GB2158842A - Surface-treated steel sheets - Google Patents

Abstract

A surface-treated steel sheet comprises a steel base having three surface layers thereon, the three surface layers consisting of a bottom layer containing at least 30 mg/m<2> of metallic chromium, a middle layer containing at least 10 mg/m<2> of metallic tin or at least 10 mg/m<2> of tin-nickel alloy containing 20 to 60 weight % of nickel and a top layer of 2 to 18 mg/m<2>, calculated as chromium of hydrated chromium oxide. The surface-treated steel sheets according to the invention have excellent weldability and corrosion resistance and are of particular use in the manufacture of cans.

Description

SPECIFICATION Surface-treated steel sheets The present invention relates to surface-treated steel sheets having an excellent weldability and an excellent corrosion resistance as well as to methods for their production.

Recently a change from the use of expensive electrotinplates to the use of cheaper tin-free steel (TFS-CT) having two layers, that is a lower layer of metallic chromium and an upper layer of hydrated chromium oxide, as well as to the use of a smaller weight of tin coating in electrotinplates has rapidly taken place in the field of, in particular, food cans. One reason for this is because the tin used for the production of tinplate is very expensive and there is concern over the exhaustion of tin resources.

An ordinary metal can, except in the use of drawn cans, consists of two can ends and a single can body. In the case of tinplate cans, the seaming of the can body is generally carried out by soldering. Using this soldering process, however, it is impossible to decrease the weight of tin coating on the tinplate to under 2.8 g/m2 because it is difficult to stabilize the soldering process when the weight of the tin coating is under 2.8 g/m2. Due to the regulation of lead content in the solder used for the seaming of the tinplate can body in food cans, the seaming of the tinplate can body is widely carried out by electric welding.

A lap seam welding, for instance, using the Soudronic process has been recently used for the seaming of the tinplate can body. in this process, it is desirable to decrease the tin coating weight in the tinplate, but the weldability of tinplate becomes poor with a decrease in tin coating weight.

On the other hand, the seaming of a TFS-CT can body is generally carried out with nylon adhesives by using the Toyo Seam (Trade name) and Mira Seam (Trade name) method. Another method of seaming a TFS-CT can body by electric welding is also well known. In the case of seaming of a TFS-CT can body by electronic welding, however, the metallic chromium layer and the hydrated chromium oxide layer must be mechanically or chemically removed from the TFS CT surface in order to easily weld the TFS-CT can body at high speed. Therefore the corrosion resistance in the welded part of the TFS-CT can body becomes remarkably poor, even if this welded part is coated with lacquer after welding.

It would thus be desirable to provide a can material which is cheaper than tinplate and is easily welded at high speed without removal of the plated layer, for use in the field of cans and particularly food cans.

Recently, various surface-treated steel sheets have been proposed as a can material which can be easily welded at high speed without removal of the plated layer. For instance, the following surface-treated steel sheets have been proposed: (a) Lightly tin-coated steel sheet (LTS) with less than about 1.0 g/m2 of tin which is reflowed or unreflowed after tin plating (Japanese Patent Publication Nos. Sho 56-3440. Sho 56-54070, Sho 57-55800, and Laid-Open Japanese Patent Application Nos. Sho 56-75589, Sho 56-130487, Sho 56-156788, Sho 57-101694, Sho-185997, Sho 57-192294, Sho 57-192295 and Sho 55-69297); (b) Nickel preplated LTS with less than about 1.0 g/m2 of tin (Laid-Open Japanese Patent Application Nos.Sho 5723091, Sho 57-67196, Sho 57-110685, Sho 57-177991, Sho 57-200592 and Sho 57203797); (c) Nickel plated steel sheet with chromate film or phosphate film (Laid-Open Japanese Patent Application Nos. Sho 56-116885, Sho 56-169788, Sho 57-2892, Sho 572895, Sho 57-2896, Sho 57-2897, Sho 57-35697 and Sho 57-35698); (d) TFS-CT having two layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide which is obtained by special methods such as cold rolling after TFS treatment (Laid-Open Japanese Patent application No. Sho-48406), porous chromium plating (Laid-Open Japanese Patent Application No. Sho 55-31113) and a cathodic treatment of a steel sheet in chromic acid electrolyte with fluoride but without anions such as sulfate, nitrate and chloride ion (Laid-Open Japanese Patent Application No. Sho 55-18542).

However, LTS and nickel preplated LTS identified as (a) and (b) above are slightly more expensive to produce than TFS-CT. Furthermore, they not only have a narrower available current range for sound welding than tinplate, but also poor lacquer adhesion compared with TFS-CT, although they can be welded without the removal of the plated layers. The reason why the available current range for sound welding in LTS and nickel preplated LTS is narrower than that in tinplate is considered to be that the amount of free tin in these is smaller than that in tinplate and also further decreases because of changes of free tin into iron-tin alloy during heating for lacquer curing.

Nickel plated steel sheets with chromate films or phosphate films identified as (c) above also have a narrower available current range for sound welding than that of LTS or nickel preplated LTS. Furthermore, the corrosion resistance of nickel plated steel sheets is poorer than that of TFS-CT, although the lacquer adhesion is good. In particular, pitting corrosion in defective parts of the lacquered nickel plated steel sheet may occur easily from acidic foods such as tomato juice because the electrochemical potential of nickel is more noble than that of the steel base and metallic chromium.

The welding of TFS-CT indicated above as (d) without the removal of the TFS-CT film is very difficult at high speed because oxide films having high electric resistance are formed by the oxidation of metallic chromium and exposed steel base through the plating pores by the dehydration of hydrated chromium oxide during heating for curing the lacquer coated on the TFS-CT can body, although TFS-CT may be welded when it is not heated before welding.

As indicated above, the previously proposed surface-treated sheel sheets (a), (b), (c) and (d) above have various problems in relation to their production cost and/or their characteristics as a can material which can be easily welded without the removal of the plated layer at high speed.

Accordingly, there is still a need to provide a surface-treated steel sheet having an excellent weldability, that is, being easily welded at high speed without removal of plated layers, and having excellent corrosion resistance after lacquering such as in a TFS-CT.

We have now developed a new surface-treated steel sheet which can be readily used with advantage in applications such as food can bodies, aerosol can bodies and miscellaneous can bodies which are lacquered, except for the part to be welded, which sheet exhibits excellent weldability, i.e. is easily welded at high speed without removal of plated layers.

This new surface-treated steel sheet can be also used in applications where lacquer coating is not carried out because it has excellent weldability. Furthermore this surface-treated steel sheet can be used in applications where excellent corrosion resistance after lacquering are required, such as can ends, drawn cans and drawn and redrawn cans (DR cans), besides can bodies.

