PROCESS AND APPARATUS FOR CLEANING BY ELECTROCHEMICAL PICKLING WITH ALTERNATING CURRENT OF SPECIFIED FREQUENCY

Abstract: A process and apparatus for the electrochemical pick­ ling of ferrous and other metal wires is disclosed. The pro­ cess, which is particularly suitable for high-carbon steel wires, involves the application of a low voltage alternating current to the wires in a pickling bath containing a selected electrolyte. The frequency of the current is modified as compared with mains frequency, the modification depending on the nature of the electrolyte. The wave-form of the current may also be modified and a suitable direct current may be superimposed.

Description: The present invention relates to a process and apparatus for electrolytically cleaning metal strands, such as ferrous wires. More particularly it relates to an improved electro­ pickling process, using alternating current, capable of effecting rapid and essentially complete removal of oxide scale, rust and other undesirable substances from the surface of iron and steel wires. The process can yield extremely clean wire surfaces suitable for subsequent finishing treat­ ments. The terms ferrous strand as used herein refers to ferrous (e.g. steel) wire, strip, sheet etc. irrespective of compo­ sition, length or cross section. The process and apparatus of the invention are however especially suitable for use in the wire industry. The process of this invention is generally applicable to the in-line cleaning of ferrous strands of variable carbon con­ tent, including low-carbon, medium-carbon and high-carbon steel wire with up to more than 1 % C. The process is parti­ cularly suitable for use in wire pickling lines and can be applied with surprising and advantageous results to carbon steel wires with more than 0.30 % C, and especially above 0.45 - 0.50 % C. In the case of steel wire manufacturing the undesirable sub­ stances found on the wire surface frequently take the form of higher oxides (such as ferric and magnetic oxides), hydro­ oxides (rust) and also foreign impurities such as dirt par­ ticles, lubricant residues, organic or inorganic matter, and often also carbonaceous and graphitic substances derived from burnt lubricant. Due to the diversity of character of these contaminants, related to the diversity of prior mill operations and the types of wire used, it is often necessary to subject the wire surface to a sequence of cleaning and pickling steps in order to ensure proper cleaning. For removing adherent scales and subsurface penetrated (oxide) particles, it is sometimes necessary to treat steel wires in strong mineral acids and to increase both pickling temperature and residence time. These prior art measures increase the cleaning cost with the need for long processing installations and high consumption of chemicals and energy. The use of an electrical current to intensify the cleaning action of a chemical pickling bath has been proposed. How­ ever, in methods making use of strong acids and employing electrolyzing current "assists" to accelerate the pickling process, there are often problems of overpickling, including significant loss of base metal, and also hydrogen embrittle­ ment which is most undesirable in high-strength applications of carbon steel wires. To overcome some of the drawbacks of prior art pickling methods, a number of specific electrolytic processes have been developed in which a direct current is applied to the steel wires, mostly in combination with specific electrolyte conditions. Known processes of this type include for example: - electrolytic D.C. pickling of steel wires in hydro­ chloric acid solutions under carefully controlled electroytic conditions. - bipolar pickling of high carbon steel wires in strong sulfuric acid solution (450 - 500 g/l of H₂SO₄) at a temperature of below 30°C with high current density (150 A/dm²). - electrolytic D.C. pickling in aqueous salt solutions (sulfates, chlorides) of sufficient conductivity. In the case of D.C. pickling, however, there is the dis­ advantage that expensive rectifying equipment is needed and also there is usually requirement for a delicate balance of electrolytic processing conditions (polarity, electrolyte temperature, electrolyte composition, etc.). A further disadvantage of direct current pickling of fer­ rous wire in hydrochloric acid is chlorine gas formation at the current densities needed to effect adequate clea­ ning. Consequently, electrolytic pickling of carbon steel wire using