EP1332245A1

EP1332245A1 - Method and device in connection with pickling

Info

Publication number: EP1332245A1
Application number: EP01975097A
Authority: EP
Inventors: Anders Eklund; Fritz Bark; Noam Lior; Nulifer Ipek
Original assignee: AvestaPolarit AB
Current assignee: Outokumpu Stainless AB
Priority date: 2000-10-20
Filing date: 2001-10-05
Publication date: 2003-08-06
Also published as: AU2001294456A1; SE0003824L; WO2002033154A1; SE519159C2; SE0003824D0

Abstract

Method and device for electrolytic, continuous treatment of a continuously formed material (1) of stainless steel, said material being made to run in an electrolyte (34) between electrodes (35, 36) lying in series, under the influence of a direct current with alternating polarity, every other electrode being anodic and every other being cathodic and every electrode being matched by an electrode of the same polarity on the opposite side of the material (1), an oxide surface layer being removed from the material in the method. According to the invention, said electrodes (35, 36) lying in series are partially, electrically insulated from each other in the electrolyte (34) by partial insulators (37) being arranged between after each other arranged electrodes of opposite polarity, preferably both on a first side and on a second side of the material (1).

Description

METHOD AND DEVICE IN CONNECTION WITH PICKLING

TECHNICAL FIELD

The present invention relates to a method of electrolytically, continuously treating a continuously formed material of stainless steel, the material being made to run in an electrolyte between electrodes lying in series, under the influence of a direct current with alternating polarity, every other electrode being anodic and every other being cathodic and every electrode being matched by an electrode of the same polarity on the opposite side of the material, an oxide surface layer with a thickness of at least 1 micrometre being removed from the material in the method. Typically, the treatment is part of a production line for stainless strip steel, which production line also comprises the steps of hot-rolling and/or cold-rolling, and annealing, during which steps said oxide surface layer is formed. The invention also relates to a device for the performance of the method according to the invention.

PRIOR ART AND PROBLEMS

When manufacturing continuously formed metal products of thin dimensions, i.e. primarily strip, from a stainless steel material, hot-rolling is generally carried out followed by the final stage, cold-rolling. The operation in that case is first hot-rolling of the material at high temperature, at which the material softens and can be rolled to a thickness of around 5-25 mm. Owing to the high temperature, an oxide scale of a thickness of around 50-500 micrometres is formed on hot-rolling, which scale consists of a mixed oxide, usually spinel, comprising at least iron and chromium. Before the material is worked further, the oxide scale has to be removed. A pretreatment can be carried out for this purpose, consisting of annealing at around 1000-1200°C, which softens up the surface, followed by cooling and then blasting which breaks up the oxide scale. After this, a pickling process is normally used, which is an electrolytic treatment stage in an electrolyte consisting of one or more mineral acids or a neutral solution and an ensuing chemical stage. What happens is that the electrolyte/acids penetrate down into cracks in the oxide scale and dissolve the chromium-depleted zone, at which the oxide scale loosens. Following rinsing, the material has a dull, so-called pickled surface. To produce thinner dimensions, the treatment continues with cold-rolling, at which the material becomes hard and brittle due to the formation of martensite. To restore the correct material attributes, stress-relieving annealing at around 1000-1200°C follows, at which however a surface layer again appears of a mixed oxide of the spinel type, this time with a thickness of around 10 micrometres. Since the oxide does not have the right stainless properties, this is also pickled away in the same way as in the earlier process. Following rinsing, the material has a dull, pickled surface. In certain cases, e.g. if further forming operations are to be carried out, the pickled surface is an advantage, but often a bright surface is desired instead, which is then produced by means of bright annealing in a reducing atmosphere, followed by smoothing rolling with only around 2% reduction of the material thickness. Blasting is not used on cold- rolled material, as the surface is destroyed by this. Instead, so-called neolyte treatment is used, which involves an electrolytic treatment with direct current, usually in sodium sulphate, at which chromium(III) is oxidized to chromiumfNI), which is easily soluble.

In SE 9800287-6 there is described a method of electrolytically, continuously treating a continuously formed material of stainless steel, at a current density of 0.1 - 3 A/cm², an oxide surface layer with a thickness of at least 1 micrometre being removed from the material. In the method there is achieved a selected surface conditioning effect in the same stage, comprising that a selected surface fineness is achieved, the method being performed by use of an electrolyte comprising sulphuric acid or salt thereof and/or phosphoric acid, or an electrolyte comprising nitric acid, the treatment stage being executed in one or more electrolytic cells lying in series, the material being made to run in the electrolyte between electrodes lying in series, under the influence of a direct current with alternating polarity, every other electrode being anodic and every other being cathodic and every electrode being matched by an electrode of the same polarity on the opposite side of the material.

