US3522154A

US3522154A - Codeposited iron and tin electroplate and a process and electroplating bath for its preparation

Info

Publication number: US3522154A
Application number: US642309A
Authority: US
Inventors: Donald Arthur Swalheim
Original assignee: EI Du Pont de Nemours and Co
Current assignee: ZACLON Inc A OHIO CORP
Priority date: 1967-05-31
Filing date: 1967-05-31
Publication date: 1970-07-28
Anticipated expiration: 1987-07-28

Description

United States Patent O 3,522,154 CODEPOSITED IRON AND TIN ELECTROPLATE AND A PROCESS AND ELECTROPLATING BATH FOR ITS PREPARATION Donald Arthur Swalheim, Hockessin, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed May 31, 1967, Ser. No. 642,309 Int. Cl. C23b /52, 5/14 U.S. Cl. 204-37 ABSTRACT OF THE DISCLOSURE Electrodeposits containing iron and tin on a metal, particularly steel, are plated using an aqueous fluoride-containing bath in which ferrous iron and stannous tin are present in controlled amounts. The bath for electroplating the iron and tin codeposit contains a stannous tin concentration of about 0.015 to about 0.1 mole/1., a mole ratio of ferrous iron to stannous tin having an upper limit defined by the equation:

and a lower limit defined by the equation:

gr? mole rat1o=0.2 +8.9e'

and a mole ratio of fluoride to ferrous iron plus stannous tin from about 1.5 to 7.5.

BACKGROUND OF INVENTION While tinplate is used extensively for packaging various types of food, beverages and other products, there is increasing commercial interest in substituting other coatings for the tin in view of the relatively high cost of tin. Furthermore, a tin coating is not essential for containers such as beer and soft drink cans since adequate protection against corrosion is provided by the lacquer. Changing techniques in fabricating the can are also important factors in selecting substitute coatings for tin. Significant cost reductions have been achieved by using nylon-type adhesives instead of solder to bond the side seam of the body of the can. Tinplate is unsatisfactory for adhesive-bonded side beams because the tin melts at the temperatures used to apply the adhesive and the bond strength is very low.

One commercial method of producing tinplate having improved corrosion resistance comprises plating the steel with a flash coating of tin (1.2 to about 6 millionths inch), refiowing the tin at a temperature suflicient to convert it into a flat plate-like iron-tin alloy, that temperature being between about 490 F. and about 850 -F., electroplating the reflowed electrotinned steel with an additional coating of tin and reflowing that additional coating of tin (U.S. Pat. 3,285,833 issued Nov. 15, 1966, to E. R. Morgan et al.). Although tinplate having good corrosion resistance is produced by this method, the investment required to convert the preplate of tin to FeSn is relatively expensive. During this melting operation, the molten tin reacts with the iron to form an interlayer consisting of the FeSn intermetallic compound. The intermetallic compound has been shown to have an important influence on the protective value of the coating. The extent of coverage of the steel is very important and the degree of coverage is probably associated with the morphology of the FeSn crystallites. The extent to which complete coverage is achieved depends primarily on the number of nucleation sites, since under normal manufacturing conditions the time available for crystal growth is less than 1 sec.

10 Claims 3,522,154 Patented July 28, 1970 No methods of producing similar coatings have been disclosed by an electroplating process. Schweikher (U.S. Pat. 2,407,579 issued Sept. 10, 1946) discloses including small amounts of the iron group, e.g., two to ten grams per liter of nickel, cobalt or iron chloride or sulfate, in the plating bath to improve the brightness of the tin deposit. However, it was found that these metals do not codeposit to any appreciable extent when present in such small amounts.

SUMMARY OF THE INVENTION According to the present invention there is provided a metal having a coating thereon of electrolytically codeposited iron and tin which is in intimate adhering contact with the metal, i.e., which is essentially inert when treated anodically in a 5% NaOH solution at a potential not exceeding 0.4 volt. The preferred ferrous basis coated prodnot is useful as a semibright, corrosion-resistant container stock (cans), particularly cans having an adhesivebonded side seam, or may be further treated or plated for other applications.

The metal products of this invention also comprise the passivated codeposit treated electrolytically in chromatetype solutions to improve the in-storage rust resistance. The metal products also include steel sheet coated with the codeposit or the passivated codeposit and organic synthetic resins of the type used to coat the interior of cans.

