US3464844A - Aluminum plating of surfaces - Google Patents

Description

United States Patent O US. 'Cl. 11746 18 Claims ABSTRACT OF THE DISCLOSURE Where an object is to be plated with aluminum alkyl decomposition in liquid phase aluminum by heating said object to a temperature above the decomposition temperature of the aluminum alkyl and thereafter contacting the heated article with a solution of aluminum alkyl, the article to be plated is first coated with a heavy hydrocarbon to protect the article from oxidation.

CROSS REFERENCE This is a continuation-in-part of my application having Ser. No. 444,392, filed Mar. 31, 1965, now abandoned.

FIELD OF INVENTION This invention relates to a method of coating a surface with aluminum. In one of its more specific aspects this invention relates to a method of protecting a surface against oxidation prior to contacting said surface With a plating batch. In another aspect this invention relates to liquid bath plating with aluminum.

BACKGROUND In the earlier filed application, I disclosed coating the article to be treated with high boiling waxes and mineral oils. Subsequent to-filing the parent application, I have found that certain diphenyl alkanes and certain dialkyl benzenes are especially suited as coating materials. While I was not restricted to separate heating in the earlier application, I wish to specifically disclosed heating the article to be plated in the coating oil in this application.

Various systems have been used to coat an oxidizable surface such as mild steel with aluminum. Cladding techniques have been employed wherein an aluminum sheet is bonded to the desired substrate or surface by extensive surface treatment of the article followed by a combination of heat and pressure. High operating pressures are necessary, thus limiting the type of articles which could be covered. Also, serious corrosion and operating problems are encountered.

The various well known vacuum techniques produce quite thin films and are subject to low production levels, batch operations and severe size limitations.

The heat decomposition of aluminum compounds is recorded in the literature. Aluminum hydride has been decomposed into aluminum and hydrogen. Vapor phase plating by decomposition of such aluminum compounds has been disclosed.

With gasor vapor phase plating, a number of practical problems are encountered. The systems must be tightly sealed against atmospheric contamination, constant pressure conditions maintained and gas flow rates controlled with narrow limits. Also, many problems exist in vaporizing decomposable materials, and in almost all cases a carrier gas must be provided to give direction to the fiow of the decomposable gas. All additives to the gas stream, such as modifiers, must also be volatile at the operating temperature and the rates of addition care- 3,464,844 Patented Sept. 2, 1969 sive process equipment (see Berger, US. Patent 3,041,-

Berger, in the US. patent supra, makes a major break through in aluminum plating by use of solution plating. In his system a heat decomposable aluminum compound in a liquid or solid solution or dispersion, said liquid or solid being stable and having a boiling point above the decomposition temperature of the aluminum compound, is put into contact with a substrate heated to a temperature above the decomposition temperature causing formation of aluminum on the .substrate. Other materials can be present, for example reducers or oxidizers. The substrate can be of any desirable material so long as said substrate can be heated to a temperature above the decomposition temperature of the aluminum compound. The deposited aluminum film can be anodized by conventional procedures.

In the process of Williams and Kostas in copending application filed Mar. 31, 1965, and having Ser. No. 444,- 431, it is disclosed that the Berger process has certain disadvantages such as excessive smoking, splattering and decomposition of the aluminum compound in the solvent. In this copending application it is shown that much superior coatings are obtained if the solvent for the aluminum compound has a boiling point below the decomposition temperature of the aluminum compound.

It is frequently desired to plate an oxidizable substrate. Such substrates are first cleaned of surface oxidation material and are heated to a temperature above the decomposition temperature of the aluminum compound and subsequently coated by the solution. It is necessary to heat the substrate in a dry box which has been flushed with a inert gas. However, these extremely hot surfaces are very active and are readily oxidized by even trace amounts of oxygen.

It is, therefore, an object of this invention to provide a method of protecting a substrate to be plated at high temperatures against oxidation at said high temperatures. Other objects and advantages of this invention will be apparent from the specification and the claims.