Thus according to the present invention we provide a surface-treated steel sheet comprising a steel base having three surface layers thereon, the three surface layers consisting of a bottom layer containing at least 30 mg/m2 of metallic chromium, a middle layer containing at least 10 mg/m2 of metallic tin or at least 10 mg/m2 of tin-nickel alloy containing 20 to 60 weight % of nickel and a top layer of 2 to 1 8 mg/m2, calculated as chromium, of hydrated chromium oxide.

According to a further feature of the present invention we provide a process for preparing a surface-treated steel sheet as defined above which comprises either a) chromium plating a steel base to form a layer of metallic chromium and hydrated chromium oxide thereon, b) tin or tin-nickel alloy plating the chromium plated steel base with a tin or a tin-nickel plating solution under conditions sufficiently acidic to substantially dissolve the hydrated chromium oxide in the solution, and c) forming a layer of hydrated chromium oxide on the tin or tin-nickel plated, chromium plated steel base of step b), steps a), b) and c) above being carried out whereby a surface-treated steel sheet as defined above is obtained;; or a) chromium plating a steel base to form a layer of metallic chromium and hydrated chromium oxide thereon, b) removing the hydrated chromium oxide formed on the chromium plated steel base by cathodic treatment in an acidic solution, c) tin or tin-nickel alloy plating the chromium plated steel base, and d) forming a layer of hydrated chromium oxide on the tin or tin-nickel plated, chromium plated steel base of step c), steps a), c) and d) above being carried out whereby a surface-treated steel sheet as defined above is obtained.

The steel base used in the surface-treated steel sheet according to the present invention can be any cold rolled steel sheet customarily used in manufacturing electrotinplate and TFS-CT.

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 product), is employed as the steel base. Preferably, the thickness of the steel base is from about 0.1 to about 0.35 mm.

The amount of metallic chromium which is in the bottom layer of the surface-treated steel sheet according to the present invention is generally from 30 to 300 mg/m2, preferably 70 to 1 50 mg/m2. If the amount of metallic chromium is below 30 mg/m2, the excellent weldability and excellent corrosion resistance are not obtained because, it is considered, the surface of the steel base is not sufficiently covered with the plated metallic chromium and a greater part of the overlying layer of tin or tin-nickel alloy changes to iron-tin alloy or iron-tin-nickel alloy having high electric resistance during heating for lacquer curing. The amount of metallic chromium is generally limited to 300 mg/m2 on economical and industrial grounds, although the oxidation of a steel base and the formation of iron-tin alloy or iron-tin-nickel alloy during heating are prevented with an increase in the amount of metallic chromium.

The amount of metallic tin or tin-nickel alloy in the middle layer of the surface-treated steel sheet according to the present invention is generally 10 to 500 mg/m2, preferably 50 to 300 mg/m2 more preferably 50 to 1 50 mg/m2. If the amount of metallic tin or tin-nickel alloy plated on the metallic chromium plated steel base is below 1 Omg/m2, excellent weldability is not obtained because chromium oxide having high electrical resistance is formed by the oxidation of metallic chromium during heating for lacquer curing, even if the oxidation of the steel base is prevented with sufficient amounts of metallic chromium. The amount of plated tin or tin-nickel alloy is generally limited to 500 mg/m2 on economic grounds, although the effect of metallic tin or tin-nickel alloy in the present invention does not change at above 500 mg/m2.

It will be appreciated that, in the present invention, the term tin-nickel alloy does not necessarily refer to a tin-nickel alloy having a stoichiometrical composition with a constant ratio of tin to nickel such as NiSn, Ni3Sn, Ni3Sn2 and Ni3Sn4, but refers to the co-deposition of tin and nickel having various ratios of tin to nickel, the co-deposited tin and nickel forming tinnickel alloy at room temperature or during heating.

In the case of tin-nickel alloy plating on the chromium plated steel base, the nickel content is generally 20 to 60 weight % based on the total weight of plated tin and nickel. Although the weldability decreases slightly with an increase in the codeposited nickel content in the plated tinnickel alloy, the surface-treated steel sheet according to the present invention, wherein tin-nickel alloy is plated on the chromium plated steel base, have an excellent weldability compared with that of TFS-CT.However, although at above 60 weight % or below 20 weight % of nickel in the tin-nickel alloy, the weldability and the corrosion resistance are excellent compared with that of TFS-CT, it is difficult to stably plate the tin-nickel alloy on the chromium plated steel base using a tin-nickel alloy plating electrolyte because the nickel content or tin content in the tin-nickel alloy changes greatly with a slight change in the plating conditions.

The surface-treated steel sheet according to the present invention wherein tin-nickel alloy is plated on the chromium plated steel base in particular show excellent corrosion resistance to sulfide stains, which stains often appear in the inside of the can when some foods containing protein such as fish and meat are packed in lacquered tin plate cans, unlacquered tin plate cans and lacquered nickel plated steel cans.

Various methods for making an alloy or codeposition layer e.g. of tin and zinc, of tin and cobalt, nickel plating after tin plating, tin plating after nickel plating, zinc plating after tin plating and tin plating after zinc plating on the chromium plated steel base have been considered in order to obtain a surface-treated steel sheet having an excellent weldability, but these methods are not suitable for industrial use because of the complexity of the process or because a special electrolyte is required.

As described above, the presence of metallic chromium as a bottom layer and metallic tin or tin-nickel alloy as a middle layer in the surface-treated steel sheet according to the present invention are indispensable in order to realize excellent weldability after heating.

Furthermore, in the present invention, the presence of a small amount of hydrated chromium oxide as a top layer is indispensable in order to prevent oxidation of the exposed steel base and the exposed metallic chromium after tin or tin-nickel alloy plating during heating for lacquer curing and to obtain excellent corrosion resistance and excellent lacquer adhesion.

The optimum range of hydrated chromium oxide is from 2 to 1 8 mg/m2, preferably 4 to 1 2 mg/m2 as chromium. If the amount of hydrated chromium oxide is below 2 mg/m2 as chromium, the corrosion resistance and the lacquer adhesion become poor. If the amount of hydrated chromium oxide is above 18 mg/m2, the weldability becomes remarkably poor because hydrated chromium oxide changes to chromium oxide having high electric resistance by dehydration during heating for lacquer curing.

The above-mentioned process, according to the invention may be used to prepare the surfacetreated steel sheets of the invention in a continuous high speed operation.

When preparing surface-treated steel sheet according to the present invention the steel base is generally subjected to degreasing with an alkali and pickling with an acid, followed by water rinsing, according to the conventional methods before the first chromium plating step.

In order to form a metallic chromium layer as the bottom layer on the surface-treated steel sheet according to the present invention, a known chromium plating electrolyte such as a Sargent bath or a chromic acid electrolyte, containing additives such as e.g. fluorine compounds, which are used for the production of TFS-CT having a lower layer of metallic chromium and an upper layer of hydrated chromium oxide, may be employed.

