US2078868A - Electroplating process - Google Patents

Description

Patented Apr. 2 7, .1937

V UNITED STATES ELECTROPLATING PROCESS Floyd F. opunger, Niagara Falls, N. r., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application January 31, 1934,

Serial No. 709,171

12 Claims. (Cl. 204-17) This invention relates to electroplating metals and more particularly to a method of producing bright, smooth coatings of electroplated tin, cadmium, zinc and their alloys.

5 Electroplated coatings of tin, cadmium, zinc and alloys of these metals when taken directly from the electroplating bath ordinarily have a dull finish and are very susceptible to fingerstaining and to tarnishing. For most purposes in a bright, non-tarnishing finish is desirable. Or-

dinarily such electroplated coating is brightened by buffing and polishing; this, however, does not substantially reduce the tendency to tarnish.

Coatings of these metals, having a bright ap- 1; pearance, also may be obtained by dipping the metal articles to be coated in a molten bath of the coating metal. In order to obtain a smooth, uniform and bright surface by this method it is usually necessary to further treat the coated articles, e. g. by rolling, wiping or other treatment to remove excess metal and smooth the coatings. While this hot dipping method. of coating metals is capable of producing a bright, smooth surface, it has a number of disadvantages. In the first place, when iron or steel articles are thus coated, careful and tedious preparation 1 of the metal surface to remove oxide and other impurities is necessary before the metal is hot dipped in order to obtain a smooth adherent coat. These preliminary steps include pickling operations and annealing. Furthermore, metals which are coated by hot dipping have a relatively thin layer of coated metal because of the fact that the molten metal has a relatively low viscosity and when the article is removed from the hot dipping bath excess metal drains ofi, leaving a relatively thin coating on the article. The thickness of the coating is further reduced when a wiping or rolling operation is used. Hot dipping methods are also in general not readily applicable to odd shaped or deeply recessed articles. Frequently great difliculty is encountered in hot dipping methods because the metal to be v coated tends to dissolve in the coating metal so 41-, that the coating metal bath becomes rapidly contaminated.

In many cases an electroplating process is preferable for coating metals because less preparation of the metal prior to plating is required and ;o a thicker coating may be obtained. However, heretofore no entirely satisfactory process has been proposed for producing bright electroplated coatings of soft metals such as tin, cadmium or zinc. While bright electroplated coatings may be 35 obtained by plating under certain carefully controlled conditions or by buffing and polishing, such plates are very easily tarnished and flngerstained. While it is possible to obtain a bright surface on a plated article by bufiing and polishing without undue removal of coated metal, it is 5 difiicult and usually impractical to obtain a uniform bright coat on articles of irregular shape because of the difficulty of buffing and polishing deep depressions and irregular surfaces.

An object of the present invention is to produce smooth, bright, electroplated coatings of fusible metals such as tin, zinc, cadmium or their alloys Without the use of mechanical polishing operations, bath addition agents, etc. A further object is to produce such coatings on iron or steel. A still further object is to produce electroplated coatings of tin, cadmium, zinc or their alloys on iron or steel which coatings have superior adherence to the base metal and increased corrosion resistance. Other objects will appear from the following description of my invention.

The above objects are accomplished by electroplating metallic articles with metals fusible below the melting point of the base and subjecting the electroplated articles to a heat treatment under substantially non-oxidizing conditions at a temperature above the melting point of the coated metal. Preferably the coated metal is heated to a temperature slightly above its melting point for a short time, for example 10 to seconds. My invention also comprises a novel method of pickling iron or steel articles prior to electroplating as hereinafter described which results in a superior electroplated coating. By

these novel methods I have obtained smooth, bright coatings of zinc, cadmium, tin and their alloys which are characterized by having a low degree of porosity.

I am aware that it has been proposed to improve electroplated coatings by the application of heat. However, so far as I am aware, heretofore it has not been proposed to electroplate metals and subsequently heat the electroplated surface to a temperature above the melting, point under substantially non-oxidizing conditions to 5 obtain the smooth, bright, oxide-free coating which characterizes my invention. r

