US3915812A - Method of manufacturing tinned plates having high corrosion resistant property - Google Patents

Abstract

In a method of electroplating tin utilizing an acidic electrolyte, prior to the electroplating step, a steel plate is subjected to a cathodic treatment in a treating bath for producing dense and continuous tin layers. The treating bath comprises an aqueous solution containing sulfuric acid and gluconic acid, water soluble salts thereof or a mixture thereof in the case of a pickling bath and caustic soda or sodium orthosilicate and one or more of water soluble oxyacid or water soluble salts thereof and a cheleting agent having cheleting capability in an alkaline region in the case of a degreasing bath.

Description

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States atent Yamagishi et al. 1 Oct. 28, 1975 [54] METHOD OF MANUFACTURING TINNED 1,995,766 3/1935 Chester 204/145 R PLATES HAVING HIGH CORROSION 2,915,444 12/1959 Meyer 2704/34 X RESISTANT PROPERTY 3,285,838 11/1966 Morgan et al -04/37 T [75] Inventors: Hidehisa Yamagishi, Yokohama;

Mizuo Tanaka, Tokyo, both of Primary ExaminerJohn H. Mack Japan Assistant ExaminerW. 1. Solomon [73] Assignee: Nippon Kokan Kabushiki Kaisha, Attorney Agent or F'mFlynn & Fnshauf Tokyo, Japan [22] Filed: June 13, 1973 21 Appl. No.: 369,707 1571 ABSTRACT In a method of electroplating tin utilizing an acidic 1 Forelgn Apphcatlon Pnonty Data electrolyte, prior to the electroplating step, a steel June 28, 1972 Japan 47-64093 plate is subjected to a cathodic treatment in a treating July 1, 1972 Ja an .1 47-66149 bath for producing dense and continuous tin layers. July 1, 1972 Japan 47-66150 The treating bath comprises an aqueous solution containing sulfuric acid and gluconic acid, water soluble [52] US. Cl 204/34; 204/28; 204/37 T; salts thereof or a mixture thereof in the case of a pick- 204/145 R ling bath and caustic soda or sodium orthosilicate and [51] Int. Cl. C25D 5/36 one or more of water soluble oxyacid or water soluble [58] Field of Search 204/29, 34, 37 T, 145 R, salts thereof and a cheleting agent having cheleting ca- 204/28 pability in an alkaline region in the case of a degreasing bath. [56] References Cited UNITED STATES PATENTS 3 Claims N0 Drawings 1,598,731 9/1926 Lee 204/145 R METHOD OF MANUFACTURING TINNED PLATES HAVING HIGH CORROSION RESISTANT PROPERTY BACKGROUND OF THE INVENTION This invention relates to a method of manufacturing tinned plates, more particularly to the improvement of a method of manufacturing highly corrosion resistant tinned plates by acidic electrolytic tinning process.

In the manufacture of corrosion resistant tinned plates by using an acidic electrolyte various methods of pretreatment have been proposed including a method of anodizing the plate in an alkaline bath of high concentration (for example, US. Pat. No. 3,075,895 and Japanese Pat. Publication No. 8558 of 1963), a method of applying a primary coating of tin in an alkaline electroplating bath (for example, Japanese Pat. Publication No. 8162 of 1962), and a method of applying a primary coating and performing pickling in a weak acidic bath containing ions of divalent metal such as tin and or nickel at a low concentration (for example, Japanese Pat. Publication No. 19362 of 1970 and No. 25603 of 1971). Although these prior art methods can improve the corrosion resistant property of tinned plate to some extent, the degree of improvement is not sufficiently high and there are many problems left to be solved. For example, the electrolyte is contaminated by the admixture of an alkaline stannate and an acidic electrolyte which is caused by insufficient washing and it is necessary to install additional apparatus for carrying out additional process steps. Furthermore, in the prior art electrolytic tinning method, an inorganic acid, for example sulfuric acid, has generally been used for pickling which step is performed after degreasing. Where an acid of high concentration, for example sulfuric acid of a concentration of 50 100 g/l is used for pickling, the surface of the iron plate to be tinned is activated excessively so that the surface will be oxidized again during a short interval in which contaminants are removed by means of a scrubber after the pickling treatment with the result that the surface of the plate will become inactive thus making it difficult to uniformly form nuclei of electroplated tin in the succeeding tinning step. Accordingly, during the succeeding reflowing step, the formation and growth of the nuclei of the alloy layer will not proceed uniformly so that it has been extremely difficult to obtain highly corrosion resistant dense and continuous electroplated tin iron alloy films.

