US2475945A - Method of chemically coating metallic articles of aluminum or predominantly of aluminm and solution for use therein - Google Patents

Description

Patented July 12,1949

UNITED STATES PATENT OFFICE Charles W. Clark, Westmount,

Collins, Montreal,

Thomas Garnet Quebec, and Quebec,

Canada, assignors to Canadian Copper Refiners Limited, Montreal East, Quebec, Canada, a corporation of Canada No Drawing. Application September 21, 1946, Serial No. 698,424

13 Claims. (Cl. 1486.16)

The present invention relates to a method for protectin the surfaces of metallic articles made of aluminum or predominantly of aluminum and a dipping solution for use therein. As such, the process is applicable both to articles which are substantially pure aluminum and also those made of aluminum alloy having a major portion of aluminum. More particularly, the invention relates to a process and solution which will provide a protective coating for metallic surfaces of the type described to prevent or substantially to retard corrosion thereof, such as would be occasioned by exposure of the metal to highly corrosive influences, such as sea Water, whether the surfaces are further protected by paint or lacquer coatings or not.

As such, the present invention is a continuation-in-part of our prior and co-pending application Ser. No. 528,358, filed March 27, 1944, now abandoned, entitled Method of producing a protective coating on aluminum articles.

Many attempts have been made in the past, with more or less success, to provide coatings for various types of metallic surfaces, most of such attempts being particularly directed to the treatment of surfaces of iron and steel articles, although some work has been done heretofore in protecting articles of aluminum or aluminum alloys. Some of these processes have been known under the conventional names of Parkerizing and Bonderizing. Many of these processes have used, alone or in combinations with other materials, a dipping solution including phosphoric acid.

In substantially all these processes, the first step has been to clean the metal in some suitable way. This is found to be necessary also in accordance with the present process, the cleaning being effected in any suitable way including mechanical cleaning to remove such impurities or foreign matter as may be removed in that fashion and also chemical cleaning. The latter may be generally classified into two main types of cleaning: (1) that using an acid such, for example, as nitric acid, which may be employed in concentrations in aqueous solution of about 1 to 20% with a preferred concentration of about and (2) that using an alkaline cleaning solution, one of which is known commercially under the name of Dow cleanin solution." This solution is made up of materials in about the following proportions: NazCOa-3 oz., NaOH-2 oz., soap- 0.1 02. These amounts of material are added to sufficient water to make up a gallon of cleaning solution. This alkaline solution is effective in removing practically all types of grease, which may usually be found on the surfaces of such articles. This portion of the process is included merely as one essential step thereof, but not as involving novelty or patentability per se. In View of this status of the cleaning step of the process, it will not be further discussed herein.

While some of the prior processes referred to have been considered reasonably effective for certain purposes, We have found that particularly good results are obtainable by following the process hereinafter described in detail, this process affording a relatively simple and cheap method of protectingaluminousmetalsurfacesmtby wgch is meant surfacespf articles of aluminum orpredominantly of alumih'uiifl and being quite effective against corrosion, whether the surfaces are further protected by a layer of paint or lacquer or not. The provision of such a novel process is a primary object of the present invention.

Further objects and advantages of the present invention will appear from the following specification, wherein the invention is set forth in detail, and Will be pointed out in the appended claims.

The invention essentially comprises a novel dipping solution and process for use thereof in which articles of the type described may be immersed after cleaning and which will provide a coating affording the desired corrosion-resisting characteristics.

The dipping solution of the present invention is aqueous in character and has three essential active ingredients which will be hereinafter considered in detail as to their character, composition, and concentration, these ingredients being (1) a selenium compound which is selected from the group which consists of selenium dioxide, selenious acid, the alkali and zinc salts thereof, selenic acid, and the alkali and zinc salts thereof; (2) a zinc compound which is selected from the group which consists of zinc chromium selenate (Crz(SeO4)3-ZnCrO4), zinc oxide, zinc selenite, zinc selenate and zinc phosphate; and (3) poses, it may be desired to add, as a fourth active' ingredient, an organic addition agent as hereinafter more particularly described. The three essential active ingredients will now be discussed in detail.

Selenium compound At least one such compound as aforesaid is considered as an essential element or ingredient of the aqueous dipping solution in this case. The preferred compound, in accordance with the present invention, is selenium dioxide. However, the other named compounds may be considered as alternatives or may be used as ingredients of mixtures, so that at least one of the compounds named must be used although two or more such compounds may be used in accordance with the present invention.