A problem in connection with electrolytic pickling, in which a continuously formed material is brought to run between electrodes of alternating polarity, has however proven to be that only a minor part of the applied current for the electrolysis runs through the continuously formed material, the main part of the current instead taking the route directly from cathode to anode, via the electrolyte, i.e. without passing the steel strip which is intended to be treated. It has been shown that normally a current efficiency of only 30% is attained, the rest constituting a loss due to short circuiting between anodes and cathodes. This means that unnecessary high voltages have to be applied over the electrodes, in order to achieve the desired pickling effect, which in turn means a considerable loss of energy. Alternatively, the run-through velocity of the continuously formed material must be lowered in order to achieve the desired pickling effect, which means that the pickling stage in the treatment line for the steel often will constitute a bottle-neck in the production. It is known from US 5,449,447 and US 5,795,460 to arrange anodes and cathodes, in the continuous treatment of carbon steel, in pickling chests which are completely separated from each other, which pickling chests exhibit an overflow that mouths in a common underlying collection chest for the electrolyte. Thanks to the shown apparatus assembly, there is achieved a close to complete electric insulation between anodes and cathodes, whereby short circuiting them between is avoided. This is however achieved at the price of a very complicated apparatus assembly, which consequently means a costly capital investment in completely new equipment, i.e. it can not be achieved by reconstruction of existing equipment for pickling. Moreover, the maintenance ought to be rendered more difficult and in addition the flexibility of the assembly is low, e.g. in connection with the treatment of varying types of continuously formed materials having different amounts of oxide. Furthermore, the shown process is mainly intended to constitute a pretreatment step before a main treatment which e.g. may consist of galvanisation.

In US 5,840,173 there is shown continuous pickling of stainless steel in connection with rolling. The pickling takes place in hydrochloric acid at a current density of 3-40 A/dm^" and a temperature of 50-95°C, in a container which comprises separate compartments for alternating pairs of anodes and cathodes, respectively. The document does however not discuss why the cells/electrodes are arranged in this way, or how the separation is performed in practice.

In US 5,804,056 too, there is shown continuous pickling of stainless steel in connection with rolling. The pickling takes place in sulphuric acid and it is shown in Fig. 1 A that a cell is used which only contains three pairs of anodes, followed by a cell which only contains one pair of cathodes. The document does not discuss the reason for this configuration any further.

In US 4,129,485 there is shown continuous pickling of carbon steel in connection with a rolling line. The pickling takes place with contact polarisation of the strip and the strip is made to run through a plurality of treatment chests which are equipped alternating with cathodic and anodic electrodes, respectively. The conditions in the pickling is that the bath consists of NaCl, the temperature is 80-100°C and the current density is 0.5-1.0

A/cm²

A major problem in connection with the complete separation between different pickling chests that each only contains one of the electrode types, is that none or at least lessened equalisation takes place between the different pickling chests. The consequence of this is that elevated concentrations of e.g. iron ions are built up and that elevated amounts of sludge are built up in separate pickling chests which constitute anodic cells. The concentration of the active components in the electrolyte too will be non-equal between the different pickling chests. All this results in that the pickling risks to be heavily impaired, in spite of the advantage which is achieved thanks to the electric insulation.

Another problem in connection with the pickling of continuously formed materials, especially strip materials, is that the material quite often is in greater need of pickling on the underneath side than on the top side. This puts requirements on different pickling on the underneath side and the top side, respectively, which may be hard to achieve with pickling equipment normally utilised today.

ACCOUNT OF THE INVENTION The present invention aims at addressing the above complex of problems. Especially, the invention aims at offering a method and device in connection with pickling of a continuously formed stainless material in connection with a production line for stainless strip material, which production line also comprises hot-rolling and/or cold-rolling of the strip. It is especially preferred that the pickling is adapted to be able to yield a selected surface conditioning effect in the same stage, comprising that a selected surface fineness is achieved.

