US3133005A

US3133005A - Process for electrodepositing metallic coatings

Info

Publication number: US3133005A
Application number: US74482A
Authority: US
Inventors: Ades Leon
Original assignee: Hooker Chemical Corp
Current assignee: Hooker Chemical Corp
Priority date: 1960-12-08
Filing date: 1960-12-08
Publication date: 1964-05-12
Anticipated expiration: 1981-05-12

Description

United States Patent No Drawing. Filed Dec. 8, 1960, Ser. No. 74,482 3 Claims. (Cl. 204-38) The present invention concerns the electrolytic coating of metals, especially of ferrous metals, for the purpose of protection or decoration.

It has especially in view the improvement of this coating and the perfection of the manner in which it has been operated up till now by combining in it a stage of preliminary phosphatization.

The customary practice at present is to phosphatize the metals in order to provide them with a protection layer which in turn is covered with a finishing coat of paint or enamel.

According to the invention it has been discovered that contrary to the accepted notion according to which these phosphate layers are electrically isolating, it is possible, by an appropriate choice of the phosphatizing agents and the operating conditions, to use the phosphate coats thus formed subsequently as a layer of anchorage for electrolytically deposited metals.

Thus, according to the invention the metals, and especially the ferrous metals, which are phosphatized by means of diacid phosphates of heavy metals with a layer weight between 0.2 and -g./m. have proved to be particularly suitable to receive subsequently an electrolytic deposit of another metal.

The object of the invention thus is a process of electrolytic coating of metals, especially of ferrous metals, which is particularly remarkable because it consists of phosphatizing these metals with an aqueous solution of diacid phosphate of heavy metal under such conditions that a weight of coating between 0.2 and 10 g./n1. is obtained and proceeding then by the manner which is usual for the electrolytic coating of metals that are thus phospha tized.

The advantages of the present process compared to pre- Vious processes of electrolytic coating are considerable, as will be seen in the following, as far as the adherence of the electrolytic coating and the improvement of the corrosion resistance of this coating is concerned.

The characteristics 'of the process according to the invention will now be examined in more detail.

The diacid phosphates of heavy metals that can be used for the stage of phosphatization are above all the diacid phosphate of Zinc and the diacid phosphate of manganese of which the first is brighter electrolytic coatings.

These phosphates in aqueous solution can be put into operation according to the usual methods of phosphatization by immersion or by spray or atomization at any temperature between the temperature of the environment and the boiling temperature of the solution.

In order to obtain the desired Weight of coating one can adjust the period of contact of the metals with the solution and the oxidizing accelerating agent which is added to the latter. In this respect and although all oxidizing agents classically used as accelerators give satisfactory results it is preferred to use the nitrates or chlorates and their mixtures, and with them present in the solution the results obtained are particularly good.

normal current density, which is electrodepositing solution.

3,133,005 Patented lVlay 12, 1964 The pieces thus phosphatized aresubsequently rinsed in water, and thereafter in a solution whose pH correspondsto that of the solution to be used to form the electrolytic coating or deposition. The surfaces are then immersed in this solution with the current on so as to avoid any redissolution of the phosphate deposit in the actual The electroplating solutions may be the classical alkaline or acid electroplating solutions used under normal working conditions, except that initially one operates under a higher current density, e,'g., up to 10 times the then reducedto normal after a few seconds.

preferred because it permits The metals that can be deposited on the phosphatized pieces are all those that are anodic with regard to the base metal or which are made anodic with regard to the latter through an increase of its potential by the phosphate layer.

On iron pieces one can thus deposit electrolytically zinc, nickel (mat or bright), chromium, copper, brass, tin, and cadmium which is normally not anodic with regard to the iron.

In order to illustrate the invention in greater detail some representative examples for carrying out the present process for coating pieces of ferrous metal or of alloys of zinc or aluminum are set forth below.