I have discovered that when heavy base metals such as iron, copper or brass are electroplated with metals having a melting point lower than the base metal, such as steel, zinc, cadmium, tin' or their alloys and the electroplated article is subsequently heated under substantially nonoxidizing conditions to a temperature above the 7 melting point of the base metal, a bright smooth Y plate is obtained which has a low degree of porosity. Such plate is similar in appearance to that obtained by the best methods of hot dipping or hot galvanizing and may be made of a much greater thickness. I have further discovered that such base metals, especially iron or steel may be plated with improved results if prior to plating the metal is treated with astrong acid to such extent that a distinct and uniform etching corrosion of the surface is obtained, followed by an alkaline treatment. With such pretreatment, the adherence of the plated coating is improved and the porosity of the plate is materially decreased. I have further found that when such pretreated metal is plated with tin, cadmium, zinc or their alloys and the plated coating is heat treated to a temperature above its melting point as described above, the resulting plate is smoother and more uniform than is the case when the base metal is not so etched and alkali treated prior to plating.

The above-mentioned etching prior to electroplating is especially useful in obtaining adherent, non-porous coatings of tin or the rust-resisting metals on cold-rolled sheet steel. Cold-rolled steel ordinarily has a bright, smooth surface. When bright, cold-rolled steel is electroplated for example with tin, the plated metal has relatively poor adherence and corrosion resistance. However, if the steel is previously treated with acid and alkali as described above, the electroplated tin has good adherence to the base and the corrosion resistance is markedly improved, without increasing the depth of the electroplated layer. These improved results are especially marked when the electroplated coating is in the neighborhood of 0.0001 inch thick or heavier.

One method of practicing my invention will be illustrated by reference to the production of a smooth, bright coating of tin or tin alloy on steel. The surface of the steel after suitably cleaning to remove grease and dirt, is first treated by immersion in a strong acid solution preferably at an elevated temperature, e. g. 100-200 F., until the surface of the metal'is distinctly etched so that a uniformly etched or corroded surface is plainly visible to the unaided eye. The steel is then treated with an alkaline solution, preferably by making the steel the anode in an alkaline cyanide bath and applying a moderate electric current for a minute or two. The treated steel is electroplated with tin or tin alloy by any suitable method to produce a tin layer of the desired thickness. I prefer to electroplate from an alkaline'solution, especially by the methods described in U. S. Patents 1,841,978 and 1,919,000. These or similar methods may be used for plating tin or tin alloys on the treated iron or steel.

After the desired thickness of tin or tin alloy has been plated on the steel, the article is heated under substantially non-oxidizing conditions to a temperature above the melting point of the tin or tin alloy coating. If the coating is of substantially pure tin (which melts at about 450 F.), heating maybe to a temperature of about 460 to 500 F. for a period of ten to thirty seconds. I

Various known means of heating under nonoxidizing conditions may be utilized in order to carry out my invention. For some work and with relatively low melting coatings I have found a bath of molten tallow to be suitable; for other pieces and especially for continuous operation,

Example 1 Samples of cold rolled sheet steel (automobile body stock) were first freed from grease and dirt by cleaning in an alkaline electrolytic cleaner containing OzJsal. Sodium cyanide 2 Caustic so 2 Trisodium phosphate 4 Temperature of solution-160-180 F. making it cathode and anode alternately; until entirely free from grease and dirt.

The samples then were pickled in a sulfuric acid solution containing by volume of sulfuric acid (66 B.) at a temperature of 70-90 C. for a period of 4 minutes. Aside from the removal of oxide or scale, the pickling was continued until the rolled surface layer was removed and a distinct etching was visible to the naked eye.

The samples then were water rinsed and further treated electrolytically in a sodium cyanide solution containing 4 oz./gal. of NaCN at 140-160 F., by making it the anode at 100 A/SF for 1 minute. After this treatment the samples again were water rinsed and then tin plated in an alkaline tin solution containing:

V 0z./gal. Sodium stannate 12 Caustic soda 1 Sodium acetate 2 Hydrogen peroxide y,

(100 vol.)

Data

Anode Straits tin Ratio of anode to cathode area 3 to 1 Cathode current density-+10 to 50 A/SF E. M. F 4.0 to 6.0 volts Temperature of solution 60 to 80 C. Time of plating 3 to minutes Thicknesses of 0.00005" to 0.0004" of tin were produced.

Following the plating, the samples were rinsed in cold and hot water and dried. The dry; samples were immersed for 10 to 20 seconds in a molten tallow bath at a temperature of 240-260 C. This treatment produced bright, smooth coatings of tin similar in appearance and hardness to that produced by hot tinning processes.

