US3537896A - Beneficial after-treatment of workpieces - Google Patents

Description

Patented Nov. 3, 1970 3,537,896 BENEFICIAL AFTER-TREATMEN T F WORKPIECES Walter Nohse, Lippstadt, Westphalia, and Gunter Fischer, Kaarst, Rhineland, Germany, assiguors to Laney Laboratories, Inc., Zelienople, Pa., a corporation of Pennsylvania No Drawing. Filed Sept. 23, 1968, Ser. No. 761,866 Int. Cl. B08b 7/04 U.S. Cl. 134-13 8 Claims ABSTRACT OF THE DISCLOSURE Workpieces being moved from a surface conditioning, treating zone or bath involving chromating or dichromating to provide a chromate conversion coating on zinc, cadmium, silver, aluminum or chromium, a pickle on magnesium, a chromate seal on aluminum, or a chromate film on copper or brass, or acid pickling or etching of copper and cuprous alloys, or acid plating, have the carried-over treatment liquid (drag-out) subjected to a valence reducing solution treatment prior to neutralization or removal of its toxic content in such a manner as to avoid damage to the metal surfaces or to a gel type film thereon containing both hexavalent and trivalent chromium. In addition to finally rendering any toxic carry-over innocuous, after-treatment desirably first reduces higher valence soluble metal compounds, such as those of chromium or copper compounds to facilitate later precipitating out and settling the metals. Hydrazine hydrate is used in the reducing, after-treating solution.

This invention relates to an improved method of reducing toxic chemical compounds that are carried over as a liquid film on workpieces that have been previously subjected to treatment in a chromium or copper ion containing surface treating solution bath.

A phase of the invention deals with reducing soluble carry-over compounds to lower valences in such a manner as to enable acontinuous in-line treatment of workpieces, prevent damage to the surfaces of the workpieces, and better convert soluble metal compounds into insoluble compounds of relatively high density form to aid fast settling or precipitation of insoluble metals.

U.S. Pat. No. 2,725,314 discloses a continuous in-line treatment for aqueous toxic carry-over or drag-out solutions, wherein such carry-over liquid may be continuously and effectively removed from or neutralized on the surfaces of workpieces during movement through treatment, wash and other baths. There has been a need for a better approach to reducing the chemical compound content of a carried-over liquid or aqueous solution in such a manner as to fit in with the continuous process of the above-mentioned patent and to enable an effective valence reducing chemical reaction on the chemical content of drag-out or carried-over liquid, without damaging the previously treated surfaces of the workpiece and in an effective manner such that subsequent chemical neutralization of the carry-over may be accomplished without interrupting continuous movement of the workpieces or necessitating a disruption of an in-line movement thereof.

Experimental work has been done in connection with a treatment solution containing a reducing chemical such as sodium hydrosulfite or sodium bisulfite, but it was determined that such a reducing chemical tends to damage the surface of a protective film on the surface of the workpiece from the standpoint of lessening its corrosion resistance, susceptibility to finger marking, etc. It was discovered that these chemicals and various other reducing chemicals tend to cause damage, both to fully exposed metal surfaces as well as to a somewhat insoluble conversion coating, for example, in the nature of a gel type of surface film containing hexavalent and trivalent chromium. It has been determined that there is a need for an approach to the problem which would enable the reduction of the valence of soluble metal compounds, without at the same time, damaging any surfaces of or protective film on surface of the workpieces with which the solution may come in contact.

It has thus been an object of the invention to provide a new and improved form of an aqueous reducing or after treating solution that may be effectively used in and be compatible with a continuous in-line process for workpieces, and, at the same time, which will accomplish its functions in such a manner that a protective film formed on the metal surfaces will remain thereon in undamaged condition to protect the workpiece during the subsequent neutralization of its carry-over and further processing of the workpiece, whether of a mechanical or chemical nature;

Another object of the invention has been to solve the problem involved in providing a reducing solution or chemical which will be effective in lowering the valency of copper and chrominum ions dissolved in a waste treatment solution and without imposing undesirable limitations on the process or damage to surfaces of the workpieces;

A further object of the invention has been to enable an efficient and effective direct application of reducing treating solution to carry-over or drag-out liquid on a previously treated workpiece, without damage to the surface of the workpiece, and to facilitate precipitating or settling out of insoluble metal compounds therefrom;

These and other objects of the invention will appear to those skilled in the art from the description and the claims.

