Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1998-06-25
2001-06-19
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S275000, C148S276000
Reexamination Certificate
active
06248183
ABSTRACT:
BACKGROUND
Chromate conversion coatings are widely used in the manufacture, repair, and refurbishment of a large number of engineering components for military and civilian applications. These surface coatings resist corrosion and promote paint adhesion. Such protective coatings are applied to a variety of substrate metals and alloys, most notably aluminum. Conversion coatings are produced by either immersion (dipping), spraying, swabbing, or brushing techniques that contain chromates and dichromates in the processing solution. Occasionally, electrolytic methods are used to obtain conversion coatings, but such methods require a great deal of maintenance and are impractical for recoating parts in the field.
Chemical conversion coatings are formed by a chemical reaction causing the surface of the metal to be converted into a tight adherent coating, all or part of which consists of an oxidized form of the substrate metal. The coating can provide high corrosion resistance as well as strong anchoring for paint. The industrial application of paint (organic finishes) to metals generally requires the use of a chemical conversion coating as a base coating, particularly when the performance demands are high.
Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys, particularly the high-copper aluminum alloys, corrode much more rapidly than pure aluminum. Thus, there is a need to treat aluminum with some form of beneficial conversion coating.
Generally two types of conversion coating processes are used in treating aluminum. The first is by anodic oxidation (anodization) in which the aluminum component is immersed in a chemical bath, such as a chromic or sulfuric acid bath, and an electric current is passed though the aluminum component and the chemical bath. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes.
The second type of process chemically produces a conversion coating by subjecting the aluminum surface to a chemical solution, but without the use of electric current. This process is commonly referred to as a chemical conversion coating. The chemical solution can be applied by using immersion or spray application and is followed by drying. When dried, the coating which is initially gelatinous (gel) hardens, and becomes hydrophobic (less soluble in water) and more resistant to abrasion. The resulting conversion coating on the surface of the aluminum component offers resistance to corrosion and a bonding surface for organic finishes, such as a paint top coat.
Commonly, a chromate conversion coating solution containing chromium ions in the hexavalent [Cr(VI)] and trivalent [Cr(III)] state is used to produce a chemical conversion coating. The Cr(VI) is partially reduced to Cr(III) during the reaction, with a concurrent rise in pH. The chemical composition of the surface coating is indefinite as it contains varying amounts of reactants, reaction products, water of hydration and other anions, such as fluorides, and phosphates. When hexavalent Cr (VI) ions are incorporated into a coating, the ions leach out when in contact with a moisture and thereby provide corrosion resistance and also impart paint adhesion properties to the coating. However, solutions containing chromium ions in the hexavalent state have been determined to be carcinogenic. The U.S. Environmental Protection Agency (EPA) has included chromium on the list of toxic chemicals for “voluntary” replacement, and has promulgated strict waste disposal standards to curtail its use.
Strict waste disposal standards and chromium's listing as a toxic chemical have created a need for alternative chemical conversion coating compounds that do not contain the Cr(VI) ion. For such a compound to be accepted as an alternative it must meet or exceed the protective properties displayed by the chromium compounds. An alternative must also be capable of being used as a substitute with very little modifications to the present process so that it is readily accepted. An alternative providing protection comparable to that of chromium without being toxic is needed.
SUMMARY OF THE INVENTION
The conversion coatings of the present invention are able to replace the currently used chromate conversion coatings without significantly altering the current methods of coating. Chromate conversion coatings are valued because they provide corrosion resistance and improved paint adhesion. During chromate coating the chromium ions, Cr(III) and Cr(VI) form precipitates that also replace the metallic cations of the substrate gel. The Cr(III) ion is thought to allow for the hardening of the gel. The Cr(VI) ion leaches out when in contact with moisture, and thereby improves the corrosion resistance and promotes self-healing when defects are introduced into the coatings. The present invention is able to provide acceptable substitute metal ions to replace the Cr(III) and Cr(VI) ions without the need for added steps or equipment.
The present invention uses a series of steps that do not substantially alter the steps currently used to produce a conversion coating on aluminum and aluminum alloy substrates. These steps include: cleaning, desmutting, steaming, conditioning, conversion coating, and sealing. Other intermediate steps may be used such as rinsing in deionized water and drying. Basically, a gel is formed on the clean surface of the substrate and then the metal ions of the gel are substituted with another metal ion having a similar atomic radius and coordination number. Once the conversion coating is formed on the substrate, subsequent inorganic or organic coatings can be applied to the coating.
Cleaning is the first step in the preparation of the aluminum surface before conversion coating. During this step organic contaminants on the surface are removed using high pH (alkaline) soluble salts. The alkaline cleaner usually contains inhibitors because aluminum is easily corroded by alkaline solutions. After cleaning, the aluminum substrate is dipped or sprayed with a deoxidizer. The deoxidizer is commonly made up of either a mixture of nitric and hydrofluoric acids, or compounds such as ferric salts, persulfates and peroxides. The deoxidizer removes any remaining surface oxides on the aluminum. The deoxidized substrate is then immersed in boiling or near boiling deionized water to form a hydrated oxyhydroxide gel on the surface of the substrate (referred to in the industry as “steaming” the substrate).
The gel coated aluminum is placed in a conversion coating bath. Once in the bath the aluminum ions of the gel are exchanged for the metal ions contained in the conversion bath solution. Manganese cations are the preferred metal ions used in the conversion process due to the ions having a similar ionic radius to that of the aluminum ion. The conversion coating bath also reacts with the aluminum substrate. The Mn (VI) ions are partially reduced during the reaction with a concurrent rise in pH. This reaction forms a uniform coating consisting of a hydrated precipitate. The aluminum substrate may be removed from the conversion coating bath once the conversion process is complete. The substrate can then be dried, sealed in a sealant bath, or prepared for final painting of a top coat. A non-rinse coating may also be formed on the substrate. A non-rinse coating skips the final deionized water rinse after the conversion process is complete.
DETAILED DESCRIPTION
The chemical conversion coating solution of the present invention is preferably composed of potassium manganate, potassium hydroxide, potassium phosphate dibasic, potassium fluoride, sodium hydrosulfide and orthophosphoric acid. The coating solution may also be mixed with wetting agents such as sulfonates that enable uniform and continuous coating. In addition, the coating solution may also contain certain additives such as acetates and nitrates that activate the surface and cont
Concurrent Technologies Corporation
Oltmans Andrew L.
Sheehan John
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