US3468769A - Process for producing tin plate of high corrosion resistance - Google Patents

Description

TEMPERATURE "F Sept. 23, 1969 SWALHIEIM ET AL 3,468,769

PROCESS FOR PRODUCING TIN PLATE OF HIGH CORROSION mzsxsmuca Filed March 15, 1967 2 Sheets-Sheet 1 j o no 0.05 550 500 new mm m szconos FIG. I

INVENTORS DONALD A. SWALHEIM ROBERT W. MACKEY BY QM ATTORNEY p 1969 o. A. SWALHEIM ETAL 3,468,769

PROCESS FOR PRODUCING TIN PLATE OF HIGH CORROSION RESISTANCE DONALD A. SWALHEIM ROBERT W. MACKEY ATTORNEY United States Patent 3,468,769 PROCESS FOR PRODUCING TIN PLATE OF HIGH CORROSION RESISTANCE Donald A. Swalheim, Hockessin, and Robert W. Mackey, Wilmington, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Continuation-impart of application Ser. No. 317,462, Oct. 21, 1963. This application Mar. 15, 1967, Ser. No. 623,401

Int. Cl. C23b /52 US. Cl. 20437 2 Claims ABSTRACT OF THE DISCLOSURE Tin plate of high corrosion resistance is produced by electrolytically depositing tin onto steel substrate, thereafter heating the plated substrate to a temperature between 520 F. and 722 F., maintaining the plated substrate at such temperature for a predetermined period of time, and thereafter rapidly quenching the heated substrate.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of US. Application Ser. No. 317,462, filed Oct. 21, 1963, now abandoned, which is a continuation-in-part of US. Aplication Ser. No. 295,612, filed July 15, 1963, now abandoned.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to an improved process for the melt after-treatment of electrolytically produced tin plate to form a superior corrosion resistant diffusion layer of tin-iron alloy at the interface of the steel and tin.

Description of the prior art It is known to after-treat tin plate by melting or flowbrightening, the tin layer. This was originally done for the purpose of brightening the tin layer to make it more attractive. In order to retain this brightness, and to operate at high speeds, the flow-brightened tin plate is quenched in relatively cool liquid, preferably water, as shown in Smith US. Patent 2,661,328.

It was also found that flow-brightening at a temperature of 475 C. and providing for a delay time of 1 to 3 seconds between flow-brightening and water quenching would improve the corrosion resistance of tin plate, as described in Frankenthal Patent 3,062,725.

The corrosion resistance of tin-plated steel is customarily measured by an accelerated test procedure consisting of measuring the electric current, in microamperes/ cm. flowing between the iron-tin (Fe-Sn alloy interlayer, between the iron and tin electroplate, and a pure tin reference electrode after hours exposure in grapefruit juice. For this test, the tin plate is stripped off the tin-electroplated steel sample to the iron-tin alloy layer, by making the samples anodic in a 5% aqueous solution of NaOH, with a stainless steel cathode, at a cell potential of about 0.4 volt. When all the free tin is removed the stripping current is automatically diminished to zero. The measured current, in microamperes/cm. between the exposed alloy layer and a pure tin reference electrode after 20 hours exposure in grapefruit juice is referred to as the ATC value. So-called Grade A plate must have an average ATC value of less than 0.05 with 95% of all values below 0.085. Moreover, for satisfactory tin can production, the weight of the alloy layer should be between 0.10 and 0.12 pound per base box.

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SUMMARY OF THE INVENTION It is an object of this invention to produce electrolytic tin plate on a steel substrate having an average ATC value of 0.05 or less.

It is a further object to produce electrolytic tin plate on a steel substrate having a uniformly thick iron-tin alloy interface between steel and tin and having a high resistance to corrosion.

Other objects of the invention will appear hereinafter.

The objects of this invention may be achieved by heating electrolytically tin-plated steel to a temperature between 520 F. and 722 F. and maintaining the structure at this temperature for a period of up to 3 seconds, and then quenching the heated structure in water. The time and temperature selected are maintained within the limits defined by the cross-hatched zone of FIGURE 3 of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a diagrammatic side elevational view of a flow-brightening apparatus which may be used in the practice of this invention.

FIGURES 2 and 3 are graphs showing time-temperature relationships for practicing this invention with the cross-hatched zone shown on FIGURE 3 defining the time and temperature limits for practicing the invention.

