum voltage obtained
 with traditional barrier film anodizing techniques is approximately 1,500
 volts, obtained by Lilienfeld (U.S. Pat. Nos. 1,986,779 and 2,013,564)
 using polyglycol borate electrolytes, which produced barrier oxide films
 on aluminum of approximately 1.5 microns in thickness.
 It has been recognized for some time that, for some applications in the
 electronics, aerospace, and chemical industries, it would be very useful
 to have the capability of producing very thick barrier-type anodic oxide
 films. It has also been widely recognized that a method of producing very
 thick (i.e., over one micron thick) barrier oxide films capable of
 withstanding very high applied voltages (i.e., over 500 volts) with
 relatively low anodizing voltage is highly desirable. Just such an
 anodizing method was developed in 1997 and is the subject of U.S. Pat.
 Nos. 5,837,121 and 5,935,408, Kinard et. al., as well as co-pending
 application Ser. No. 09/090,164, now U.S. Pat. No. 6,149,793 and Ser. No.
 09/265,593.
 This method of producing barrier-type anodic oxide films of unlimited
 thickness on valve metals at relatively low anodizing cell voltages
 (dubbed, Non-Thickness-Limited or N-T-L anodizing by the inventors) was
 also described in a technical paper, The Non-Thickness-Limited Growth of
 Anodic Oxide Films on Valve Metals, published in Electrochemical and Solid
 State Letters, Vol. 1, No. 3, September 1998, pp. 126-129.
 Non-Thickness-Limited anodizing, as described in U.S. Pat. Nos. 5,837,121
 and 5,935,408, Kinard et. al., consists of the application of relatively
 low voltage (about 30 volts or less) to a valve metal object immersed in a
 glycerine solution of dibasic potassium phosphate containing less than
 about 0.1% water and at a temperature above about 150.degree. C. in order
 to produce a barrier anodic oxide film on the surface of the valve metal
 object. Basic salts, other than dibasic potassium phosphate, were found to
 result in fairly rapid polymerization of the glycerine to polyglycerine
 accompanied by the evolution of water.
 It was found that thermally stable acid salts giving a solution pH of 4-7
 may be employed (in place of the dibasic potassium phosphate) in
 combination with the glycerine solvent for non-thickness-limited anodizing
 of valve metals, as described in co-pending application Ser. No.
 09/090,164.
 It was found that, after a period of days at temperatures above 150.degree.
 C., the glycerine-based electrolyte solutions employed for
 non-thickness-limited anodizing contain so little water (below 0.05%) that
 the N-T-L anodizing may prove difficult to initiate. It was found that a
 thin anodic oxide film applied to the valve metal substrate prior to N-T-L
 anodizing, such as a 3-volt anodic oxide film applied in room temperature
 dilute phosphoric acid, provides a film sufficiently thick to then be
 converted readily to non-thickness-limited anodizing kinetics upon
 immersion in an N-T-L electrolyte above 150.degree. C. and applying
 voltage (i.e., the valve metal substrate with the preformed film gives
 rise to N-T-L anodizing more readily in low water content N-T-L
 electrolytes than does a valve metal substrate without a thin pre-formed
 film). This phenomena is described in co-pending application Ser. No.
 09/265,593, which is primarily concerned with the use of constant current
 anodizing to produce a predictable anodic oxide film thickness under
 non-thickness-limited anodizing conditions.
 Unfortunately, some valve metals, most notably niobium and niobium alloys,
 have proven difficult to anodize under non-thickness-limited anodizing
 kinetics due to the difficulty in initiating N-T-L film growth with these
 materials. The electronic leakage current through the native or passive
 oxide film which forms on the surface of niobium and alloys such as
 Nb/1%Zr is sufficiently high that little or no ionic current (necessary
 for anodic film growth) flows through the passive film upon application of
 voltage in N-T-L anodizing solutions at the required temperatures (i.e.,
 in 10 wt. % dibasic potassium phosphate solution in glycerine containing
 less than 0.1 wt. % water and maintained at a temperature above
 150.degree. C.).
 SUMMARY OF THE INVENTION
 The invention is directed to a method of non-thickness-limited anodizing
 valve metals and alloys, in particular niobium and niobium-containing
 alloys.