D.C. is not generally satisfactory and such methods are not industrially important in comparison to the use of hydrochloric acid solutions in the non-elec­ trolytic chemical pickling of wire. As an alternative to D. C. electrolytic treatments, the use of alternating current has been proposed but prior attempts to use ordinary A.C. (mains frequency 50 - 60 Hertz) as a cheap source of electrical power to electrolyze steel wires in a hydrochloric acid pickling solution have been largely unsuccessful. One particularly serious problem with the use of an electrolyzing alternating current in the pickling of medium and high-carbon steel wires, e.g. containing at least about 0.30 % C and more especially above 0.45 % C, is the occurrence of a smut layer on the wire surface. This smut is difficult to remove and gives the wire a dark to black appearance. It is unacceptable because poor adhesion of sur­ face coatings applied subsequent to said pickling operation may result. The real nature and cause of the smut phenomenon are not yet fully understood (overpickling, anodic action and surface oxidation, debris of carbonaceous origin, alloying elements or steel impurities may all play a part) but its occurrence imposes severe limits on the applicability of A.C. electrolyzing current in acid cleaning of steel wires, parti­ cularly medium and high-carbon steel wires. Hence, A.C. pickling of medium and high-carbon steel wire in HCl-solu­ tions (in theory a most attractive process) has not been industrially important up to the present time. A primary object of the present invention is to provide an electrolytic pickling process for the treatment of steel wire which utilises an alternating electrolyzing current and which is applicable to steel wires in general including medium and high-carbon steel wires to provide wires having surfaces of improved cleannesss and increased reactivity as compared to prior art pickling methods and more especially to provide wires having surfaces free form smut layers as described above. A further object is to provide an economical pickling process utilising alternating current which is suitable for the elec­ trolytic pickling of steel wires of low, medium and high­ carbon contents. The present invention is based upon the surprising discovery that the effectiveness of electrolytic cleaning processes utilising alternating current can be improved by the modifi­ cation of the frequency of the alternating current as com­ pared with mains frequency (50 - 60 Hertz) and that by means of such modification of the frequency of the alternating current steel wires including such wires having medium and high carbon contents can be treated to provide wire having smut-free surfaces of improved cleanness and increased reac­ tivity as compared to those obtained by prior art pickling methods. According to one feature of the present invention, there is provided a process for the surface cleaning of ferrous and other metal strands by means of an electrochemical pickling method in which the ferrous strand to be cleaned is passed through an electrolytic pickling bath characterised in that the ferrous strand is subjected to the action of a low vol­ tage alternating current the frequency of which is modified as compared with mains frequency (50 - 60 Hertz) whereby the effectiveness of the cleaning process is improved. In a preferred process the metal strand is continuously con­ veyed past a plurality of spaced apart electrodes arranged along the path of travel of the strand so that the strand passes in turn within a predetermined close distance of each of the said electrodes, electrolyte being provided in the space between each of the said electrodes and the portion of the strand which is adjacent thereto whereby the strand serves as an intermediate conductor between adjacent elec­ trodes. According to a further feature of the present invention, there is provided apparatus for use in the surface cleaning of ferrous or other metal strands by a process according to the invention as hereinbefore defined which apparatus com­ prises: - at least one electrolytic picking cell which in use con­ tains the aqueous acid electrolyte; - conveying and supporting means which in use enable the strand to be passed through the said electrolyte in the electrolytic pickling cell; - a plurality of spaced apart electrodes arranged along the path of travel of the strand so that in use the strand passes in turn within a predetermined close distance of each of the said