WO1999045177A2

WO1999045177A2 - Anodic spark coating

Info

Publication number: WO1999045177A2
Application number: PCT/US1999/004555
Authority: WO
Inventors: Sten A. Wallin
Original assignee: The Dow Chemical Company
Priority date: 1998-03-06
Filing date: 1999-03-02
Publication date: 1999-09-10
Also published as: AU2888599A; WO1999045177A3; TW408187B

Abstract

A spark anodization process is claimed which provides improved coatings for such applications as adhesive bonding and the treatment of aluminum brake cylinders.

Description

ANODIC SPARK COATING

This application claims the benefit of U.S. Provisional Application Serial No. 60/077,062, filed March 6, 1998.

The present invention relates to anodic spark coating ("ASC") processes, electrolyte baths useful in such processes, and coatings produced by utilizing such processes. More specifically, the present invention relates to anodic spark coating processes that utilize novel electrolyte baths containing low concentrations of phosphate to produce thin, smooth, predominately oxide coatings on metal substrates, especially substrates made of aluminum and aluminum alloys.

ASC processes are known in the art. Generally, a variety of these processes have been used on metal substrates to improve surface characteristics, such as appearance (for example, coloring), durability (for example, corrosion resistance, wear resistance), and improved adhesion (for example, adhesion to lacquer or epoxy coatings). In the prior art, ASC processes have been referred to as anodic spark deposition ("ASD"), electrodeposition, spark anodization ("SA"), a microplasma method, etc.

Typically, an ASC process is carried out by positioning the metal substrate to be coated as an anode in an electrolytic solution (generally aqueous). The electrolytic solution is frequently referred to as an electrolyte "bath." A cathode is also positioned in the bath and a voltage is imposed across the anode and cathode, causing an electric current to flow between the anode and cathode.

The electric current is allowed to flow until the desired coating thickness is obtained on the anode. The electric current is typically controlled by maintaining a constant electrical current between the anode and cathode or by maintaining a constant voltage across the anode and cathode. During the process of coating the anode, sparking is generally visibly observed at the anode.

In the prior art, many different electrolyte bath compositions have been tried with varying degrees of success. Electrolyte baths have been used which contain aluminates, silicates, tungstates, molybdates, chromates, phosphates, fluorides, vanadates, titanates, niobates, carbonates, borates, and the like. It is also known that dispersed particles, such as oxide ceramics, pigments, and metal sulfides, can also be present in the electrolyte bath.

In many ASC processes of the prior art, the objective is to deposit ions (usually metal ions) or an oxide from the electrolyte bath onto the surface of the substrate to be coated. Typically, this is the objective of what some of the prior art refers to as ASD or electrodepositing.

ASC processes involve many different parameters, including the composition of the substrate, the composition of the bath, the pH of the bath, the temperature of the bath, the voltage used, the current used, and the duration that the current is applied. The prior art teaches many different ASC processes that have attempted to improve coatings by altering one or more of these parameters.

While the prior art teaches many different ASC processes, there are still some deficiencies in processes and coatings taught in the prior art. For example, it has been generally observed that thicker coatings produced by ASC processes of the prior art are not as smooth as thinner coatings and often demonstrate a propensity to crack. Hence, there is still a need in industry for coatings that are smoother relative to coatings of the same thickness made by ASC processes of the prior art. Smoother coatings can be advantageous in the industry, especially in bonding applications. There also continues to be a need in industry for coatings having increased wear resistance, corrosion resistance, and reduced surface roughness to be used on articles such as master brake cylinders.

In one embodiment, the present invention is a novel ASC process. In another embodiment, the present invention is a novel composition of an electrolyte bath useful in ASC processes. In yet another embodiment, the present invention is a novel coating of a metal substrate.

Electrolyte baths of the present invention are generally non-toxic, inexpensive to make, easy to make, and easy to maintain. Electrolyte baths of the present invention can be used in ASC processes to produce high quality, uniform coatings on a wide range of aluminum alloys under a wide tolerance of process parameters (for example, current density, bath temperature, and duration of current application). Electrolyte baths of the present invention comprise low concentrations of phosphate anions in an alkaline solution. By alkaline, it is meant that electrolyte baths of the present invention have a pH greater than or equal to 7. Preferably, electrolyte baths of the present invention have a pH greater than about 8.