Eabample 2 Samples of cold rolled sheet steel were cleaned and then treated with acid and cyanide solution as described in Example 1. The treated samples were plated in an acid tin solution made up as follows:

Oz./gal. Sodium stannate 6 Sulfuric acid 16 Cresylic acid 1 Glue V 9,018,868 3 Data Example 7. -"w n Strait-Jilin Samples of sheet brass and copper were first Ratioof amde to catmde area 3 to I cleaned by making them cathodes for 3 to 5 Catlmde current density 6 A/SF minutes at 160 to 180 F. in an alkaline solution E. M. F 2.0 to 4.0 volts containing. Temperature of solution oz Mal I Normal room temperature sodium cyanide 2 Time of plating 3-0 to minutes msodmm f 2 Thickness of coatings 0.00005" to 0.0004" Caustic soda 4 Following the plating, the samples were heated I in a w a as described in Example 1, with iii/2t? iitif witii tiniitftte iiniiefwiifl Substantially identical resultspickled at 140 to 160 F. for 2 to 4 minutes in a solution containing: Example 3 Part by volume Cold rolled sheet steel samples were treated. Water 1 plated and heat treated as described 111 Exa p e Muriatic acid 1 t f gg g ig f g acid solution was The samples then were water rinsed and immersed for 3 minutes at 140 F. in a solution HCl Da t by vo ume 1 containing 4 oz./gal. of sodium cyanide. The Water part by vo um 1 treated samples then were plated and heat treated Temperature of solution C- 70 to 90 as in Example 1 Time of immersion minutes 2 to 5 Example 8 x mp 4 Samples of sheet brass and copper, cleaned as The procedure of Example 3 was repeated described in Example 7, were treated in the folcept that the following acid solution was used to lowing solutim' etch the steel: v o Cc./L

HNO; (cone) 1 part by volume Stnfuric .acid F 53o Sulfuric (cone) 1 Nitric acid (42 Be.) 160 Water 4 Hydrochloric acid (24 B.) 0.75 Temperature Normal room temperature 'Water f 320 Time of immersion seconds to 2 minutes After the acid treatment, the samples were immersed in a sodium cyanide solution contain- Example 5 ing 4 oz./gal. of sodium cyanide for 3 minutes Samples of hot rolled steel were first freed from 1 a temperature of Th mpl h grease and dirt by cleaning at 160 to 180 F. in were plated and heat treated as described in an alkaline electrolytic cleaner containing: p e With similar results.

ozJgag Example 9 22: :32 SgSE E LIIIIII 2 Pieces of cold-rolled sheet steel were first Trisodium phosphate L 4 cleaned to remove grease and dirt as in Exampie 1. .They were then plated without previous The work was made cathode and anode alternateacid etching or alkali treatment in a tin 801w 1y until entirely free from grease and dirttion similar to that of Example 1 to which had The samples were then pickled in a sulfuric been added about i f cadmium oxide acid olu i containing by volume of S111- dissolved in sodium cyanide solution. The plated furic acid at a temperature of to coatings had a thickness of 0.0001" to 0.0004". 90 for a period of 1 to 3 minutes 50 that an fire After plating, the coatings were treated in molten scale was removed and etching was plainly vistallow at about C Evidenceof the preslble to t e naked y ence of cadmium in the coating was presented I The samp s w e e water rinsed and by the fact that the coatings melted below 231 ther treated in a warm sodium cyanide solution 0,, th melting point of ti containing 8 oz./gal. of NaCN at 140 to 160 E, Bright deposits of cadmium-tin alloys were by making them anodes at a current'denslty of produced in all cases by the heat treatment. 50 to 100 A/SF for 1 minute. After this-treat- However, the coatings were lumpy, due to the ment, the samples were plated and heat treated omission of the acid etch and alkali treatment.

as in Example 1, with substantially the same resuits- Example Example 6 Pieces of cold-rolled sheet steel were degreased In place of the anodic cyanide treatment men- 2 32122 1 353% g -3:5 :6 2; 2 tioned in the foregoing examples, sheet steel 0 0 0 2 4 samples, after being etched in various acid solu- 32:23: 5 g gg fg z f 5225 :2: 3: $8, 22: ::gfigggiggggfiigf treated anodically in a sodium cyanide solution i oz lgal. (6 oz./gal.) at 140 to 160 F., using a current Sodium hydroxide 2 density of 50 to 100 A/SF for 1 minute. After sodium carbonate v 4 water rinsing the pieces were then plated in the tin-cadmium bath described in Example 9. Upon The anodic treatment was continued for 1 to 2 t treatment in tanow as described in Exanb minutes at a c e density of 50 to 100 pie 9, adherent, bright coatings of cadmium-tin The samples were then plated and heat treated alloy were obtained. These coatings were smooth as in Example 1, with the same results. and entirely free from lumps.

