EP0894158A1

EP0894158A1 - Descaling of metal surfaces

Info

Publication number: EP0894158A1
Application number: EP97916580A
Authority: EP
Inventors: Neil Mcmurray; John Mcdonald Duncan; Sandy Francoise Lancelot; Edward Pugh
Original assignee: Maysonic Ultrasonics Ltd
Current assignee: Maysonic Ultrasonics Ltd
Priority date: 1996-04-15
Filing date: 1997-04-15
Publication date: 1999-02-03
Also published as: CA2251782A1; AU2519097A; KR20000005451A; JP2000508711A; WO1997039167A1; GB9607810D0; PL329292A1

Abstract

The surface of a metal body is descaled by subjecting the body to electrolysis in a bath of an electrolyte and also to ultrasonic agitation. The electrolysis comprises applying a pulsed electric potential to the metal body while the metal body is present in the bath, and the ultrasonic agitation is carried out while the body is still wet after the electrolysis.

Description

Descaling of Metal Surfaces

The present invention is concerned with the removal of scale from surfaces of metal bodies, such as steel, in the form of steel wires, rods or the like.

When mild steel is in air at a temperature in the range 575 to 1370C, a surface scale in the form of oxide scale (or heat scale) results on the steel surface. Before steel wires can be drawn or surface treated (such as by tinning or galvanising), such oxide scale must be removed. Currently, the most common method for removal of heat scale is to "pickle" the wire in dilute (5 to 40%) mineral acid, such as sulfuric acid or hydrochloric acid. Similar pickling methods are used for removal of surface coπiaminants from other metals (such as brass).

This pickling method has several drawbacks, including slow speed (pickling times of up to several minutes), the possibility of hydrogen evolved during pickling diffusing into the metal and causing embrittlement, and the rapid consumption of acid through the dissolution of scale (with simultaneous production of high concentrations of water-soluble heavy metal salts), causing a significant effluent problem.

Electrolytic pickling was first introduced in the 1930's, and originally the aim was to use electrolysis as a way to enhance conventional acid pickling techniques. Consequently, the electrolytes used have always tended to be derivatives of the main mineral acids used during the conventional pickling processes (sulfuric and hydrochloric acid).

In electrolytic pickling, an applied electric potential causes a current to flow between a pickling solution and a metal surface. The current may be anodic or cathodic and will typically be of a density of 1 to 200 amps dm².

Initially electrolytic pickling was introduced in order to make acidic pickling faster and more efficient and therefore the pickling solutions used changed very little. However, recently the advantages of lowering of acid concentration and temperature were fully recognised. Electrolytic pickling has several advantages over acid pickling, namely shorter pickling times, minimisation of hydrogen embrittlement, and removal of heat scale without dissolution, with consequent lower consumption of pickling solution and less heavy metal contaminated effluent. Another significant advantage of using an electrolytic process over more traditional methods is that the reactions involved are much more controllable.

A review of the use of electrolysis is given in "Wire Journal International", June 1985, pages 62-67. An example of the use of electrolytic pickling is described in GB 1571308, in which the process is followed by ultrasonic rinsing. A process involving electrolysis and simultaneous ultrasonic agitation is described in SU 916618; this is for use in an electroplating process.

The pickling time is minimised when the scale is porous or cracked (either initially or following pickling). The pickling time can also be lowered by agitation of the pickle liquor, as this can loosen insoluble scale and increase the rate at which solution at the scale surface is replenished.

Another type of surface scale for metals such as steel is a solid lubricant, such as graphite. Graphite is an effective die lubricant for wire drawing, but the subsequent removal of a compacted solid graphite layer from a metal surface is difficult. The currently preferred method is generally wet shot blasting or acid pickling.

