US3708406A - Tinning high carbon ferrous metals coated with iron using alkaline plating baths with chelating agents - Google Patents

Abstract

HOT DIP TINNING A HIGH CARBON FERROUS METAL SUCH AS CAST, GREY, MALLAEABLE, MOTTLED, WHITE, OR WROUGHT IRON BY FIRST CLEANING FROM THE METAL ANY SOIL WHICH MIGHT PREVENT ELECTROPLATING ON IT, THEN ELECTROPLATING ON IT AN ELECTRODEPOSIT OF IRON FROM AN ALKALINE IRON-PLATING BATH CONTAINING AN ORGANIC SEQUESTERING AGENT AND AN IRON COMPOUND.

Description

"United States Patent C Patent No. 3,547,787, dated Dec. 15, 1970. This ap-' plication Oct. 6, 1970, Ser. No. 78,562 v Int. Cl. C2311 5/04, 17/00 US. Cl. 204-38 S 20 Claims ABSTRACT OF THE DISCLOSURE Hot dip tinning a high carbon ferrous metal such as cast, grey, malleable, mottled, white, or wrought iron by first cleaning from the metal any soil which might prevent electroplating on it, then electroplating on it an electrodeposit of iron from an alkaline iron-plating bath containing an organic sequestering agent and an iron compound.

This application is a continuation-in-part of my copending application Ser. No. 172,118 filed Feb. 9, 1962 and now abandoned, and a division of my copending application Ser. No. 627,309 filed March 31, 1967 now Pat. No. 3,547,787 issued Dec. 15, 1970.

This invention concerns the hot tin dipping, i.e., hot tinning, of coarse-surfaced ordinarily relatively high carbon content ferrous metals and alloys, such as cast iron as grey cast iron, malleable iron, and wrought iron, the hot tinning of which heretofore could not be accom lished readily and at best could be carried out only by undesirable methods involving considerable hazard and expense and yielding only erratic results.

More specifically the invention is that of hot tinning a coarse-surfaced ferrous metal by initially cleaning it of any soil which could prevent adherence of a fresh metal electrodeposit on it; electroplating on it a firmly adherent at least masking electrodeposit of nascent iron from an alkaline iron electroplating bath having a pH up to about 14 and containing an organic sequestering agent, and thereafter hot tinning it in the usual way used, for example, on cold rolled steel.

The high carbon content ferrous metals such as cast iron as grey cast iron, malleable iron, and wrought iron ordinarily, that is to say when clean (e.g., free of molding sand or scale) and before being surface ground or polished, have a coarse surface, apparently because of their coarse grain structure as distinguished from that of the low carbon steels which are easily rolled and shaped and still present a substantially overall flat, smooth surface.

Accordingly, the said coarse-surfaced high carbon content ferrous metals and alloys which include among them also those having high silicon content such as the acidresistant ferrous metals as the one long known by its trademark Duriron, conveniently can be called ferrous metals ordinarily having a coarse surface or singly an ordinarily coarse-surfaced ferrous metal. The Duriron alloy is a ferrous cast alloy containing, in addition to iron, 14.5% silicon, 0.85% carbon, and 0.65% manganese.

Heretofore, considerable difficulty was encountered in trying to accomplish regularly satisfactory practical hottinning of an ordinarily coarse-surfaced ferrous metal. Generally, it has been highly difficult, and for the most part possible only in a limited way and under special conditions, dependably to hot tin coat an ordinarily coarseice surfaced ferrous metal, especially cast iron such as grey cast iron.

British Pat. 11,698 of 1906 to John Swain describes coating cleaned cast iron articles with a layer of pure iron from an electrolytic bath prepared from iron or steel turnings, hydrochloric acid and lye, preferably using 2 pounds of turnings to 4 pints of hydrochloric acid of 1.14 specific gravity, and half an ounce of alkali preferably caustic soda. The latter is said to combine with the hydrochloric acid to give sodium chloride, and that common salt may be used instead.

