EP0025396B1 - Process for elaborating a material for manufacturing metal packages, particularly foodcans, constituted by a protected steel sheet - Google Patents

Description

The present invention relates generally to mild steel sheets of the type usually used for the manufacture of metal packaging, and in particular of canned food.

Among the questions raised by the manufacture of tin cans, one relates to their embodiment, the other to the protection of the sheet which constitutes them.

As regards their production, the cans generally have a body closed by two bottoms.

Traditionally, the body is produced by rolling a strip, the lips of which should be assembled which are superimposed at the end of such rolling; the funds both form separate parts of such a body, which it is necessary to then assemble to the latter by crimping.

Admittedly, it is known to ensure the production in one piece of one of these bottoms with the body, by stamping and / or stretching the assembly from the same blank, but this embodiment requires a aptitude for particular shaping.

For the assembly of the lips of a box body formed by rolling a strip, one usually proceeds by stapling, most often with counter-welding, or organic bonding, or by electric welding with a seam.

Compared, in particular, to stapling with counter-welding, electric welding has the advantage of being satisfied with a lower consumption of sheet metal, of requiring no addition of any material, and of leading to favorable minimum thicknesses. proper execution of the subsequent setting of the funds, which explains its current development; unfortunately it can only be used if the treated sheet has a reduced contact resistivity, which in practice limits its field of application to date, as specified below.

It will be recalled that electric welding with a thumbwheel, which constitutes a particular embodiment of resistance welding, in order to produce sealed linear welds, consists in passing the sheets to be welded along the weld line to be produced, between two electrodes. form of knurled tight against each other, and to pass between the knobs of repeated current pulses so that the welding spots partially overlap. This resistance welding mode has particular constraints, linked to running. The clamping pressure passes through a maximum at the point of contact of the rollers, in coincidence with the point of welding; an approach phase, or prior pressurization, cannot be provided to reduce the contact resistances between sheets and electrodes before the current flows. The proximity of the soldering points requires that the current pulses are well calibrated and that the heated zones are well localized, to avoid either erratic overheating or lack of soldering. It is understood, and experience confirms, that the contact resistances between the rollers and the sheets, parasitic resistances, must be low and regular for the welds to be reliable. Already with conventional white irons, in order to prevent the alteration of the surfaces of the knurls causing variations in contact resistance, copper wires running with the sheets and forming electrodes are used, according to French patent No. 1,258,185. auxiliary, making contact with the knobs which have grooves of suitable profile.

With regard to the protection necessary to prevent corrosion of the sheet in service to the detriment of its qualities and / or the flavor of the preserved food, it has been customary until a recent past to ensure it by an operation of tinning, possibly followed by a varnishing.

As is known, the tinned material obtained is usually called tinplate.

It is made of a mild steel sheet coated on the surface with a protective tin layer; the latter may remain free or, by combination with the iron of the underlying sheet, be transformed in whole or in part into an alloy of iron and tin, for example by reflow.

But the current cost of tin, due in particular to an exhaustion of world resources in this metal, has now favored another method of protecting such a steel sheet, which consists in applying to it an electrolytic treatment operation. , by passing through a chromium plating bath formed from a dilute aqueous solution of chromic anhydride containing, for example, sulfuric acid as catalyst, referred to below, for convenience, chromium plating operation.

Such a protection process is notably described in French patent N ° 1,365,368.

It leads to the formation of a material, commonly known as TIN FREE STEEL, (literally: tin-free steel), by the abbreviation TFS, formed from a sheet of mild steel coated on the surface with a protective layer containing metallic chromium and chromium oxide.

In addition to the tin economy to which it leads, such a material has various advantages, and in particular that of good adhesion of varnishes.

However, it has a major drawback, which is a particularly high contact resistivity, making any electrical welding with a knurl impracticable.

This is the reason why, in canned food, its use is currently reserved for the manufacture of bottoms and certain stamped bodies, while electric welding with a seam is only practiced with tinplate, except in cases where we agree to apply an additional preparation operation to the TFS.

In general, the subject of the present invention is a process for obtaining a material comprising, like TFS, a composite layer of metallic chromium and chromium oxide, but having in particular good aptitude for electric welding with a seam thanks to the limitation of the chromium oxide content of its coating and the presence of a thin tin underlay.

In addition, the materials according to the invention have a corrosion resistance greater than that of TFS due to the formation by diffusion in the solid state of a layer of tin-iron alloy during a heat treatment for polymerization of organic coatings.

