CA1138373A - Electrolytically tin-plating steel, remelting and alkaline cathodic treating - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An electrolytic tin-plated steel sheet excellent in long-term paint adhesion and a process for manufacturing same, in which a steel sheet is subjected to an electrolytic tin plating treatment to form a tin plating layer thereon, then the resultant tin plating layer is subjected to a remelting treatment to produce noncrystallized tin oxides on the plating layer. The resultant noncrystallized tin oxides are subjected to a cathodic electrolytic treatment to eliminate, by reduction, tin oxides which crystallize into tetragonal .alpha.-Sno and tetragonal SnO2 on heating at the time of painting and baking so as to leave only a tin oxide which crystallizes into orthorhombic SnO on heating at the time of painting and baking in an amount within the range of from 1.0 to 6.5 millicoulomb/cm2.

Description

.31~3~

REFERENCE TO PATENTS, APPLICATIONS AND PUBI,ICATIONS
-PERTINRNT TO TIIE INVENTION

So :Ear as we know, prior documents pertinent to the present invention are as follows:

(1~ A paper by Mr. S.C. Britton and Mr. K. Bright entitled "An Examination of Oxide Films on Tin and Tinplate"
appearing on page 163 et seqq. in the "Metallurgia", a British periodical, vol. 56, issued in October 1957;

(2) Japanese Patent Publication No. 48,389/74 of December 20, 1974;

(3) Japanese Patent Publication No. ~,691/75 of April 7, 1975;

(4) A paper by Mr. P.R. Carter entitled "Some Factors Affecting the Surface Chromium Content of Electro- ~-chemically Treated Tin Plate" appearing on page 782 e-t seqq. in the "Journal of the Electrochemical Society", an American periodical, vol. 108, No. 8, issued in August 196l.;

(5) U.S. Patent No. 3,647,650 oE March 7, 1972;

(6) Japanese Patent provisional Publication No. 68,933/75 of June 9, 1975; and .,~i ' ~ .

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3~

,

(7) British Patent No. 1,483,305 published on August 17, 1977.

The details of the prior arts disclosed in the ,~bove-mentioned prior documen-ts will be commented on in the "BACK-GROUND OF THE INVENTION" presented herebelo~.

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~3~3~3 FIELD OF THE INVENTION

The present invention rela;tes to an electrolyti.c tin-plated steel sheet excellent not only in paint adhesion, but also in smudge resistance, oxidation resis-tance, sulfurization resistance and solder-ability, these properties being not only ~alid immediately after manufacturing but also maintained with almost no degradation even after holding or storing for a long period of time, and a process of manufacturing same.

. BACKGROUND OF THE INVENTION_ Electrolytic tin-plated steel sheets are widely used as materials for various canning containers. With a view to preventing corrosion, rusting, discoloration and other defects on the i.nner and outer surfaces oE a canning container and to maintain a beautiful appearance, an electrolytic tin-plated steel sheet is used as the material for the canning container after subjecting the surface of its tin plating layer to painting and baking. More specifically, an electrolytic tin-plated steel sheet for canning containers is subjected, after painting and baking, to such high-speed can-making operations as cutting, blanking, bending and assembling, in the course of which it is exposed to such strong physical actions as Eriction, impact and bending. The can thus manufactured, when containing a food, is subjected -to a sterilization treatment at a high temperature under a high pressure for hours, in ......... . ' ,. ' ` ' ` ~;;
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the course of which the electrolytic tin-plated steel sheet is subjected to various chemical actions by the constituents of the contents.
Therefore, an electrolytic tin-plated steel sheet for canniny containers ls required to have an excellent paint adhesion to withstand the afore mentioned severe physical and chemical actions so that the baked paint may not peel off.
A tin plating layer of a tin-plated steel sheet as manufactured by the conventional continuous electrolytic tin plating process, being an aggregate layer of Eine tin grains from the microscopic point of view, has a poor adhesion to the steel sheet surface ancl has no gloss. For the purpose of imparting a gloss to the tin plating layer and improving sulfuri~ation resis-tance and oxidation resistance, it is the usual practicel following a continuous electrolytic tinning, to continuously heat the resultant electrolytic tin-plated steel sheet in a heating furnace to rapidly melt the formed tin plating layer, and then, immediately cool the heated tin plated steel sheet continuously in a cooling tank to rapidly solidify said molten tin plating layer. The above-mentioned treatment of the tin plating layer comprising a rapid melting and a rapid solidification of the tin plating layer is hereinaftex referred to as the "remelting trea-tment".
When the above-mentioned remelting treatment is applied to the tin plating layer of an electroLytic tin-plated steel sheet, tin oxides ' ." , ~ .

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:~ : ~ ~ ' . ; , ~L13~ 3 are produced on the surface of said tin plating layer in an amount of from about 3 to about ~ millicoulomb/cm , as converted into a quantity of electricity required in the measurement by cathodic reduction. The amount of such tin oxides is expressed by the quantity of electricity per square cen-timetre required until tin oxides on the surface of the tin plating layer are completely eliminated by cathodic reduction in a water solution of hydrogen bromide of 10 molarity. The amount of tin oxides will be hereinafter expressed by the above-mentioned quantity of electricity in all cases.
In general, since an electrolytic tin-plated s-teel sheet immediately after the remelting treatment of the tin plating layer has an appropriate amount of tin oxide produced on the surface of the tin plating layer, these tin oxides exert an effect as a bonding agent, thus giving an excellent paint adhesion. However, when the electrolytic tin-plated steel sheet having a remelting treatment of the tin plating layer is held or stored for a period of several months without applying paint and ,,.~ . . , ;,', ~- '; `, , ~
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t' . , ~ 3~3~3 baking, said tin oxides gradually grow, and as a result, not only the surface of the electrolytic tin-plated steel sheet becomes discolored~. but also the paint adhesion becomes degraded.

To inhibit growth of the aforementioned -tin oxides on the surface of the tin plating layer, an electrolytic or non-electrolytic post-treatment has conventionally been applied to the surface of an elec-trolytic tin-plated steel sheet, and the following post-treatments are known:

~1) A non-electrolytic post-treatment of a tin-plated steel sheet disclosed in a paper by Mr. S.C. Britton and Mr. K. Bright en-titled "An Examination of Oxide Films on Tin and Tinplate", appearing on page 163 et seqq. .
in the "Metallurgia", a Bri-tish periodical, vol. 56, lS issued in October 1957, which comprises:

immersing a tin-plated steel sheet into a hexa-valent chromic ion-based aqueous solu-tion under the .
following conditions:

Composition of aqueous solution: an aqueous solu-tion containing 1% chromate;
Temperature of aqueous solution: 85~C;
Immersion time : up to 60 minutes;
(hereinafter referred to as -the "prior art (1)").

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n electrolytic post~treatment o an electrolytic tin-plated steel sheet disclosed in Japanese Patent Publication No. 48,389/74 of December 20, 1974, which compriseso after an electEolytic tin plating treatment or after a remelting treatment of the ti.n plating layer following said tin plating, applying an anodic ¦ electrolytic treatment to the electrolytic tin-plated steel sheet in a hexavalent chromic ion-~ased aqueous solution under the following conditions:

Composition of aqueous solution: An aqueous solution containing 2 to 100 g/Q hexavalent chromic ion, or an aqueous solution contain-. ing 2 to 100 g/Q hexavalent chromic ion and . 15 0.1 to 5 g/Q phosphoric acid and/or phosphate;
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pH of aqueous solution: 1.O to 9.5;
Temperature of aqueo~s solution: 20 to 80C;
. Quantity of electricity: 0.05 to 0.23 millicoulomb/
. dm2;
.. ~.. . 20 (hereinafter referred to as the "pr1or art (2)").

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: (3) An electrolytic post-treatment of an electrolytic .
~ tin-plated steel sheet disclosed in Japanese Patent : ~ Publication NoO 8,691/75 of April 7, 1975 which . ,.
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comprises:
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After an elec-trolytic tin plating treatment or after a remelting treatment of the tin plating layer following said tin plating, applying an anodic electro-lytic treatment to the electrolytic tin-plated steel sheet in an alkaline aqueous solution, under the following conditions:

Composition of aqueous solution: an aqueous solution containing 0.5 to 30 g/~ at least one compound selected-from the group con-sisting of organic and inorganic alkali me-tal compounds and ammonium compo~mds;
Temperature of aqueous solution: 20 to 80C;
Quantity of electr~city: 0.05 to 0.23 millicoulomb/dm2;
~hereinafter referred to as the "prior art (3)").