That is a very confused direction giving a bath apparently uncertain as to operation because 4 pints of 1.14 specific gravity hydrochloric acid provides only 1.3 pounds of hydrogen chloride, That amount is only half of the equivalent needed for 2 pounds of iron or steel turnings and thereby wholly inadequate when one part of ferrous chloride dissolves in one part of water only in the presence of hydrochloric acid. That may explain why no use appears to have been made of that procedure, but instead other methods were tried.

Thus, to a limited extent hot tinning has been carried out with some very few ordinarily coarse-surfaced ferrous metals, for example, by first subjecting the such ferrous metal article to a special pre-treatment in a molten bath of one of a few available proprietary inorganic cleaning and descaling compositions. This involves preliminarily preparing the surfaces of such ferrous metal products for the hot tin dipping operation by immersing them, for eX- ample, in a catalyzed molten salt reduction bath (having a melting point of about 500 F.) and at an operating range between about 850-950 F., and passing a current through the molten bath.

Such a procedure is overly costly due to the energy required to maintain the bath molten. It also is highly hazardous to the operators working about such bath, for

example, from dragout on the articles leaving such hot I bath. In addition the results are inadequate for there appears to be a too frequent lack of uniformity in the coating, so that the percentage of rejects is undesirably too high for general practical operation.

The foregoing disadvantages and others are overcome by the process of the invention which is a significantly less costly, relatively much quicker, and considerably safer method of hot tinning. Also, it provides a resulting more regularly and uniformly dependably tin coated product.

Considered broadly, the process of the invention is that of applying a hot tin coating to an ordinarily coarse-surfaced ferrous metal, by removing from the surface of said metal any soil (including rust) which initially should be removed, electroplating on the clean coarse-surfaced ferrous metal in an alkaline iron electroplating bath having a pH up to about 14 and in the presence of a sufficiently effective quantity of a squestering agent, a firmly adhering, substantially continuous, at least masking iron electrodeposit; removing the thus electroplated ferrous metal from the plating bath and any plating solution adhering to the plated metal; and thereafter dipping the thus plated ferrous metal, with or without intermediate immersion in a tinning flux bath, into the molten tin bath to apply the tin coating to that iron electro-plated ferrous metal.

The foregoing sufficiently effective quantity of a sequestering agent is an amount substantially sufiicient for iron in any iron compound in the bath to be present as the iron chelate of the sequestering agent.

The masking electrodeposit is essentially pure carbonfree iron. In describing this electrodeposit of iron as firmly adhering, substantially continuous, at least masking the expression firmly adhering means that the deposit adheres to the ferrous base metal without flaking and is non-peeling from that ferrous base metal.

The expression at least masking means that the deposit is thick enough at least to mask the generally overall finely spotted or matted surface of the ordinarily coarse surface of the high carbon content ferrous metal, to the extent that the finely spotted or matted surface is replaced by a substantially uniform, continuous silvery-grey clean iron appearance.

The method of the invention can be carried out by depositing the firmly adherent, substantially continuous, at

least masking electrodeposit of iron on the coarse-surfaced ferrous metal from an alkaline iron plating bath having a pH up to about 14 and containing a sufficiently effective concentration of a sequestering agent. Obviously, before starting the electrodeposition, any rust or other soil which could prevent uniform adherence of the electrodeposit, should be removed from the surface to be hot tin coated.

In many cases wherein the residual molding sand or oil or grease, or rust are not extraordinarily excessive, advantageously such soil can be removed in the same bath in which the iron electrodeposition is to be carried out, and especially so if there is no oil or grease or possibly only an insignificant amount of them.

Effective as such iron plating bath for the firmly adhering electrodeposit of iron is any of a wide variety of aqueous alkaline iron plating baths having a pH up to about 14 and dissolved therein various effective amounts of one or more sequestering agents which form a watersoluble chelate with iron, whether ferrous or ferric, along with an effective concentration of an alkali metal hydroxide, such as sodium or potassium hydroxide, or ammonium hydroxide.