These and other objects of the invention are obtained by coating a substrate prior to heating With a high boiling liquid hydrocarbon.

The surface to be coated should be prefectly clean and free of any oxides. This cleaning can be mechanical such as by abrasion with steel wool, sandblasting, sandpaper, and the like. The cleaning can also be by chemical means such as ammonia solution of citric acid, dilute hydrochloric acid, dilute phosphoric acid and the like. The chemical cleaning agent can be washed off with water and dried with suitable drying agents such as acetone or methanol.

The solvents suitable in the practice of this invention are fully disclosed by Berger and the copending application supra. They include alkanes, aromatics and paraflins, ethers and substituted compounds of this type. These include n-dodecane, 1,2,3,4 tetramethylbenzene, tetralin, naphthalene, diphenyl, anthracene, parafiin distillate, toluene, ethyl benzene, xylene, kerosene and the like. However, hydrocarbons are generally preferred. Suitable bath temperature will depend upon the solvent used and the aluminum compound. The temperature should be below the boiling point of the solvent and also below the decomposition temperature of the aluminum compound. In general, the solution temperature will be between 50 C. and about C. We prefer a solvent that has a boiling temperature below that of the decomposition temperature of the aluminum compound and prefer to maintain the bath at a temperature between 50 and 150 C.

Any aluminum-containing organic compound, capable of decomposing under heat to liberate aluminum and which can be dissolved or suspended in a solvent medium, can be utilized to product an aluminum film on a substrate in the practice of this invention. As a practical matter, due to cost, availability, safety, and the temperature to which the Substrate may be heated, the aluminum alkyls are generally preferred. Examples of such aluminum compounds include aluminum trimethyl, aluminum tri ethyl, tripropyl aluminum, aluminum tributyl, aluminum tri-isobutyl, aluminum tri-isopentyl, aluminum tri-isooctyl and aluminum tridodecyl. Also operable are substituted alkyls, preferably wherein the substituent is a saturated aliphatic radical. Substituents tending to react with the newly deposited aluminum coating would ordinarily be avoided. Also monoand dialkyl aluminums can be used. Examples include ethyl aluminum dihydride and diethylaluminum hydride, di-isopropylaluminum hydride, aluminum dibutyl hydride, aluminum di-isobutyl hydride and the like. In general, the preferred material is a trialkylaluminum or a dialkylaluminum hydride wherein each alkyl radical contains one to four carbon atoms, and, in particular, we prefer aluminum triethyl and diethyl aluminum hydride.

The aluminum alkyl in solvent can vary from to 90 weight percent, and most generally will be present in the range 30 to 70 weight percent.

As has been indicated, the use of various minor components in solvent medium can be employed to cause modification of the physical and chemical properties of the film deposited. Such materials as wetting agents to promote adhesion, oxidizing and reducing compounds can be employed. The precoating of the substrate by the practice of this invention protects the substrate from such compounds and permits it to modify only the coating.

The substrate treatment coating material of thi invention is a heavy hydrocarbon having a boiling point in excess of the decomposition temperature of the aluminum alkyl. They include the well known mineral oils and waxes, diphenyl alkanes which contain two phenyl groups distributed randomly along a normal paraffin, preferably of a C to C backbone and dialkylbenzenes, preferably with each alkyl containing 10 to 14 carbon atoms and mainly meta and para isomers prepared by alkylation of benzene with chloroalkanes. These materials are obtained from petroleu m, generally by fractionation in various temperature range cuts and have no fixed chemical makeup. Such compounds generally have a boiling range from 250 to 350C. It would be expected, therefore, that the coating material would be completely evaporated under the temperature conditions of the furnace (400-550" C.). At these": temperatures one would expect to find decomposition of the mineral oil and fouling of the substrate; however, it has been found that no decomposition occurs, and the substrate is substantially free of oxides. The resulting aluminum plate i far superior to plate prepared under identical conditions except for the coating. Examples of suitable coating materials then include, but are not limited to, mineral oils, paraflin waxes, diphenyl decane, diphenyl undecane, diphenyl dodecane, diphenyl tridecane, diphenyl tetradecane, didecyl benzene, diundecyl benzene, didodecyl benzene, ditridecyl benzene and ditetradecylbenzene.