Admittedly, a combination of a tin-based layer and a chromium-chromium oxide-based layer has already been proposed, but it is remarkable that the ability to weld electrically with a thumb wheel 'was not mentioned correlatively, despite the known interest of this process in the manufacture of metal packaging.

In the French patent application filed on August 18, 1977 under No. 77 25340 and published under No. 2,362,056, on the one hand, the tin-based layer forming a first protective layer, is remelted and is therefore made of an iron tin alloy; and, on the other hand, the coating which surmounts this first protective layer, and which itself forms a second protective layer resulting from a chroming operation of the type of that explained above, consists essentially of oxide of chromium, the metallic chromium content of this coating preferably being zero and always less than 0.005 g / m ² , ie 0.5 _kt g / cm ² , of surface treated.

If such an arrangement can effectively lead, as sought in this case, to materials having a good aptitude for organic bonding and a good aptitude for forming, it is not the same as regards electric welding, which n is also not envisaged.

Indeed, the preferential presence of chromium oxide in the surface coating of this material leads for the latter to resistivity values incompatible with such an electric welding, except before proceeding to a difficult and expensive grinding of the coating.

The same applies to the material described in the French patent application filed on August 24, 1977 under No. 77 25886 and published under No. 2,362,943, which differs from the previous one only in that the tin-based underlay consists of free tin.

In Japanese Patent No. 48 35136, such a layer based on free tin forms the surface coating of the material concerned, the layer based on chromium and chromium oxide then forming an underlayer underlying it. coating; but, in this case, the establishment of the tin layer can only be done to the detriment of the protective performance expected elsewhere from the undercoat based on chromium and chromium oxide.

In the French patent application filed on March 31, 1978 under No. 78 09425 and published under No. 2,385,818, the first protective layer is exclusively an alloy of tin and iron, the tin initially deposited being systematically remelted for this purpose, and a chromium-based passivation coating is applied to the underlay thus produced.

This coating, the characteristics of which are not moreover specified, is obtained either by passage through a conventional cathodic passivation bath of electrolytic white iron based on sodium dichromate, or by passage through a conventional chromium plating bath of the type of that indicated above for obtaining a TFS material.

In the first case, the deposit based on chromium and chromium oxide obtained is so small that the corrosion resistance which results therefrom is insufficient; in the second case, the deposit obtained must, without others, have a chromium oxide content which is too high for an electrical welding of the material obtained to be effectively possible.

French patent application No. 70 23705 published under No. 2,053,038 relates to the deposition of mixed chromium / chromium oxide coatings on sheet substrates, in particular zinc-plated or tinned, with the essential aim of improving the resistance to corrosion, the chromium / chromium oxide weight ratio being able to vary within wide limits, 4/1 to 1/30. It appears from this request that the study of mixed coating deposits focused on zinc-plated substrates as a priority. In particular on such substrates, the mixed coating would provide, without specifying the conditions for mixed coating, in addition to an improvement in corrosion resistance, less aggressiveness with respect to spot welding electrodes, a conventional process for sheet metal. 'steel. It can be understood that this refers specifically to the difficulties of welding galvanized sheets, where the volatilization of zinc causes degradation of the electrodes. Furthermore, with regard to tin covering, the cited application envisages only improvements in the corrosion resistance of tin-plated sheets, with tin layers of conventional thickness for white irons, and without distinction tin not alloyed or alloyed by fusion with the underlying iron.

The present invention more specifically relates to a process for the protection of a sheet of mild steel leading to a material which can be welded by means of the wheel, the protective coating comprising a tin underlayment of very reduced thickness.

To this end, the invention provides a process for the preparation of a material for the manufacture of metal packaging, in particular cans, from mild steel sheets, the material having, with good corrosion resistance, a electrical contact resistance sufficiently low to allow electrical seam welding during the manufacture of packaging, a process according to which a layer of tin of between 0.1 and 1.5 g / m ² is deposited on the surface of the sheet surface covered, and on the tinned sheet a composite layer of chromium metal and chromium oxide, characterized in that, on the tin layer not alloyed with steel, the said composite layer is deposited so that it contains at least 5 micrograms of chromium metal per square centimeter of covered surface (5 µg / cm ² ) and between 0.6 and 2.5 µg / cm ² of chromium in the form of oxide, limits included.