Baths of the present invention should have a sufficient amount of phosphate to eliminate burning due to low bath conductivity. Baths of the present invention contain at least about 0.01 molar of phosphate anions.

Baths of the present invention contain no more than about 0.1 molar phosphate anions. Preferably, baths of the present invention contain no more than about 0.06 molar of phosphate anions.

In a preferred embodiment of the present invention, the electrolyte bath comprises approximately equal concentrations of orthophosphate and pyrophosphate. In a particularly preferred embodiment of the present invention, the electrolyte bath comprises approximately 0.03 molar orthophosphate and approximately 0.03 molar pyrophosphate.

Other anions may be present in electrolyte baths of the present invention.

However, these anions must be present only in concentrations low enough so as not to significantly alter the coatings that result from utilizing the bath in an ASC process. For example, if silicates are present in electrolyte baths of the present invention, they should be present in concentrations sufficiently low that essentially no SiO2 is deposited on the surface of the substrate during the ASC process.

Coatings of the present invention are thin, dense coatings of primarily aluminum oxide. Although these coatings are primarily comprised of aluminum oxide, minor amounts of phosphorous may sometimes be found in the coating. If phosphorous is present in the coating, it is generally found near the outer surface of the coating. Coatings of the present invention are between approximately 0.5 micrometer and 10 micrometers thick. Additionally, coatings of the present invention have less surface roughness than coatings in the prior art of equal thickness. Coatings of the present invention have been found to enhance the corrosion resistance and wear resistance of the substrate surfaces coated. The present invention further offers unexpected advantages for use on anode configurations including deep bores, and the inside of narrow tubes, for example, which exist in cast aluminum parts used, for example, in automobiles, such as master brake cylinders. The present invention also provides improvement in bonding metal substrates to other materials, such as another metal or a plastic, with the use of an adhesive. Coatings of the present invention can be useful as a surface treatment for an aluminum surface to be adhesively bonded or painted.

Generally, processes of the present invention are carried out by positioning the metal substrate to be coated as an anode in an electrolyte bath. A cathode is also positioned in the bath and a voltage is imposed across the anode and cathode, causing an electric current to flow between the anode and cathode. The electric current is allowed to flow until the desired coating thickness is obtained on the anode. The electric current is typically controlled by maintaining a constant electrical current between the anode and cathode or by maintaining a constant voltage across the anode and cathode. During the process of coating the anode, sparking is generally visibly observed at the anode. For purposes of the present invention, the voltage measured at the time a spark is first visible will be referred to as V_Spark.

It should be noted that when electrolyte baths of the present invention are used, processes of the present invention demonstrate a remarkable insensitivity to the particular current density imposed. That is, in processes of the present invention, significant changes in the current density do not significantly change the properties (for example, surface roughness) of the resulting coatings. It appears that coating properties are instead more dependent on the product of the current density and the run time (that is, the total charge). In contrast, the surface roughness of prior art coatings seems to be much more closely dependent on the current density used during the process of producing the coating.

Electrolyte baths of the present invention comprise low concentrations of phosphate anions in an aqueous alkaline solution. By alkaline, it is meant that electrolyte baths of the present invention have a pH greater than or equal to 7. If the pH of the bath is too low, there will be some "burning" of the anode substrate. "Burning" happens when the spark discharge occurs with excessive frequency in a localized region on the anode substrate, resulting in a poor quality, non-uniform coating. Preferably, electrolyte baths of the present invention have a pH greater than about 8. Coatings of the present invention have been successfully produced using baths having a pH as high as about 11. It may be possible to utilize baths with a pH greater than 11.

Lower concentrations of anions in the bath cause the bath to have lower conductivity, and therefore, less efficiency. If the concentration of phosphate is too low, there will be a higher propensity for burning due to the lower conductivity. Thus, baths of the present invention should have a sufficient amount of phosphate to eliminate burning due to low bath conductivity. Baths of the present invention contain at least about 0.01 molar of phosphate anions.