The metal products also include steel sheet having a coating of electrolytically codeposited iron and tin and a coating of tin plated over the codeposit which may or may not be heated to melt the tin; if heated, the reflow should be at a temperature of from about 485 to 720 F.

There is also provided a process for preparing electroplated metal articles which comprises simultaneously comole ratio =0.2+14.7e' set) and a lower limit defined by the equation:

Sn? mole ratio=0.2]8.9e 511+) and the mole ratio of fluoride to ferrous iron plus stannous tin is from about 1.5 to 7.5, preferably about 3 to 7.

DETAILED DESCRIPTION OF THE INVENTION The products of this invention may be prepared by following the general procedures used in Halogen Tin electroplating described in Schweikher U.S. Pat. 2,407,579. It is essential, however, that the plating bath be modified to control the mole ratio of ferrous iron to stannous tin as well as the mole ratio of fluoride to total moles of ferrous iron plus stannous tin within the limits set forth. The mole ratio of ferrous iron to stannous tin will vary depending on the mole concentration of stannous tin in the bath. In addition, plating should preferably be carried out at a current density in excess of about 20 amperes per square foot.

While codeposits of iron and tin are produced over the entire 1.5 to 7.5 range of mole ratio of fluoride (F) to total metal (Sn+++Fe++), the appearance of the codeposits improves as the ratio increases. A mole ratio of 5:1 is most preferred since no discernible improvement is noted as the ratio is increased from 5:1 to 7.5: 1.

The thickness of the iron-tin codeposit can be readily controlled by varying the coulombs supplied to the bath as is well known in the art Within the range of about 5 to 150 coulombs/ftf however, depending upon the desired end use, the thickness produced is estimated to varry from about 0.1x inch to about 4.5 10' inch.

In order to further inhibit corrosion, and especially to retard the formation of oxide films during storage, the codeposited iron-tin plate can be given a typical, electrolytic chromate passivating treatment in an acidic or basic chromate bath. Generally, such baths contain about 1 to 75 g./1. of a water-soluble chromate.

The chromate passivating treatment is particularly desirable, although not essential, when preparing a synthetic organic resin coated plate useful for beverage containers. Among the various organic coatings which can be used are resins well documented in the art, but include such resins as phenolics, expoxies, organosols, vinyls and combinations thereof, particularly an epoxy phenolic lacquer.

In end uses where tin is necessary, electroplating and reflowing a coating of tin on the codeposit can be effected by conventional techniques. Of course, many other of the conventional tin plate practices can be employed such as pickling the base or oiling the coating.

In practicing the preferred processes of the invention, polyalkylene glycols of the type described in U.S. Pat. 2,457,152 issued to Raymond A. Hoffman on Dec. 28, 1948, and the patents identified therein are employed. The polyethylene glycols sold under the trademark Carbowax are employed in the examples which follow. Useful polyalkylene glycols have number average molecular weights from about 1500 to 20,000, preferably about 4000 to 20,000 and are employed in the bath at concentrations ranging from about 0.01 to 5 g./l., preferably from about 0.02 to 1 g./l.

Example No.

C ontrol.

In addition to the materials described in preparing the baths of the following examples, other materials can be used in combination or in place of the bath ingredients. The tin salts can be selected from stannous sulfate, stannous fluoride, or stannous chloride as Well as other watersoluble stannous salts. Likewise, the iron salts can include ferrous chloride as well as ferrous sulfate and other water-soluble ferrous salts. To improve conductivity of the bath, such materials as sodium chloride or sodium sulfate can be used.

In the examples which follow, the plating characteristics of different electrolytes are readily determined in the Hull Cell which has a current density range over the 4-inch wide cathode from essentially zero at the low end to over 150A/SF at the upper end closest to the anode. Polished brass cathodes were used as Hull Cell panels for the tests, and, unless otherwise indicated, the panels were plated using 3 aniperes for a period of 5 seconds. The Du Pont rotating cathode used in some examples is described by Swalheim in Trans. Electrochem-Soc. 86, 395 (1944).

The examples also use Anodic Stripping to determine whether there is an iron-tin codeposit that is in adhering contact with the metal. In this test, any free tin present is readily removed from the iron-tin codeposit by treating the plated panel anodically at a controlled potential not exceeding 0.4 volts in a 5% solution of sodium hydroxide. Current continues to flow until the free tin is completely removed. The iron-tin codeposit does not dissolve anodically in the caustic solution. Based on the results of the salt spray test and underfilm test, 'which are de scribed later in the examples, and the above-described Anodic Stripping test, the electrolytically deposited coating is distinctly different from substantially pure tin and substantially pure iron. It is a codeposit of iron and tin wherein tin is a major constitoent.