Particularly effective organic sequestering agents are the various monohydroxy or polyhydroxy, monoor polycarboxy lower aliphatic acids having from two through seven carbon atoms such as gluconic acid, citric acid, tartaric acid, glucoheptonic acid, and its isomers galactoheptonic acid, fructoheptonic acid, and the mixed hexahydroxyheptoic acids, and saccharic acid, or an amino, polyhydroxy lower aliphatic acid such as 3-amino-2,4, 5,6,7-pentahydroxyheptoic acid, or other sugar acid, as well as the alkali metal and ammonium salts of any of those acids and the alkaline earth (including magnesium with them, and at least for ferric iron) salts of any of those polycarboxylic acids.

Any of these various six or seven carbon atom polyhydroxy acids or any of the sugar acids can be admixed with one another or with any of the other sequestering agents, and advantageously with from about one-third to three times its quantity of a hexitol such as sorbitol or mannitol.

The sequestering agents include also the polyalkylene polyamine polyacetic acid compounds and their monoand divalent metal salts, for example, diethylenetriamine pentaacetic acid and any of its alkali metal and ammonium salts or even any of its alkaline earth salts (at least for ferric iron) as its calcium or magnesium salts, and any of the mono-hydroxyethyl-tetra-carboxymethyl diethylenetriamines or dihydroxyethyl-tricarboxymethyl diethylenetriamines, any any of the corresponding same salts of any of them, as well as any of the free acid and salt form sequestering agents disclosed in US. Letters Patent 2,831,885, 2,848,469, 2,859,104 and 2,906,762.

While individual alkaline plating baths can be prepared using any one of the foregoing and other effective sequestering agents along with a suitable amount of alkali metal hydroxide to give an effective pH value up to about 14, more than one of any of the applicable sequestering agents can be used. There can be included various amounts of ethylenediamine tetraacetic acid or any of its monoto tetra-alkali metal or ammonium salts as well as any of its alkaline earth metal salts (including magnesium among them), and generally to the extent of no more than about one-half the amount of the other sequestering agent, or of a lower alkanolamine such as mono-, di-, or triethanolamine and like propanolamines.

The aqueous alkaline baths can be used at any suitable pH beneficially above 11 depending on the sequestering agent or agents and the nature of any other alkaline agent or agents included, and up to about pH 13.8 and possibly somewhat higher. These various alkaline baths exhibit advantageously good throwing power, and serve effectively in the preliminary cleaning as referred to above.

When the preliminary cleaning is not going to be conducted in the plating bath or when the amount of rust to be removed is low, the aqueous plating bath should contain in solution at least a sufficient amount of a compatible iron salt or chelate, which is soluble at the pH of the plating bath to avoid exhausting its iron if no iron-bearing anode is being used. It is advantageously, however, to use some suitable iron-bearing anode. Thus generally, the starting composition of the bath need contain only very little of a water-soluble iron compound such as an iron salt or chelate when the bath will be used to clean a sufficient amount of rust from the cast iron articles which are to be given the iron electrodeposit, or ironbearing anodes are to be used.

To provide the initial iron content of the bath when iron-bearing anodes are used, it can contain a relatively small amount such as about one-tenth percent of such Water-soluble iron salt as a ferrous or ferric salt soluble of the pH of the bath, such as ferrous or ferric sulfate, chloride, or nitrate, or as thus far noted to be advantageous ferric acetate, as well as any of the iron chelates of any of the applicable sequestering agents and soluble at the pH of the bath at least to the extent sufiicient to enable electrodeposition of iron to be initiated therein.

