US2971826A - Chemical polishing method - Google Patents

Description

Feb. 14, 1961 c. c. 'COHN ETAL 2,971,826 CHEMICAL POLISHING METHOD Filed Nov. 3, 1954 INVENTOR. CHARLES C. GOHN 8 SAMUEL L. GOHN fiwii m,

ATTORNEYS United States Patent OfiFice 2,971,826 Patented Feb. 14, rest CHEMICAL POLISHING METHOD Charles C. Cohn, Atlantic City, NJ., and Samuel L. Cohn, Philadelphia, Pa., assignors to Samuel L. Colin and Charles C. COllH, copartners, trading as Colonial Alloys Company, Philadelphia, Pa.

Filed Nov. 3, 1954, Ser. No. 466,556 2 Claims. (Cl. 41-42) This invention relates to article treating method and apparatus and has particular reference to the interruption of chemical actions on metals.

In the art of treating metals and metal alloys, as in etching, brightening, plating, electrochemical polishing, and the like, there are used treating baths whose compositions continue to react with the metal surfaces being treated after withdrawal of the surfaces from the treating bath. These continued reactions are often objectionable in that they impair the desired surface by etching it to too great an extent or otherwise changing the characteristics of the subsequent coating or of the basis metal. The evolution of noxious or dangeroues fumes or sprays also makes these continued reactions undesirable.

For example, there may be considered the treatment of surfaces such as those of aluminum or aluminum alloys which frequently involve the necessity for treatment during only a limited time. The attainment of the desired action in some cases may be critical as to time, and then it is desirable at the expiration of such time to remove the article from the treating bath as rapidly as possible and subject it to rinsing usually by immersion in a rinsing bath or by washing the reacting material from the article by a stream of rinsing liquid. When manual transfer is used, the time involved between removal from the active bath and insertion into rinsing liquid may be small and relatively insignificant if the operator works rapidly. However, where an automatic operation is involved with the articles being treated moved through the treating bath on a conveyor, the conveyor speed is generally set by the time of treatment required and the transfer from the treating bath to a rinsing bath is necessarily slow unless mechanically complex devices are resorted to which, for example, transfer the articles from a slow moving conveyor to a rapidly moving one, or the like.

Furthermore, it is not practical to add the time of transfer to the treating time in the bath on the supposition thatadhered liquid will continue to react properly, simply because the adhering liquid will generally channel on the metal surface, becoming irregularly distributed, and with certain portions becoming exhausted more rapidly than others, resulting in non-uniform action. Furthermore, in many'instances, the conditions of such further action change. For example, some actions give proper results only at high temperatures and at lower temperatures produce undesired results. In this connection there may be cited the chemical brightening of aluminum and its alloys through the use of mixtures of phosphoric and nitric acids as described in the application of Charles C. Cohn, Serial No. 404,436, filed January 18, 1954, now US. Patent No. 2,729,551. For bright specular finishes of the most desired types, the action, which is a rapid one, desirably takes place at quite high temperatures in the region of or exceeding 100 C. As the temperature of the treatment drops, the action tends more and more to become one of production of diffuse and then matte surfaces, and the latter only are produced at or around room temperatures. Accordingly, if aluminum articles are thus being treated and are removed from the bath operating at proper temperature, if the solution is not very promptly removed and is permitted to continue action while the article is suspended in the air and while the temperature substantially drops, the continued action may destroy the specular surface produced in the bath and result in the formation of an unsatisfactory matte surface.

Furthermore, exposure to the atmosphere in many cases plays a part. While in the treating bath, the limited exposed surface of the bath and the bath composition itself may effect practical isolation from the oxygen in the atmosphere; whereas if the article is Withdrawn from the bath and is undergoing draining therefrom of the bath liquid, the exposure to atmospheric oxygen may rapidly discolor or otherwise adversely affect the surface finish.