It is specified that the limits of thickness of the tin layer, expressed in the unit usually used by those skilled in the art, correspond, for the lower limit, to the minimum essential for obtaining the technical result sought, and, for the upper limit , to what should not be exceeded to save tin, the reduction in contact resistivity, with the specified composition of the chromium-based composite layer, becoming insignificant beyond. However, it should be noted that the layer of tin must not have given rise to iron-tin intermetallic compounds.

Preferably the composite layer will contain at most 10 µg / cm2 of chromium metal.

Tinning can be achieved by electrolysis; it is a known technique which will not be described here in all its details.

It will suffice to recall that it is usually carried out after degreasing in an alkaline medium, rinsing with water, pickling in an acid medium and a new rinsing with water, and that it uses any electrolyte of acid tinning, such as stannous sulfate, stannous chloride and stannous fluoborate for example, or any alkaline tinning electrolyte, such as sodium stannate or potassium stannate for example.

• - concentration du bain en ions stanneux : 1,5 à 40 g/1
• - concentration du bain en acide, qui peut être de
  l'acide sulfurique ou un acide phénolsulfonique par exemple : 1,0 à 20 g/I équivalents H₂SO₄
• - concentration du bain en produits d'addition,
  sulfones, diphones, ou produit connu sous la dénomination
  ENSA par exemple : inférieure à 30 g/1
• - température du bain : 30 °C à 60 °C
• - densité du courant : 5 à 50 A/dm²

With an acidic tinning electrolyte, the tinning conditions are preferably as follows:

• - concentration of stannous ions in the bath: 1.5 to 40 g / 1
• - concentration of acid in the bath, which can be
  sulfuric acid or a phenolsulfonic acid for example: 1.0 to 20 g / I H ₂ SO ₄ equivalents
• - concentration of the bath in addition products,
  sulfones, diphones, or product known by the name
  ENSA for example: less than 30 g / 1
• - bath temperature: 30 ° C to 60 ° C
• - current density: 5 to 50 A / dm ²

The steel sheet, previously degreased, pickled and rinsed is treated as a cathode, the anode being made of pure tin.

But the tinning can advantageously be carried out by a tinning pickling, as described in particular in French patent N ° 1,500,185 of August 8, 1966.

In this case, it is simply preceded by degreasing in an alkaline medium and rinsing with water, the acid pickling being carried out simultaneously with the desired tinning.

The advantage of such a tinning pickling is in particular to lead to a slight deposit of free tin, particularly adherent and uniform, and well suited to obtaining the desired material according to the invention.

The tinning pickling bath used consists, like an electrolytic tinning bath, of an aqueous acid solution containing stannous ions, but with a lower concentration of stannous ions and a higher acid content; the steel sheet, previously degreased and rinsed, is treated therein as a cathode, as previously, but, this time, the anode is made of a material having a high resistance to anodic dissolution, graphite or ferrosilicon for example.

• - concentration du bain en ions staneux : 0,3 à 1,5 g/I
• - concentration du bain en acide, qui peut être
  de l'acide sulfurique ou un acide phénosulfonique : 15 à 100 g/I équivalents H₂SO₄
• - température du bain : 20°C à 80 °C
• - densité de courant : 10 à 60 A/dm²

The implementation conditions are preferably as follows:

• - bath concentration in stanous ions: 0.3 to 1.5 g / I
• - concentration of acid in the bath, which can be
  sulfuric acid or phenosulfonic acid: 15 to 100 g / I H ₂ SO ₄ equivalents
• - bath temperature: 20 ° C to 80 ° C
• - current density: 10 to 60 A / dm ²

In practice, it is important that the concentration of hydrogen ions from the acid in the bath be large enough with respect to the concentration of stannous ions for the cathode current yield of deposited metal to be low, less than 50%, and preferably between 5 and 25%.

In all cases, electrolytic tinning or tinning pickling, no remelting of the deposited tin is practiced, this tin therefore remaining in the form of free tin, that is to say in the form of unalloyed tin.

Chrome plating in principle can also follow a known technique: the chrome plating bath used is then an electrolytic bath of the type described in French patent No. 1,365,368 mentioned above, that is to say say an electrolytic bath which is formed of a dilute aqueous solution of chromic anhydride, containing for example sulfuric acid as catalyst, and in which the worked sheet is taken as cathode.