(4) An electrolytic post-treatment of an electrolytic tin-plated steel sheet disclosed in a paper by Mr. P.R. Carter entitled "Some Factors Affecting the Surface Chromium Content of Electrochemically 'rreated Tin Plate" appearing on page 782 et seqq.
in the "Journal of the Elec-trochemical Society", an American periodical, vol. 108, No. 8, ;.ssued in ~ 9 _ . . .
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August 1961, which comprises~

applying a cathodic electrolytic treatment to a -tin-plated steel sheet in a hexavalent chromic ion-based aqueous solution under the following conditions;

Composition of aqueous solution: an aqueous solution containing 20 to 30 g/Q sodium bichromate;
Temperature of aqueous solution: 46 to 52C;
: Quantity of electricity: 10.8 to ~1 . millicoulomb/cm2;
(hereinafter referred to as the "prior art (4)~

(5) Another electrolytic post~treatment of an electrolytic tin-plated steel sheet disclosed in the paper presented above by Mr. P.R. Carter entitled "Some Factors Affect-ing the Surface Chromium Content of Elec-trochemically Treated Tin Plate" appearing on page 782 et seqq. in the "Journal of the Electrochemical Society", an American periodical, vol. lOa, Mo. 8, issued in August 1961, which comprises:

applying a cathodic electrolytic treatment to a tin-plated steel sheet in a hexavalent chromic ion-based aqueous solution under the following conditions:

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Composition of aquebus solution: an aqueous solutlon eontaining 20 to 30 g/~ sodium ~ichroma~e;
Temperature o~ aqueous solutlon: 46 to 52C; .:
Quantity of eleetricity: 10.8 to 41 millicoulombtcm2;

and, direetly thereafter, applying an anodie eleetro- ;
lytic treatment with a quantity of eleetrieity of 2.7 :
millieoulomb/em2 to said tin~plated steel sheet applied with said cathodic electrolytic treatment in the same aqueous solution as -that used in said cathodic electrolytic treatment;
(hereinafter referred to as the prior art (5)").

(6) An electrolytic post-treatmen-t of an electrolytic tin-plated or electro-gal~anized steel sheet dis-closed in U.S. Patent No. 3,647,650 of March 7, 1572, which comprises:

applying a first cathodic electrolytic treatment to an electrolytic tin-plated or electro-gal~anized steel sheet in an alkaline aqueous solution, under ~:
the following condi-tions:
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~i~373 Composition of aqueous solution: an aqueous solution containinq 1 to 3 g/~ at least one compound selected from the group con-sisting of alkali metal carbonates, alkali metal borates, alkali metal phosphate and alkali metal hydroxide;
Temperature of aqueous solution: 20 to 70 Ci Quantity of electricity: 1 to 20 coulomb/dm ;
and directly thereafter, subjecting said so-treated electrolytic tin-plated or electro-galvanized steel sheet to a second cathodic electro-lytic treatment in a hexavalent chromic ion-based aqueous solution under the following conditions:

Composition of aqueous solution: an aqueous solution containing 2 to 20 g/.~ hexavalent chromic ion and 3 to 20 g/~ at least one water soluble saturated carboxylic acid selected from the group consisting of monobasic aliphatic carboxylic acid, dibasic aliphatic carboxylic acid, aromatic carboxylic acid and salts thereof;
Temperature of aqueous solution: 30 to 70 C;
Curren-t density: 1 to 10 A/dm ;

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Energizing time: 0.5 to 5 seconds;
(hereinafter re~erred to as the "prior art ~6)").

(7) ~n electrolytic post~treatment of an electrolytic tin-plated steel sheet disclosed in Japanese Patent Provisional Publication No. 68,933/75 of June 9y 1975, which comprises:

, applying a ixst cathodic electrolytic treat-ment to an electrolytic tin-plated steel sheet in an alkaline aqueous solution, under the following conditions:

Composition of aqueous solution: an aqueous solution containin~ l to 30 g/Q alkali ~Letal carbonate and/or ~kal}~e borate and 1 to 30 g/Q at least one complexing agent selected from the group consisting of taxtaric acid, gluconic acid, citric acid, condensed phosphoric acid and salts thereof;
Temperature of aqueous solu-t.ion: Z0 to 60~C;
Current densit~: 4 to 10 A/dm2;
Energizing time: 0.5 to 1 secondsi and directly thereafter, subjecting said ~ t~eated electrolytic tin-plated steel sheet to a second ~ -~'t~

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I cathodlc electrolyt.~c treatment in a he~avalent ~ chromic ion-based aqueous solution under the fo1low-j ing condi~ions: .
.
Composition of aqlleous solution: an aqueous solution containing 2 to 20 gj~ hexavalent chromic ion, or, . an aqueous solution containing 2 to 20 g/~
hexavalent ahromic ion and 3 to 20 g/~ at least one compound selected from the group consisting o~ monobasic saturated lower fatty acid, dibasic saturated lower fatty acidj ~romatic saturated carboxylic acid . l ~:
and s~lts thereof;
Temperature o aqueous solution. 30 to 70C;~
Current density: 1 to 10 A/dm2 Energizing time: 0.5 to S seconds;
; thereina~ter re~erred to as the "prior art ~7)"~
I
~8~ An electrolytîc post-treatment of an electxolytic j ~ :
tin-plated steel sheet disclosed in British Patent No. 1,483,305 published on ~ugust 17, 1977, which . comprises~ ¦
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, ~31~3 applying a first cathodic electroly-tic -treatmen-t to an elec-trolytic tin-plated steel sheet in an alkaline aqueous solution, under the following conditions:
Composition of aqueous solu-tion: an aqueous solution containing l to 50 g/,~ at least one compound selected from the group consisting of sodium hydroxide, potassium hydroxide, and alkali metal carbonate, bicarbonate, silicate, borate, phosphate, acetate, oxalate, citrate and gluconate;
pH of aqueous solution: 7 to lOi Temperature of aqueous solution: 15 to 70 C;
Current density: l to 20 A/dm ;
Energizing time: 0.1 to 2 seconds;
and subjec-ting, as required, said so-treated electrolytic tin-pla-ted steel sheet to a ~second cathodic electrolytic treatment in a hexa-valent chromic ion-based aqueous solution under the following conditions:
Composition of aqueous solution: an a~ueous solution containing 2 to 20 g/~ hexavalent chromic ion, 15 ~

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an aqu~ous solukion containing 2 to 20 g/Q
hexavalent chromic ion and 3 to 20 g/Q at least one compound selected from the group consisting o acetic acid, propionic acid, butyric acid, malonic acid, succinic acid, glutaric acid, adip.ic acid, benzoic acid, phthalic acid and salts thereof;
Temperature of aqueous solution: 30 to 70C;
Current density: 1 to 10 A/dm2;
Energizing time: 0.5 to 5 seconds;

said electrolytic tin-plated steel sheet, to which had been app].i.ed said first cathodic electrolytic treat-m~nt,being immersed, as required, for 1 to 3 seconds in an aqueous solution at a temperature of 50 to 80C ` :
containing 0.05 to 0.2 mQ/~ acetic acid directly after said first cathodic electrolytic treatment and be~ore said second cathodic electrolytic treatment;
(hereinater referred to as the "prior art (8)").

According to the above-mentioned prior arts tl) to .

(8), it is certainly possible to improve paint adhesion, oxidation resistance and sulfurization resistance of an electrolytic tin-plated steel sheet to some extent.
However/ since, in the prior art (1), a tin-plated steel ':

..

sheet is only immersed into a hexavalent chromic ion-based aqueous solution for example, sodium bichromate aqueous,solution, a tin-plated steel sheet post-treated in accordance with the prior art (1) is inferior . in the effect of inhibiting the growth of tin oxides produced on the surface of the tin plating layer. In the prior art:s (2) and (3), an anodic electrolytic treatment being applied to an electrolytic tin-plated steel sheet in a hexavalent chromic ion-based aqueous solution or in an alkaline aqueous solution, the tin oxides produced on the surface of the tin plating layer by the remelting treatment remain withou-t being eliminated, with further addition of new tin oxides. The effect of inhibiting the crystal growth of tin oxides of the electrolytic tin-plated steel sheet is not satisfactory. Thus, a tin-plated steel sheet applied with an electrolytic or non-electrolytic post-treatment in accordance with any of the prior arts (1) to (3), though being excellent in paint adhesion immediately after manufacturing, inevitably has a decreasing paint adhesion after being held or stored for a long period of time according as the tin oxides on the surface of the tin plating layer grow with the lapse of time: ;
The prior art (4) is the method that has been most popularly used as an electrolytic post-treatment of an electrolytic tin-plated steel, sheet. In the prior art (4), in which an electrolytic tin-plated steel sheet is subjected to a cathodic elec-trolytic treatment in a hexa-valent chromic ion-based aqueous solution, a chromated film is formed on the surface of the tin plating layer. Too much of this chromate film ., . . ~ ' N 17 ` h ,:, . ~ : .