It is to be understood, however, that the compositions given are illustrative only and the operating conditions are specific for the given solutions-only. Other'oxidizing agents, concentrations .and operating conditions can be used to produce the desired 0.2-10 grams per square meter coating and, once it is rinsed in a solution having the acidity of the contemplated electroplating solution under a similar current density condition, the phosphate coated surface is prepared to receive an electrodep'osit.

Example I G./liter Diacid zinc phosphate 18 Zinc nitrate s 7 4 Phosphoric acid .24.. 3 Sodium chlorate Y r 7.5

The characteristics of thisrbath are as follows: i Free acidity points 4 to 4.8 Total acidity do 27 to 30 Temperature C 65 to Time of treatment per immersion minutes 5 to 10 The weight of the-layer obtained-is from 1.5 to 8.g./m.

Example II V i i v G./liter Diacid zinc phosphate 7.5 Zinc nitrate 10 Phosphoric acid 3.4

The characteristics of this bath are as follows:

Total acidity points 20 Temperature C to G./liter Diacid manganese phosphate 48 Manganese nitrate 6 Phosphoric acid 7 3 The characteristics of this bath are as follows:

Free acidity points 9 to 11 Total acidity do 60 Temperature C 95 Period of immersion minutes 15 The weight of the layer obtained is from 2 to 10 g./m.

The weight of the layer obtained is from 1 to 6 g./m.

Aiter phosphatiziation in any one of the aforementioned baths, the pieces are rinsed in water, then with a diluted solution of sodium cyanide (it the bath of electrolysis is alkaline) or of hydrochloric acid (it the bath of electrolysis is acid). The quantity of the sodium cyanide or hydrochloric acid used in the aqueous solution was controlled so as to adjust the pH of the aqueous solution to a value approximating the pH of the subsequent electroplating solution. The aqueous solution containing the sodium cyanide or hydrochloric acid at the appropriate pH was contacted with the phosphated surface of the workpiece electrically charged as a cathode under a current density con -'tion-corresponding substantially to that to be employed in the subsequent electroplating step.

These pieces are subsequently immersed under current in any one of the following baths of electrolytic deposit depending upon the coating that one wishes to obtain.

Example V-Cadmium- Bath l Example VIII-Brass Bath Copper cyanide g./liter Zinc cyanide do 10 Sodium cyanide do 50 Sodium carbonate do 30 Density of normal current I amp./=drn. 0.3 Temperature C 20 to 30 Tension ..v 2

Variable period of treatment: Fixed at 20 minutes for the tests.

Example IXTinning Bath Sodium stannate g./ liter 80 Caustic soda do 13 Density of normal current "amp/din? 1 Tension v 4 to 6 Temperature C i p 75 Variable time: fixed at 15 minutes for the tests.

Example X-Nickeling Bath Nickel sulfate -g./liter 250 Nickel chloride do 40 Boric acid do 40 Density of normal current a1np./dm. 2 Tension v Temperature C to Variable time: Fixed at 10 minutes for the tests.

The tests described in these examples support the observation that in all the cases the adherence of the electrolytic coating is excellent. However, the deposits made on zinc phosphate according to Examples I, II and IV are brighter than on manganese phosphate according to Example HI. 7

Moreover, these tests clearly show the following advantages of the present process as compared to previous Double cadmium and sodium cyanide g./liter 40 Sodium cyanide do 30 Caustic soda do 10 Usual brightening agent do 3 Density of normal current ..a.mp./dm. 1 to 1.5 Tension v 1 to 3 Temperature of the environment. 7 Variable time: Fixed at 20 minutes for the tests.

Example VI-Zinc Bath Zinc cyanide g./liter ,60 Sodium cyanide do 42 Soda ..do.. Usual brightening agent -cm./ liter..- 20 Density of normal current .amp./dm. 3 Tension v 3 to 4 Temperature otthe environment. Variabletime: Fixed at 20 minutes for the tests.