SUMMARY OF THE INVENTION It is an object of this invention to provide a novel method of manufacturing tinned plates having dense alloy layers and continuous plated tin iron alloy films with high corrosion resistant property.

Another object of this invention is to provide a novel method of manufacturing tinned plates which provides a highly activated surface having a tendency of being oxidized again during a short interval in which surface contaminants are removed by means of a scrubber thereby preventing uniform formation of nuclei of electroplated tin and of alloy layers.

Still another object of this invention is to provide an improved method of manufacturing tinned plates having excellent corrosion resistant property without providing any additional equipment for the production line and without decreasing the speed of the production line.

A further object of this invention is to provide a novel acidic treating bath for carrying out a cathodic treatment of a steel strip which can eliminate the conventional pickling process.

In accordance with this invention these and further objects can be accomplished by providing a method of manufacturing a tinned plate having a high corrosion resistant property wherein a steel strip is pretreated to clean its surface, removed of surface contaminants by means of a scrubber and then electroplated with tin in an acidic electrolyte, characterized in that the steel strip is subjected to a cathodic treatment in a treating bath prior to treatment in an electrolytic vessel.

The treating bath comprises an aqueous solution con taining from 10 to 50 g/l of caustic soda or sodium orthosilicate and from 5 to 50 g/l of one or more members selected from the group consisting of such water soluble oxyacids as citric acid, gluconic acid, water soluble salts of the oxyacids, a cheleting agent having a cheleting capability in an alkaline region such as ethylenediaminetetraacetate and diethylenetriaminepentaacetate or an aqueous solution containing from 1 to 10 g/l of sulfuric acid and from 5 to 50 g/l of gluconic acid, water soluble salts of gluconic acid or a mixture thereof.

The cathodic treatment is carried out at a bath temperature of from 15 to 60C in the case of an acidic bath and from 40 to C in the case of an alkaline bath and at a current density of 5 to 15 A/dm for 0.5 to 3 seconds, these parameters varying dependent upon the composition of the treating bath.

DESCRIPTION OF THE PREFERRED EMBODIMENTS When tinning steel strips with an acidic electrolyte some of the strips are greatly rusted while the other are rusted only a little. To carry out the method of this invention, steel strips which are rusted greatly are degreased and pickled according to standard processes whereas those are not rusted appreciably are degreased but not pickled. After removing contaminants on the surface by means of a scrubber and immediately prior to tinning, the strips are subjected to a cathodic treatment in the following treating bath A or B for 0.5 to 3 seconds at a current density of from 5 to 15 A/dm for reactivating the surface of the strip which has been oxidized during a short interval in which the surface contaminant is removed by the scrubber. Then the strip is immediately plated with tin by using an acidic electrolyte so as to uniformly form nuclei of the electroplated tin and a metal layer thus providing highly corrosion resistant tinned plate provided with dense and continuous alloy layers. A. an aqueous solution of sulfuric acid having a concentration of l 10 g/l and maintained at a temperature of 15 60C, or said aqueous solution incorporated with from 5 to 50 g/l of gluconic acid or a water soluble salt thereof. B. An aqueous solution of a mixture of 10 50 g/l of caustic soda or sodium orthosilicate, and 5 50 g/l of one or more members selected from the group consisting of citric acid, sodium gluconate, ethylenediaminetetraacetate, diethylenetriaminepentaacetate, and maintained at a temperature of from 40 to 95C.

