Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating
Reexamination Certificate
1999-12-30
2001-07-03
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating
C204S225000, C204S226000, C204S272000, C204S275100
Reexamination Certificate
active
06254759
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the art of electrolytic formation of coating on metallic parts. More specificically, it relates to electrolytic formation of a coating on a metallic substrate by cathodic deposition of dissolved ions in the reaction medium (electrolyte) onto the metallic substrate (cathode), or anodic conversion of the metallic substrate (anod) into an adherent ceramic coating (oxide film).
BACKGROUND OF THE INVENTION
It is well known that many metallic components or parts need a final surface treatment. such a surface treatment increase functionality and the lifetime of the part by improving one or more properties of the part, such as heat resistance, corrosion protection, wear resistance, hardness, electrical conductivity, lubricity or by simply increasing the cosmetic value.
One example of a part that is typically surface treated is the head of aluminum pistons used in combustion engines. (As used herein an aluminum component is a component at least partially comprised of aluminum, including aluminum alloys.) Such piston heads are in contact with the combustion zone, and thus exposed to relatively hot gases. The aluminum is subjected to high internal stresses, which may result in deformation or changes in the metallurgical structure, and may negatively influence the functionality and lifetime of the parts. It is well known that formation of an anodic oxide coating (anodizing) reduces the risk of aluminum pistons performing unsatisfactorily. Thus, many aluminum piston heads are anodized.
There is a drawback to anodizing piston heads. Conventional anodizing with direct current or voltage, increases the surface roughness of the initial aluminum surface by applying an anodic coating. The increase in surface roughness can be as high as 400%, depending on the aluminum alloy and process conditions. The amount of VOC (Volatile Organic Compounds) in the exhaust of a combustion engine is correlated with the surface finish of the anodized aluminum piston: higher surface roughness reduces the efficiency of the combustion, because a greater proportion of organic compounds can be trapped in micro cavities more easily. Therefore, a smooth surface is required, which may not always be provided by anodization.
A typical prior art power supply for the conversion of metallic aluminum into a ceramic coating (aluminum oxide or alumna) provides direct current, normally between 3 and 4 A/dm2. Typically, a film thickness of 20 to 25 microns is reached after 30 to 40 minutes.
Convention anodizing includes subjecting the aluminum to an acid electrolyte, typically composed of sulfuric acid or electrolyte mixed with sulfuric and oxalic acid. The anodizing process is generally performed in electrolytes containing 12 to 15% v/v sulfuric acid at relatively low process temperature, such as from −5 to +5 degrees C. Higher concentrations and temperature usually decrease the formation rate significantly. Also, the formation voltage decreases with higher temperature, which adversely affects the compactness and the technical properties of the oxide film.
Performing anodizing process at (relatively) low temperature and fairly high current density increases the compactness and technical quality of the coating performance (high hardness and wear resistance). The anodization produces a significant amount of heat. Some heat is the result of the exothermic nature of the anodizing of aluminum. However, the majority of the heat is generated by the resistance of the aluminum towards anodizing. Typically, the reaction polarization is high, such as from 15-30 volts, depending upon the composition of the alloying elements and the process conditions. Given typical current densities, from 80% to 95% of the total heat production will be resistive heat.
The electrolyte is acidic, and thus chemically dissolves the aluminum oxide. Thus, the net formation of the coating (aluminum oxide) depends on the balance between electrolytic conversion of aluminum into aluminum oxide and chemical dissolution of the formed aluminum oxide.
The rate of chemical dissolution increases with heat. Thus, the total production of heat is a significant factor influencing this balance and helps determines the final quality of the anodic coating. Heat should be dispersed form areas of production toward the bulk solution at a rate that prevents excess heating of the electrolytic near the aluminum part. If the balance between formation and dissolution is not properly struck, and dissolution is favored, the oxide layer may develop holes, exposing the alloy to the electrolyte. This often happens in prior art anodization methods and is known as a “burning phenomena”.
Heat produced at the aluminum surface is dispersed by air agitation or mechanically stirring of the electrolyte in which the oxidation of aluminum is taking place, in the prior art, to help reach the desired balance.
Another way of dispersing the heat is by spraying the electrolyte toward the aluminum surface (U.S. Pat. No. 5,534,126 and U.S. Pat. No. 5,032,244). The electrolyte is sprayed toward the aluminum surface at an angle of 90 degrees, moving heat toward the areas of production, and then symmetrically dispersed away from the aluminum surface.
Another way to disperse heat is to pump the electrolyte over the aluminum substrate (U.S. Pat. No. 5,173,161). The electrolyte is moved parallel to the aluminum surface, moving heat from the lower part of the aluminum substrate over the entire surface before it is finally dispersed away from the aluminum surface.
A steady state transport mechanism in electrochemical analysis (not anodization) techniques based on wall jet processes can be achieved by either rotating the working electrode, or by directing the flow toward a stationary electrode, at an angle of between 60 and 70 degrees. By angling the jet stream of the reaction medium to 60-70 degrees where steady state conditions are obligatory, electrochemical analysis can be made. Steady state conditions in a jet stream orthogonal to the working electrode is less suitable for wall jet electrochemical analysis. The inventor is not aware of this information having been applied to an electrolytic process.
The driving force of the charge-transfer reaction taking place at the substrate surface in the process described in U.S. Pat. Nos. 5,032,244, 5,534,126 and 5,173,161, was direct current. The reaction medium was a solution of sulfuric acid or a combination of sulfuric and oxalic acid in U.S. Pat. No. 5,032,244. The electrolyte formulation was 180 g/l sulfuric acid and the process temperature was +5 degrees C. A current density of 50 A/dm2 produced a coating with a thickness of 65 microns in 3 minutes. The microhardness of the obtained coating was between 200 and 300 HV.
A second process included the addition of 10 g/l oxalic acid at the same current density. A coating having a thickness of more than 60 microns and having a microhardness greater than 400 HV was obtained in 5 minutes.
After anodizing, the aluminum parts are typically rinsed and dried. Both anodizing, rinsing and drying is made in the same process chamber in all three US patents mentioned above. Some chambers have at least two aluminum parts (see U.S. Pat. Nos. 5,534,126 or 5,173,161). Others have a single part in each chamber (see U.S. Pat. No. 5,032,244).
Conventional batch anodizing has used square wave alternation of current or potential. This allows anodizing to be performed at higher current densities compared to anodizing with direct current. The pulse anodizing is characterized by a periodically alternation between a period with high current or voltage, during with the film is formed, and a period with low current or voltage, during which heat is dispersed (U.S. Pat. No. 3,857,766). This technique utilizes the “recovery effect”, after a period of high formation rate (a pulse period), heat is allowed to disperse during the following period with low formation rate (a pause period) and defects in the coating are repaired before the current increases during the next puls
Corrigan George R.
Phasge Arun S,.
Pioneer Metal Finishing
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