If the phosphate concentration exceeds about 0.1 molar, thicker, less uniform coatings are formed. At concentrations above about 0.1 molar, the required voltage is lower, and the rate at which the coating is formed is higher. It has been found that coatings produced using baths comprising phosphate concentrations greater than about 0.1 molar are thicker, less uniform, and rougher than coatings produced using baths with lower concentrations of phosphate. These thicker, less uniform coatings are not as useful in many applications, such as a pretreatment for adhesive bonding, painting, or coating master brake cylinders. Preferably, baths of the present invention contain no more than about 0.06 molar of phosphate anions.

In a preferred embodiment of the present invention, the electrolyte bath comprises approximately equal concentrations of orthophosphate and pyrophosphate. It has been observed that electrolyte baths comprising approximately equal concentrations of orthophosphate and pyrophosphate allow for a wider range of useful process parameters (for example, current density, bath temperature, and duration of current application) than baths not having approximately equal concentrations of orthophosphate and pyrophosphate. In a particularly preferred embodiment of the present invention, the electrolyte bath comprises approximately 0.03 molar orthophosphate and approximately 0.03 molar pyrophosphate. Other anions may be present in electrolyte baths of the present invention. However, these anions must be present only in concentrations low enough so as not to significantly alter the coatings that result from utilizing the bath in an ASC process.

More specifically, when certain anions are present in electrolyte baths in sufficiently high concentrations, oxides of these anions can get deposited on the anode substrate during the ASC process. For the purposes of the present specification, these anions will be referred to as "depositable anions." Depositable anions are oxoanions for which there can be associated well-known binary oxide phases. Depositable anions include, for example, silicates, vanadates, molybdates, tungstates, aluminates, borates, and phosphates.

For example, if silicates are present in electrolyte baths of the present invention, they should be present in concentrations sufficiently low that essentially no SiO₂ is deposited on the surface of the substrate during the ASC process. When SiO₂ is deposited on the surface of the substrate, the surface roughness is higher than if no SiO₂ is deposited on the surface of the substrate. Of course, small amounts of SiO₂ deposits may be tolerable, depending on the intended application of the surface coating.

It is believed that oxides of depositable anions are formed during the phase of an ASC process when sparking is visible. More intense sparking produces larger deposits of oxides derived from the depositable anions.

The upper limit on the concentration of depositable anions may vary slightly depending on the particular anion used, the intensity of the sparking, pH of the bath, temperature of the bath, etc. The precise upper limit on concentration of any particular anion, in use with other parameters, can be determined without undue experimentation. This upper limit on concentration of depositable anions has been observed to be approximately 0.04 molar. Preferably, no depositable anions are present in electrolyte baths of the present invention.

For purposes of the present specification, "non-depositable anions" are anions for which no associated well-known stable binary crystalline oxide phase can be associated. Non-depositable anions include, for example, carbonates, sulfates, and fluorides. Non-depositable anions can be tolerated in baths of the present invention in higher concentrations than depositable anions. Generally, electrolyte baths of the present invention can contain up to approximately 0.1 molar of non-depositable anions. Preferably, no non-depositable anions are present in baths of the present invention.

Once desired concentrations of anions are known, the method of making electrolyte baths of the present invention is not critical. Given the desired concentrations of anions in a bath, a person of ordinary skill in the art would readily know how to make the bath.

The temperature of electrolyte baths of the present invention is not particularly critical. Typically, the bath temperature will rise somewhat during a process of the present invention. Excellent coatings have been produced using bath temperatures from 15°C and to 35°C. As previously indicated, the range of acceptable bath temperatures is even greater when the electrolyte bath contains approximately equal concentrations of orthophosphate and pyrophosphate. It is expected that bath temperature should generally be kept between about 10°C and 40°C.