In the present invention, it is preferable to employ the following electrolytic chromium plating conditions for the formation of a metallic chromium layer on the steel base: Concentration of chromic acid: 30-300 g/l, more preferably 80-300 g/l Concentration of additive: 1.0-5.0, more preferably 1.0-3.0 weight % of concentration of chromic acid Additives: at least one compound selected from fluorine compounds and sulfur compounds Temperature of the electrolyte: 30-60"C Cathodic current density: 10-100 A/dm2 Generally, the amount of hydrated chromium oxide formed during chromium plating decreases with an increase in the concentration of chromic acid in the suitable weight ratio of additives to chromic acid.It is not desirable to use an electrolyte having below 30 g/l of chromic acid for the chromium plating because the current efficiency for the deposition of metallic chromium decreases considerably. A concentration of chromic acid above 300 g/l is also not desirable from an economic point of view.

The presence of additives such as fluorine compounds and sulfur compounds in the chromium plating electrolyte is indispensable for uniform chromium deposition. If the weight % of additives to chromic acid is below 1.0 or above 5.0, the current efficiency of the deposition of metallic chromium remarkably decreases, as well as there being a decrease in the uniformity of the deposited metallic chromium layer. Particularly, at below 1.0 weight % of additives to chromic acid, the insoluble hydrated chromium oxide formed significantly prevents the formation of a uniform metallic tin or tin-nickel alloy layer in the following tin or tin-nickel plating step.

It is preferable that the additives be at least one compound selected from fluorine compounds such as e.g. hydrofluoric acid, fluoboric acid, fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, an alkali metal fluoride, ammonium fluoborate, an alkali metal fluoborate, ammonium fluosilicate, an alkali metal fluosilicate, aluminum fluoride and sulfur compounds such as e.g. sulfuric acid, ammonium sulfate, an alkali metal sulfate, chromium sulfate, aluminum sulfate, phenolsulfonic acid, ammonium phenolsulfonate, an alkali metal phenolsulfonate, phenoldisulfonic acid, ammonium phenoldisulfonate, an alkali metal phenoldisulfonate, ammonium sulfite, an alkali metal sulfite, ammonium thiosulfate and an alkali metal thiosulfate.

The amount of hydrated chromium oxide formed during chromium plating decreases with an increase in the temperature of the electrolyte. A temperature of the electrolyte above 60"C is not, however, suitable from an industrial point of view because the current efficiency for the deposition of metallic chromium decreases remarkably. A temperature of the electrolyte below 30"C is also not desirable because a long time is necessary for the removal of the large amount of hydrated chromium oxide formed during chromium plating.

With an increase in cathodic current density, the current efficiency for the deposition of metallic chromium increases and the amount of hydrated chromium oxide formed during chromium plating decreases. It is desirable, therefore, in the present invention that the range of the cathodic current density for the deposition of metallic chromium is 10 to 100 A/dm2, more preferably 40 to 80 A/dm2 because metallic chromium does not deposit significantly at below 10 A/dm2 of the current density and the current efficiency for the deposition of metallic chromium does not increase significantly at above 100 A/dm2 of the current density.

In the present invention, conditions for chromium plating wherein good current efficiency for the deposition of metallic chromium is obtained and a small amount of hydrated chromium oxide is formed should preferably be selected in order to minimise the treatment necessary to subsequently remove the hydrated chromium oxide. Thus the presence of hydrated chromium oxide prevents the formation of a uniform tin or tin-nickel alloy layer in the following tin or tinnickel alloy plating. However, some hydrated chromium oxide is always formed on a deposited metallic chromium layer during chromium plating.

Under conditions of higher concentration of chromic acid, higher current density and higher temperature of the electrolyte, the amount of hydrated chromium oxide formed on the deposited metallic chromium is about 3 to 10 mg/m2 as chromium. On the other hand, under conditions of lower concentration of chromic acid, lower current density and lower temperature of the electrolyte, it is about 10 to 50 mg/m2 as chromium.

When a large amount of hydrated chromium oxide is formed during chromium plating, it is possible to decrease the amount by leaving the chromium plated steel base in the chromium plating electrolyte for a few seconds. However, hydrated chromium oxide in an amount of about 3 to 5 mg/m2 as chromium still remains on the surface of the chromium plated steel base, even if the chromium plated steel base covered with hydrated chromium oxide is left in the chromium plating electrolyte for a long time.

In the present invention, such hydrated chromium oxide must be removed before the subsequent tin or tin-nickel alloy plating because the presence of hydrated chromium oxide prevents the deposition of a uniform tin or tin-nickel alloy layer on the metallic chromium layer.

The following methods are conventionally considered suitable for the removal of hydrated chromium oxide on deposited metallic chromium layers: (A) Immersion of the chromium plated steel base, before drying, into a high concentration of an alkaline solution such as an alkali metal hydroxide and an alkali metal carbonate at high temperature of 70 to 90'C-it is, however, difficult to industrialize this method because the alkaline solution may be mixed into the following tin or tin-nickel alloy plating electrolyte; (B) Immersion of the chromium plated steel base, before drying, into an acid solution such as sulfuric acid and hydrochloric acid-this method is not suitable in the present invention because hydrated chromium oxide formed during chromium plating is not sufficiently dissolved by an immersion into acid solution for a short time; ; (C) Mechanical removal of hydrated chromium oxide by a brushing roll or wiper in an alkaline solution or an acid solution before drying the chromium plated steel base-the hydrated chromium oxide formed on the chromium plated steel base is not uniformly removed by this method.

These methods (A), (B) and (C) are not, as indicated above, suitable for the removal of hydrated chromium oxide before the subsequent tin or tin-nickel alloy plaing.

In the present invention, we have found the following methods are preferable for the removal of hydrated chromium oxide formed on the metallic chromium layer. Thus according to one method the chromium plated steel base is cathodically treated in an acid solution such as e.g. in sulfuric acid or hydrochloric acid having a pH of 0.5 to 2.0, before tin or tin-nickel alloy plating.

According to an alternative method the tin or tin-nickel alloy plating is carried out at the same time as removal of hydrated chromium oxide formed on the metallic chromium layer by using plating conditions sufficiently acidic to substantially dissolve the hydrated chromium oxide. In this alternative method either a tin plating electrolyte having a low concentration of stannous ion or a tin-nickel alloy plating electrolyte is used, the electrolytes having a low current efficiency for the deposition of tin or tin-nickel alloy.

The conditions for the removal of hydrated chromium oxide by the former method using cathodic treatment in an acidic solution are generally as follows: Electrolyte: An acid solution containing at least one acid selected from sulfuric acid, hydrochloric acid, hydrofluoric acid, fluoboric acid and fluosilicic acid having a pH of 0.5 to 2.0.

