US3308042A - Electrolytic tin plating - Google Patents

Description

March 7, 1967 R. LOZANO ETAL 3,308,042

ELECTROLYTIC TIN PLATING Filed June 11, 1963 Inventors Robert Loano Cog S. flam- Charles C. Marsha" Jack E. Jogce 3a 196M H-kornegs United States Patent f 3,308,042 7 ELECTROLYTIC TIN PLATlNG Robert Lozano, Hammond, Coy S. Ham, Munster,

Charles C. Marshall, Gary, and Jack E. Joyce, Chesterton, Ind, assignors to Inland Steel Company, Chicago, Ill., a corporation of Delaware Filed-June 11, 1963, Ser. No. 287,138 1 9 Claims. (Cl. 20428) The present invention relates generally to an electrolytic method of producing tin plate,.and-more particularly to a method of producing electrolytic acid tin plate having superior corrosion resistance,

Most tin plate used in themetal container industry is electrolytic tin plate produced either from an acid plating solution or an alkaline solution. The acid'electrolytic plating systemis considered by many to be a faster, simpler and a more economical technique. Therefore, a greater proportion of tin plate is produced by the acid plating system than by the alkaline plating system. However, alkaline tin plate which has satisfactory special property test values, such as pickle lag, alloy tin couple, and iron solution values, consistently demonstrates superior corrosion resistance; whereas acid tin plate having similarly satisfactory special property test values characteristically demonstrates only average corrison resistance. Based on these differences, tin plate having superior cor-. rosion resistance is designated Grade A plate; whereas tin plate having average corrosion resistance is designated Grade B plate. Grade A electrolytic tin plate has at least one-third better corrosion resistance than hot-dip (Common Coke) tin plate. Acid electrolytic tin plate is usually rated Grade B, and at present no commercial electrolytic acid tin plate manufacturer in the United States is consistently able to produce Grade A ac d tin plate.

Prior attempts to produce Grade A electrolytic acid tin plate have involved changes which result in the formation of a more continuous tin layer by increasing the size of the tin crystal. More recently, a procedure has been described for producing Grade A acid tin plate in which the acid tin plate before flow brightening is subjected to a sub-fusion heat treatment to provide a substantially continuous tin-iron alloy layer between the ferrous metal base and the outer free tin layer, and it has been postulated that a continuous tin-iron alloy layer is essential in order to obtain an electrolytic acid tin plate having superior corrosion resistance with most canned products. The introduction of a sub-fusion heat treatment step in a continuous electrolytic tin line prior to flow brightening, however, involves a great deal of expense and is not feasible in most existing electrolytic acid tin plate lines.

The present invention solves the problem of producing Grade A electrolytic tin plate, and particularly acid tin plate, in an extremely simple and economical manner which has not heretofore been attempted with electrolytic tin plate.

It is therefore an object of the present invention to provide an improved and more economical method of consistently producing electrolytic tin plate having superior corrosion resistance and improved quality.

It is a further specific object of the present invention to provide an improved method of consistently producing Grade A electrolytic acid tin plate.

Another object of the present invention is to provide a method of consistently producing electrolytic acid tin plate having improved corrosion resistance which involves 3,308,042 Patented Mar. 7, 1967 a minimum of changes in a continuous production line for converting steel strips into finished electrolytic acid tin plate.

Still another object of the present invention is to provide a method of forming a steel surface which has a positive, a negative, or no measurable pickle lag.

Other objects of the present invention will be apparent to those skilled in the art from the following detailed description and claims when read in conjunction with the accompanying drawing showing a schematic diagram of a preferred embodiment of the present invention,

It has been discovered that Grade A electrolytic acid tin plate can be consistently produced by providing on the surface of a steel base strip prior to electrolytic tinning a thin diffused layer containing an element of the group phosphorus, sulfur, or nitrogen, and particularly good results have been achieved by providing a very thin diffused layer containing phosphorus.

The incorporation of elemental phosphorus in steel is known to increase the adherence of a hot-dip metal coating, such as zinc. Also, heretofore ferrous metal surfaces have been treated to incorporate a small quantity of the element phosphorus in the surface layer thereof to effect a strong bond with hot-dip zinc or other hot-dip metal coating. Phosphorus has also been incorporated in metal surfaces for the purpose of rust-proofing a ferrous metal.

