Method for pretreatment in the production of tin-free steel

A method for the pretreatment of tin-free steel which comprises an anodic treatment or an anodic treatment after a cathodic treatment of an acid pickled steel sheet in an alkaline electrolyte with a pH of above 8, said steel sheet consisting of an upper layer of hydrated chromium oxide and a lower layer of metallic chromium. This pretreated tin-free steel has excellent lacquer adhesion after aging in hot water and under retort conditions.

BACKGROUND OF THE INVENTION 
The present invention relates to a pretreatment method in a manufacturing 
process of a tin-free steel (TFS) having a first layer of metallic 
chromium on a steel base, and a second layer of hydrated chromium oxide on 
the first layer. 
DESCRIPTION OF THE PRIOR ART 
Recently, lacquered TFS has largely been used for manufacturing carbonated 
beverage cans and beer cans, since it exhibits excellent lacquer adhesion. 
In the case of lacquered TFS, the seaming of the can body is mainly 
carried out with a nylon adhesive. The adhered part of the lacquered TFS 
can body not only has an acceptable bonding strength at a normal 
temperature, but also has a bonding strength which can satisfactorily 
withstand internal pressure caused by the contents, such as beer and 
carbonated beverages. 
However, when a can having a TFS can body seamed by a nylon adhesive after 
lacquering is used as a container for foods such as fruit juices which are 
immediately hot-packed after pasteurization at a temperature of 
90.degree.-100.degree. C., or as a container for foods such as coffee, 
meat and fish, which are pasteurized by hot steam at a temperature above 
100.degree. C. in a retort after being packed in the can, the lacquer film 
may be peeled off from the TFS surface. Thus, a drop in the degree of 
vacuum in the can may occur due to the partial loss of adhesion in the 
adhered parts of the can body, because the lacquer adhesion of 
conventional TFS becomes poor after aging in hot water and under retort 
conditions. Therefore, it is not possible to use conventional TFS cans 
seamed with nylon adhesive after lacquering, for pasteurizing the contents 
of the cans packed at high temperature. 
Recently, methods to provide TFS having an excellent lacquer adhesion after 
aging in hot water and under retort conditions have been proposed by us. 
One method relates to a pretreatment which is characterized by a cathodic 
treatment after an anodic treatment of said steel sheet in an acidic 
chromate electrolyte prior to conventional electrolytic chromic acid 
treatment. Another method relates to a composition of hydrated chromium 
oxide in TFS which is characterized by the restriction in the amount of 
sulfur and fluorine incorporated in the hydrated chromium oxide layer or 
by the restriction in the amount of oxygen existing as a hydroxyl radical 
and bonded water (water bonded to trivalent chromium) in the hydrated 
chromium oxide layer. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide TFS having an excellent 
lacquer adhesion after aging in hot water and under retort conditions by 
an improvement in the pretreatment of the TFS. 
In the present invention, the steel sheet is subjected to an anodic 
treatment or an anodic treatment after a cathodic treatment in an alkaline 
electrolyte having a pH of above 8, after degreasing by an alkaline 
solution and pickling by sulfuric acid or hydrochloric acid. The steel 
sheet is then subjected to the conventional electrolytic chromic acid 
treatment. 
The alkaline electrolyte having a pH above 8 used for the pretreatment 
according to the present invention contains at least one alkaline compound 
selected from the group consisting of a hydroxide, a carbonate, a 
bicarbonate, a silicate and a borate of an alkali metal and ammonium, or 
furthermore contains at least one compound selected from the group 
consisting of a phosphate, an acidic phosphate, an oxalate and an acetate 
of an alkali metal and ammonium. 
Generally, the amount of metallic chromium is in the range of 50-150 
mg/m.sup.2 and that of hydrated chromium oxide is in the range of 8-25 
mg/m.sup.2 as chromium in TFS, namely the thickness of metallic chromium 
is about 70-210 .ANG. and that of hydrated chromium oxide is about 70-220 
.ANG.. In consideration of the roughness of the surface of the steel 
sheet, these are very thin films. 
Furthermore, the growth of metallic chromium depends strongly on the 
crystallographic orientation in the surface of the steel sheet because the 
crystal structure of metallic chromium is the same body centered cubic 
lattice as that of iron and the lattice constants of those are very close, 
that is, 2.884 .ANG. in metallic chromium and 2.866 .ANG. in iron. The 
rate of crystal growth of metallic chromium is also different by the 
crystallographic orientation of steel. 