According to the present invention there is provided a process of descaling the surface of a metal body, in which said metal body is subjected to electrolysis in a bath of an electrolyte and also to ultrasonic agitation, the electrolysis comprising applying a pulsed electric potential to the metal body while the latter is present in the bath, and the ultrasonic agitation being carried out while the body is still wet. The body may still be wet from the electrolyte or from another suitable intermediate solution. It is advantageous that the body is not allowed to dry prior to ultrasonic agitation, as drying hardens the scale and hinders successful descaling.

The electrolysis and the ultrasonic agitation may be carried out simultaneously or the electrolysis may be followed by a separate ultrasonic agitation step.

Typically, the process is carried out with the electrolyte bath at a substantially neutral pH.

The process is typically used for removal of scale from steel, which is typically in the form of wire, rod or other continuously formed article. Preferably the pulsed electric potential has a current density is in the range 0.1 to 10 amp cm², more preferably in the range 0.5 to 5 amp cm².

According to a first embodiment of the present invention, the electrolysis is carried out in a substantially aggressive electrolyte bath (that is a bath containing anions of a strong acid, such as a chloride, sulfate, nitrate or the like).

Preferably, the aggressive bath comprises a solution of an ammonium or alkali metal chloride or sulfate. In such salts, the alkali metal is typically sodium.

It is preferred that the electric potential is predominantly anodic, with the anodic pulse duty cycle being preferably at least 67% (such as at least 75%). It is particularly preferred for the anodic pulse duty cycle to be at least 90%.

According to a second embodiment of the present invention, the process is carried out in a substantially non-aggressive electrolyte.

Preferably, the non-aggressive electrolyte comprises a solution of an ammonium or alkali metal tripolyphosphate; typically, the alkali metal is sodium.

The electric potential may be predominantly anodic or cathodic, typically with an anodic pulse duty cycle of 5 to 95% such as 45 to 75%.

It is advantageous to use a tripolyphosphate electrolyte, because scale removed from the surface of the metal body using the process according to the invention is left as a solid in the electrolytic solution and can easily be removed by filtration, therefore allowing the electrolytic solution to be re-used. The phosphate also forms a protective layer on the cleaned surface, which inhibits corrosion.

Features of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a schematic illustration of an exemplary process and apparatus according to the invention;

Figure 2 is a diagrammatic illustration of a typical pulsed current waveform used for electrolytic current in a process according to the invention (preferably in the first embodiment of the invention);

Figure 3 is a graph showing cleaning times for removal of oxide from heat scaled carbon steel wire as a function of pH and current density in a process according to the first embodiment of the invention; Figure 4 is a graph showing cleaning times for removal of oxide from heat scaled carbon steel wire as a function of pH and current density in a process, not according to the invention, involving no ultrasonic treatment;

Figure 5 is a 3-dimensional plot showing cleaning times for the same wire in 10% aqueous NaCl solution at 65'C, as a function of the frequency and the anodic duty cycle;

Figure 6 is a graph showing the percentage of scale (graphite) remaining at the wire surface, as a function of time, for current densities of 0.5 and 2.5 amps cm"², according to the first embodiment of the present invention;

Figure 7 is a graph showing the time to clean as a function of current density at pH's of 0, 1 and 7, respectively;

Figure 8 is a graph showing the influence of anodic duty cycle on the time- dependent cleaning at pH 7 with a current density of percentage of scale (graphite) remaining at the wire surface, as a function of time, for current densities of 1 amp cm'*;

Figure 9 is a 3-dimensional surface plot of descaling time as a function of current density and temperature in neutral sodium chloride solution with an anode duty cycle of 95%;

Figure 10 is a 3-dimensional surface plot of descaling time as a function of current density and pH in sodium chloride solution with an anode duty cycle of 95% and interspersed ultrasound;

Figure 11 is a 3-dimensional surface plot of descaling time as a function of current density and temperature in neutral sodium chloride solution with an anode duty cycle of 95% and interspersed ultrasound;

