US3514343A - Regeneration of chromate conversion coating solutions - Google Patents

Description

US. Cl. 1486.2

United States Patent 3,514,343 REGENERATION OF CHROMATE CONVERSION COATING SOLUTIONS 1 Frank A. Bauman, Cleveland Heights, Ohio, assignor to The Lubrizol Corporation, Wickliife, Ohio, :1 corporation of Ohio No Drawing. Filed Dec. 13, 1966, Ser. No. 601,344 Int. Cl. C231? 7/26 3 Claims ABSTRACT OF THE DISCLOSURE Chromate conversion coating baths for metal surfaces, especially ferrous metal and aluminum surfaces, are regenerated and the dissolved metal ion and trivalent chromium are removed therefrom by adding an alkaline reagent, preferably ammonium hydroxide, until a pH of about 5.5- 8.0 (5.5-9.0 in the case of ammonium hydroxide) is reached. The metal is precipitated as the hydroxide and is separated from the solution, which is then restored to its I original active ingredient content.

This invention relates to the formation of chromate coatings on metal surfaces, and more particularly to an improved method for the regeneration of aqueous chromate coating solutions. The invention is applicable to aqueous acidic conversion coating solutions containing chromium, which solutions gradually increase in metal ion concentration due to dissolution of metal therein from the surface of the object being coated, and it involves regenerating said solution by adding an alkaline reagent contacting the metal object with an aqueous solution containing hexavalent chromium in combination with one or more auxiliary substances such as nickel ion, fluoride ion, phosphate ion, ferrocyanide or ferricyanide and the like. The coating solution must be acidic, and preferably has a pH of about 1.52.5. v

During the formation of the coating, metal from the surface of theobject being coated dissolves in the chromate solution, reducing part of the hexavalent chromium .to trivalent chromium. This reduces the working concentration of the bath, and the dissolved metal (especially aluminum) often interferes with the coating process.The result is decreased efliciency of the solution and an increase in the time necessary to produce a coating of the desired weight or thickness. Eventually, the coating solution becomes so ineflicient that it must be discarded, polluting streams and lakes in the vicinity; or else themetal must be removed to render the solution suitable for further use.

A principal object of the present invention, therefore, is to provide an improved method for forming chromate conversion coatings on metal objects.

A further object is to provide a convenient method for the regeneration of aqueous chromate solutions used for forming conversion coatings on metal objects.

A still further object is to provide a regeneration method which is simple to operate and which may be usedv in either batch or continuous systems.

Other objects will in part be obvious and will in part appear hereinafter.

The method of this invention may be used in connection with chromate coating of any metal to which such coatings are ordinarily applied. The principal metals which are thus treated are aluminum, ferrous metals and gal- 'vanized metals. The method is particularly effective in connection with the coating of aluminum surfaces.

Coating solutions which may be regenerated by the method of this invention include generally all such solutions which are acidic and contain hexavalent chromium. The hexavalent chromium is generally added in the form of chromic acid (CrO or a water-soluble hexavalent chromium salt (e.g., potassium chromate, potassium dichromate or sodium dichromate). The chromium concentration, expressed as chromic acid, is not critical but should preferably be between about 1 and 50 grams per liter. Other ingredients may be and usually are present; typical ones include fluoride ion (which may be present in the form of hydrofluoric acid, a soluble fluoride or a soluble complex fluorine-containing salt), phosphate ion, ferrocyanide or ferricyanide ion, auxiliary metal ions such as nickel, and the like. The pH of the coating solution is usually between about 0.8 and 4.5.

The following examples illustrate typical chromate coating baths which are suitable for use with the metnod of this invention.

EXAMPLE 1 Fluoride Chromic acid Phosphate EXAMPLE 2 Chromic acid, potassium ferricyanide and hydrofluoric acid are dissolved in water to prepare a solution containing the following concentrations of ingredients (expressed as grams per liter):

Chromic acid 5.0 Potassium ferricyanide 0.6 Fluoride 1.0

The bath has a pH of 1.5.

' EXAMPLE 3 A dry mixture is prepared from 3.6 grams of nickel carbonate, 8.8 grams of sodium bifluoride, 51.5 grams of potassium dichromate, 8.8 grams of sodium fluosilicate, 8.8 grams of sodium fluoborate, 17.5 grams of sodium nitrate, and 1.0 gram of water. A 1% (by weight) solution of this mixture in dilute aqueous nitric acid is prepared; this solution has a pH of 2.0,and contains active ingredients in the following concentrations (expressed in grams per liter): 7 I

Chromic acid Nickel Fluoride According to this invention, an alkaline reagent is added to the chromate solution in an amount sufiicient to precipitate substantially all of the metal but less than that required to form a soluble complex metal ion. Alkaline substances which may be used include the alkali metal hydroxides, carbonates, bicarbonates and the like, anhydrous ammonia, ammonium hydroxide and other water-soluble alkaline substances such as amine, Ammonium hydroxide is preferred.