Coatings of the present invention on aluminum substrates were subjected to microprobe analysis. The analysis showed that the coatings comprised aluminum oxide. Thus, coatings of the present invention are thin, dense coatings of primarily aluminum oxide. Although these coatings are primarily comprised of aluminum oxide, minor amounts of phosphorous may sometimes be found in the coating. If phosphorous is present in the coating, it is generally found near the outer surface of the coating.

Coatings of the present invention are between approximately 0.5 micrometer and 10 micrometers thick. If two coatings are produced using identical baths, the thicker coating will have greater surface roughness. However, coatings of the present invention have less surface roughness than coatings in the prior art of equal thickness.

The data in the tables below further illustrate some of the embodiments of the present invention. For the data in the tables below, all the coatings were produced using DC power supplies. In Table 1, the data pertains to coatings that were produced on 1 ft. (30.48 cm) x 1 ft. (30.38 cm) panels made of Al 5052. In Table 2, the data pertains to coatings that were produced on 2.5 inches (6.35 cm) x 3 inches (7.62 cm) panels made of Al 1100.

Other aluminum substrates have been successfully coated according to the present invention. For example, aluminum substrates made from 1000 series, 2000 series, 3000 series, 5000 series and 6000 series wrought aluminum alloys have been successfully coated, as well as substrates made from aluminum casting alloys such as 356 alloy.

Table 1

Bath Composition Bath pH Current Run * spark Observation Density Time

0.03M K₂HP0₄ 10.3 30 mA/cm² 4 min 360V Uniform beige 0.03M Na₄P,0₇ coating

0.03M K₂HP0₄ 9.9 30 mA/cm² 4 min 360V Uniform beige 0.03M Na₄P₂0₇ coating O.OlM NaF

0.03M K₂HP0₄ 9.4 30 mA/cm² 4 min 240V More burning 0.03M Na₄P₂0₇ than sparking - O.OlM NaF fractal pattern 0.04M NH₄V0₃ of dark 0.04M NaOH deposits

0.03M K₂HP0₄ 9.7 30 mA/cm² 4 min 250V More burning 0.03M Na₄P₂0₇ than sparking - O.OlM NaF fractal pattern 0.04M NH₄V0₃ of dark 0.05M Si0₂ deposits

0.018M K₂HPO₄ 10.3 30 mA/cm² 4 min 320V Uniform 0.018M Na₄P₂O₇ brown-beige 0.006M NaF coating a small 0.024M NH₄V0₃ black deposit 0.03M Si0₂ near a corner

0.06M K₂HP0₄ 11.7 30 mA/cm² 4 min 200V Heavy crusty O.OlM NaF deposits on edges and corners

Table 2

Bath Composition Bath pH Current Run V_Spark Observation Density Time

0.02M K₂HP0₄ 9.9 30 mA/cm² 4.5 380 V Uniform beige 0.02M Na₄P₂0₇ min. coating

0.03M K₂HP0₄ 9.9 30 mA/cm² 4.1 345V Uniform beige 0.03M Na₄P₂0₇ min. coating

0.03M K₂HP0₄ 9.9 30 mA/cm² 4.0 360V Uniform beige 0.03M Na₄P₂0₇ min. coating O.OlM NaF

0.04M K₂HP0₄ 10.0 30 mA/cm² 4.0 300V Uniform beige 0.04M Na₄P₂0₇ min. coating

0.045M K₂HP0₄ 10.0 30 mA/cm² 4.0 300V Uniform beige 0.045M Na₄P₂0₇, min. coating

0.05M K₂HP0₄ 10.1 30 mA/cm² 4.0 300V Uniform beige 0.05M Na₄P₂O₇, min. coating

0.055M K₂HP0₄ 10.2 30 mA/cm² 4.0 300V Onset of 0.055M Na₄P₂0₇ min. localized burning

0.03M K₂HP0₄ 10.3 40 mA/cm² 3.0 300V Uniform beige 0.03M Na₄P₂0₇ min. coating

0.018M K₂HPO₄ 40 mA/cm² 4 min. 250V More burning 0.018M Na₄P₂O₇ than sparking - 0.04M NH₄V0₃ fractal pattern 0.030M Si0₂ of dark deposits