Figure 12 is a 3-dimensional surface plot of descaling time for removal of graphite scale as a function of anodic duty cycle and frequency at 60°C in neutral sodium sulfate with a current density of 1 Acm²;

Figure 13 is a 3-dimensional surface plot of descaling time for removal of graphite scale as a function of current density and pH at 60°C in sodium sulfate with an anode duty cycle of 95% and interspersed ultrasound;

Figure 14 is a 3-dimensional surface plot of descaling time for removal of graphite scale as a function of current density and pH at 60°C and 1Hz in sodium sulfate with an anode duty cycle of 95% and continuous ultrasound; Figure 15 is a 3-dimensional surface plot of the descaling time for oxide heat scale as a function of the anodic duty cycle and frequency at 60°C and 1 Hz in a 10% sodium tripolyphosphate solution at pH7 and 1.6 Acm ² current density;

Figure 16 is a 3-dimensional surface plot of descaling time again for oxide heat scale as a function of current density and pH at 60°C and 1Hz in a 10% sodium tripolyphosphate solution with an anodic duty cycle of 95% and interspersed ultrasound;

Figure 17 is a 3-dimensional representation of the descaling time (again for oxide heat scale) vs electrolyte concentration and solution temperature, with data collected under the conditions of pH7, 1Hz at 95% anodic duty cycle; and

Figure 18 is a 2-dimensional graph of descaling time (again for oxide heat scale)vs anodic duty cycle at 1Hz frequency (for which the cleaning conditions were pH7, adjusted using orthophosphoricacid, 60°C, 10% sodium tripolyphosphate, samples being high carbon Si-Mn wire, pickled and subsequently scaled in air and 900°C for various times);

Referring to Figure 1, there is shown a computer 1 which is operatively connected to a voltage waveform controller 11 to generate a voltage waveform with an amplitude not greater than ±_ one volt. This voltage waveform was used to control the galvanostat 2 which passed a current of proportionate amplitude between respective electrodes 3 and 4.

Electrode 3 (in the form of wire) is the sample to be cleaned and electrode 4 is a graphitic carbon counter electrode. Both electrodes were mounted in a beaker 5 of electrolyte 6 which was in turn placed in an ultrasonic bath 7 containing water 10. The bath itself is thermostatically controlled in order to maintain a constant temperature. During the testing, the exposed area of the electrode 3 is submerged in the electrolyte 6. A Luggin capillary 8 was placed in contact with electrode 3 so that a reference electrode 9 could be used to measure the potential of electrode 3. This potential data was fed back to the Data Acquisition card in the computer 1 and recorded continuously.

The electrolytic current was applied to the cell using a pulsatile alternating current (a typical waveform being shown schematically in Figure 2). Experimental Method

The combined ultrasonic-electrolytic surface cleaning of wires was carried out using the cleaning cell apparatus shown schematically in Figure 1. The wire 3 to be cleaned was suspended in a volume of electrolyte 6 contained within a thermostated ultrasonic agitation bath 7. Electrolytic current was passed between the electrode wire 3 and a graphite counter electrode 4; in all cases the area of the electrode wire exposed to electrolyte was determined, in order to calculate surface current density. The flow of electrolytic current was established using a voltage controlled current source (galvanostat) 2 which was in turn actuated by voltage waveform controller 11. The electrolytic current passed in the cleaning cell was usually in the form of a pulsatile alternating current and a typical current waveform is shown schematically in Figure 2.

The ultrasonic agitation was either carried out continuously and simultaneously with the electrolytic current (referred to as simultaneous electrolysis-ultrasonication) or intermittently and in alternation with periods of electrolysis (referred to as interspersed electrolysis-ultrasonication): the object of these procedures was to determine whether or not any synergistic effects could be detected in the case of simultaneously applied ultrasound and electrolysis. The progress of cleaning was followed by the periodic withdrawal of the wire sample and visual examination of the surface. Two types of estimate of surface cleanliness were made:

1) Whether the surface scale had been completely removed or not: in this case the only quantity recorded was the "time to clean".

