BACKGROUND OF THE INVENTION
The present invention concerns a process for cleaning metals. In particular, the present invention concerns a rapid and inexpensive method for cleaning metal parts including high-precision machine parts to remove corrosion without substantial change in the dimension of the part. The present invention also concerns a process for removing corrosion from metal whereby the resistance of the metal to further corrosion is improved.
Corrosion, a constant destroyer of many metals, is a serious problem to industry where it is responsible for the continuous deterioration and loss of expensive metal parts. The rusting or other corrosion of machined parts, particularly high-precision machined parts, the replacement of which is very costly, is of special concern to modern industries. High-precision parts are expensive to produce; when they become corroded, it is usually necessary to replace them by completely new parts due to their narrow tolerance requirements. The corroded precision part which cannot be reused is then discarded at a complete loss. Thus, rust and other forms of corrosion result in significant economic waste, not only with respect to monetary losses but also with respect to our metal resources.
Many cleaning and anticorrosion processes are known in the art. However, most procedures available for cleaning metals and rendering them resistant to corrosion cause a substantial change in dimensions to parts which have undergone treatment; where high-precision parts are required such methods are worthless.
The high cost of many cleaning processes makes them uneconomical for most applications. In addition, known cleaning and anticorrosion processes require a relatively long period of time to complete; under these conditions, spare parts must be kept on hand otherwise there is a risk of machinery being idle and nonproductive for a substantial period of time. Another significant disadvantage of almost all known cleaning and particularly derusting processes is that the products which are obtained do not have the qualities or appearance of newly machined parts; parts subjected to known cleaning procedures tend to be dull without shine or luster.
SUMMARY OF THE INVENTION
An object of the present invention is the provision of a highly economical and highly effective process for cleaning metal.
Another object of the present invention is the provision of an economical process for cleaning and particularly for derusting corroded machined parts of narrow tolerance whereby the initial dimensions of the machined parts are substantially unchanged and whereby the treated products have the appearance of newly machined parts.
Another object of the present invention is the provision of a fast, economical process for cleaning brass, copper, bronze, steel and cast iron.
A further object of the present invention is the provision of a process for cleaning corroded metal parts wherein the part is then resistant to further corrosion.
It has been discovered that corroded metal and metal parts are rendered free of corrosion rapidly by treating the metal with an aqueous solution of hydrochloric acid, rinsing the part with water and then treating the metal part with a dilute aqueous solution of an alkali metal cyanide. The metal part treated in this manner is corrosion free and corrosion resistant; the metal surface is relatively porous and has good properties of adherence for finishes such as lacquers and paints.
It has also been found that the luster and appearance of the original, newly milled part is restored to the used part by further treatment which comprises buffing the part with a mixture of lubricant and fine abrasive, cleaning the part in an aqueous alkaline electrolytic bath wherein the metal part acts as the cathode and treating the part again with a aqueous solution of an alkali metal cyanide. The part is then rinsed, dried and coated with a lubricant.
Corroded parts which have been treated in accordance with the present invention are bright and lustrous and have the appearance of a newly milled product.
The entire process may be carried out rapidly requiring about 10 to 25 minutes depending on the size of the part to be treated. The time of treatment per part may be only a fraction of the total time since a plurality of parts can be treated simultaneously.
Measurements indicate that there is almost no change in dimensions of precision parts which have been treated in accordance with the present invention.
DESCRIPTION OF THE INVENTION
According to the present process, and particularly when treating used metal parts, it is desirable to degrease the part with a suitable organic solvent prior to treatment with acid. Any solvent which dissolves grease readily may be used, such as hydrocarbons, for example common gasoline and petroleum ether, ethers and ketones including acetone, paint or lacquer thinner, and chlorinated hydrocarbons such as trichloroethylene. Generally the part is simply dipped into the solvent to remove the grease.