0.019M K₂HPO₄ 30 mA/cm² 4 min. 320V Uniform 0.019M Na₄P₂O₇ brown-beige 0.0144M NH₄VO₃ coating 0.05M SiO,

Claims

CLAIMS:

1. An electrolyte bath for forming a coating on the surface of a metal substrate, comprising:

A. phosphate in a concentration greater than 0.01 molar and less than 0.1 molar; B. depositable anions in a concentration of from 0 to 0.04 molar;

C. non-depositable anions in a concentration from 0 to 0.1 molar; and

D. water, wherein the electrolyte bath has a pH greater than or equal to 7.

2. An electrolyte bath according to Claim 1, wherein said phosphate comprises approximately equal amounts of orthophosphate and pyrophosphate.

3. An electrolyte bath according to Claim 1, wherein said phosphate comprises 0.03 molar orthophosphate and 0.03 molar pyrophosphate.

4. An electrolyte bath according to Claim 1 , wherein the electrolyte bath has a pH greater than 8.

5. An electrolyte bath, consisting essentially of:

A. phosphate in a concentration greater than 0.01 molar and less than 0.1 molar; and

B. water, wherein the electrolyte bath has a pH greater than or equal to 7.

6. An electrolyte bath according to Claim 5, wherein said phosphate comprises approximately equal amounts of orthophosphate and pyrophosphate.

7. An electrolyte bath according to Claim 5, wherein the phosphate comprises 0.03 molar orthophosphate and 0.03 molar pyrophosphate.

8. An electrolyte bath according to Claim 5, wherein the electrolyte bath has a pH greater than 8.

9. A method for forming a protective coating on the surface of a metal substrate by spark discharge performed in an electrolyte bath, comprising the steps of:

A. immersing the metal substrate in an electrolyte bath comprising: i. phosphate in a concentration greater than 0.01 molar and less than 0.1 molar; ii. depositable anions in a concentration of from 0 to 0.04 molar; iii. non-depositable anions in a concentration from 0 to 0.1 molar; and iv. water, wherein the electrolyte bath has a pH greater than or equal to 7;

B. immersing a cathode in the electrolyte bath; and

C. imposing a sufficient electric voltage potential between the cathode and the metal substrate to cause the metal substrate to become an anode and to cause an electric current to flow between the cathode and the metal substrate anode; and

D. maintaining a sufficient voltage potential between the cathode and anode until a desired coating thickness is formed on the metal substrate anode.

10. A method according to Claim 9, wherein the electrolyte bath comprises approximately equal amounts of orthophosphate and pyrophosphate.

11. A method according to Claim 9, wherein the electrolyte bath comprises 0.03 molar orthophosphate and 0.03 molar pyrophosphate.

12. A method according to Claim 9, wherein the metal substrate comprises aluminum.

13. A method according to Claim 9, wherein the electrolyte bath has a pH greater than

8.

14. A method for forming a protective coating on the surface of a metal substrate by spark discharge performed in an electrolyte bath, comprising the steps of:

A. immersing the metal substrate in an electrolyte bath consisting essentially of: i. phosphate in a concentration greater than 0.01 molar and less than 0.1 molar; and ii. water, wherein the electrolyte bath has a pH greater than or equal to 7;

B. immersing a cathode in the electrolyte bath; C. imposing a sufficient electric voltage potential between the cathode and the metal substrate to cause the metal substrate to become an anode and to cause an electric current to flow between the cathode and the metal substrate anode; and D. maintaining a sufficient voltage potential between the cathode and anode until a desired coating thickness is formed on the metal substrate anode.

15. A method according to Claim 14, wherein the electrolyte bath comprises approximately equal amounts of orthophosphate and pyrophosphate.

16. A method according to Claim 14, wherein the electrolyte bath comprises 0.03 molar orthophosphate and 0.03 molar pyrophosphate.

17. A method according to Claim 14, wherein the metal substrate comprises aluminum.

18. A method according to Claim 14, wherein the electrolyte bath has a pH greater than 8.