It is the general object of the present invention, therefore, to provide a method and apparatus for the treatment of metallic surfaces whereby a desired treating reaction may be abruptly discontinued particularly under conditions, for example, of automatic conveyor operation, where it is impractical or costly to secure a rapid transition from the treating bath to a rinse or quenching bath. However, the invention is not limited to automatic conveyor operation, but is applicable to manual operations wherein it permits more leisurely manipulation of the work, prevents spoilage in the event that through some mismanipulation the handling of the Work is abnormally slowed down, and serves to protect the operator against noxious or dangerous fumes. In accordance with the invention, the articles immediately upon removal from the active bath, either automatically or manually, are subjected to an atmosphere which promptly inhibits further action of detrimental type while permitting the articles to move relatively slowly in the transition stage between the treating bath and a rinsing or quenching bath. Usually, the last type of bath is desirably used though, in some cases, it may be omitted. In accordance with the invention, furthermore, there is only negligible introduction into the treating bath of materials which will dilute the bath or add deleterious substances which would shorten its effective life. The objects of the invention relate to the attainment of results of the type indicated, and these broader objects together with detailed objects will become apparent from the following description read in conjunction with the accompanying drawing in which:

Figure 1 represents diagrammatically a typical type of apparatus which may be utilized for carrying out the invention; and

Figure 2 represents diagrammatically an apparatus wherein the invention is applied to manual Work handling.

The apparatus will be first described without specific reference to the chemical aspects of what is involved.

The treating tank 2 contains the bath B in which the articles to be treated are immersed. The articles are conventionalized at A and are supported by the conveyor chains 4, two such chains generally being provided with cross members 5 by which the articles A are supported. The articles represented by A may be quite general:

ported by hooks or otherwise from the members 5, or

" A may be taken to represent racks of suitable metal on which the articles are supported in any suitable fashion. The articles to which the invention relates are those presenting metallic surfaces to be treated, and may or may not be completely metallic. They may, in fact, comprise several metals of which only one is to undergo treatment, and in such case areas or metals which are not to be treated may be coated with suitable resists.

The conveyor carrying the articles A is guided over and under sprockets 6 and 8 to provide immersion of the articles in the bath B. it may be assumed that a substantial time of immersion in the bath is required, and if the length of the tank 2 is of reasonable extent, this will generally mean that the conveyor may advance only at a relatively low speed. The conveyor 4 is caused to take a path to lift the articles out of thetank by being trained about the sprockets ill and 12, and it will be evident that the removal of the article in itself necessarily requires a considerable time in proportion to the time of passage through the tank unless the bath is quite shallow in a relatively long tank.

What has been so far described would be typical of conveyor operation. In the absence of special provisions provided by the invention, the articles would have to be subjected to a substantial time interval between removal from the bath tank 2 and insertion in a rinse or quenching bath. During such interval, draining would take place with the production of irregularity of coating, exposure to air, possible partial drying, local exhaustion of the active solution, change of temperature, or the like,

previously described as detrimental in the great majority of processes. An attempt to wash off the bath solution by a stream or spray of rinsing or quenching liquid would obviously, unless the application thereof was delayed, result in entry of the rinsing or quenching solution into the bath which would either rapidly dilute the bath or render it inoperative by neutralization of its active ingredients.

in accordance with the present invention there is provided at the discharge end of the tank 2 an enclosure 14 into and through'which the articles are progressed during the passage of the conveyor chains about the sprockets indicated at 16, 12, i6 and 18. This enclosure 14 is closed to the maximum extent possible consistent with the entry and exit of the conveyor and the articles carried thereby. As shown, it has bottom openings which overlap the discharge end of the tank 2 and the receiving end of a tank 29 which contains the rinsing medium R. To complete the picture of the path of the conveyor and the articles, reference may be made to the sprockets 22 and 24 which in conjunction with sprockets 18 serve to provide for progress of the articles through the rinsing bath and removal therefrom.

In accordance with the present invention there are provided, in its preferred form, nozzles 26 which discharge a rinsing or quenching medium into the enclosure 14, the medium being provided through the manifold 2%.