With the treating bath A, if the concentration of sulfuric acid is higher than 10 g/l, excessive activation would be resulted so that the surface of the steel strip would be oxidized again before tinning, whereas when the concentration of sulfuric acid is less than 1 g/l, de rusting power is not sufficiently large. Accordingly, a range of from 2 to 7 g/l is preferred. Where the concentration of gluconic acid or a water soluble salt thereof is less than g/l, it would be impossible to improve the derusting power and electroconductivity, whereas concentrations exceeding 50 g/l would not increase proportionally the derusting power and electroconductivity. For the purpose of improving the derusting power and electroconductivity, it is desirable to maintain the treating bath A at a temperature of from 40 to 55C but temperatures higher than 60C would result in an excessive activation of the surface of the steel strip, whereas temperatures lower than 15C would decrease both the derusting power and electroconduc tivity.

With regard to the treating bath B, if the concentration of the mixture of caustic soda or sodium orthosilicate and such water soluble oxyacids as citric acid and gluconic acid or water soluble salts thereof, or a cheleting agent having cheleting capability in an alkaline region such as ethylenediaminetetraacetate (EDTA) and ethylenetriaminepentaacetate were less than the lower limit, or 5 g/l, it would be impossible to improve the corrosion resistant property of the tinned plate. On the other hand, higher concentrations of the mixture not only improve proportionally the corrosion resistant property, but also make it difficult to remove the treating bath by washing with water. When the treating bath is maintained at a temperature lower than 40C, the de rusting power is not sufficiently large, whereas temperatures higher than 95C would not result in any additional improvement.

If the current density and the treating time for the cathodic treatment were decreased below respective lower limits described above, the effect of removing oxide would be greatly decreased whereas current desities and treating times exceeding respective upper limit described above do not result in any additional improvement.

Further, the invention is characterized by the steps of degreasing the steel strip in a manner well known in the art, subjecting the steel strip to a cathodic treatment in a treating bath consisting of an aqueous solution containing sulfuric acid with a low concentration of l 10 g/l, and 5 to 50 g/l of gluconic acid [CH Ol-l(Cl-lOH)- .,COOH]and or a water soluble salt of gluconic acid, for example sodium gluconate at a temperature of from to 60C and at a current density of from 5 to 15 A/dm for 0.5 to 3 seconds, washing the steel strip, removing surface contaminants with a scrubber, and tinning the strip with an acidic electrolyte. Such treatment can prevent reoxidation of the active surface which occurs during removal of the surface contaminants with a scrubber due to excessive activation of the surface of the steel strip, and promotes uniform formation of the nuclei of the plated tin and of the alloy layers. This enables formation of dense, continuous and highly corrosion resistant tin films by using an acidic electrolyte.

If the concentration of sulfuric acid contained in the treating bath were higher than 10 g/l, an excessive activation would be resulted, whereas concentrations less than 1 g/l decreases the derusting power and electro conductivity. For this reason, an adequate concentration ranges from 2 to 7 g/l. The purpose of using gluconic acid or a water soluble salt thereof is to prevent the decrease in the derusting power caused by decreasing the concentration of sulfuric acid and to prevent the rise of the bath voltage caused by the decrease in the electroconductivity. The reason for limiting the concentration of these ingredients in the range of from 5 to 50 g/l is that concentrations lower than 5 g/l do not provide sufficiently large derusting power as well as high electroconductivity, whereas concentrations higher than 50 g/l do not increase proportionally the derusting power and electroconductivity. in order to improve the derusting power and electroconductivity it is essential to maintain the treating liquid at a temperature of from 15 to 60C. Temperatures higher than 60C are not advantageous because such high temperatures activate excessively the surface of the steel strip to be tinned whereas temperatures lower than 15C lower both the derusting power and electroconductivity. For this reason, an adequate temperature ranges from 40 to 55C.