The invention further aims especially at providing a method and device in connection with the pickling of a stainless continuously formed material, said material being made to run in an electrolyte between electrodes lying in series, under the influence of a direct current with alternating polarity, every other electrode being anodic and every other being cathodic and every electrode being matched by an electrode of the same polarity on the opposite side of the material, a partial electric insulation being achieved at the same time, between electrodes of opposite polarity arranged after each other, by partial insulators being arranged them between. Such insulators are preferably arranged between each after each other arranged pairs of electrodes, suitably on both sides of the material, i.e. on a first side of the material and on a second side of the material. Preferably, these insulators are, at least on the first side of the material, displaceable for a varying insulating effect in the electrolyte between the electrodes that are arranged on said first side of the material. Thereby, there is also provided prerequisites for the achieving of a good, but variable, mixing in the electrolyte bath which is common for the entire pickling chest. By the term "displaceable" is in this connection meant that the effective insulating area of the insulator can be changed in the electrolyte bath. This may be achieved in many ways, as will be described in greater detail further down. Typically, the position of the insulator and its area between the electrodes may be changed so that the electrodes are variably shielded from each other. The insulating area may, whether the insulator is fixed or arranged to be displaceable, be 20-95%, preferably 30-90% and even more preferred 40-85%, counted on the total cross sectional area in the electrolyte bath on one side of the strip.

The term "electrode" here and in the following means one single electrode or a bunch of electrodes, the latter being the most usual in the industry and meaning that each bunch of electrodes in practice operates as a single electrode in that one bunch of electrodes has a specified polarity (cathode or anode). The electrodes may e.g. be made of lead, titanium, stainless steel or Si-steel.

In connection with the present invention, it has surprisingly been shown that a very good improvement may be achieved even with an electric insulation between electrodes of opposite polarity in the electrolyte, which insulation is not complete. This enhanced current efficiency may therefore be attained at the same time as a good mixing in an electrolyte bath in a pickling chest which is common for all electrode pairs. The reason for this being possible is that it, in connection with the development of the invention, has been shown that the potential in the electrolyte drops exponentially with the distance to the electrode. Therefore, electrodes of opposite polarity may be electrically insulated/shielded from each other in one and the same electrolyte bath, by introducing insulators which need not extend more than just beyond the electrodes, as seen in their extension in the direction towards the continuously formed material. Closer to the continuously formed material, the potential has dropped so much that the current never the less rather seeks its way into this material than takes the short circuit route directly between the electrodes.

According to one aspect of the invention, said insulators are made of an electrically insulating material, preferably chosen from the group that consists of polymeric and ceramic materials, most preferred plastics, rubber, ceramics or plexiglass. Such insulators are arranged according to the invention between all or essentially all electrode pairs in the device. The solution according to the invention brings about many advantages. The primary advantage is, as has been stated, that short circuiting is prevented to a great extent, whereby the current efficiency is improved, which in turn leads to an optimised pickling process with a lowered energy consumption and/or enhanced capacity. At the same time there is achieved a good mixing and concentration equalisation in the electrolyte bath. These advantages may furthermore be achieved according to the invention with a relatively simple apparatus construction which is also suitable for so called "retrofit", i.e. the reconstruction of existing equipment, with comparatively small changes in the equipment. In the preferred embodiment of the invention, the electric insulation and the concentration equalisation in the electrolyte are furthermore controllable, thanks to displaceable insulators, which gives a valuable flexibility to the set-up. As an example, it may be mentioned that if one desires to pickle a steel strip having a heavy but easily pickled oxide layer which gives rise to a large formation of sludge in the electrolyte, there may be used only a minor shielding between the electrodes, an thus an enhanced concentration equalisation, while one, if one desires to pickle a steel strip having a less heavy but hard pickled oxide layer which does not give rise to large amounts of sludge, may use a larger, perhaps almost complete, shielding between the electrodes.

Advantageously, the invention is used in connection with a pickling process of the type which is described in SE 9800287-6, which document is hereby incorporated by reference. Accordingly, this means that the pickling is performed so that there may be achieved a selected polishing effect of the steel in the same step, instead of the dull, pickled surface which is normally achieved in pickling. Preferably, there is used an electrolyte comprising sulphuric acid or salt thereof and/or phosphoric acid and optionally hydrofluoric acid or salt thereof. The most preferred range is 0-95 volume-% sulphuric acid and 5-100 volume-% phosphoric acid, but it is to be realised that a great number of other varieties are also possible, according to what is described in SE 9800287-6. The electrolyte may also mainly comprise nitric acid, which however signifies an undesirable negative effect on the environment. It should however be realised that the invention is not limited to the use of these electrolytes, but works well for all types of electrolytes.