The following examples further illustrate the present invention.

EXAMPLES 1-3 Mole ratio of fluoride to Fe -I-Sn++ Example No Control Control F- to Fe+++Sn++ Mole Ratio. 511012, g.ll F8804. 7H20, g./l

Na ,g./l- Polyethylene glyeols, g. /l

The appearance of Hull Cell panels plated from the above baths and appearance of the panels after stripping are given in Table II.

TABLE II Mole ratio, Appearance, After Anodic F-/Fe+++Sn++ Appearance, Hull Cell Panels Stripping White Deposit, 0-1A/SF No Deposit, 0-18A/SF. Semi-Bright, 18-36A/SF. Semi-Bright, 18-36A/SF. Dark Streaks, 36150A/SF Dark Steaks, 36-150A/SF. White Deposit, 0-18A/SF. No Deposit, O-lSA/SF.

Semi-Bright, 18150/ASF. Dark Deposit, 140-150A/SF White Deposit, O-18A/SF No Deposit, 0-18A/SF.

Semi-Bright, 1890A/SF Semi-Bright, 18-90/ASF. {Dark Deposit, 90-150A/SF Dark Deposit, 90-150/ASF. 5 {Semi-Bright, 18150A/SF Semi-Bright, 18-150A/SF. 7. 5 Similar to No. 2 Similar to N0. 2.

White Deposit, 0-18A/SF- No Deposit, 0-18A/SF.

3 Semi-Bright, 18-140A/SF It is readily apparent that presence of fluoride in the bath is needed in order to produce satisfactory deposits. Concentrations of fluoride required will vary with the total moles of Fe++ and Sn++ present in the bath within the range of about 1.521 to 7.5: 1.

EXAMPLES 4-17 Mole ratio of ferrous iron to stannous tin TABLE IIL-BATH COMPOSITIONS [Varying Mole Ratio of Fe++ to Sn++, Ferrous Iron Maintained Constan at 0.17 Mole/1.]

Example 4 5 6 7 Control Control Fe++/Sn++ Mole Ratio 7. 5 5 3. 5 2. 5 2 FeSO4. 7H2O, g./l. 48 48 48 48 48 SnCle, g./l 4. 35 6. 5 9. 2 13 16. 3 NaI-IFQ, g./l 20. 6 31 3 33. 4 36. 4 39 NaCl, g./l 15 15 15 15 15 15 Polyethylene glyeols, 5.11... 0. 38 0.38 0. 38 0. 38 0.38 0.38

Results of Hull Cell plating tests are given in Table IV.

TABLE IV Example Mole Ratio, No. Fe++/Sn++ Appearance, Hull Cell Panels Appearance, After Anodic Stripping 4 10 Semi-Bright 150A/SF Thin Deposit Over Most of Panel. 7 5 {White Deposit, 0-18A/SI No Deposit, 0-18A/SF.

Semi-Bright, 18-150AISF Semi-Bright, 18-150A/SF. 6 5 Similar to No. 5 Similar to N o. 5

No Deposit, o-2oA sr. Semi-Bright, 20-150A/SF With Dark Streaks 20-40A/SF. No Deposit, 0-24A/SF.

7 3 5 {White Deposit, A7

""""" Semi-Bright, 20-150A/SF tWhite Deposit, 0-24A/SF 5 emi-Bright, 24-l50A/SF Semi-Bright, 24-150A/SF With Dark Streaks 24-75A/SF.

D0 2 {White Deposit, 0-30A/SI No Deposit, O-30A/SF.

"- Serni-Bright, 30-150A/SF Semi-Bright, 30-150A/SF With Dark Streaks Over Most of Area.

tin mole ratio was maintained at 5. The composition of With a ferrous iron concentration of about 0.17 mole/l. the baths studied are given in Table VII. good quality codeposits are obtainable over a Fe++ to 15 TABLE vr TH COMPOSITIONS Sn++ mole ratio range Of about 10 t0 3. At mole ratios [Varying Mole Ratio of Fe++ to Sn++ Stannous Tin Maintained Constant lower than 3 to 1, the plating range becomes restricted at 0104 Mme/L] and the codeposits are somewhat dark in appearance The Example 13 14 15 16 17 mole ratio range of 10 to 3 corresponds to a stannous tin geH/SnH M016 Ram 25 5 1 2 BSO47H20,g /l.. 2.9 7.2 14.5 29 58 concentration of 0.017 to 0.057 mole/1. SNOIQ, g./l 20 2o 20 20 20 In the next series of tests, the stannous tin content NaHFQi 5 20 24 43 Pol etii lene 1 eols, .1 0. 38 0.38 0.38 0.38 0. as was maintained constant at 0.052 mole/l. and the mole y y gy g/ Results of the Hull Cell plating tests are shown in Table VIII.

