US3531380A - Method of pretreating ferrous metal substrates prior to electroplating with an aluminum-containing coating - Google Patents

Description

US. Cl. 20434 11 Claims ABSTRACT OF THE DISCLOSURE Improved aluminum-containing electrodeposited coatings are produced on ferrous metal substrates by pretreating the substrate with hydrogen halide, iron halide, or ammonium halide to activate the surface, and then electrodepositing th aluminum-containing coating thereon.

This invention relates to a novel method of electrodepositing an aluminum-containing coating on a ferrous metal substrate. In one of its more specific variants, the invention is concerned with a novel pretreatment of the ferrous metal substrate for the purpose of activating the surface prior to electrodepositing the aluminum-containing coating thereon.

The invention will be illustrated and described hereinafter with specific reference to the electrodeposition of aluminum or aluminum-manganese alloy coatings on a ferrous metal strip using an inorganic fused salt electrolyte containing predominate proportions of aluminum chloride. However, it will be apparent to those skilled in the art that the principles of the invention are applicable to other base metals, forms of base metals, aluminum-containing coatings, or other electrolytes for electrodepositing aluminum.

A fused salt electrolyte having a composition of approximately 7585% by weight of aluminum chloride, and the remainder of sodium and/or potassium chloride, is useful in producing aluminum-containing electroplated coatings. The electrolyte composition may be of the binary or ternary type. A satisfactory binary electrolyte is exemplified by a bath containing by weight 80% aluminum chloride and 20% sodium chloride or potassium chloride, and a satisfactory ternary electrolyte is exemplified by a bath containing by weight 80% aluminum chloride, sodium chloride and 10% potassium chloride. A bath containing 8085% by weight of aluminum chloride produces some objectionable visible fuming in the presence of water, and electrolytes containing more than 85% aluminum chloride fume excessively and the plating results usually are not improved by increasing the aluminum content above this value. If the aluminum content falls below about 75% by weight, poor plating results when using unmodified direct current in the absence of special plating conditions. However, it is possible to use electrolyte compositions containing less than 75% by weight aluminum chloride and the remainder alkali metal chloride when using special plating conditions and/or when using low current densities. For example, nonfuming baths maintained in the molten condition at a temperature of about 190200 C. or higher, and containing less than 70% by weight of aluminum chloride, i.e., substantially no free aluminum chloride, may be used to electroplate satisfactory adherent nonpowdery aluminum coatings when using straight direct current at current densities less than one ampere per square foot, when using polarity reverse current at current densities up to two amperes per square foot, when using pulsed direct current, and up to amperes per square foot when using a hot cathode maintained at a temperature nited States Patent 0 3,531,380 Patented Sept. 29, 1970 much higher than the bath temperature. At higher bath temperatures within the operative range, current densities may be raised to some extent without causing the deposition of powdery coatings or other imperfections. With a slightly fuming aluminum chloride-alkali metal chloride bath, i.e., electrolyte baths containing about 70% by weight aluminum chloride and the remainder alkali metal chloride, which have a free aluminum chloride content of about 05-15%, the current density may be up to 5 amperes per square foot when straight direct current is used, up to 10 amperes per square foot when polarity reverse current is used, above 15 amperes per square foot when using a hot cathode, and considerably above 18 amperes per square foot when using pulsed direct current. When using a fuming aluminum chloride-alkali metal chloride bath, such as a bath containing by weight aluminum chloride, 10% sodium chloride and 10% potassium chloride, current densities up to amperes per square foot and often as high as 100-400 amperes per square foot, may be used with straight direct current, and the current densities for special plating conditions are equally satisfactory or even better. Thus, it is apparent that a wide variety of aluminum chloridealkali metal chloride electrolyte baths are suitable for the electrodeposition of aluminum on a ferrous metal base, and that any suitable bath of this type may be used in practicing the present invention. Suitable electrolyte baths and plating conditions for use in practicing the present invention are disclosed in US. Pat. Nos. 3,136,709, 3,167,403; and 3,259,577, the teachings of which are incorporated herein by reference. Aluminummanganese alloy coatings containing 1090% and preferably 1030% by weight of manganese, may be electrodeposited by using aluminum-manganese alloy anodes and/or incorporating about 0.55% by weight of a soluble manganese salt, such as manganese chloride, in the above baths, as disclosed in Pat. No. 3,167,403.

