US3419419A - Nickel-plating bath for thorium - Google Patents

Description

United States Patent C) 3,419,419 NICKEL-PLATING BATH FOR THORIUM Ralph R. Wright, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Feb. 19, 1965, Ser. No. 445,865 1 Claim. (Cl. 117-130) ABSTRACT OF THE DISCLOSURE A chemical reduction bath for nickel-plating thorium having a pH of 9.0 to 9.5 and containing 0.05-0.15 mole per liter nickelous sulfate, 0.3 to 0.6 mole per liter ammonium sulfate, 0.05 to 0.30 mole per liter succinic acid, 0.1 to 0.4 mole per liter sodium acetate, and 0.2 to 0.5 mole per liter sodium hypophosphite.

My invention relates in general to methods of coating a metallic article with a protective layer of another metal and specifically to methods of coating thorium with nickel.

A thorium body can be protected from corrosion by covering its surface with a metal which does not react with thorium or with the environment to which the body is exposed. As is well known, various metals including thorium can be nickel-plated by chemically reducing nickel in the presence of the metal. In these chemical reduction methods a metal body to be plated is immersed in an aqueous solution containing nickel and hypophosphite ions under conditions whereby the nickel is reduced and deposited in an adherent layer onto the surface of the metal.

The nickel-plating of thorium by this technique has :been only partially successful. The known nickel-plating =baths corrode the thorium surface before an adherent protective layer of nickel is deposited thereon. Attempts have been made to plate thorium in accordance with the method disclosed in copending application Ser. No. 374,857, filed June 12, 1964, now abandoned, for Nickel Plating Bath by Robert J. Clouse and John O. Dodson. While it has been possible to plate thorium using the plating bath disclosed therein, bodies so plated characteristically have black deposits at the thorium-nickel interface, and the nickel plating can often be removed merely by scraping.

It is accordingly one object of my invention to provide an improved nickel-plating bath.

It is another object to provide a plating solution useful in depositing strongly adherent nickel coatings onto thorium surfaces.

Other objects of my invention will be apparent from the following detailed description and the claims appended hereto.

In accordance with my invention I have provided a nickel-plating solution having a pH of 9.0 to 9.5 and containing in the concentrations indicated: nickelous sulfate, 0.05 to 0.15 mole per liter; ammonium sulfate, 0.3 to 0.6 mole per liter; succinic acid, 0.05 to 0.3 mole per liter; sodium acetate, 0.1 to 0.4 mole per liter; and sodium hypophosphite, 0.2 to 0.5 mole per liter.

This bath is free of ions such as fluoride and chloride which have a tendency to corrode a thorium surface, and permits plating at a relatively low temperature, thus further minimizing thorium corrosion. An excellent bond between thorium and nickel is achieved by use of this bath.

The process of plating nickel onto a thorium body is preceded by a process of cleaning the thorium body. This cleaning process must provide an active thorium surface free of oxides, hydrides and other foreign material. Steps of alternately electropolishing, rinsing and etching with acids will produce a surface having the desired properties.

The cleaned thorium body is then placed in the plating solution. During the initial period of plating, i.e., until a ice continuous protective film of nickel is on the thorium body, the plating solution is maintained at a relatively low temperature, i.e., below about 30 C. and preferably at a temperature of 22 to 28 C. At a temperature of about 25 C. the nickel is deposited at a rate of about 0.055 mil per hour and at 30 C. the rate is about 0.085 mil per hour.

While the entire coating of nickel may be deposited at these rates, it is normally desirable to deposit the final layer of nickel at a higher rate. This may be accomplished by plating an initial protective layer of nickel onto the thorium in my bath and then either raising the temperature of the bath or immersing the article in a conventional high temperature electroless nickel-plating bath.

Having thus described my invention, the following examples are offered to illustrate it in more detail. Example I gives the composition of my bath and Example II gives a method of using my bath including a method of cleaning the thorium surface.