Accordingly, the thickness of the metallic chromium layer which is 
deposited by an electrolytic chromic acid treatment, differs depending on 
the crystallographic orientation of the individual steel and the thickness 
of the hydrated chromium oxide layer formed on the metallic chromium layer 
also varies with the effect of the crystallographic orientation of the 
steel base. Therefore it is considered that the distribution in the 
thickness of the metallic chromium layer and hydrated chromium oxide layer 
becomes nonuniform. 
Especially, it is considered that a nonuniform TFS film is formed in the 
case of the conventional process of manufacturing TFS. Such conventional 
process comprises a series of degreasing by an alkaline solution, pickling 
by a sulfuric acid solution, rinsing with water and an electrolytic 
chromic acid treatment. The effect of the crystallographic orientation of 
the steel base is magnified by pickling because the surface of the steel 
base is activated. 
On the other hand, a uniform TFS film consisting of metallic chromium and 
hydrated chromium oxide can be formed by an electrolytic chromic acid 
treatment as a result of the pretreatment according to the present 
invention. This is presumed to be because the effects of the 
crystallographic orientation of the steel base are reduced by the 
pretreatment according to the present invention, in which the surface of 
the steel base activated by pickling is inactivated by an anodic treatment 
in an alkaline electrolyte. 
Although it may be considered that the pretreatment according to the 
present invention is the same process as that of the conventional process, 
in which pickling is omitted, there is a clear difference in uniformity of 
surface oxide films. Pickling is necessary in the present invention 
because the nonuniform oxide film, formed in the annealing process or the 
other process cannot be removed sufficiently and uniformly the first 
conventional alkaline degreasing. 
In the present invention, it is important that the surface of the steel 
base is inactivated uniformly by an anodic treatment in an alkaline 
electrolyte after the nonuniform oxide film formed on the steel base in 
the annealing process is sufficiently removed by pickling. 
Although a cathodic treatment or a cathodic treatment after an anodic 
treatment in an alkaline electrolyte has been considered, TFS having an 
excellent lacquer adhesion after aging in hot water and under retort 
conditions, which is the object of the present invention, cannot be 
obtained. This is becasue the surface of the steel base activated by 
pickling is not inactivated by a cathodic treatment in an alkaline 
electrolyte and the surface of the steel base, inactivated by an anodic 
treatment, is activated again by a cathodic treatment in an alkaline 
electrolyte since the oxide film on the steel base is reduced by the 
cathodic treatment. 
As described above, in the present invention, the uniformities of a 
metallic chromium layer and a hydrated chromium oxide layer in TFS are 
important factors influencing the lacquer adhesion after aging in hot 
water and under retort conditions, which is the object of the present 
invention. The uniformities of a metallic chromium layer and a hydrated 
chromium oxide layer are improved by the pretreatment according to the 
present invention.

After one piece of TFS 3 having a thick lacquer film of an epoxy-phenolic 
type 4, and another piece of TFS 3 having a thin lacquer film of an 
epoxy-phenolic type 5 are adhered with a nylon adhesive 6 on the edges, 
the resultant adhered specimen is fixed in the channel 2 in a bent state 
as shown in the FIGURE. 
DETAILED DESCRIPTION OF THE INVENTION 
In general, two types of manufacturing processes are well known for the 
production of conventional TFS. Namely, one is a one-step process in which 
metallic chromium and hydrated chromium oxide are simultaneously formed by 
using one electrolyte. The other is a two step process in which metallic 
chromium is formed at first using a chromium plating solution and then 
hydrated chromium oxide is formed on the metallic chromium layer by using 
other electrolytes. 
The pretreatment step of the present invention is applicable to both the 
one-step and two-step processes and can improve the lacquer adhesion of 
TFS after aging in hot water and under retort conditions. 
It is an essential condition that the alkaline electrolyte used for the 
pretreatment of the present invention should be maintained above a pH of 
8, although it is not necessary to strictly control the concentration of 
hydroxide, carbonate etc., salts of the alkali metal and ammonium radical. 
The concentration of the alkaline electrolyte is preferably in the range of 
10-100 g/l, if they are restricted. 