The invention is further characterized in that the prior art pretreatment step including a cleaning step by an alkaline solution followed by a pickling step can be eliminated and that the steel strip is tinned by the steps of preparing a treating bath consisting of an aqueous solution containing 10 to 50 g/l of caustic soda or sodium orthosilicate and 5 to 50 g/l of one or more of the members selected from the group consisting of such water soluble oxyacids as citric acid, gluconic acids or water soluble salts thereof, cheleting agents having cheleting capability in an alkaline region such as ethylenediaminetetraacetate (EDTA) and diethylenetriaminepentaacetate (DTPA), maintaining the temperature of the treating bath at a temperature of from 40 to C, subjecting the steel strip to a cathodic treatment for 0.5 to 3 seconds at a current density of from 5 to 15 A/dm washing the treated steel strip with water, removing surface contaminants by means of a scrubber, and then electroplating the steel strip with tin by using an acidic electrolyte, thereby producing a highly corrosion resis tant tinned plate formed with dense and continuous alloy layers. Conventional tinned plate is prepared by the steps of degreasing, pickling, washing with water, removing surface contaminants by means of a scrubber, electroplating tin, reflowing, subjecting to formation process and oiling. However, in accordance with this invention, a special treating bath is prepared and grease and a thin oxide film on the surface of the steel strip are removed by subjecting the steel strip to a cathodic treatment in the treating bath under conditions specified just above. Then without performing pickling process, the steel strip is washed with water, the surface contaminant is removed by a scrubber and finally tin films are applied with an acidic electrolyte. The tinned plate is then subjected to reflowing, formation and oiling steps.

The concentration of caustic soda or sodium orthosilicate is desirable to be in a range of from 10 to 50 g/l. With concentrations less than 10 g/l, it is difficult to perfectly degrease whereas with concentrations higher than 50 g/l, it is difficult to remove alkali adhered to the surface of the steel strip. Each of the oxyacids such as citric acid, gluconic acid and water soluble salts thereof, and cheleting agent having cheleting capability in the alkaline region such as ethylenediaminetetraacetate (EDTA) and diethylenetriaminepentaacetate (DTDA) have both oxide removing action and a capability of improving the corrosion resistant property and their concentrations are desirable to be from 5 to 50 g/l. With concentrations less than 5 g/l it is difficult to efficiently remove the oxide thus fails to improve the corrosion resistant property, whereas with concentrations higher than 50 g/l the corrosion resistant property is not improved proportionally, rather removal of the treating liquid with water washing becomes difficult.

It is advantageous to maintain the treating bath at a temperature of from 40 to 95C. Preferred current density for the cathodic treatment ranges from 5 to l5 A/dm and the preferred treating time is from 0.5 to 3 seconds. Treating conditions lower than the lower limits described above greatly decreases the efficiency of removing grease and oxide, and those higher than the upper limits do not provide any additional advantage.

Following Examples are given by way of illustration and not limitation.

EXAMPLE 1 A rusted cold rolled steel strip for tinning (T4-CA) and having a thickness of 0.23 mm was degreased, pickled and removed of surface contaminants by means of a scrubber. Prior to the tinning process the cleaned steel strip was subjected to a cathodic treatment in the following treating bath and under the following conditions.

Treating bath aqueous solution of 5 g/l sulfuric acid Temperature of the treating bath C Current density 8 Aldm Time of cathodic 1.5 sec.

treatment Immediately thereafter tin was electroplated with an acidic electrolyte until depositing tin at a rate of 11.2 g/m Then the plated tin was reflowed by resistance heating.

The ATC value (to be described later) of the tinned plate prepared in this manner was 0.033 p.A/cm but that of the tinned plate prepared under the same conditions except the special treatment of this invention was 0.1 12 ,uA/cm The ATC test (Alloy-tin couple test) is one of the methods of determining the corrosion of tinned plates for manufacturing not coated cans caused by acidic foodstuffs. This test is used to correctly predetermine example, on the assumption that the speed of corrosion (rate of dissolving tin) is determined essentially by the quantity of current flowing between layers of tin, alloyed tin and the exposed surface of the steel strip. A pure tin electrode and a test piece (in this case, a tinned plate with its alloyed tin layer exposed over an area of 3.6 cm was used and the temperature of the bath was maintained at a constant value of 26C) were dipped in a bath filled with an acidic foodstuff (grape fruit juice in this example) and an ammeter was connected between the pure tin electrode and the test piece. ATC current (pA/cm is expressed by an equation Voltage/area x resistance. Tinned plates having low ATC currents have a high corrosion resistant property against acidic foodstuffs. A maximum value of ATC of 0.12 .tA/cm and a mean value per month of 0.05 ,uA/cm are used as a measure of the capability of the alloy layer for limiting the rate of dissolving tin.