Heretofore, it was thought to be necessary to provide fast acting reducing compounds for converting hexavalent chromium into trivalent chromium or for reducing copper oxides on metal surfaces in a continuous or in-line treatment process. Also, a relatively high excess above a stoichiometric quantity is needed to assure the completion of the desired reactions. While it has been desirable to reduce chromic acid or chromate compounds and copper hydroxides in the wet film carried by the workpieces as a drag-out solution from the previous treatment bath, harm must not be imparted to the surface finish whether of a chromium or copper type. Ordinarily, fast reaction types of reducing chemicals and as used in excess have been found to cause such damage.

The invention deals with the provision of an aqueous treatment solution of a reducing nature which, although of a rather strong reducing type, will enable continuous, in-line movement of but will not cause damage to the surface of the workpiece as when solution comes in contact therewith or is used directly on such surfaces as applied to the toxic solution carry-over thereon. The problem is to substantially fully reduce and precipitate chromates and chromium compounds in a drag-out solution disturbing, destroying or adversely changing protective compounds'formed as an adherent film on surfaces of the workpiece. The surface film is a somewhat fragile, extremely thin chemical coating consisting of various chemical compounds as reaction products with the metal surface, itself. Thus, such a film is called a conversion coating since an outer metal surface is converted from a metallic to a partially or completely chemical film.

A chemically formed film such as previously mentioned is usually very thin and may be visible or invisible depending upon its content. For example, zinc, cadmium, aluminum, and magnesium chromate films are visible while films such as formed on copper and brass may be invisible. Even the visible films are more often in the range of only a few Angstroms such that they provide interference fringes rather than noticeable color.

The invention deals principally with copper and its alloy films as Well as chromate conversion films on various metals such as zinc, cadmium, aluminum, magnesium, copper and cuprous alloys, silver, tin, etc. As above indicated, such chromium films are basically reaction products of the metal surface with the chromate chemical and consist of a variety of hydrated metal chromates, depending on the basic metal, for example, zinc chromate, cadmium chromate, etc. Such films may consist of precipitate-d reduced chromium salts also in hydrated form. It appears that the metal chromates, whether in the form of a visible or invisible film, are both absorbed and adsorbed and contain some water of hydration. On chemical analysis such a film may yield small quantities of both hexavalent and trivalent chromium on being subjected to a leaching process.

It has been discovered that the problem could be solved by the use of an aqueous hydrazine chemical compound in the nature of a hydrazine hydrate washing-treating solution maintained at a near neutral or an alkaline pH range of about 6 to 13, as applied to the drag-out on surfaces of a workpiece, while the workpiece is being moved continuously from a chemical treating bath, zone or solution into, through and out of a treatment wash bath, zone or solution. Such a wash-treating is used in such a manner that it acts to reduce the dissolved chemical content of the drag-out, directly on the moving workpieces, as it is washed off into the bath, and finally in a treatment solution tank or reservoir at which the washingtreating solution is formed and reconstituted for use in the wash-treating bath. It has been determined that such a solution has a slow reducing reaction rate on the dragout that is, on the one hand, sufficient to enable a continuous inline movement of the workpieces, such that they have a normal immersion period of 15 to 30 seconds in the washing-treating bath and, on the other hand, without any deleterious effect on the surfaces or chemically formed films thereon. In fact, about 32 times a stoichiometric equivalent for the completion of the reaction has been successfully employed in this connection.

It has been determined that hydrazine hydrate (N H -H O) used in an alkaline pH range has a somewhat slowing down effect on reaction time for the reduction of, for example, chromic acid or copper hydroxide, and that it can be used without harm to copper or chromium surfaces of workpieces, and particularly to metal workpieces having a protective film or conversion coating thereon.

By way of example, an after treatment solution containing about 2 g./l. of N H (3.13 ml. of a 64% solution concentrate) in a liter of treatment solution at a pH of 9.65 requires about one hour to completely reduce 250 mg. of CrO contained in a one ml. solution which was added to this after treatment solution. Using hydrazine hydrate in stoichiometric excess in the after or washing-treating solution, it was discovered that dwell times in the process rinse of longer than 30 seconds, even up to 120 seconds, did not in any way adversely effect the corrosion resistance of a dichromate film on a zinc Plated surface. On the other hand, a similar film is completely ruined in one minute if immersed in a treatment solution of equal concentration having an excess of a conventional reducing chemical, such as sodium bisulfite or sodium hydrosulfite. It was further determined that hydrazine hydrate, as applied in such a solution, only acted on the drag-out or carry-over and not on the surface of the metal workpiece, whether or not it has a protective film or is fully exposed as pure metal.

