US3445351A

US3445351A - Process for plating metals

Info

Publication number: US3445351A
Application number: US405362A
Authority: US
Inventors: Donald A Swalheim; Robert W Mackey
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1964-10-21
Filing date: 1964-10-21
Publication date: 1969-05-20
Anticipated expiration: 1986-05-20
Also published as: DE1496960A1; GB1124107A

Description

United States Patent 3,445,351 PROCESS FOR PLATING METALS Donald A. Swalheim and Robert W. Mackey, Wilmington, Del., assignors to E. I. du Pout de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Oct. 21, 1964, Ser. No. 405,362 Int. Cl. C23b 5/52, 5/50, 5/14 US. Cl. 204-37 3 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the production of electrolytic tin plating having high corrosion resistance. More particularly, the invention relates to applying an ultra-thin preplate deposit of nickel-tin or nickel onto a base metal surface prior to plating the tin.

Recent efforts to improve corrosion resistance of tin plate to provide what is referred to as Type K or Grade A plate have been directed mainly toward melting or flowbrightening the tin layer. Embodiments of this technique are described in US. Patents 2,661,328 and 3,062,275. An improvement in such a process is described in our copending application Swalheim & Mackey, 'Ser. No. 317,462, filed Oct. 21, 1963. In this application the improved process comprises flow-brightening the tin plate by heating it at an elevated temperature and thereafter quenching the tin plate within a predetermined peroid of time. While the improved process does provide high quality plate, it would be most desirable to have greater flexibility in the use of processing conditions and, in particular, to be able to reduce the melting temperature and hold-up time between melting and quenching the tin plate in the flow-brightening ste I is, therefore, a primary object of this invention to provide an improvement in the process of tin plating which gives high quality plate using a moderate temperature and a reduced delay time between heating and quenching. Other objects will be apparent from the detailed description which follows hereinafter.

The objects of this invention are accomplished by our discovery that an ultra-thin, i,e., from about 2X10 to 65 X 10- inch, coating of nickel-tin alloy or nickel applied to the base metal prior to the tin plating provides substantial improvements in the quality of tin plate over a wide range of temperature and delay times. This coating or preplate may be applied using conventional electroplating procedures. The necessity for controlling the thickness of the preplate within the limits specified is graphically illustrated by the family of curves shown on the accompanying drawing which will be discussed in detail later herein.

In carrying out the process of our invention, the preplate may be applied by using a conventional electroplating unit and one of the many known nickel or nickel-tin plating baths. Since a very thin coating is desired, the current in the electroplating unit must be carefully controlled. Suitable apparatus such as a high current transistor switching device capable of delivering a current pulse of controlled duration, e.g., 5 to 100 milliseconds, may be used. After application of the preplate, the sheet is then electroplated in the usual manner to provide a tin plating of desired thickness, preferably 0.25 to 1.5 pounds per base box. The plated sheet is then passed through a heatice ing device where it is heated to a temperature from about 540 to 850 F., preferably 540 to about 700 F. The temperature is maintained for a very short period of time, e.g., up to 3.0 seconds and preferably 0.25 to 1.0 second, after which the sheet is quenched, bringing the temperature rapidly to a level of about 200 F. or lower.

The utility of this invention can be best demonstrated in terms of its application. In succeeding paragraphs data are presented to show that very thin layers of nickel or nickel-tin alloy give substantial improvements in corrosion resistance when the tin plating and flow brightening operations are carried out in an otherwise normal manner. As a further advantage, these improvements can be readily incorporated in high-speed electrotinning operations.

The corrosion resistance of tin plate is measured by the ATC (Alloy Tin Couple) test. The results have been shown to correlate well with shelf life experience for tin cans packed with corrosive foods. In the ATC test, the free tin layer is stripped from a test panel exposing the iron-tin alloy covering the steel base. The stripped test piece is connected to a thin anode and placed in a cell where conditions simulating those found inside sealed cans are maintained. After twenty (20) hours in this environment, the electric current flowing between the test piece and the anode is measured and, when expressed as microamperes per square centimeter, is called the ATC value. Lower ATC values indicate better corrosion resistance. Type K or Grade A tin plate must have an average ATC value less than 0.05 with of all tests less than 0.085.

Referring to the drawing, the curves shown illustrate the effect of thickness (d) of a nickel-tin preplate on the ATC value of a 0.75 pound per base box tin plate. The curves are grouped in families to show that a similar improvement in corrosion resistance (decrease in ATC value) is obtained at typical reflow temperatures (T) and delay times (0) within the ranges described above. A substantial improvement is obtained with very thin preplates. Maximum improvement is obtained at thicknesses of about 65 X 10- inch and the effect diminishes as the thickness of the preplate increases.

The data used in establishing the curves shown on the drawing were obtained from a large number of experiments which will be described in the following examples:

Example I Type L 220-finish low carbon steel of the type customarily used for tin plate was used for preparation of samples. A 6" x 12" steel cathode strip was mounted on the rotor of a Du Pont rotating cathode cell which is described by Swalheim in Trans. Electrochem. Soc. 86, 395 (1944) and, after conventional cleaning and pickling, the strip was plated with a preplate of nickel-tin. The cathode was then removed from the preplating cell, rinsed in water and placed in another Du Pont rotating cathode cell to plate the tin. A tin plate of 0.75 pound per base box was applied, using a conventional plating bath.

