CA1238293A - Method for producing tin-free steel strips having improved lacquer adhesion - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Tin-free steel sheets are produced by chromium plating a thin steel sheet through cathodic electrolysis in a chromium-containing bath to deposit a metallic chromium layer, reversely electrolyzing the chromium plated steel steet by a successive anodizing treatment in the same or another chromium-containing bath, and subjecting the reversely electrolyzed steel sheet to an electrolytic chromate treatment in an aqueous chromate solution. The adhesion of lacquer to the TFS sheet is improved by carrying out the electrolytic chromate treatment in a chromate solution which contains chromic acid, chromates or dichromates as a main ingredient and sulfuric acid, sulfates, thiocyanates, sulfonic acids or sulfonates as an assistant ingredient, in a molar ratio of H2SO4/CrO3 between 1/1000 and 1/100 provided that the main and assistant ingredients are converted into CrO3 and H2SO4, respectively.

Description

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Method for Producing Tin-Free Steel Strips Having Improved Lacquer Adhesion BACKGROUND OF THE INVENTION
This invention relates to a method for producing tin-free steel strips or sheets having improved lacquer adhesion.
Electrolytic chromates treated steel sheets also known as tin-free steel (IFS) of chromium type have improved properties as can-forming material and are regarded as a substitute for tin plates, and the demand for them is increasing in recent years.
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Since IFS has a two-layer coating of metallic chromium and hydrated chromium oxide layers on the surface, it does not possess sufficient weld ability. A Cannes fabricated from a IFS sheet by coating the sheet with an epoxy-phenol .: : :
resin and cementing the mating edges of the coated sheet wlth~a~polyamide adhesive to form a can carrel.
Recently, the extent of application of IFS cans has been further spread. that is, IFS cans are not only used for low-temperature packs prepared my packing contents such as carbonated beverage and beer in cans at relatively low 20~ temperatures, but Al soused for the so-called hot packs prepared by packing contents such as fruit juice in scans at . .