Example 11 Pieces of cold rolled sheet steel were degreased as described in Example 1. They were then acid etched as described in Example 13. Following this the same anodic treatment as in Example 13 was used.

The pieces were then plated with tin as described in Example 1 to a thickness of 0.0001" to 0.0003". Followingthis, they were plated with cadmium from a solution containing:

Oz./ gal. Cadmium oxide 4 Sodium cyanide 10 Sodium hydroxide 1 Data.

Anodes Cadmium Cathode current density 20 to 30 H/SF Temperature normal room temperature Coatings of cadmium 0.0001" to 0.0003" thick were applied to the previously plated tin coating so that the total thickness of the coatings varied between 0.0002 and 0.00006". These coatings were then heat treated in tallow at 200 to 300 C.

Adherent bright coatings of tin-cadmium alloy, free from lumps or runs were obtained.

Emample 12 Pieces of cold rolled steel were prepared and plated with 0.0001 to 0.0003" of tin as described in Example 11. Following the tin plating, the samples were zinc plated in a solution containing:

Oz./gal. Zinc cyanide 5 Sodium cyanide 3 Sodium hydroxide 4 Data Anodes s Zinc Cathode current density 20 to 30 H/SF Temperature 40 to 50 C.

The tin plated pieces were zinc plated with 0.0001" to 0.0002" of zinc.

Upon being heat treated in tallow at 200 C. to 300 C., adherent, bright coatings of tin-zinc alloy were obtained. The coatings were free from lumps and other imperfections.

Example 13 Pieces of hot rolled sheet steel were degreased in an alkaline electrolytic cleaner and than acidpiokled to remove rust and fire scale until a clean surface was obtained. Following the pickling, the pieces were immersed in a NaCN solution (6 oz./ gal.) and cleaned anodically for 1 minute using 50 to 100 A/SF. The samples were then plated as described in Example 11 with 0.0002" to 0.0003" of cadmium. The cadmium plated pieces were then plated in a zinc solution as described in Example 12 with 0.0001" to 0.0002" of zinc. The double plated pieces were then treated in tallow at 260 to 300 C. Adherent bright coatings of cadmiumzinc alloy were obtained free from lumps and other imperfections.

As mentioned above, in practicing my process I prefer to treat the metal to be plated, especially iron or steel, first with a strong acid under such conditions and for such length of time that a decided visible etching corrosion of the metal is obtained and then to treat the metal with an alkaline solution. This method of pretreatment of iron and steel markedly decreases the .po-

rosity of the electroplated coating and hence increases the corrosion reaistance of the plate. This effect is not limited to the low-meltinl metals such as tin, zinc or cadmium; for example, I have found that the corrosion resistance of nickel plate is improved by this pre-treatment. Furthermore, when it is desired to subsequently heat treat the electroplated coating by the herein described method, this preliminary etching and alkaline treatment assists in producing a smooth coating of the final product, especially when the electroplated coating is relatively thick. If such relatively thick coatings are heat treated subsequent to plating without this preliminary treatment, the final coating sometimes has certain irregularities or lumpiness, apparently caused by an uneven flow of the molten electroplated coating during the heat treatment and/or a failure of the plated metal to adhere well to the base metal. Also, if the metal is thus etched but the alkaline treatment is omitted, the desired satisfactory results will not be obtained. I have found that when my preliminary treatment is used, such irregularities may be substantially entirely eliminated.

The herein described preliminary etching and alkaline treatment is suitable for treating various heavy metals that are to be electroplated, e. g. steel, cast iron, brass and copper. It is especially advantageous when it is desired to produce bright coatings on cold rolled steel or cast iron by electroplating followed by heat treatment, as herein described. The tendency for the plated metal to fail to adhere properly to the base metal and form an irregular or lumpy coating during the heat treatment is most marked when the base metal is cold-rolled steel. For this reason, my preliminary acid etching and alkali treatments usually are necessary in order to produce a good coating on such steel by my invention. In the case of hot-rolled steel and the non-ferrous metals, the etching and alkali treatments often may be omitted or replaced by the usual cleaning operations with more or less satisfactory resiflts. However, it generally is preferable to use the etching and alkali treatment, since it tends to improve the quality of the plate in any-case.

