GB2046304A

GB2046304A - Tin-free steel

Info

Publication number: GB2046304A
Application number: GB791957A
Authority: GB
Original assignee: Toyo Kohan Co Ltd
Current assignee: Toyo Kohan Co Ltd
Priority date: 1979-04-05
Filing date: 1979-09-14
Publication date: 1980-11-12
Also published as: FR2453225A1; DE2935314A1; GB2046304B; IT7968767A0; IT1126779B; DE2935314C2; JPS55134197A

Abstract

Tin-free steel comprises a first layer of metallic chromium on a steel base and a second layer of hydrated chromium oxide on the first layer, in which the atomic ratio of oxygen existing as a hydroxyl radical and bonded water to the sum of chromium, oxygen, sulfur and fluorine in the second layer is 60-75 atomic percent. This tin-free steel can be used for a nylon-adhered can body to be subjected to a hot-packing or retort treatment, since it has excellent lacquer adhesion after aging in hot water and under retort conditions. The layers are electrodeposited in two stages or a single stage from a solution containing chromic acid and at least one F or S compound: eg HF, ammonium (bi)fluoride, alkali metal fluoride, HBF4, H2SiF6, ammonium or alkali metal fluoborate or fluosilicate, H2SO4, phenolsulfonic acid, ammonium or alkali metal sulfate, phenolsulfonate, sulfite or thiosulfate.

Description

SPECIFICATION Tin-free steel The present invention relates to 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, which can be used for a nylon-adhered can body requiring excellent lacquer adhesion after aging in hot water and under retort conditions.

Recently, lacquered TFS, rather than electrotinplates, has largely been used for manufacturing carbonated beverage cans and beer cans, since it exhibits lacquer adhesion which is superior to that exhibited by electrotinplates.

The ordinary metal can consists of the two can ends and a can body. In the case of lacquered TFS, the seaming of the can body is mainly carried out with nylon adhesive by using the Toyo Seam and Mira Seam methods. In these cases, the nylon adhesive is inserted not between the plan TFS surfaces, but between the lacquered TFS surfaces. An epoxy-phenolic type of lacquer is generally applied to the TFS. Therefore, the bonding strength of the adhered part of the lacquered TFS can body is the sum of the bonding strength between the surface of the TFS and the lacquer film, and the bonding strength between the lacquer film and the nylon adhesive.

The nylon adhered part of the lacquered TFS can body not only has an acceptable bonding strength in a normal state, i.e. at room temperature and atmospheric pressure, but also a bonding strength which can satisfactorily withstand internal pressure caused by the contents of the can, such as beer and carbonated beverages.

However, when a can having a TFS can body seamed by 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 90-1 00 C, or as a container for foods such as coffee, meat and fish, which are pasteurized by hot steam at a temperature above 1 00,C in a retort after being packed in the can at about 1 00 C, 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 partial loss of adhesion between 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 temperatures.

It is assumed that the deterioriation of the lacquer adhesion of conventional TFS, after aging in hot water and under retort conditions, depends on the properties of the hydrated chromium oxide in the TFS.

In general, there are two well-known types of manufacturing processes for the production of commercial TFS. The first type is a one-step process in which metallic chromium and hydrated chromium oxide are formed in one operation by using one electrolyte composition. The second type is a two-step process in which first metallic chromium is formed by using one electrolyte composition as a chromium plating solution, and then hydrated chromium oxide is formed on the metallic chromium layer by using another electrolyte composition. In both types of processes, addition agents such as sulfuric acid and fluoride are added to the electrolyte compositions in amounts which result in incorporation of substantial amounts of sulfur and/or fluorine into the hydrated chromium oxide layer.

It is an object of the present invention to provide TFS which can be used for producing a nylon-adhered can body having excellent lacquer adhesion after aging in hot water and under retort conditions.

This object can be accomplished by restricting the amount of oxygen existing as a hydroxyl radical and bonded water (water bonded to trivalent chromium) in the hydrated chromium oxide layer formed on the metallic chromium layer during electrolytic chromic acid treatment.

The term "oxygen existing as a hydroxyl radical and bonded water" means the total of the oxygen existing as a hydroxyl radical and the oxygen existing as bonded water.