These nozzles are preferably of the well-known fog type producing a dispersion of very fine droplets of the rinsing or quenching liquid. As is known, such nozzles will produce a fog of a slowly settling type, due to the very small size of the droplets, but at the same time provides a high concentration of the rinsing or quenching liquid per unit volume of the fog. There is thus provided an atmosphere through which the articles pass in their progress into, through, and out of the enclosure 14. To prevent the entry of any substantial amount of the rinsing or quenching liquid into the bath B, the enclosure 14 is provided with an exhaust pipe 36 through which flow is induced by a pump 32. By suitable location and direction of the fog nozzles 26 relative to the entrance to the enclosure 14 and the path taken by the articles, and by the proper control of the removal of the atmosphere within the enclosure 14 by the pump 32, the air drawn into the enclosure at the entrance and exit openings will serve to act as a barrier preventing settling of the fog particles into the bath B and also into the rinsing or quenching bath R, though settlement of the fog particles into the latter may not be objectionable. It will be evident to those skilled in the art that the location of the fog within the enclosure 14 may be readily controlled to secure 'a proper location thereof as may be observed, for example, by providing transparent windows in the enclosure to enable proper adjustments to be made. If the fog is of the desirable nonsettling type, a considerable proportion of the fog will be withdrawn through the exhaust passage 30 which is desirably provided with a sump 34 for the collection of the fog liquid which may settle out, and desirably the remaining portion of the fog passing the pump 32 is caused to settle out in a settling chamber 38 provided with baffles, with the result that substantially only air will emerge from the exhaust passage 42. The liquid collecting in the sump 34 and in the chamber 33 may be drawn ofi through the respective pipes as and 43 and, in general, will be suitable'for reuse, being returned to the manifold 28. This is possible since that tog which emerges through exhaust passage 3% will, in general, not

have been affected by the liquid carried by the articles out of the bath B.

Between the bath B and the rinse or quench bath R there is desirably provided a catcher 21 for liquid which drains from the articles. This liquid will comprise to a considerable extent liquid carried from the bath by the articles undergoing treatment and, in addition, will contain the fog liquid which has settled out on the article and has admixed with the bath liquid to stop its action. Generally, therefore, the liquid which separates out at 21 is not suitable either for direct return to the bath B or for recirculation to the manifold 23, but may be drawn off through the pipe 23 for the possible recovery of any constituents of value which it may contain. It may be remarked, however, as will shortly appear, that the liquid provided to the fog nozzles may be merely water or a mixture of water containing one or more of the bath constituents, in which case the liquid leaving at 23 may possibly be concentrated to give a liquid which essentially has the composition or partial composition of the bath B and may be returned thereto or has a composition constituting a suitable makeup or partial makeup for the bath B. 7

Considering the various aspects which the invention may take, the simplest operation, and one which is a possibility in various instances, involves merely the introduction into the enclosure 14 of air which has been cooled or quite substantially refrigerated, in which case fog nozzles are, of course, unnecessary and may be replaced by such nozzles as will merely produce an air blast. This type of operation may be used when the action of the bath B takes place at elevated temperatures and the activity of the bath which might be detrimental may be caused to be rendered substantially ineffective merely by cooling. The passage of the articles through the normal ambient atmosphere at room temperature in many cases will not suifice to produce sufi'lciently rapid cooling to terminate the action of the bath, and in such cases refrigerated air will lower the temperature of the article and the carried out bath liquid to such a. point that detrimental action will cease;

In other cases, and constituting the next simplest situation, the action of the bath may be terminated merely by substantial dilution with water of the bath solution which may be carried out by the articles. In such case, water may be admitted through the manifold 28 and dispersed in the fog nozzles 26. By the arrangement which is shown, this water, transformed into a fog the location of which may be controlled as set forth above, will not pass down into the bath B so as to dilute it and render it ineffective. If water is so used, the dilute bath liquid will drip from the articles as they pass over the catcher 21, and the diluted solution will be collected and rejuvenated by evaporation. In such a case, the rinsing or quenching bath would probably also consist merely of water or possibly of some dilute solution of an acid or alkali and no particular care need then be exercised to prevent water from the fog from entering this bath.

If water is used as just described, it may, of course, be cooled to provide such low temperature as may in aim many cases serve to aid in terminating the reaction'of the bath on the surfaces being treated in addition to the results from dilution.