Heretofore, however, no one has discovered that incorporating a very small amount of phosphorus in the surface layer of a steel base prior to electrolytic tinplating, and specifically incorporating phosphorus by diffusion into the surface of a steel base used for electrolytic acid tin plating, would make it possible without otherwise materially changing the electrolytic tin plating procedure to produce consistently electrolytic tin plate, and partic ularly acid tin plate having the superior corrosion resistance which is characterized as Grade A tin plate. The foregoing discovery is particularly valuable to the electrolytic tin plating art because of the insignificant added expense required to incorporate the discovery in any standard continuous electrolytic tin plating line. Thus, the present discovery with very little added cost makes it possible to produce consistently commerical Grade A acid tin plate on electrolytic acid tin plate lines which heretofore have been unable to produce consistently Grade A acid tin plate under the best commercial operating practice known in this country.

i It will be evident from the following detailed description that the innovation of the present invention comprises incorporating in an electrolytic tin plating process a simple and inexpensive chemical treating step at a stage prior to annealing which provides on the surface of the steel strip to be electrolytically tin plated a film or coating containing a reactive phosphate ion or an equivalent phosphorus compound and thereafter diffusing said coating into the surface of the strip by heating in a reducing atmosphere. In the preferred embodiment of the present invention the chemical treatment step is carried out by incorporating a small amount of a phosphoric acid or a mixture of phosphoric acid and a Water soluble phosphate salt, such as tetrasodium pyrophosphate or sodium acid phosphate, in the final pre-annealing aqueous rinse, prefera-b-ly through the final ore-annealing rinse water sprays, to provide a dilute acidic phosphate treating bath immediately following the conventional pre-annealing cleaning treatment and without requiring any other alterations in the metal composition or in the manner of treatment of a Co the said strip before, during or after electrolytic acid tin plating.

The phosphorous-containing chemical bath in which the strip is immersed or otherwise contacted preferably comprises a dilute phosphoric acid solution having a pH below 7 and preferably not substantally lower than about pH 2. The preferred range of pH values is from pH 3 to pH 5. The aqueous phosphorus-containing solution normally contains between about 500 and 1500 ppm. phosphate ion and preferably below 1000 p.p.m. On a percentage basis the phosphorus concentration has a range of between about and 0.2% by weight and preferably below 0.1%. The chemical treatment with the herein described phosphate ion-containing solutions provides on the surface of the strips treated therewith an amount of phosphorus equivalent to between about .00015 and .00035 gram P0 per square fot, as shown by analysis of several phosphate treated annealed products. The control strip of low carbon steel from the same melt as the treated test strips and processed in the same way except that the strip was not exposed to the phosphate-ion containing solution, when analyzed in the same manner for phosphorus, had no measurable phosphorus in the surface thereof.

While the chemical treating solution is conveniently provided by means of addingphosphoric acid (H PO or one of the other phosphoric or phosphorous acids to the continuous anneal pre-furnace final water rinse, it is also possible to add to the rinse water a water soluble phosphate salt, such as sodium acid phosphate (Na HPO sodium dihydrogen phosphate (Naf-I PO or tetrasodium pyrophosphate (Na P O along with sufficient phosphoric acid or other acid of phosphorus to bring the pH of the bath within a range of pH 2 to 6 and preferably to pH 3 to 5. It is further contemplated that a non-phosphorus acid could be used to adjust the pH of the phosphate containing solution, such as nitric acid or sulfuric acid. When an all aline phosphate salt is used with a phosphorus acid, a higher total phosphate concentration is possible without exceeding the desired pH range. Thus, a total phosphate-ion concentration of 1500 ppm. can be present in'the treating solution when comprised of tetrasodium pyrophosphate and sufficient phosphoric acid to adjust the pfl to below 7 and preferably between pH 3 and 5.

If preferred, it is possible to treat the steel strip with the acid phosphate solution or equivalent phosphorus compound at a point prior to annealing other than immediately before the annealing step. For example the strip might be immersed or sprayed with an active acidic phosphorus solution or composition in the final water rinse just after the first acid pickling bath treatment and immediately before the cold rolling mill treatment.

