US2862860A - Cadmium electroplating - Google Patents

Description

cAnMnJM ELECTROPLATING Peppino N. Vlannes, Fort Washington Forest, Md., and Simon W. Strauss and Benjamin F. Brown, Washington, D. C.

No Drawing. Application September 5, 1957 Serial No. sszmn 8 Claims. (Cl. 20450) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a process for the electrodeposition of cadmium and to new electroplating baths therefor.

Experience with cadmium plated high-strength steels has shown that fracture may occur in service after a period of time under an essentially constant stress much lower than the yield strength. This phenomenon, described variously as delayed fracture or static fatigue, is known to be associated with the presence of hydrogen in the steel. In recent years there has been a mounting incidence of service failures in which cadmium electroplated high-strength steel articles fail by delayed fracture.

It has long been known that hydrogen is introduced into the steel and other metals in the electroplating with cadmium from the cyanide bath. It is a characteristic of the cyanide bath process that both hydrogen and cadmium are deposited at the surface of the cathode. Part of the deposited hydrogen is eliminated by bubblingaway, but part of it becomes dissolved in the metal undergoing plating. The presence of this dissolved hydrogen reduces the fatigue strength of the metal and renders it susceptible to delayed fracture through hydrogen embrittlement.

It is an object of the present invention to provide a new process for electroplating cadmium. It is a further object to provide a process for cadmium electroplating in which deposition of hydrogen and consequent hydrogen embrittlement of the metal is markedly reduced over that occurring in the cyanide bath process. It is another object to provide new electroplating baths. It is also an object to provide new electroplating baths which are non-cyanide in composition.

We have found that the above and other objects can be accomplished in accordance with our invention by electrodepositing cadmium from a bath comprising an aqueous ammoniacal solution of a cadmium salt other than a cyanide and an alkyl monoaminomonocarboxylic acid of 2 to carbon atoms in which the molar ratio of amino acid to cadmium is from 1:1 to 2:1 and the pH may range up to about 11.2, preferably up to about 10, and optimally is from about 9 to 10. In the alkaline range of pH, the amino acid is present in the aqueous ammoniacal solution as a complex with cadmium.

It is known that the ease with which hydrogen is discharged in an electrode process depends partly on the concentration of hydrogen in the bath, i. e., on the pH of the bath. To lessen hydrogen discharge, therefore, it is preferable to work in the alkaline range. However, over a wide range of pH, cadmium tends to precipitate from ammoniacal'solution as the hydroxide unless it is properly complexed. The presence of cadmium in the electroplating baths of our invention as a complex with the amino acid permits working in the alkaline range acid, etc.

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without precipitation of cadmium as the hydroxide. The presence of the cadmium-amino acid complex also contributes to obtaining a good plate. The complex formed in the ammoniacal solution between the cadmium and the amino acid is a chelate and is thus distinguished from the complex of cadmium and ammonia which is characterized by being a salt in which four NH groups are attached to cadmium.

The electroplating baths of the invention can be made up in simple manner by dissolving the cadmium salt and the amino acid in a volume of water or of dilute ammonium hydroxide which is slightly less than that of the final volume of the solution and adjusting the pH and/ or volume of the solution to the desired level by addition of concentrated ammonium hydroxide and water as required. Suitable cadmium salts for preparation of the new baths are, for example, cadmium sulfate, chloride, bromide, acetate, citrate, tartrate, etc. Suitable amino acids therefor are, for example, glycine, /3-alanine, a amino-n-butyric acid, a-amino-isobutyric acid, [it-aminon-valeric acid, 'y-amino-n-valeric acid, ,B-amino-isovaleric The concentration of c-adimum in the bath may range between about 0.5 and 2 molar and preferably is between about 1 and 2 molar. The amino acids can be present in the bath in molar concentrations from one to two times that of the cadimum. The molar ratio of cadmium and amino acid in the bath, as stated above, may range between 1:1 and 1:2, with no apparent advantage to be had by the use of higher ratios of amino acid to cadmium.

The composition of typical electroplating baths in accordance with the invention are contained in the table below in which the weight of the starting components is in grams per liter of aqueous ammoniacal solution.