2) The fraction of surface scale remaining at the time; in this case the fraction of surface covered with scale was estimated by viewing through a millimetre grid and the " % scale remaining" recorded as a function of time.

In both the above cases "time" is the total time for which the electrolytic current is flowing at the sample surface. In experiments involving interspersed electrolysis- ultrasonication, the electrolytic current was interrupted and followed by a period of ultrasonication to remove any loosened scale immediately prior to visual evaluation of surface cleanliness. Unless otherwise stated all experiments were conducted with a 1Hz (1 cycle per second) squarewave electrolytic waveform i.e. with an anodic duty cycle (as defined with reference to Figure 2) of 0.5. Example 1 Oxide heat scale (aqueous sodium chloride)

Figure 3 shows cleaning times for oxide removal from heat scaled carbon steel wire in 10% aqueous sodium sulfate solution at 65°C as a function of pH and current density. Figure 4 shows cleaning times for the same system subject to ultrasound electrolysis. Figure 5 show cleaning times for the same wire in 10% aqueous sodium chloride solution at 65°C as a function of frequency and anodic duty cycle (duty cycle shown as a percentage figure); identical cleaning times were measured for the same system subject to interspersed ultrasound-electrolysis. Using interspersed ultrasound-electrolysis under neutral conditions (pH7) at 65°C, with a current density of 2 amp cm^"2 and an anodic duty cycle of 95%, cleaning was complete in approximately twenty seconds. xt may be seen from Figures 3 and 4 that cleaning time in the sulfate medium decreases with increasing current density and decreasing pH; cleaning times at pH3 were immeasurably long (> 30 minutes). Figures 3 and 4 also show that a synergistic effect exists between ultrasound and electrolysis, in that cleaning times are about 30% shorter in the case of simultaneous electrolysis-ultrasonication.

It may be seen from Figure 5 that cleaning times in the chloride medium are effectively independent of the frequency of the electrolytic current but decrease markedly with increasing anodic duty cycle. The observation that there are negligible differences in cleaning times for the cases of simultaneous and interspersed ultrasonication-electrolysis in neutral chloride implies that little or no synergistic effect exists between ultrasound and electrolysis under these conditions. It was found that making the electrolytic current entirely anodic (i.e. d.c) resulted in increases in cleaning time together with significant amounts of anodic chlorine evolution due to chloride electrolysis; however, pulsing the d.c. current (with no cathodic half cycle) gave a marginal improvement over the fastest pulsed a.c. cleaning times with little chlorine evolution. It was concluded from these findings that:

1) The rate determining step for oxide scale removal was the anodic dissolution of underlying metal.

2) That the competing reaction (anodic chlorine evolution) was discouraged by pulsed a.c. or d.c. electrolysis, possibly by a depassivation of the metal surface during the zero current or cathodic part of the cycle. (Here, "passivation" means the covering of the metal surface with a dissolution resistant, electrolytically grown, oxide layer.) 3) Ultrasound alone has no effect.

4) Oxide scale removal, leaving a clean, satin textured, metal surface is possible using combined ultrasound and anodic d.c. electrolysis in aqueous sodium sulfate solutions at pH < 3.

5) Oxide scale removal, leaving a clean, satin textured, metal surface is possible using combined ultrasound and anodic d.c. electrolysis in aqueous sodium chloride solutions at pH7 but with significant anodic chlorine evolution.

6) Pulsing the electrolytic current gives significantly faster cleaning than the d.c. method and greatly reduced the amount of chlorine evolved in aqueous sodium chloride.

7) Electrolysis alone loosens the oxide layer but does not remove it.

8) Oxide detachment appears to occur by the anodic dissolution of a thin layer of the underlying metal.

Example 2

Graphite drawing lubricant (aqueous sodium sulfate).