The aluminum plate produced by the process of this invention is flexible, the coated material, e.g. iron strip, can be bent and twisted repeatedly without breakingthe coating-The coating is more resistant to tape-pull-test than are other coatings. This test comprises pressing tape on the coating and pulling it off. Bright, shiny, lustrous aluminum coatings are obtained.

The substrates to be treated by the method of this invention are ones that can be oxidized. In general this includes metal substrates and particularly metal strips. Most frequently, the metal to be treated will be in iron alloy such as mild steel. However, other metals such as copper strips or articles can be advantageously treated by the method of this invention.

The plating can be done by any suitable method. The plating bath (aluminum compound in solvent) will be maintained at a temperature below the boiling point of the solvent and below the decomposition temperature of the aluminum compound and generally above ambient temperature, however, the initial bath temperature can be at room temperature. The bath temperature will be determined largely by the nature of the solvent, the aluminum compound and the temperature differential of bath and substrate desired. As a general rule a temperature in the range 50 to C. will be employed; however, any temperature up to the boiling point of the solvent can be employed.

The substrate, after cleaning and removal of oxides will be coated with the heavy hydrocarbon and then heated to a temperature above the decomposition temperature of the aluminum compound. This temperature will, of course, depend upon the particular aluminum compound and will be sufficiently high to offset the cooling effect of the bath. The temperature ordinarily employed ranges from about 400 to 550 C. When the substrate is heated to much below 400 C., the coating is tough and resist flexing, however, such coatings are unusually thin. Several treatments can improve the thickness. When the temperature exceeds about 550 C., the coatings tend to flake or crack on flexing. Where these limitations are not important, we can use temperatures from about 300' to 1000 C.

The article to be plated is then contacted by the plating bath while at this high heat. The contacting can be done by dipping the article in the bath, passing it through the bath, spraying with the bath or any other suitable means. Since frequently these solvents are flammable, it is preferred that the bath be in an oxygen-free atmosphere. It is also within the scope of the invention to heat the substrate in the oxygen-free atmosphere. However, with the coating treatment of this invention, the operation can be performed in the natural atmosphere and in small amounts of oxygen in the oxygen-free atmosphere can be tolerated. That is, the system would not require the taking of extensive precautions to exclude all oxygen. However, even in oxygen-free atmosphere, more uniform and stronger coatings have been obtained than when the coating was not applied. When the coating material has sutficiently high boiling point, the coating material can be held at the desired temperature and the article heated in the coating bath and then contacted with the plating bath.

To further illustrate the invention, the following examples are given. It is to be understood that these serve to illustrate the invention and are not to be considered limiting.

EXAMPLE I Several runs were made wherein thin strips of mild steel were plated. The plating operation was carried out in a nitrogen purged dry box. The heating furnace was also in the dry box.

A 50% solution of diethylalminum hydride in a hydrocarbon wash oil was heated in an oil bath to about 150 C. Two clean mild steel coupons (1 x 3 x 0.03 inch) were coated with a thin film of mineral oil. They were then placed in a muffle furnace and heated to 500 C. The coupons were then removed from the furnace and dipped into the plating bath, i.e. the alkyl aluminum solution.

The plated coupons were repeatedly bent and twisted with no adverse effect.

EXAMPLE II The above run was repeated except the two coupons were coated by dipping into a 20% solution of mineral oil in toluene.

Again the strips showed no adverse effect from twisting and bending.

EXAMPLE III This run was the same as run 1 except the coupons were not first coated.