As discussed in more detail later on, various TFS samples having a first layer of 80-120 mg/m2 of metallic chromium and a second layer of 12-20 mg/m2, as chromium, of hydrated chromium oxide were prepared by varying the conditions under which electrolytic chromic acid treatment was carried out, and the atomic ratio of oxygen, existing as a hydroxyl radical and bonded water, to the sum of the element chromium, oxygen, sulfur and fluorine in the second layer was measured by using X-ray photoelectron spectrometer (XPS). At the same time the lacquer adhesion (1) in the normal state, (2) after aging in hot water and (3) under retort conditions, of these TFS samples was tested.As a result, it was confirmed that the degree of lacquer adhesion on TFS having a hydrated chromium oxide layer restricted in the amount of oxygen existing as a hydroxyl radical and bonded water according to the present invention was superior to the degree of lacquer adhesion in conventional TFS.

Thus the invention provides a tin-free steel comprising a steel base, a first layer on said steel base and a second layer on said first layer, said first layer being metallic chromium, said second layer being hydrated chromium oxide, the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in said second layer to the total of chromium, oxygen, sulfur and fluorine in said second layer being 60-75%.

The steel base to be subjected to electrolytic treatment to produce the TFS of the present invention can be any cold rolled steel sheet customarily used in manufacturing electrotinplate and tin-free steel. Preferably, a type of steel base for electrotinplate, as set out in ASTM A 623-76 of 1 977 (standard specification for general requirements for tin mill products), is employed as the steel base.

Preferably, the thickness of the steel base is from about 0.1 to about 0.35 mm.

The TFS for use in a nylon-adhered can body according to the present invention is characterized by a hydrated chromium oxide layer which satisfies the following formula: 60 atomic % S O-4S/Cr + 0 + S + F X 100 S 75 atomic % Namely, this formula shows that the atomic ratio of oxygen existing as a hydroxyl radical and bonded water to the sum of the four elements, chromium, oxygen, sulfur and fluorine, in the hydrated chromium oxide layer, is at least 60 atomic percent and not greater than 75 atomic percent.

In order to efficiently produce TFS having a uniform metallic chromium layer and a uniform hydrated chromium oxide layer, it is indispensable to add at least one addition agent such as sulfuric acid and fluoride to the chromic acid electrolyte solution. The addition agent thus becomes incorporated into the hydrated chromium oxide formed on the metallic chromium layer.

For instance, in the case of the addition of sulfuric acid and sulfate, or thiosulfate and sulfite which are oxidized to sulfate, these addition agents are incorporated as sulfate radicals into the hydrated chromium oxide. Therefore, oxygen in the sulfate radicals incorporated in the hydrated chromium oxide layer is also measured together with oxygen existing as a hydroxyl radical and bonded water in the hydrated chromium oxide layer by XPS.

In the above formula, 0 shows the total oxygen in the hydrated chromium oxide layer and 4S shows the oxygen bonded to sulfur as a sulfate radical incorporated in the hydrated chromium oxide layer. Although the atomic ratio of hydrogen, which is one component of the hydrated chromium oxide layer, should be restricted, it is represented by the atomic ratio of oxygen, since hydrogen exists as a hydroxyl radical and bonded water, because the quantitiative analysis of hydrogen contained in hydrated chromium oxide is very difficult. It is therefore apparent that the atomic ratio of hydrogen is in fact thus restricted.

It is assumed that the bonding strength between the surface of the TFS and the lacquer film is mainly dependent on hydrogen bonding between the hydroxyl radical or bonded water in the hydrated chromium oxide and the active radical in the lacquer film. It is also assumed that this bonding strength decreases when the hydroxyl radicals or bonded water is substituted by sulfate radicals incorporated into the hydrated chromium oxide.

If water or organic acids penetrate into the interface between the TFS and the lacquer film, the bonding strength decreases remarkably. Furthermore, under the heat conditions encountered during such operations as hot-packing or retort pasteurization, a remarkable deterioration of the bonding strength is also observed. Especially, if a high amount of sulfate radical is incorporated into the hydrated chromium oxide formed by an electrolytic chromic acid treatment, as in conventional TFS, the deterioration of the bonding strength is even more remarkably accelerated.