According to a further aspect of the invention, the treatment time is 2 sec - 5 min, preferably 10 sec - 3 min and even more preferred 30 sec ~ 2 min. Suitably, the treatment time is 30 sec - 5 min, preferably 1 min - 3 min and even more preferred around 2 minutes when the starting material is hot-rolled and annealed, or 2 sec - 2 min, preferably 5 - 90 sec. and even more preferred 10 - 60 sec. when the starting material is cold-rolled and annealed, with the treatment being able to be carried out in one stage or divided into two or more stages. The anodic current density during electrolysis is 0.1-3 A/cm², preferably 0.3-2.5 A/cm² and even more preferred 0.5-2 A/cm², and the temperature is 50-100°C, preferably 60-90°C and even more preferred 65-80°C.

The stainless material which is pickled may be ferritic, martensitic, duplex, austenitic or superaustenitic, for example. The composition for these stainless types of steel is defined in "Stainless Steel, The New European Standards", 2^nd Edition, 1997-02, Avesta Sheffield, but other types of stainless steel can also be treated according to the invention.

According to yet another aspect of the invention, the treatment according to the invention can be preceded and/or followed by a chemical surface treatment with mixed acid in one or more cells, preferably using nitric acid and hydrofluoric acid.

SHORT DESCRIPTION OF DRAWINGS

In the following, the invention will be described in greater detail, with reference to the drawings, of which:

F Fiigg.. 1 1 is schematically showing the current distribution in an electrolysis chest according to prior art, without any insulation between anode and cathode,

Fig. 2 is showing a preferred line for annealing including following pickling according to the invention,

Fig. 3 is schematically showing a pickling chest with a lid, which comprises insulators according to the invention,

Fig. 4 is showing a cross section of a device according to a first, preferred embodiment of the invention,

Fig. 5 is showing yet two conceivable embodiments of the invention,

Fig. 6-8 is showing the concentration of iron, chromium and nickel, respectively, at different degrees of insulation,

Fig. 9 is showing the current in the strip as a function of the total amount of applied current, at different degrees of insulation.

DETAILED DESCRIPTION OF THE INVENTION In Fig. 1 there is shown the current distribution between anode 35 and cathode 36 in an electrolysis cell of laboratory size, in which a stainless steel strip 1 has been mounted. The assembly shown is without the inventive insulators, i.e. it is an assembly according to prior art. It is evident that the current, in the position between anode and cathode, takes the route directly them between, without taking the route via the strip 1. This leads to a poor utilisation of the current, i.e. a low current efficiency.

Fig. 2 shows a production line for treatment of a hot- or cold-rolled strip material of stainless steel. In the following description of the drawing, it is assumed that the starting material is a hot-rolled strip which thus has an oxide layer of the spinel type. The hot-rolled strip 1 is first placed on a so-called uncoiling capstan 2, after which it runs forward to a cut 3 and a welding device 4, which has the function of welding one strip at its end to the start of a new strip, so that production can continue without any major stoppage to exchange the strip. A stretching and setting mechanism 5 then follows for stretching the strip and adjusting its line speed, which is preferably high. In the next stage, the strip passes through a furnace at a temperature of around 1050- 1150°C, the function of which is to soften up the oxide on the surface of the strip. A cooling stage 7 then follows, and a blast 8, the objective of which is to break up the oxide scale so that the electrolyte, in a later stage, can penetrate as far as the chromium- depleted zone which lies inside the oxide scale. Following blasting 8 comes the stage which is covered by the present invention, namely the pickling stage which preferably also constitutes a combined polishing stage. Here, this stage is divided into three cells 9, 10, 11. In the cells is an electrolyte according to the previous description, the most preferred having the composition 5 mol/1 sulphuric acid, 8-8.5 mol/1 phosphoric acid and iron (dissolved). Electrolysis in the cells is carried out using partially insulated cathodes and anodes, respectively, and using a direct current at a preferred current density of 0.5-2 A/cm², a temperature of 70°C and a total time of around 2 minutes, a bright surface being obtained on the strip. The principle of how the electrolytic cells can be constructed is evident in greater detail from Fig. 3-5. Following treatment according to the invention in the cells 9, 10, 11, rinsing equipment 12 follows for rinsing of the strip and then a so-called coiling capstan 13. The strip may then advance for cold-rolling.