- EXAMPLE 2 Treating bath aqueous solution containing 2.5 g/l of sulfuric acid and 30 g/l of sodium gluconate Temperature of the 50C treating bath Current density 10 A/dm Time of Cathodic treatment 0.5 sec.

Thereafter, tin was electroplated at a rate of 11.2 g/m with an acidic electrolyte, and then the plated tin was reflowed by resistance heating.

The ATC value of the product of the novel method was 0.031 uA/cm whereas that of a control prepared under the same conditions excepting the novel treatment was 0.098 uA/cm EXAMPLE 3 The process steps of Example 2 were repeated except that the following treating bath and conditions of cathodic treatment were used.

Treating bath aqueous solution containing 25 g/l of caustic soda and 15 g/l'of citric acid Temperature of 55C.

treating bath Current density lOA/dm Time for cathodic 1.5 sec.

treatment The ATC value of the tinned plate prepared by this example was 0.025 psA/cm whereas that of a control prepared under the same conditions except that the novel treatment was not used was 0.102 [LA/C1112.

The ATC value of the tinned plate of a modified method wherein 15 g/l of citric acid in the treating bath was substituted by 25 g/l of sodium gluconate was 0.037 ,uA/cm EXAMPLE 4 Again the process steps of Example 2 were repeated except that the cathodic treatment was carried out in the following treating bath and under the following conditions.

Treating bath aqueous solution containing 40 g/l of sodium orthosilicate and 30 g/l of ethylenediaminetetraacetate Temperature of the 45C treating bath Current density 8 A/dm Time of cathodic 2 sec.

treatment The ATC value of the product of this example was 0.041 uA/cm whereas that of a control prepared under the same conditions except the novel treatment was 0.087 uA/cm v Where 20 g/l of diethylenetriaminepentaacetate was substituted for 30 g/l of ethylene diaminetetraacetate in the treating bath and where a mixture of 20 g/l of ethyl enediaminetetraacetate and 15 g/l of diethylenetriaminepentaacetate was substituted for the 30 g/l of ethylenediaminetetraacetate the ATC value were 0.043 ,uA/cm and and 0.027 uA/cm respectively.

Examples 1 to 4 illustrate the method of this invention. According to this method, the oxide formed on the surface of the steel strip while the surface contaminant is being removed by a scrubber after the strip has been subjected to the degreasing and pickling processes is removed immediately prior to the tinning process, so that sufficiently activated surface of the steel strip is presented to the tinning process thereby assuring uniform creation of the nuclei of the plated tin and the alloy layers. As a result, it is possible to produce high quality tinned plate having dense and continuous alloy layers that can manifest high corrosion resistant property. Where the steel strip is not rusted in any appreciable extent, it is possible to omit the pickling process which otherwise is performed subsequent to the degreasing step so that it is possible to manufacture at a low cost tinned plates having a high corrosion resis tant property.

EXAMPLE 5 aqueous solution containing 3 g/l of sulfuric acid and 30 g/l of sodium gluconate.

Treating bath Temperature of the 40C treating bath Current density A/dm Time of cathodic treatment 1.5 sec.

Table 1 below shows a comparison between ATC val ues of the product of this example and a control tinned plate having the same quantity of tin deposited and prepared by the conventional method wherein after the pickling process (electrolysis in an aqueous solution containing 50 g/l of sulfuric acid) the same steel strip was electroplated with tin.

Table 1.

Test piece ATC value (pA/cm control 0.108 0045 product of Example 5 EXAMPLE 6 aqueous solution containing 7 g/l of sulfuric acid and 10 g/l of sodium gluconate Treating bath Treating temperature 50C Current density 7 A/dm Time of cathodic 2 sec.

treatment The ATC values of the tinned plate produced by the method of this example and of a control prepared by pickling (electrolytic pickling in a high concentration aqueous solution of sulfuric acid containing g/l of sulfuric acid) followed by electroplating tin in an acidic electrolyte until the same quantity of tin was deposited are shown in the following table 2.

Table 2.

Test piece ATC value [LA/C1112) control 0.092 product of example 6 0.054.