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leads to a degraded paint adhesion of the electrolytic tin-plated steel sheet. This is attributed to the easy occurrence of peeling Oll the interface between the tin plating layer and the thick chromate film.
On the other hand, a too thin chromate film results in a poor effect of inhibiting the grow-th of tin oxides produced on the surface of the tin plating layer. As a result-, the electrolytic tin-plated steel sheet, being satisfactory in paint adhesion immediately after manufacturing, has a decreasing paint adhesion after being held or stored for a long period of time as tin oxides on the surface of the tin p]ating layer grow with the lapse of time. Thus, in an electrolytic tin-plated steel sheet applied with an electrolytic post-treatment in accordance with the prior art ~4), variations may be caused in paint adhesion immediately after manufacturin~, depending on the conditions of said post-treatment.
Even if paint adhesion is satisfactory im~ediately after manufacturing, paint adhesion may gradually degrade after a long period of storage.
In the prior axt (5), an electrolytic tin-plated steel sheet is subjected to a cathodic electrolytic treatment in a hexavalent chromic ion-based aqueous solution and directly thereafter, said electrolytic tin-plated steel sheet having been applied with said cathodic electrolytic treatment is subjected to an anodic electrolytic treatment in the same aqueous solution. Therefore, tin oxides produced on the surface of the tin plating layer through the remelting treatment of the tin plating layer .. '~ . :.
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are mostly ellminated by reductlon by the cathodic electrolytic treatment, and then new tin oxldes are produced on the surface of the tin plating layer as a result of the subsequen-t anodic electrolytic treatmen-t following said cathodic electrolytic treatment. These new tin oxides impair, by a cause not as yet known, paint adhesion of the electrolytic tin-plated steel sheet. Because of this, an electrolytic tin-plated steel sheet applied with an electrolytic post-treatment based on the prior art (5) has a poor paint adhesion even immediately after manufacturing.
In the prior arts (6) to (8), an electrolytic tin-plated steel sheet is subjected to a first cathodic electrolytic treatment in an alkaline aqueous solution, and directly thereafter, said electrolytic tin-plated steel sheet having been applied with said first cathodic electrolytic treatment is subjected to a second ca-thodic electrolytic treatment in a hexavalent chromic ion~based aqueous solution. Since, in the prior arts (6) to (8), the electrolytic tin-plated steel sheet is subjected twice to cathodic electrolytic treatments as described above, and no careful con-templation is taken to ensure a high paint adhesion and a satisfactory long-term maintenance thereof, there is used a relatively large quantity o~ electricity. For these reasons, tin oxides produced on the surface of the tin plating layer as a result of the remelting treatment of said tin plating layer are mostly eliminated by reduction, so that the surface of the tin plating layer is covered directly by a chromate film. Therefore, . ~ , .

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the electrolytic tin-plated steel sheets having been applled with an electrolytic post-treatment by any of the prior arts (6) to (3~ nd directly covered by chromate films show poor paint adhesion even immediately after manufacturing.

SUf~ RY OF THE INVENTION

An ob~ect of the present invention is therefore to provide an electrolytic tin-plated steel sheet excellent not only in paint adhesion but also in smudge resistance, oxidation resistance, sulfur-ization resistance and solderability, and a process of manufacturing same.
A principal object of the present invention is to provide an electrolytic tin-plated steel sheet of which -the aforementioned properties remain valid not only immediately after.manufacturing but also afte:r holding or storing for a long period of time with almost no degradation, and a process of manufacturing same.
In accordance with one of the features of the present invention, there is provided an electrolytic tin-plated steel sheet having tin oxides produced on the surface of a tin plating layer as a result of a remelting treatment of said tin plating layer directly following an electrolytic tin plating treatment for forming said tin plating layer on at least one surface of a steel sheet; said electrolytic tin-plated steel sheet being ~ 20 -~, .
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characterized in that:
said tin oxides on the surface consist essentially of a tin oxide crystallizable into orthohombic SnO and the amount of said ~t rv so-crystallizable tin oxide is ~i~t-hin-the range of from 1.0 to ~.5 millicoulomb/cm .
In accordance with another feature of the present invention, there is provided a process of manufacturing an electrolytic tin-plated steel sheet, which comprises:

. subjecting a s-teel sheet to an electrolytic tin plating treatment to form a tin plating layer on at least one surface of said steel sheet; then, subjecting said tin plating layer to a remelting treat-ment to cause formation of tin oxides on the surface of said t:in plating layer; and then, subjecting said remelted electrolytic tin-plated steel sheet to a cathodic electrolytic treatment in an alkaline aqueous solution;
said process being characterized in that:
said cathodic electrolytic treatment is applied under the following conditions:

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~3~3t73 Composition of aqueous solution: an alkaline aqueous solution containing from 1 to 40 g~Q at least one compound selected from the group consisting of sodium bicarbonate ~NaTICO3), sodium carbonate (Na2CO3~ and soclium hydroxide (NaOE-I);
pH of aqueous solution: from 7.0 to 12.0;
Temperature of aqueous solution: from 40 to 60C;
Quantity of electricity: from 0.05 to 5 coulomb/dm2;
whereby those tin oxides which crystallize into tetragonal a-SnO and tetragonal SnO2 are substantially eliminated by reduction leaving only a tin oxide crystallizable into orthorhombic SnO; and the amount of said remaining tin oxide crystallizable into orthorhombic SnO is adjusted to be equivalent to the range of from 1.0 to 6.5 millicoulomb/cm2;
and ' ,.
as required, directly thereafter, subjecting said so-treated electrolytic tin-plated steel sheet to a second cathodic electrolytic treatment in an aqueous solution selected from the group consis,ting of the following (a), (h~ and (c):

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ta) a hexavalent chromic ion based chromate aqueous solution containlng from lO to 30 g/,~sodium bichromate (Na2Cr207.21120) tb) a phosphate aqueous solution containing from 15 to 30 g/Q diammonium hydrogen phospha-te [(Ml3ll)2HPOI,]i and (c) a mixed chromate and phosphate aqueous solution containing from 15 -to 30 g/~ sodium bichromate and from 15 to 30 g~
diammonium hydrogen phosphate;
under the following conditions:
pH of aqueous solution: from 4 to 6;
Temperature of aqueous solution: from 40 to 60 C;
Quantity of electricity: from 0.01 to 0.5 coulomb/dm .

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~ 3~ 37 3 BRIEF DESCRIPTION OF T~E D~wINGS -Fig. 1 (~) is an electxon microphotograph ~3,000 magnifications) of tin oxides crystallized by heating on the surface of the tin plating layer of an electrolytic tin-plated steel sheet immediately aE-ter the application of a remelting treatment o~ the tin plating layer;

Fig. 1 (B) is an electron microphotograph (3,000 magniEications) of tin oxides crystallized by ~:
heating on the surface of the tin plating layer of an electrolytic tin-plated steel sheet immediately after the application o~ a second cathodic electrolytic treat-ment with ~ quantity of electricity of 0.26 coulomb/dm2 - ~
in a hexavalen-t chromic ion-based aqueous solution, :;`
following a first cathodic electrolytic treatmen-t in :~
an alkaline aqueous solution;

Fig. 1 (C) is an elec-tron microphotograph ~3,000 magnifications) of tin oxides crystallized by heating on the surface of the tin plating layer of an electrolytic tin-plated steel sheet immediately af-ter the application oE a second cathodic electrolytic treat-ment with a quantity of electrici-ty of 2.5 coulo~b/dm in d hexavalent chromic ion~based aqueous solution, following a first cathodic electrolytic treatment in - 2~ -`''.`,, . ` ~ , :

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an alkaline aqueous solutlon;
Fig. 2 ls a graph illustrating the relation between the amount of tin oxides on the surface of the tin plating layer and the peel resistance of an electrolytic tin-plated steel sheet having been applied with a post-treatment in accordance with a prior art process whlch peel resistance represents the paint adhesion of the electrolytic tin-plated steel sheet;
Fig. 3 is a graph illustrating the relation between the amount of a tin oxide crystallizable into orthorhombic SnO on the surface of the remelted tin layer of an electrolytic tin-plated steel sheet and peel resistance which represen-ts the paint adhesion of the electrolytic tin-plated steel sheet; and Fig. 4 is a graph illustratiny -the relation between the quantity of electricity in a second cathodic electrolytlc treatment of an electrolytic tin-plated steel sheet in a hexavalent chromic ion-based aqueous solution, following a first cathodic electrolytic treatment in an alkaline aqueous solution and the peel resistance which represents the paint adhesion of the electrolytic tin-plated steel sheet.