In some cases, initial content of such iron salt or chelate can be quite small and at times avoided when the bath is to be used for the preliminary cleaning of the cast iron or other ordinarily coarse-surfaced ferrous metal which is to receive the iron electrodeposit. That is so because such preliminary cleaning, carried out advantageously by periodic reverse current procedure, results in providing an adequate initial amount of dissolved iron in the bath sutficient to enable electrodeposition of iron on the cathode to progress by continued dissolution of iron from the particular iron-containing anode used to enable depositing the necessary firmly adhering electrodeposit of iron on the originally coarse-surfaced ferrous metal cathode.

For regularly dependable deposition of iron, the total dissolved solids in the bath can range from about one to about four pounds per gallon (i.e., about to about 450 grams per liter). A generally good practical concentration, bearing in mind such factors as conductivity, plating rate, and dragout, is in the neighborhood of about two to about 2.5 pounds per gallon. However, for higher conductivity with certain solutions (e.g., sodium gluconate without any other added salts), it is more desirable to work with solutions of at least two pounds and nearer about four pounds per gallon.

The concentration of the sequestering agent, whether one or a mixture of them, can vary widely, generally from about two to about one hundred percent of the total solids content, and preferably from about fi ve to about ninety-five percent of it, depending on providing the required pH value or range.

Grey cast iron, or black iron or other cast iron is very satisfactory for anodes of ferrous material to replenish iron to the bath as it is plated out, to provide a consistently uniform masking iron electrodeposit on the initially ordinarily coarse-surfaced ferrous metal cathode. Electrolytic iron anodes also are suitable. At times even cold rolled steel anodes can be used. To avoid interference with the quality of the iron deposit by suspended carbon particles released from a high carbon content ferrous metal anode over continued use, it is desirable to enclose such anodes in Orlon or other suitable anode bags, as preferable over periodic or continuous filtration of the bath.

Consideration should be given to the relationship of anode area to that of the articles being plated and thus serving as cathodes. Generally, it is advisable that the anode area be significantly greater than that of the part to be plated, and even up to double its area particularly if the cathode part has deep hollows.

The bath may be operated over a wide temperature range, even as low as ambient (i.e., room) temperature, but at such level the plating rate is very slow (e.g., at about 80 F.), and the voltage needed for suitable current density is excessive, being from 12 to 15 volts or even more. A presently indicated most practical temperature range is from about 140 to about 180 F., although there is no discernible difference in the adhesion and general-1y desirable character of the iron electrodeposit even at the lower temperatures. Where conditions permit, very satisfactory practical results occur at as high as 190 F. and are obtained even at 200 F. and can be obtained also at possibly even a higher point. It appears generally advisable, of course, to work safely below the baths boiling point.

Current density, for generally good results, should range from about to about 80 amperes per square foot, and under many conditions can be as high as 100 amperes per square foot. However, for cathode articles having sharp points or projections, it may be advisable to operate somewhat under 80 amperes per square foot to avoid burning at such points. Ordinarily the lower the temperature, the higher should be the current density.

Thus, the maximum current density for any particular bath should be just under that at which the electrodeposit would begin to show signs of burning. However, the current density generally would have to be well over 100 amperes per square foot before any indication of burning or other undesirable injury can occur to the iron electrodeposit on the originally coarse-surfaced ferrous metal article cathode, or a flaky (and thus undesirable) electrodeposit can be produced.

Elcctrodeposition time has to vary with the character of the surface to be electroplated, bath composition, temperature, current density, and any other plating condition. With some combinations of conditions, possibly as little as about seven minutes or so may be adequate, and in others more than that and possibly up to about fifteen, or even an about twenty-five minute or more plating cycle may be needed; the rougher the surface, the lower the temperature, the longer the time required ordinarily. Also the heavier or bulkier the work piece, the longer the deposit time.

The method of the invention is operable readily in quantity production scale. Quite often the alkaline bath used for the plating step of the method of the invention can be used for the preliminary treatment to remove the average ordinary amounts of soil and rust encountered on the general run of articles which will need to be iron plated in the bath. As already indicated, such soil can be removed by subjecting the articles to preliminary electrolytic treatment, including periodic reverse current, for a time sufficient to remove the soil and rust. That will depend on the type and extent of soil and rust, the bath, and the treatment.