Still other types of aluminum electroplating baths are known and may be used when practicing the present invention. For instance, one or more of the chloride salts of the baths mentioned herein may be replaced with other suitable halide salts of the same metal, in the molar proportions existing in the above mentioned baths, such as aluminum bromide, sodium bromide and potassium bromide. Also, the aluminum chloride may be complexed with complexing agents such as ammonia and organic amines or their salts in a manner somewhat analogous in effect to the complexing of aluminum chloride in the foregoing baths with potassium and/or sodium chloride. An aluminum chloride-ammonia bath contains complexed aluminum chloride which is thought to be in the form of AlCl -NH or possibly higher members of the ammonia series with varying amounts of free aluminum chloride being dissolved therein in the case of a fuming bath, or a nonfuming bath may be used as discussed above for the more common aluminum chloride-alkali metal chloride baths. Additionally, aluminum chloride may be complexed by means of organic amines and their salts such as ethyl pyridinium ibromide, to produce complex ethyl pyridinium bromide-A101 This complex may have free aluminum chloride dissolved therein, or it may be a nonfuming bath such as discussed above with little or no free aluminum chloride.

The fused salt aluminum electroplating baths discussed herein may be considered to be, broadly speaking, electrolyte baths containing aluminum halide and a complexing agent therefor, with the aluminum halide complex being in the molten state and preferably having free aluminum halide dissolved therein. Such fused salt electrolyte compositions presently known to the art are suitable for electroplating aluminum-containing coatings and may be used in practicing the present invention.

Organic type baths for electroplating aluminum are also known. Such baths may contain one or more organic compounds such as an organic solvent or organo metallic compounds. The composition, preparation and use of organic baths for the electrodeposition of aluminum-containing coatings are described in numerous publications and patents, including US. Pats. No. 1,911,122; 1,939,397; 2,170,375; 2,446,349; 2,651,608; 2,763,605; 2,849,349; 2,902,416 and 3,268,421.

It is understood that when substituting other halide salts for chloride salts in the electroplating baths discussed herein, the substitutions should be made in molar proportions rather than in proportions by weight, so as to provide the same molar proportion of aluminum halide, the complexing agent and the free aluminum halide. Unless stated to the contrary, the parts and percentages of the ingredients of the electrolyte are calculated by Weight.

When electroplating an aluminum-containing coating in accordance with the present invention, the electroplating baths described herein may be used without departing from the prior art operating conditions. No changes in the electroplating conditions are necessary, and it is only necessary that the ferrous metal base be pretreated in accordance With the invention prior to the electrodeposition of the aluminum-containing coating thereon.

The above described electroplating baths are often contaminated initially due to impurities in the ingredients, or the baths become contaminated with impurities with use. In such instances, it is difficult to obtain uniform, high quality, nonporous and adherent coatings. The nature of the impurities which cause the difficulty is not fully understood at the present time. However, it is known that impure aluminum chloride, which normally produces an unsatisfactory bath, may be purified by sublimination to produce a sublimed aluminum chloride product which results in satisfactory electrodeposited coatings. It has been further discovered unexpectedly that plating difficulties due to impurities in the bath may be overcome by pretreating the ferrous metal substrate with certain halogen-containing treating agents to be descirbed more fully hereinafter, which activate the surface in some unknown but surprisingly effective manner. The pretreatment of the invention is effective in the electrodeposition of pure aluminum coatings, as well as in the electrodeposition of aluminum-manganese alloy coatings, or other types of aluminum-containing coatings.

It is an object of the present invention to provide a novel method of electrodepositing aluminum-containing coatings on ferrous metal substrates.

It is a further object to provide a novel method of pretreating ferrous metal substrates to activate the surface and assure the electrodeposition of adherent, nonporous, more uniform and higher quality aluminum-containing coatings thereon from baths containing impurities which normally have an adverse effect.