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LED DESCRIPTION OF PREFERRED EMBODIMENTS

From the aforementioned point of view, we have carried out - extensive studies on the effect of various tin oxides produced on the surface of the tin plating layer of a remelted electrolytic tin-plated steel sheet on the paint adhesion to said electrolytic tin-plated steel sheet. What was ascertained first as a result of these studies is that tin oxides in an amount of from about 3 to about 8 millicoulomb/cm are produced on the surface of the tin plating layer of an electrolytic tin-plated steel sheet immediately after the application of a remelting treatment of the tin plating layer.
We then studied the structure of said tin oxides. The electron diffraction is not applicable for said tin oxides which initially are amorphous or noncrystalline. We therefore carried out an electron micro-scopic observa-tion and an electron diffraction after crystallizing said tin oxides by heating under a non-oxidizing atmosphere. ~ -Fig. 1 (A) is an electron microphotograph (3,000 magnifications) of tin oxides crystalllzed as mentioned above on the surface of the tin plating layer of an electrolytic tin-plated steel sheet immediately after the application of a remelting treatment of the tin plating layer.
In Fig. 1 (A), the small particles are orthorhombic SnO and the petal-like .

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dark and bright portions are tetragonal ~-SnO. In addition, there is observed a small quanti-ty of tin oxide which is confirmed to be -tetra-gonal SnO2. More specifically, as is clear from -the electron mlcro-photograph in Fig. 1 (A), the tin oxides crystallized on the surface of the tln plating layer of an electrolytic tin-plated steel sheet immediately after a remelting treatment of the tin plating layer mostly comprise orthorhombic SnO, with small amounts of coexisting tetragonal ~-SnO and tetragonal SnO2.

An electron diffraction was effected on polycrystal and single-crystal tin oxides crystallized as mentioned above on the surface of -the tin plating layer of a remelted electrolytic -tin-plated steel sheet immediately after the application of a remelting.treatment of the tin plating layer. Table. 1 shown the results of said electron diffraction in comparison with -the results of X-ray diffraction of orthorhombic SnO
in the ASTM Standard. ~

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T~le . _ _ Elect~on di~ractio~ X~ray di~rac-tlon (ASTM) ., _ Polycrystal SLngle cxyst~l O~thorhombic SnO ~
. ~_ ., Distance Intens- Dis-tance Miller Distance Intens MilIer between ity between indices between ityindices lattice lattice lattice ratio pl~nes planes planes .(~A~ (I) (dA) (hkl) (dA)(C/Io) (hkl) _ . . . _ :: .
5.53 M 5.56 10 002 3.75 ~ 3.71 10 102 3.68 111 3.58 30 111 3.17 S 3.25 112 3.12 100 112 2.88 M 2.86 20 020 2,73 021 ~
2.68 S ~ 2.78 40 00~ - ~, 2.50 30 200 2O29 20-3 2.25 20 023 2.13 S 1.994 10 024 1.888 20 220 .876 VS 1.857 60 20 1.8~0 22-2 1.757 S 1.734 130 1.773 ~0 130 1.681 30 223 1.662 20 116 1.621 S 1.625 -22 1.610 133 1.610 20 133 1.550 W 1.540 30 312 ~ -1.~87 31-4 1.493 S 1.429 ~0 0~0 1.405 M 1.429 042 1.393 20 008,0~2 1.326 S 1.342 10 043 1.223 S 1.251 20 400 . ~ _ , ~, ,~ :.
- 28 - ~

.

~31~73-~ n the column of the Intensity (I) of Polycrystal in the electron diffraction given in Table I, VS indicates "very strong", S, "strong", M, "medium" and W, "weak".
As shown in Table I, the distance between lattice planes of the polycrystal tin oxide measured by electron diffraction almost agrees with the distance between lattice planes of orthorhombic SnO
measured by X-ray diffraction in the ~STM Standard. No conflict is observed between the distances between lattice planes and the Miller indices of the single-crystal tin oxides measured by electron diffraction.
10 This reveals, as suggested also by the results of the electron diffraction, that most oE the tin oxides crystalli~ed on the surface of the tin plating layer of an electrolytic tin-plated steel sheet after a remelting treat-ment o the tin plating layer are orthorhombic SnO.
It was thus ascertained from the results of observation by electron microscope and the results of electron diffraction that, although being amorphous or non-crystallinet the tin oxides that are produced on the surface of the tin plating layer of an electrolytic tin-plated steel sheet immediately after the remelting treatment of the tin plating . ~:

layer, mostly comprise tin oxides which crystallize into orthorhombic SnO, with a small amount .

~ 29 -: - . .. . :~ , - .. . ..

:- : . ~ : ' ' :::: `,, ~ ~ .' :' -11;~ 3 ..
' I .

o coexistinSi tin o~ldes whi~h crystalli~ie into tet~gonal a-SnO and tet~gonal ~n~2.

Then, we inYestigated the relation between the amount of tln oxides on the surface of the tin platins layer and peel resistance representing paint adhesion, on electrolytic tin-plated steel sheets having been applied :
each with a post-treatment in accordance with the pxior arts (1), ~4) and (6) to (8) as representatives of the aforementioned prior arts (1) to (8). Fig. 2 is a graph 10 . illustrating the results of said investigation. In Fig. 2, . the mark "ol- represents an electrolytic tin-plated steel sheet-applied with a non-electrolytic post-treatment in accordance with the prior art (1) which comprises subjecting an electrolytic tin-plated steel sheet to an immersion -treatment in a hexavalent chromic ion based aqueous solu~
tion; the mark "~" indicates an electrolytic tin-plated.
steel sheet applied with an electrolytic post~treatment in accordance with prior art (4) which comprises subject-ing an electrolytic tin-plated steel sheet to a cathodic electrolytic treatment in a hexavalent chromic ion-based :-. : aqueous solution; and the mark "O" shows an electrolytic ti~plated steel sheet applied with an electrolytic post- ' .
treatment in accordance with any of thie prior arts (6) . to ~8) which comprises subjecting an electrolytic tin- , plsted steel sh0et to a i'irst cathodic electrolytic , . ' ' ` ' .~ . .

:,' . '.:

... ...

~3~3 ~ :
,;

.
.

treatment in an alkaline aqueous solutlon~ and directly therea~ter, subjecting said elect~ol~tic tin-plated steel sheet having been applied wlth sald flrst cathodic electro-, lytic treatment to a second cathodic electrolytic treatment ! 5 in a hexa~alent chromic ion-based aqueous solution.

Fig. 2, the amount o~ tin o~:ides is xepresented ~y the quantity of electricity required in the measurement by cathodic reduction. Peel resistance which represents paint adheslon is determined by: baking at 205C for lO
minutes two specimens cut rom an electrolytic tin-plated steel sheet applied with an epoxy phenol paint for cans I ;
in an amount of S0 ~ 5 mg/dm~ each on one side; bonding the painted and baked sur~aces of said two specimens with a bonding agent comprising a linear polyamid~ which has a melting point lower than that of tin, peeling o~ the bonded surfaces in accordance with the T-peel test speciEied in ASTM D 1876-72; and expressing the force (kg/cm)required for said peeling per centimetre of bonded width.

As shown in Fig. 2, in the electrolytic tin-plated steel sheets ha~ing been applied with a post-treatment in accordance with the prior arts~(l), (4) and (6~ to (8 peel resistance representing paint adhesion decreases according as the amount of ~in oxides on the surface of the tin plating layer increases with the lapse o time.
. '' l~
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Moxe specificall~, ln the electrolytic tin~plated steel sheet subjected to a non~electrolytic post~treatment in accordance with the prior art tl) (indicated by "o"
in Fig. 2), which shows a peel resistance of almost 8 kg/cm S immediately after said post~treatment, peel resistance decreases according as the a~ount of tin oxides on the surface o~ the tin plating layer increases. This is con-sidered attributable to the following fact: because an appropriate amount of tin oxide ~rystallizable into orthorhombic SnO by heating at the time of painting and baking is present on the surface of the electrolytic tin-plated steel sheet immediately after the post-treatment by the prior art (1), said orthorho~bic SnO serves as a bonding agent between the tin plating layer and the paint at the time o painting and baking, thus imparting i an excellent peel resistance, i.e.~ an excellent paint adhesion. ~owever, when the electrolytic tin-plated steel sheet i5 held or stoxed for a long period of time, the tin oxides on the sur~ace of the tin plating layer absor~ atmospheric humidity. This causes a consl~erable increase of the tin oxides which crystallize into ~;
tetragonal a-SnO and tetragonal SnO2 which have initially bee~ present in a small amount, whereaR the tin oxide which crystallize into orthorhombic SnO does not increase so much because o~ the relative difficulty o~ growth.

.1:
- 32 ~
... , . , ' ..