More usually, however, there is required for the rapid termination of the bath reaction the treatment of the wet metallic surface with some inhibiting chemical in solution. Such an inhibiting chemical will sometimes be quite detrimental to the treating bath B, and consequently the matter of preventing'its entrance into the bath B is of major importance. The invention attains particular value in a case such as this, since the inhibiting solution may be fed to the fog nozzles 26 to produce a fog which is effectively isolated from the bath B by the control of its placement as described. In such case, the inhibiting reagent will generally act chemically upon the adhering liquid either to neutralize it or, at least, to neutralize its activity, possibly by rendering the metal surface inert to further bath action. The result is that the liquid dripping from the articles after they enter the fog will not be suitable for simple reclaiming for return to the bath B. The liquid collected in 21, therefore, may either pass to waste, or may be chemically treated for recovery of any valuable constituents therein. In some cases, of course, it may be used as a part makeup for the inhibiting liquid entering the manifold 28, though, in general, a certain amount will have to be removed to prevent undue concentration of the bath constituents.

In such cases, the rinsing or quenching bath R may or may not be detrimentally affected by the fog liquid, and depending upon the particular circumstances there may or may not be necessary precautions to prevent the fog liquid from entering this rinsing or quenching bath. In many cases, the rinsing or quenching bath R may have the same composition as what is introduced through the nozzles 26, in which case it would be immaterial whether the fog does or does not pass into the bath R. In fact, it may happen in certain instances that a suitable rinsing or quenching bath R may have the same active constituents as the liquid desirably entering into manifold 28, in which case the fog may be collected in the bath R or what is collected, as in the sump 34 and collector'38 may be returned directly to the bath and may, in fact, constitute the makeup material forvthis bath.

In other instances, however, the inhibiting chemical may be a constituent of the bath B. For example, consider the brightening of aluminum as carried out by the treatment of aluminum or its alloys as described in the application of Charles C. Cohn referred to above at high temperatures by a mixture of phosphoric and nitric acids. As already pointed out, if this mixture is allowed to act at lower temperatures an etching action will occur which will destroy the bright surfaces produced at high temperatures. However, nitric acid added with or without water dilution at the lower temperatures will inhibit the etching action which is'due to the phosphoric acid. (The low percentage of nitric acid originally in the hot bath is not of sufficient concentration to inhibit the phosphoric acid action in the cold.) Accordingly. in this case, cold nitric acid, either concentrated or dilute, may constitute the liquid fed to the nozzle 26 to provide the fog and will efiectively inhibit the undesired reaction which would otherwise continue. For example, the inhibiting composition may consist of 15% by weight of 70% HNO and 85% of water, though this solution may vary greatly in composition. But in this case the inhibiting agent, the nitric acid, is a desirable constituent of the treating bath B as well as of the rinsing bath R, and, consequently, the drained material at 23 and the recovered fog at 36 and tion, if desired) for makeup of this bath, or the recovered fog may be returned to rinse R from which, in fact, there may be drawn the liquid to supply the nozzles. In particular, in this fashion the relatively costly H PO carried out of the treating bath may be recovered and used as makeup for the treating bath, suitable amounts of H PO and/or nitric acid and/or water being added to secure a suitable makeup liquid. Thus, various 'cycles of the components may be provided, depending upon the concentrations which it is desired to maintain.

Conversely, in the brightening of copper, nickel or the like by the use of hot phosphoric-nitric acid mixtures, it is the nitric acid which, at lower temperatures, has an undesirable etching effect while phosphoric acid acts as an inhibitor. In such case the matters discussed in the last paragraph would be applicable but with an interchange of the nitric and phosphoric acids; i.e., dilute phosphoric acid would be the fog-forming inhibitor and also, if desired, the constituent of the rinsing bath.

It will now be evident that the invention is of very broad applicability and is obviously not limited to any particular treatments, classes of treatments, materials used, or classes of materials used. Examples, however, will be informative, though they are not be regarded as suggesting limitations on the method or apparatus.