relatlvely~higb temperatures for sterlllzatlon. IFS is ; also used in those cans requiring a high temperature . :
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3L~3~293 retorting treatment for sterilization at the end of packing of coffee, for example. In the latter applications, there often occurred accidents of rupture of can barrels.
This can barrel rupture occurs in cemented IFS cans during hot packing and retorting treatment because hot water penetrates through the coating at the barrel junction to deteriorate the interracial adhesion between the lacquer film and the IFS substrate to eventually separate the lacquer film from the IFS.
The inventors previously proposed in Japanese Patent Application Cook No. 57-177998, laid open to the public on November 1, 1982, (Applicant Cossack Steel Corporation) a method for producing a tin-free steel sheet having improved lacquer adhesion by chromium plating a steel strip, subjecting the strip to reverse electrolysis with the strip made the anode, and thereafter subjecting the strip to an electrolytic chromates treatment in an aqueous solution containing hexavalent chromium.
The reverse electrolysis may be carried out in the chromium plating solution or in another aqueous solution containing chromium ion, with similar results with respect to lacquer adhesion. It was found that the method characterized by inserting the reverse electrolysis step between the chromium plating and electrolytic chromates treatment steps is fully effective in improving lacquer adhesion. Continuing research, we found that reversely electrolyæed steel strips tend to suppress the growth of a chromium oxide layer thereon during the subsequent electrolytic chromates treatment, as long as the chromates Jo ~;~3~3~93 bath contains at least one selected from chronic acid android, chromates and dichromates and concomitant anions such as sulfate (S042 ), fluoride (F-) and chloride (Of-) ions. The electrolytic chromates treatment then requires a large quantity of electricity in order to deposit the chromium oxide layer to a sufficient thickness to insure improved lacquer adhesion, resulting in an economic loss in commercial production.
The surface state of steel strips after chromium plating and reverse electrolysis was examined by the electron spectrocopy for chemical analysis (ESCA) to find that as compared with the surface state observed after chromium plating, the quantity of i sulfur (probably in the form of sulfate) code posited in the chromium oxide layer is reduced and the bond energy of 1S is shifted to a lower energy level, that is, from 531.4 eve to 530.1 eve The proportion of of linkages cry in the chromium oxide layer is reduced and the proportion of ox linkages tCr-O) is increased. This is the reason why the growth of a chromium oxide layer is suppressed on reversely electrolyzed steel strips.
The inventors have studied how to promote the growth of a chromium oxide (Or ) layer on reversely electrolyzed steel strips without detracting prom subsequent lacquer adhesion. We have found that electrolytic chromates treatment nay be Jo advantageously carried out in an aqueous - ;
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chromates solution which contains at least one chromium compound selected from chronic acid android, chromates and dichromates as a main ingredient and at least one selected from sulfuric acid, sulfates, thiocyanates, sulfonic acids, and sulfonates as an assistant ingredient.
It is, therefore, an object of the present invention to provide an electrolytic chromates solution which allows chromium oxides (Crux) to be effectively deposited on reversely electrolyzed steel strips without detracting from subsequent lacquer adhesion.
SUMMARY OF THE INVENTION
In one broad aspect, the present invention relates to a tin-free steel strip having improved lacquer adhesion, comprising method for producing a chromium plating thin steel strip through cathodic electrolysis in a chromium ion-containing aqueous solution to form a layer consisting essentially of metallic chromium, reversely electrolyzing the chromium plated steel strip by a successive anodizing treatment in said aqueous solution or in another chromium ion-containing aqueous solution, and subjecting the reversely electrolyzed steel strip to an electrolytic chromates treatment in an~aqaeous solution containing as a main ingredient at least one selected from the group consisting of chromlc acid, chromates and dichromates, and as an assistant ingredient at least one selected from the group consisting of sulfuric acid, sulfates, t~iocyanates, sulfonic acids and sulfonates, wherein the solution n which said electrolytic ohromate treatment is carried out has a . - -: , , ::: . , ': ' -1; :3~2~3.~

molar ratio of H2SO4/CrO3 in the range between 1/1000 and 1/100 provided that the mow of the main ingredient it calculated as Crow and the mow of the assistant ingredient is calculated as H2S04.
IFFY DESCRIPTION OF THE DRAWINGS
The above and other objects, features, and advantages of the present invention will be readily understood by reading the following description in conjunction with the accompanying drawings, in which:
Fig. 1 is a diagram showing lS peaks measured by the ESCA on the surface of chromium plated steel and reversely electrolyzed steel;
I, Fig. 2 shows the amount of hydrated chromium oxides as a I; function of the molar ratio of H2SO4/CrO3 in the electrolytic chromates solution;
i Figs. pa and 3b illustrate a bonded sample of lacquered Jo ;: pieces and a testing holder having the sample mounted therein for the evaluation of lacquer adhesion.
; DETAILED DESCRIPTION OF TOE INVENTION
The present invention is directed to IFS strips or sheets having a layer of metallic chromium (Crimea ranging from 50 Tao my per square meter on each sheet surface and a layer of hydrated chromium oxides (Crux) ranging from 8 to 30 my of Or per square , meter on the metallic chromium I: :