After plating, strips (2" x 6") were cut from the larger strips and were heated to a carefully controlled given temperature by electrical resistance heating and, after a controlled, predetermined delay time, were dropped into a water quenching bath.

After quenching, the remaining tin plate was stripped from the samples, leaving the steel panels with an exposed iron-tin alloy coating. This stripping was done by immersing the panels in a 5% NaOH solution using a constant voltage power supply to maintain a constant potential of 0.4 volt between the panels and a stainless steel electrode. When all the free tin was removed, the stripping current automatically diminished to zero. The stripped panels were then subjected to tests to determine ATC 3 values in accordance with the test procedure previously described.

The nickel-tin alloy was plated from a bath having the following composition:

Grams/liter SnCl 36 NaF 34 NaHF 16.9 NaCl 35 The pH of the bath was adjusted to within the range of 2.0 to 2.5. The thickness of the nickel-tin was controlled by selecting the prescribed plating time required for passing of the coulombs to deposit the thickness of nickel-tin alloy desired.

The nickel-tin alloy plating bath deposited an alloy containing approximately 35% Ni and 65% Sn. The baths were operated at a temperature of 160 F. The results showing the effect of temperature and delay time on ATC values using preplate deposits from the bath described above at thicknesses of 6.5, 36.5, 65.5, 200 and 455 x 10- inch, respectively, are recorded in the table which follows:

The results obtained, using various thicknesses of pieplate, are set forth in the following table:

The data illustrates similar advantages to those obtained in Example I when using thin Ni-Sn preplates. A relatively dilute bath offers the additional advantage from a commercial standpont of diminishing drag-out of ingredients.

Example JII A series of experiments was conducted in which nickel preplates were applied to the steel substrates pior to ap- TABLE 1 ATC value (average) preplate thickness of- Delay time Melt temp.

(8%.) F.) 6.5)(10' 36.5)(10 65.5X 200x10 455x10- TABLE 2 [No Ni-Sn Preplate] Melt temperature (F.) ATC value (avg) Delay time (sec.)

It will be noted from the foregoing table that Type K plate, i.e., plate having an ATC value of less than 0.05 was not even produced at the reflow temperature of 595 C., with a relatively long delay time of 1.0 second.

Example II The experiment described in Example I was repeated except that the thin nickel-tin preplates were coated from a dilute bath having the following composition:

SnCl Grams/liter 3.6 NaF do 3.4 NaHF do 1.69 NaCl do 3.5 NiCl -6H O do- 22.5

plying the tin plate. A nickel plating bath having the following composition was used:

Panels 2" x 6" were cleaned and pickled in the conventional manner and were then plated with nickel in a small laboratory cell under essentially static conditions. A very thin nickel preplate was obtained by using equipment to provide a plating time in the order of 0.0025 second and a current density during discharge of a set of condensers of about 400 amperes per square foot. After plating, the panels were plated with 0.75 pound per base box of tin (45 10 inch) and were then flow brightened at a temperature of about 605 F. Time of heating was 0.55 second and the delay time between power shut-off and quenching in water was 0.1 second. The data obtained from the experiments are set forth in the following table:

Example IV In another series of experiments, nickel preplates were deposited by plating onto 6" x 12" steel panels using the DuPont rotting cathode cell. The preplated panels were then given a 0.075 pound per base box tin plating, using the procedure described in Example IH. The nickel plating bath used in the series of tests had the following composition:

NiCl -6H O Grams/liter H BO d 7.5 P-4 Addition Agent ml./liter 3.5 pH 3.5

Proprietary Additive Supplied by Harshaw Chemical Comparty for Decorative Nickel Plating.

The date obtained from the series of experiments are recorded in the table which follows:

TABLE 5 Preplate thickness ATC value X 10- inch) (Average) None 0.279

scope thereof, it is to be understood that this invention is not to be limited to the specific embodiment thereof except as defined in the appended claims.

1. In a process for producing a bright tin coating on a base metal strip which comprises immersing said strip in an electrolytic cell containing a tin plating solution and thereafter flow brightening the resulting tincoated strip by heating the coated strip to a temperature from about 540 to 850 F., the improvement comprising precoating the base metal in an electrolytic cell to provide an ultra-thin metal preplate having a thickness in the range from about 2X10" to about X 10'- inch, said preplate being selected from the group consisting of nickel and nickel-tin alloy.

2. The process of claim 1 wherein said temperature is maintained for a period of time not greater than 3 seconds, after which the coated strip is rapidly quenched.

3. The process of claim 2 wherein said tin coating has a thickness from about 0.25 to about 1.5 pounds per base box.

References Cited UNITED STATES PATENTS 2,085,543 6/ 1937 Oplinger 204-17 2,266,330 12/1941 Nachtman 20428 2,303,035 1l/l942 Fink, 204-36 2,381,778 8/1945 Schoonmaker et a1 204-37 2,315,740 4/ 1943 Schoonmaker et a1. 204-37 3,260,580 7/1966 Kamm et al 29196.4 3,334,030 8/ 1967 Notman 20437 3,285,838 11/1966 Morgan et a1 204-37 FOREIGN PATENTS 297,161 9/1928 Great Britain.

473,479 10/ 1937 Great Britain.

484,909 5/ 1938 Great Britain.

JOHN H. MARK, Primary Examiner. W. B. VANSISE, Assistant Examimer.

US. Cl. X.R. 29196.4; 204-40