For some combinations of these conditions, including mild soil and/or rust, two or three minutes of periodic reverse current treatment may be adequate. Slightly heavy soil and rust, possibly may need from about ten to almost fifteen minutes. For heavy soil and/or rust conditions, even up to about thirty minutes or so may be required.

For such preliminary cleaning treatment in an alkaline bath, and possibly more so for organic soils, it may be helpful to include an alkali metal carbonate in an amount below that at which its concentration could interfere with the quality of the resulting iron electrodeposit. Excess such carbonate even can present entirely the iron deposition. Present indications are that such carbonate content should 6 be restricted to no more than about ten percent of the total dissolved solids in the bath.

For some soils, perhaps more so with oil and grease, it can help to include a small content, generally under onehalf percent and possibly more often about half of that or less, of a synthetic detergent, nonionic or anionic and at times even cationic, or a mixture of any of them, as specific conditions may dictate.

As stated earlier above, the iron electrodeposit needs to be sufficient to mask the initial ordinarily coarse, generally grainy surface of the cathode article. Because of the relatively rough and irregular surface of even the initial coarse-surfaced ferrous metal base, in that it is not fully fiat and smooth as in the case of low carbon steel, no specific numerical minimum electrodeposit thickness can be given for each different ordinarily coarse-surfaced ferrous metal. However, the electrodeposit thickness appears to be adequate when the cleaned surface is covered with the plated iron to the extent that the plated surface appears to be a firmly adherent, overall substantially continuous masking iron electrodeposit.

Such adherent, overall substantially continuous minimum deposit then is sufiicient to resist flaking or being destroyed or burned away at the temperature of the molten tin bath used in the hot tinning operation, as tin melts so far below these ferrous metals. Ordinarily, the iron electrodeposit thickness does not have to be much more than that just described above, even though the thus plated surface then may not be entirely flat. A slightly thicker deposit even below 0.0001 inch could be more practical.

However, it is diflicult also to set a numerical maximum electrodeposit thickness applicable to all surfaces of the various coarse-surfaced ferrous metals. While about 0.0001 inch thick might be more than enough for most conditions, yet, where particularly needed or desired, it could be as much as up to about 0.0002 inch. Generally, there does not appear to be any particular need to plate a deposit that thick or thicker than that.

While the method of the invention is applicable to hot dip tinning of any ordinarily coarse-surfaced ferrous metal article, it is applicable particularly to such tinning of cast iron such as grey cast iron. Accordingly, the invention will be more fully illustrated below, but is not intended to be restricted, by a description of details of its application to hot dip tinning of grey cast iron. It applies equally to such tinning of any other coarse-surfaced high carbon content ferrous metal and even nodular iron.

Cast iron castings of a rectangular prismatic box (25 inches long by 4 inches square inside cross-section) open at one side were cleaned of adhering loose mold sand, in customary manner. They then were hung suitably spaced from one another from horizontal arms of a cathode rack and were immersed, properly spaced from grey i-ron anodes (of about double the cathode area), in an aqueous alkaline iron plating bath held at about 200 F. and containing per liter 170 grams of sodium hydroxide, 60 grams of sodium gluconate, and 9.5 grams of sodium carbonate.

Since these cast boxes were dirty and rusted in areas, they were subjected in this bath for fifteen minutes to periodic reverse current of five seconds direct current to the cathode, and ten seconds the reverse, to clean them. Without removing them from this bath, direct current (set to deliver amperes per square foot of cathode area) then was passed from the anodes to these boxes as cathodes to deposit iron on them for fifteen minutes. The castings then were removed, rinsed adequately with warm water and allowed to air dry. They showed a continuous and uniform electrodeposit of iron firmly adhering to and completely masking the original speckle spotted cast grey iron surface.