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layer. IFS sheets generally have a metallic chromium layer of 50 to 200 mg/m2 because thinner layers of less than 50 mg/m2 have poor corrosion resistance. Thicker layers exceeding 200 mg/m2 do not provide an additional improvement in corrosion resistance.
IFS sheets have a layer of hydrated chromium oxides of 8 to 30 mg/m2 (calculated as metallic chromium) because thinner layers of less than 8 mg/m2 do not provide the necessary lacquer adhesion. Thicker layers exceeding 30 mg/m2 have a poor appearance and are prone to cracking during subsequent processing and thus impractical.
The chromates solution in which the electrolytic chromates treatment was carried out contains as a main ; ingredient at least one chromium compound selected from ; 15 chronic anhydrlde, chromates and dichromates. Examples of the chromates and dichromates used herein include potassium chromates K2CrO4, sodium chromates Nicker, ammonium cremate NH4)2CrO4, potassium dichromate K2Cr2O7, sodium dichromate Nicker, and ammonium dichromate (N~4)2Cr2O7. The Jo 20 concentra~lon of the chromium compound should be limited to the range of 20 to 200 g of Crow per liter of the solution.
At chromates concentrations of less than 20 g/l of Crow, the bath voltage must be increased, leading to an electric power loss. Chromates concentrations beyond 200 g/l of Cry are costly because the loss of Crow due to entrainment of I:
solution with outgoing steel strips is increased.

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The assistant ingredient which is intentionally added to the chromates solution is at least one compound which is selected from the group consisting of sulfuric acid, sulfates, thiocyanates, sulfonic acids, and sulfonates.
Examples of the sulfates, thiocyanates, and sulfonic acids and sulfonates used herein include potassium sulfate K2SO4, sodium sulfate Nazi, and ammonium sulfate (NH4)2SO4;
potassium thiocyanate KSCN, sodium thiocyanate NaSCN, and ammonium thiocyanate NH4SCN; and phenoldisulfonic acid and catecholdisulfonic cold and the potassium, sodium, and I; ammonium salts thereof.
The following experiment was carried out to determine the optimum range of the assistant ingredient concentration in the chromates solution. Like the chromates converted into Cry, the assistant ingredient is converted into H2SO~ for I; calculation purpose The molar ratio of H2SO4/CrO3 was I::
varied over the range between l/2000 and Lowe. A number ; of reversely electrolyzed steel pieces were electrolytically chromates. The amount of hydrated chromium oxides (Or X) formed thereon was determined and lacquer adhesion was tested.
The results are plotted in Fig. 2 which shows that the assistant ingredient should be present in such concentrations that the molar ratio of SUE to Crow is in the range between Lowe and Lowe In region I where the assistant ingredient is present in concentrations to give a molar ratio of Hiker of lower than Lowe the .

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~313~3 assistant ingredient it not effective to assist a chromium oxide layer in growing, failing to achieve the objects of the invention. On the other hand, in region III where the assistant ingredient is present in concentrations to give a molar ratio of H2SO4/CrO3 of higher than l/l00, the chromates film is stained so that the resulting products become commercially unacceptable. or this reason, the assistant ingredient should be added to the chromates solution in an amount (calculated as H2SO4) such as to give a molar ratio of H2SO4/CrO3 between l/l000 and l/100 as shown by hatched region II in Fig. 2.
It should be noted that the present invention is directed to a method for producing a tin-free sleet strip comprising chromium plating, reverse electrolysis, and 15~ electrolytic chromates treatment. Unless the reverse electrolysis is interposed between the chromium plating and electrolytic chromates treatment, tuner steel strips having improved lacquer adhesion cannot be obtained even if the electrolytic chromates bath contains the assistant I ingredient in a concentration to give a molar ratio of cry in the range between l/l000 and Lyon Since a controlled amount of S042 is intentionally .
added to the electrolytic chromates bath according tooth method of the invention, a great amount of metallic chromium is deposited during the electrolytic chromates treatment. The amount of metallic chromium (Cry) deposited is greater on the presence of reverse electrolysis than in , : , ' ` ` ' : . :