Examples 5 and 6 illustrate the method of this invention. According to this method, the concentration of sulfuric acid contained in the pickling bath is reduced and the steel strip is subjected to a cathodic treatment under prescribed treating conditions in a treating bath containing sulfuric acid and gluconic acid or a water soluble salt thereof, so that it is possible to prevent reoxidation of the activated surface of the steel strip which occurrs during the removal of the surface contaminants by a scrubber caused by an excessive activation of the surface of the steel strip, whereby nuclei of the plated tin and alloy layers are formed uniformly. The use of gluconic acid or a water soluble salt thereof as one of the components of the treating bath effectively prevents decrease in the derusting power caused by the decrease in the quantity of sulfuric acid used as well as the increase of the bath voltage caused by the decrease in the electroconductivity. These functions contribute to the production of tinned plates having dense and continuous alloy layers and manifesting extremely high corrosion resistant property.

EXAMPLE 7 A cold rolled steel strip for producing a tinned plate (T4-CA) and having a thickness of 0.23 mm was subjected to a cathodic treatment in a treating bath having the following composition under the following treating conditions, washed with water, removed of surface contaminants by using a scrubber, electroplated with tin in an acidic electrolyte and the plated tin layers were reflowed by resistance heating.

Treating bath aqueous solution containing 50 g/l of caustic soda and 20 g/l of sodium gluconate.

The ATC value of a control prepared by the same steps and under the same conditions except that the sodium gluconate in the treating bath was omitted was 0.089 uA/cm whereas that of the product produced by the method of this example was 0.048 [LA/CD12.

After being subjected to the steps described above, the steel strip to be plated was pickled again by subjecting it to an additional conventional cathodic treatment in an aqueous solution containing 50 g/l of sulfuric acid at normal temperature for 1 second and at a current density of A/dm and then the plate was electroplated with tin in an acidic electrolyte. The tin layers were reflowed. The ATC value of the resulting product was 0.052 ,uAlcm This test shows that an additional conventional pickling step carried out following alkaline treatment according to this invention, does not improve the ATC value.

EXAMPLE 8 The process steps of Example 7 were repeated except that the same steel strip was subjected to a cathodic treatment in a treating bath having a composition as described hereunder under the following conditions.

Treating bath aqueous solution containing 30 g/l of sodium orthosilicate and 30 g/l of citric acid Temperature of bath 80C Current density 10 A/dm Treating time 1.5 sec.

EXAMPLE 9 The process steps of Example 7 were repeated except that the steel strip was subjected to a cathodic treatment in a treating bath having the following composition under the following treating conditions.

Treating bath aqueous solution containing 30 g/l of caustic soda and 25 g/l of ethylenediaminetetraacetate Temperature of 85C treating bath Current density 7.5 A/dm Treating time 2.5 sec.

The ATC value of this product was 0.049 ,uA/cm whereas that of a control which was prepared by a similar method except that ethylenediaminetetraacetate.

was not incorporated into the treating bath was 0.102 ,uA/cm A product prepared by a modified method wherein ethylenediaminetetraacetate was substituted by 30 g/l of diethylenetriaminepentaacetate showed an ATC value of 0.045 ,uA/cm and where 25 g/l of ethylenediaminetetraacetate and 20 g/l of diethylenetriaminepentaacetate was incorporated to caustic soda, an ATC value of 0.041 .LA/cm was obtained.

Examples 7 to 9 illustrate the method of this invention. According to this method, it is possible to provide an improved method of manufacturing tinned plate having excellent corrosion resistant property without the necessity of installing any additional equipment and without decreasing the line speed and hence the pro ductivity. Furthermore, since the steel strip which has been subjected to the novel pretreatment of this invention and washed with water can be immediately plated with tin without the necessity of subjecting it to a pickling step as in the prior art method, it is possible to prevent the surface of the steel strip which has been activated by the pickling process from being reoxidized during a short interval in which surface contaminants are removed by using a scrubber thereby making it possible to uniformly form the nuclei of the electroplated tin and of the alloy layers. For this reason, it is possible to form dense and continuous alloy layers between the tin layers and the steel strip.

Although the invention has been described in terms of several preferred embodiments thereof it will be clear that many changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.