If the concentration of the alkaline electrolyte used for the present 
invention is below 10 g/l, a waste of electric power results because of 
the higher electric resistance of the alkaline electrolyte. The 
concentration is limited to 100 g/l from the economical point of view, 
although the effect of the present invention is not reduced if the 
concentration is above 100 g/l. The concentration of the phosphate, acidic 
phosphate, oxalate and acetate of the alkali metal and ammonium radical 
added to the alkaline electrolyte is also desirably in the range of 10-100 
g/l and in this case, the pH of the alkaline electrolyte should be still 
maintained above 8. 
In the case of an acidic or weakly alkaline electrolyte below a pH of 8, 
the effect of the present invention cannot be accomplished, because the 
surface of the steel base is not inactivated. 
The effect of the present invention is not reduced as far as the alkaline 
electrolyte according to the present invention is maintained above a pH of 
8, even if a small amount of sulfuric acid or hydrochloric acid is brought 
into the alkaline electrolyte of the present invention because of 
insufficient rinsing after pickling. Furthermore, the effect of the 
present invention is not reduced, even if the surface active agent, which 
is usually added to the alkaline solution for degreasing of the steel 
sheet, is added to the alkaline electrolyte according to the present 
invention. Although it is not necessary that the temperature of the 
electrolyte used for the pretreatment of the present invention be strictly 
controlled, it is preferably below 90.degree. C. from an energy savings 
point of view. 
The manner in which the electrolytic treatment is carried out is most 
important in the present invention. Namely electrolytic treatment must be 
either anodic or anodic after cathodic. 
It is necessary to accomplish the object of the present invention that the 
quantity of electricity for the anodic treatment be in the range of 1-200 
coulombs/dm.sup.2, more preferably in the range of 5-50 coulombs/dm.sup.2. 
If the quantity of electricity is below 1 coulomb/dm.sup.2, the effect of 
the present invention is not obtained because the inactive dense oxide 
film is not uniformly formed on the surface of the steel base. 
The use of a quantity of electricity above 200 coulombs/dm.sup.2 is not 
desirable in the present invention, because the inactive oxide film formed 
on the surface of the steel base is not reduced or removed sufficiently by 
the following electrolytic chromic acid treatment and may induce a surface 
stain. 
For the high speed production of TFS, it is reasonable that the 
electrolytic time is 0.1-5 seconds, and the range of the current density 
is 1-100 A/dm.sup.2 in the anodic treatment of the pretreatment. 
In the case of an anodic treatment after a cathodic treatment, it is not 
necessary to control strictly the conditions of cathodic treatment since a 
cathodic treatment prior to an anodic treatment has less effect on the 
formation of inactivated oxide film. However, it is naturally limited from 
the point of the high speed production of TFS. 
From an industrial point of view, the application of the following methods 
has been considered in conjunction with the pretreatment of the present 
invention, and it has been discovered that the effect of the present 
invention does not change by use of these methods. The first is a method 
in which the cycle consisting of the anodic treatment after the cathodic 
treatment is repeated several times. The second is a method in which water 
rinsing is carried out between the cathodic treatment and the following 
anodic treatment. 
It is needless to say that the pretreatment according to the present 
invention is applicable not only to the electrolytic chromic acid 
treatment, but also to the metal plating of the steel sheet in which the 
uniformity and the denseness of the metal layer is required. 
The present invention is illustrated by the following examples, in which a 
duplex layer consisting of a lower layer of metallic chromium in the 
amount of 80-120 mg/m.sup.2 and an upper layer of hydrated chromium oxide 
in the amount of 12-20 mg/m.sup.2 as chromium is formed on a cold rolled 
steel sheet having a thickness of 0.23 mm under various conditions of 
electrolytic chromic acid treatment. 
EXAMPLE 1 
A cold rolled steel sheet was electrolytically degreased in a solution of 
70 g/l sodium hydroxide. After rinsing with water, the steel sheet was 
pickled by immersion into a solution of 100 g/l sulfuric acid. After 
rinsing with water, the steel sheet was pretreated under the following 
conditions. 
______________________________________ 
Conditions of Pretreatment 
______________________________________ 
Composition of electrolyte 
Sodium hydroxide 60 g/l 
pH of electrolyte 14 
Temperature of electrolyte 
30.degree. C. 