The principle for treatment of a cold-rolled material is the same as that shown with reference to Fig. 2. The only difference is that a cold-rolled material is not blasted, since this destroys the surface of the strip, in which case a so-called neolyte stage can be used instead as a pretreatment before the pickling and polishing stage. The neolyte stage can be formed by an electrolytic cell with only slight agitation which contains an electrolyte consisting of e.g. sodium sulphate. The neolyte stage is executed using direct current at a current density of around 1-10 A/dm^". When treating a cold-rolled strip material, the furnace also has a somewhat different function, namely to stress- relieve anneal the material.

The line shown in Fig. 2 can be executed to advantage as a so-called combined mill, the strip material being made to run through the line twice, first in the hot-rolled state and then in the cold-rolled state.

In Fig. 3 the principle of the device according to the invention is schematically shown. A pickling chest 9, 10, 11 is built up from a lower part having side walls 31 and bottom 32, and an upper part 33 which constitutes a lid. In the chest there is an electrolyte bath 34 according to the description above. The continuously formed steel material (the strip) 1 runs into the chest and out of the same via sealings (not shown) in the side walls 31 of the chest. Electrodes of alternating polarity are arranged in the lower part of the chest, every other electrode being anodic 35 and every other being cathodic 36. Electrodes 35, 36 are also arranged in the upper part of the chest in a corresponding way, each electrode being matched by an electrode of the same polarity on the opposite side of the material. According to the invention, the electrode pairs are partially, electrically insulated from each other in the electrolyte by partial insulators 37 being arranged on both sides of the material, between each after each other arranged pairs of electrodes 35, 36 of opposite polarity. By the term "partial" it is here meant that these insulators not completely divide the chest into different, from each other completely separate compartments, but that a certain open cross-sectional area for an electrolyte flow remains between the different "rooms". Ordinarily, this electrolyte flow is achieved by means of a conventional pump device (not shown) which circulates the electrolyte. The open cross-sectional area is suitably arranged in connection with the strip 1, but it may also be conceived that it in addition, or instead, is arranged a certain open cross- sectional area above the upper electrodes and/or below the lower electrodes, i.e. in connection with the bottom 32 of the chest. One conceivable solution is e.g. that an open cross-sectional area is arranged in connection with the strip 1 at every other lower insulator 37, but that there also is arranged an open cross-sectional area in connection with the bottom 32 at the other insulators 37, so that the turbulence of the electrolyte flow is increased. A corresponding open cross-sectional area may naturally also be arranged at every other insulator 37, above the upper electrodes.

Fig. 4 is showing a cross-section of a preferred device according to the invention, cross the run-through direction of the strip 1, i.e. the strip 1 runs into and out from, respectively, the plane of the paper in the drawing. Here, the insulator 37 is of fixed type, i.e. not displaceable, its construction instead having to be adapted to a given pickling process. The insulator exhibits a total height H₂, which extends from the bottom 32 of the pickling chest and up to the lid or to the liquid level. It is also conceivable that it ends a small distance below the liquid level, but always over the level of an upper electrode 35. The extension of the insulator from the bottom 32 of the chest and up to a centre line for the strip 1 is denoted Hi. At the bottom of the chest, the insulator 37 exhibits an opening which defines an open area hi x bl, where bl equals or essentially equals the total width B of the chest. In the case that an opening in the insulator 37, for the strip 1, exhibits a height b.3 which in principle equals the thickness of the strip, which may be achieved by the insulator having a rubber lip which drags against the strip, for example, the share of insulating area below the strip is defined as (1-hl/Hι) or h2/Hι, h2 constituting the effective height of the insulator below the strip. Above the strip, the insulating area is in principle one hundred percent if H₂ is larger than the liquid level, or partial if H₂ is less than the liquid level, the share of insulating area above the strip in the latter case being calculated in a way corresponding to the calculation below the strip. The distance between the electrodes 35 in the electrode pair is denoted h4, in which case h3 < h4, h4 typically being about 150-300 mm, preferably 200-250 mm. If h3 is larger than the thickness of the strip 1, this should be taken into account when calculating the share of insulating area.