TABLE VIII Example Mole Ratio, No. Fe to Sn++ Appearance, Hull Cell Panels Appearanee,AiterAnod1c Stripping 13 0 1 {\NhiteDeposit,036A/SF NoDeposit,045A/SF. Milky Deposit, 36-150AISF Dark Deposit, 45-150. 14 0 {White Deposit, 036A/SF No Deposit, 0-36A/SF.

Semi-Bright, 36-150A/SF Semi-Bright With Dark Streaks, 36-150. 15 0 5 {White Deposit, 0-36A/SF No Deposit, 0-36A/SF.

Semi-Bright, 36-150A/SF Semi-Bright With Dark Streaks,36150. 16 1 {White Dep0sit,0-36A/SF No Deposit,036A/SF.

lSerni-Bright, 36150A/SF Semi-Bright With Dark Streaks, 36-150. 17 2 {WhiteDepositfi-SSA/SF. NoDeposit,036A/SF.

------- Serni-Bright, 36150A/SF- Semi-Bright With Dark Streaks, -150.

ratio of Fe++ to Sn++ was varied. The mole ratio of F The results of the plating tests given in Table VIII to moles of F++ plus Sn++ was maintained constant at 5. for baths containing 0.104 mole/l. of stannous tin show TABLE V.BATH COMPOSITIONS [Varying Mole Ratio of FE++ to Sn++, Stannous Tin Maintained Constant at 0.052 Mole/1.]

Example No Control Control 8 9 10 11 12 Control Control FE++ sn++Mo1e Ratio1 0. 052 o. 125 0. 25 0. 5 1 2 4 e 8 FOSO4-7H20, g.ll 0. 9 1.8 3.6 7. 2 14. 5 29 5s 87 115 81101), g./l 10 10 1o 10 10 1o 10 1o 10 NaHFe, g./l s. s s. s 9.8 12. 3 1e 24 5e 72 NaCl, g./l 15 15 15 15 15 15 15 15 15 P y y g yc -l 38 1 Same. Results of Hull Cell plating tests are given in Table that the plating range is somewhat restricted and the VI. deposits are somewhat dark after stripping.

TABLE VI Example Mole Ratio No. Fe++ to Sn Appearance, Hull Cell Panels Appearance, After Anodic Stripping C 1 {White Deposit, 0-30A/SF No Deposit, 0-36A/SE.

Milk Deposit, 30-120A/SF Dark Deposit, 36-120A/SF.

D 125 {White Deposit, 0-24AlSF No Deposit, 0-30A/SF.

Milky Deposit, 24-130A/sF Dark Deposit, 30120A/SF. 8 0 25 {White Deposit, 0-24A/SF No Deposit, 0-24A/SF.

Semi-Bright, 24135A/SF Semi-Bright, 24-135A/SF With Dark Cast. 9 0 5 {White Deposit, 0-24A/SE. No Deposit, 0-24A/SF.

Semi-Bright, 24-140A/SF. Semi-Bright, 24-140A/SF. 0 1 {White Deposit, 024A/SF No Deposit, 0 24A/SF. 1 Semi-Bright, 24-140A/SF. Semi-Bright, 24-140A/SF. 11 2 {White Deposit, 0-18A/SF. No Deposit, 0-18A/SF.

Semi-Bright, l8l50A/SF Semi-Bright, 18-150A/SF. 2 4 {White Deposit, 0-18A/SF. No Deposit, 0-18A/SF. 1 Semi-Bright,l8150A/SF se r ii i iglgh g, 18-150A/SE With Dark Streaks,

s C t 1 6 {\VhiteDeposit, 0-18A/SE-.. NoDepesit,024A/SF.

Semi-Bright,18150A/SF Semi-Bright With Dark Streaks, 24-150A/SF;

D 8 {White Deposit, 0i8A/SF 1 No Deposit, 030A/SF.