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3~2~3 the absence of reverse electrolysis. The method of the invention thus has the additional advantage of a reduced quantity of electricity required for chromium plating.
The adhesion of lacquer to chromates steel is evaluated by carrying out both U- and T-peel tests in the following manner.
U-peel test As shown in Fig. pa, a phenol-epoxy lacquer was applied to one surface of a treated steel sheet 1 to a build-up of 60 mg/m2 and baked for 12 minutes at 210C to form a lacquer film lay The same lacquer was applied to one surface of another treated steel sheet 2 to a build-up of 25 mg/m2 and baked under the same conditions to form another lacquer film PA. The sheets were cut into pieces ox 70 mm wide by 60 mm long. The longitudinal ends of two . different pieces were overlapped eye drove a distance of 8 mm with a nylon film 3 of logjam thick interposed there between as shown in Fig. pa. Using a hot press, these two pieces were preheated at 200C for 120 seconds and 20~ pressure cemented at 200C for 30 seconds under a pressure of~3~kg/cm2. Ten samples 4 were cemented in this manner for each run. The cemented samples 4 were mounted in a rectangular holder S as shown in Fig. 3b and placed in a retort kettle filled with steam at 130C.~ Some samples separated at the junction under retorting after 300 minutes. In this U-peel test, lacquer adhesion was evaluated good when zero to one sample separated after 300 minute retorting.

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' Lo 33 of test A phenol-epoxy lacquer was applied to a treated steel sheet to build up of 50 mg/m2and baked for 12 minutes at 21~ C.
The baked sheet was cut into pieces of 5 mm wide. A sandwich of two pieces with a nylon adhesive interposed between the lacquer films was cemented under a hot press at 200 C for 30 seconds.
The cemented samples were immersed for 7 days in a 0.4% citric acid aqueous solution at I C. Thereafter, T-peel tensile strength was measured to evaluate secondary or wet adhesion of lacquer. In this T-peel test, samples whose T-peel tensile strength was 2.5 kg/5 mm or more after 7-day immersion in 0.4%
citric acid solution at 90 C were evaluated good.
Samples which passed both the U- and T-peel tests were evaluated to have excellent secondary or wet adhesion of lacquer.
Examples of the present invention are presented by way of ~;~ illustration and not by way of limitation.
i Exam Cold rolled steel sheets (T4CA) having a thickness of ; 0.22 mm were electrolytically decreased in a 5% HOMEZARINE*
(decreasing agent manufactured by Kayo OK of Japan solution at 80 C, rinsed with water immersed in a 10% H2SO~ for 5 seconds for pickling, rinsed again with water, and then subjected tooth following treatments in sequence:
(1) chromium plating, :: :

(2) reverse;elect~oLysis, and

(3) electrolytic~chromate treatment.

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Steps (1) and (2) were successively carried out in the same electrolytic bath. These steps were carried out in the following conditions.
(l) chromium plating Plating bath composition Bath A at a temperature of 50C
Crow 150 g/l SO 0.8 g/l Na2SiF6 7 g/1 Cry+ 3 g/l Bath B at a temperature of 60C
Crow 250 g/l H2SO4 2.5 g/l g/l :
Electrolytic conditions Cathodic treatment Current~density50 Adam Time 1.2 seconds (2) Reverse electrolysis I.
The steel sheets were subjected to reverse electrolysis at a current density of 5 Adam for 0.2 seconds or at a current density of 15 A/dmG for Owl seconds while the aye made the anode in the same bath as used for chromium plating. For comparison sake, some steel sheets were not subjected to reverse electrolysis.

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313~9;3 (3) Electrolytic chromates treatment Cathodic treatment was carried out with an electricity quantity of lo to 20 coulomb/dm2 using various chromates baths based on Crow and Nicker and containing varying concentrations of the assistant (H2SO4) at a temperature of 40C. For comparison sake, one chromates bath was free of H2SO4.
By carrying out (1) chromium plating, (2) reverse electrolysis, and (3) electrolytic ehromate treatment in succession, there were prepared IFS sheets The amount of chromium oxides (Crux) formed on the IFS sheets was determined by X-ray fluorometry and the amount of metallic chromium (Or ) was determined by electrolytic analysis.
The U- and T-peel tests were carried out to evaluate lacquer adhesion after retorting. The results are shown in Table l.