Anodic current density 20 A/dm.sup.2 
Anodic treating time 1 sec. 
______________________________________ 
After rinsing with water, the pretreated steel sheet was subjected to 
electrolytic chromic acid treatment under the following conditions and was 
then rinsed with hot water and dried. 
______________________________________ 
Conditions of Electrolytic Chromic Acid Treatment 
______________________________________ 
Composition of electrolyte 
Chromic trioxide 50 g/l 
Sodium fluoride 3 g/l 
Temperature of electrolyte 
50.degree. C. 
Cathodic current density 
20 A/dm.sup.2 
______________________________________ 
EXAMPLE 2 
The same kind of steel sheet degreased and pickled as in Example 1 was 
pretreated under the following conditions after rinsing with water. 
______________________________________ 
Conditions of Pretreatment 
______________________________________ 
Composition of electrolyte 
Sodium orthosilicate 30 g/l 
pH of electrolyte 14 
Temperature of electrolyte 
80.degree. C. 
Anodic current density 30 A/dm.sup.2 
Anodic treating time 2 sec. 
______________________________________ 
After rinsing with water, the pretreated steel sheet was subjected to 
electrolytic chromic acid treatment under the following conditions and was 
then rinsed with hot water and dried. 
______________________________________ 
Conditions of Electrolytic Chromic Acid Treatment 
______________________________________ 
Composition of electrolyte 
Chromic trioxide 30 g/l 
Ammonium fluoride 1.5 g/l 
Temperature of electrolyte 
30.degree. C. 
Cathodic current density 
20 A/dm.sup.2 
______________________________________ 
EXAMPLE 3 
The same kind of steel sheet degreased and pickled as in Example 1 was 
pretreated under the following conditions after rinsing with water. 
______________________________________ 
Conditions of Pretreatment 
______________________________________ 
Composition of electrolyte 
Sodium hydroxide 8 g/l 
Sodium phosphate 30 g/l 
pH of electrolyte 10 
Temperature of electrolyte 
60.degree. C. 
Electrolytic method 
Anodic treatment after cathodic treatment 
Anodic and cathodic current 
Each 5 A/dm.sup.2 
density 
Anodic and cathodic treating 
Each 1 sec. 
time 
______________________________________ 
After rinsing with water, the pretreated steel sheet was plated with 
metallic chromium by using an electrolyte containing 250 g/l of chromium 
trioxide and 2.5 g/l of sulfuric acid in water under 30 A/dm.sup.2 of 
cathodic current density at an electrolyte temperature of 50.degree. C. 
After rinsing with water, the chromium plated steel sheet was subjected to 
electrolytic chromic acid treatment under the following conditions and was 
then rinsed with hot water and dried. 
______________________________________ 
Conditions of Electrolytic Chromic Acid Treatment 
______________________________________ 
Composition of electrolyte 
Chromic trioxide 30 g/l 
Sulfuric acid 0.2 g/l 
Sodium fluoride 1.0 g/l 
Temperature of electrolyte 
30.degree. C. 
Cathodic current density 
20 A/dm.sup.2 
______________________________________ 
EXAMPLE 4 
The same kind of steel sheet degreased and pickled as in Example 1 was 
pretreated under the following conditions after rinsing with water. 
______________________________________ 
Conditions of Pretreatment 
______________________________________ 
Composition of electrolyte 
Sodium borate 15 g/l 
Potassium hydroxide 20 g/l 
pH of electrolyte 13.5 
Temperature of electrolyte 
60.degree. C. 
Anodic current density 10 A/dm.sup.2 
Anodic treating time 2 sec. 
______________________________________ 
After rinsing with water, the pretreated steel sheet was treated by using 
an electrolyte containing 90 g/l of chromic trioxide and 6 g/l of sodium 
fluoride in water under 40 A/dm.sup.2 of cathodic current density at an 
electrolyte temperature of 50.degree. C. The treated steel sheet was then 
further treated with this electrolyte diluted to one-third its original 
concentration under 10 A/dm.sup.2 of cathodic current density at an 
electrolyte temperature of 35.degree. C., and was then rinsed with hot 
water and dried. 