DESCRIPTION OF THE INVENTION In carrying out the process of this invention, tin is electrodeposited in an amount from about 0.15 to about 0.75 pound per base box upon a steel substrate using either an alkaline or an acid electrolyte. A preferred process is the halogen tin process described by Schweikher in US. Patent 2,407,579.

Referring now to FIGURE 1 of the drawings, a tin plated sheet 3 travels from the electroplating unit (not shown) through the flow-brightening and water quenching apparatus. The sheet 3 passes around guide rolls 4 and 5 and then through a conventional heating device 6, for example, a resistance or induction heating device. In passing through the heating device 6, the sheet is rapidly brought to a temperature of between 520 F. and 722 F. and is maintained at this temperature for a period of up to 3 seconds as determined from the time-temperature relationship shown in FIGURE 3. The lower limit for this relationship is shown graphically in FIGURE 2 and for periods of time up to about 2 minutes is defined by the equation:

in which T represents any desired temperature between 520 F. and 722 F. and 6 is the minimum delay time, in seconds. The delay time is that time interval which begins when the temperature of the plated steel reaches the desired flow-brightening temperature and heating ceases and terminates upon quenching the flow-brightened plate.

Immediately after the heating and flow-brightening period, the sheet 3 is passed under the sink roll 7 in the water quenching tank 8 where the sheet is substantially instantaneously brought to a temperature below 200 F. Results of tests conducted on 0.75 lb. plate showing effect of temperature and delay time on ATC values of halogen tin plate are shown graphically in FIGURE 3. The numbers shown in FIGURE 3 above and below the line, designated Grade A plate, represent average ATC values obtained from Table XIII. These results indicate that production of Grade A plate requires a refiow temperature of at least 520 F. and maintaining the strip at this temperature for a period of at least 1.5 seconds before quenching. Correspondingly shorter delay times are required as the temperature is increased. For example, at about 670 F., Grade A plate will be produced with a 3 delay time of about 0.3 second. At a temperature of about 705 F., Grade A plate will be produced with a delay time of 0.1 second or less.

As stated previously, the operable zone for time and temperature in flow-brightening the plated steel is illustrated by the cross-hatched zone shown in FIGURE 3. At temperatures below the zone, it is impossible to form a satisfactory iron-tin alloy by the process described herein. At elevated temperatures and when using prolonged heating periods, oxidation and buckling of the steel strip occur.

At flow-brightening temperatures below 520 F., although an FeSn alloy layer is formed above the melting point of tin, 450 R, such alloy layer has been found to be unsatisfactory for tin plate to be used, particularly for corrosive food packing, in tin can production. At flow-brightening temperatures below 520 F. and delay times up to about 3 seconds, the alloy layer is comprised of relatively large crystals with relatively wide spacing between crystals and the alloy thickness will be insufficient to obtain an average ATC value as low as 0.05. It is not desirable to maintain flow-brightening temperatures much longer than 3 seconds since the alloy layer thickness formed will be too great to permit satisfactory soldering or can crimping.

With a delay time of 0.1 second, the average ATC values for 0.15, 0.25 and 0.5 lb./base box tin plate reflowed at temperatures of 595 F. and 650 F., as shown in Tables VIII, IX and X, below, are somewhat lower than the average ATC values shown for 0.75 lb. plate. The deviation or lower values is not considered of real significance since the spread of ATC values for different panels reflowed under these conditions varies to a considerable extent. Also it requires about 0.1 second for the laboratory mechanism to release the framework before the strip is quenched in water. A very small increase in delay time could account for the lower values as noted by the steep slopes of these curves. However, at 705 F., the average ATC values for 0.15, 0.25 and 0.5 lb. plate are essentially the same as the average values shown for 0.75 lb. plate.

With a delay time of 0.8 second, the average ATC values for the lighter coatings reflowed at a temperature of 595 F. do not deviate as widely as the values shown with a delay time of 0.1 second. At 650 F., the ATC values for the lighter coatings are in close agreement with the average ATC value shown for 0.75 lb. plate.

It may be concluded from the above that the ATC values are not influenced to any significant degree by the thickness of the tin plate within the range investigated.

There was no significant drop in the temperature of the strip between the time the power was shut off and when the strip was quennched in water. The temperature drop with 0.3 second delay time from a temperature of 600 F.

was in the order-of-rnagnitude range of to F. based upon decrease in millivolts as estimated by the millivolt profile curves recorded during temperature measurement studies.