electrodes; - a source of low voltage alternating current for the said electrodes; and - means for modifying the frequency of the low voltage alternating current supply from the said source as compared with mains frequency (50 - 60 Hertz) whereby in use the effectiveness of the cleaning process is improved. The method of this invention is not restricted to the use of an alternating current with the usual sinusoidal wave form. It also includes the use of modified forms, such as pulsed A.C. current, e.g. with anodic and cathodic current periods of equal length or of dissimilar length, and also the use of A.C. of rectangular shape and other variants. According to one preferred feature of the invention, the alternating current of modified frequency (and if desired also modified wave form) is combined with a superimposed direct current of predetermined voltage (related to the applied current density) so as to modify the anodic character of the A.C. pickling system. An appropriate electrolytic bath for high-speed A.C. pickling of steel wire preferably comprises an aqueous solution of hydrochloric acid, the concentration of which can for example vary from less than 100 to more than 250 g/l of HCl. The aqueous hydrochloric acid electrolyte usually (but not necessarily) contains ferrous chloride in variable amounts of up to 150 g/l FeCl₂ and preferably less than 140 g/l expressed as weight amount of iron ion (g/l of Fe²⁺). When employing an electrolyte based on hydrochloric acid for per­ forming the electrolytic pickling of high-carbon steel wire of above 0.45 % C, the modified frequency for obtaining effective cleaning without formation of smut will be substan­ tially higher than the mains frequency and will in general be at least 200 Hertz, preferably at least 400 Hertz and more preferablly in the range of from about 500 to 2000 Hertz. A suitable electrolyte composition may thus for example con­ tain from about 50 to 300 g/l of HCl, preferably 100 to 300 g/l of HCl, and 0 to 120 g/l of Fe²⁺ and the electrolytic pickling can in general be operated at an economic tempera­ ture below 60 - 65°C using high frequency alternating current as described above to effect smut-free, rapid and efficient pickling of high-carbon steel wires. When using electrolyte baths different from the aqueous hydrochloric baths described above, e.g. electrolytic baths based on concentrated salt solutions, mixtures of acids and the like, the modification to the alternating current fre­ quency which is necessary in order to obtain improved effec­ tiveness of cleaning in accordance with the present invention will vary according to the composition of the electrolytic bath. With the knowledge of the criticality of the alterna­ ting current frequency, the optimum modified frequency for any particular composition of electrolytic bath can be rea­ dily determined. for example, with electrolytic baths based upon aqueous solutions of Na₂SO₄ or K₂SO₄ or mixtures thereof the desired frequency has been found to be signifi­ cantly lower than the mains frequency of 50 - 60 Hertz and the frequency used preferably does not exceed 10 Hertz. Suitable concentrations of Na₂SO₄ range from 100 to 200 g/l at a pH of from 5 to 7. Effective electrolytic A.C. pickling of steel wires in aqueous sulfuric acid solutions is most advantageously car­ ried out at a low frequency of not more than 5 Hertz. Sui­ table concentrations of H₂SO₄ are from 100 to 500 g/l. The method of the present invention is not restricted to the above mentioned electrolytes. NaCl, KCl or mixtures thereof, mineral acid mixtures (e.g. HCl + H₂SO₄, HCl + HNO₃) and various acid/salt mixtures may also be used and the optimum modified frequency determined by simple experiment. Suitable current densities in electrochemical pickling sys­ tems according to the invention are from 25 to 500 A/dm² of submerged wire surface. The rapid and unexpectedly efficient cleaning action together with the total absence of pickling smut on the surface of in particular medium and high-carbon steel wires, obtainable by subjecting said wires to the A.C. pickling method of this invention, in particular in the presence of aqueous hydro­ chloric acid electrolytes, is most surprising in the light of the prior art practice and knowledge, which generally teaches that A.C. pickling of ferrous wires is less effective and that A.C. pickling of high-carbon steel wires results in unavoidable smut formation. In one preferred emboidment of this invention, steel wires are