The boxes on half of the racks then were immersed in the customary tinning flux bath. Then these and the boxes on the other half of the racks were submerged in customary manner and for the usual time in the molten tin bath, and removed and allowed to cool. Both batches of the boxes showed uniformly overall adherent dependable tin coatings.

Any of the various combined plating bath operating conditions in the foregoing more fully described illustrative operation can be changed at least within the various ranges disclosed herein to be practical. So also the same bath at any combination of suitable operating conditions can be used for cleaning and also to plate a corresponding masking iron electrodeposit on any other such castings of the same iron or any other ordinarily coarse-surfaced ferrous metal of the type disclosed herein.

As already stated above, the already described firmly adherent, masking, overall substantially continuous electrodeposit of iron can be plated out of any other suitable alkaline iron plating bath used in this invention, such as are illustrated by, but not restricted to, the aqueous iron plating bath compositions containing respectively the following ALKALINE BATHS Grams per liter Sodium glucoheptonate 45 Sodium hydroxide 185 Sodium carbonate This bath shows pH 13.8. Its sodium carbonate content can be reduced or even omitted for it is not essential to obtaining from it a good iron electrodeposit. In a mixture of the foregoing three as dry ingredients in their same proportions, the sodium carbonate included serves to prevent caking of the mixture.

Grams per liter Citric acid 45 Sodium hydroxide 195 In this aqueous bath, obviously 28 grams of the sodium hydroxide neutralizes the citric acid so that the composition of the aqueous solution is equivalent in grams per liter to about 60 of sodium citrate and 167 of sodium hydroxide. It shows pH 13.6.

Grams per liter Sodium diethylenetriamine pentaacetate 68 Sodium hydroxide 40 The sodium diethylenetriamine pentaacetate used in making up this bath (of pH 13.6) was used as its available 34% aqueous solution.

Grams Sodium glucoheptonate 0.3 Sorbitol 0.1 Sodium hydroxide Water 84.6

The free alkali in any of these alkaline baths can be replaced by the equivalent or other amount of any other herein indicated applicable alkaline agent. The quantity of any such alkaline agent in any of the foregoing baths, or any modifications of them, can be varied by reasonable increase or decrease so long as the pH beneficially is above 11.

Likewise, the sequestering agent in any of those baths can be replaced in part or as a whole by the same or other quantity of any other herein applicable sequestering agent indicated to be effective in an alkaline bath and within the indicated range and beneficially with the bath having a pH over 11.

The sodium hydroxide of baths (a) to (d) can be replaced by any other herein indicated applicable alkaline agent, even compatible amine, e.g., diethylamine or a mono, dior triethanolamine or mixtures of them so long as the bath pH still is above 7 and beneficially at least about 11.

The sodium glucoheptonate of baths (a) and (d) and the citric acid of bath (b) can be replaced in whole or part by any quantity of the other of them or of any other hereinabove disclosed sequestering agent which is sufficiently soluble in water at the pH of the bath for it to be within the recently above indicated pH range.

The sodium diethylenetriamine pentaaeetate of bath (0) can be replaced in Whole or part by its corresponding salt of any other alkali metal or its ammonium. or other above-indicated amine salt, or by the corresponding tetra-, tri-, or di-acetate, or by any other herein disclosed sequestering agent sufficiently soluble in water for the pH of the bath to be within the above disclosed range.

In addition, that originally included alkaline bath in that above fully described illustrative complete operation, showing the overall treatment of cast iron boxes, can be replaced by any other alkaline bath respectively specifically identified in any of the foregoing specific baths or any above explained possible modifications of them.

However, where it is possible to conduct the electrolytic cleaning in the same bath from which the iron deposit is to be plated out, the bath should be at a high enough pH to enable a practical rate of such cleaning to occur.