The reasons why the lacquer adhesion after aging in hot water and under retort conditions is deteriorated by the incorporation, into the hydrated chromium oxide, of the addition agents used in the electrolytic chromic acid treatment, such as sulfuric acid or fluoride, are considered to be as follows: (1) The amount of hydroxyl radicals or bonded water in the hydrated chromium oxide layer, which are needed to form hydrogen bonds with the active radicals in the lacquer film to ensure lacquer adhesion, is decreased because such hydroxyl radicals or bonded water are substituted by the addition agents incorporated into the hydrated chromium oxide layer.

(2) The formation of the hydrated chromium oxide is substantially disturbed, or the coordinate bond in the hydrated chromium oxide is broken, since the sulfate radical incorporated into the hydrated chromium oxide has the same volume as trivalent chromium coordinated by a hydroxyl radical or bonded water with a coordination number of 6.

Fluorine incorporated into the hydrated chromium oxide layer does not disturb the construction of the hydrated chromium oxide as much as does the sulfate radical, because fluorine has nearly the same volume as the hydroxyl radical or bonded water.

For the production of TFS having excellent lacquer adhesion even after aging in hot water and under retort conditions, the amount of addition agent added to the chromic acid electrolyte which is used for the formation of the hydrated chromium oxide layer should be decreased as much as possible below the amount used in producing conventional TFS, because as indicated above, the incorporation of addition agents into the hydrated chromium oxide layer causes a decrease in the content of hydroxyl radicals or bonded water in the hydrated chromium oxide layer, thus reducing the number of sites for hydrogen bonding between the chromium oxide layer and the lacquer film.However, in order to efficiently produce TFS having a uniform metallic chromium layer and a uniform hydrated chromium oxide layer, it is indispensable to add at least one addition agent selected from the group consisting of sulfur compounds (e.g. sulfuric acid, phenolsulfonic acid or an ammonium or alkali metal sulfate, phenolsulfonate, sulfite or thiosulfate) and fluorine compounds (e.g. an ammonium or alkali metal fluoride, fluoborate or fluosilicate, or acid thereof, i.e. hydrofluoric acid, fluoboric acid, fluosilicate acid, ammonium bifluoride or an alkali metal bifluoride) to the chromic acid electrolyte solution.

In the case of a one-step process in which metallic chromium and hydrated chromium oxide are formed in one operation on the steel base, the amounts of addition agents such as sulfuric acid and fluoride added to the electrolyte solution for the electrolytic chromic acid treatment should be suitably controlled according to the amount of chromic acid employed and in consideration of the current efficiency to be realized during the formation of the metallic chromium layer and hydrated chromium oxide layer.

In the present invention, if the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in the hydrated chromium oxide layer is below 60 atomic percent, the lacquer adhesion after aging in hot water and under retort conditions is not improved beyond that exhibited by conventional TFS, because, it is considered, the uniformity of the hydrated chromium oxide layer becomes poor and that portion of the surface of the metallic chromium layer which is not covered with the hydrated chromium oxide is oxidized.The upper limit of the atomic ratio of oxygen existing as a hydroxyl radical and bonded water is set at 75 atomic percent because it is difficult to produce TFS having a hydrated chromium oxide layer in which the atomic ratio of oxygen existing as hydroxyl radical and bonded water is above 75 atomic percent in commercially stable production, though it should not be restricted in tersm of the effects of the present invention.

For example, in order to produce TFS having a hydrated chromium oxide layer in which the atomic ratio of oxygen existing as a hydroxyl radical and bonded water is 60-75 atomic percent, the sulfuric acid should be added in an amount of not greater than 0.2 g/l to the electrolyte consisting of 20-150 g/l of chromic acid. However, this electrolytic solution, having such a low sulfate content, is not practical for the commercial production of TFS, because of the low current efficiency realized during the formation of metallic chromium.In such a case, therefore, it is desirable to add a suitable amount of, for example, a fluoride, to the electrolyte, instead of additional sulfuric acid, because fluorine incorporated in the hydrated chromium oxide has less deleterious effect on the lacquer adhesion after aging in hot water and under retort conditions than does the sulfate radical, as described above.

It is more desirable to use a fluorine compound (e.g. a fluoride) electrolyte, for example those disclosed in Japanese Patent Publication No. Sho 49-25537, without using any sulfur electrolyte.