In Fig. 5 there is shown yet two varieties of how the partial electrical insulation according to the invention may be solved in practice. In the left part of the figure it is shown that the insulators may be constituted by rotatable rolls 37', which can be raised and lowered. This solution has the advantage that the strip 1, on its underneath side, may rest on the rolls 37', so that the strip 1 may be brought closer to the electrodes 35, 36 on its underneath side, when these rolls 37' are lowered. In that way, the pickling may be enhanced on the underneath side of the strip. In this embodiment, there is an open cross-sectional area i.e. not insulating, above and below the rolls 37' on the top side and the underneath side of the strip, respectively. In the right part of the figure there is instead arranged a partition wall 37'" beneath the rolls 37' on the underneath side of the strip, which partition wall also constitutes an insulator. Here, the rolls 37' are suitably not displaceable, but fixedly mounted, in which case the partition walls 37'" may instead be arranged to form a, possibly variable, open area at the bottom 32 of the chest. Above the strip, it is shown how a partition wall 37" may be arranged to be displaceable so that it may be partially pulled out through the lid 33. This partition wall 37^" may, as well as the partition wall 37'" on the beneath side, be constituted by a flexible rubber membrane, e.g. a rubber cloth. In Fig. 6-8 there is shown the result, in diagrams, of experimental tests being performed in laboratory equipment corresponding to that which is shown in Fig. 4, the strip however being stationary and the electrolyte being exposed to mixing in the anode and the cathode cell, respectively. The electrolyte used, the temperature, the current density etc. was according that which has been previously defined as preferred in connection with the invention.

In Fig. 6, the concentration of iron ions in the entire chest is shown as a function of the time for the electrolysis. The curve Fei shows the concentration without insulation, while the curve Fe_2a shows the concentration in the anodic part of the chest when the insulation was 70%, the curve Fe_2c shows the concentration in the cathodic part of the chest when the insulation was 70%, the curve Fe_3a shows the concentration in the anodic part of the chest when the insulation was 100% and the curve Fe_3c shows the concentration in the cathodic part of the chest when the insulation was 100%.

In Fig. 7 the concentration of chromium ions in the entire chest without insulation is shown as curve Crj, while the curve Cr_2a shows the concentration in the anodic part of the chest when the insulation was 70%, the curve Cr _c shows the concentration in the cathodic part of the chest when the insulation was 70%, the curve Cr _a shows the concentration in the anodic part of the chest when the insulation was 100% and the curve Cr_3c shows the concentration in the cathodic part of the chest when the insulation was 100%.

In Fig. 8 the concentration of nickel ions in the entire chest without insulation is shown as curve Nil, while the curve Ni_2a shows the concentration in the anodic part of the chest when the insulation was 70%, the curve Ni_2c shows the concentration in the cathodic part of the chest when the insulation was 70%, the curve Ni _a shows the concentration in the anodic part of the chest when the insulation was 100% and the curve Ni_3c shows the concentration in the cathodic part of the chest when the insulation was 100%.

In all curves it is clear that a considerably better concentration equalisation is achieved between the cathodic and anodic part of the chest, at partial insulation when compared to 100%) insulation being used. At the same time, the partial insulation gives a considerably enhanced current efficiency when compared to the case when no insulation is used at all, which is clear from Fig. 9. Accordingly, Fig. 9 shows the current in the strip as a function of the total current which was applied on the electrolyte cell in the tests shown in Fig. 6-8. The best current efficiency is achieved at 100%o insulation between anode and cathode, but at the price of a non existing concentration equalisation between the anodic and the cathodic part of the chest. At 70% insulation, a much improved current efficiency is achieved at the same time as the concentration equalisation is good. At 0%o insulation, the current efficiency is very poor.

The invention is not limited by the embodiments shown above, but may be varied within the scope of the following claims. For example, the skilled man will easily realise that additional varieties are conceivable when it comes to the practical achievement of the insulators, and that the varieties shown here also may be combined with each other, to a certain extent.

Claims

1. Method of electrolytically, continuously treating a continuously formed material (1) of stainless steel, said material being made to run in an electrolyte (34) between electrodes (35, 36) lying in series, under the influence of a direct current with alternating polarity, every other electrode being anodic and every other being cathodic and every electrode being matched by an electrode of the same polarity on the opposite side of the material (1), an oxide surface layer with a thickness of at least 1 micrometre being removed from the material in the method, characteri s ed in that said electrodes (35, 36) lying in series are partially, electrically insulated from each other in the electrolyte (34) by partial insulators (37) being arranged between after each other arranged electrodes of opposite polarity, preferably both on a first side and on a second side of the material (1).