electrolytically cleaned by passing one or more wires parallel in a (preferably) horizontal plane through an elec­ trolytic bath past a sequence of immersed electrodes and subjecting said wires to an alternating current of modified frequency, preferably by the method of non-contact current flow whereby the wires act as intermediate conductors between adjacent electrodes of opposite polarity which are serially arranged, at a preset close distance from the wire, in the longitudinal bath direction and connected to the terminals of a suitable low-voltage A.C. power supply of the desired modified frequency. In one particularly preferred embodiment of the apparatus of the invention, the electrolytic processing zone of the pick­ ling apparatus is designed as a single overflow bath divided into a plurality of successive electrolytic cells with cont­ nuous circulation of electrolyte, each of said cells contai­ ning a plate electrode on its bottom and separated from the adjacent cell (containing a similar plate electrode of oppo­ site polarity) by partition means disposed in said bath transverse to the wire travel and longitudinal cell direc­ tion. The partitions prevent undesirable direct flow of A.C. between adjacent electrodes and cause the current to be con­ ducted via the moving wires. In addition they can act as supporting elements for the spaced-apart travelling wires so as to maintain the immersed wires at the required distance from the electrodes. Electrolyte flow is maintained by sui­ table pumping means for supplying and circulating the elec­ trolyte from a central tank to the electrolytic cells, from which the electrolyte overflow returns to the tank. The electrolytic cells may advantageously comprise a one-bath integrated construction containing the required number of distinct electrode compartments forming a sequence of spaced-­ apart electrodes, past which the immersed wires are conti­ nuously moved so as to be effectively electrolyzed over the available treatment length. An alternative apparatus arrangement comprises a sequence of separate overflow cells or baths whereby each bath contains one or more spaced-apart electrodes suitably connected to their corresponding A.C. power terminal, e.g. a sequence of single-electrode cells or a sequence of separate baths each containing two or more spaced-apart electrodes. In this appa­ ratus arrangement only the moving wires form a continuous conduction path between adjacent baths thereby excluding current leakage between electrodes of adjacent cells. In case of a line-stop or a wire stop (e.g. incidental wire break), however, the non-immersed wire portions are readily oxidized and may even heat up, giving a burnt surface. There­ fore, in multiwire operations, a one-bath multi-cell appara­ tus providing entire wire immersion over the total pickling length is most preferred. In one particularly preferred embodiment of the apparatus according to the invention as described above, means are provided for supplying a single phase low voltage alternating current of specified frequency. The supply conveniently comprises means for stepping down the mains voltage to a required low voltage and means for converting the mains fre­ quency so as to generate an alternating current of the desired modified frequency in a desired wave form and also means for regulating the current density. In addition means for supplying a predetermined low-voltage D.C. to the wires may be provided whereby said auxiliary D.C. is superimposed on the main A.C. power supply so as to change the active electrolyzing voltage and/or wave form of the applied A.C. in a desired way. According to our findings, the pickling results for steel wires subjected to a modified high frequency as hereinbefore described in a hydrochloric acid electrolyte are further unexpectedly improved by cathodic superposition of a D.C. voltage, which may vary in height in relation to the ampli­ tude of the applied A.C. voltage and is preferably adapted to attain a fraction of the peak height of said A.C. voltage in a specified range 0.05 to 0.50. Said cathodic super­ position of suitable intensity presumably reduces or compen­ sates the possibly harmful effect of a too active anodic period (depending on actual system electrolytic equilibrium) by a controlled extension of the cathodic period relative to the anodic period of the A.C. cycle. Said cathodic shift may amount to 50 % of the peak height of the applied A.C. voltage. A preferred range in A.C. pickling of