- . , . ~ . ~ `,. . .
... ~...... . . .
: ~ :, .~

~3~3 As a result, the surface of the tin plating layer becomes mostly covered by the tin oxides which crystallize into tetragonal~d;sno and tetragonal SnO2, leading to a relatively lowered bonding effect of the othorhombic SnO acting as a bonding agent, hence to a decreased peel resistance, i.e., an inferior paint adhesion of the electrolytic tin-plated steel sheet.
In the electrolytic tin-plated steel shee-t subjected to an electrolytic posttreatment in accordance with the prior art (~) so far most popularly applied (shown by "~" in Fig. 2), peel resistance is as low as about 3.6 kg/cm even immediately after said post-treatment. Peel resistance is much lower in the electrolytic tin-plated steel sheets having been subjected to an electrolytic post-treatment in accordance with any of the prior arts (6) to (8) (represented by ~" in Fig. 2).
This is considered to be due to the fact that, in the prior arts (4) and ~6) to ~8), the electrolytic post-treatment causes an almost complete elimination, by reduction, of not only the tin oxides crystallizable into tetragonal ~-SnO and tetragonal SnO2 but also the tin oxide crystallizable into orthorhombic SnO which acts as a bonding agent on the surface of the tin plating layer.
We consider that peel resistance of at least 4 kg/cm ' '.

~ 33 ~

. . .

~3~ 3 is practically necessary for an electrolytic tin-plated steel sheet.
This conclusion was derived from our study in which we carried out a peel test comprising conducting painting and baking of -the surface of the tin plating layer of an electrolytic tin-plated steel sheet with a paint having a relatively weak adhesion selected from among the commonly utili~ed epoxy phenol paints, cutting checkered notches on said palnted surface, and peeling off said painted surface with a bonding tape, and a measurement of peel resis-tance of the electrolytic tin plated steel sheets having had any portion of the painted surface peeled off in said peel test by the T-peel test showed a value of 4 kg/cm.
Another test was then carried out on the relation between the amount of tin oxide which crystallizes into orthorhombic SnO on the surface of the tin layer and peel resistance. Fig. 3 is a graph illustrating the results of said test.
As shown in Fig. 3, when the amount of the tin oxlde which crystallizes into orthorhombic SnO on the surface of the tin plating layer decreases to under 1.0 millicoulomb/cm or increases to over 6.5 millicoulomb/cm2, peel resistance of the electrolytic tin-plated steal , ,. .~ : , , ~ :

~ , . . ' ' ' ' ~3e~

sheet decreases to under 4 kg/cm.
The results of the above-mentioned test revealed that it is necessary to adjust the amount of the tin oxide which crystallizes into orthorhombic SnO on the tin plating layer of an electrolytic tin-plated steel sheet to wlthin the range of from 1.0 to 6.5 millicoulomb/cm . It is not as yet clear why an electrolytic tin-plated steel sheet with an amount of such tin oxide crystallizes into orthorhombic SnO on the surface of the tin plating layer within the range of from 1.0 to 6.5 millicoulomb/cm shows an excellent peel resistance, i.e., an excellent paint adhesion.

However, just as a too much or too small an amount of bonding agent cannot in general give an excellent bonding effec-t, too much of this tin oxide crystallizable into or-thorhombic SnO is considered to cause aggregation/
fracture in the tin oxide, which becomes more brittle, thus leading to peeling of the paint together with the thus embrittled tin oxide, while too small an amount of this tin oxide which crystallizes into orthorhombic SnO is considered to prevent the resulting orthorhombic SnO from acting as a bonding agent.
As a result of our further studies based on the results of the various tests described above, we have successfully obtained an electrolytic tin-plated steel sheet which shows excellent paint adhesion not only .
~ 35 ~3~3~3 immediately After manufacturing but a~so after holding or storing for a long period of time by substantially eliminating by reduction, through a cathodlc electrolytic treatment in an alkaline aqueous solution, those tin oxides which crystallize into tetragonal ~SnO and tetragonal SnO2 which are liable to grow during holding or storage for a long period of time, from among tin oxides on the surface of the tin plating layer of . ;~
the electrolytic tin-plated steel sheet, the latter tin oxides being produced as a result of the remelting treatment of said tin plating làyer . following an electrolytic tin plating treatment for forming said tin plating layer on at least one surface of the steel sheet, thereby substantially leaving only the tin oxlde which crystallizes into orthorhombic SnO, and by adjusting the amount of said tin oxide which crystallizes into orthorhombic SnO within the range of from 1.0 to 6.5 millicoulomb/cm2. .
Now, the process of manufacturing the electrolytic tin-plated ;
steel sheet of the present invention is described below.
In the process of the present invention, a steel sheet is subjected to a conventional electrolytic tin plating treatment to form a tin plating layer on at least one surface of said steel sheet, and , , : . -~.
- : -:: :
, ~

~L3~ 3 directly thereafter, said tin plating layer is subjected to a conventional remelting treatment. Then, following said reme1ting treatment, said electrolytic tin-plated steel sheet is subjected to a cathodic electrolytic treatment in an alkaline aqueous solution, under the following conditions:
Composition of aqueous solution: an alkaline aqueous solution con-taining from 1 to 40 g/~ of at least one compound selec-ted from the group consisting of sodium bicarbonate (NaHC03), sodium carbonate (Na2CO3) and sodium hydroxide (NaOH);
pH of aqueous solution: from 7.0 to 12.0;
Temperature of aqueous solution: from 40 to 60 C:
Quantity of electricity: from 0.05 to 5 coulomb/dm .
Through the above-mentioned cathodic electrolytic treatment in the alkaline aqueous solution, those tin oxides which crystallize into tetra-gonal ~-SnO and tetragonal Sn02, which are detrimental to paint adhesion, are substan-tially eliminated by cathodic reduction from among the tin oxides produced on the surface of the tin plating layer as a result of the remelting treatment of the tin plating layer, thus substantially leaving only the tin oxide which crystallizes into orthorhombic SnO, which acts as 37 - ' : : : : ,,,, ., ., s ~ ~ ., , , ." ~ , " , ",. ..

3~3 - a bonding agent, and the amount of said tin oxide whlch crystallizes into orthorhombic SnO is adjusted within the range of from l.O to 6.5 millicoulomb/cm .
A pH of the alkaline aqueous solution of under 7.0 or over 12.0 during this step eliminates by reduction not only the tin oxides which crystallize into tetragonal~ SnO and tetragonal SnO2 but also the desirable tin oxide that is crystallizable into orthorhombic SnO on the tin plating layer, and in addition, causes etching of -the tin layer. It is therefore necessary to adjust the pH-value of the alkaline aqueous solution within the range of from 7.0 to 12.0 by limiting the composition of the alkaline aqueous solution as mentioned above. It is also desirable that the temperature of the alkaline aqueous solution be within the range of from ~0 to 60C as in conventional practice.
A quantity of electricit~ of under 0.05 coulomb/dm in said cathodic electrolytic treatment cannot sufficiently eliminate by reduction the tin oxides crystallizable into tetragonal~Z-SnO and tetragonal SnO2.
On the other hand, a quantity of electricity of over 5 coulomb/dm leads ;
to an elimination by reduction of not only the tin oxides crystallizable into tetragonal~-SnO and tetragonal SnO2 but also the tin oxide .~

, .
. :
. .
: . ~:

~3~73 crystalli~able into orthorhombic SnO, thus nullifying the efEect under the present invention. ~'he quanti.ty of electricity in said cathodic electro-lytic treatment should therefore be limited within the range of from 0.05 to 5 coulomb/dm .
In an alkaline aqueous solution with a pll of 10.3 having the above-mentioned composition, an electrolytic tin-plated steel sheet after a remelting treatment of the tin plating layer, was subjected to a cathodic electrolytic treatment under the various quantities of el.ecticity as shown in Table 2 below, and then,a~ter holding said electrolytic tin-plated steel sheet in an atmosphere with a relative humidity of 95% at 50C for 96 hours to accel-erate aging, the amount of tin oxides produced on the surEace of the tin plating layer of said electrolytic -tin-plated steel sheet was measured. Then, said electrolytic tin-pla-ted steel sheet was painted and baked. Then, the peel resistance, representing paint adhesion of said electrolytic tin-pla_ed steel sheet thus painted and baked, was measured. Table 2 gives the .
results of said measurement. Peel resistance was measured by the afore-mentioned T-peel test.

_ 39 _ :
~.....