Considering, for example, a bath B which may be strongly acid, or even moderately acid, the action of such a bath will usually be terminated by an increase in pH. In such a case, there is, of course, indicated the introduction through the nozzle 26 of an alkaline solution which will raise the pH to such value as will stop the action.

Conversely, if the bath B is alkaline, its action may be terminated by .introducing an acidic solution to form the fog. In this connection, the term acid is used quite broadly, since, for example, if the bath B contained as its active or energizing constituent a caustic alkali, and would be rendered completely or substantially inactive if the alkalinity was that of an alkali carbonate, it will be evident that the reaction might well be terminated by the introduction of a bicarbonate solution.

Again there may be introduced a solution of a material which has what might be considered an inhibiting action as described above.

It will be evident that the invention is quite broadly applicable to the treatment of numerous metals. Besides the brightening of aluminum as described in my application referred to above through the use of hot mixtures containing phosphoric acid and nitric acid, with or with out sulfuric acid, the invention is applicable to the following procedures which are merely typical of a great number which may be used:

The treatment of aluminum with hot caustic soda for etching of its surface is common and in this case the action may be inhibited by providing an artificial atmos phere which may consist solely of cold air, may consist of a fog of water, or may consist of a fog of an acid material capable of neutralizing the caustic soda to an extent stopping its action on the aluminum. For this purpose, there may be used a solution such as that of sodium bicarbonate or other weakly acidic material.

Alternatively, the fog may consist of a solution contain-- ing an alkali chromate or chromic acid which will have an inhibiting effect on the action of the caustic soda.

In the case of treatment of magnesium or zinc with nitric or sulfuric acid, the atmosphere may be provided in the form of a fog of water or a fog of an aqueous soIution of an alkali capable of neutralizing the acid.

In the treatment of copper, brass, nickel, German silver, or the like, with mixtures of phosphoric and nitric acids, with or without acetic acid or other materials, the inhibiting atmosphere may be provided by a water fog or by the fog of a solution of an alkali or phosphoric acid as described above which will inhibit the action of the nitric acid, the major active constituent of the bath.

Aluminum or its alloys may also be brightened using hydrofluoric acid with or without nitric acid. The inhibiting atmosphere in such case may take the form of a water fog or a fog of an alkali capable of neutralizing the acid or acids or the fog of a chromate solution which will inhibit the action of the bath material.

Numerous other treatments in accordance with the invention will be evident to those skilled in the art.

Aside from inhibiting the action of the dragged-out bath solutions or compositions, the invention has the further important aspect of gettingrid of the hazards of noxious or dangerous fumes which are produced in many treating processes and, in particular, surround the materials as they are withdrawn from the bath. The artificial atmosphere producing arrangement not only draws ofi such vapors but, in the case of the production of fogs, the material constituting the fog will dilute or react with the fumesubstances to produce safe working conditions. A

Reference may now be made to Figure 2 which shows the modification of a conventional type of treating tank assembly for the manual handling (usually, by the use of crane-providing devices) of materials treated in a bath. In this figure, 44 represents the usual tank containing the treating bath, and it may be. assumed that the operator stands to the rigt of this bath to handle the materials undergoing treatment. It is customary in the case of a bath which gives rise to fumes to surround the upper edge of the tank with a trough-like venting system connected to a suction fan to exhaust the fumes generated at the surface of thebath. Additionally, above the tank, there is provided a hood vent connected to an exhaust fan for the removal of fumes which rise from the material as it is lifted from the tank. In accordance with the present invention there is produced an atmosphere above the bath and below the hood 48 which is of the type previously discussed and into which the materials undergoing treatment are raised as they emerge from the bath. Headers 52 and 56 running along the edges of the tank are provided with nozzles 50 and 54 which, depending upon the treatment involved, may either produce merely a cold atmosphere of air'or a fog of water or of a neutralizing or inhibiting solution as has been previously described. In view of the exhaust conditions provided at 46 and by reason of the hood 48, this fog is maintained directly above the tank and is drawn, in particular, upwardly into the hood and may be separated as in the modification shown in Figure 1 by apparatus arranged in the exhaust conduit. By reason of this arrangement, not only is the action of the bath liquid arrested in the fashions already described but noxious or dangerous fumes are diluted or neutralized, or merely diminished by virtue of cooling action, so as not to spread into the regions about the tank whereat they might reach and be dangerous to the operator. With the arrangement of, nozzles as indicated, the arresting action beginsalmost as soon as an arti le or a portion thereof is raised from the surface of the bath. With an exhaust system provided as illustrated, the amount of fine fog material which will enterthe' bath will usually be negligible so that the fog may be produced by a solution of a material which in subsantial quantities might be deleterious to the bath.