Temperature of the electolyte: 30-70"C Cathodic current density: 20-50 A/dm2 Treating time: 0.5-5.0 seconds Although the main component of the electrolyte is an acid such as e.g. sulfuric acid or hydrochloric acid, provided the pH of the electrolyte is kept between 0.5 to 2.0, various ions which are not deposited on the surface of the chromium plated steel base and do not oxidize the surface of the chromium plated steel base, may be present in the electrolyte. It is not necessary for preferred operation that the temperature of the electrolyte be strictly controlled provided it is kept between 30 to 70"C. If the temperature of the electrolyte is above 70"C, evaporation of water is increased.At below 30"C cathodic treatment for a long time is required for the sufficient removal of hydrated chromium oxide.

At below 2 A/dm2 of current density, hydrated chromium oxide is not sufficiently removed, even if the chromium plated steel base is cathodically treated for a long time. An upper limit of current density is preferably 50 A/dm2 because removal is not increased at a current density above 50 A/dm2.

If the treating time is below 0.5 seconds, hydrated chromium oxide is not sufficiently removed from the metallic chromium layer, even if a higher current density is applied. A treating time above 5.0 seconds is not generally suitable in the high speed production of the surface treated steel sheets according to the invention.

The conditions for the latter method of hydrated chromium oxide removal wherein tin or tinnickel alloy plating is carried out at the same time as the removal of the hydrated chromium oxide are generally as follows: In tin plating, the following conditions are preferable.

Concentrations of stannous ion: 2-10 g/l pH of the electrolyte: 0.5-3.0 Temperature of the electrolyte: 30-60"C Cathodic current density: 3-50 A/dm2 Generally, a tin plating electrolyte such as a ferrostan bath or a halogen bath having about 20 to 40 g/l of stannous ion is used for the industrial product of electrotinplate. Such tin plating electrolytes are not, however suitable for use where the tin plating is carried out simultaneously with removal of hydrated chromium oxide formed during chromium plating because a uniform tin layer is not then formed on the chromium plated steel base and the coarsely and dendritically plated tin may be peeled off from the chromium plated steel base.Such tin plating electrolytes can, however, be used for tin plating where the removal of hydrated chromium oxide on the chromium plated steel base is carried out by a cathodic treatment in an acidic solution.

Where tin plating is carried out simultaneously with the removal of hydrated chromium oxide formed during chromium plating a tin plating electrolyte having a low concentration of stannous ion such as 2 to 10 g/l, i.e. having a low current efficiency for the deposition of tin is required.

The reason why a uniform tin layer is not formed on the chromium plated steel base in this case using a tin plating electrolyte having a high concentration of stannous ion is that the greater part of electricity is consumed for the deposition of tin and not for the removal of hydrated chromium oxide. It is therefore desirable to decrease the concentration of stannous ion to below 10 g/l in the present invention in order to obtain a uniform tin layer on the chromium plated steel base where removal of hydrated chromium oxide is carried out simultaneously with the tin plating.

However, a concentration of stannous ion below 2 g/l is not suitable in the present invention, because the current efficiency for the deposition of tin decreases remarkably and becomes unsuitable due to the presence of a small amount of ions such as chromium ion and iron ion which build up in the electrolyte by dissolution of the hydrated chromium oxide and the steel base.

Stannous ion is generally supplied by the addition of stannous sulfate, stannous chloride, stannous fluoride and/or stannous fluoborate or by the dissolution of a soluble tin anode.

The pH of the electrolyte is also very important for tin plating on the chromium plated steel base with simultaneous removal of hydrated chromium oxide. The pH range of the electrolyte should be from 0.5 to 3.0, and preferably 0.5 to 1.5 in a ferrostan bath containing sulfuric acid or phenolsulfonic acid or preferably 2 to 3 in a halogen bath containing stannous chloride, sodium fluoride, potassium bifluroide and sodium chloride.

At a low pH such as 0.5 to 3.0, the surface of the chromium plated steel base is uniformly activated because hydrated chromium oxide formed during chromium plating is easily removed from the chromium plated steel base with the evolution of a large amount of hydrogen during dissolution by acid. A uniform metallic tin layer is therefore formed on the metallic chromium layer. A pH of below 0.5 is not desirable in the present invention, because a part of the metallic chromium may be dissolved. A pH of above 3.0 is also not desirable because a uniform tin layer is not generally formed on the metallic chromium layer due to the insufficient dissolution of hydrated chromium for a short time.Furthermore it becomes difficult to produce the surfacetreated steel sheet according to the present invention in a stable manner because the pH of the electrolyte then changes greatly on only a slight change in the concentration of stannous ion and acid.

The pH of the electrolyte is generally controlled by the addition of sulfuric acid, phenolsulfonic acid, hydrochloric acid, hydrofluoric acid, fluoboric acid, fluosilicic acid or alkali metal salts thereof. Various ions, which do not give rise to adverse effects in tin plating and in the dissolution of hydrated chromium oxide, may be contained in the electrolyte provided the pH of the electrolyte is kept in the range of from 0.5 to 3.0.

Additives such a ethoxylated a-naphthol sulfonic acid, ethoxylated a-naphthol, ss-naphthol and gelatine which are used in known tin plating electrolytes such as ferrostan baths and halogen baths for the production of electrotinplate can be used to improve the uniformity of the plated tin layer in the present invention.

It is suitable in the present invention that the range of the cathodic current density is 3 to 50 A/dm2, more preferably 10 to 30 A/dm2. If the current density is below 3 A/dm2, the current efficiency for tin plating becomes so low that a long time is necessary for the deposition of the required amount of tin. If the current density is above 50 A/dm2, a tin layer having an excellent adhesion to the chromium plated steel base is not obtained.

The optimum range for the temperature of the electrolyte is from 30 to 60"C, more preferably 30 to 50"C. At below 30"C, hydrated chromium oxide is not dissolved sufficiently, so that a uniform tin layer is not plated on the chromium plated steel base. At above 60"C, metallic chromium also tends to be disolved with the dissolution of the hydrated chromium oxide.

In tin-nickel alloy plating with simultaneous removal of hydrated chromium oxide, the following two types of electrolyte may be used. The first type of electrolyte is a pyrophosphate bath consisting of an alkali metal pyrophosphate, stannous chloride, nickel chloride and additives. The second type of electrolyte is a halogen bath consisting of a stannous halide, nickel halide, an alkali metal halide and additives.

