Claims
- 1. A process for improving the corrosion-resistance of a chromium plated article comprising
- polishing a chromium plated article,
- drying the article to provide a moisture free chromium surface,
- friction finishing said chromium surface by mechanically applying a friction finishing compound under conditions which cause the temperature of said chromium surface to be increased to about 500.degree. F.,
- said friction finishing compound comprising abrasive particles having an average particle size of less than 15 microns dispersed in a water free organic vehicle.
- 2. A process in accordance with claim 1 wherein the temperature of the chromium surface is increased to above 700.degree. F.
- 3. A process in accordance with claim 2 wherein the friction finishing compound is applied by a buffing wheel and wherein the relative surface velocity between the wheel and the chromium surface is between about 6,500 and about 10,000 SFM.
- 4. A process for improving the corrosion-resistance of chromium plated and polished steel bar stock comprising
- providing a chromium plated and polished bar stock the chromium surface of which is free of moisture,
- rotating said bar stock in a fixture,
- applying to the chromium surface of the rotating bar stock a friction finishing compound by means of a rotating buffing wheel under conditions which cause the temperature of the chromium surface to be increased to above 500.degree. F.,
- said friction finishing compound comprising abrasive particles having an average particle size of less than 15 microns dispersed in a water free organic vehicle.
- 5. A process in accordance with claim 4 wherein the temperature of the chromium surface is increased to above 700.degree. F.
- 6. A process in accordance with claim 5 wherein the relative surface velocity between the wheel and the chromium surface is between about 6,500 and 10,000 SFM.
Parent Case Info
This application is a Continuation-In-Part of Ser. No. 749,429, filed June 27, 1985, abandoned.
This invention relates to the finishing of chromium plated products, and more specifically to a method of friction finishing the chromium surface of a steel plated product to improve corrosion resistance.
Hard chromium plating is produced by electrodeposition from a solution containing chromic acid and catalytic anions in proper proportion. The metal deposit thus produced is extremely hard and the chromium coating per se provides some corrosion resistance. The chromium deposit that results from the electroplating process contains cracks, nodules, and other surface defects which must be removed before the product can be used in many applications, for example, piston rods. These microscopic defects detract from the surface appearance, and can serve as corrosion sites when the product is exposed to a corrosive environment. Polishing of the chromium surface with aluminum oxide is conventionally practiced to minimize the detrimental effects of these defects. The aluminum oxide polishing operation levels the nodules and tends to fill in the cracks in the chromium deposit. The polishing operation results in a significant improvement in the surface appearance, and the corrosion resistance of the product is also improved. The quality and corrosion resistance of the polished hard chromium deposit depends upon the size of the aluminum oxide particles used, the type of wheels or belts used, polishing speeds, and a variety of other processing parameters. The chromium surface produced using aluminum oxide polishing is sufficient for many industrial applications.
Some applications require that the hard chromium plated product have a greater level of corrosion resistance than that which is provided by aluminum oxide polishing. For example, to improve corrosion resistance, some specifications state that a layer of nickel plate must be deposited underneath the chromium plating. The nickel layer provides a barrier protecting the base metal from the environment in areas where defects in the hard chromium deposit occur. The present invention improves the corrosion resistance of the plated chromium surface to a degree that a nickel plate undercoat is not required to prevent corrosion. This reduces the cost of the product and makes the hard chromium plated product acceptable for a wider range of applications.
It has been discovered that a secondary friction finishing treatment using particular buffing compounds results in significant improvements in the corrosion resistance of an electroplated chromium surface and also results in improvements in the overall finish of the surface. The buffing compound comprises an organic material containing long chain molecules and fine abrasive particles as additives. It is the combined action of the fine particles and the organic material that result in a chromium surface possessing improved corrosion resistance. The improved corrosion resistance is believed to be the result of the sealing of the microscopic defects in the chromium surface.
It has also been discovered that the protection provided by this friction finishing treatment is not affected by subsequent cleaning of the treated surface with solvents or detergents. Hence the friction finished steel part can be thoroughly cleaned prior to assembly into a larger machine, and the corrosion protection will not be diminished.
It is well-known that chromium oxidizes rapidly in air to produce an adherent oxide surface coating. The formation of this tough, adherent oxide film prevents further oxidation of the chromium. The chromium oxide film is transparent and the chromium surface has a lustrous metallic appearance even though it is oxidized. While the presence of the oxide film is generally beneficial, it prevents the application of additional protective coatings. Hence a hard chromium plated surface which contains microscopic defects is difficult to protect from corrosion because the oxide layer prevents organic materials from adhering to the surface.
The chromium oxide film forms instantaneously when the plated product is removed from the plating bath and exposed to oxygen in the air. The oxide film is removed temporarily during polishing, but it reforms again very rapidly. Hence a protective coating cannot be effectively applied after polishing due to the presence of an oxide film which interferes with surface adhesion.
It has been discovered that removing the oxide film at elevated temperatures with fine abrasive particles in the presence of organic compounds makes it possible to effectively bind the organic compounds to the chromium surface. When the organic molecules come into contact with the oxide free chromium surface they are tightly held in place by intermolecular forces at the surface. Thereafter, a fresh chromium oxide film forms around the organic molecules further adhering these molecules to the surface. The organic molecules effectively seal the cracks and other microscopic defects in the plated chromium surface thereby preventing corrosion. Also, the organic molecules are so tightly adhered to the surface that they cannot be removed by solvents. Hence, the parts can be solvent cleaned without diminishing the corrosion protection.