Milky Deposit, 18150A/SF Semi-Bright With Dark Cast, 30-150A/SF.

In a bath containing 0.052 mole/l. of stannous tin, The results from Tables III through VIII can be sumdeposits having the deslred quahty charafltenstlcs t marized as expressed in the previously defined equations. Produced Over a ferrous to stannous 1 2 The mole concentration of stannous tin has a marked 8% aboilt to The preferred range mo 6 18 effect on the concentration of iron required. At low mole to concentrations of stannous tin in the range of 0.02 to Another series of Hull Cell tests were made to determine the effect of the ferrous iron to stannous tin mole the mole who of to stannous can ratio in a bath containing 0.104 mole/l. of stannous tin. y Over f broad f Thls represents the p The mole ratio of fluoride to ferrous iron+stannous ferred conditions of operation.

7 EXAMPLE 18 Effect of .polyalkylene glycols Hull Cell plating tests were made to determine the effect of polyalkylene glycols on the plating characteristics of the electrolyte. The bath compositions are given below:

/S Example 18.-. {White Deposit, O-IBA/SF... No Deposit, -18AISF.

Semi-Bright, l8-150A/SF..- Semi-Bright, l8-150A/SF.

Although codeposits are produced in the absence of polyalkylene glycols, the brightness and general appearance of the codeposits are improved by adding the polyalkylene glycols to the bath.

EXAMPLE 19 Effect of pH on plating characteristics A bath having the following composition was used for this series of tests:

Composition:

SnCl 6.5 g./1. FeSO -7H O-48 g./l. NaHF -3 1.3 g./l.

NaCl-15 1g./l. Polyethylene glycols-0.38 g./l.

The mole ratio of Fe to Sn++ in this bath is 5. The mole ratio of F to Fe+++Sn++ is also 5. The pH was changed by either adding NaOH or HCl. The pH was determined colorimetrically using 'benzo yellow indicator.

Brass Hull Cell panels were plated. After plating, the panels were treated anodically in a NaOH solution. A description of the panels before and after stripping is given in Table IX.

TABLE IX.-EFFECT OF pH ON PLATING Codeposits are produced over the range of 2-4 pH. Based on results shown in Table IX, a pH range of 2.5-3.5 is preferred.

EXAMPLE 20 Effect of temperature on plating characteristics The composition as shown in Example 19 was used in this series of tests. Hull Cell tests were conducted in a similar manner. Results are given in Table X.

TABLE X.-EFFECT OF TEMPERATURE ON PLATING CHARACTERISTICS Appearance of Hull Cell Panels After Anodically Stripping No Deposit, 0-18A/SF Temp, F. As Plated Milky Deposit, 012A/SF "ISeIni-Brrght, 12150A/SF Semi-Bright, 18-150A/SF. {White Deposit, 0-18A/SF No Deposit, 0-18A F.

Semi-Bright, 18l50A/SF Semi-Bright, 18-150A/SF. White Deposit, 0-18 No Deposit, 0-18.

Semi-Bright, 18150 Semi-Bright, 18-150. {White Deposit, 0-18..- No Deposit, 0-18.

Semi-Bright, 18-150..- Semi-Bright, 18-150. {Dull White Deposit, 0- No Deposit, 0-24.

Semi-Bright 24l50 Semi-Bright, 24-150. 200 {Dull White eposit, 0-24 No Deposit 0-24.

------- Semi-Bright, 24-15o Semi-Bright, 24-150.

The operating temperature range is very broad as shown in Table X, and therefore not critical. The preferred range is 100-l60 F.

EXAMPLE 21 Plating at high strip speed In most of the commercial operations involving coating of strip steel with protective coatings, the strip travels through solutions at relatively high strip speeds. For example, strip steel is electroplated with tin at speeds up to 2000 ft. min. The Du Pont rotating cathode cell provides a convenient method of evaluating plating characteristics at high strip speeds. Steel panels were plated in the cell at 1500 ft./min. using a bath having the following composition:

NaCl-35 g./l.

Polyethylene glycols0.34 g./l. Temperature 150 F.

The time of plating was 0.17 second with a current density of 360 amperes per square foot. This quantity of current was equivalent to 60 coulombs/sq.ft. The plated deposits were semi-bright in appearance and were essentially inert when treated anodically in a 5% NaOH solution at a potential not exceeding 0.4 volt.