It is suitable to employ the following conditions for tin-nickel alloy plating using the first type of electrolyte: Concentration of stannous ion: 2-40 g/l Concentration of nickel ion: 4-20 g/l Concentration ratio of stannous ion to nickel ion: 0.1-3 Concentration of an alkali metal pyrophosphate: 80-300 g/l pH of the electrolyte: 8-10 Temperature of the electrolyte: 40-60 C Cathodic current density: 1-30 A/dm2 It is suitable to employ the following conditions for tin nickel alloy plating using the second type of electrolyte: Concentration of stannous ion: 2-70 g/l Concentration of nickel ion: 4-80 g/l Concentration ratio of stannous ion to nickel ion: 0.1-0.8 pH of the electrolyte: 0.5-3 Temperature of the electrolyte: 30-60"C Cathodic current density: 1-30 A/dm2 In tin-nickel alloy plating on the chromium plated steel base with simultaneous removal of hydrated chromium oxide using an alkaline pyrophosphate bath or an acidic halogen bath, it is very important that the concentration ratio of stannous ion to nickel ion be kept within the range described above in order to obtain a uniform tin-nickel alloy layer containing 20 to 60 weight % of nickel on the chromium plated steel base.

In tin-nickel alloy plating, generally the nickel content increases under conditions of higher current density, lower temperature of the electrolyte, lower concentrations of stannous ion and nickel ion, lower concentration ratio of stannous ion to nickel ion and higher pH of the electrolyte.

At below the lower limit for the concentration of stannous ion and nickel ion, the current efficiency for the codeposition of tin and nickel decreases remarkably. A concentration of above the upper limit of stannous ion and nickel ion is not generally desirable from an economic point of view because the drug out loss of stannous ion and nickel ion increases. Furthermore, if the concentration ratio of stannous ion to nickel ion is below the lower limit or above the upper limit, it is difficult to plate tin-nickel alloy having 20 to 60 weight % of nickel on the chromium plated steel base.

Stannous ion and nickel ion are generally supplied by the addition of stannous chloride and nickel chloride or by the dissolution of a soluble tin anode and nickel anode, respectively.

The pH of the electrolyte is generally controlled by the addition of hydrochloric acid and an alkali metal chloride in an acidic halogen bath and by the addition of pyrophosphoric acid, an alkali metal pyrophosphate, hydrochloric acid, an alkali metal chloride or an alkali metal hydroxide in an alkaline pyrophosphate bath.

Additives such as glycine and ethylene glycol which are used in known tin-nickel alloy plating electrolytes can also be used to improve the uniformity of the plated tin-nickel alloy layer in the present invention.

In the case of tin or tin-nickel alloy plating after the removal of hydrated chromium oxide by cathodic treatment in an acidic solution, the tin or tin-nickel plating may also be carried out using the same electrolyte and the same plating conditions described above. In this case, a conventional tin plating electrolyte such as a ferrostan bath or halogen bath having a high concentration of stannous ion, for instance, a ferrostan bath consisting of 30 g/l of stannous sulfate (as stannous ion), 30 g/l of phenolsulfonic acid (60% solution) and 6 g/l of ethoxylated a-naphthol sulfonic acid or a halogen bath consisting of 30 g/l of stannous chloride (as stannous ion), 30 g/l of sodium fluoride, 50 g/l of sodium chloride and 3 g/l of gelatine can be used for tin plating.

For the formation of the hydrated chromium oxide layer as the top layer of the surface treated steel sheet according to the present invention, a known electrolyte such as the acidic chromate electrolyte used for the post-treatment of electrotinplate or a chromic acid electrolyte containing a small amount of additives such as fluorine compounds and sulfur compounds, used for the production of TFS-CT having a lower layer of metallic chromium and an upper layer of hydrated chromium oxide, may be employed.

In the present invention, two types of electrolyte may be used for the formation of the hydrated chromium oxide layer. The first type of electrolyte consists of an acidic chromate electrolyte without addition of additives such as fluorine compounds and sulfur compounds. The second type of electrolyte consists of a chromic acid electrolyte with additives such as fluorine compounds and sulfur compounds.

It is suitable to employ the following conditions for the formation of hydrated chromium oxide of 2 to 1 8 mg/m2 as chromium using the first type of electrolyte: Concentration of hexavalent chromium ion: 5-30 g/l Temperature of the electrolyte: 30-70"C Cathodic current density: 1-20 A/dm2 Quantity of electricity: 1-40 coulombs/dm2 If the concentration of hexavalent chromium ion is below 5 g/l, a waste of electric power results because of the higher electrical resistance of the electrolyte. The concentration of hexavalent chromium ion is limited to 30 g/l from the viewpoint of conserving resources, although the effect of the present treatment is not decreased in a concentration above 30 g/l.

It is highly desirable that the electrolyte should be acidified. In the case of an alkaline electrolyte, the efficiency for the formation of hydrated chromium oxide is so low that a long time is necessary for the formation of satisfactory hydrated chromium oxide. Therefore an electrolyte containing only a chromate of an alkali metal or ammonium is not generally used in the present invention.

In the above case it should be acidified by the addition of chromic acid. It is also possible to add a hydroxide of an alkali metal or ammonium to a chromic acid electrolyte within the desired range.

At least one chromate selected from chromic acid, a chromate and dichromate of an alkali metal, ammonium chromate and ammonium dichromate is generally used for the first type of electrolyte within the desired acid range in the present invention. It is not necessary that the temperature of the electrolyte be strictly controlled provided it is generally kept between 30 to 70"C. If the temperature of the electrolyte is above 70"C, evaporation of water is increased.

At a current density below 1 A/dm2, a long time is necessary for the formation of a satisfactory hydrated chromium oxide. At a current density above 20 A/dm2, control of the amount of the hydrated chromium oxide formed may be difficult, although a satisfactory amount of hydrated chromium oxide may be formed by a cathodic treatment for a short time.

If the quantity of electricity is below 1 coulombs/dm2, it is difficult to form a suitable amount of hydrated chromium oxide. At above 40 colombs/dm2 of electricity, the weldability of the surface-treated steel sheet according to the present invention becomes poor because of the formation of a thicker layer of hydrated chromium oxide.

It is desirable to employ the following conditions for the formation of hydrated chromium oxide using the second type of electrolyte: Concentration of chromic acid: 10-50 g/l Weight % of additives to chromic acid: 0.2-1.0 Additives: Sulfur compound and/or fluorine compound Temperature of the electrolyte: 30-60"C Cathodic current density: 1-10 A/dm2 Under the conditions described above, the weight % of additives to chromic acid and current density are very important in the present treatment because at a higher weight percent of additives to chromic acid and higher current density, metallic chromium, which imparts a bad effect to the weldability, is deposited on the tin or tin-nickel alloy plated steel base.The weight percent of additives to chromic acid is therefore preferably limited to 1.0 and the cathodic current density is preferably limited to 10 A/dm2. However, if the weight percent of additives to chromic acid is below 0.2, the weldability becomes poor because a thick layer of hydrated chromium oxide is formed. Under a current density below 1 A/dm2, a long time is necessary for the formation of a satisfactory hydrated chromium oxide layer. Furthermore, the ranges in the concentration of chromic acid, the quantity of electricity and the temperature of the electrolyte are limited as mentioned above in relation to the first type of electrolyte for the same reasons.