As a further modification of the present invention, it is possible to provide the required diffused coating or fil-m containing phosphorus on the surface of the metal strip to be electrolytically tin plated by contacting the metal strip with a phosphorus-containing gas under conditions which form a substantially continuous and uniform coating or film of phosphorus or phosphorus compound on the surface of the strip and diffusing the film into the surface of the strip prior to electrolytically tin plating. For example, the strip while heated at a temperature of about 1300 F., as when being annealed, can be contacted by phosphine (PH gas which decomposes on the surface of the strip to provide a film of elemental phosphorus and which diffuses into the surface of the strip at the annealing temperature. Other atmospheres or gases containing phosphorus which provide a phosphorus-con-taining surface film difiusable into the steel strip can also be used in place of phosphine.

Following the chemical treatment of the steel base strip it is essential that the strip having the phosphorus ion-containing film or an iron phosphate film on the surface thereof be heated to a moderately high temperature in a reducing atmosphere to effect diffusion of the film into the surface of the strip. The required heating in a reducing atmosphere is most conveniently carried out by passing the strip at the normal rate of travel (between approximately 800 and 1250 ft. per min.) continuously through a standard continuous annealing furnace having a normal hydrogen-nitrogen reducing atmosphere (5% 1-1 -95 N and at the normal operating temperature of around 1200-1500 F. Other types of continuous annealing apparatus or equivalent apparatus can be used, such as an open coil annealing equipment or a continuous normalizer or similar equipment employing a heated reducing atmosphere. If desired, however, the coiled strip can be box annealed for a period of from about 6 to 48 hours in the presence of a reducing atmosphere, depending on the degree or depth of diffusion desired or the special properties required of the base metal strip.

While it is preferable that the surface coating formed from the acidic phosphorus solution be exposed to a reducing atmosphere at an elevated temperature between about 1200 F. and 1500 F. for a period of about 30 to seconds, the strip can be heated longer at a lower temperature or for a shorter period at a higher temperature without adversely affecting the results. It is not necessary to have any particular reducing atmosphere in the annealing chamber and any of the commonly available reducing gases can be used, such as hydrogennitrogen, dissociated ammonia, and the like. Nor is it essential that the atmosphere have a particular dew point. The temperature, time of treatment and atmosphere in the annealing chamber is in each instance, however, regulated and controlled so as to effect diffusion of the phosphorus-containing film into the surface of the strip to form an integral continuous phosphorus-containing diffused film while the said strip remains in the annealing chamber. The phosphorus film preferably is diffused into the strip to a depth greater than the depth of surfaces, scratches or markings which are formed in the strip following annealing and prior to tin plating. The depth of the diffused layer is primarily dependent on the time the strip is held at the elevated diffusing temperature.

In practicing the preferred embodiment of the present invention as schematically illustrated in the drawing, a hot rolled band 5 about 0.1 inch thick is continuously immersed in a conventional sulfuric acid pickle bath 10 and passed through a tandem rolling mill 11 to provide a full hard, low carbon, cold rolled steel strip 12 which generally has a thickness of between about .005 to .02 inch and is referred to as black plate. The steel strip 12 traveling at a rate of between about 800 and 1250 feet per minute is subjected to a continuous pre-annealing chemical cleaning treatment at a cleaning station 13 to remove rolling oils and other surface contamination. In the pre-annealing continuous chemical cleaning treatment, the strip is continuously dipped in an alkaline cleaning bath 14, then passed through an electrolytic alkaline cleaning solution 15, and finally through a scrubber 16 where the surface of the strip 12 is contacted by brushes and sprayed to remove any firmly adhering particles. Each of the foregoing cleaning steps can be any of the conventional types presently used in the art.

The cleaned steel strip 12 which is normally given a final pre-annealing water rinse following the foregoing chemical cleaning treatment before passing through a continuous annealing furnace is, in accordance with the preferred embodiment of the present invention, next chemically treated at a treating station 18 by spraying, dipping, wiping, or the like with an aqueous treating bath 20 comprising a dilute acidic aqueous solution of a phosphorus compound, preferably containing a phosphate ion or an equivalent phosphorus oxide ion, to provide on the surface of the strip 12 an integral thin film or coating containing phosphorous, and with said phosphorus compound being diffusable into the surface of the steel strip 12 when subjected to elevated temperatures in a reducing atmosphere.