A person skilled in the art knows, with such a bath, exactly controlling the amount of metallic chromium deposited as a function of the experimental conditions used, and in particular the current density, this amount of metallic chromium deposited according to the law of Faraday with a relatively poor but known and constant yield.

It is therefore easy to ensure that the quantity of metallic chromium deposited is that provided for according to the invention, as specified above.

A person skilled in the art also knows how to obtain compliance with the contents, specified above, of the composite layer, of chromium oxide, either that it conducts the chromium-plating operation accordingly, or that the latter results in a value higher than that which is sought, it implements a corrective process of dissolution by more or less prolonged stay of the treated sheet in the chromium plating bath without current flow, capable of bringing this content to the desired value.

According to the invention, it is preferable to operate with a chromic anhydride chromating bath, optionally with sulfuric acid as catalyst, the content expressed as chromium being between 25 and 50 g / l. In addition, it is preferable that the content of the trivalent chromium Cr- ³ bath, especially at the start of the chromium plating operation, is at most equal to 0.5 g / l, to avoid the appearance of coloring zones (layers interference). We can therefore provide, at least for the first bath if the chromium plating is carried out in several successive baths, a continuous circulation of the bath with passage over cation exchange resins, to maintain its concentration of Cr ⁺³ ions at a value lower than the previously mentioned limit.

In addition, it is preferable that the chromium plating begins as soon as the tinned sheet is immersed in the chromating bath, the sheet being cathodic before entering the bath, or in the first bath if several successive baths are used.

These operating particulars have been specified by the Applicant during industrial development tests.

In all cases, it is sought to reduce the chloride content of the chromium plating bath as much as possible, in order to avoid excessive deposition of chromium oxide, favored by the presence of chloride ions.

In all cases, the chrome plating operation applied to the tinned sheet is, as usual, followed by at least one rinsing, drying and surface lubrication, for example using sebacate of dioctyle or cotton oil.

Before completing the description of the invention with examples, the contact resistivity test method will be specified by which the suitability for seam welding is determined.

Measuring, at 25 ° C, the voltage drop between two copper electrodes having a contact surface of 10 mm ² well erected, applied on either side of a sample; the measurement is carried out when a direct current of 1 ampere flows between the two electrodes and a contact pressure gradually applied to them is applied up to 1,400 bars (ie a clamping force of 1,400 newtons). It is essential that before the current flow the force has not exceeded the specified value, to reproduce conditions similar to those of a seam welding.

As the TFS type materials are still used after varnishing their two faces, two resistance measurements are carried out, one on the material leaving the manufacturing process, and the other on a sample of the same material after baking for 30 min at 200 °. C, simulating the curing of varnish polymerization.

The two resistances of the material, in the state and after baking will be noted respectively R ₂₅ and R _2oo . It will be noted that during the baking, the tin not originally alloyed partially diffuses into the steel substrate to give intermetallic iron-tin compounds, so that, predictably, the resistance R ₂₀₀ will be greater than resistance R ₂₅ .

As, under the usual conditions for the manufacture of metal packaging such as cans made of TFS materials, seam welding will take place after varnishing, it is the resistance R ₂₀₀ which will be indicative of the ability to weld electrically. the wheel. In correlation with practical trials welder seam, it was established that this welding became random and inconsistent with serial manufacturing for superior resistance to 100 x 10 ⁵ ohms, and practically impossible for higher resistance 1000 _X 10- ⁵ ohms (resistances measured under the conditions specified above on the material at the time of use).

By way of nonlimiting examples, various examples of the processes leading to a tinned material according to the invention will be given below:

Example 1

A cold reduced mild steel sheet having a thickness of 0.21 mm is electrolytically degreased in a sodium carbonate solution at 10 g / l, then this sheet is pickled in dilute sulfuric acid.

• - Composition du bain d'électrolyse :
  • Etain stanneux : 30 g/I
  • Acide paraphénolsulfonique (en équivalent H₂SO₄) : 15 g/I
  • Ethoxylated Alpha-Naphtol Sulfonic Acid (ENSA) : 2 g/l
• - Température du bain d'électrolyse : 45 °C
• ― Densité de courant cathodique : 5 A/dm²

After rinsing with water, the sheet is subjected to an electrolytic tinning operation under the following conditions:

• - Composition of the electrolysis bath:
  • Stannous tin: 30 g / I
  • Paraphenolsulfonic acid (in H ₂ SO ₄ equivalent): 15 g / I
  • Ethoxylated Alpha-Naphtol Sulfonic Acid (ENSA): 2 g / l
• - Temperature of the electrolysis bath: 45 ° C
• - Cathodic current density: 5 A / dm ²

Under these conditions, the weight of the coating of free tin deposited is 0.25 g / m ² .