The additives may also be selected from those given above in respect of the chromium plating electrolyte.

In the treatment using this second type of electrolyte, it is very important to select conditions wherein metallic chromium is not deposited on the tin or tin-nickel alloy plate surface. However, under conditions where metallic chromium is deposited, the maximum amount of metallic chromium should be limited to 10 mg/m2, although the amount of the deposited metallic chromium should ideally be zero.

In both the above-described process for preparing the surface-treated steel sheets according to the invention it is desirable that the plated base be rinsed with water between each step.

In summary, therefore, the surface-treated steel sheets of the present invention may, according to the invention be prepared by treating the steel base as follows: (1) Degreasing with an alkali and pickling with an acidowater rinsingochromium pla tingowater rinsingetin or tin-nickel alloy platingowater rinsingochromate treatmentowater rinsingedrying, or;; (2) degreasing with an alkali and pickling with an acid ywater rinsingochromium pla tingowater rinsingethe removal of hydrated chromium oxide by a cathodic treatment in an acid solutionowater rinsingetin or tin-nickel alloy plating ywater rinsingochromate treat mentowater rinsingedrying.

The present invention is illustrated by the following non-limiting examples.

In Example 1 to Example 5, a cold rolled steel sheet having a thickness of 0.22 mm was treated by the following process (denoted A) after electrolytically degreasing in a solution of 70 g/l of sodium hydroxide, water rinsing and then pickling in a solution of 100 g/l of sulfuric acid, following by rinsing with water.

Process A--Chromium platingowater rinsing tin or tin-nickel alloy plating (with removal of hydrated chromium oxide formed during chromium plating)owater rinsingochromate treat mentowater rinsingdrying.

In Example 6 and Example 7, the same kind of steel sheet pretreated as in Example 1 to Example 5 was treated by the following process (denoted B).

Process BChrnmium platingowater rinsing yremoval of hydrated chromium oxide formed during chromium plating by a cathodic treatment in an acidic solutionewater rinsing tin platingowater rinsing chromate treatmentowater rinsingedrying.

In each Example, the conditions were as shown in detail below.

Example L Conditions for chromium plating Composition of electrolyte CrO3 120 g/l HBF4 0. 8 g/l H2SO4 0.5 g/l Temperature of electrolyte 600C Cathodic current density 50 A/dm2 Conditions for tin plating Compositon of electrolyte SnSO4 10 g/l as Sn2+ Phenolsulfonic acid (60% solution) 20 g/l Ethjoxylated a-naphthol sulfonic acid 5 g/l pH 1.1 Temperature of electrolyte 400C Cathodic current density 5 A/dm2 Conditions for chromate treatment CrO3 50 g/l H2SO4 0.1 g/l Temperature of electrolyte 550C Cathodic current density 3 A/dm2 Example 2 Conditions for chromium plating Composition of electrolyte Cr03 250 g/l H2SO4 3 g/l Temperature of electrolyte 450C Cathodic current density 20 A/dm2 Conditions for tin plating Composition of electrolyte SnSO4 3 g/l as Sn2+ Phenolsulfonic acid (60% solution) 20 g/l Ethoxylated &alpha;-naphthol sulfonic acid 3 g/l pH 0. 9 Temperature of electrolyte 450C Cathodic current density 8 A/dm2 Conditions for chromate treatment Na2Cr207.2H20 60 g/l Temperature of electrolyte 400C Cathodic current density 20 A/dm2 Example 3 Conditions for chromium plating Composition of electrolyte Cr03 50 g/l NaF 2 g/l Temperature of electrolyte 550C Cathodic current density 40 A/dm2 Conditions for tin-nickel alloy plating Composition of electrolyte SnCl2 50 g/l as Sn2+ NiCl2 6H20 75 g/l as Ni2+ Ethylene glycol 80 g/l HC1 30 g/l Ratio of Sn2+ to Ni2+ 0.67 pH 0.5 Temperature of electrolyte 450C Cathodic current density 10 A/dm2 Conditions for chromate treatment Composition of electrolyte Na2Cr2O7 # 2H2O 30 g/l Temper ate of electrolyte 400C Cathodic current density 10 A/dm2 Example 4 Conditions for chromium plating Composition of electrolyte CrO3 100 g/l HF 3 g/l Temperature of electrolyte 600C Cathodic current density 30 A/dm2 Conditions for tin-nickel alloy plating Composition of electrolyte SnCl2 26 g,/1 as Sn2+ NiC12 . 6H20 60 g/l as Ni2+ NaHF2 35 g/l NaF 28 g/l Ratio of Sn2+ to Ni2+ 0.43 pH 2.9 Temperature of electrolyte 400C Cathodic current density 2 A/dm2 Conditions for chromate treatment Composition of electrolyte K2Cr207 50 g/l Temperature of electrolyte 550C Cathodic current density 5 A/dm2 Example 5 Conditions for chromium plating Composition of electrolyte CrO3 200 g/l NaF 6 g/l NSiF6 1 g/l Temperature of electrolyte 500C Cathodic current density 40 A/dm2 Conditions of tin-nickel alloy plating Composition of electrolyte SnCl2 15 g/l as Sn2+ Nici2 . 6H20 7 g/l as Ni2+ Glycine 28 g/l K4P2O7 # 3H2O 150 g/l Ratio of Sn2+ to Ni2+ 2.1 pH 8.1 Temperature of electrolyte 500C Cathodic current density 5 A/dm2 Conditions for chromate treatment Composition of electrolyte CrO3 30 g/l NSiF6 0.3 g/l Temperature of electrolyte 550C Cathodic current density 10 A/dm2 Example 6 Conditions for chromium plating Composition of electrolyte CrO3 100 g/l HF 3 g/l Temperature of electrolyte 60"C Cathodic current density 80 A/dm2 Conditions for the removal of hydrated chromium oxide Composition of electrolyte H2 504 pH 0.5 Temperature of electrolyte 600C Cathodic current density 20 A/dm2 Treating time 0.5 seconds Conditions for tin plating Composition of electrolyte SnSO4 30 g/l as sun Phenoldisulfonic acid (60% solution) 25 g/l Ethoxylated a-naphthol 6 g/l pH 0.8 Temperature of electrolyte 400C Cathodic current density 15 A/dm2 Conditions for chromate treatment Composition of electrolyte K2Cr207 20 g/l Temperature of electrolyte 400C Cathodic current density 5 A/dm2 Example 7 Conditions for chromium plating Composition of electrolyte Cr03 40 g/l NaF 1.5 g/l Temperature of electrolyte 550C Cathodic current density 30 A/dm2 Conditions for the removal of hydrated chromium oxide Composition of electrolyte HC1 pH 1.5 Temperature of electrolyte 450C Cathodic current density 5 A/dm2 Treating time 3 seconds Conditions for tin plating Composition of electrolyte SnC12 28 g/l as Sn2+ NaF 30 g/l NaCl 50 g/l Gelatine 3 g/l pH 2.5 Temperature of electrolyte 600C Cathodic current density 15 A/dm2 Conditions for chromate treatment Composition of electrolyte Cur2 07 . 2H2 30 g/l Temperature of electrolyte 600C Cathodic current density 3 A/dm2 Comparative Example 1 The same kind of steel sheet pretreated as in Example 1 was treated under the following conditions and was then rinsed with water and dried.