The rust or other form of corrosion is removed principally by treatment with dilute hydrochloric acid. The part is immersed in an aqueous solution containing about 10 to 20 percent by weight of hydrochloric acid, more preferably a 15-18 percent solution is used. To avoid the formation of hydrocyanic acid in the next step of the process, the part is rinsed with water to remove residual hydrochloric acid. The part is immersed in hydrochloric acid for a time sufficient to remove the rust. If only surface rust is present, immersion for about a second may be sufficient; on the other hand, if the part is pitted and the rust is deep, immersion may be continued for 5 minutes or more.
The metal part is then subjected to treatment with aqueous solution of an alkali metal cyanide. Sodium and potassium cyanide are generally preferred for economic reasons. The cyanide solution is prepared by mixing about 2.5 to 3.5 oz. of alkali metal cyanide per gallon of water, most preferably 3 oz. per gallon of water are employed. The part is dipped into this solution for a short time, i.e. about 30 seconds to 2 minutes. It is believed that the alkali metal cyanide removes impurities from the surface of the metal parts and such impurities are often the locus for corrosion. In any event, metals which have been subjected to this treatment are more resistant to corrosion than untreated metal.
After this procedure, the part may be put into service again. Metal which has been subjected to the foregoing treatment has a porous surface and good adhesion for coatings such as paints and lacquers; thus, parts treated in accordance with this invention are particularly suitable for the application of such permanent coatings.
The steps of the foregoing treatment are usually carried out in rapid sequence at room temperature and require only a short time to complete. Generally this treatment is complete within about 2 to 5 minutes depending upon the equipment used and the size of the parts; one part, or a plurality of parts may be processed simultaneously.
According to the most preferred method of the present invention, the part is then buffed with wax or grease which has been impregnated with a fine abrasive, using a cloth-covered buffing wheel. Preferably, the abrasive is a fine emery of about 0.00 grit. For small parts having a regular configuration, i.e. parts of having a diameter about 2.5 to 5 inches, the buffing operation is complete in a few seconds; 10 to 20 seconds is usually sufficient to return the parts to the bright luster of the original machined part without change in the dimension of a precision part of narrow tolerance.
It is desirable to rinse the parts with water prior to the buffing operation, in order to remove residual alkali metal cyanide. Most preferably, the part is also rinsed with hot water so that it dries rapidly and may then be buffed immediately.
To move the wax and grit residue after the buffing operation, the part is cleaned by means of an aqueous alkaline electrolytic bath. Although not essential, the electrolytic bath is preferably maintained at an elevated temperature, most preferably at about 200°-225° F. While most alkaline substances which do not damage the metal may be used in the bath sodium and potassium carbonate are preferred. The exact concentration of alkaline material in the electrolytic bath is not critical; generally about 1 to 3 oz. of sodium or potassium carbonate are used; however, more concentrated solutions are sometimes desirable when treating large parts. A steel anode is provided; the metal part which is to be treated acts as the cathode. A standard plating generator operating at about 2 to 10 volts provides an electric current of 1.5 to 3.5 amps. The metal part is permitted to remain in the electrolytic solution until the part becomes hot or until some electrolysis has taken place as evidenced by the formation of bubbles at the surface of the part. The part is then rinsed with water.
A second treatment with an aqueous solution of an alkali metal cyanide is then employed. In this treatment, the part is again immersed in a solution of an alkali metal cyanide, preferably more concentrated that the first cyanide treating mixture. This cyanide solution generally contains about 5-14 oz. of sodium or potassium cyanide per gallon of water. The period of immersion in the second cyanide solution is about 1 to 2 minutes.
After a final rinse, the part is dried and coated with a lubricant of oil or grease. Preferably, hot rinse water is used in order to provide for the fast drying of the part, and the drying operation is usually completed with hot air.
It is difficult to distinguish a used part which has been treated in accordance with this process from a newly milled product. It has a bright lustrous appearance and measurements indicate that there is substantially no change in the dimensions of the product as a result of the treatment.