The plating was uniform but failed when bent or twisted.

EXAMPLE IV Oven Conditions Flex Time Temp., 0. Tape pull test test Run:

1 1min... 500 Good Good. 2 1min 500 Having thus described the invention, what is claimed 1. A process for plating aluminum on a metal substrate which comprises preparing a solution of an aluminum capable of decomposing under heat in an organic solvent having a boiling point below the decomposition temperature of the aluminum alkyl, coating said substrate with a heavy hydrocarbon having a boiling point above the decomposition temperature of said aluminum alkyl, heating the coated substrate to a temperature above the boiling point of said heavy hydrocarbon and contacting the heated substrate with said solution.

2. The process of claim 1 wherein said heavy hydrocarbon is a mineral oil.

3. The process of claim 2 wherein the aluminum alkyl is diethylaluminum hydride.

4. The process of claim 2 wherein the aluminum alkyl is triethylaluminum.

5. The process of claim 2 wherein the aluminum alkyl is tri-isobutylaluminum.

6. The process of claim 2 wherein the aluminum alkyl is di-isobutylaluminum hydride.

7. The process of claim 2 wherein the aluminum alkyl is triethylaluminum.

8. The process of claim 2 wherein the aluminum alkyl is di-n-butylaluminum hydride.

9. A process for plating aluminum on a metal substrate comprising preparing a solution of an aluminum alkyl in a hydrocarbon solvent having a boiling point below the decomposition temperature of said aluminum alkyl, said aluminum alkyl being present in the range of 10 to weight percent; coating said substrate with a film of a heavy hydrocarbon having a boiling point above the decomposition temperature of said aluminum alkyl, heating the coated substrate to a temperature above the decomposition temperature of the aluminum alkyl and above the boiling point of said heavy hydrocarbon; and thereafter contacting the hot substrate with said solution.

10. The process of claim 9 wherein the substrate is heated to a temperature in the range 300-1000 C.

11. The process of claim 4 wherein the substrate is heated to a temperature within the range 450550 C.

12. The process of claim 10 wherein the aluminum alkyl is diethylaluminum hydride.

13. The process of claim 12 wherein said substrate is contacted by spraying.

14. The process of claim 10 wherein the aluminum alkyl is triethylaluminum.

15. The process of claim 10 wherein the aluminum alkyl is tri-isobutylaluminum.

16. The process of claim 10 wherein the aluminum alkyl is di-isobutylaluminum hydride.

17. The process of claim 10 wherein the aluminum alkyl is tripropylaluminum.

18. The process of claim 10 wherein the aluminum alkyl is di-n-butylaluminum hydride.

References Cited UNITED STATES PATENTS 2,523,461 9/1950 Young et a1. 11747 2,619,433 12/1952 Davis et al 117-107.2 X 2,698,811 1/1955 Legg 11746 3,041,197 6/1962 Berger 11747 3,075,858 1/1963 Breining 117107 3,155,532 11/1964 Basile 117 47 3,198,167 8/1965 Bakish et a1. 117-l07.2 X 3,305,386 2/1967 Hafiver et al. 117107.2 X

ALFRED L. LEAVITT, Primary Examiner A. GOLIAN, Assistant Examiner US. Cl. X.R. 1l7-50, 105,

iii/ 1f UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3, 464,544 Dated September 2, 1969 Inventofls) Billv J. Williams It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line +3, "disclosed" should be --disclose-.-. Column 2, line 55, "a" should be --an--. Column 3, line 4, "product" should be --produce--. Column 5, line 13, before "alkyls" insert --aluminuJn--. Claim 1, line 3, before "capable" insert --alkyl--. Claim 7, line 2, "triethylaluminum" should be --tripropylaluminum--.

SIGNED AND SEALED MAY 5 1970 (SEAL) Attest:

d d M. Fletcher, 11-

mum: E. *saaumm, m. Attefl g Comisaioner o1 Patents