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: ~ ' ' ' ':: : . . ' ' ~3~3~3 . , ,;
...
.' Table 2 .................. ................................... . .
_ _ ~ :
Conditions fox cathodic A~ter holding in atmospher electr~lytic treat~ent with relatl~e humidity o in alkaline aqueous 95% at 50C for 96 hours solution Quantity of Cur~ent Amount o~ Peel electricity density x time tin oxides resistance ~coulomb~ (A~dm2 x(milli~ 2(kg/cm) dm~) seconds)coulomb/cm ) . _ .
0.03 0.05 x 0.6 9.2 1.4 50.0S 0.05 x 1.0 4.8 7.0 0.50 0.5 x 1.0 ~.0 7.6 1.00 1.0 x 1.0 4.0 ~.6 ~.50 2.5 x 1.0 3.3 8.5 S.00 2.5 x 2.0 4.7 5.1 1010.00 5.0 x 2.0 6.5 1.4 _......... . .__ As shown in Table 2, in the case oi a quantity of el~ctricity of 0.03 coulomb~dm2, the insufficient elLmination by reduction of tin oxides on the surface of the tin plating layer causes a considerable increase of the remaining tin oxides Which crystallize~ into tetra-gonal ~-SnO and tetragonal SnO2 which are detrimental to the paint adhesion, this resulting in a relative decrease of the tin oxide ~hich: erystalli~ into orthorhombic SnO which acts as a bondin~ agent. As a result, even 2U after the acceleration of change with time, peel resis-tance of the electrolytic tin-plated steel sheet is as low as 1.4 ~g/cm. On the other hand, with a quantity . . :
:
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:

of electricity of 10 coulomb/dm , not only the tin oxides which crystallii~e into -tetragonal ~-SnO and tetragonal SnO2 which are detrimental to paint adhesion but also the tin oxide which crystallii2es into orthorhombic SnO which acts as a bonding agent are eliminated by reduction, and most of tin oxides produced thereafter during aging are tin oxides which crystallii7e into tetragonal ~Z-SnO and tetragonal SnO2 which are detrimental to paint adhesion. As a result, peel resistance of the electrolytic tin-plated steel sheet after accelerated aging is as low as 1.4 kg/cm. On the contrary, in the case of a quantity of electricity within the range of from 0.05 to 5 coulomb/dm , almost all the tin oxides are those which crysta].lii~e into orthorhombic SnO acting as a bonding agent in an appropriate amount. As a result, peel resistance of the electrolytic tin-plated steel sheet is as high as from 5.1 to 8.5 kg/cm.
It is known also from the above-men-tioned results of measurement given in Table 2 that it is necessary to limit the quan-tity of electricity for a cathodic electrolytic treatment in an alkaline aqueous solution within the range of from 0.05 to 5 coulomb/dm .
In the present invention, it is possible to impart further stabilized long-term paint adhesion, smudge resistance, oxidation Xesistance, sulfuxization ~esistance and solderabilitx~ to an electxolytic tin~plated steel sheet by subjecting the electrolytic tin~plated steel sheet having been applied with the above~mentioned cathodic electrolytic treatment in an alkaline aqueous solution to a second cathodic electrolytic treatment in an aqueous solution selected from the group consisting of -the following ~a), (b) and (c):

ta): a hexavalent ~chromic ion-based~chromate aqueous solution containing from 10 to 30 g/~ sodium bichromate (Na2Cr207 2H2O);

(b) a phosphate aqueous solution containing from 15 to 30 g/Q diammonium hydrogen phosphate ((NH4)2~PO4); and (c) a mixed chromate and phosphate aqueous solution containing from 15 to 30 g/~ sodium bichromate and from 15 -to 30 g/~ diammonium hydrogen phosphate;

under the following conditions:
pH of aqueous solution: from 4 to 6;
Temperature of aqueous solution: from 40 to 60C;
Quantity of electricity: from 0.01 to 0.5 co ulomb/dm2 .

~2 _ , :

~L~3~3~

By the above-mentloned second cathodic electrolytic treatment, a chromate film, a phosphate film or a mixed chromate and phosphate film is formed on the tin oxide which crystallizes into orthorhombic SnO which has been adjusted by the first cathodic electrolytic treatment in an alkaline aqueous solution as mentioned previously within the range of from 1.0 to 6.5 millicoulomb/cm , almost without reducing said tin oxide.
A pH of under 4 of the aqueous solution in the above-mentioned second cathodic electrolytic treatment leads to a lower adhering effect of the chromate film, the phosphate film or the mixed chromate and phosphate film. In order to obtain a desired amount of -these films, therefore, it becomes necessary to increase the quantity of electricity, which in turn eliminates by reduction the tin oxide which crystallizes into orthorhombic SnO. On the other hand, a pH of over 6 results in an unstable aqueous solution, which is not desirable. It is therefore necessary to adjust the pH-value of the aqueous solution used in the second cathodic electrolytic treatment within the ran~e of from 4 to 6 by limiting the composition of said aqueous solution as mentioned above.

It is desirable that the temperature of sald aqueous solution is within the range of from 40 to 60 C.

~ ~r - 43 ~

... . .

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3~;~

A quantity of electricity of under 0.01 coulomb/dm2 in said second cathodic electrolytic treatment makes it di~ficult to control the electric current by a rectifier in line operations. On the other hand, with a quantity of electricity of over 0.5 coulomb/dm~, the tin oxide which crystallizes into the orthorhombic SnO is eliminated by reduction, thus nullifying the effect of the present invention. Therefore, the quantity of electricity in saicl second cathodic elêctrolytic treatment should be limited within the range of from 0.01 to 0.5 coulomb/dm2.
Electrolytic tin-plated steel sheets to which had been applied the first cathodic electrolytic treatment in an alkaline aqueous solution were subjected to the second cathodic electrolytic treatment, under the various quantities of electricity as shown in Fig~ 4, in a chromate aqueous solution having the above-mentioned composition, and then, painted and baked. Then, for said electrolytic tin-plated steel sheets, we investigated the relation between the quantity of electricity in said second cathodic electrolytic treatment and peel resistance representing paint adhesion. Fig. 4 is a graph illustrating the results of said investigation. Peel resistance was measured by the aforementioned T-peel test.
In Fig. 4, the solid line connecting the p]ots "o" shows the bm~

.;
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- :
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. '' ~ .

electrolytic tln~plated steeI sheets p~inted and baked ' `
immedi~tely a~ter the secvnd cathodlc electrolytic treatment, and the dotted llne connecting the plots "~"
shows the electrolytic tin~plated steel sheets palnted S and baked six months after the second cathodic electro-lytic treatment.

As shown in Fig. 4, with a quantity of electricity of over about O.5 coulomb/dm2 in the second cathodic electrolytic treatment, peel resistance of the electrolytic tin-plated steel sheet painted and baked immediately after the second cathodic electrolytic treatment (represented by the solid line connecting the plots "o" in Fig. 4) decreases to under 4 kg/cm. This is because, with a quantity of electricity of over about 0.5 coulomb/dm2, the tin oxide which crystallizes into orthorhombic SnO
acting as a bonding agent is eliminated by reduction. ~;
In contrast, with a quantity of electricity of under about 1-0.5 coulomb/dm2, the electrolytic tin-plated steel sheet painted and baked immediately after the sesond cathodic electrolytic treatment has a p~el resistance of over 4 l ;
kg/cm. As shown in Fig. 4, a smaller quantity o~ elec-triclty leads to a hlgher peel resistance whereas a quantity of electricity decreased to under 0.01 coulomb/
am~ makes it difficult to control the electric current ~s T
. . . . , ~
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.. .. "

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. I

with a rectifier and to cont~l the line speed, Also as i5 clear from Fig. 4, the electrolytic tin~plated steel sheet painted and bakad six ~onths ater the second cathodic electrolytic treatment ~sho~n by the dotted line connecting the plots "~" in Fig.'4) has higher peel resistance than the electrolytic tin-plated steel sheet painted and haked immediately after the second cathodic electrolytic treatment. This is considered attributable to the fact that t,he amount of the tin oxide which crystallizes into orthorhoT~ic SnO acting as a bonding agent on the surface o~ the tin plating layer of an , '~
electrolytic tin-plated steel sheet approaches the optimum value described abo~e with reference to Fig. 3 during 5ix months.
. I .
Fig. l (B) and Fig. l (C) are both electron microphotographs (3,000 magnifications) of tin oxides crystallized on the surface of the tin plating layer oE ' an electrolytic tin-plated steel sheet immediately after the second cathodic electrolytic treatment'in a hexavalent ' , 20 chromic ion-based chromated aqueous solution having the above-mentioned composition. In Fig. 1 tB), which shows an electron ~icrophotograph in the case of a quantity of electricity of 0.26 ~oulomb/dm2,within the scope of the ~resent in~ention in the s~cond cathodic electrolytic ¦
~5 treatment, the crystallized tin oxides consist essentially ,~
. .. lf .
.~ -46- ;

n , .