2. Method according to claim 1, characteri s ed in that said electrodes (35, 36) are insulated to 20-95%>, preferably 30-90% and even more preferred 40-85%, calculated on the total cross-sectional area in the electrolyte (34) on one side of the material.

3. Method according to claim 1 or 2, characteri s ed in that said insulators (37), at least on the first side of the material (1), are displaceable for a variable electrical insulating effect in the electrolyte (34) between the electrodes (35, 36) which are arranged on the corresponding side of the material.

4. Method according to any one of the preceding claims, characteri sed i n that said insulators (37) are made of an electrically insulating material, preferably chosen from the group that consists of polymeric and ceramic materials, most preferred plastics, rubber, ceramics or plexiglass.

5. Method according to any one of the preceding claims, characterised i n that said electrolyte (34) comprises sulphuric acid or salt thereof and/or phosphoric acid, or comprises nitric acid, the electrolyte preferably also comprising hydrofluoric acid or salt thereof.

6. Method according to any one of the preceding claims, ch arac teri sed i n that said electrolyte (34) comprises sulphuric acid in a concentration of 2 — 12 mol/1, preferably 2 — 10 mol/1 and even more preferred 2 — 6 mol/1, and phosphoric acid in a concentration of 2 — 14 mol/1, preferably 4 — 12 mol/1 and even more preferred 4 - 9 mol/1.

7. Method according to any one of the preceding claims, characteris ed in that the method is operated at a treatment time of 2 sec - 5 min, preferably 10 sec - 3 min and even more preferred 30 sec - 2 min, at a current density of 0.1-3 A/cm², preferably 0.3-2.5 A/cm² and even more preferred 0.5-2 A/cm², and a temperature of 50-100°C, preferably 60-90°C and even more preferred 65-80°C.

8. Method according to any one of the preceding claims, ch aracteri sed in that the treatment constitutes a part stage (9, 10, 11) in a process line for production of said continuously formed material of stainless steel, which process line also comprises hot-rolling and annealing (6) and/or cold-rolling and annealing (6).

9. Device for electrolytic, continuous treatment of a continuously formed material (1) of stainless steel, said device comprising a pickling chest (31, 32) with a lid (33), for an electrolytic bath (34), and in series lying electrodes (35, 36) arranged in this chest with a lid, which electrodes are arranged to have alternating polarities under the influence of a direct current, every other electrode being anodic and every other being cathodic and every electrode being matched by an electrode of the same polarity on the opposite side of the material (1), characterised in that said electrodes (35, 36) lying in series are partially, electrically insulated from each other in the electrolyte (34) by partial insulators (37) being arranged between after each other arranged electrodes of opposite polarity, preferably both on a first side and on a second side of the material (1).

10. Device according to claim 9, characteri s ed in that said electrodes (35, 36) are insulated to 20-95%, preferably 30-90% and even more preferred 40-85%, calculated on the total cross-sectional area in the electrolyte (34) on one side of the material.

1 1. Device according to claim 9 or 10, characteri s ed in that the insulators (37), at least on the first side of the material (1), are arranged so that their effective insulating area between the electrodes (35, 36) is variable.

12. Device according to any one of claims 9-1 1 , characteri s ed i n that said insulators (37), at least on the first side of the material (1), are arranged to be displaceable for a variable electrical insulating effect in the electrolyte (34) between the electrodes (35, 36).

13. Device according to claim 12, characterised in that said insulators (37) are constructed as partition walls (37", 37'") which are arranged in the lower part of the chest and/or at said lid (33), which partition walls are displaceable in a direction which is perpendicular to the run-through direction of the material (1) in the device, said lid (33) preferably exhibiting openings for the partition walls (37"), so that these may be partially displaced out of the device or into the lid of the chest.

14. Device according to claim 9, characteri sed in that said insulators (37) are fixedly attached to said lid (33), at the second side of the material (1).

15. Device according to any one of claims 9-12, characteri s ed in that said insulators (37) are constituted by rotatable rolls (37'), at least on the first side of the material (1), which rolls are preferably so arranged on the first side of the material that it rests and rolls on the same during the treatment process.

16. Device according to any one of claims 9-15, characteri s ed in that said insulators (37) are made of an electrically insulating material, preferably chosen from the group that consists of polymeric and ceramic materials, most preferred plastics, rubber, ceramics or plexiglass.