Conditions of electrolytic chromic acid treatment Composition of electrolyte Cr03 80 g/l HBF4 0.5 g/l H2SO4 0.5 g/l Temperature of electrolyte 450C Cathodic current density 20 A/dm2 Comparative Example 2 The same kind of steel sheet pretreated as in Example 1 was plated with tin under the following conditions.

Conditions for tin plating Composition of electrolyte Sus04 30 g/l as Sn2+ Pbenolsulfonic acid (60% solution) 20 g/l Ethoxylated a-naphthol 5 g/1 pH 0.8 Temperature of electrolyte 40 C Cathodic current density 8 A/dm2 After rinsing with water, the tin plated steel sheet was treated under the following conditions and was then rinsed and dried.

Conditions for chromate treatment Composition of electrolyte Na2Cr207 . 2 H2 30 g/l Temperature of electrolyte 600C Cathodic current density 10 A/dm2 Comparative Example 3 The same kind of steel sheet pretreated as in Example 1 was plated with tin under the same conditions as in Comparative Example 2. After rinsing with water, the tin plated steel sheet was treated under the same conditions as in Comparative Example 1 and was then rinsed with water and dried.

The weldability, lacquer adhesion and corrosion resistance of the steel sheets treated as in the above described Examples and Comparative Examples were evaluated by the following testing methods after measurement of the amounts of metallic chromium, metallic tin, metallic nickel and chromium in the hydrated chromium oxide by the fluorescent X-ray method.

The results are shown in the attached Table.

(1) Weldability The weldability is usually evaluated by the available range of secondary current in welding as shown in the report by N.T. Williams (Metal Construction, April 1977, page 157-160), that is to say, the wider the secondary current range in welding, the better the weldability. The upper limit in the available secondary current range cqrresponds to the welding conditions in which some defect such as splashing is found and the lower limit corresponds to the welding conditions in which breakage occurs in the welded part on tearing tests.

However, in oder to determine the available range of secondary current in welding to be used in each sample, a large number of samples are necessary.

Therefore, the weldability was evaluated by the electric contact resistance according to the following method because the electrical contact resistance has an apparent correlation to the available range of secondary current in welding as shown in the report by T.Fujimura (Journal of the Iron and Steel Institute of Japan, vol. 69, No. 13, September 1983, page 181), that is, the lower the electric contact resistance, the wider the secondary current range in welding.

Accordingly, if the electric contact resistance is lower, the weldability is better.

Firstly, the sample was cut to a size of 20mm X 100mm after baking at 210"C for 20 minutes.

The electric contact resistance of the sample was calculated from the change of voltage between a pair of copper disk electrodes (diameter: 65mm, thickness 2mm) wherein 5 amperes of direct current was employed and 50 kg of load was added, when a pair of samples were inserted between the pair of the copper disk electrodes rotating at 5 m/min.

(2) Lacquer adhesion The sample was baked at 210"C for 12 minutes after coating with 60 mg/dm2 of an epoxyphenolic type of lacquer. The coated sample was cut into a circular blank having a diameter of 80 mm by a punch press, and the blank was deeply drawn to form a cup.

The lacquer film in the side wall of a cup was peeled off by an adhesive tape. The adhesion of the lacquer film was divided into 5 grades, namely, 5 was excellent, 4 was good, 3 was fair, 2 was poor and 1 was bad.

(3) Corrosion resistance after lacquer coating The sample was baked at 210oC for 12 minutes after coating with 60 mg/dm2 of an epoxyphenolic type of lacquer. The coated sample was immersed into a solution containing 1.5% of citric acid and 1.5% of sodium chloride for 7 days at 50"C, after the surface of the coated sample was cross-hatched by a razor.

The corrosion in the scratched part of the coated sample was divided into 5 grades, namely, 5 was excellent, 4 was good, 3 was fair, 2 was poor and 1 was bad.

TABLE

Ex.1 Ex.2 Ex.3 Ex.4 Ex.5 Ex.6 Ex.7 Comp Comp. Comp.

Ex.1 Ex.2 Ex.3 Process A A A A A B B - - Amount of metallic chromium (mg/m2) 33 120 105 78 151 291 115 121 - 128 (Bottom layer) Sn Sn Sn Sn Sn Sn Sn Sn Sn Amount of Sn & Ni 492 35 324 64 6 15 31 - 38 34 (mg/m2) Ni Ni Ni Ni Ni Ni Ni Ni Ni (Middle 1 ayer) 0 0 86 42 7 0 0 - 0 0 Amount of hydrated chromium oxide (calculated as 18 8 12 8 6 4 10 8 7 8 chromium) (mg/m2) (Top layer) Electric contact resistance (m#) 5 2 10 18 15 1 4 321 150 301 Lacquer adhesion 4 4 5 5 5 5 4 5 4 5 Corrosion resistance 4 5 5 5 5 5 5 5 4 4 Remarks * bottom layer is metallic tin and middle layer is metallic chromium in Comp. Ex. 3.

Claims (25)

1. A surface-treated steel sheet comprising a steel base having three surface layers thereon, the three surface layers consisting of a bottom layer containing at least 30 mg/m2 of metallic chromium, a middle layer containing at least 10 mg/m2 of metallic tin or at least 10 mg/m2 of tin-nickel alloy containing 20 to 60 weight % of nickel and a top layer of 2 to 18 mg/m2, calculated as chromium of hydrated chromium oxide.

2. A surface-treated steel sheet according to claim 1 wherein the bottom layer contains from 30 to 300 mg/m2 of metallic chromium and the middle layer contains from 10 to 500 mg/m2 of metallic tin or of tin-nickel alloy.

3. A surface-treated steel sheet according to claim 1 or claim 2 wherein the amount of metallic chromium in the bottom layer is from 70 to 1 50 mg/m2, the amount of metallic tin or tin-nickel alloy in the middle layer is from 50 to 300 mg/m2 and the amount of hydrated chromium oxide in the top layer is from 4 to 1 2 mg/m2 calculated as chromium.

4. A surface-treated steel sheet according to any preceding claim wherein the steel base has a thickness of from about 0.1 to about 0.35 mm.