.. ,. . .. : . :

. . "

3~3 .
of orthorhombic SnO, and this electrolytic tin-plated steel sheet has an excellent paint adhesion. In Fig. 1 (C), which shows an electron microphotograph in the case of a quantity of electricity of 2.5 coulomb/dm outside the scope of the present invention in the second cathodic electrolytic treatment, almost no orthorhombic SnO is observed, and this electrolytic tin-plated steel sheet has an inferior paint adhesion.
The results of investigation represented by the graph in Fig. 4 mentioned above, and also the electron microphotographs of Fi~. 1 (B) and Fig. 1 (C) indicate the necessity to limit the quantity of electricity in the second cathodic electrolytic treatment within the range of from O.Ol to 0.5 coulomb/dm .
In the process of the present invention, a result as good as that in the second cathodic electrolytic treatmen-t is obtained by subjecting the electrolytic tin-plated steel sheet after the first cathodic electrolytic .
treatment in an alkaline aqueous solution to a non-electrolytic immersion treatment immediately.following said first cathodic electrolytic treatment, in place of the above-mentioned second cathodic electrolytic treatment, in an aqueous solution having the same composition as the aqueous solution used in said second 3~3 cathodic elect~ol~tlc t~e~tment~ l.e., in ~n aqueous solution selected from the ~roup cons~sting o the following (a), (bi and (c):

(a) a hexavalent chromic ion~based chromate aqueous solution containing from 10 to 30 g~ sodium :
bichromate. ~Na2Cr2O7-2E~2O);

(b) a phosphate aqueous solution containing from 15 to 30 g/~ diammonium hydrogen phosphate (~N~4)2llPO4): and (c) a mixed chromate and phosphate aqueous solution containing from 15 to 30 g/~ sodium bichromate and from 15 to 30 g/~ diammonium hydrogen phosphate;

under the following condi-tions:
pH of aqueous solution: from 4 to 6;
Temperature of aqueous solution: from 40 to 60C
Immersion time: from 0.1 to 10 secondsO

Now, the present invention is described in more detail wi-th reference to examples.

EXA~PI.E
An electrolytic tin-plated s-teel sheet (correspond-ing to ~25 tinplate specified in JIS G 3303) was prepared . .
. ~- , - .. .
.

: . : . , ... :: :

.. . ..

; . , ~ : . : - .

~ ~3~7~

by subjeetlng ~ 0,32 mm thick steeI sheet to an eleet~o~
lytic degreasing, R ~ate~ rins~ng and a plekling, and then ~orming a tin plating laye~ in an amount o~ 2.8 g~m2 ~;
, on one slde o~ said steel sheet by a eon~entional electro-lytie tin plating treat~ent, and directly thereafter, said tin plating layer was s-ubjected to a conventional remelt-ing treatmen-t. Then, said electrolytie tin-pla-ted steel sheet was s~jeeted -to the cathodie eleetrolytic treatment i in an alkaline aqueous solu-tion of a pH oE 10.3 a-t a temperature o:E 50C containing 30:g/~ sodi.um biearbonate (NaHCO3) with a quantity of electricity of 2.5 coulomb/dm O

-- : .
An electrolytie tin-plated steel sheet prepared and applied with the eathodic eleetrolytic treatment in an alkaline aqueous solution following the remelting treat-ment of the tin plating layer in accordance with Example 1, was immedi.ately subjected to the immersion trea-tment for 1 second in a chromate aqueous solution oE a pH of 4.5 at a temperature of 45C containing 30 g/~ sodium bichromate (Na2Cr2O7 2lI2O).

EX~PLE 3 An electrolytic tin~plated steel sheet prepared and applied with the cathodic electrolytic treatment in .
,i' ; !

-- !
` ~3~3~3 . . . .

an alkaline aqueous salution following the remelting treatment o~ the tin plating layer in accordance with Example l, was i~mediately su~jected to the second cathodic electrolytic treatment in a chromate aqueous solution o a pH of S.0 at a temperature of 50C co~taining 30 g/~
sodium bichromate (Na2Cr2O7~2H2O) with a quantity of electricity Oæ 0,26 coulomh~2.
'':

_ ,:
The second cathodic electrolytic treatment was ef~ected i~ a chromate aqueous solution under the same conditlons as in Example 3, except for the quantity of electricity chansed to 0.46 coulomb/dm2.
. : .
EX~MPLE 5 ~ ~
. 1, ~
An electrolytic tin-plated steel sheet prepared and applied with the cathodic electrolytic treatment in an alkaline aqueous solution following the remelting treatment o~ the tin plating layer in accordance with , Example l, was immediately subjected to the immersion ~;
~reatment for l second in a phosphate aqueous solution of a pH of 4.5 at a temperature of 45C containing 30 g/Q
diammonium hydrogen phosphate ((N~4)2HP04). ¦

50 ~

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, :.,: , :: . :

~l~ 3~37~

EXAMæLE
An electroly-tic tin plated steel sheet prepared and applied with the cathodic electrolytic treatment in an alkaline aqueous solution following the remelting treatment of the tin plating layer in accordance with Example 1, was i~mediately subjected to the second cathodic electrolytic treakment in a mixed chromate and phosphate aqueous solution of a pH of S.0 at a tempera-ture of 50C
containing 15 g/~ sodium bichromate (Na2Cr2O7~2E~2O) and 15 g/~ dic~ onium hydroyen phosphate ((NH4)2HPO~) with a quantity of electricity o~ 0~26 coulomb/dm2.
Now, xe;ference c~ses 1 to 7 conducted ~or the purpose of co~paring with the present invention are des- :
cribed belo~. Re.ference 1 represents an electrolytic tin-plated steel sheet post-treated in accordance with the aorementioned prior art (1), References 2 -to 4, in accordance with the prior art (4), and References 5 to 7, in accordance with the prior arts (6) to (8).
.

RE~ERENCE
:, An electxolytic tin-plated steel sheet (correspond-ing to #25 tinplate specified in JIS G 3303) was prepared by sub~ecting d O . 32 mm thick steel sheet to an electro-lytic degreasing, a water rinsing and a pickling, and then forming a tin plating layer in an amount of 2.8 g/m2 : . , . ~ , . . . .;, , , ~ .

~3~73 on one slde o~ said steel sheet by a conventional electro-lytic tin plating treatment, and directly thereafter, said tin plating layer ~as subjected to a conventional remelt-ing treatment. Then, said electrolytic tin-plated steel sheet ~as subjected to the immersion treatmen-t for l second in a chromate aqueous solution con-taining 30 g/Q sodium bichromate (Na2Cr2O7 2H2O) of a pH of 4.5 at a temperature of ~5C.

REFE.RENCE 2 An electrolytic tin-plated steel sheet prepaxed and applied with the remelting treatment of the tin plating layer in accordance with Reference l, was immediately subjected to the cathodic electrolytic treatment in a chromate aqueous solution of a pH of 5.0 at a -temperature of 50C containing 30 g/~ sodium bichromate (Na2Cr2O7 2H2O~
with a quantity o elec-tricity of 1.04 coulomb/dm2.

The cathodic electro].ytic treatment was carried out in a chromate aqueous solution under the same conditions as in Re~erence 2 e~cept for the quantity of elect changed to 2.5 colllomb/dm .

, , .

:, : . .. ,: , ,. ::.. ; -: .:

3~3~3 The cathodic electrolytic treatment was carried ~:
out in a chromate aqueous solution under the same conditions as in Reference 2 except for the quantity of electricity chanyed to 4 coulomb/dm2.

An electrolytic tin-plated steel sheet prepared and applied with the remelting -treatment of the tin plating layer in accordance with Reference 1, was immediately, subjected to the cathodic electrolytic treat~
ment in an alkaline aqueous solution containing 30 g/Q
sodium bicarbonate (NaHCO3) of a pH of 10.3 at a temper-ature of 50C with a quantity of electricity of 2.5 coulomb/dm2.

Then, the electroly-tic -tin-plated steel sheet applied with said cathodic electolytic treatment in an alkaline aqueous solution was subjected, directly there-after, to the second cathodic electrolykic treatment in a chromate aqueous solution of a pE~ of 5.0 at a temper-ature of 50C conkaining 30 g/Q sodium bichromate (Na2Cr2O7-2H2O) with a quantity of electricity of 1.04 coulomb/dm2.

; . ` ~. , . . ' ' ~ " ,' . . "

~131~73 ~E~E~ENCE 6 The second cathodlc electrolytic treatment was carried out in a chromate aqueous solution under the same conditions as in Reference 5 except for the quantity S of electricity changed to 2.5 coulomb/dm2.