The concentration of the selenium compound in the solution is preferabl calculated as chemical equivalents of selenium dioxide (SeO2), the calculation preferably also including all the selenium present in the solution irrespective of how it is introduced. On this basis, the concentration (expressed as SeOz) should be from a minimum of about grams to a maximum of about 300 grams per liter, the preferred concentration being about 200 grams per liter.

The effect of concentration in general is that higher concentrations of the selenium compound are effective either to produce a heavier coat in a given time or to permit the shortening of the dipping or immersion time for the production of a coat of a given weight. The minimum concentration as set forth numerically above is not particularly critical as it is necessary in accordance with the present invention to have a concentration sufficiently high so that effective results are produced. The temperature of the bath will be considered hereinafter and has no particular special effect in connection with the concentration or presence of the selenium compound other than its general effect as hereinafter stated.

Zinc compound While several such compounds as aforesaid have been tried and have been found to be successfully operative, it is preferred in accordance with the present invention to use zinc chromium selenate. From a broader point of view, however, any of the zinc compounds listed hereinabove are to be considered within the purview of the present invention and any mixtures of two or more such compounds also within such invention.

In considering the concentration of the zinc compound used, all the zinc compounds in the solution should be taken into account whether such compounds be, for example, zinc selenate, zinc selenite, or some other compound which is also to be considered in calculating the selenium concentration. With the preferred zinc compound, zinc chromium selenate, the concentration range contemplated for use is from about 2 to about 250 grams per liter, with a preferred concentration of about 200 grams per liter. On the other hand, calculation may also be made of all the zinc present in the solution as equivalents of zinc oxide (ZnO). On this basis, satisfactory results may be obtained between minimum and maximum concentrations of about 10 and about 200 grams per liter, respectively, with a preferred concentration of about 100 grams per liter. In both these instances, the minimum concentration is not particularly critical, as it is governed by the principle that there should be at least an effective amount of zinc present. The maximum concentration is usually determined by the amount of such compound or compounds used which can be gotten into and maintained in solution. This last is interrelated with the phos- 4 phoric acid concentration as hereinafter set forth.

In general, the effect of concentration of zinc compound or compounds used is about the same as that given above for selenium, i. e., that higher concentrations will be effective to produce a heavier coating in a given time, or from another point of view, a given weight of coating may be produced in a shorter time with higher concentrations. It is further usual that relative concentrations of selenium and zinc compounds be kept more or less parallel, i. e., if a high concentration of selenium is used, it is customary, although not necessarily required, that a relatively high concentration of the zinc compound or compounds be used. This last principle is usual, but not an essential element of the present invention. Other known inorganic salts of zinc, such as zinc chloride, zinc nitrate and zinc sulphate, do not give desirable results, possibly for the reason that these compounds are too highly ionized in aqueous solution.

The temperature and time factors as to their effect on, or the manner in which they are affected by, the zinc compound are similar in character and direction to those stated above in connection with the selenium compound ingredient.

Phosphoric acid This ingredient has no equivalents as far as we have ascertained up to this time. The concentrations required for use are preferably relatively high. There are, however, no particular low limits except, first, that there shall be an effective amount present, i. e., enough to accomplish the objects of providing a satisfactory coating, and second, that there shall be enough phosphoric acid present at least to dissolve all the zinc compound present in accordance with the teaching given above. It is further preferable, in accordance with the present invention, that some excess of phosphoric acid be present in the solution in addition to that required to dissolve the zinc compound introduced as aforesaid, so that the solution shall be positively acidic in character. Based on these principles, the concentration of phosphoric acid (HIiPOl) may be between a low limit of about 12 /2 and a high limit of about 625 grams per liter in the solution with a preferred concentration of about 300 grams per liter.

As to the relative effects of concentrations of the different ingredients, it is essential as aforesaid that there be sufiicient phosphoric acid to dissolve all the zinc compound present, so that as the zinc compound ingredient is increased in concentration, the phosphoric acid concentration must usually be more or less correspondingly increased. Furthermore, as a practical upper limit, the excess of phosphoric acid provided as aforesaid beyond that amount required to dissolve the zinc compound present is contemplated to be not more than about grams per liter. Furthermore, it has been found that as a rule of thumb, the dipping time varies in reverse relationship in respect to the concentration of phosphoric acid, which is a further reason for the use of relatively high concentrations of this ingredient, so that the time may be reduced to a practicable period.