The steel strip 12 preferably has its surface uniformly coated with a film of an acidic phosphate ion-containing solution by immersing the strip 12 in the treating bath 20, passing the strip between conventional squeegee rolls 21 which remove excess solution, and rapidly drying the strip by means of conventional gas driers 24 to form the dry film or coating 22 on the surface of the steel strip 12. The steel strip 12.with the coating 22 on the surface can be passed directly into the continuous annealing furnace 25, or the treated strip can be placed in a box annealing chamber (not shown) of any conventional type, if it is necessary to provide a deeper diffused phosphoruscontaining coating or special metal properties are desired. If desired, however, the annealing can take place at a subsequent time and the strips can be coiled for storage.

. Thereafter in the preferred embodiment the steel strip 12 having a thin dry film or coating 22 of a phosphorus compound on the surface thereof is continuously passed through a conventional continuous annealing furnace 25 having therein a reducing atmosphere, such as 5% hydrogen and 95% nitrogen, in which the strip 12 remains for the usual period of between about 30 and 50 seconds and during which period the phosphorus -com pound is reduced and diffused into the surface of the steel strip 12 to form a diffused phosphorus-containing surface layer 27.

When the steel strip 12 having the phosphorus-containing film diffused into the surface thereof is to be electrolytically coated with tin, the annealed strip 12 is temper rolled, as at 28, and then cleaned and passed through a conventional sulfuric electrolytic pickle bath normally containing about 3% by volume sulfuric acid to remove any surface oxides which have formed subsequent to annealing or may be passed directly to electrolytic acid tinning, if no objectionable oxides or contamination are present on the surface of the strip 12. However, any conventional acid electrolytic tinning line can be used to tin plate the treated annealed strip, such as the preferred Ferrostan line 30 which normally includes an electrolytic alkaline phosphate dip 31, an electrolytic sulfuric acid pickle 32, cleaning by one 'or more scrubbers 33, immersion in an acid tin plating bath contained in a plurality of electrolytic acid tin plating tanks 34, followed by fusion at 35, and chemical treatment, such as a cathodic dichromate treatment at 36. The resultant acid tin plated strip 12 is thereafter generally oiled in the usual manner and coiled as at 37 or cut into sheets, as desired.

The amount of phosphorus diffused into the surface of the base strip during the process of the present invention can be accurately determined by careful chemical analysis, but it is also possible to determine the overall changes effected by the present process indirectly by means of simpler tests which indicate small alterations in the base metal surface structure, crystal boundary composition or the like variables which are known to have an effect on the plating characteristics of a metal strip. One such test which is found to have close correlation with the chemical treatment step of the present invention is the pickle lag test. The pickle lag phenomenon and the methods of determining said values are discussed by E. L. Koehler in Transactions of the ASM, vol. 44 (1952), p. 1076. It was found by Koehler that some steels will be attacked or pickled at substantially a constant rate throughout the test. However, many steels display an initial period in the early part of the test in which the steel is attacked at a considerably lower rate, with the steel thereafter exhibiting a substantially linear or constant final rate of pickling characteristic of the metal base. The rate of pickling may be determined either by weight loss or by measurement of the hydrogen evolved. The initial period of low rate of pickling or increasing 6 rate of weight loss during pickling is called the pickle lag period. The quantity of metal or surface skin which is removed during the pickle lag period, i.e. until the rate of Weight loss becomes substantially constant, is designated as the pickle lag layer. Generally speaking, the depth of the pickle lag layer may be from about .0001 inch to about .001 inch, as calculated from the quantity of metal removed during pickle lag tests.

While the effectiveness of the instant process in pro ducing Grade A electrolytic acid tin plate is determined by long term performance pack tests therewith, it is possible to predict the corrosion resistance of the tin plate toward most materials with reasonable accuracy by means of certain laboratory tests. Two such tests which are used to predict the service life of tin plate are the Modified Iron Solution Value (MISV) and the Alloy Tin Couple (ATC) tests. Both of these tests measure the corrosion properties of tin plate on the interior of a C011? tainer and the test results have been shown to have a high degree of correlation with the actual service life (i.e. pack performance) of the tin plate when used under highly corrosive conditions.