steel wire in HCl is selected as follows in accordance with applied current density: These and other aspects of the invention will be more fully understood by reference to the accompanying drawings and the examples which show the results of a selection of pickling treatments as applied to steel wires including high-carbon steel wires containing 0.70 to 0.85 % C. References in the text to "degree of pickling" refer to the following scale, where a series of standard conditions are used to obtain samples of pickled high-carbon steel wires for comparison. Ferrous chloride (FeCl₂) content is always expressed as a weight amount of ferrous ion (g/l of Fe²⁺). Referring to the drawings, Figures 1 to 5 summarize some of the pickling results obtained by performing the method of the present invention on steel wire. Figures 6 to 9 schema­ tize apparatus embodiments suitable for carrying out the method of this invention. Figure 1 shows the effect of frequency (f in Hertz) on the achievable degree of pickling (P) in an electrolytic solution of 250 g/l of HCl + 20 g/l of FeCl₂ at 45°C. Below a fre­ quency of 250 Hertz the pickling results become unreliable and often very poor. Above 1500 Hertz there is little addi­ tional gain. The shaded band is the normally encountered degree of pickling with conventional HCl-pickling and the line L corresponds to the best result according to prior art HCl-practice. Figure 2 shows the effect of HCl-concentration varying between 100 and 300 g/l (in the presence of 20 g/l of FeCl₂ and at 45°C) on the obtainable degree of surface cleanliness when subjecting high-carbon steel wire to an A.C. current of 1000 Hertz with a current density rating of resp. 100, 150 and 200 A/dm². Figure 3 illustrates the effect of total pickling time (40 % chemical + 60 % electrolytical, resulting from the electrode configuration used) on the degree of pickling obtained by the method of this invention (1000 Hertz - 200 g/l HCl + 40 g/l FeCl₂ at 45°C). Figure 4 shows the additional effect of superimposing a cathodic D.C. voltage (expressed as fraction p of applied A.C. voltage) on the degree of pickling at 1000 Hertz in 200 g/l at 45°C. An optimum p-range related to applied A.C. current density is observed. A current density of 200 A/dm² gives extremely good results when p is in the range 0.10 to 0.30. Figure 5 shows the degree of pickling obtainable by combining an A.C. base current of 1000 Hertz with a superimposed catho­ dic voltage of respectively 1/20, 1/6 and 1/5 of the peak height of the applied A.C. voltage for current densities of 100, 150 and 200 A/dm² respectively. Pickling bath : 200 g/l HCl at 45°C, variable FeCl₂ content. Best results are obtained from 40 to about 120 g/l FeCl₂. Figures 6a and 6b show schematically a non-contact electro­ pickling cell, having respectively two (Fig. 6a) and three (Fig. 6b) spaced-apart electrodes connected to a single phase A.C. power supply. A power supply 10, electrodes 11 and a partition wall 12 which also supports the moving wire W are immersed in an electrolytic bath with electrolyte level 13. Figure 7 shows schematically an apparatus arrangement for use with a three-phase current supply 10' of delta connection with phase terminals A, B and C. The apparatus can be used for non-contact electropickling according to the invention. Figure 8 illustrates a simplified arrangement for combining a suitable supply of A.C. base current 10 with superposition of a cathodic D.C. voltage or current 14, according to a preferred feature of the invention. Example 1 Drawn steel wire of 0.70 % C and 1.5 mm diameter was first isothermally transformed (lead patenting) to pearlite and stored in ambient air for different times up to 6 months. The wire surface contained higher oxides, rust and small residues of burnt lubricant in varying amounts. The wires were immersed in a hydrochloric acid bath of 180 g/l of HCl at a temperature of about 60 - 65°C and pickled in different ways: a) chemical pickling for 30 seconds b) electrolytic pickling for 5 seconds with alternating current of varying frequency. The pickling results were as follows: Example 1 clearly demonstrates that high-frequency pickling of high-carbon steel wires in HCl in accordance with the present invention gives a considerable improvement in cleaning capability and wire surface purity as compared to conventional chemical pickling and to electrolytic A.C. pickling at mains frequency. Example 2 This example illustrates a specific embodiment of the method of