The minimum amount of alkaline reagent to be added is determined by the amount of metal ion present in solution and the maximum amount which is tolerable in the chromate bath under the conditions of operation. Where the metal is treated by spray-coating or by dip-coating over a short time period (e.g., a few seconds to a minute or less) only a small amount of aluminum (preferably less than 0.4 gram per liter and desirably less than 0.2 gram per liter) is permissible. When the metal object is to be dip-coated and the coating time may be longer than specified above, more metal is permissible in the solu tion, but in general it should not contain more than about 1 gram per liter.

The amount of metal removed by the addition of the alkaline reagent is dependent upon the pH of the solution after said addition. When aluminum is the metal being treated and the chromate solution of Example 3 is used, aluminum remains in solution to the extent of about 0.55 gram per liter at a pH of 5.5. When the pH is increased to 6.0, the aluminum ion concentration drops to 0.32 gram per liter, and at 6.5 it is slightly below 0.2 gram per liter. In the treatment of ferrous metals with the solution of Example 3, most of the iron in solution precipitates as ferric hydroxide at a pH of 5.5 or higher. The trivalent chromium is also precipitated as chromium hydroxide at this pH. In general, the pH of the solution after alkali treatment should be between about 5.5 and 8.0 when an alkali metal hydroxide is used as the regenerating agent; when ammonium hydroxide is used, a somewhat higher upper limit (about 9.0) is possible. The preferred pH range, for a chromate bath used for spraycoating of aluminum, is about 6.5-7.0. For dip-coating baths where contact time is not as critical, a pH range of about 5.5-7.0 is preferred although 6.5-7.0 is desirable.

If too much alkaline reagent is added, there is danger of redissolving the metal; this is especially true when the metal being removed is aluminum and sodium or potassium hydroxide is used, since an aluminate is formed if excess base is present. Therefore, in no event should enough alkaline reagent be added to redissolve the metal. If the pH limits already described are maintained, there is no danger of aluminate formation in the solution as a whole, but there may be localized regions of higher pH if sodium or potassium hydroxide is added rapidly to a poorly circulated solution. This should be avoided by vigorous agitation and slow, careful addition of base. The problem is less critical when ammonium hydroxide is used.

The precipitated metal hydroxides are usually gelatinous in character. They may be removed from the solution by filtration, decantation, centrifugation or any other suitable method. If filtration is to be used, it is usually necessary to add a filter aid material. Generally, however, it is convenient merely to allow the precipitate to settle and then to siphon olf the supernatant liquid and return it to the metal treatment vessel.

After the precipitate has been removed, the original concentrations of the active ingredients of the chromate coating solution are restored by the addition of chromic acid or a chromate, fluoride ion and the like, and the solution is acidified to its original pH. It is then suitable for continued use. treatment of chromate coating solutions according to It will be apparent to those skilled in the art that the the method of this invention may be accomplished in several ways. Thus, the entire coating bath may be treated with alkali and the precipitate removed therefrom all at once. Preferably, however, a portion of the bath liquid is removed and treated and is then returned to the bath. This may be conveniently done by continuously circulating the bath liquid through a second vessel which is maintained at the desired pH for metal ion removal by continuous metering of alkali into the same. The liquid, thus treated may then be passed through a suitable filter, centrifugal separator or the like and the purified liquid returned continuously to the coating vessel.

The method of this invention is illustrated by the treatment of a chromate coating bath containing fluoride ions which is similar to the bath described hereinabove in Example 3. After some hours of use for coating aluminum, this bath has a pH of 2 and contains 1.98 grams per liter of hexavalent chromium, 0.4 gram per liter of trivalent chromium, and 0.66 gram per liter of aluminum. The aluminum and trivalent chromium are removed by adding ammonium hydroxide to a pH of 7.0; after filtration, the solution is found to contain 1.89 grams per liter of hexavalent chromium, 0.089 gram per liter of aluminum, and 0 gram per liter of trivalent chromium. After the addition of nitric acid to reduce the pH to about 2, the liquid is suitable for further use.

What is claimed is:

1. In a method for forming a chromate conversion coating on an aluminum object which comprises contacting said object with an aqueous acidic solution containing hexavalent chromium and fluoride ion, which solution has an original pH of about 2.0 and gradually increases in aluminum and trivalent chromium ion concentration due to dissolution of aluminum therein from the surface of said object and reduction of hexavalent chromium; the improvement which comprises regenerating said solution by adding ammonium hydroxide in an amount sufficient to increase the pH of said solution to between about 5.5 and 9.0, thereby precipitating essentially all of said aluminum and trivalent chromium; removing the precipitate containing said aluminum and trivalent chromium; acidifying said solution and restoring the active ingredient content thereof.

2. The method of claim 1 wherein the ammonium hydroxide is added in an amount suflicient to increase the pH of the solution to between about 6.5 and 7.0.

3. The method of claim 2 wherein the aqueous acidic solution additionally contains nickel ion.

References Cited UNITED STATES PATENTS RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 1486.27