The Modified Iron Solution Value test measures the amount of iron dissolved from tin plate using a specific acid solution under controlled test conditions with the test specimen having tin previously removed to expose the iron-tin alloy layer thereof. Grade A tin plate will have a Modified Iron Solution Value less than twenty. A complete description of the foregoing test is set forth in a Technical Service Division Memorandum of the American Can Company, Maywood, Illinois, entitled The Modified Iron Solution Test, ,October 1960. The Modified Iron Solution Values of Grade A tin plate are less than about 20 and generally about 10 or below.

In the Alloy Tin Couple (ATC) test which is less adapted to production control use but is considered more accurate than the Modified Iron Solution Value test, a tin plate specimen stripped of free tin to expose the alloy layer is coupled to a reference tin electrode and the electrode immersed in deaereated grapefruit juice to which has been added p.p.m. stannous chloride. The current generated between the two electrodes is measured after 20 hours and the results are expressed as microamperes per square cm. of alloy layer exposed. (See Corrosion Resistance of Electrolytic Tin Plate-Part II, The Alloy Tin Test-A New Research Tool, G. G. Karnm et al., Corrosion, l7, 84t92t (1961) February.) The ATC values of Grade A tin plate are less than .l0;ta./cm. and generally about .OS a/cm. or below.

The following specific examples are for the purpose of further illustrating the present invention but should in no way be construed as limiting the invention to the particular materials or conditions specified.

Example 1 A low carbon steel strip of the type generally designated black plate and having a thickness of .01 inch, which is well suited for the production of electrolytic acid tin plate and adapted for use in the manufacture of tin cans for packing fruit and juices, while moving at a rate of about 1000 feet per minute was thoroughly cleaned by a conventional pre-annealing cleaning treatment comprising an alkaline dip, an electrolytic cathodic cleaning step and scrubbing and immediately thereafter was continuously passed through a phosphoric acid final pre-annealing aqueous rinse solution containing sufiicient phosphoric acid (H PO to provide a pH ranging between 2.2 and 3.1. The phosphoric acid was introduced into the final water rinse through the final pre-annealing rinse water sprays 19 which were equipped with proportioning pumps to add continuously the required amount of phosphoric acid to maintain the pH between about 2 and 3. The steel strip at a temperature of about F. remained in contact with the dilute phosphoric acid solution also at a temperature of about 180 F. for a period of between about 1 and 3 seconds. After passing the strip through rolls to form a uniform film and drying the strip by a conventional dryer, the strip was then passed directly into a continuous annealing furnace maintained at a temperature of about 1350 F. In the annealing furnace a reducing atmosphere was maintained which comprised about 4% hydrogen and 95% nitrogen. The dew point of the annealing atmosphere was maintained between plus 30 F. and plus 45 F. The steel strip having the phosphate ion-containing film on the surface thereof on emerging from the annealing furnace had a phosphorus-containing layer diffused into the surface thereof, exhibited a pickle lag of from to +4 seconds, whereas initially the pickle lag value for the strip Without the phosphate treatment was to +16 seconds. The treated and annealed steel strip Was then passed at the normal rate of travel through a conventional Ferrostan electrolytic acid tin plating line employing a solution of stannous sulphate in phenolsulfonic acid with conventional additives and conventional plating conditions. The resultant fused tin exhibited MISV test values between 3 and 9, ATC test values between .03 and .04 ,ua./cm. and was found to be Grade A tin plate.

Example II A low carbon strip of black plate as in Example I traveling at a rate of about 1000 feet per minute after a conventional pro-annealing cleaning treatment was continuously passed through a dilute phosphoric acid final pre-annealing rinse solution having a total phosphate ion concentration of 500 ppm. (parts per million) and a pH of 3.4 with said acid being introduced trdo-ugh the find rinse sprays. Following continuous annealing as in Example l, the steel strip exhibited pickle lags values of 0 seconds. Thereafter the strip was tin plated by a standard electrolytic alkaline tin plating process to provide a thin tin coating and the resultant tin plate was rated at Grade A tin plate.