If TFS having a hydrated chromium oxide layer incorporating too much sulfate radical is produced by using an electrolyte composition containing a correspondinly high amount of sulfate, the result may be that the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in the hydrated chromium oxide layer will be below 60 atomic percent. In this case, such atomic ratio of oxygen can be increased to 60 atomic percent or above by treating the TFS with hot water having a temperature above 50"C, preferably above 70"C, for at least one second, preferably 1-10 seconds, because the sulfate radical may be easily substituted by hydroxyl radicals or bonded water.The use of steam having a temperature above 100"C is also effective for this purpose, but, from the viewpoint of energy cost and heat resistance of equipment, the temperature should desirably not exceed 100"C.

In the case of a two-step process, chromium deposition is carried out by using a high concentration of chromic acid electrolyte containing a suitable amount of addition agents such as sulfuric acid and fluoride. In this case, it is desirable to use a chromium plating solution having a low sulfuric acid content and a high fluoride content, because sulfuric acid and fluoride are incorporated into a thin hydrated chromium oxide layer formed on the metallic chromium layer during chromium deposition, i.e. during the first step.It is also necessary to (a) dissolve the hydrated chromium oxide, formed during chromium deposition, by immersing it in the chromium plating solution or (b) treat the hydrated chromium oxide layer with hot water of above 50"C, preferably above 70"C, or (c) remove the hydrated chromium oxide layer mechanically, before carrying out the second step of the two-step process.

For the second step, i.e. the formation of the hydrated chromium oxide layer after metallic chromium deposition, the same attention is needed as in the one-step process. In this second step, it is desirable to use a chromic acid solution containing one or more addition agents for the formation of the hydrated chromium oxide layer.

The amount of hydrated chromium oxide which is formed on the metallic chromium layer is desirably in the range of from about 8 to about 30 mg/m2, as chromium. If the amount of hydrated chromium oxide is below 8 mg/m2 as chromium, the lacquer adhesion after aging in hot water and under retort conditions is not improved, even if the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in the formed hydrated chromium oxide layer is 60-75 atomic %, because the metallic chromium layer is not sufficiently covered by the hydrated chromium oxide layer. If the amount is above 30 mg/m2, the lacquer adhesion after a forming operation, such as drawing, becomes slightly poor.

The amount of metallic chromium which is formed on the steel base is desirably in the range of from about 50 to about 200 mg/m2. If the amount of metallic chromium is below 50 mg/m2, the corrosion resistance after lacquering and forming becomes poor. An amount above 200 mg/m2 is not suitable for the high speed production of TFS.

The present invention is illustrated by the following examples, in which a duplex layer consisting of a lower layer of metallic chromium of 80-120 mg/m2 and an upper layer of hydrated chromium oxide of 12-20 mg/m2, as chromium, is formed on a cold rolled steel sheet having a thickness of 0.23 mm by various treating conditions.

Example 1 A cold rolled steel sheet was treated by using an electrolyte composition consisting of 50 g/l of CrO3, 0.1 g/l of H2SO4 and 1.8 g/l of NaF in water under 20 A/dm2 of cathodic current density at an electrolyte temperature of 40"C. The thus treated steel sheet was rinsed with water at room temperature and dried.

Example 2 A cold rolled steel sheet was plated with metallic chromium by using an electrolyte composition consisting of 100 g/l of CrO3 and 6 g/l of NaF in water under 60 A/dm2 of cathodic current density at an electrolyte temperature of 50"C. After the current was turned off, the steel sheet was left in the electrolyte solution for 3-5 seconds to remove the very thin hydrated chromium oxide layer which had formed on the metallic chromium layer. After rinsing with water, the chromium plated steel sheet was treated by using an electrolyte composition consisting of 30 g/l of CrO3 and 1.2 g/l of NaF in water under 20 A/dm2 of cathodic current density at an electrolyte temperature of 30"C, and was then rinsed with water at room temperature and dried.

Example 3 A cold rolled steel sheet was treated by using an electrolyte composition consisting of 90 g/l of CrO3 and 5 g/l of KF in water under 40 A/dm2 of cathodic current density at an electrolyte temperature of 55"C. After the current was turned off, the steel sheet was left in the electrolyte solution for 3-5 seconds to remove the very thin hydrated chromium oxide layer which had formed on the metallic chromium layer. The thus treated steel sheet was further treated with this electrolytic solution diluted to one-third its original concentration and having added thereto 0.3 g/l of H2SO4, under 10 A/dm2 of cathodic current density at an electrolyte temperature of 35"C, and was then treated for 3 seconds with hot water at 90"C, and dried.