 The invention is particularly directed to a method of non-thickness-limited
 anodizing of a valve metal or valve metal alloy substrate comprising
 immersing the substrate in a first glycerine-based electrolyte comprising
 more than 0.1 wt % water, preferably about 1 to about 3 wt % water, and at
 a temperature of at least 150.degree. C., and applying sufficient first
 anodizing potential to form an oxide film on the substrate; then immersing
 the substrate in a second glycerine-based electrolyte having less than
 about 0.1 wt % water and at a temperature of at least 150.degree. C., and
 applying sufficient second anodizing potential to form a non-thickness
 limited oxide film on the substrate.
 In a preferred embodiment of the invention, the water in the first
 glycerine-containing electrolyte is evaporated to form the second
 glycerine-containing electrolyte.
 It is to be understood that both the foregoing general description and the
 following detailed description are exemplary and explanatory only and are
 not restrictive of the present invention as claimed.
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention is directed to the production of anodic oxide films
 on valve metals via anodic polarization in a liquid electrolyte under
 conditions which result in the production of adherent, coherent,
 non-porous films of unlimited thickness at a fixed and relatively low
 (less than 100 volts) D.C. potential. This type of "non-thickness limited"
 anodizing stands in contrast to traditional anodizing, which produces
 anodic films having a thickness in direct proportion to the applied
 voltage and absolute temperature of the electrolyte.
 The present invention is particularly directed to producing such
 non-thickness-limited films on valve metals such as niobium that are
 difficult to initiate non-thickness-limited anodizing.
 It was observed that niobium and its alloys are resistant to undergoing
 non-thickness-limited anodizing kinetics. Even with a significant
 thickness of traditionally formed anodic oxide present, niobium and
 niobium alloy anode surfaces tend to exhibit electronic leakage currents
 to the point of preventing the flow of ionic current necessary for anodic
 oxide formation when electrified in non-thickness-limited anodizing
 electrolytes. The films formed by exposure to the atmosphere and by
 traditional anodizing methods and electrolytes are not sufficiently
 electrically insulating and thermally stable to support
 non-thickness-limited anodizing kinetics.
 It was discovered that electrolytes, suitable for non-thickness-limited
 anodizing (i.e., thermally stable glycerine solutions of ionogens) but
 containing more than about 0.1 wt. % water, in particular 1 to 3 wt %
 water, may be used to produce anodic oxide films on the surfaces of
 niobium and niobium alloys which are significantly more thermally stable
 than films produced in aqueous electrolytes.
 First, an anodic oxide film is grown on a niobium or niobium alloy
 substrate by immersing the substrate in a non-thickness limited
 electrolyte solution having more than 0.1 wt % water, preferably about 1
 to about 3 wt % water, and at a temperature of at least 150.degree. C. An
 anodizing potential is applied, while maintaining the solution at or above
 about 150.degree. C. The anodizing potential applied to initiate the oxide
 film growth is about 5 to about 30 volts, preferably about 20 to about 30
 volts.
 After the initial oxide film growth, the substrate is transferred to a
 non-thickness-limited anodizing electrolyte having water content below
 about 0.1 wt. % water at a temperature above about 150.degree. C. Voltage
 is then applied to produce non-thickness-limited anodic films.
 Alternatively, the water in the electrolyte solution containing more than
 0.1% water is allowed to evaporate to achieve the non-thickness-limited
 anodizing electrolyte having water content below about 0.1 wt. % water. In
 other words, once the substrate is coated with the initial anodic oxide
 film, the temperature is maintained in excess of 150.degree. C., and the
 voltage is applied continuously while the water content of the electrolyte
 solution is reduced by evaporation. As the water content drops below about
 0.1%, the anodizing kinetics change to non-thickness-limited kinetics and
 a thick, uniform barrier oxide coating is produced.
 A non-thickness-limited anodizing electrolyte and has a water content of
 less than about 0.1 wt % water. The electrolyte used to initiate oxide
 growth on the niobium or niobium alloy substrate is the same
 non-thickness-limited anodizing electrolyte with a water content of about
 1 to about 3 wt % water. It is preferred that both electrolytes are the
 same but for the water-content. However, different electrolyte solutions
 are also contemplated for the first and second glycerine-containing
 electrolytes.
 The method of the invention is very effective to obtain the desired
 non-thickness-limited oxide growth on niobium substrates. This is in
 contrast to the negative results obtained with niobium and high niobium
 alloys if the non-thickness-limited anodizing electrolyte (for example, 10
 wt. % dibasic potassium phosphate in glycerine) containing less than about
 0.1 wt. % is used to anodize niobium anode materials at temperatures below
 about 150.degree. C. (e.g., 100.degree. C.) with the temperature then
 being increased to 150+.degree. C. in an attempt to initiate
 non-thickness-limited anodizing kinetics.