Example III A clean low carbon steel strip as in Example I was continuously passed through a preannealing final water rinse to which phosphoric acid had been directly added in an amount sufficient to provide a total phosphate ion (P0 concentration of 900 ppm. and a pH of about 3.5, followed by the standard annealing treatment as in Example I in which the hydrogen reducing atmosphere had a dew point of plus 45 F. The treated annealed steel strip exhibited a pickle lag of plus 6 seconds. The tin plate produced after conventional electrolytic acid tin plating on a Ferrostan line exhibited a pickle lag of about plus 6 seconds, Modified Iron Solution Values of between 3 and 9, and ATC values of between .03 and .05 ira/cm? Example I V A clean low carbon steel strip as in Example I was continuously passed through a pie-annealed water rinse solution containing sufficient phosphoric acid to provide a total phosphate ion (P0 concentration of 1200 ppm. and a pH of 2.4. Following the standard annealing treatment in Which the reducing hydrogen atmosphere exhibited a dew point of plus 39 F., the treated annealed steel strip exhibited pickle lag values of between 0 and 3 seconds, whereas the strip before the phosphate treatment had pickle lag values between 12 and 16 seconds. The Modified Iron Solution Values and the ATC values of the tin plate produced by conventional electrolytic acid tin plating as in Example I were values indicating Grade A tin plate.

Example V A low carbon steel strip as in Example I was continuously passed at a rate of about 1000 feet per second through a preannealing final water rinse bath following the conventional pre-annealing continuous cleaning with said bath having directly added thereto sufllcient phosphoric acid to provide a total phosphate ion (P0 concentration of about 1400 ppm. and a pH of about 1.8 and a small amount of a wetting agent. Following the usual continuous annealing treatment, Grade A acid tin plate was produced by passing the annealed strip through a Ferrostan acid electrolytic tin plating line.

Example VI A clean low carbon steel strip in Example I was continuously passed through a pro-annealing final water rinse tank solution containing the usual volume of water and to which was added directly without using the rinse sprays five liters of phosphoric acid and 25 lbs. of tetrasodium pyrophosphate to provide a total phosphate ion (P0 concentration of between 900 and 1400 ppm. and provide an initial pH of 3.3. After an endless steel strip was continuously passed through the phosphate solution for 2 hours 15 minutes without further addition of phosphoric acid or phosphate salt, the pH of the solution rose to pH 5.6. The phosphate treated strip was then continuously annealed, as in Example I. The portion of the steel strip which Was treated when the solution had a pH of 3.3 exhibited a piclde lag after annealing of minus 3 seconds, and the portion Which was treated when the solution had a pH of 5.6 exhibited a pickle lag of plus 6 seconds. The treated strip after annealing was subjected to electrolytic alkaline phosphate (2-4 oz./gal.) cleaning and electrolytic pickling in a. 5% by volume sulfuric acid aqueous solution, followed by scrubbing to tho-roughly clean the strip. Thereafter the thoroughly cleaned strip was electrolytically acid tin plated in the conventional manner, as described in Example I. The Modified Iron Solution Values of the acid tin plate product ranged between 3 and 9 with with these average test values being lower than any previously processed acid tin plate, and the pro-duct exhibited ATC values between 0.3 and 0.4 a./cm. The acid tin plate product was rated as Grade A tin plate.

In the foregoing specific examples particularly good results were obtained when the final pro-annealing rinse solution had a total ion phosphate concentration below 1000 p.p.m. and with a pH below 6. In certain of the phosphate chemical treating runs, particularly Where the total phosphate ion concentration was about 1000 ppm. and no precautions were taken to remove any surface solids, a White precipitate which has been identified as a ferric phosphate, probably ferric orthophosphate or ferric pyrophosphate, increased in concentration during the chemical treatment of the strip and tended to remain on the surface of the strip as a visible deposit. When the steel strip was annealed with a significant excess amount of ferric phosphate on the surface, as when there was no final rinse sprays used to fiush the strip as the strip left the phosphate treating bath, the electrolytic acid tin plate produced from the annealed strip having the ferric phosphate on the surface assumes a dull appearance which is objectionable for some uses of tin plate. The foregoing objectionable result was corrected, however, by subjecting the annealed strip to a more intensive pickling treatment immediately prior to electrolytic tinning (see Example VI).