What is claimed is:

1. In a method of electroplating a steel strip to produce a highly corrosion resistant tinned plate wherein a steel strip is treated according to the sequence: degreased, pickled electrolytically, washed, scrubbed and electroplated with tin in an acidic electrolyte, the improvement which comprises conducting said electrolytic pickling with a cathodic treatment bath consisting of l-10 g/l of sulfuric acid and 5-50 g/l of gluconic acid, water-soluble salt thereof or a mixture thereof at a cathodic bath temperature of from 15 to 60C. and at a current density of from 5 to 15 A/dm for from 0.5 to 3 seconds.

2. The method of claim 1, wherein the cathodic treating bath contains gluconic acid.

3. In a method of electroplating a steel strip to produce a highly corrosion resistant tinned plate wherein a steel strip is treated according to the sequence: degreased, washed, scrubbed, and electroplated with tin in an acidic electrolyte, the improvement which comprises subjecting said steel strip, following said scrubbing and before said electroplating with tin, to a cathodic treatment in a cathodic treatment bath consisting of l-l 0 g/l of sulfuric acid and 5-50 g/l of gluconic acid, water-soluble salt thereof or a mixture thereof at a cathodic bath temperature of from 15 to 60C. and at a current density of from 5 to 15 A/dm for from 0.5

to 3 seconds.

- Page 1 of 2 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,915,812

DATED October 28, 1975 |NVENT0R(5) HIDEHISA YAMAGISHI et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Abstract, line 10: replace "chelating" with chelating lines 49 and 53: replace "tin iron" with tin-iron Column 1,

Column 2, lines 19 and 20: replace "cheleting" with chelating Column 3, lines 20-21: replace "cheleting" with chelating Column 3, line 21: replace "cheleting" with chelating Column 3, lines 37-38: replace "desities" with densities Column 3, line 38: after "exceeding",- insert the Column 3, line 38: replace "limit" with limits Column 4, lines 28-29: "cheleting" should be chelating Column 4, line, 50: after "performing", insert a Column 4, line 62: replace "cheleting" with chelating (two occurrences) Column 5, line 32: replace "Aldm with A/dm r Page 2 of 2 UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTI PATENT NO. 3,915, 812

DATED October 28, 1975 lN\/ ENTOR(S) I HIDEHISA YAMAGISHI et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below;

Column 5, line 44: after "except", insert for Column 5, line 48: replace "not coated" with uncoated Signed and Scafle twenty-second Day 0? June1976 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Ojjicer Commissioner oflalems and Trademark;

Claims (3)

1. IN A METHOD OF ELECTROPLATING A STEEL STRIP TO PRODUCE A HIGHLY CORROSION RESISTANT TINNED PLATE WHEREIN A STEEL STRIP IS TREATED ACCORDING TO THE SEQUENCE: DEGREASED, PICKLED ELECTROLYTICALLY, WASHED, SCRUBBED AND ELECTROPLATED WITH TIN IN AN ACIDIC ELECTROLYTE, TH IMPROVEMENT WHICH COMPRISES CONDUCTING SAID ELECTROLYTIC PICKLING WITH A CATHODIC TREATMENT BATH CONSISTING OF 1-10 G/L OF SULFURIC ACID AND 5-50 G/L OF GLUCONIC ACID, WATER-SOLUBLE SALT THEREOF OR A MIXTURE THEREOF AT A CATHODIC BATH TEMPERATURE OF FROM 15* TO 60*C. AND AT A CURRENT DENSITY OF FROM 5 TO 15 A/DM2 FOR 0.5 TO 3 SECONDS.

2. The method of claim 1, wherein the cathodic treating bath contains gluconic acid.

3. In a method of electroplating a steel strip to produce a highly corrosion resistant tinned plate wherein a steel strip is treated according to the sequence: degreased, washed, scrubbed, and electroplated with tin in an acidic electrolyte, the improvement which comprises subjecting said steel strip, following said scrubbing and before said electroplating with tin, to a cathodic treatment in a cathodic treatment bath consisting of 1-10 g/l of sulfuric acid and 5-50 g/l of gluconic acid, water-soluble salt thereof or a mixture thereof at a cathodic bath temperature of from 15* to 60*C. and at a current density of from 5 to 15 A/dm2 for from 0.5 to 3 seconds.