Temperature The temperature in this case is not absolutely critical. On the other hand, it has a major effect upon the action of the solution in forming the desired coating. This is shown from the fact that swim indifferent results which can best be described as a-thin patchy coating can, be. obtained at room temperature after about sixteen hours immersion in a given bath, but with the temperatures of the bath and the articleto be coated near the boiling point of the bath, a good coating can be obtained in about minutes. Boiling'of the bath solu-. tion does not harmfully afiect the coating properties of the bath. The results progressively 'vary between the two limits just given. In view'of this data, it is preferred in accordance withthe present invention that the temperature of the bath, and hence that of the articles to be coated during'the coating operation, be maintained in a temperature range of about 90 C. to the boiling point of the solution. It is usually desired, how-.- ever, to maintain the dipping solution slightly below its boiling point as the vaporizationof some of the materials of solution, particularly the selenium compounds, might create an undesired condition from the point of view of workmen nearby, even thoughthe bath composition is not substantially impaired thereby.

Time of immersion While the time of immersion is determined principally by other factors, theprimary consideration is the thickness of the coating whichis to beformed on the articles inquestion, longer immersion giving greater thickness. 'Also, as above indicated,;this time is to a certainextenta function of the temperature of the bath, so that with higher temperatures, shorter immersion may be used to effect the same results. The timeof immersion is also a function of the concentrations used as aforesaid, higher concentrations within the limits given being effective to reduce the time required to attain any given result.

An exampleof a time actually used can be given in that a satisfactory coating may be obtained using the preferred composition as above set forth, 1. e., (SeOz) about 200 grams per liter, zinc chromium selenate (Crz(SeO4)3-ZnCrO4), about 200 grams per liter, and phosphoric acid about 300 grams per liter, the bath being maintained at a temperature between about 90C. and its boil-l ing point, the article being dipped being pure aluminum, and a time of immersion of about 10 minutes.

Added material While the dipping bath aforesaid'will givede sirable results in forming coatings upon any aluminous articles, and particularly desirable re- 'sults when the article is substantially pure aluminum, it has sometimes been found desirable, 'es pecially in treating aluminum alloy articles, to add to the dipping bath one or the other of two particular organic materials, these materials b'eing phthalic anhydride and propylene glycol, In this case, the materials are not to be considered strictly as equivalents of one another, but rather as alternatives. The theory of operation or action of these materials is not definitely known. However, it is believed that they act in some way as surface activators. 'Of the two materials, phthalic anhydride is preferred, particularly when aluminum alloys are to be coated. Phthalic anhydride, if used, may be used in any 'concentratio'n up to about 100 grams per liter, higher concentrations" given no further advantageous results, but not being positively undesirable. The preferred concentration is, however,

about 10 grams per liter.

th s. in egers the following data has been determined. Phthalic anhydride is not essential, but does not harm or otherwise interfere with the operation of the bath when treating pure aluminum. In treating aluminum'alloys, the results without using this ingre-. dient are to some extent desirable, but are not as desirable as when some phthalic anhydride is usedin accordance with the above teaching. For example, using. an alloy known as AC 17s,". which contains in addition to the aluminum as alloying ingredients, manganese 0.5%, copper 4.0% and magnesium 0.5%, highly desirable re-. sults were obtained using a bath containing phthalic anhydride. Somewhat less desirable but still advantageous results were obtained using phthalic anhydride in coating an alloy known as -AC .573, which is one containing in addition to the aluminum, manganese 0.1%, copper 0.1% and magnesium 2.2-2.8%. Propylene glycol, if used, is effective in a some-, what similar way. -In this case. it is contemplated to use this compound in concentrations between about 1 and about 5 grams per liter with a preq ferred concentration of about 2 grams per liter. It has been found, for example, that concentrations as high as grams per liter are definitely undesirable as causing etching of the surface.

Character of the coating The exact chemical composition of the coating produced as aforesaid is not known. It is be-, l ieved tocontain selenium and zinc and probably also the P04 radical in some combination. The coating is quite light in weight and probably is further reduced in weight by subsequent treat- .40 thepho'sphoric acid, possibly with the evolution tion per se.

of some hydrogen, .and that there is some chemical interchange and/or deposit of either selenium or zinc or compounds of one or both of these materials. In any event, the exact character of the coating and the reactions by which it is formed are not herein depended upon in support of the patentability of the appended claims, which are directed to the process by which it is formed and to the dipping composition or solu- Farther process details Following the formation of the coating by the clipping process aforesaid, it is usual to wash the article in water. This removes excess dipping solution from the surface and is particularly necessary in the event that a dipping bath ineluding chromium compounds is used, such as the zinc chromium selenate aforesaid, for under these circumstances some green chromium salts are found on or adjacent to the surface, particularly when high concentrations of selenium dioxide and phosphoric acid are used. This washing operation serves to dissolve off or otherwise remove these green chrbmium salts. On the other hand, if

some other zinc salts are used, so that the bath does not contain chromium, the washing operation may be minimized and in some cases wholly omitted. The process is to be considered from this pointof view, so that a process omitting the washing step is within the purview of the present invention considered in its broader aspects.