Example 4 A cold rolled steel sheet was treated under the same conditions as in Example 2. The thus treated steel sheet was treated with hot water at 95"C for 5 minutes. This Example was carried out to obtain the maximum value for the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in the hydrated chromium oxide layer, though it is not a commercially practical method.

Comparative Example A cold rolled steel sheet was treated by using an electrolyte composition consisting of 80 g/l of CrO3, 0.35 g/l of H2SO4 and 0.4 g/l of HBF4 in water under 40 A/dm2 of cathodic current density at an electrolyte temperature of 58"C. The thus treated steel sheet was rinsed with water at room temperature and dried.

The atomic ratio of oxygen existing as a hydroxyl radical and bonded water to the sum of the elements, chromium, oxygen, sulfur and fluorine in the hydrated chromium oxide layer of each resultant TFS prepared in Examples 1, 2, 3 and 4 and in the Comparative Example was measured by XPS, and the characteristics of each TFS were evaluated by the following test methods (1)-(3). The results are shown in the Table set forth below.

The measurement of chromium, oxygen, sulfur and fluorine in the hydrated chromium oxide layer by XPS was carried out at normal temperature in a vacuum. The absorbed water existing on the surface of TFS has no effect on the measured values of each element, because it is easily desorbed in vacuum. The spectrum of chromium is obtained in a partly overlapped state of two spectra of trivalent chromium in the hydrated chromium oxide layer and of metallic chromium under the hydrated chromium oxide layer. Therefore, the measured value of trivalent chromium can be obtained by the separation of the overlapped spectra according to the intensity ratio of each spectrum.The composition ratio of each element in the hydrated chromium oxide layer is finally obtained by dividing the integral value of each spectrum which is rectified by sensibility, by the sum of each measured value of chromium, oxygen, sulfur and fluorine in the hydrated chromium oxide layer.

(1) Lacquer adhesion in a normal state in the part adhered with nylon adhesive: Two pieces of the treated sample were prepared. One piece of the treated sample was baked at 210"C for 12 minutes after coating with 60 mg/dm2 of an epoxy-phenolic type lacquer, and the other piece was baked under the same conditions after coating with 25 mg/dm2 of the same lacquer. The two differently coated sample pieces were each cut to a size of 5 mm x 100 mm and bonded together by using a nylon adhesive having a thickness of 100 ym at 200"C for 30 seconds under 3 kg/cm2 of pressure by a hot press after pretreating at 200"C for 1 20 seconds. The bonding strength of the assembly, in 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"C for 3 days. The bonding strength of the assembly was measured in 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 in width and 60 mm in length, and were bonded so as to overlap each other by 8 mm in the longitudinal direction under the same conditions as described in (1). Ten assembled samples were prepared in this manner. 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 in width, as shown in the drawing, in which one piece of TFS 3 having a thick lacquer film 4, and another piece of TFS 3 having a thin lacquer film 5, are adhered with nylon adhesive 6 on the edges, and the resultant adhered specimen is fixed in a channel 2 in a bent state.The ten fixed samples were set in a retort into which steam, heated to 125-130"C under a pressure of 1.6-1.7 kg/cm2, was blown for 1 50 minutes or 300 minutes. The lacquer adhesion under the retort conditions was evaluated by the number of the samples which had peeled.

TABLE CHARACTERISTICS OF TREATED STEEL SHEET Compara tive Example 1 Example 2 Example 3 Example 4 Example Amount of metallic Cr 115 107 98 112 110 (mg/m2) Amount of hydrated Cr 15 17 19 13 15 oxide as CF (mg/m2) CrandOinhydrated Cr 18.9 22.5 20.1 21.2 20.7 Cr oxide by XPS (Atomic %) 0 71.8 70.5 74.6 76.8 71.2 O-4S/Cr + 0 + S + F x 100 (Atomic %) 63.8 67.3 69.4 74.8 54.0 Lacquer adhesion in normal state (kg/5 mm) 6.8 6.7 7.0 6.6 6.8 Lacquer adhesion after aging in hot water 2.4 2.9 2.7 3.0 0.2 (kg/5 mm) Lacquer adhesion 150 min. 1/10 0/10 0/10 0/10 7/10 under retort conditions* 300 min. 2/10 0/10 1/10 0/10 10/10 A A shows the number of the peeled assemblies.