. Example .VIl..-Copper Bath v Copper cyanide g./liter 30 Sodium cyanide do 37.5 Carbonate do 25 Caustic soda do 10 Rochelle salt do 45 Density of normal current amp./dm. 2to 3 Temperature C 50 to 55 processes of electrolytic coating. The objects w ch are phosphatiz/ed and coated electrolytically according to the.

invention show a resistance to corrosion which is much greater than that of objects which are coated simply electrolytically. Thus the pieces which were phosphatized according to any one of the Examples I to IV and cadmium-plated according to Example V and on which an initial point of corrosion was marked have resisted 288 hours in an industrial acid atmosphere as compared to 104 hours for pieces which were only cadmium-plated according to Example V and provided with an initial point of corrosion.

Besides, it should be emphasized that the preliminary phosphatization of the pieces which are to be coated electrolytically is actually not an additional operation, for it replaces advantageously the electrolytic degreasing and the anodic attack in acid environment normally used. Finally, the phosphate layer which is deposited according to the invention favors a better dispersion of the lines of the current and hence a better penetration of the electrolytic deposits. (The needle effect being distributed over the whole surface of the pieces.)

What is claimed is:

l. A process tor forming electrodeposited coatings on metal surfaces which comprises the steps of contacting said metal surface with an aqueous acidic phosphate coating solution selected, from the group consisting of zinc and manganese phosphate solutions to form thereon a phosphate coating having a weight in the rangeot 0.2-10 grams per square meter, thereafter contacting said phosphate coated surface as a cathode with an aqueous solution containing a soluble compound selected from the group consisting of hydrochloric acid and sodium cyanide in an amount to provide a pH of a value herein.- a-fiter defined and for a period of time sufficient to effect stabilization of said phosphate-coated surface, thereafter electroplating a metallic, coating directly on said phos: phate coating, said aqueous solution having been adjusted 3,133,005 5 6 to the approximate pH of the electroplating solution and References Cited in the file of this patent said aqueous solution being applied under approximately UNITED STATES PATENTS similar current density conditions as employed in said electroplating step. 2,132,438 Rflmig 00L 1938 2. A process in accordance with claim 1 wherein said 5 FOREIGN PATENTS phosphate coated metal is electroplated with a metal 345119v F i 8 19 selected firom the group consisting of zinc, nickel, chrorance May 39 mium, copper, brass, tin and cadmium. OTHER REFERENCES 3. A process in accordance with claim 1 wherein said L0 Presti: Metal Finishing, October, 1942, pages 533- phosphate coating comprises a zinc phosphate coating. 10 536.

Claims

1. A PROCESS FOR FORMING ELECTRODEPOSITED COATINGS ON METAL SURFACES WHICH COMPRISES THE STEPS OF CONTACTING SAID METAL SURFACE WITH AN AQUEOUS ACIDIC PHOSPHATE COATING SOLUTION SELECTED FROM THE GROUP CONSISTING OF ZINC AND MANGANESE PHOSPHATE SOLUTIONS TO FORM THEREON A PHOSPHATE COATING HAVING A WEIGHT IN THE RANGE OF 0.2-10 GRAMS PER SQUARE METER, THEREAFTER CONTACTING SAID PHOSPHATE COATED SURFACE AS A CATHODE WITH AN AQUEOUS SOLUTION CONTAINING A SOLUBLE COMPOUND SELECTED FROM THE GROUP CONSISTING OF HYDROCHLORIC ACID AND SODIUM CYANIDE IN AN AMOUNT TO PROVIDE A PH OF A VALUE HEREINAFTER DEFINED AND FOR A PERIOD OF TIME SUFFICIENT TO EFFECT STABILIZATION OF SAID PHOSPHATE-COATED SURFACE, THEREAFTER ELECTROPLATING A METALLIC COATING DIRECTLY ON SAID PHOSPHATE COATING, SAID AQUEOUS SOLUTION HAVING BEEN ADJUSTED TO THE APPROXIMATE PH OF THE ELECTROPLATING SOLUTION AND SAID AQUEOUS SOLUTION BEING APPLIED UNDER APPROXIMATELY SIMILAR CURRENT DENSITY CONDITIONS AS EMPLOYED IN SAID ELECTROPLATING STEP.