Thus, in accordance with the present process, workpieces may be moved directly from a toxic treatment zone or bath into and through an after treatment bath or zone, utilizing a solution of the present invention and in such a manner as to fully convert the soluble metal content of the carry-over or drag-out into a lower valence carry-over or solution. The after treatment solution may be circulated between a bath, zone or container in the workpiece treatment line that follows the chemical treatment bath, zone or container and a treatment reservoir. Additional hydrazine hydrate may be added to the solution in a hold or treatment reservoir as needed and above a stoichiometric amount before the aqueous reducing solution is returned as reconstituted to the after treatment bath. The movement of the reducing solution with its drag-out solution content is progressive to the hold reservoir as is the movement of the reconstituted reducing solution being returned from the reservoir to the after treating bath or zone. A neutralizing chemical which may be either acidic or alkaline, may be added to the soluttion in the reservoir to facilitate the precipitating and settling of its reduced metal compound content, in addition to a hydrazine salt for providing a suitable 6 to 13 pH in the solution. It will be recognized that a weaker acid may be added to raise the pH of a solution containing a strong acid. Precipitated reaction products may be settled in the hold or treatment reservoir and removed therefrom periodically.

We claim:

1. In a compatible intermediate processing step in an integrated in-line process for reducing and precipitating toxic material of the nature of chromates and chromium compounds in an aqueous drag-out solution from workpieces being chemically processed in a treatment line wherein the workpieces as treated have surfaces of the class consisting of chromium, copper and copper alloys and a chromium compound based protective surface film, such as on zinc, cadmium, aluminum, magnesium, silver, copper or brass surfaces, the steps of preparing and maintaining an aqueous chemical reducing solution having a hydrazine chemical compound in solution, applying such reducing solution directly to surfaces of the workpieces and to the aqueous drag-out solution carried over thereby from a toxic treatment zone while moving the workpieces from a toxic treatment zone in and through a reducing solution application zone, employing the reducing solution to reduce any dissolved hexavalent chromium and higher valent metal salt content of the drag-out solution, and precipitating reduced metal compounds from the reducing solution.

2. In a process as defined in claim 1, after precipitating and settling out reduced metal compounds from the reducing solution, then reconstituting the reducing solution and again applying it to the surfaces of the workpieces that are being moved from the toxic treatment zone.

3. In a process as defined in claim \1, maintaining the aqueous reducing solution at a pH within a range of about 6 to 13 during the treatment of the drag-out solution.

4. In a process as defined in claim '1, supplying hydrazine hydrate to the reducing solution by the application of water soluble inorganic hydrazine salts thereto. v

5. In a process as defined in claim 1, moving the workpieces continuously from the toxic treatment zone into and through a bath containing the reducing solution, progressively moving the reducing solution with its dragout content from the bath into a holdingreservoir and 5 there separating reduced metal compounds from the reducing solution, and then progressively returning the solution to the bath.

6. In a process as defined in claim 5, reconstituting the reducing solution within the holding reservoir before returning it to the bath.

7. In a process as defined in claim 1, applying the reducing solution to the surfaces of the workpieces and removing the aqueous drag-out solution thereon within the reducing solution application zone, while maintaining the reducing solution being applied to the workpieces Within a pH of about 6 to 13.

'8. In a process as defined in claim 7, removing the reducing solution containing drag-out solution therein from the reducing solution application Zone, applying a neutralizing (alkali or acid) chemical to the removed solution and precipitating and settling out insoluble metals 6 therefrom, and thereafter reconstituting and returning the reducing solution to the solution application zone for reuse.

References Cited MORRIS O. WOLK, Primary Examiner J. T. ZATARGA, Assistant Examiner US. Cl. XJR. 21050 ggg UN'I'IED STATES PA'JFENT OFFICE 'CIiH'lIFICATE OF CUI EREC'IION Patent No. 3 537 896 Dated November 3 1 970 n Walter Nohse and Gunter Fischer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 2, change "of", first appearance, to --or--; line 30, correct the spelling of "chromium" Column 3, line 1, before "disturbing" insert --without--.

olumn 1;, line 30, correct the spelling of "solution".

SIGNED AN'D stein w; 29 1970 5mm Attest:

Edward M. mew mum E. m. A Officer fiomissiozmof Patents