As comparative examples, the same kind of steel sheet was degreased and 
pickled as in Example 1. After rinsing with water, Comparative examples 1, 
2, 3 and 4 were subjected to electrolytic chromic acid treatment or 
electrolytic chromic acid treatment after chromium plating under the same 
conditions as in Examples 1, 2, 3 and 4, respectively, without the 
pretreatment according to the present invention, and were then rinsed with 
hot water and dried. 
The amount of metallic chromium and hydrated chromium oxide as chromium in 
TFS which was prepared in Examples 1, 2, 3 and 4 and in Comparative 
examples 1, 2, 3 and 4 was measured and the characteristics of each 
resulting TFS were evaluated by the following test methods, the results of 
which are shown in the Table 1. 
(1) Lacquer adhesion at a normal temperature. 
Two pieces of the treated sample were prepared. One piece of the treated 
sample was baked at 210.degree. C. for 12 minutes after coating with 60 
mg/dm.sup.2 of an epoxy-phenolic type lacquer and the other piece was 
baked two times under the same conditions as described above, before and 
after coating with 25 mg/dm.sup.2 of the same lacquer. 
The two differently coated sample pieces were each cut to a size of 5 
mm.times.100 mm and bonded together using a nylon adhesive having a 
thickness of 100 .mu.m at 200.degree. C. for 30 seconds under 3 
kg/cm.sup.2 of pressure by a Hot Press after preheating at 200.degree. C. 
for 120 seconds. 
The bonding strength of the assembly which is shown as kg/5 mm was measured 
by a conventional tensile testing machine. 
(2) Lacquer adhesion after aging in hot water. 
The assembly prepared by the method described in (1) above, was peeled by a 
conventional tensile testing machine after the assembly was immersed in a 
0.4% citric acid solution at 90.degree. C. for 3 days. The bonding 
strength of the assembly was shown as kg/5 mm. 
(3) Lacquer adhesion under retort conditions. 
Two pieces of the differently coated samples prepared by the method 
described in (1) above, were each cut to a size of 70 mm width and 60 mm 
length, respectively, and were bonded in such a way as to overlap each 
other by 8 mm in a longitudinal direction under the same conditions as 
described in (1). 
Ten assembled samples were prepared as described above. 
Each assembled sample was curled to a radius of 100 mm as for a can body, 
and then fixed in a channel of 70 mm width. 
After that, the ten fixed samples were set in a retort in which hot steam, 
heated to 125.degree.-130.degree. C. under a pressure of 1.6-1.7 
kg/cm.sup.2, was blown for 150 minutes or for 300 minutes. The lacquer 
adhesion under retort conditions was evaluated by the number of the 
samples which had peeled to the total ten assembled samples. 
As shown in Table 1, it is evident that there are very clear differences 
between the Examples of the present invention and the Comparative examples 
in the lacquer adhesion after aging in hot water and under retort 
conditions, although there is no difference between the Examples of the 
present invention and the Comparative examples in the lacquer adhesion at 
a normal temperature. It is recognized from these Examples that the 
pretreatment of the present invention has the remarkable effect of 
improving the lacquer adhesion after aging in hot water and under retort 
conditions. 
TABLE 1 
__________________________________________________________________________ 
CHARACTERISTICS OF TREATED STEEL SHEET 
Example Comparative Example 
1 2 3 4 1 2 3 4 
__________________________________________________________________________ 
Amount of metallic 
112 115 106 110 115 122 100 108 
Cr in mg/m.sup.2 
Amount of hydrated 
16 15 14 18 15 13 17 18 
Cr oxide (as Cr) 
in mg/m.sup.2 
Lacquer adhesion at 
6.8 6.2 6.4 6.6 6.7 6.4 6.3 6.6 
a normal temperature 
(kg/5 mm) 
Lacquer adhesion 
3.2 3.1 2.8 3.0 1.8 1.5 0.9 1.3 
after aging in hot 
water (kg/5 mm) 
Lacquer 
150 min. 
0/10 
0/10 
0/10 
0/10 
1/10 
0/10 
0/10 
0/10 
adhesion 
300 min. 
0/10 
1/10 
3/10 
1/10 
7/10 
8/10 
10/10 
7/10 
under 
retort 
conditions 
(A/B)* 
__________________________________________________________________________ 
*A shows the number of the peeled assembly. B shows the total number of 
the assembly.