As previously emphasized, the temperature to which the tin-plated steel is heated and the delay time between power shut off and quenching have profound effects on the ATC values and weights of the alloyed tin layer. Referring to the data given in Table XIII showing average ATC values and alloy weights in pounds per base box for 0.75 lb./base box tin plate heated to a temperature of about 595 F., delay time has significant efliects on the ATC values and alloy weights. Time of heating was controlled at 0.3 second in obtaining the data shown in Table XIII. With a delay time of 0.1 second, the ATC values averaged 0.259 and weight of the alloy was 0.086 lb./base box. With a delay time of 0.4 second, the ATC values averaged 0.078 and weight of the alloy was 0.112 lb./base box. No data are given for ATC values and alloy coating weight with a delay time of 0.3 second.

Although the mechanism of alloy formation is not fully understood, rates of diffusion to form the FeSn alloy must be extremely rapid to explain formation of the alloy layer. Increase in delay time, in addition to forming a heavier alloy layer, may provide better coverage of the alloy over the steel surface thru re-orientation of the FeSn crystals at the steel-alloy interface.

The following examples are given to show the high degree of corrosion resistance obtained by the practice of this invention.

In the examples, the tin electroplating, flow-brightening and ATC value determinations were carried out as follows:

Low carbon steel of the type customarily used for tin plate was used for preparation of samples.

After conventional cleaning and pickling, 2 in. x 6 in. panels were tin-plated to a thickness of 0.75, 0.50, 0.25 and 0.15 pound per base box (a base box corresponds to 435 sq. ft. of surface area) using the halogen tin process described in US. Patent 2,407,579.

The tin-plated samples were heated to a carefully controlled given temperature by relatively high voltage alternating current resistance heating and after a carefully controlled given delay time were dropped into a water quenching bath.

After quenching, the remaining tin plate was stripped from samples leaving the steel panels with an exposed iron-tin alloy coating. This stripping was done by immersing the panels in a 5% NaOH solution using a constant voltage power supply to maintain a potential of 0.4 volt between the panels and a stainless steel electrode. When all free tin was removed the stripping current automatically diminished to zero.

The stripped panels were then subjected to tests to determine ATC values in accordance with the test described above. The results are tabulated below: In the tabulated results the ATC values are given in groups of 3 panels cut adjacently from each 2 in. x 6 in. panel:

TABLE I.EFFECT OF FLOW BRIGHTENING (TEMP. CA.

460 F.) ON ATC VALUES 0F HALOGEN TIN PLATE-0.75

LBJBASE BOX [Delay time before quenching with ATC values given below] TABLE II.EFFECT OF FLOW BRIGHTENING CONDITIONS (TEMP. CA. 475 F.) ON ATC VALUES OF HALO GEN TIN PLATE0.75 LBJBASE BOX [Delay time before quenching with ATC values given below] Example Number 3 4 5 6 7 8 Delay Time (See) 0. 4 0. 8 1. 2 2.0 3.0

Average ATC Values 0.463 0.385

TABLE III-EFFECT OF FLOW BRIGH'IENIN G CONDI' TIONS (TEMP. CA. 515 F.) ON A'IC VALUES OF HALOGEN TIN PLATE0.75 LBJBASE BOX {Delay time before quenching with ATC values given below] TIN PLATE0.75 LBJBASE BOX [Delay time before quenching with ATC values given below] Example Number 9 10 11 12 13 14 5 Example Number 36 37 38 39 DeleyTime(Sec.) 0.1 0.4 0.3 1.2 2.0 3.0 De1aYTime(Se') 0.307 0.303 0.135 0.030 0.033 0. 053 8-833 3833 0.440 0.327 0.170 0.088 0.007 0.058 0'038 0-023 0-028 Average ATCVelnes.--. 0.430 0.323 0.133 0.143 0.070 0. 5 Average ATC Values (L038 L031 TABLE IV.EFFECT OF FLOW BRIGHTENING OONDI' TIN PLATE-41.75 LBJBASE BOX [Delay time before quenching with ATC values given below] TABLE VlIL-EFFECT OF FLOW-BRIGHTENING CONDI- TIONS ON ATO VALUES OF HALO GEN TIN PLATE-COAT- ING WEIGHT ABOUT 0.15 LBJBASE BOX [Delay time before quenching with ATO values given below] Tegnp. Ter np. Temp. ExampleNumber 15 10 17 13 10 20 595 2i 40 41 42 43 44 Delay'11me(Sec.) 0.1 0.4 0.8 1.2 2-0 3.0 33.30%:03253 0.1 0.0 0.1 0.8 0.1