this invention wherein a combination of low frequencies (outside the mains frequency range) and a particular electrolyte are used. Electrolytic pickling of 0.70 % C patented steel wire in sodium sulphate solution: 150 g/l of Na₂SO₄, immersion time of 5 seconds, temperature of 40°C, current density of 100 A/dm². Example 3 Lead-annealed drawn low-carbon steel wires (0.15 % C - 2.4 mm) containing oxide scale and partially carbonized lubricant residue on their surfaces were subjected to comparative pickling treatments: The pickling results were as follows: I. pickling degree (surface cleanliness) of 3 to 2 (dark grey to grey) - black spots are sporadically present. II. pickling degree of 2 (grey), absence of black spots. III. pickling degree of 2 to 1 : iron grey to silvery white. Surface cleanliness obtainable by the method of this inven­ tion is clearly superior. The method of this invention is thus suitable for the pickling of low-carbon steel wire, though with less spectacular results than obtainable on high-­ carbon steel wires. In a second series of tests the process of the invention was tested more systematically on high-carbon steel wires, in particular the high-carbon range 0.50 - 1.0 % C which covers most steel grades for high-performance applications. Said carbon steel wires are generally prone to smut formation on pickling in hydrochloric acid solutions and require careful pickling conditions to obtain adequate surface cleanliness. Activation of the pickling process with normal A.C. (mains frequency) unavoidably causes black smut and surface dete­ rioration. Therefore, a most important object of the present invention was to solve this persistent problem. Further objects were to improve the surface treatment of steel wires with regard to pickling speed, reliability and ultimate degree of cleanliness as compared to the present limits of conventional chemical pickling in HCl. Example 4 (Figures 1 to 3) Patented 0.80 % C steel wire of 1.25 mm diameter normally has a black oxidized surface. Conventional pickling in a hydrochloric acid bath usually attains a degree of pickling of about 3 - 3.5 (grey to light dark grey surface) ; in the best conditions, referring to a two-bath system (precleaning HCl-bath followed by desmutting bath of increased HCl concen­ trations up to 250 - 300 g/l) a degree of pickling of 2 - 3 is obtainable with total immersion times of about 15 - 20 seconds and bath temperatures of 60 - 70°C. The wires were treated according to the electropickling method of this invention at a specified high frequency above 200 Hertz, more particularly in the range 250 to 5000 Hertz in which the preferred working range was from 500 to 1500 Hertz. A.C. current density applied to the wires was varied from about 50 to 500 A/dm² in an aqueous acid electrolyte containing from 100 to 300 g/l of HCl and up to 140 g/l of FeCl₂. Excellent pickling results (significantly better than degree 3) were obtained in the following conditions: - frequency 1000 Hertz, current density 100 to 200 A/dm² - pickling bath : 150 to 250 g/l of HCl, up to 50 g/l of FeCl₂; temperature 40 to 60°C. A really outstanding surface cleanliness, comparable to degree 2 (light iron grey) and mostly better (silver grey to white), hitherto unachievable with conventional HCl pickling, was consistently obtained in a short time of 8 to 10 seconds by treating the wires at 1000 Hertz and 200 A/dm² in a pickling bath containing 200 g/l of HCl at 45°C (or 150 g/l at 60°C) with an optional amount of FeCl₂ (usually 20 - 40 g/l) which may rise to about 100 g/l without affecting sur­ face purity. Example 5 Drawn 0.65 % C steel wire of 1.50 mm diameter was alkaline degreased and lead annealed at 450°C, leaving oxide and burnt lubricant residues on the wire surface. Conventional high-­ speed in-line pickling for about 3.5 - 4 seconds in a hydro­ chloric acid bath (200 - 250 g/l of HCl + 40 - 60 g/l of FeCl₂ at 60°C) gives a degree of pickling of at best 3 whereby the wire surface has an irregular lustrous aspect. The same wire material was subjected to electrolytic pickling in an aqueous electrolyte of 200 g/l HCl + 40 g/l FeCl₂ at 45 and 60°C with an A.C. current of 1000 Hertz and 150 - 200 A/dm². In these conditions a surface cleanliness of 2 to 3 was reliably achievable in 2 to 3 seconds; in addition the wire surface showed a uniform satin grey aspect pointing to an improved and regular surface reactivity which is more beneficial for subsequent finishing than conventionally pickled