EXAMPLE 22 chromate-type passivating treatment Protective coatings are frequently treated in chromatecontaining solutions to passivate the surface and improve the corrosion resistance. Salt spray tests are used quite extensively as accelerated tests to measure rust resistance of coatings.

Panels plated from the bath of Example 21 were treated cathodically for 1 sec. at a current density of 50 A/SF in a solution containing 27.75 g./l. CrO and maintained at 150 F. After rinsing and drying, the panels along with panels plated with the codeposit but given no chromate treatment were exposed in a standard 5% NaCl salt spray chamber for 2 hours. After exposure for 2 hours, the panels were rinsed with water, dried and examined for rusting. The codeposit treated cathodically in the chromic acid solution was coated with a very thin film of rust, whereas the codeposit given no chromate passivating treatment was heavily rusted.

EXAMPLE 23 Underfilm corrosion The interior surface of beer and beverage cans is coated with enamel to prevent direct contact of the contents with the metal. Any significant amount of underfilm corrosion during storage cannot be tolerated because this would lead to excessive exposure of the metal. For example, introduction of a few p.p.m. of iron salts in beer produces turbidity and has an adverse effect on flavor of the beer.

The following procedure was used to evaluate underfilm corrosion. Steel panels were plated using the following bath:

Bath composition:

SnCl g./l. FeSO -7H O48 g./l. NaHF 45.3 g./1. NaCl-35 g./l. Polyethylene glycols-0.3 8 g./l. pH2.4

Panels were plated at 360 amp/sq. ft. in the rotating cathode cell at 1500 ft./min. Quantity of current applied was 40 coulombs/ sq. ft.

Test strips cut from the panels were coated with a 0.2 mil thick enamel. A Stoner and Mudge No. 2799V enamel, a typical epoxy phenolic lacquer used for beverage containers, was used for these tests. The panels were heated in an oven maintained at 400 F. for a period of 12 minutes to cure the lacquer.

Strips A x 4" were scribed with an X through the organic coating and the codeposit thereby exposing the steel basis metal. Strips were then placed in a test medium consisting of an aqueous solution containing 1.5% NaCl and 1.5% citric acid for 5 days at 80 F. Any undercutting in excess of 0.1 mm. is considered excessive. Results of these tests showed that underfilm corrosion for the plated panels was less than 0.1 mm. and substantially better than with 0.25# tinplate or plain steel.

EXAMPLE 24 Preplate structure The characteristics of the codeposit were evaluated by studying the structure using the electron microscope. A bath having the following composition was used to plate the codeposit:

The rotating cathode cell was used to plate the codeposit at a strip speed of 1300 ft./min. A cathode was plated applying current equivalent to 28 coulombs./ft. After plating, a 2" x 6" strip cut from the plated panel was heated to a temperature of 540 F. controlling total heat-up time at 0.3 sec. and allowing a delay time of 0.25 sec. between power shutoff and quenching the strip in water. After heat-treating, the strip was treated anodically in 5% NaOH at a controlled potential not exceeding 0.4 volt. The current dropped rapidly to zero indicating no significant amount of free or uncombined tin in the deposit. An electron photo micrograph (20,000 X) of the coating showed that the structure of the codeposit was extremely fine grained and showed good coverage of the steel surface.

Another series of panels were made using the rotating cathode cell for plating. The cell was operated at a strip speed of 600 ft./min. Conditions of plating and treating the samples are given in Table XI.

The coulombs/ sq. ft. shown for plating tin deposits a coating equivalent to a thickness of approximately 45 millionths of an inch. This thickness is commonly referred to as 0.75 lb./base box of 435 sq. ft. of surface.

After plating and refiowing, strips from each panel were treated anodically in 5% NaOH to remove free tin from the codeposited iron and tin. Electron photomicrographs were then taken of the surfaces to compare differences in structure. The electron photomicrographs taken at 20,000 magnification showed that the structure of the Fe-Sn interlayer for panel No. 1 in Table XI is coarse grained and typical of reflowed tinplate produced in the conventional manner. An extremely thin preplate layer of the codeposit prior to plating the tin and refiowing the composite coating as on panel No. 2 showed no significant change in structure. However, substantial changes in the structure are shown for the thicker preplate layers of panels Nos. 35. The electron photomicrographs for these panels showed a fine-grain type of structure with good coverage over the steel surface which is highly desirable for superior corrosion resistance.