Still other objects and advantages of the invention will be apparent to those skilled in the art upon reference to the following detailed description and the examples.

It has been discovered that the adverse effects of electrolyte impurities on the quality of the electrodeposits produced therefrom may be reduced markedly or eliminated by pretreating the ferrous metal substrate with a halogen-containing substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof to activate the surface. The preferred halides are the chlorides and bromides. It is also preferred that the halogen content of the halide be the same as that used in preparing the electrolyte. For example, when an aluminum chloride-alkali metal chloride fused salt bath is used, it is preferred that a chloride be employed in activating the substrate surface.

When a gaseous hydrogen halide such as gaseous hydrogen chloride is used, the ferrous metal surface is intimately contacted therewith for a sufficient period of time to result in activation, such as 1-60 and preferably 1030 seconds. The gaseous hydrogen chloride may be mixed with air or an inert gas such as nitrogen or helium, or substantially pure hydrogen chloride may be used. A small amount of moisture, which may be in the form of steam or other source of water or water vapor, may be present as this seems to increase the rate of activation. As a general rule, the ferrous metal surface is sufficiently activated when a blue to blue-green haze appears thereon. In most instances, it is preferred that the atmosphere contacted with the substrate contain about 10-50% by volume of the gaseous hydrogen halide, about 1-10% by volume of water vapor, and the remainder a gas or gaseous mixture which does not interfere with the activation such as air, helium, nitrogen or other inert gas. The substrate surface may be contacted with the gaseous mixture at ambient temperature, but higher temperatures may be employed such as, for example, up to the operating temperature of the electrolyte bath, or 10-100 F. thereabove.

It is also possible to use aqueous hydrogen halide, such as aqueous hydrochloric acid and hydrobroniic acid. Preferably, the acid is diluted with water to provide an aqueous solution containing 1-10% and preferably about 5% by weight of the hydrogen halide. The ferrous metal substrate is activated by contacting it with the aqueous acid such as by spraying the acid on the surface, or by immersing the substrate in a bath of the acid. The wet ferrous metal substrate is dried before electroplating so as to form a dried film thereon of the reaction product of the acid and the ferrous metal. The substrate should not be washed after the pretreatment as this renders the activation ineffective.

The ferrous metal substrate may also tie activated by immersion in or spraying with an aqueous solution of an iron halide or ammonium halide. Examples of preferred iron and ammonium halides include ferrous chloride and bromide, ferric chloride and bromide, and ammonium chloride and bromide. The aqueous solutions may contain 1-10% and preferably about 5% by weight of the dissolved iron halide or ammonium halide and the remainder water. After it has been intimately contacted with the aqueous iron or ammonium halide solution, the wet substrate is dried to deposit a film of the dried reac tion products thereon. As is true of the previous treat ments, the metal surface should not be washed as this tends to remove the deposited substances which are responsible for the activation.

The concentrations of the substances which are used in treating the substrate, whether in liquid or gaseous form, may be varied over wide ranges. It is only necessary that the ferrous metal substrate be treated with the treating agent in a concentration and for a period of time sufiicient to impart the desired activation. At lower concentrations, longer periods of treatment are required, whereas at higher concentrations, shorter periods of treatment are necessary. Additionally, for a given concentration of treating agent, shorter periods of treatment are needed at higher temperatures, whereas at lower temperatures longer periods of treatment are required. The specific treating conditions to be selected in a given instance are within the skill of the art once the principles of the invention are understood. It is only necessary that the substances originally present in the treating agent, or the reaction products thereof with a clean ferrous metal surface, be deposited thereon in an amount and for a sufilcient period of time to activate the strip surface. When the aqueous solutions of the various treating agents discussed above are used, the usual film thickness remaining on the ferrous metal surface, e.g., about 0.0005 0.001 inch, will provide a satisfactory treatment when dried. Still another method of determining whether or not satisfactory activation is obtained is by observing the appearance or color of the clean ferrous metal surface before receiving the activation treatment and after receiving it. For instance, the activated surface has a bluish to bluish-green haze or color thereon, as distinguished from the initial bright metallic appearance.