5. A surface-treated steel sheet according to claim 1 substantially as herein described.

6. A surface-treated steel sheet according to claim 1 substantially as herein described with reference to the Examples.

7. A process for preparing a surface-treated steel sheet according to claim 1 which comprises: a) chromium plating a steel base to form a layer of metallic chromium and hydrated chromium oxide thereon, b) tin or tin-nickel alloy plating the chromium plated steel base with a tin or a tin-nickel plating solution under conditions sufficiently acidic to substantially dissolve the hydrated chromium oxide in the solution, and c) forming a layer of hydrated chromium oxide on the tin or tin-nickel plated, chromium plated steel base of step b), steps a), b) and c) above being carried out whereby a surface-treated steel sheet according to claim 1 is obtained.

8. A process according to claim 7 wherein the chromium plated steel base is plated with a tin plating solution at a temperature of 30 to 60"C and under a cathodic current density of 3 to 50 A/dm2, the tin plating solution having a pH of 0.5 to 3.0 and containing 2 to 10 g/l of stannous ion.

9. A process for preparing a surface-treated steel sheet according to claim 1 which comprises: a) chromium plating a steel base to form a layer of metallic chromium and hydrated chromium oxide thereon, b) removing the hydrated chromium oxide formed on the chromium plated steel base by cathodic treatment in an acidic solution, c) tin or tin-nickel alloy plating the chromium plated steel base, and d) forming a layer of hydrated chromium oxide on the tin or tin-nickel plated, chromium plated steel base of step c), steps a), c) and d) above being carried out whereby a surface-treated steel sheet according to claim 1 is obtained.

10. A process according to claim 9 wherein, in step b), removal of hydrated chromium oxide formed on the chromium plated steel base is carried out at a temperature of 30 to 70 C, under a cathodic current density of 2 to 50 A/dm2 and for a treating time of 0.5 to 5 second in an acidic electrolyte containing sulfuric acid, hydrochloric acid, hydrofluoric acid, fluoboric acid and/or fluosilicic acid and having a pH of 0.5 to 2.0.

11. A process according to claim 9 or claim 10 wherein the chromium plated steel base is plated with a tin plating solution at a temperature of 30 to 60"C and under a cathodic current density of 3 to 50 A/dm2, the tin plating solution having a pH of 0.5 to 3.0 and containing 2 to 40 g/l of stannous ion.

1 2. A process according to any one of claims 7 to 11 wherein, in step a), chromium plating of the steel base is carried out at a temperature of 30 to 60"C and under a cathodic current density of 10 to 100 A/dm2 in an electrolyte containing 30 to 300g/l of chromic acid and at least one additive selected from fluorine compounds and sulfur compounds, the amount of said additive being 1 to 5 weight percent based on chromic acid.

1 3. A process according to claim 12 wherein the additive is at least one compound selected from hydrofluoric acid, fluoboric acid, fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, an alkali metal fluoride, ammonium fluoborate, an alkali metal fluoborate, ammonium fluosilicate, an alkali metal fluosilicate, aluminum fluoride, sulfuric acid, ammonium sulfate, an alkali metal sulfate, phenolsulfonic acid, ammonium phenolsulfonate, an alkali metal phenolsulfonate, phenoldisulfonic acid, ammonium phenoldisulfonate, an alkali metal phenoldisulfonate, ammonium sulfite, an alkali metal sulfite, ammonium thiodisulfate, an alkali metal thiosulfate, aluminium sulfate and chromium sulfate.

14. A process according to any one of claims 7, 9, 10, 12 and 1 3 wherein the chromium plated steelbase is plated with a pyrophosphate tin-nickel alloy plating solution at a temperature of 40 to 60"C and under a cathodic current density of 1 to 30 A/dm2, the tin-nickel alloy plating solution having 2 to 40 g/l of stannous ion, 4 to 20 g/l of nickel ion, 0.1 to 3 concentration ratio of stannous ion to nickel ion, 80 to 300 g/l of an alkali metal pyrophosphate and a pH of 8 to 10.

1 5. A process according to any one of claims 7, 9, 10, 1 2 and 1 3 wherein the chromium plated steel base is plated with a tin-nickel alloy plating solution at a temperature of 30 to 60"C, under a cathodic current density of 1 to 30 A/dm2, the tin-nickel alloy plating solution having 2 to 70 g/l of stannous ion, 4 to 80 g/l of nickel ion, 0.1 to 0.8 concentration ratio of stannous ion to nickel ion and pH of 0.5 to 3.

1 6. A process according to any one of claims 7 to 1 5 the wherein the layer of hydrated chromium oxide is formed on the tin or tin-nickel alloy plated steel base by a cathodic treatment in an acidic electrolyte containing at least one compound selected from chromic acid, a chromate and a dichromate of an alkali metal, ammonium chromate and ammonium dichromate.

1 7. A process according to claim 1 6 wherein the cathodic treatment is carried out at a temperature of 30 to 70"C and under a cathodic current density of 1 to 20 A/dm2, using a quantity of electricity of 1 to 40 coulombs/dm2 in an acidic electrolyte containing 5 to 30 g/l of hexavalent chromium ion.

1 8. A process according to any one of claims 7 to 1 5 wherein the layer of hydrated chromium oxide is formed on the tin or tin-nickel alloy plated steel base by a cathodic treatment in an acidic electrolyte containing 10 to 50 g/l of chromic acid and at least one additive selected from fluorine compounds and sulfur compounds, the amount of the additive being 0.2 to 1.0 weight percent based on chromic acid.

1 9. A process according to claim 1 8 wherein the cathodic treatment is carried out at a temperature of 30 to 60"C and under a cathodic current density of 1 to 10 A/dm2 using a quantity of electricity of 1 to 20 coulombs/dm2.

20. A process according to claim 1 8 or claim 1 9 wherein the additive is selected from hydrofluoric acid, fluoboric acid, fluosilicic acid, ammonium bifluoride, an alkali metal bifluoride, ammonium fluoride, an alkali metal fluoride, ammonium fluoborate, an alkali metal fluoborate, sulfuric acid, ammonium sulfate, an alkali metal sulfate, phenolsulfonic acid, ammonium phenolsulfonate, an alkali metal phenolsulfonate, phenoldisulfonic acid, ammonium phenoldisulfonate, an alkali metal phenoldisulfonate, ammonium sulfite, an alkali metal sulfite, ammonium thiosulfate, an alkali metal thiosulfate and chromium sulfate.

21. A process according to any one of claims 7 to 20 wherein the plated base is rinsed with water between each step.

22. A process for preparing a surface-treated steel sheet according to claim 1 substantially as herein described.

23. A process for preparing a surface-treated steel sheet according to claim 1 substantially as herein described with reference to the Examples.

24. A surface-treated steel sheet whenever prepared by a process according to any one of claims 7 to 23.

25. Each and every novel method, process, composition and product herein disclosed.