The following results were all obtained from graphite drawn carbon steel wire in 10% aqueous sodium sulfate solution at 50°C.

Figure 6 shows the percentage of scale (graphite) remaining at the wire surface as a function of time, for current densities of 0.5 and 2.5 amps cm ², with and without simultaneous ultrasound.

Figure 7 shows time to clean as a function of current density at pH 0, 1 and 7; and Figure 8 shows the influence of anodic duty cycle on the time dependent cleaning curve at pH7 with a current density of 1 amp cm².

Under neutral conditions (pH7) at 50°C, with a current density of 2 amp cm ² and an anodic duty cycle of 95 % , cleaning was complete in approximately ten seconds.

Figure 6 shows that, although the shapes of the cleaning curves are different for the cases of simultaneous electrolysis-ultrasonication and interspersed electrolysis- ultrasonication, there is no significant influence of simultaneous ultrasonication on time to clean (also see Figure 7). Figure 7 reveals that graphite removal is most rapidly accomplished at low pH but that the influence of pH is reduced a higher current densities. Figure 8 shows that cleaning rates increase markedly with increasing anodic duty cycle; however, it was also found that making the electrolytic current entirely anodic i.e. d.c. resulted in large increases in cleaning time together with significant amounts of anodic oxygen evolution due to water electrolysis.

Further results of descaling of graphite scale are illustrated in Figures 9 to 14. It was concluded from the results descibed above that:

1) The rate determining step for graphite removal was the anodic dissolution of underlying metal.

2) That the competing reaction (anodic oxygen evolution) was discouraged by pulsed a.c. electrolysis, possibly by a depassivation of the metal surface during the cathodic half cycle.

3) Ultrasound alone has no effect.

4) Combined ultrasound and anodic d.c. electrolysis in aqueous sodium chloride solutions results in partial graphite removal, leaving a highly pitted metal surface, with significant concomitant chlorine evolution.

5) Combined ultrasound and anodic d.c. electrolysis in aqueous sodium sulfate solutions results in graphite removal, leaving a clean satin textured metal surface, with significant concomitant oxygen evolution.

6) Pulsing the electrolytic current in aqueous sodium sulfate solutions, with alternating anodic and cathodic half cycles, gives significantly faster cleaning than the d.c. method with no significant concomitant oxygen evolution.

7) Electrolysis alone loosens the graphite layer but does not remove it.

8) Graphite detachment appears to occur by the anodic dissolution of a thin layer of the underlying metal.

Example 3

Oxide heat scale (sodium tripolyphosphate).

A 10% sodium tripolyphosphate bath adjusted to pH 7 and raised to 60°C was set up. The current density for each sample was 1.6 Acm^'2, representing lcm length of metal surface exposed for descaling. The electrical properties were methodically varied, the anodic city cycle adjusted from 5 to 95% and the frequency of pulsed ranging from 0.3 to 1000 Hz. Descaling times obtained were compiled and arranged into a 3-dimensional graph shown in Figure 15. Optimum conditions appear to be obtained with an anodic duty cycle of 45-75% and frequencies 0.3 to 100 Hz. For these particular set of conditions, fastest cleaning times are achieved at an anodic duty cycle of 75% and at the lowest frequency of 0.3 or 1 Hz.

Figure 16 shows a 3-dimensional plot of the results compiled from a 10% sodium tripolyphosphate bath raised to 60°C and the potentiostat set at an anodic duty cycle of 95% with a frequency of 1 Hz. The acidity of the solution was varied from pH 3 to 12 and the current density adjusted systematically from 0.5 to 2.5 Acm'². Orthophosphoric acid was used to adjust the pH.

Lowest descaling times were clearly obtained at highest current density and lowest pH value of 3. At pH 12 where the solution is very alkaline, descaling becomes slow and inefficient with cleaning times reaching values of a several minutes as the current density is decreased below 2 Acm^"2. Under neutral conditions (pH7), descaling times are acceptably rapid, only a few seconds slower than under the more acidic conditions of pH3.