REFERENC'E 7 The second cathodic electrolytic treatment was carried out in a chromate aqueous solutlon under the same conditions as in Reference 5 except Eor khe quantity of electricity changed to 4 coulomb/dm20 For the electrolytic tin~plated steel sheets applied with the non-electrolytic and/or electrolytic treatment in the above-mentioned Examples l to 6 and Reference l to 7, we measured peel resistance represent-ing paint adhesion; the amount of tin oxides and the amoun-t of a chromate film on the tin plating layer which represent o~idation resistance and sulfurization resistance; smudge resis-tance; and solder-rise value representing solderability.

The results of said measurement are shown in rrable 3. In Table 3, peel resistance was measured in accordance with the 'r~peel test described above with reference to Fig. 2. The amount of tin oxides is ~ 5~ - .

.. . ~ . . . . .
. . ~

, . : , . .

- ~3~

expressed by the quantit~ of electricity required in ~he measurement b~ cathodic reduction~ Peel resistance and the amount o~ tin oxid~s Were measured immediately after the post-treatment of the electrol.ytic tin-plated steel sheets and six months after said post-treatment.
Smudge resistance was e~aluated by firing at 210C for 10 minutes in open air an electrolytic tin-plated steel sheet subjected to the post-treatment, rubbing once the surface of said electrolytic tin~plated steel sheet with a permanent magnet enclosed in Japanese paper oVer a length of 80 cm, and, grading stains of the paper into five grades with the slightest stain as 1 and the most serious stain as 5. Solder-rise value was obtained by firing at 210C ~or 10 minutes in open air an electro-lytic tin-plated steel sheet applied with the post-treatment r folding in~o two said electrolytic tin-plated steel sheet with the surface subjected to ~aid tin plating treatment inside so as to cause a capillary phenomenon, immersing the lower portion of said electrolytic t.in-plated steel sheet thus folded into a solder bath of pure tin at 290C, and, measuring the height of molten solder rising in the gap of said electrolytic tin-plated steel sheet thus folded due to a capillary phenomenon from the ;~
soldering bath surface. Incidentally, in Table 3, "c/dm2"
i5 the abbreviation o coulomb/dm2, and "mc/cm2~,millicoulomh/cm2.

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As shown in Table 3, in ~xamples 1 to 5 within t~e scope of the present invention, peei resistance immediately after the post-treatment of the electrolytic tin-plated steel sheet is over 4.0 kgjcm in all ca~es, and furthermore, peel resistance six months a~ter the ~ po~t-treatment is improved by about 32 to about 76% from ! that immediately after the post-treatment. Thus, the electrolytic tin-plated steel sheets in Examples 1 to 6 - ;~
have a stable and excellent paint adhesion not only ' 10 immediately after manufacturin~ but also after holding ¦ or storing for a period o six mollths.
In References 1 to 7, in contrast, peel resistance immediately a~ter the post-treatment of the electrolytic ~ tin-plated s~eel sheet is as low as 3.6 kg/cm or under j lS except for 7.4 kg~cm in Reference 1, and furthermore, peel I resistance six months a~ter the post-treatment is largely ` decreased from tha~ immediately after the post-treatment.
¦ Especially in Re~erences 5 to 7, in which the second cathodic electrolytic treatment was efEected in a chromate~
aqueous solution after the first cathodic electrolytic 1 treatment in an alkaline aqueous solution, the quantity ¦
of eleotricity in said second cathodic electrolytic ~¦
treatment is larger than that in the present invention. -As a result, the tin oxide crystalliz~ble into orthorhombic SnO acting as a bonding agent on the sur~ace `
' ' ' , . ;
~ ~ 58 ~ ~, .~'0 . ~ . . ~

~31~3~3 of the tin plating layer is eliminatecl by reduction. Therefore, peel resistance of the electrolytic tin-plated steel sheets in Refc!rences 5 to 7 is very low, both immedi.ately after the post-treatment and six months after the post-treatment. Thus, in the e].ectrolytic tin-plated steel sheets in ReEerences l to 7, paint adhesion is, even if excellent immediately after manufacturi.ng, seriously decreased after holding or storing for a period of six months.
As is evident from Table 3, smudge resi.stance of the e].ectro-lytic tin-plated steel sheets in Examples l to 6 is far superior -to the smudge resistance of the electrolytic tin-p].ated steel sheets in References 1 to 7. Solder-ri.se value of the electrolytic tin-plated steel sheets in Examples l to 6 shows a high and stable value as from 15 to 17 mm.
Furthermore, oxidation resistance and sulfuri~.ation resistance of the electrolytic tin-plated steel sheets in Examples l to 6 were ascertained to be well comparable with those of the electrolytic tin-plated steel sheets in ReEerences l to 7.
According to the present invention, as described above in detail, it is possible to obtain an electrolytic tin-plated steel sheet that is excellent, not only in pain-t _ 59 _ , ' ~ " '~ ' ~'' ' '' .

~3~37~

adhesion ~ut also ln smudge resist~nce, oxidation resis-tance, sulfurization resistance and solderability, which aforementioned properties remain not only immediately after manufacturing but a].so after holding or storing for a long period of time with almost no degradation, thus pro~iding industrially useful effects.

60 ~

Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrolytic tin-plated steel sheet having tin oxides produced on the surface of a tin plating layer as a result of a remelting treatment of said tin plating layer directly following an electrolytic tin plating treatment for forming said tin plating layer on at least one surface of a steel sheet; said electrolytic tin-plated steel sheet being characterized in that:
said tin oxides consist essentially of a tin oxide which crystallizes into orthorhombic SnO by heating at the time of painting and baking and the amount of said tin oxide which crystallizes into orthorhombic SnO by heating at the time of painting and baking is equivalent to the range of from 1.0 to 6.5 millicoulomb/cm2.

2. A process of manufacturing an electrolytic tin-plated steel sheet, which comprises subjecting a steel sheet to an electrolytic tin plating treatment to form a tin plating layer on at least one surface of said steel sheet; then, subjecting said tin plating layer to a remelting treatment to cause formation of tin oxides which crystallize into orthorhombic SnO, tetragonal .alpha.-SnO and tetragonal SnO2 by heating at the time of painting and baking, on the surface of said tin plating layer; and then, subjecting said electrolytic tin-plated steel sheet having been subjected to said remelting treatment to a cathodic electrolytic treatment in an alkaline aqueous solution;
said process being characterized in that:
said cathodic electrolytic treatment is applied under the following conditions:

(a) composition of aqueous solution: an alkaline aqueous solution containing from 1 to 40 g/? at least one compound selected from the group consisting of sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), and sodium hydroxide (NaOH);
(b) pH of aqueous solution: from 7.0 to 12.0;
(c) temperature of aqueous solution: from 40 to 60°C;
(d) quantity of electricity: from 0.05 to 5 coulomb/dm2 whereby the tin oxides which crystallize into tetragonal .alpha.-SnO and tetragonal SnO2 on heating at the time of painting and baking are substantially eliminated by reduction so as to leave only a tin oxide which crystallizes into orthorhombic SnO on heating at the time of painting and baking; and the amount of said tin oxide which crystallizes into orthorhombic SnO on heating at the time of painting and baking is adjusted to be equivalent to the range of from 1.0 to 6.5 millicoulomb/cm2.

3. The process as claimed in claim 2, wherein:
said electrolytic tin-plated steel sheet after said cathodic electrolytic treatment is immediately subjected to a second cathodic electrolytic treatment in an aqueous solution selected from the group consisting of:
(a) a hexavalent chromic ion-based chromic aqueous solution containing from 10 to 30 g/? sodium bichromate (Na2Cr2O7.2H2O):
(b) a phosphate aqueous solution containing from 15 to 30 g/? diammonium hydrogen phosphate [(NH4)2HPO4]; and (c) a mixed chromate and phosphate aqueous solution containing from 15 to 30 g/? sodium bichromate and from 15 to 30 g/? diammonium hydrogen phosphate;
under the following conditions:
pH of aqueous solution: from 4 to 6;
temperature of aqueous solution: from 40 to 60°C;
quantity of electricity: from 0.01 to 0.5 coulomb/dm2.

4. The process as claimed in claim 2, wherein:
said electrolytic tin-plated steel sheet after said cathodic electrolytic treatment is immediately subjected to an immersion treatment in an aqueous solution selected from the group consisting of:
(a) a hexavalent chromic ion-based chromate aqueous solution containing from 10 to 30 g/? sodium bichromate (Na2Cr2O7.2H2O);
(b) a phosphate aqueous solution containing from 15 to 30 g/? diammonium hydrogen phosphate [(NH4)2HP04]; and (c) a mixed chromate and phosphate aqueous solution containing from 15 to 30 g/? sodium bichromate and from 15 to 30 g/? diammonium hydrogen phosphate; under the followmg conditions:
pH of. aqueous solution: from 4 to 6;
temperature of aqueous solution: from 40 to 60°C;
immersion time: from 0.1 to 10 seconds.