Corrosion resulting from air oxidation such as rust as well as corrosion due to other cause, for example contact with chemicals is removed from metal by means of the present process.
The present procedure is particularly economical; no special equipment or costly reagents are employed. Ordinary tanks may be used for the various baths and due to its simplicity, the present procedure is readily carried out at the place where the metal is stored and/or used. The availability of this process at the point of storage and/or use and the mobility of the operation of this process result in an enormous saving. Costs of handling and the transporting material to and from the point of treatment are eliminated.
Further, the present process may be carried out in a single continuous operation.
By means of this treatment otherwise unusable material is salvaged. Metal products on scrap piles can be returned, after treatment to the usable stock piles of an installation, and this can be done at a fraction of replacement costs and replacement time.
In addition, this process can be applied to new material, to prevent corrosion from starting. This can be done to material in stock, and to material as it leaves the production line at the point of manufacture.
The following example further illustrates the best mode currently contemplated for carrying out the present invention, however this example must not be considered as limiting this invention in any manner whatsoever. Unless otherwise indicated, all of the process steps are carried out at room temperature.
Twenty-five roller bearings each having a diameter of 2.5 inches are placed in a wire basket. The roller bearings pitted and so rusted that they no longer revolve are composed of 6 bearings made of hardened high carbon steel held together by washers or rings of cold rolled steel. The basket containing the roller bearings is immersed in trichloroethylene to remove the grease. The bearings are then withdrawn from the organic solvent, which evaporates from the bearings rapidly and still in the basket the bearings are dipped into a 17 percent aqueous hydrochloric acid solution contained in a 5 gallon ceramic tank. The bearings are maintained in the hydrochloric acid solution for about 2 minutes and are then rinsed with tap water. Next, the basket containing the bearings is placed in an aqueous solution containing 3 oz. of sodium cyanide per gallon of water and is permitted to remain in the solution for about 1 minute.
After the cyanide treatment, the bearings are rinsed in cold and then hot water so that they dry rapidly.
The bearings are then removed from the basket and placed, about five or six at a time on a wooden bar, where they are subjected to buffing with a buffing wheel covered with a cotton cloth and coated with LEE compound, which is a mixture of beeswax and 25 percent by weight of 0.00 emery grit. The wheel which rotates at about 2,800 to 3,000 r.p.m. is covered with a cotton cloth. After about 15 seconds of such polishing, the bearings are bright and lustrous.
After all of the roller bearings have been buffed, they are placed in a hot electrolytic bath maintained at a temperature of 200°-225° F. The electrolytic bath is prepared by dissolving 2 oz. of "PER MAG No. 50", a commercial form of potassium carbonate, per gallon of water. A steel anode is provided in the electrolytic bath; the roller bearings act as the cathode. An electric current of 3 amps., provided by a standard plating generator operating at about 5 volts is passed through the bath. The electrolysis is continued until the bearings become hot, as evidenced by the appearance of bubbles at the surface thereof. This usually requires about 2 or 3 minutes.
The roller bearings are again placed in a basket and rinsed with tap water and are then dipped into a sodium cyanide solution containing 6 oz. of sodium cyanide per gallon of water. After the bearings have remained in this solution for about 1 to 2 minutes, they are removed, rinsed with hot water and dried with hot air. The roller bearings are then coated with an oil or grease and are ready for use.
The entire process requires about 15-20 minutes to complete. The bearings have a shiny, lustrous finish and look like a newly milled product. Measurements of the bearings with a micrometer indicate that there is a change in the outer dimension of less than 0.0001 percent in each bearing compared to the bearing before treatment.
While this process has been illustrated with particular reference to steel roller bearings, the process is effective for cleaning and removing corrosion from a part made of brass, copper, bronze and cast iron. This process is, of course, applicable to parts having an outer metal surface and an interior core of some other material.