wire. Example 6 (Figures 4 and 5) To further improve cleaning efficiency and surface quality achievable by performing the method of this invention the applied electrolyzing A.C. of specified frequency may be combined with a superposed D.C. A cathodic D.C. voltage of suitable magnitude was applied to a typical A.C. hydrochloric acid pickling system according to the invention. The magnitude of the cathodic shift may vary, but is prefe­ rably related to the amplitude of the applied A.C. current cycle in a way so as to encompass a predetermined fraction of the peak voltage value of applied A.C. At 1000 Hertz we found the following values (Fig.4) to be satisfactory for said cathodic superposition when pickling steel wire in a HCl-bath containing 150 to 250 g/l of HCl and up to 140 g/l of FeCl₂ with a basic A.C having a den­ sity of 100 to 200 A/dm²: - 100 A/dm² : cathodic shift corresponding to about 1/50 to 1/6 of total A.C. voltage with optimum improvement range of 1/20 to 1/8. - 150 A/dm² : cathodic shift corrresponding to about 1/20 to 1/5 of total range, and most preferable range of 1/15 to 1/6. - 200 A/dm² : cathodic shift corresponding to about 1/15 to 1/3 of total voltage and most preferably 1/8 to 1/4. As a result of said optimum cathodic superposition we were able to obtain a significant additional improvement in sur­ face cleanliness often amounting to about one unit on the comparative degree of pickling scale. The achievable degree of pickling often corresponds to the maximum value of surface purity (value 1 of the degree of pickling scale) as illus­ trated in Fig. 5, in particular for high-carbon steel wire pickled at 1000 Hz in a solution of 200 g/l of HCl with varying FeCl₂-content. The process and apparatus of the present invention may also be used for the electrochemical pickling of strands other than ferrous strands. Examples of other metals to which the invention can be applied include non-ferrous metals (for instance aluminium) and non-ferrous alloys (for instance nickel-based heat-resistant alloys). Our copending Application No. 85-17605 in Great Britain with the same priority date (12th July 1985) of today's applica­ tion (priority GB 85-17606) further describes process condi­ tions and apparatus suitable for use in the present inven­ tion, the process conditions being especially suitable for the treatment of low carbon ferrous strands. The description and drawings of Application No. 85-17605 are to be understood as part of the description and additional drawings for the present application.

Claims: 1. A process for the surface cleaning of ferrous and other metal strands by means of an electrochemical pickling method in which the strand to be cleaned is passed through an electrolytic pickling bath and subjected therein to the action of a low voltage alternating current the frequency of which is substantially different from the normal mains fre­ quency, characterized in that said frequency is comprised of a specified range of higher, respectively lower frequencies as compared with mains frequency which is adapted as a func­ tion of the kind of strand material and used electrolyte whereby the effectiveness of the cleaning process is conside­ rably improved. 2. A process as claimed in claim 1 wherein the strand is continuously conveyed past a plurality of spaced apart electrodes arranged along the path of travel of the strand so that the strand passes in turn within a predetermined close distance of each of the said electrodes, electrolyte being provided in the space between each of the said elec­ trodes and the portion of the strand which is adjacent there­ to whereby the strand serves as an intermediate conductor between adjacent electrodes. 3. A process as claimed in claim 1 or claim 2 wherein the strand is steel wire containing a variable amount of carbon of from less than 0.1% to up to more than 1%. 4. A process as claimed in claim 3 wherein the steel wire contains at least 0.30 % C, in particular high-carbon steel wire with from about 0.45% to more than 1% C. 5. A process as claimed in claim 3 or claim 4 wherein the electrolytic pickling bath contains from 50 to 300 g/l of hydrochloric acid and from 0 to 150 g/l of ferrous chlo­ ride (expressed as g/l of Fe²⁺-ion). 6. A process as claimed in claim 5 wherein the electro­ lytic bath contains from 100 to 300 g/l of hydrochloric acid and from 0 to 120 g/l of iron as ferrous chloride. 7. A process as claimed in claim 5 or claim 6 wherein the frequency of the alternating current is modified to be at least 200 Hertz. 