US2132438A - Method of coating metal - Google Patents

Description

Patented Oct. 11, 1938 UNITED STATE-S METHOD OF COATING METAL Gerald G. Romig, Elkins Park, Pa., assignor to American Chemical Paint Company, Ambler, Pa., a corporation of Delaware No Drawing. Application December 11, 1933,

Serial No. 701,869

Claims.

This invention relates to the art of coating metals and is particularly concerned with a process for providing metal with a non-metallic coating. More specifically it is directed to an 5 electro-plating process, the principal objects of which are to provide metal with a coating which will protect it from corrosion, to ensure an endur- .ing bond between the metal and an applied finish,

to prevent the development of defects in the apl0 plied finish, to provide metal with a stainless coating and, in general, to greatly improve the art of coating metal so as to afford the maximum degree of protection for a corrodible metal in a minimum of time and at a minimum expense.

A further object is to provide a coating for metal which will be a comparatively poor conductor of electricity when in its dry condition but which, if wet, will have sufllcient conductivity to enable the article to be electro-plated from alkaline electrolytes with copper and other metals. More particularly it is an object of my invention to provide metal with a water-insoluble phosphatio coating.

In its broader aspects the invention consists in using the metal to be coated asan electrode in an electrolyte and passing an electric current into and out of the electrolyte by means of said electrode. A coating produced in accordance with my method will be free from discoloration or stains and will have a' very great ability to retain an applied finish'such as paint, lacquer, japan, synthetic enamels or varnish.

Almost any kind of an electric current may be employed provided, however, that it changes in direction at least once during the coating operation.- Currents that fiuctuate in direction, even though not periodically, will produce a coating in accordance with my invention although,- from a commercial standpoint, I prefer the alternating 0 current of standard frequency (say from 25 to Nevertheless I do not ciently long to give the required ampere-minutes.

I prefer a current density which is in the neighborhood of 50 amperes per square foot of over a wide range without changing the nature or the thickness of the resultant coating.

The metals which I have successfully coated by my improved process are in a class or group which 55 comprises iron, steel, zinc, cadmium, brass, nickel,

metal to be coated although this may be altered chromium, tin and magnesium. All of these metals are capable of forming water-insoluble phosphates. The process has not been found to work on lead, copper, silver, gold or platinum. My experience, therefore, strongly indicates that the process will work on metals which are less noble than lead and which are capable of forming water-insoluble phosphates.

The cations of the electrolyte may be one or more of the metals in that class including iron, zinc, cadmium, calcium, nickel, cobalt and manganese. Cations of the alkali metals may be present without doing harm.

The anions of the electrolyte may be wholly phosphate ions or phosphate ions associated with any or all of the ions included in that class comprising the ions of sulphate, nitrate, chloride and arsenate.

By way of specific example, I will describe my improved process as it may be applied to the coating of a piece of cold rolled sheet steel as, for instance, an automobile fender or other stampmg.

In the first place the stamping is first freed from extraneous matter. The manner in whic this is accomplished depends, of course, on t e nature of the extraneous matter. This in itself forms no part of the present invention although it might be said that scale and rust may be removed with an acid pickle followed by washing with water; oil and grease may be removed by wiping or washing in an oil solvent such as petroleum spirits followed by wiping with a dry cloth; and paint may be removed by boiling in a bath of caustic soda solution or other alkaline cleaning bath and then washing with water.

An electrolyte is then provided, using materials a in the following proportions:

Formula No. 1

Zinc oxide pounds 0.150 75% orthophosphoric acid gallon 0.050 Water to make gallons 1.000

foregoing electrolyte is placed in the vat to completely submerge the stamping which is connected to one terminal of a cycle alternating electric circuit the other terminal being connected to the steel vat. The circuit is then completed and the current adjusted to a density of approximately 50 amperes for every square foot of stamping surface exposed to the electrolyte.

At first there is a rapid evolution of gas from the stamping, but this gradually subsides and reaches a minimum in about four minutes. During'this time a coating gradually forms on the stamping and reaches a maximum thickness about the time that the gassing reaches a minimum. The stamping is then removed from the vat and disconnected from the electric circuit, washed with water and permitted to dry, thus completing the job. I

The electrolyte may be employed at any desired temperature up to its boiling point although I prefer a temperature in the neighborhood of about 180 F. to which end the vat may be supplied with some suitable type of heating means such for example as steam coils. In this connection it should be pointed out that the resistance is less at higher temperatures which, of course, is an advantage although the use of a temperature which is too high will make for excessive evaporation. A compromise therefore should be efl'ected and I find that a temperature of about 180 F. is a good temperature to use.

The time required to produce the coating depends on factors such as current density, the nature of the metal being coated, the concentration of the electrolyte, the ions associated with the phosphate ions and the temperature of the electrolyte. So long as the electrolyte contains the proper cations and anions, the composition, concentration, temperature and current density, are not critical. In practice very satisfactory results are obtained by adjusting the factors so that the coating is completed in about four minutes.

When the coating has completely formed, continuing the electrolyzing does not increase its thickness or alter its composition nor the electrical resistance of the circuit; hence metal that has been coated by the process may be used as a permanent electrode to co-act with a succession of pieces of metal to be coated. This being the case, a steel vat may be used for holding the electrolyte and at the same time may be used as an electrode.