It will be evident that the invention may be carried out in other fashions without departing from the principles of the invention as defined in the. following claims.

What is claimed is: a

1. A method comprising the steps of immersing 'an article of aluminum or an alloy of aluminum in a bath comprising phosphoric acid, nitric acid and water having a brightening action on the surface of said article,- withdrawing said article from said bath at substantially. the time of completion of the desired extent of said chemical action, and treating said article immediately and while it carries adhering liquid from the bath with an aqueous solution of nitric acid adapted to stop further chemical action on the surface of the article. a

2. A method comprising the steps of immersing an article of aluminum or an alloy of aluminum in a bath comprising phosphoric acid, nitric acid and water having a brightening action on the surface of said article, with drawing said article from said bath at substantially the. time of completion of the desired extent of said chemical action, treating said article immediately and while it carries adhering liquid from the bath with an aqueous solution of nitric acid adapted to stop further chemical action on the surface of the article, and thereafter using the last mentioned treating solution as a makeup con stituent of the first mentioned bath.

References Cited in the file of this patent UNITED STATES PATENTS 1,908,487 Powers May 9, 1933 2,086,993 Barrett July 13, 1937 2,172,171 Meyer Sept. 5, 1939 2,437,528 Hodil Mar. 9, .1948 2,541,901 Zademach et a1 Feb. 13, 1951 2,593,447 Hesch Apr. 22, 1952 2,632,693 Jenkins Mar. 24, 1953 2,650,157 Cochran Aug. 25, 1953 2,689,785 Simon Sept. 21, 1954 2,699,382 Altenpohl Jan. 11, 1955

Claims (2)

1. A METHOD COMPRISING THE STEPS OF IMMERSING AN ARTICLE OF ALUMINUM OR AN ALLOY OF ALUMINUM IN A BATH COMPRISING PHOSPHORIC ACID, NITRIC ACID AND WATER HAVING A BRIGHTENING ACTION ON THE SURFACE OF SAID ARTICLE, WITHDRAWING SAID ARTICLE FROM SAID BATH AT SUBSTANTIALLY THE TIME OF COMPLETION OF THE DESIRED EXTENT OF SAID CHEMICAL ACTION, AND TREATING SAID ARTICLE IMMEDIATELY AND WHILE IT CARRIES ADHERING LIQUID FROM THE BATH WITH AN AQUEOUS SOLUTION OF NITRIC ACID ADAPTED TO STOP FURTHER CHEMICAL ACTION ON THE SURFACE OF THE ARTICLE.

2. A METHOD COMPRISING THE STEPS OF IMMERSING AN ARTICLE OF ALUMINUM OR AN ALLOY OF ALUMINUM IN A BATH COMPRISING PHOSPHORIC ACID, NITRIC ACID AND WATER HAVING A BRIGHTENING ACTION THE SURFACE OF SAID ARTICLE, WITHDRAWING SAID ARTICLE FROM SAID BATH AT SUBSTANTIALLY THE TIME OF COMPLETION OF THE DESIRED EXTENT OF SAID CHEMICAL ACTION, TREATING SAID ARTICLE IMMEDIATELY AND WHILE IT CARRIES ADHERING LIQUID FROM THE BATH WITH AN AQUEOUS SOLUTION OF NITRIC ACID ADAPTED TO STOP FURTHER CHEMICAL ACTION ON THE SURFACE OF THE ARTICLE, AND THEREAFTER USING THE LAST MENTIONED TREATING SOLUTION AS A MAKEUP CONSTITUENT OF THE FIRST MENTIONED BATH.

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