It is mentioned above to include a water-soluble iron salt in a plating bath to facilitate initiating iron electrodeposition in it especially when that same bath is not used initially to clean the article to be plated. In such case, any water-soluble iron salt or chelate (e.g., ferrous gluconate) for the foregoing purpose advantageously can be included in a dry mix containing one or more such salt or chelate or other iron (ferrous or ferric) salt directly soluble or by chelation in the plating bath at its specific pH, along with the selected sequestering agent or agents, and also any alkaline substance which may be needed to adjust the bath to the required pH after the mix is dissolved in water. If cost is not a deterrent, the soluble form of iron and the required chelating agent or agents could be combined and included in the mix as the iron chelate equivalent of the quantity of sequestering agent content required.

The separate iron salt or chelate then could be included in any such dry mix in an amount to provide a minimum of, say, from about 0.02 to about 0.1 percent of iron in the aqueous plating bath. That will provide about 0.2 to about one gram of iron per liter of such aqueous bath. The dry mix then can contain enough of the iron salt or chelate to provide a minimum of, say, from about 0.2 to about 1.25 grams of iron per hundred grams of dry mix to be added at from about to 600 grams (of mix) per liter of water.

More water-soluble iron salt can be included in such mix, up to a maximum which conveniently can be set at the amount of iron required to form the iron chelate with all of the sequestering agent content of the mix. However, it is advisable to keep the iron content of the mix below its chelate equivalent of all of its sequestering agent content.

The dry mixes are illustrated by, but not restricted to, the following compositions:

This whole mix dissolved in a liter of water shows a pH about that of bath (a) above.

Grams Citric acid 45 Sodium hydroxide Ferric citrate 10 Sodium carbonate 10 240 grams of this mix dissolved in a liter of water has a pH about that of bath (b) above.

The ferrous gluconate or ferric citrate in any of these dry mixes can be increased or decreased within the hereinabove indicated range of content of iron in the dry mixes, or can be replaced in part or as a whole by the iron equivalent amount of the other of them or of any other ferrous or ferric chelate of any other chelating agent, such as those shown in any of the patents identified above, or by any ferrous or ferric salt or salts soluble in the bath, e.g., ferric acetate.

Any of the sodium glucoheptonate or citric acid used in any of these dry mixes can be replaced in part or as a whole by a corresponding amount of any other alkali metal or ammonium salt or other hereinabove indicated salt of any of them or of any other applicable sequestering agent available in solid discrete particle form, with or without any minor amount of sodium or other alkali metal carbonate which may need to be added to prevent caking. All such dry mixes are embraced herein as a part of this invention.

The method of the invention works also with the low carbon ferrous metals such as the low carbon steels which can be cold rolled and shaped by such operations as forging and spinning. However, ordinarily such low carbon ferrous metals readily can be hot tinned.

Thus, the method of the invention is beneficial primarily with those ferrous metals, including their alloys, with which some difliculty or disadvantage is met in attempts to hot tin coat them so that they generally cannot be hot tin coated readily or else require some special preliminary treatment more diflicult, hazardous, and/or costly than merely preliminary electrodeposition of iron.

Herein and in the appended claims, the expression ferrous metal is to be broadly construed as including the ordinary commercial forms of the various irons as well as its common alloys composed highly predominately of iron.

While the invention has been explained more extensively by detailed description of certain specific illustrative embodiments of it, it is understood that various modifications and substitutions can be made in any of the thus described embodiments within the scope of the appended claims which are intended also to include equivalents of any such embodiments.

What is claimed is:

1. The method of hot dip tinning a ferrous metal selected from the class consisting of cast iron, grey iron, malleable iron, mottled iron, white iron, wrought iron, and any other ordinarily coarse-surfaced ferrous metal which ordinarily can be so tinned only in a limited way and under specially different conditions, which method comprises (a) initially cleaning from the surface of such ferrous metal any soil which could prevent electroplating thereon an adherent electrodeposit of iron;

(b) electroplating on said surface a firmly adherent substantially continuous, at least masking electrodeposit of iron while said cleaned ferrous metal is a cathode immersed in an aqueous alkaline iron-plating bath having (i) a maximum pH of about 14 and (ii) dissolved therein an organic sequestering agent in an amount substantially suflicient to hold the iron in any iron compound in the bath dissolved therein as the iron chelate of said sequestering agent, and for a time sutficient for said electrodeposit to be plated on said metal;

() removing the thus iron-plated ferrous metal from said bath and any plating solution adhering to said electrodeposit; and

(d) thereafter contacting said iron-plated surface of the ferrous metal with molten tin and thereby applying over said surface a continuous coating of tin, and allowing said tin coating to cool to solidify.