If the metal to be coated is an electrode in the specified electrolyte, the electrical circuit may be modified for convenience or economy. For example, in addition to using a single phase twowire alternating circuit where the vat is one electrode and the metal to be coated the other,- the vat may be made of wood or other non-conducting material and a piece of metal to be coated take its place as the opposing electrode, thus coating two pieces of metal simultaneously; a single phase three-wire circuit may be used where two of the electrode. terminals are pieces of metal to be coated and the other the steel vat; or the three electrode terminals are all different pieces of metal, thus coating three pieces at one time; a two phase three-wire circuit may be used where the vat is one terminal electrode and two separate pieces of metal to be coated, the other two, or pieces of metal to be coated may be the three ter minal electrodes; or a three phase three-wire circult, of either the delta or the Y formation may be used where one of the wires terminates at metal to be coated, another at the metal vat and the third at an aurriliary electrode or one of the wires may be connected to the metal vat and each of the other two, to separate pieces of metal to becoated, or each wire may be connected to a different piece of metal to be coated, thus coating three pieces of metal simultaneously. In all these variations the kind and quality of the coating will be the same, the cost of current however will of course vary inversely with the number of pieces coated with the same current.

In all these variations the pieces to be coated are made electrodes in the electrolyte and every piece is connected in electrical series with the solution, the power source and one or more opposite electrodes, whether these latter be a con ducting vat, an auxiliary electrodeor other pieces of work in process, and the current is passed into an out of the electrolyte by means of the pieces acting as electrodes. Irrespective of the variation selected, the kind and quality of the coating, will be the same, but the cost of the electric current will vary inversely with the number of pieces coated simultaneously with the same current.

Where a steel vat is usedas one of the electrodes, it becomes coated and protected from further action, and, since the coating when wet with the electrolyte remains an electrical conductor, the vat may be used continuously for coating stampings without either wasting away or otherwise corroding. It is also possible where the vat is made of wood or brick or some other non-conducting material, to use some, sort of a plate of conducting material, as one of the electrodes and .the work as the other or, as above indicated, two

stampings. may be immersed in the electrolyte at the same time, acting as oppositeelectrodes so as to coat them both at the same time with the same current.

It, for some purposes, the finished dry coating produced by Formula N0, 1 has too high an electrical resistance, a coating of less electrical resistance may be produced by using an electrolyte made in accordance with the following formula:

Formula N0. 2

Cadmium hydroxide pounds 0.338 75% orthophosphoric acid gallons 0.050 Water to make do 1.000

Various formulas for the electrolyte may be substituted for those already given and still produce satisfactory results without departing from the invention, as illustrated in the following:

Any convenient source of the cation in the electrolyte may be substituted for that given in the formulas such as free metal, oxide, hydroxide, carbonate or any salt whose anion may be tolerated in the electrolyte and the same considerations apply to the anions of the electrolyte.

The process produces a gray water insoluble phosphate coating. Iis composition however varies according to the composition of the electrolyte used and the nature of the metal coated. Generally stated Formula No. 1 produces a zinc phosphate coating; Formula No. 2, a coating containing both cadmium phosphate and metallic cadmium; Formula No. 3, a zinc phosphate coating; Formula No. 4, an iron phosphate coating; Formula No. 5, a coating containing zinc phosphate, cadmium phosphate and metallic cadmium; Formula No. 6, a coating containing zin phosphate and iron phosphate.

After the metal has been coated according to this invention and then washed and dried, it is in condition to receive an applied organic finish such as oil, paint, lacquer, synthetic enamel, japan or varnish, which'may be applied by dipping, brushing or spraying. This may be allowed to air dry or it may be dried in an oven provided its temperature is not high enough to injure the organic finish. Repeated tests indicate that the organic finish holds better to the coating of this invention than it does to the bare metal and remains entirely free from the development of defects usually met-with when the finish is applied directly to the metal, hence the invention is useful in preparing the metal for painting, etc.

As the chemicals used in preparing the electrolyte are inexpensive and commercially available, as the amount of electrical energy consumed is relatively small, and as, the coatings produced are superior to those of the prior art, the process affords a maximum protection against corrosion at a minimum cost' and constitutes a unique advance in the art of protecting .metal phate, the method which comprises electrode-.

positing the coating by using the metal as an electrode in an electrolyte containing phosphate anions and cations of metal of the coating and passing a reversing current through said electrolyte by means of said electrode.

'2. The method of claim 1 wherein the current is a periodically alternating current.

3. In the art of coating metals less noble than lead with water insoluble zinc phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing phosphate anions and the cations of zinc and passing a reversing current through said electrolyte by means of said electrode.

4. The method of claim 3 wherein the current is a periodically alternating current.

5. In the art of coating metals less noble than lead with water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode- Zinc oxide pounds 0.150 75% orthophosphoric acid gallons 0.050 Water to make gallons 1.000

and passing a reversing current through said electrolyte by means of said electrode.

8. The method of claim 7 wherein the electrolyte is employed at a temperature in the neighborhood of about 180 F. and wherein the current is a periodically alternating current.

9. In the art of coating steel with water insoluble metallic phosphate, the method which comprises electrodepositing the coating by using the metal as an electrode in an electrolyte containing the following in approximately the proportions indicated:

Cadmium hydroxide pounds 0.338 75% orthophosphoric acid gallons 0.050 Water to make gallons i 1.000

and passing a reversing current through said electrolyte by means of said electrode.

10. The method of claim 9 wherein the current is a periodically alternating current.

GERALD c. ROMIG.