The extent to which the base metal should be acid etched during the preliminary treatment may vary, depending on the nature of the base metal That is, in some cases, the minimum etching may be suflicient for the best results. The proper minimum time for treatment in the etching acid may be readily determined by simple trials, but usually the etching is carried on till a visible appearance of etching is noted.

Various acidic materials capable of corroding iron, steel or other base metals are suitable for the preliminary etching step in accordance with my invention. For example, sulfuric acid, hydrochloric acid, nitric acid or mixtures of these acids may be utilized. I prefer to use sulfuric acid in concentrations of 15 to 30% by weight. If desired, the acid may be used as electrolyte and the article to be etched made the anode in an electrolytic system. However, this electrolytic method of etching ordinarily offers little or no advantage over simple immersion in the acid solution. The time and temperature of the etching operation as well as the concentration of the acid in the solution may be varied over a wide range, provided that the operation-is continued over such length of time as is necessary to produce the required etching effect. I pre- 2 normal to 6 normal strength and carry out the etching operation at an elevated temperature,

for example 100 to 200 F.

The time required to etch the metal with acid.

and/or the type of acid or acid mixture required may vary depending on the nature of the metal. The kind of acid required to corrode a .depend partly upon the constitution of the steel and partly on the heat treatment and mechanical working to which it has been subjected. The best method of etching a given sample may easily be determined by simple trials with various acids or mixtures at various temperatures. When it is desired to' produce a mirror-like plate by the herein described heat treatment after plating, the steel should not be etched too drastically. However, even when the steel is drastically etched, the heat treatment subsequent to plating produces a distinct brightening and a smooth, dense plate.

Various alkaline aqueous solutions are suitable for the alkali treatment following the above described etching operation. I have found that in general the best results are obtained when the alkaline bath contains substantial amounts of an alkali metal cyanide; if desired, a solution of alkali metal cyanide alone may be used or sodium hydroxide or other alkaline materials may be added. I prefer to use a cyanide solution containing 2 to 10 oz; per gal. of sodium cyanide as an electrolytic cleaning bath, wherein the work to be treated is preferably made the anode and using an anode'current density of up to around 100 amps. per sq. ft. The best results generally are secured by keeping the bath hot, e. g.. at a temperature of 140 to 160 F. and using an anode current density of 50 to 80 amps.

.per sq. ft. Various other alkaline solutions may -be used for such electrolytic treatment.

The time of treatment may be varied widely; ordinarily to 3 minutes is suflicient when the electrolytic method is used- If the alkaline bath is used without the aid of the electric current, a somewhat longer time of treatment usually will be required, e. g. up to around 30 minutes.

As mentioned above, my process is suitable for coating metals with alloys of tin, cadmium, or zinc as well as the pure metals. Various other relatively fusible metals or alloys, e. g. lead or lead alloys, may be utilized in my invention. The alloy coatings may be obtained by electroplating the alloy from electrolytes containing salts of alloy constituents. An alloy coating also may be obtained in accordance with my inventibn by plating out the metals to be alloyed in two or more separate layers and'then subjecting the electroplated article to the above described heat treatment under non-oxidizing conditions. For

' example, an article may be plated with a layer of tin and then with a layer of cadmium or the tin may be deposited on the cadmium plate and the article thus double plated subjected to a heat treatment under non-oxidizing conditions at a temperature slightly above the melting point of the resulting tin-cadmium alloy. Likewise, other metals may be alloyed by this method with tin, cadmium or zinc; for example,- a tin-copper alloy may be made by first plating the article with tin and then with a light copper plate and subjecting the electroplated articles to the above described heat treatment. In this case, in order to produce a tin-copper alloy at the surface, it

is essential that the copper be plated over the tin. If thetin is plated over the copper, I have found that" complete alloying of the copper and tin cannot be obtained without the use of a heat treatment at at'emperature far above the melting point of tin. Such high temperatures are in many cases impracticable to use because of the effect they may have on the base metal. Hence in general when it is desired to produce an alloy of tin, cadmium or zinc with another metal such as copper or silver, which has a markedly higher melting point, by means of the multiple plating method, I prefer to plate the higher melting metal as the exterior coat prior to heat treating.