 As mentioned above, the present invention uses glycerine-based electrolytes
 which are useful for non-thickness limited anodizing above 150.degree. C.
 Due to its low pH, these electrolytes are not susceptible to
 polymerization of the glycerine. Such glycerine-based electrolytes are
 described in U.S. Pat. Nos. 5,837,121 and 5,935,408, and in co-pending
 Ser. No. 09/265,593, each of which is incorporated by reference in its
 entirety. For example, the glycerine-based electrolytes may comprise
 phosphate salt ionogens or acid salt ionogens. Solutions having acid salt
 ionogens typically have a pH of less than 7. The glycerine-based
 electrolytes are then modified by the addition of water to provide a water
 content of 1 to 3 wt %.
 U.S. Pat. Nos. 5,837,121 and 5,935,408 describe electrolytic solutions of
 dibasic potassium phosphate in glycerine. Such electrolytic solutions can
 be prepared, for example, by mixing the phosphate and glycerine together
 at room temperature such as by stirring. The dibasic potassium phosphate
 is added in amounts of about 0.1 to 15 wt %, preferably about 2 to 10 wt
 %, based on the total weight of solution.
 In co-pending application Ser. No. 09/265,593, electrolytes suitable for
 non-thickness-limited anodizing are produced by dissolving an organic acid
 salt, an inorganic acidic salt, or mixtures thereof in glycerine or by
 producing acidic salts in situ via addition of acidic and basic ionogen
 components to the glycerine. By mixtures thereof, it is meant a mixture of
 acidic salts, a mixture of basic salts, or a mixture of acidic and basic
 salts. The solution is then heated to above about 150.degree. C. and the
 water content is reduced to below 0.1 wt %. The pH level is below about 7,
 and preferably between about 4 and 7.
 Alternatively, suitable acidic salts are formed in situ via addition of
 acidic and basic ionogen components. The salt nature of the ionogen
 prevents consumption of the acidic component of the electrolyte in the
 production of esters with the elimination of water as occurs with straight
 acid solutions above 150.degree. C. Preferably an organic salt is combined
 with a non-volatile organic or inorganic acid. Suitable salts include
 potassium acetate, sodium bicarbonate and potassium formate. Suitable
 inorganic acids and salts include sulfuric acid and potassium hydrogen
 sulfate. Suitable organic acids include P-toluene sulfonic acid, and
 tartaric acid. Preferably potassium acetate is mixed with sulfuric or
 tartaric acid to form, for example monobasic potassium tartrate.
 The process of the invention is particularly useful for niobium and its
 alloys which have been difficult to anodize with the non-thickness-limited
 process describe in the co-pending application. Preferably, the niobium
 alloys contain at least about 50 atomic % niobium. The process may be used
 to produce anodic films on other types of metals including other "valve"
 metals such as aluminum, tantalum, titanium, zirconium, silicon, although
 the two-step anodization process of the invention may not be necessary for
 these other metals.
 After the initial oxide film is formed, anodic films, prepared with the
 non-thickness-limited electrolytic solution may be produced at constant
 voltage, with the film thickness being approximately proportional to the
 time held at voltage at a constant temperature above about 150.degree. C.
 The rate of film growth in these solutions is a function of both the
 applied voltage and electrolyte temperature. There is no known upper limit
 to the thickness of a film produced in accordance with the invention.
 There are unlimited applications for the electrolytic solution of the
 invention including the production of electrolytic capacitors, rectifiers,
 lightning arrestors, and devices in which the anodic film takes the place
 of traditional electrical insulation, such as special transformers,
 motors, relays, etc. In addition, because of the uniformity obtained with
 the invention, the process of the invention may be used in the production
 of surgical implants where a minimum of induced currents is desirable. The
 rapid rate of growth achieved with the invention also allows for the
 production of practical anti-seize coatings for connectors and plumbing
 fabricated from valve metals and alloys.

EXAMPLES
 The invention will be further described by reference to the following
 examples.
 These examples should not be construed in any way as limiting the
 invention.
 Example 1 (Comparative)
 In order to demonstrate the difficulty in initiating non-thickness-limited
 anodic oxide formation on niobium and alloys of high niobium content by
 coating the anode with a thin layer of traditional anodic oxide prior to
 anodizing in the non-thickness-limited mode, as described in co-pending
 application Ser. No. 09/265,593, the following test was conducted.