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iL~31~3X~33 In Example 1 (comparison), chromium plating (1) is followed by reverse electrolysis (2) and then by electrolytic chromates treatment in a bath containing H2SO4 in such a lower concentration as to give a molar ratio of H2SO4/CrO3 of 1/2000. A smaller quantity of chromium oxide (Crux) was present and the wet (secondary) lacquer adhesion was unsatisfactory.
Unlike Example 1 (comparison), Example 2 used an electrolytic chromates bath having a proper H2SO4 concentration to give a molar ratio of H2SO4/CrO3 of 1/200, yielding a sufficient quantity of chromium oxide (Crux) which ensures the improved wet lacquer adhesion.
Example 3 (comparison) used an electrolytic chromates bath having the same H2SO4/CrO3 ratio as in Example 2, but omitted reverse electrolysis (2). The elimination of reverse electrolysis (2) resulted in inferior wet lacquer adhesion irrespective of an increased quantity of chromium oxide.
Example 4 (comparison) carried out chromium plating (I) and reverse electrolysis (2) followed by electrolytic ehromate treatment in a bath containing H2SO4~in such a higher concentration as to give a molar ratio of H2SO4/CrO3 of 2/100. Although wet lacquer adhesion was satisfactory, products were stained to a commercially unacceptable level.
2~5 yin Examples S through 7, chromium plating (1) and reverse electrolysis (2) were followed by electrolytic chromates treatment in a bath having a molar ratio of :::

, 31~29~3 H2SO4/CrO3 in the range of 1/1000 to 1/100, that is, the specific range according to the present invention.
Sufficient quantities of chromium oxide were present and the wet lacquer adhesion was superior.
Examples 8 and 9 (comparison) carried out reverse electrolysis (anodic treatment) (2) and electrolytic chromates treatment (cathodic treatment) (3) without chromium plating (cathodic treatment) (1), failing to provide improved lacquer adhesion.
These data suggest that improved lacquer adhesion cannot be accomplished unless the series of three essential steps, that is, chromium plating (1), reverse electrolysis (2), and electrolytic chromates treatment (3) are carried out and unless an assistant ingredient capable of generating sulfate ion is intentionally added to the ; electrolytic chromates bath in such an amount as to give a molar ratio of sucker in the range of 1/1000 to 1/100.
; With these requirements met, IFS sheets having improved lacquer adhesion can be manufactured at a commercially ;

acceptable cost.
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Although in the above-described embodiment, reference is made to electrolytic chromates treatment in a bath containing Crow and Nicker as the main ingredient and H2SO~ as the assistant ingredient, it should be understood that similar effects are obtained when other compounds are selected as the main and assistant ingredients from the above-described groups as long as the molar ratio of H2S04/CrO3 falls within the optimum range as specified in the present invention.
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Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for producing a tin-free steel strip having improved lacquer adhesion, comprising chromium plating a thin steel strip through cathodic electrolysis in a chromium ion-containing aqueous solution to form a layer consisting essentially of metallic chromium, reversely electrolyzing the chromium plated steel strip by a successive anodizing treatment in said aqueous solution or in another chromium ion-containing aqueous solution, and subjecting the reversely electrolyzed steel strip to an electrolytic chromate treatment in an aqueous solution containing as a main ingredient at least one selected from the group consisting of chromic acid, chromates, and dichromates, and as an assistant ingredient at least one selected from the group consisting of sulfuric acid, sulfates, thiocyanates, sulfonic acids, and sulfonates, wherein the solution in which said electrolytic chromate treatment is carried out has a molar ratio of H2SO4/CrO3 in the range between 1/1000 and 1/100 provided that the mol of the main ingredient is calculated as CrO3 and the mol of the assistant ingredient is calculated as H2SO4.

2. The method according to claim 1 wherein the chromium layer is formed to a build-up of 50 to 200 mg per square meter on each surface.

3. The method according to claim 1 wherein the electro-lytic chromate treatment is carried out to form a chromium oxide layer to a build-up of 8 to 30 mg of Cr per square meter on each surface.