Once the ferrous metal surface has been activated in accordance with the invention, the remaining steps in the prior art processes for the electrodeposition of aluminum-containing coatings, including aluminum-manganese alloys, on ferrous metal substrates may be practiced. Prior art processes are disclosed, for example, in U.S. Pats. No. 3,259,557; 3,136,709; 3,167,403 and 3,268,421. In general, the ferrous metal substrate to be electroplated is cleaned electrolytically in an alkaline solution, such as in a solution of orthosil, anodically and cathodically, followed by a cold water rinse, electrolytic pickling in dilute sulfuric acid, such as 17% sulfuric acid, followed by a cold water rinse, a hot water rinse, and drying of the wet strip. The strip surface is then activated in accordance wth the invention, followed by immersion in the electrolyte and electrodeposition of the aluminum-containing coating thereon.

When using a fuming fused electrolyte described herein containing 7585% by weight of aluminum chloride and the remainder sodium and/or potassium chloride, the operating temperature of the bath is about 250400 F., and for better results about 300350 F. Additionally, it is preferred that the pretreated ferrous metal strip be heated as disclosed in US. Pat. No. 3,136,709, to a temperature within about 25 F. of the operating temperature of the electrolyte before the electrodeposition commences. It is also preferred that the iron salt concentration be 0.001-0.02% and preferably 0.0030.01% by weight of EXAMPLE I Ferrous 'metal strip of tinplate gauge is cleaned by a prior art process such as described in Pat. No. 3,259,557 to provide a clean dry steel surface which is free of contaminates. The strip is then pretreated by contacting it with 50% by volume of gaseous hydrogen chloride in air for about -20 seconds. The resulting pretreated surface of the strip has a blue haze thereon, as distinguished from the original bright metallic appearance.

The pretreated or activated steel strip is heated to approximately 350 F., and passed into an enclosed body of a fused salt electrolyte containing by weight 80% aluminum chloride, 10% sodium chloride and 10% potassium chloride. The electrolyte also has dissolved therein an iron salt in an amount of 0.01% by weight and it is prepared from impure commercial aluminum chloride which has not been purified :by sublimination. The electrolyte is maintained at a temperature of 325 F.

The strip is passed between spaced aluminum anodes immersed in the electrolyte and a current density of 42.5 amperes per square foot is used to thereby electroplate an aluminum coating 30 millionths of an inch thick thereon. After plating, the coated strip is passed from the electrolyte, washed with water and dried.

The resulting aluminum-coated strip is tested for corrosion resistance, porosity and adherency of the coating by prior art test procedures. The aluminum coating is uniform, highly adherent to the steel surface, nonporous, and provides excellent corrosion resistance and protection for the steel substrate.

EXAMPLE I[ The procedure of Example I is repeated in this example with the exception of eliminating the pretreatment of the invention, i.e., the gaseous hydrogen chloride treatment.

The resulting electroplate is tested following the same prior art procedures as in Example I.

The electroplate produced in this example is noticably not as uniformly coated, nor is the appearance as good as that produced in Example I. Upon testing for porosity of the aluminum coatings and corrosion resistance, the electroplate of this example is found to be much inferior in these respects to the electroplate of Example I.

EXAMPLE III The procedure of Example I is followed in this example with the exception of eliminating the gaseous hydrogen chloride pretreatment and pretreating the ferrous metal strip with the following aqueous solutions:

(A) 5% by weight aqueous hydrochloric acid; (B) 5% by weight aqueous ferric chloride; and (C) 5% by weight ammonium chloride.

The strip is prepared by passing it through a bath of one of the above aqueous treating agents, then passing it through a pair of squeegee rolls to thereby provide about 0.0005-0.001 inch of liquid on the strip, and drying. The resulting activated strip has a bluish or blue-green cast.

The strip is electroplated with aluminum following the procedure of Example I, and the electroplate is tested for corrosion resistance and adherence of the coating following the same procedures. Test results comparable to those of Example I are obtained with the exception of with the aluminum chloride treatment, which is not quite as effective.