 A coupon of dimensions 4".times.1" was cut from 0.01" thick Cabot niobium/1
 wt. % zirconium alloy foil. The coupon was rinsed with acetone to remove
 any rolling oils or other organic materials. The coupon was then immersed
 to a depth of 2" in a 1 vol. % electrolyte solution of phosphoric acid at
 room temperature and a positive bias of 5 volts was applied to the coupon.
 The coupon rapidly aged-down in current at 5 volts. After 10 minutes at 5
 volts, the current had decayed from an initial value of over 12
 milliamperes to a value of 0.023 milliampere, indicating the presence of a
 5 volt traditional anodic oxide film having high electrical resistance.
 The coupon was then transferred to a non-thickness-limited electrolyte
 solution (10 wt. % dibasic potassium phosphate/90 wt. % glycerine)
 containing less than 0.1 wt. % water and maintained at a temperature above
 150.degree. C.
 No additional anodic oxide was produced upon the application of 0.2, 0.4,
 and 0.8 milliamperes/cm.sup.2 of coupon surface, the current being
 consumed as electronic leakage current.
 Example 2 (Comparative)
 In order to further demonstrate the difficulty of initiating
 non-thickness-limited anodic oxide production on niobium and high niobium
 alloy anode materials, a coupon was prepared from the Cabot niobium/1%
 zirconium foil, as used in Example 1. This coupon was anodized
 traditionally, at room temperature, in 1 vol. % phosphoric acid, as in
 Example 1 except that the bias applied was increased to 30 volts positive
 bias on the coupon (with respect to the anodizing electrolyte). After 10
 minutes at 30 volts, the leakage current through the anodic oxide film on
 the coupon decreased from an initial value of approximately 20
 milliamperes to approximately 0.53 milliampere, indicating the presence of
 an insulating, traditional anodic oxide film. This produced a film
 equivalent to 30 anodizing volts or 5-10 times thicker than has been found
 necessary for the transition to non-thickness-limited anodic oxide growth
 with tantalum anode materials.
 The coupon was then transferred to the same non-thickness-limited anodizing
 electrolyte that was used in Example 1, again at a temperature above the
 approximately 150.degree. C. initiation point for non-thickness-limited
 anodic oxide production. A current of approximately 0.4
 milliampere/cm.sup.2 was applied for 10 minutes. During this exposure to
 non-thickness-limited anodizing conditions, the voltage across the cell
 (mainly voltage drop across the oxide film) was observed to decrease, from
 approximately 18 volts initially to approximately 2.25 volts at the end of
 10 minutes. The coupon was then removed from the non-thickness-limited
 electrolyte, washed, and examined.
 The oxide did not grow appreciably thicker (same interference color as
 before exposure to N-T-L conditions). The edges of the coupon were found
 to have oxide damage or gray-out present due to the passage of current
 through the oxide.
 The above examples illustrate the difficulty of applying the method of
 pre-anodizing anode materials conventionally prior to
 non-thickness-limited anodizing for the purpose of facilitating initiation
 of non-thickness-limited anodic oxide growth (as described in co-pending
 Ser. No. 09/265,593) to niobium and high niobium content alloys.
 Example 3 (Invention)
 In order to illustrate the efficacy of the method of the invention, a
 coupon was cut from Cabot niobium/1% zirconium foil, as in Examples 1 and
 2. This coupon was acetone washed, as in Examples 1 and 2. The coupon was
 then immersed in the same non-thickness-limited anodizing electrolyte used
 in Examples 1 and 2, with approximately 25 cm.sup.2 immersed in the
 electrolyte. The electrolyte temperature was maintained between
 155.degree. C. and 165.degree. C. for the duration of the test. The
 electrolyte water content was initially below 0.1 wt. %.
 The coupon was biased positive, with an available current density of 0.4
 milliampere/cm.sup.2. After 5 minutes, the voltage had risen from 0.98
 volts to only 1.12 volts. Essentially no anodic oxide growth was observed.
 At this time, 1% water was added to the electrolyte (as a 50% solution in
 glycerine to prevent boiling). The voltage began to rise immediately,
 reaching 3.27 volts within 1 minute and 9.32 volts within 20 minutes of
 the water addition. Twenty minutes after the first water addition, an
 additional 1% water was added to the electrolyte solution. Twenty minutes
 after the second addition, a third addition of 1% water was made to the
 electrolyte solution. Although the anodizing efficiency was low and the
 current unstable during this traditional anodizing portion of the anodic
 oxide formation (probably due to the very high anodizing temperature and
 inherent instability of anodic niobium oxide), within 3 hours of the third
 water addition, the current had decayed to approximately 0.18
 milliampere/cm.sup.2.