• - Composition du bain d'électrolyse :
  • Anhydride chromique : 60,0 g/I
  • Acide sulfurique : 0,6 g/I
• - Température du bain d'électrolyse : 50 °C
• ― Densité de courant cathodique : 55 A/dm²

After rinsing with water, the sheet thus covered with a layer of free tin is subjected to an electrolytic chromium plating operation, by cathodic route, under the following conditions:

• - Composition of the electrolysis bath:
  • Chromic anhydride: 60.0 g / I
  • Sulfuric acid: 0.6 g / I
• - Temperature of the electrolysis bath: 50 ° C
• - Cathodic current density: 55 A / dm ²

Under these conditions, the layer based on chromium and chromium oxide deposited contains 9.2 μg / cm ² of metallic chromium and 2.1 μg / cm ² of chromium oxide (adjusted by redissolution).

After chromium plating, the treated sheet is rinsed with water, dried, and coated with a thin film of dioctyl sebacate.

The electrical resistance of the material obtained is, under the experimental conditions specified above, of

Example II

The experimental conditions are substantially the same as those of Example I, with the following modifications:

• - Pour le chromage la composition du bain est :
  • Anhydride chromique : 82 g/I
  • Acide sulfurique : 0,8 g/I
  • tons chrome trivalent : 0,2 g/I
• - Température du bain : 52 °C
• ― Densité de courant cathodique : 48 A/dm²

For tinning, the cathodic current density is 20 A / dm ² , and the weight of the deposited tin coating is 1.05 g / m ² .

• - For chrome plating the composition of the bath is:
  • Chromic anhydride: 82 g / I
  • Sulfuric acid: 0.8 g / I
  • trivalent chrome tones: 0.2 g / I
• - Bath temperature: 52 ° C
• - Cathodic current density: 48 A / dm ²

Under these conditions, the composite layer comprises 7.4 μg / cm ² of chromium metal and by subsequent dissolution, the content of chromium oxide is reduced to 1.6 μg / cm ² .

The resistances measured are:

Example III

• - Composition du bain d'électrolyse :
  • Sulfate stanneux : 63,3 g/I
  • Acide paraphénolsulfonique (en équivalent H₂SO₄) : 15 g/I
  • Sulfones 6 g/l
  • Agent mouillant : 0,1 g/l
• - Température du bain d'électrolyse : 40 °C
• - Densité de courant cathodique : 8 Aldm²

The experimental conditions are the same as those of Example I, except as regards those of the tinning operation, which are as follows:

• - Composition of the electrolysis bath:
  • Stannous sulphate: 63.3 g / I
  • Paraphenolsulfonic acid (in H ₂ SO ₄ equivalent): 15 g / I
  • Sulfones 6 g / l
  • Wetting agent: 0.1 g / l
• - Temperature of the electrolysis bath: 40 ° C
• - Cathodic current density: 8 Aldm ²

Under these conditions, the weight of the coating of free tin deposited is 0.25 g / m ² .

The metallic chromium content of the chromium and chromium oxide layer deposited during the following chromium plating operation is 7.1 µg / cm ² and its chromium oxide content of 1.9 µg / cm ² .

The electrical resistances of the material obtained are:

Example IV

• - Composition du bain d'électrolyse :
  • Sulfate stanneux : 1,2 g/I
  • Acide, par exemple acide sulfurique : 100 g/l
• - Température du bain d'électrolyse : 35 °C
• ― Densité de courant cathodique : 15 A/dm²

The experimental conditions are the same as those of Example I, except as regards those of the tinning operation, which is ensured by electrolytic tinning stripper, under the following conditions, so that the sheet treated n '' is beforehand the object only of a degreasing followed by a rinsing:

• - Composition of the electrolysis bath:
  • Stannous sulphate: 1.2 g / I
  • Acid, for example sulfuric acid: 100 g / l
• - Temperature of the electrolysis bath: 35 ° C
• - Cathodic current density: 15 A / dm ²

Under these conditions, the weight of the coating of free tin deposited is 0.3 g / m ^2.