EXAMPLE IV The procedures of Examples I through III are repeated in this example with the exception of employing aluminum-20% maganese by weight alloy anodes and an electrolyte containing 1% by weight of dissolved manganese chloride to thereby deposit an aluminum-manganese alloy on the strip surface. The resulting electroplated products are then tested following the procedures of Examples I through III.

The electroplated products of Examples I and III have uniform, nonporous and highly adherent and corrosion resistant alloy coatings thereon which provide excellent protection for the steel substrate. However, the alloy coating on the electroplate produced in accordance with Example II is nonadherent, and thus the electroplate of Example II is unsatisfactory for use as a high quality container stock.

What is claimed is:

1. In a method of electroplating an aulminum-containing coating on a ferrous metal substrate wherein the substrate is electroplated while immersed in a body of fused salt electrolyte comprising a major amount of aluminum halide, a complexing agent therefor and impurities which normally adversely affect the quality of the electroplate, the improvement which comprises pretreating the ferrous metal substrate by intimately contacting it with a halogen-containing substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof to activate the surface with reaction products produced by the pretreatment and improve the quality of electroplate produced therefrom, and immersing the pretreated ferrous metal substrate in the electrolyte and electroplating the activated surface thereof with an aluminum-containing coating selected from the group consisting of aluminum and aluminummaganese alloys, the activated surface of the pretreated substrate having a dry film of said reaction products on its surface when immersed in the electrolyte and the substrate being immersed in the electrolyte in the absence of a water washing step after pretreating with said halogen-containing substance and prior to immersing in the electrolyte.

2. The method of claim 1 wherein the electrolyte comprises a major amount of aluminum chloride and the 7 halogen of said halogen-containing pretreating substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof is chlorine.

3. The method of claim 2 wherein the ferrous metal substrate is pretreated by intimately contacting it with gaseous hydrogen chloride.

4. The method of claim 2 wherein the ferrous metal substrate is pretreated by intimately contacting it with aqueous hydrogen chloride.

5. The method of claim 2 wherein the ferrous metal substrate is pretreated by intimately contacting it with aqueous iron chloride.

6. The method of claim 1 wherein the substrate is electroplated while immersed in a fused salt electrolyte containing about 75-85% by Weight of aluminum chloride and about 25-15% by Weight of'alkali metal chloride selected from the group consisting of sodium chloride and potassium chloride.

7. The method of claim 6 wherein the halogen of said halogen-containing pretreating substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof is chlorine.

8. The method of claim 7 wherein the substrate surface is contacted with said halogen-containing pretreating substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof until it has a blue to blue-green appearance.

9. The method of claim 7 wherein said halogen-containing pretreating substance selected from the group 30 consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof is gaseous hydrogen chloride,

and the gaseous hydrogen chloride is intimately contacted with the substrate surface in the presence of moisture.

10. The method of claim 7 wherein said halogen-containing pretreating substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof is aqueous hydrogen chloride, and a film of the aqueous hydrogen chloride is applied to the substrate surface and is dried thereon prior to immersing the substrate in the electrolyte.

11. The method of claim 7 wherein said halogencontaining pretreating substance selected from the group consisting of hydrogen halide, iron halide, ammonium halide and mixtures thereof is aqueous ferric chloride, and a film of the aqueous ferric chloride is applied to the substrate surface and is dried thereon prior to immersing the substrate in the electrolyte.

References Cited UNITED STATES PATENTS 2,728,696 12/1955 Singer 148-6.14 X 2,899,367 8/1959 Veeder 204-29 X 2,971,899 2/1961 Hanink et a1. 20429 3,007,854 11/1961 Smith et al. 204-34 X 3,167,403 1/1965 Smith et al. 29196.2 3,259,557 7/1966 Smith et a1. 204-39 X 3,268,421 8/1966 McGraw 20439 WINSTON A. DOUGLAS, Primary Examiner O. F. CRUTCHFI'ELD, Assistant Examiner