Af'further step which is, however, essential in accordance with the present invention is that of drying or maturing the' coating. This is pref-,

erably accomplished by subjecting the articles, in an oven or other suitable apparatus, to a temperature in the order of about 120 to about 150 C. for a time period preferably at least about two hours. Articles have been baked at this temperature up to 48 hours or more without any harmful results so that overbaking is not productive of undesirable results.- It has been found that this is an essential step, for if the coating formed as aforesaid is not thus dried and matured, it may be at least partially rubbed off; while coatings prepared in accordance with the present invention, including this drying or baking step for maturing the coatings, are adherent to such an extent that coated panels may be bent until they break without the coating chipping off.

The coating formed as aforesaid, either before or after the maturing operation, may, however, be removed from the articles by either of the two principal methods hereinabove given for cleaning the articles, i. e. either by treating the articles with a nitric acid or an alkaline solution including Na2CO3NaOH. Thus panels or other articles may be recleaned or recoated if desired an indefinite number of times.

The present coatings are to be distinguished from other prior art coatings, some of which have included selenium compounds, but which are heavy, rough and black in character and sometimes accompanied by severe pitting and etching of the material surfaces. The coating of the present invention, by way of contrast, is relatively thin, dark gray in color, satin-smooth and very adherent, in no way marring the surface of the material coated. At the same time, the coating of the present invention is effective practically to protect the material for substantial periods of time against many common corrosive influences and is similarly effective whether used alone or whether the articles be thereafter further protected by a coating of paint or lacquer.

While we have disclosed herein the preferred embodiment of our invention and certain alternatives now known to us which will be effective to produce desirable results, we do not wish to be limited except by the appended claims, which are to be construed validly, as broadly as the state of the prior art permits.

What is claimed is:

1. The process of protecting the surfaces of metal articles composed principally of aluminum, which comprises the steps of cleaning the surfaces of the metal to be protected; immersing the cleaned articles to be treated, for a time predetermined in accordance with the thickness of the coating to be formed thereon, in an aqueous solution, the essential active ingredients of which, per liter of solution, consists of: (a) at least one selenium compound selected from the group which consists (as introduced into said aqueous solution) of selenium dioxide, selenious acid, the alkali and zinc salts thereof, selenic acid, and the alkali and zinc salts thereof, and in such amount that the total concentration of selenium in the final solution, calculated as SeOz, will be about to about 300 grams, (b) at least one zinc compound selected from the group which consists (as introduced into said aqueous solution) of zinc chromium selenate (Cr2(SeO4)3 ZnCrO4) zinc oxide, zinc selenate, zinc selenite and zinc phosphate, and in such amount that the total concentration of zinc in the final solution, calculated as ZnO, will be about 10 to about 200 grams, and (c) phosphoric acid in a concentration of from about 12 to about 625 grams, such concentration of phosphoric acid being one which will be sufficient at least to dissolve all the zinc compound present, said solution containing materials resulting from the simultaneous presence in aqueous solution of the ingredients aforesaid; and thereafter maturing the coating thus formed by drying the articles at a temperature in the range of about 120 C. to about 150 C. for at least about two hours.

2. The process in accordance with claim 1, wherein the total concentration of selenium in the said aqueous solution, calculated as SeOz, is about 200 grams per liter.

3. The process in accordance with claim 1, wherein the total concentration of zinc in said aqueous solution, calculated as ZnO, is about grams per liter.

4. The process in accordance with claim- 1, wherein the total concentration of selenium in said aqueous solution, calculated as SeOz, is about 200 grams per liter, and wherein the total concentration of zinc in said aqueous solution, calculated as ZnO, is about 100 grams per liter.

5. The process in accordance with claim 1, wherein the selenium compound ingredient is selenium dioxide.

6. The process in accordance with claim 1, wherein the zinc compound ingredient is zinc chromium selenate [Crz(SeO4)a-ZnCrO4l, and comprising the additional step, performed following the immersion step and prior to the maturing step, of washing the articles to remove green chromium salts from the coating surfaces thereof.