Average ATC Values. 0.425 0.157 0.116 0.046 0.049 0.044 Average ATC Values @212 52 (L092 1023 0,036

Example Number 21 Delay Time (See) 0. 1

Average ATG Values o Q00 c 000 o o \IDOOO cu moow 5 g N @0101 q mean: 53 a:

TABLE VI.-EFFECT OF FLOW BRIGHTENING C O NDITIO NS (TEMP. 651

F.) ON ATC VALUES OF HALOGEN TIN PLATE0.75 LB/BASE BOX [Delay time before quenching with ATC values given below] Example Number 28 Delay Time (See) 0.1

Average ATO Values .0. 152

From the above results a mathematical equation can be derived from which may be calculated the relative temperature, in F., and delay time, in seconds, between the limits of 520 F. and 722 F. and between and 3 seconds, to obtain any desired ATC value.

This equation describing effect of refiow conditions on ATC values of tin plate is given below:

r s r no The parameters A, B, C, m, n, and p were obtained in the following fashion. First, consider the boundary condition, T :450 F. No significant amount of alloy is formed until T is greater than 450 F. Therefore, at 450 F. the ATC value is a constant equal to the inherent corrosion resistance of unheated tin-plated steel, which has been experimentally determined to be about 0.7. The Equation 1 reduces to the following under the above conditions (T:T :450 F.):

2 ATC:e

Using an ATC value of 0.7, A is calculated to be 0.357.

The coefficients B, C and the powers m, n, 7 shown in Equation 1 account for the non-linear character of the functional relationship between temperature, delay time and ATC (refer to FIGURE 3). Values for B and m can be calculated at the boundary condition where the delay time between reaching peak refiow temperature and quenching the strip in water is zero (0:0). Data shown in FIGURE 3 indicate that melting the tin-plated steel to a temperature of about 720 F. (with 0:0) will produce Grade A plate (ATC values of 0.05). When the temperature is about 550 F. and 8:0, the ATC values are approximately 0.6.

What a value of 6:0, Equation 1 reduces to:

given above, this equation can now be solved for B and m. The calculated values are:

where:

The parameters A, B and m have now been estimated. The remaining parameters, C, n and p must be estimated from the experimental data. Referring to FIGURE 3, Grade A plate should be produced under the following approximate conditions:

T:700 F., 6:0.1 sec. T:600 F., 0:0.65 sec. T:520 E, 0:2.0 sec.

Three simultaneous equations using the above values can be solved to obtain the following:

C=123 n:1.8 p:1.4

The unknowns in Equation 1, describing ATC values as a ftmction of delay time and temperature have now been estimated from the experimental data. Using these calculated values of the 6 parameters, Equation 1 reduces to:

The above-referred to equation:

is a simplified close approximation to the above Equations 1 to 5 and is also derived from the results of the above examples as shown in the curve illustrated in FIG- URE 2. This simplified equation solved for 0 to give delay time in seconds where T is temperature in degrees F. is:

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

We claim:

1. The process for improving the corrosion resistance of electrolytically deposited tin plate which comprises electroplating a steel substrate to provide a coating of tin thereon having a weight between about 0.15 to 0.75 pound per 'base box of tin plate, heating the tin plate to a tem peratul'e between 520 F. and 722 F., maintaining said tin plate at said temperature for a period up to 3 seconds, said time and temperature being selected from the limits defined by the cross-hatched zone of FIGURE 3 of the drawing, and thereafter rapidly quenching the heated tin plate.

2. The process of claim 1 wherein the tin is deposited onto a steel strip from a halogen tin electro-plating bath.

References Cited UNITED STATES PATENTS 1,776,603 9/1930 Schulte 204-37 XR 3,062,725 11/ 1962 Frankenthal 204-37 3,087,871 4/1963 Kamm 204-36 3,174,917 3/1965 Lesney et a1 204-37 3,285,838 11/1966 Morgan et al. 204-37 JOHN H. MACK, Primary Examiner W. B. VANSISE, Assistant Examiner US. Cl. X.R. 29-1964

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