 During the course of the anodizing, after the third de-ionized water
 addition, the electrolyte solution decreased in water content due to the
 high electrolyte temperature (160+.degree. C.). After approximately 3
 hours, the electrolyte was sufficiently low in water content (i.e., below
 approximately 0.1 wt. %) for N-T-L anodic oxide formation to be detectable
 by an increase in the cell current. The anodizing current rose steadily
 over the next 3 hours as the electrolyte dried further. The final current
 had risen to 0.28 milliampere/cm.sup.2.
 The coupon, which had undergone non-thickness-limited anodic oxide
 formation for at least 3 hours (as indicated by the increasing current
 through the anodizing cell), was bent double, so as to crack the anodic
 oxide, then the coupon was subjected to scanning electron microscope
 examination. The anodic oxide film was found to be approximately 2.8
 microns thick. This film is, then, over 30 times thicker than would be
 expected for a traditionally formed anodic oxide film on niobium.
 This example demonstrates raising the water content of an
 non-thickness-limited type of anodizing electrolyte solution to 1-3 wt %,
 anodizing a niobium or high niobium content alloy anode material in the
 electrolyte at this water content, then allowing the water content to be
 reduced through evaporation at a temperature above about 150.degree. C.
 with positive bias applied to the anode material, produces a sufficiently
 stable anodic film so that the transition to non-thickness-limited anodic
 oxide formation.
 Example 4 (Comparative)
 In order to demonstrate that the successful transition from traditional
 anodic oxide growth to non-thickness-limited anodic oxide growth requires
 the addition of water to the non-thickness-limited electrolyte and cannot
 be produced by merely reducing the non-thickness-limited electrolyte
 temperature to significantly below 150.degree. C., anodizing the niobium
 material in the reduced temperature/low water electrolyte solution, then,
 raising the electrolyte temperature above about 150.degree. C. with
 positive bias applied, the following experiment was conducted.
 A 10 wt. % solution of dibasic potassium phosphate in glycerine was
 prepared and was dried by heating to 156.degree. C. to 158.degree. C. for
 17 hours. The electrolyte temperature was then reduced to 100.degree. C.
 to 110.degree. C.
 A coupon was cut from Cabot niobium/1% zirconium foil and acetone washed as
 in the first three examples. The foil coupon was immersed in the
 electrolyte solution and a current of 0.4 milliampere/cm2 was applied. The
 voltage across the cell increased to 30 volts (the voltage set point)
 within 11 minutes and the current decayed, as is the case with traditional
 barrier anodic oxide film formation. Within 30 minutes of the application
 of positive bias to the coupon, the current had decayed to less than 0.04
 milliampere/cm.sup.2.
 At this point (30 minutes after the first application of voltage bias to
 the coupon), the electrolyte solution temperature was increased. As the
 temperature rose to approximately 160.degree. C., the current increased to
 the 0.4 milliampere/cm.sup.2 set point and the voltage dropped to less
 than 2 volts.
 The coupon was then held at 0.4 milliampere/cm.sup.2 for over 2 hours at a
 temperature above 150.degree. C. and with an electrolyte solution water
 content of less than 0.1%. At the end of this time, the coupon was
 examined and was found to have grayed-out badly (seriously flawed anodic
 oxide) with no evidence of non-thickness-limited anodic oxide growth.
 Example 5 (Invention)
 In order to illustrate the method of the present invention with a niobium
 substrate, a coupon was cut from niobium foil, 99.8%, and was
 acetone-rinsed to remove any rolling oils, etc.
 The coupon was then suspended partially immersed in a 10 wt. % solution of
 dibasic potassium phosphate in glycerine contained in a stainless steel
 beaker. This electrolyte solution had previously been dried to reduce the
 water content to less than 0.1 wt. % water by heating at 150-160.degree.
 C. for approximately 7 hours.
 The coupon was connected to the positive pole, and the beaker to the
 negative pole of a constant current/constant voltage power supply set to
 deliver a maximum voltage of 30 volts and a maximum current such that the
 maximum current density available was 0.35 milliampere/cm.sup.2 of coupon
 surface.
 Current was then applied to the cell. After 5 minutes with 0.35 mA/cm.sup.2
 current flow, the voltage across the cell was approximately 1.5 volts and
 was essentially the same for the 5 minute hold time (i.e., no evidence of
 anodic film growth).