Weight Weight pH of Amino Acid Amino 30 (18 04.81350 solution Acid Glycine 305 9-10 fl-ala-nine 320 465 910 a-amino-n-butyric acid. 150 9-10 a-amino-isobutyric acid. 150 190 9-10 on iron, copper and copper-rich alloys and on other metals to be plated with cadmium. From the practical standpoint, the electroplating is carried out at room temperature, although the process may also be conducted at temperatures which are considerably below or above room temperature, as low as about 4 C. and as high as the boiling point of the ammoniacal solution of the amino acid-cadmium complex, with the production of a uniform, adhering deposit of cadmium on the metal to be plated using current densities of from .0027 to .027 amps/cm. (2.55 to 25.51 amps/ft?) An average current density for operation of the electrodeposition process of the invention is 15 amps/ft? Comparative tests involving electroplating a cylindrical notched bar of A181 4340 steel of approximately 285,000 p. s. i. ultimate tensile strength as substrate from baths of the composition set forth in the table above and from a standard cadmium-cyanide bath demonstrated the marked reduction in hydrogen deposition and hydrogen embrittlement which can be obtained by electroplating with the process of the invention. The baths of the .3. composition of the invention were at near saturation with respect to the amino acid and ammoniated to the optimum pH range. The current density therefor was 15 amps/ft. in each case. The current density for the standard cadmium-cyanide bath was 25 amps/ft The platings were carried out at room temperature for a plating time of 15 minutes. The plate thickness on the barrel of the specimen was about .0007 inch.

For the delayed fracture test of the plated specimens, the load applied was 75% of the ultimate tensile strength of the unplated bar. This represents very severe test conditions. The steel bars were sandblasted rather than pickled before being plated, simply to eliminate the embrittlement known to be caused by pickling alone. All specimens in the series were loaded immediately after being plated and examined microscopically. Criteria for acceptable plating of the tensile bars included coverage by cadmium of the root of the notch as judged by microscopic examination of a metallographic section. The delayed fracture test used for determination of hydrogen embrittlement was that of Raring and Rinebolt, ASTM Bulletin No. 213, pages 74-76, April 1956. From the standpoint of reduced hydrogen embrittlement, glycine and a-amino-nbutyric acid are preferred amino acids for preparing electroplating baths of the invention.

Additives which have been heretofore employed in cadmium electroplating, such as wetting agents, e. g., iso propyl naphthalene sulfonic acid, brighteners, e. g., dextrose, polyvinyl pyrrolidine, may be employed in the electroplating baths of the invention for particular im provement effects in the plating.

Since the principle of the invention herein described may be variously practiced without departing from the spirit or scope of the invention, it is to be understood that specific embodiments appearing in the above description are to be taken as illustrative and not in limitation except as may be required by the appended claims.

What is claimed is:

1. In the electrodeposition of cadmium from aqueous bath, the improvement which comprises electrodepositing cadmium from a cyanide-free aqueous ammoniacal bath of pH up to about 11.2 in which is dissolved an alkyl rnonoaminomonocarboxylic acid of from 2 to 5 carbon atoms and a cadmium salt in a molar ratio of from about 1:1 to 2: 1.

2. In the electrodeposition of cadmium from aqueous bath, the improvement which comprises electrodepositing cadmium from a cyanide-free aqueous ammoniacal bath of pH up to about 10 in which is dissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and a cadmium salt in a molar ratio of from about 1:1 to 2:1.

3. In the electrodeposition of cadmium from aqueous bath, the improvement which comprises electrodepositing cadmium from a cyanide-free aqueous ammoniacal bath of pH up to about 10 in which is dissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and a cadirnum salt in a molar ratio of about 2:1.

4. In the electrodeposition of cadmium from aqueous bath, the improvement which comprises electrodepositing cadmium from a cyanide-free aqueous ammoniacal bath of pH up to about 10 in which is dissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and cadmium sulfate in a molar ratio of from about 1:1 to2:1.

5. In the electrodeposition of cadmium from aqueous bath, the improvement which comprises electrodepositing cadmium from a cyanide-free aqueous ammoniacal bath of pH up to about 10 in which is dissolved an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and cadmium sulfate in a molar ratio of about 2:1.

6. A cyanide-free aqueous electroplating bath comprising an aqueous ammoniacal solution of an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and a cadmium salt in the molar ratio of from about 1:1 to 2:1 in which the pH is up to about 11.2.

7. A cyanide-free aqueous electroplating bath comprising an aqueous ammoniacal solution of an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and a cadmium salt in the molar ratio of from about 1:1 to 2:1 in which the pH is up to about 10.

8. A cyanide-free aqueous electroplating bath comprising an aqueous ammoniacal solution of an alkyl monoaminomonocarboxylic acid of from 2 to 5 carbon atoms and cadmium sulfate in the molar ratio of from about 1:1 to 2:1 in which the pH is up to about 10.

References Cited in the file of this patent UNITED STATES PATENTS 2,377,228 Harford May 29, 1945

Claims (1)

1. IN THE ELECTRODEPOSITION OF CADMIUM FROM AQUEOUS BATH, THE IMPROVEMENT WHICH COMPRISES ELECTRODEPOSITING CADMIUM FROM A CYANIDE-FREE AQUEOUS AMMONIACAL BATH OF PH UP TO ABOUT 11.2 IN WHICH IS DISSOLVED AN ALKYL MONOAMINOMONOCARBOXYLIC ACID OF FROM 2 TO 5 CARBON ATOMS AND A CADMIUM SALT IN A MOLAR RATIO OF FROM ABOUT 1:1 TO 2:1.