8. A process as claimed in claim 7 wherein the modified frequency is in the range of from 250 to 5000 Hertz and wherein the electrochemical pickling is carried out at a current density of from 25 to 500 A/dm². 9. a process as claimed in claim 8 wherein the modified frequency is in the range of from 500 to 2000 Hertz and the said current density from 50 to 300 A/dm². 10. A process as claimed in any of claims 1 to 4 wherein the frequency of the alternating current is modified to be less than 50 Herz and wherein an aqueous electrolytic pickling bath containing an appropriate electrolyte other than hydrochloric acid is used, in particular an electrolyte of the kind Na₂SO₄ or K₂SO₄ (and mixtures thereof), of NaCl of KCl (and mixtures thereof), of sulfuric acid and/or other suitable acids incl. mixtures thereof. 11. A process as claimed in claim 10 wherein an elec­ trolytic pickling bath containing from 100 to 200 g/l of Na₂SO₄ with a pH of from 5 to 7 is used and wherein the modified frequency is from 1 to 10 Herz. 12. A process as claimed in claim 10 wherein the electrolytic bath contains from 100 to 500 g/l of sulphuric acid and wherein the modified frequency is not more than 5 Herz. 13. A process as claimed in any of the preceding claims in which single phase alternating current is used. 14. A process as claimed in any of claims 1 to 12 in which three phase alternating current is used. 15. A process as claimed in any of the preceding claims wherein the normal wave form of the alternating current is modified to a rectangular, trapezoidal or pulsed wave form. 16. A process as claimed in any of the preceding claims wherein a direct current, preferably of cathodic voltage is superposed on the basic alternating current of modified fre­ quency. 17. A process as claimd in claim 16 wherein the catho­ dic voltage of the superimposed D.C. current amounts to up to 50 % of the peak amplitude of the basic A.C. voltage. 18. A process as claimed in claim 16 or claim 17 for the electrochemical pickling of high-carbon steel strands in a HCl-electrolyte, wherein the applied A.C. current has a frequency of from 500 to 1500 Hertz and a current density of 50 to 300 A/dm² and wherein the cathodically superposed D.C. voltage is between 0.02 and 0.35 times the peak voltage amplitude of the A.C. electrolyzing cycle. 19. A process as claimed in any of the preceding claims wherein the strands to be cleaned are conveyed along recti­ linear and horizontal paths through an electrolytic overflow bath past a series of two or more spaced-apart electrodes submerged therein and connected to a suitable supply of single or three phase alternating current of modified fre­ quency so as to form a planar sequence of electrodes of al­ ternating polarity or phase in the longitudinal bath direc­ tion. 20. Apparatus for use in the surface cleaning of fer­ rous or other metal strands by a process as claimed in any one of claims 1 to 19, which apparatus comprises: - at least one electrolytic picking cell which is use con­ tains the aqueous acid electrolyte; - conveying and supporting means which in use enable the strand to be passed through the said electrolyte in the electrolytic pickling cell; - a plurality of spaced apart electrodes arranged along the path of travel of the strand so that in use the strand passes in turn within a predetermined close distance of each of the said electrodes; - a source of low voltage alternating current (single phase or three-phase) for the said electrodes; and - means for modifying the frequency of the low voltage al­ ternating current supply from the said source as compared with mains frequency (50 - 60 Hertz) whereby in use the effectiveness of the cleaning process is improved. 21. Apparatus as claimed in claim 20 which includes a multi-compartment overflow pickling cell with continuous electrolyte circulation containing a planar sequence of spaced-apart electrodes whereby adjacent electrodes are separated by submerged partitions of suitable height which partitions support the immersed moving strands out of contact of said electrode plates and also prevent current leakage between adjacent electrodes. 22. Apparatus as claimed in claim 20 or claim 21 which includes a source of direct current for superimposition on the low voltage alternating current.

Inventors: ADRIAENSEN, LUDO, DECOUTTERE, BERNARD
Applicants: N.V. BEKAERT S.A.
Application year: 1986
Main classification: C25F1
IPC codes: C25F7, C25F1