 With the current applied, 1.6 wt. % water was added to the cell (stirred
 with a magnetic stirring bar) as a 50% glycerine solution. The voltage
 began to rise immediately with the water addition as follows:

Time After H.sub.2 O Addition Voltage Current
 (0) 1.5 volts 0.35 mA/cm.sup.2
 5 minutes 9.1 volts 0.35 mA/cm.sup.2
 10 minutes 18.2 volts 0.35 mA/cm.sup.2
 15 minutes 28.0 volts 0.35 mA/cm.sup.2
 16 minutes 30.2 volts dropping
 20 minutes 30.2 volts 0.115 mA/cm.sup.2
 25 minutes 30.2 volts 0.090 mA/cm.sup.2
 The above data is typical of traditional anodic oxide films formed in
 organic electrolyte solutions at this temperature (155-160.degree. C.).
 The solution/coupon were held at temperature with voltage applied across
 the cell in order to allow the water to evaporate so as to reduce the
 water content of the electrolyte to less than about 0.1 wt. %.
 Upon thermally reducing the water content of the solution to the level
 required for the onset of non-thickness-limited anodizing behavior (i.e.,
 below approximately 0.1 wt. %), the current began to increase, eventually
 reaching the preset limit of the power supply, at which time the voltage
 level required to drive the current through the anodic oxide (producing
 N-T-L oxide growth) also decayed. The voltage/current history is as
 follows:

Time After H.sub.2 O Addition Voltage Current
 1 Hr. 35 min 30 volts (dropping) 0.35 mA/cm.sup.2
 2 Hrs. 28 volts 0.35 mA/cm.sup.2
 3 Hrs. 28.5 volts 0.35 mA/cm.sup.2
 4 Hrs. 29 volts 0.35 mA/cm.sup.2
 5 Hrs. 24 volts 0.35 mA/cm.sup.2
 6 Hrs. 8.5 volts 0.35 mA/cm.sup.2
 7 Hrs. 7.0 volts 0.35 mA/cm.sup.2
 8 Hrs. 6.8 volts 0.35 mA/cm.sup.2
 9 Hrs. 7.0 volts 0.35 mA/cm.sup.2
 10 Hrs. 7.0 volts 0.35 mA/cm.sup.2
 11 Hrs. 6.5 volts 0.35 mA/cm.sup.2
 12 Hrs. 3.0 volts 0.35 mA/cm.sup.2
 13 Hrs. 2.1 volts 0.35 mA/cm.sup.2
 14 Hrs. 1.0 volts 0.35 mA/cm.sup.2
 15 Hrs. 1.0 volts 0.35 mA/cm.sup.2
 16 Hrs. 1.0 volts 0.35 mA/cm.sup.2
 Note: Temperature maintained at 155-160.degree. C. during the test.
 It may be seen from the above data that N-T-L anodizing behavior was
 induced by the addition of water to the N-T-L electrolyte to produce a
 traditional anodic oxide film. The water content was decreased to the
 point that N-T-L anodizing behavior ensued. The voltage decreased as the
 water content of the electrolyte solution dropped due to evaporation.
 At 16 hours after the initial addition of 1.6 wt. % water, an additional
 1.6 wt. % water was made to the N-T-L electrolyte (as a 50% aqueous
 glycerine solution). The voltage again began to increase immediately,
 reaching the 30 volt preset power supply limit, followed by decay of the
 current to 0.019 mA/cm.sup.2 within 30 minutes of this water addition.
 Thus the film growth is of the non-thickness-limited variety and ceased
 upon increasing the water content of the electrolyte solution above about
 0.1 wt. % water.
 The coupon was then removed from the anodizing cell and rinsed to remove
 the electrolyte. The coupon was bent in order to crack the anodic oxide
 and then was examined with a scanning electron microscope. This
 examination revealed a relatively smooth and uniform anodic oxide was
 present, having a thickness of approximately 12 microns. This is the
 approximate equivalent of an anodic oxide film grown at 5000-6000 volts by
 traditional methods. (This voltage is an extrapolation based upon 20-25
 angstroms per volt for anodic niobium oxide. It is not currently possible
 to grow a uniform anodic film on niobium above a few hundred volts using
 traditional anodizing techniques and electrolytes.)
 It will be apparent to those skilled in the art that various modifications
 and variations can be made in the compositions and methods of the present
 invention without departing from the spirit or scope of the invention.
 Thus, it is intended that the present invention covers the modifications
 and variations of this invention provided they come within the scope of
 the appended claims and their equivalents.