2. The method as claimed in claim 1, wherein after 10 removing any plating solution from the plated ferrous metal removed from the plating bath and before contacting the iron plated surface with the molten tin, the plated ferrous metal surface is contacted with tinning flux.

3. The method as claimed in claim 1, wherein the sequestering agent is hydroxy-substituted lower aliphatic, soluble in the bath, and includes at least one group --COOR wherein R is a cation which can be replaced by iron at whichever valence it exists in the bath and for it to form a chelate with the ligand portion of said sequestering agent.

4. The method as claimed in claim 3, wherein said sequestering agent is polyhydroxy.

5. The method as claimed in claim 4, wherein said polyhydroxy-substituted sequestering agent has from 6 to 7 carbons.

6. The method as claimed in claim 5, wherein the sequestering agent has 6 carbon atoms in a straight chain and a hydroxyl group attached to each of the 5 carbons other than in the group COOR.

7. The method as claimed in claim 6, wherein said sequestering agent is an alkali metal gluconate.

8. The method as claimed in claim 7, wherein said alkali metal is sodium.

9. The method as claimed in claim 5, wherein the sequestering agent content of the bath comprises and R is defined as in claim 5.

10. The method as claimed in claim 9, wherein the sequestering agent is an alkali metal glucoheptonate.

11. A method as claimed in claim 10, wherein the alkali metal is sodium.

12. A method as claimed in claim 9, wherein said bath also contains a hexitol dissolved therein in an amount from about one-third of to three times the weight of said sequestering agent.

13. A method as claimed in claim 12, wherein the sequestering agent is sodium glucoheptonate and the hexitol is sorbitol.

14. The method as claimed in claim 9, wherein the sequestering agent is sodium glucoheptonate and the bath contains from about forty to about fifty grams per liter of it and enough alkali metal hydroxide to have a pH of from about ten to about fourteen.

15. The method as claimed in claim 3, wherein the sequestering agent has from six to nine carbon atoms, from zero to one hydroxyl group, from two to five carboxyl groups, and is not otherwise substituted.

16. The method as claimed in claim 15, wherein the sequestering agent content of the bath comprises HzO-COOR HO- 'COOR HzCOOR wherein R is a cation which can be replaced by iron at whichever valence it exists in the bath and for it to form a chelate with the ligand portion of said sequestering agent.

17. The method as claimed in claim 16, wherein the sequestering agent content consists essentially of an alkali metal citrate.

18. The method as claimed in claim 1, wherein prior to direct current deposition of iron on the ferrous metal, soil is removed from it by subjecting it to the effect of periodic reverse current in the plating bath for a time suflicient to remove any soil from its surface; and then subjecting the thus cleaned ferrous metal as a cathode while in the same bath to direct current to electroplate on it said electrodeposit of iron.

19. The method as claimed in claim 18, wherein the sequestering agent is selected from sodium citrate, sodium gluconate, sodium glucoheptonate, and sodium diethylenetriamine pentaacetate.

References Cited UNITED STATES PATENTS Ramage 20438 S Nourse 20438 8 Braddock 20438 S Tsu et a1. 20443 Poor 20448 12 7/1955 Meyer 20448 9/1894 Schaag 20438 S FOREIGN PATENTS 3/1907 Great Britain 20438 S 12/1893 Great Britain 20438 S 5/1918 France 20448 11/1931 Italy 20448 1/1960 U.S.S.R 20448 10 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. 01. X.R.