EXAMPLE I Grams per liter Moles per liter N ickelous sulfate 37 0. 14 Ammonium sulfate 67 0.51 Suceinic acid 30 0.25 Sodium acetate (anhydrous 30 0. 37 Sodium hypophosphite 50 0. 47

d A thorium coupon was cleaned by the following proceure:

1) The coupon was electropolished for 8 minutes in a bath containing 120 ml. per liter 96% sulfuric acid and 500 ml. per liter phosphoric acid at a temperature of 55 C. and a current density of 2 amperes per square inch using lead cathodes;

(2) The coupon was then raised in flowing water at less than 25 C. for 1 minute;

(3) The coupon was then placed in a solution of 900 ml./liter glacial acetic acid and 100 ml./1iter 37 percent hydrochloric acid;

(4) The coupon was then etched for 7 minutes by placing it in a solution of 3 normal nitric acid containing 23 grams per liter of a mixture of potassium dichromate and potassium fluoride which functioned as a deoxidizer;

(5) Step (2) was repeated;

(6) The coupon was anodically etched for 5 minutes in a 1.2 normal HCl solution at 25 C. and a current density of 0.4 ampere per square inch using titanium cathodes;

(7) The coupon was then etched by placing it in a 3.6 normal H solution at 25 C. for 5 minutes;

(8) Step (4) was carried out for 20 minutes;

(9) Step (2) was repeated;

(10) Step (6) was repeated;

(11) Step (7) was repeated;

( 12) Step (4) was carried out for 20 minutes;

(13) Step (2) was repeated;

(14) The coupon was pickled for 4 minutes in a solution of 58 rnl./liter of 96% H 80 at 25 C.;

(15) Step (2) was repeated.

The cleaned coupon was placed in the bath of Example I at 22 to 28 C. for 2 hours while the bath was agitated. The resulting coupon having a nickel coating about 0.13 mil thick was then placed in a conventional nickel-plating bath maintained at 96 C., having a pH of 4.5, and containing in the concentrations indicated: nickel sulfate,

0.08 mole per liter; sodium hypophosphite, 0.23 mole per liter; propionic acid, 0.03 mole per liter; lactic acid, 0.03 mole per liter; tartaric acid, 500 p.p.m.; lead, less than 3 p.p.m.; and a wetting agent consisting of a sulfonated saturated long-chain hydrocarbon, 50 ppm. The coupon was held in this hath until a plating 3.5 mils thick was attained.

The resulting coupon was tested for adherence and continuity of the nickel coating by subjecting it to 100 percent relative humidity at 200 F. At the end of 90 hours the coupon showed no evidence of failure. At 900 hours the plating exhibited some discoloration, but showed no signs of peeling or porosity. At the end of 913 hours of exposure the plating burst from oxide build-up.

Other coupons plated as above were heated red hot with a torch and then quenched in cold water. No bursting or peeling of the plating occurred.

The above examples are offered to illustrate, not to limit, my invention. For example, the cleaning step does not constitute a part of my invention and is offered merely to show a procedure that may be used to clean a thorium surface. My invention should be limited only in accordance with the following claim.

Having thus described my invention I claim:

1. In a process for depositing nickel onto a thorium article the step of immersing said article into a nickel plating bath maintained at a temperature between 22 C. and 30 C., having a pH of 9.0 to 9.5 and consisting essentially of the following in the concentrations indicated: nickelous sulfate, 0.05 to 0.15 mole per liter; ammonium sulfate, 0.3 to 0.6 mole per liter; succinic acid, 0.05 to 0.30 mole per liter; sodium acetate, 0.1 to 0.4 mole per liter; and sodium hypophosphite 0.2 to 0.5 mole per liter.

References (Iited UNITED STATES PATENTS 2,532,283 12/1950 Brenner et a1 1061 2,819,187 1/1958 Gutzeit et al 117-430 2,916,401 12/1959 Puls et a1. 1061 JULIUS FROME, Primary Examiner.

L. HAYES, Assistant Examiner.

U.S. Cl. X.R. 1061