B shows the total number of the assemblies.

As shown in the Table, there are very clear differences between the products of the Examples of the present invention and that of the Comparative Example, in terms of the lacquer adhesion after aging in hot water and under retort conditions, although there is no substantial difference between these products in the lacquer adhesion in a normal state. It is apparent from these Examples that TFS having a hydrated chromium oxide layer in which the atomic ratio of oxygen existing as a hydroxyl radical and bonded water to the sum of the elements chromium, oxygen, sulfur and fluorine is restricted according to the present invention, exhibits remarkable effects in terms of improved lacquer adhesion after aging in hot water and under retort conditions.

Claims

1. A tinfree steel comprising a steel base, a first layer on said steel base and a second layer on said first layer, said first layer being metallic chromium, said second layer being hydrated chromium oxide, the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in said second layer to the total of chromium, oxygen, sulfur and fluorine in said second layer being 60-75%.

2. A tin-free steel as claimed in claim 1, wherein the amount of metallic chromium in said first layer is from 50 to 200 mg/m2.

3. A tin-free steel as claimed in claim 1 or claim 2, wherein the amount of hydrated chromium oxide in said second layer is from 8 to 30 mg/m2, calculated on the basis of chromium.

4. A process for preparing a tin-free steel as claimed in claim 1, which comprises heating, in water having a temperature above 50"C, a tin-free steel in which the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in said second layer to the total of chromium, oxygen, sulfur and fluorine in said second layer is less than 60%.

5. A process as claimed in claim 4, wherein said water has a temperature above 70"C.

6. A process for preparing a tin-free steel as claimed in claim 1, which process comprises subjecting said steel base to electrolytic treatment in an aqueous electrolytic solution containing chromic acid and at least one addition agent selected from the group consisting of a fluorine compound and a sulphur compound, the amount of said addition agent incorporated in said hydrated chromium oxide layer during said electrolytic treatment are restricted in a manner such that the atomic ratio of oxygen existing as a hydroxyl radical and bonded water in said hydrated chromium oxide layer to the total of chromium, oxygen, sulfur and fluorine in said hydrated chromium oxide layer is 60-75%.

7. A process as claimed in claim 6, wherein said addition agent is at least one fluorine compound.

8. A process as claimed in claim 6, wherein said addition agent is a combination of at least one fluorine compound and at least one sulfur compound.

9. A process as claimed in claim 8, wherein the amount of said sulfur compound is not greater than 0.2 gram per liter of said electrolytic solution.

1 0. A process as claimed in any one of claims 6 to 9, wherein said fluorine compound addition agent is at least one compound selected from the group consisting of hydrofluoric acid, ammonium bifluoride, ammonium fluoride, an alkali metal bifluoride, an alkali metal fluoride, fluoboric acid, ammonium fluoborate, an alkali metal fluoborate, fluosilicic acid, ammonium fluosilicate and an alkali metal fluosilicate.

11. A process as claimed in any one of claims 6, 8 or 9, wherein said sulfur compound addition agent is at least one compound selected from the group consisting of sulfuric acid, ammonium sulfate, an alkali metal sulfate, phenolsulfonic acid, ammonium phenolsulfonate, an alkali metal phenolsulfonate, ammonium sulfite, an alkali metal sulfite, ammonium thiosulfate and an alkali metal thiosulfate.

1 2. A process as claimed in any one of claims 6 to 11, wherein said electrolytic treatment is carried out in two stages, the first stage of which results in formation of said metallic chromium layer on said steel base, and the second stage of which results in formation of said hydrated chromium oxide layer on said metallic chromium layer.

1 3. A process as claimed in any one of claims 6 to 11, wherein said electrolytic treatment is carried out in a single stage which results in formation of said metallic chromium layer on said steel base and formation of said hydrated chromium oxide layer on said metallic chromium layer.

14. A tin-free steel as claimed in claim 1, substantially as described herein in any one of the Examples.

1 5. A process for producing a tin-free steel, substantially as described herein any any one of the Examples.

1 6. A can having a can body made of a tin-free steel as claimed in any one of claims 1 to 3 and 14.