It is postulated that in the foregoing treatment the acid phosphate ion or equivalent phosphorus ion may react with the iron in the surface of the strip 12 to form an iron phosphate surface film which may thereafter be partially or completely reduced by the reducing atmosphere and annealing treatment to form a diffused layer of iron phosphide. It is also possible, however, that the phosphate ion or other reactive phosphorus compound may be directly reduced to elemental phosphorus which might then be diffused into the surface of the steel strip to form an iron-phosphorus alloy during the annealing treatment. Whatever the precise chemical form of the phosphorus in the surface of the annealed strip 12, the acid tin plate after conventional surface fusion of the tin to flow brighten has a normal amount of iron-tin alloy formed between the surface of the steel strip and the free tin layer. Thus, the iron phosphate which may be reduced to iron phosphide or iron phosphorus alloy diffused in the surface of the strip 12 does not appear to inhibit the formation of the conventional iron-tin alloy layer, and the improved results achieved by the present invention would not appear to be due to any change in amount of the iron-tin alloy layer. The present invention, however, is not-dependent on any theory of operation, since the improved results are achieved by employing the herein disclosed combination of treating steps in the production of electrolytic tin plate, and particularly acid tin plate.

From the preceding examples and discussion it will also be evident that the present invention provides an improved method of controlling the pickle lag of a steel strip to any desired value below the normal pickle lag of the strip. In general the decrease in the pickle lag is a function of the acidity of the phosphate solution (see Example VI). In some instances where the pH of the phosphate solution is relatively low or the P concentration relatively high, there is an actual increase in the rate of pickling above the constant rate of pickle of the base metal. When the latter phenomenon occurs, the strip is herein designated having a negative or minus pickle lag, as contrasted with a positive or plus pickle lag when the strip has an initial rate of pickle slower than the constant rate of pickle of the base metal. It has been observed that a negative or minus pickle lag valuecan be changed to a zero or positive pickle lag value by subjecting the herein treated annealed strip to a pickle treatment with sulfuric acid. The herein disclosed chemical treatment provides a further means for adjusting or controlling the pickle lag of a metal strip to a desired low or negative value, where the art considers this necessary or helpful to provide a more reactive or improved base for any surface coating to be applied on the strip.

The term acid tin plate, as used herein, designates electrolytic tin plate, either matte or flow-brightened, wherein the tin is electrolytically deposited onto a steel base from an aqueous acidic solution. This procedure is old and well known in the art; and in general involves immersing a conventional thin gauge black plate steel strip in an acidic aqueous solution of stannous ions while making the steel a cathode and a tin bar immersed in the solution the anode, and passing a current between the cathode and anode through the solution. The stannous ions being positively charged migrate to the cathodic steel base and are plated thereon as a matte deposit of metallic tin. The acid bath commonly comprises a solution of stannous sulphate in phenolsulfonic acid with conventional additives, although other acids can be used, including various combinations of halide salts and halogen acids.

It should be further understood that the herein disclosed improvement in the process of producing electrolytic tin plate, and particularly acid tin plate, with few exceptions does not obviate the necessity of using the art recognized good plating techniques in the several stages of the electrolytic plating process in order to produce Grade A tin plate. Thus, while it is possible to use in the annealing furnace a reducing atmosphere having a higher dew point than heretofore considered possible and still achieve good results, it remains important to have the steel strip thoroughly cleaned and to use proper electrolytic plating bath conditions when employing the present invention for the production of electrolytic acid tin plate.

We claim:

1. A process of producing electrolytic tin plate having superior corrosion resistance which comprises; contacting the surface of a steel strip with a fluid containing phosphorus which forms a surface film containing phosphorus on said strip and which at an elevated temperature in a reducing atmosphere provides a diffused layer containing phosphorus in the surface of said strip, holding said strip with said film at an elevated temperature in a reducing atmosphere until a diffused layer containing'phosphorus is formed in the surface layer of said strip, and applying a coating of tin over said diffused layer by immersing said strip in an electrolytic tin plating bath; whereby a more economical electrolytic tin plate is produced which has corrosion resistance properties characterized as Grade A tin plate.

2. A process of producing electrolytic acid tin plate having superior corrosion resistance which comprises; contacting the surface of a steel strip with an acidic solution containing a phosphorus compound which at an elevated temperature in a reducing atmosphere provides a diffused layer containing phosphorus, holding said strip at an elevated temperature in contact with a reducing atmosphere until a diffused layer containing phosphorus is formed on the surface of said strip, and applying a coating of tin over said diffused layer by immersing said strip in an electrolytic acid tin plating bath; whereby a more economical electrolytic acid tin plate is produced which has corrosion resistance properties characterized as Grade A tin plate.