Various known methods of heat treating under non-oxidizing atmosphere may be used; I prefer to use a bath of hot tallow, or other high boiling liquids, fused salts or the like, or a furnace which is continuously provided with an atmosphere of oxygen-free hydrogen. Obviously these preferred methods may be modified by using other suitable non-oxidizing liquidor gaseous media. v

An advantage of my herein described inven-- tion is that it results in a smooth, bright plated coating which has superior corrosion resistant properties. The coating has the density, hardness, low degree of porosity and bright appearance which are characteristic of hot dipped coatings and furthermore, my coatings may be made of greater thickness than by hot dipping methods. Also, the step of heat treating subsequent to plating in accordance with my invention simplifies the electroplating operation, since it eliminates the need of originally obtaining plates having the best .color and general appearance. The heat treatment brightens the dullest plates substantially as well as the brighter ones. Hence it is necessary only to regulate the electroplating bath to obtain plating suitable for heat treatment.

Furthermorevmy improved method of treating metal prior to plating is advantageous not only in producing superior coatings by heat treating subsequent to plating, but also improves the electroplate itself as regards adherence and corrosion resistance. Hence,- improved results may be obtained by this method without heat treating subsequent to plating.

I claim: 1 i

1,. A process for electroplating a metal comprising treating said metal in an acid solution until the metal is visibly etched;\thereafter subjecting said metal to an anodic treatment in an alkaline solution, electroplating the treated metal with a metal or alloy having a melting point below that of said treated metal and subsequently heating the electroplated surface under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated coating.

2. A process for electroplating steel comprising treating said steel in a 2 to 6 normal acid solution containing at least one of the acids of the group sulfuric acid, hydrochloric acid and nitric acid until the metal is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkali metal cyanide solution and electroplattially non-oxidizing conditions to a temperature of 460 to 500 F. i

3. A process for electroplating cold-worked steel comprising treating said steel in a 2 to 6 normal acid solution containing at least of sulfuric acid until the metal is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkali metal cyanide solution and electroplating the treated metal with tin and subsequently heating the electroplated surface under substantially non-oxidizing conditions to a temperature of 460 to 500 F. for 10 to seconds.

4. A process for electroplating cold rolled steel comprising treating said steel in an acid solution containing at least one of the acids of the group sulfuric acid, hydrochloric acid and nitric acid until said steel is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkali metal cyanide solution and electroplating the treated metal with a tin coating and immersing the electroplated metal in a hot, substantially non-oxidizing liquid, whereby said tin coating is fused to form a smooth, bright coating.

5. A process for electroplating steel comprising treating steel in an acid solution until the steelis visibly etched, thereafter subjecting said steel to an anodic treatment in an alkaline solu--' tion, electroplating the treated steel with a metal or alloy having a melting point lower than that of said steel and subsequently heating the electroplated surface under substantially non-oxidizing conditions to 'a temperature above the melting point of the electroplating coating.

6. A process for electroplating steel comprising treating steel in an acid solution until the steel is visibly etched, thereafter subjecting said steel to an anodic treatment in an alkaline solution, electroplating the treated steel with tin and subsequently heating the electroplated surface 40 under substantially non-oxidizing conditionsto a temperature above the melting point of the electroplated coating.

- '7. A process for electroplating cold-worked steel comprising treating said steel in an acid solution until the steel is visibly etched, thereafter subjecting said steel to an anodic treatment in an alkaline solution, electroplatingthe treated steel with tin and subsequently heating the electroplated surface under substantially nonoxidizing conditions to a temperature above the melting point of the electroplated coating.

8. A process for electroplating cold-rolled steel comprising treating said steel in an acid solution until said steel is visibly etched, thereafter subjecting said metal to an anodic treatment in an alkaline cyanide solution, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated tin.

9. A process for electroplating steel comprising treating steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solution for a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplawd tin.

10. A process for electroplating steel comprising treating steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solution containing alkali metal cyanide for a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated tin.

11. A process for electroplating cold-worked I steel comprising treating said steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solutionfor a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electroplated tin.

12. A process for electroplating cold-worked steel comprising treating said steel in an acidic solution until said steel is visibly etched, thereafter treating said steel with an alkaline solution containing alkali metal cyanide for a period of at least three minutes, electroplating the treated steel with tin and subsequently heating the electroplated steel under substantially non-oxidiz-' ing conditions to a temperature above the melting point of the electroplated tin.

FLOYD F. OPLINGER.