The metallic chromium content of the chromium and chromium oxide-based layer deposited during the following chromium plating operation is 7.0 µmg / cm ² and its chromium oxide content of 1.9 µg / cm ² .

The electrical resistances of the product obtained are:

It can be seen by the preceding examples, that the resistor R ₂₅ is less than 10 x 10- ⁵ ohms, and the resistance R ₂₀₀ of less than 100 x 10- ⁵ ohms.

Comparative tests were carried out between a conventional tinplate, samples of materials obtained according to examples II and IV, samples of materials produced according to the teachings of application 78 09425, that is to say by forming an intermetallic compound. iron-tin by remelting on line between the tinning operations and chromium plating (noted prior art) and samples of TFS, respectively in the state (noted TFS-1) and after selective removal of the chromium oxide layer (noted TFS-2), by analytically measuring the thickness of the layer of unalloyed (or free) tin after baking at 200 ° C for 30 minutes.

The results are reported in the following table:

It will be noted that the materials obtained by the process according to the invention show, after baking, a resistance at most of the same order as the resistance of the material of the prior art in the raw state, this under conditions where the baking has caused the apparently complete disappearance of free tin, while the material of the prior art, by steaming, saw its contact resistance considerably increase. It would therefore seem that the presence of free tin during the deposition of the composite layer leads to an improvement in the electrical weldability with the thumbwheel, even when the free tin has disappeared, or at least when this free tin is no longer detectable. .

It is not without interest to note that the TFS samples used in the tests had undergone chromium plating under the same conditions as those of Example IV. In addition to the results confirming that the combination of free tin, metallic chromium, chromium oxide has effects without comparison with the added effects of the elements isolated or taken in pairs, it will be understood that an installation intended to carry out the process according to he invention is capable of producing TFS in its part relating to chromium plating.

Of course, the invention is not limited to the examples described, but embraces all of the variant embodiments. In particular, the conditions for depositing the tin or the chromium chromium oxide composite layer may be subject to variants such that a person skilled in the art will know how to use them on the basis of the data indicated in the description.

For example, it is possible to envisage depositing a different quantity of tin on the two faces or even a conventional tinning on one of the faces.

On the other hand, it is quite obvious that, if the method according to the invention aims to obtain a material having particular aptitudes for electric welding with a seam. the invention is not limited to the use of this material in assemblies by seam welding.

Claims (7)

1. A process for the production from mild steel sheets of a material suitable for the manufacture of metal containers, in particular food cans, the material possessing, together with good corrosion resistance, an electrical contact resistance which is sufficiently low to permit roller spot welding during the manufacture of the containers, in which a layer of tin of from 0.1 to 1.5 g/m² of the surface area of the sheet steel is deposited on the surface of the sheet steel, and then a composite layer of metallic chromium and chromium oxide is deposited on the tin-coated sheet steel, characterized in that on the layer of tin, which is unalloyed to the steel, the said composite layer is deposited so that it contains at least 5 micrograms per square centimetre (5 mg/cm²) of the surface area of the coated material of metallic chromium and from 0.6 to 2.5 mg/cm² of chromium in oxide form, inclusive of the limits.

2. A process according to Claim 1, characterized in that the composite layer is deposited so that it contains at most 10 mg/cm² of metallic chromium.

3. A process according to Claim 1 or 2, in which the deposit of the composite layer comprises passing the tin-coated sheet steel, which acts as the cathode, through an electrolysis bath containing a dilute solution of chromium trioxide, characterized in that, after the composite layer of metallic chromium and chromium oxide has been deposited, the content of chromium oxide in this layer is reduced to within the specified limits.

4. A process according to any one of Claims 1 to 3, wherein the deposit of the composite layer comprises passing the tincoated sheet steel, which acts as the cathode, through at least one electrolysis bath containing a dilute solution of chromium trioxide, characterized in that in at least the first electrolysis bath, the content of Cr⁺³ ions is kept at a value of at most 0.5 g/litre.

5. A process according to Claim 4, characterized in that the specified content of Cr³⁺ ions is maintained by passing the bath over a cation exhange resin.

6. A process according to Claim 4 or 5, characterized in that the total content of chromium in the bath, calculated as chromium, is from 25 to 50 g/litre, inclusive of the limits.

7. A process according to any one of Claims 4 to 6, characterized in that the tin-coated sheet steel is immersed, under tension relative to an anode, at least in the first bath.