3. A process of producing electrolytic acid tin plate having superior corrosion resistance which comprises; contacting the surface of a steel strip with an acidic solution containing a phosphate ion, holding said strip having a surface film containing a phosphate ion at an elevated temperature in a reducing atmosphere until a diffused layer containing phosphorus is formed on the surface of said strip, and applying a coating of tin over said diffused layer by immersing said strip in an electrolytic acid tin plating bath; whereby a more economical electrolytic acid tin plate is produced which has corrosion resistance properties characterized as Grade A tin plate.

4. A process of continuously producing electrolytic acid tin plate having superior corrosion resistance which comprises; continuously contacting the surface of a steel strip with an acidic solution containing a phosphate ion, continuously passing said strip having a surface film containing a phosphate ion through a treating zone heated to a temperature between about 1200 and 1500 F. and having a reducing atmosphere for a period of between about 30 and 50 seconds until a diffused layer containing phosphorus is formed in the surface of said strip, applying a coating of tin over said diffused layer by continuously passing said strip through an electrolytic acid tin plating line and fusing the tin coating to flow-brighten the said tin coating; whereby a more economical electrolytic acid tin plate is produced which has corrosion resistance properties characterized as Grade A tin plate.

5. An electrolytic acid tin plating process as in claim 4, wherein said solution is a solution of a phosphorus acid with a pH below 7 and not substantially lower than 2.

6. An electrolytic acid tin plating process as in claim 4, wherein said solution is a dilute aqueous solution of phosphoric acid having a pH between about 2 and 6.

7. An electrolytic acid tin plating process as in claim 4, wherein the phosphorus content of said diffused layer is equivalent to between about .00015 and .00035 gram P0 per square foot of surface area of said strip.

8. An electrolytic acid tin plating process as in claim 6, wherein said solution also contains a water soluble phosphate salt.

9. A process of producing electrolytic acid tin plate having superior corrosion resistance which comprises; contacting the surface of a steel strip with a gaseous fluid containing phosphorus, holding said strip having a surface filrn containing phosphorus at an elevated temperature in a reducing atmosphere until a diffused layer containing phosphorus is formed on the surface of said strip, and applying a coating of tin over said diffused layer by immersing said strip in an electrolytic acid tin plating bath; whereby a more economical electrolytic acid tin plate is produced which has corrosion resistance properties characterized as Grade A tin plate.

References Cited by the Examiner UNITED STATES PATENTS Billeter et al. 204-29 Keller et al. 148-615 Miller 20429 Ades et al. 20429 Schmidt et al. 1486.15

JOHN H. MACK, Primary Examiner.

T. TUFARIELLO, Assistant Examiner.

Claims (1)

1. A PROCESS OF PRODUCTING ELECTROLYTIC TIN PLATE HAVING SUPERIOR CORROSION RESISTANCE WHICH COMPRISES; CONTACTING THE SURFACE OF A STEEL STRIP WITH A FLUID CONTAINING PHOSPHORUS WHICH FORMS A SURFACE FILM CONTANING PHOSPHORUS ON SAID STRIP AND WHICH AT AN ELEVATED TEMPERATURE IN A REDUCING ATMOSPHERE PROVIDES A DIFFUSED LAYER CONTAINING PHOSPHORUS IN THE SURFACE OF SAID STRIP, HOLDING SAID STRIP WITH SAID FILM AT AN ELEVATED TEMPERATURE IN A REDUCING ATMOSPHERE UNTIL A DIFFUSED LAYER CONTAINER PHOSPHORUS IS FORMED IN THE SURFACE LAYER OF SAID STRIP, AND APPLYING A COATING OF TIN OVER SAID DIFFUSED LAYER BY IMMERSING SAID STRIP IN AN ELECTROLYTIC TIN PLATING BATH; WHEREBY A MORE ECONOMICAL ELECTROLYTIC TIN PLATE IS PRODUCED WHICH HAS CORROSION RESISTANCE PROPERTIES CHARACTERIZED AS GRADE A TIN PLATE.

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