An additional advantage of the friction finishing process is a significant improvement in surface finish. When fine abrasive particles are used in the friction finishing compound the scratch pattern on the surface can be refined and the surface topography reduced. This improves the product's appearance and reduces the surface coefficient of friction. These surface changes are generally considered beneficial to the product.
The friction finishing process used in the present invention consists of several essential steps. The chrome plated article must first be dried because the presence of water during the friction finishing diminishes the quality of the finish. The preferred method of drying of work is with an air spray, but any drying method which removes the water would be acceptable.
The friction finishing compound is then applied to the surface of the part under conditions which cause the temperature of the surface of the part to be elevated. The surface temperature of the part should be raised to at least 500.degree. F. (260.degree. C.) during application of the finishing compound, preferably to at least 750.degree. F. (400.degree. C.). The upper temperature limit is that temperature at which the friction finishing compound applicator ignites.
Elevation of the surface temperature of the part is created by friction between the surface of the part and the finishing compound applicator. In most instances, and particularly when the part is round, e.g., a round bar, the bar is rotated in a suitable fixture and the applicator is a buffing wheel. The buffing wheel preferably includes a hub which acts as a heat sink to prevent the wheel from igniting. Since the fixture holding the bar conventionally operates at a fixed speed, for example 295 surface feet per minute, the rpm of the bar being treated will vary depending upon its diameter; a one inch diameter bar will be rotated at approximately 1127 rpm and a six inch diameter bar will be rotated at approximately 188 rpm.
A preferred form of applicator is a sisal buffing wheel. To create the requisite friction to elevate the surface temperature of the bar, the buffing wheel is operated at a high surface velocity relative to the bar stock being treated. Since a sisal buffing wheel wears away during use, the surface velocity of the wheel applied to the rotating bar stock will vary. With the bar stock and wheel rotating in the same direction the relative surface velocity between the two, i.e., the difference between the surface velocity of the wheel and the surface velocity of the bar stock, is betweeen about 6500 and 10,000 surface feet per minute (SFM). The buffing wheel is urged into contact with the bar stock under pressure which further increases the friction and the temperature. This may be accomplished using a conventional air actuated biasing linkage. If desired, as the sisal applicator wheel wears and its diameter is reduced, the pressure may be increased and/or the wheel speed may be increased to maintain the desired surface temperature.
The surface temperature of the part after friction finishing should be between about 125.degree. and about 150.degree. F. (50.degree.-65.degree. C.), preferably about 140.degree. F. (60.degree. C.). If this temperature is measured, for example with an optical pyrometer, after disengagement between the part and applicator, this temperature measurement can be used to control the process.
The friction finishing compound is selected in order to provide a desired finish on the part. The finishing compound comprises abrasive particles dispersed in a suitable organic vehicle which may be composed of fatty acids and/or fatty acid esters having 8 to 18 carbon atoms, animal fat, vegetable fat, lanolin and mixtures thereof. A surfactant may be added if desired, as well as other ingredients typically found in finishing and polishing compounds, for example additives for pH control and gelling agents. In order to achieve the improved corrosion resistance of the invention, it is necessary that the finishing compound be essentially free of water.
The abrasive particles in the finishing compound may be any of those typically used, for example oxides, carbides or nitrides of iron, chromium, aluminum, titanium, silicon, magnesium or calcium. In order to provide the desired result, the average particle size of the abrasive should be less than 15 microns, and preferably 90 percent of the abrasive particles should have a particle size of less than 15 microns. Typically the abrasive particles comprise between about 50 and about 90 weight percent of the finishing compound.
The use of various polishing compounds to finish surfaces has been suggested in the prior art. For example, U.S. Pat. No. 1,986,388--Calcott et al. describes a metal polish composed of fatty acids, an organic solvent which is immiscible with water, and abrasive and a wetting agent. This invention differs from the present invention in several aspects including the use of water with the polish and the method of application. The process described in the Calcott et al. patent does not teach working the compound into the surface with pressure to seal the surface and thereby provide corrosion resistance. Instead the polish described leaves behind a waxy film to provide protection. This waxy film can be removed by solvents while the protection provided by the present invention cannot be removed by solvents.
U.S. Pat. No. 3,419,902--Gerber et al. describes another polishing compound made from fatty acids with a mild abrasive. However, this compound is to be applied using a polishing cloth, and the essential aspects of working the compound into the surface with pressure were not included in the techniques described in the Gerber et al. patent.
U.S. Pat. No. 3,619,962 describes a polishing compound which contains fatty acids and abrasive compounds. However, this patent describes a liquid compound which consists mostly of water. The present invention teaches that the presence of water during finishing detracts from the quality of the finished surface.
The prior art contains several references to polishing compounds containing fatty acids and mild abrasives. However, the novel aspect of the present invention lies not in the composition of the polishing compound, but in the method of applying it to the surface of a steel workpiece which has been chromium plated. Sufficient pressure must be applied to remove the hard chromium oxide and work the friction finishing compound into the microscopic defects in the chrome plate. Simply applying the polish to the surface by hand will not provide a surface with sufficient corrosion resistance.
US Referenced Citations (9)
Continuation in Parts (1)
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Number |
Date |
Country |
| Parent |
749429 |
Jun 1985 |
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Patent Grant
4755191