EXAMPLE 25 Effect of inert atmosphere The following bath was used to plate the codeposit: SnCl 5 g./ 1. FeSO -7H O--48 g./1. NaHF 23.2 g./ 1. NaC135 g./1. Polyethylene glycols0.38 g./ 1.

In the first test, the cylinder in the rotating cathode cell was revolved at a peripheral speed of 1500 ft./min. Under these conditions, the bath becomes thoroughly entrained with air. The solution was titrated for SnCl at the start of the test and after 1.5 hrs. of operation. Results are shown below:

Intial SnCl 4.93 g./ 1. After 1.5 hrs.2.56 g./ 1.

In the next test, the top of the cell was covered with a split methyl methacrylate cover and N was introduced continuously at a rate sufiicient to exclude air from coming in contact with the bath. Results of this test are given below.

Initial SnCl 4.93 g./ 1. After 1.5 hrs-4.93 g./ 1.

This example shows that it is preferred to operate the bath under an inert atmosphere in order to avoid the entrainment of air which oxidizes the ferrous iron, thus necessitating replenishing the bath with both stannous tin and ferrous iron.

What is claimed is:

1. A process for preparing a composite coated metal article which comprises simultaneously codepositing iron and tin on a metal by electrodepositing the iron and tin from an aqueous, fluoride containing electroplating bath having as essential ingredients (1) a stannous tin concentration of about 0.015 to about 0.1 mole/1., (2) a mole ratio of ferrous iron to stannous tin having an upper limit defined by the equation:

Fe++ W mole ratio=0.2+14.7e-

and a lower limit defined by the equation:

2. The process of claim 1 wherein the stannous tin concentration is from about 0.02 to about 0.05 mole/l. and the mole ratio of fluoride to ferrous iron plus stannous tin is from about 3 to 7.

3. The process of claim 1 wherein the bath additionally contains from about 0.01 to about 5 g./l. of polyalkylene glycols having a molecular weight from about 1500 to 20,000.

4. The process of claim 1 wherein the bath temperature is from about to F.

5. The process of claim 1 wherein the bath is maintained under an inert atmosphere.

6. The process of claim 1 wherein the codeposited iron and tin coating is passivated.

7. The process of claim 1 wherein the codeposited iron and tin coating is overcoated with an organic synthetic tin.

8. The process of claim 1 wherein the codeposited iron and tin is heat-treated at a temperature of from about 485 to 720 F.

9. An electroplating bath comprising an aqueous fluoride, ferrous iron and stannous tin containing bath having as essential ingredients (1) a stannous tin concentration of about 0.015 to about 0.1 mole/1., (2) a mole ratio of ferrous iron to stannous tin having an upper limit defined by the equation:

and a lower limit defined by the equation:

12 and (3) a mole ratio of fluoride to ferrous iron plus stannous tin from about 1.5 to 7.5, said bath having a pH of about 2 to 4.

10. The electroplating bath of claim 9 wherein the bath additionally contains from about 0.01 to about 5 g./l. of polyalkylene glycols having a molecular weight from about 1500 to 20,000.

References Cited UNITED STATES PATENTS 2,407,579 9/1946 Schweikher 20454 2,758,075 8/1956 Swalheim 204-28 3,082,157 3/1963 Francisco et al 20454 3,138,548 6/1964 Ham et a1. 20432 3,334,030 8/1967 Notman 20437 J. H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner U.S. Cl. X.R.

mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: ua ggg i r Dated July as, 1970 Inventor-(s) Donald A. Swalheim It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I In Claim 1 at column 10, line 65, the portion after the second formula has been omitted. Claim 1 in its entirety should read:

1. A process for preparing a composite coated metal article which comprises simultaneously codepositing iron and tin on a metal by electrodepositing the iron and tin from an aqueous, fluoride containing electroplating bath having as essential ingredients (1) a stannous tin concentration of about 0.015 to about 0.1 mole/l. (2) a mole ratio of ferrous iron to stannous tin having an upper limit defined by the equation:

26(Mole/l. Sn++) W Mole Ratio 0.2 l ifie and a lower limit defined by the equation:

-72(Mo1e/1. sn g Mole Ratio 0.2 8.9e

and (3) a mole ratio of fluoride to ferrous iron plus stannous tin from about 1.5 to 7.5, said bath having a pH of about 2 to I 121 1 m Attestmgofficflr -188mm of Patent.