Figure 17 shows a 3-dimensional representation of the descaling time results vs the tripolyphosphate concentration and solution temperature. The tripolyphosphate concentration was varied from 1-15% and the temperature of the bath adjusted at 20-60°C. The pH value was kept constant at 7 and the anodic duty cycle fixed at 95 % with a frequency of 1 Hz.

Figure 18 visually summarises results obtained on descaling times using sodium tripolyphosphate with varying heat scale thickness.

A furnace was allowed to reach the temperature of 900°C before being filled with argon gas. Samples were laid out flat on a ceramic boat, separated from each other, and subsequently left in the furnace for 15 minutes so as to allow them to reach 900°C. The furnace was subsequently flushed through with a fast stream of air for a period of 20 seconds and the samples were left to oxidise for 1-60 minutes. Once sealed for the required period of time, the boat was removed from the furnace and placed on a ceramic fibre mat to cool in air at room temperature. Samples were left to oxidise for 1,5, 10,15,30,45 and 60 minutes, to ensure a considerable increase in the scale thickness obtained. Using a 10% sodium tripolyphosphate electrolyte bath adjusted to pH7 with orthophosphoric acid, the cleaning solution was raised to 60°C and exposed to ultrasound for a minimum period of 15 minutes prior to experimentation. Electrical properties were set at IA and the current pulse fixed at 1 Hz. The anodic duty cycle was varied between 5-95% and its efficiency testing for the descaling of wire of various oxide thicknesses.

A general trend is evident with the descaling times at their lowest values when a 5% anodic duty cycle is used, irrespective of the scale thickness.

Samples oxidised in air for a period of 1 to 15 minutes show very similar cleaning time requirements.

As the oxidation periods of the samples increase to 30 to 60 minutes, suddenly a significant increase in cleaning times is observed.

Optimum descaling conditions for fastest descaling the metal samples were obtained at high electrolyte concentrations (10-15%) and high temperatures of 50-60°C.

Claims

Claims:

1. A process of descaling the surface of a metal body, in which said metal body is subjected to electrolysis in a bath of an electrolyte and also to ultrasonic agitation, characterised in that said electrolysis comprises applying a pulsed electric potential to said metal body while said metal body is present in said bath, and said ultrasonic agitation is carried out while said body is still wet.

2. A process according to claim 1 , wherein said electrolysis and said ultrasonic agitation are performed simultaneously.

3. A process according to claim 1, wherein said electrolysis is followed by a separate ultrasonic agitation step.

4. A process according to any of claims 1 to 3, wherein said bath of electrolyte is at a substantially neutral pH.

5. A process according to any of claims 1 to 4, wherein said metal is steel.

6. A process according to any of claims 1 to 5, wherein said body comprises a continuously formed article.

7. A process according to any of claims 1 to 6, wherein said pulsed electric potential has a curcent density in the range 0.1 to 10 amp cm².

8. A process according to claim 7, wherein said current density is in the range 0.5 to 5 amp cm ².

9. A process according to any of claims 1 to 8, wherein said electrolyte bath is substantially aggressive.

10. A process according to claim 9, wherein said aggressive bath comprises a solution of an ammonium or alkali metal chloride, nitrate or sulfate.

11. A process according to claim 10, wherein said alkali metal is sodium.

12. A process according to any of claims 9 to 11, wherein said electric potential is applied predominantly in anodic pulses.

13. A process according to claim 12, wherein said electric potential has an anodic duty cycle of at least 67%.

14. A process according to claim 13, wherein said duty cycle is at least 75%.

15. A process according to any of claims 1 to 8, wherein said electrolyte bath is substantially non-aggressive.

16. A process according to claim 15, wherein said bath comprises a solution of an ammonium or alkali metal tripolyphosphate.

17. A process according to claim 16, wherein said alkali metal is sodium.