7. The process in accordance with claimv 1, wherein the selenium compound ingredient and the zinc compound ingredient are respectively selenium dioxide and zinc chromium selenate [Cr2(SeO4)s-ZnCrO4l, and comprising the additional step, performed following the immersion step and prior to the maturing step, of washing the articles to remove green chromium salts from the coated surfaces thereof.

8. The process of protecting the surfaces of metal articles composed principally of aluminum, which comprises the steps of cleaning the surfaces of the metal to be protected; forming a coating on the cleaned surfaces by immersing the metal in an aqueous solution, the essential active ingredients of which, per liter of solution, consist (as introduced into thesolution of: (a) about 200 grams selenium ioxide, (b) about 200 grams zinc chromium sel nate (Cr2(SeO4)a-ZnCrO4), and (0) about 300 grams phosphoric acid, said solution containing materials resulting from the simultaneous presence in aqueous solution of the ingredients aforesaid; washing the articles in water to remove green chromium salts from the coated surfaces thereof; and maturing the coating thus formed by drying the metal at a temperature in the range of about C. to about C. for at least about two hours.

9. The process in accordance with claim 1, wherein said aqueous solution contains as an additional active ingredient, phthalic anhydride, in an amount suflicient to provide in the solution as prepared a concentration of this ingredient from zero to about 100 grams per liter.

10. The process in accordance with claim 1, wherein the aqueous solution contains, as an additional active ingredient, phthalic anhydride, in an amount sufficient to provide in the solution as prepared a concentration of this ingredient of about 10 grams per liter.

11. The process of protecting the surfaces of metal articles composed principally of aluminum.

4 9 which comprises the steps of cleaning the surfaces of the metal to be protected; forming a coating on the cleaned. surfaces by immersing the metal in an aqueous solution, the essential active ingredients of which, per liter of solution, consist (as introduced into the solution) of: (a) about 200 grams selenium dioiiide, (b) about 200 grams zinc chromium selenate (Cr2(SeO4) 3 ZnCrOi) (c) about 300 grams phosphoric acid, and (d) about 10 grams phthalic anhydride, said solution containing materials resulting from the simultaneous presence in aqueous solution of the ingredients aforesaid, maintaining said aqueo s solution at a temperature in the range of about 90 C. to its boiling point during the immersion of the articles therein; washing the articles in water to remove green chromium salts from the coated surfaces; and maturing the coating thus formed by drying the metal at a temperature in the range of about 120 C. to about 150 C. for i at least about two hours.

12. A dipping solution for forming a protective coating on the surfaces of cleaned metallic articles composed principally of aluminum, said solution being an aqueous solution,,the essential active ingredients of which, per liter of solution, consist (as introduced into the'solution) of: (a)

about 200 grams selenium dioxide, (1)) about 200 grams zinc chromium selenate (Crz (SeOr) s-ZnCrO4) (0) about 300 grams phosphoric acid, and (02) about 10 grams phthalic anhydride, said solution containing materials resulting from the simultaneous presence in aqueous solution of the ingredients aforesaid.

13. The process of protecting the surfaces of articles of metallic alloy composed principally of aluminum, which comprises the steps of cleaning the surfaces of the metal to be protected, form- 10 ing a coating on the cleaned surfaces by immersing the metal in an aqueous solution, the essential active ingredients of which, per liter of solution, consist (as introduced into the solution) of: (a) about 200 grams selenium dioxide, (1)) about 200 grams zinc chromium selenate.

(0) about 300 grams phosphoric acid, and (d) about 10 grams phthalic anhydride, said solution containing materials resulting from the simultaneous presence in aqueous solution of the ingredients aforesaid; maintaining said solution during the, immersion of the metallic articles v therein'at a temperature in the range of about C. to its boiling point; washing the coating thus formed on the articles in Water to remove green chromium salts therefrom; and maturing the coating thus formed on the articles at a temperature of about C. to about C. for at least about two hours. 4 CHARLES W. CLARK. THOMAS GARNET COLLINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,961,030 Bengough et al May 29, 1934 2,066,611 Christy Jan. 5, 1937 2,186,085 Wein Jan. 9, 1940 2,303,350 Fuller Dec. 1, 1942 OTHER REFERENCES Lange, Handbook of Chemistry, 6th edition, 1946, page 276, published by Handbook Publishers, Inc., Sandusky, Ohio.

W. H. Banks, Journal of the Chemical Society (London), 1934, pages 1010-1012.