CA2251782A1 - Descaling of metal surfaces - Google Patents

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

CA 022~1782 1998-10-13 Decl~linP of Metal Surfaces The present invention is concerned with the removal of scale from s~ s of metal bodies, such as steel, in the form of steel wires, rods or the like.
When mild steel is in air at a ~enlpe~dtule 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 hydf~chloric acid. Similar pickling methods are used for removal of surface conr~min~nt~ from other metals (such as brass).
This pir~ling method has several drawbacks, including slow speed (~)icl~ling times of up to several ...;n..l~s), the possibility of hydrogen evolved during pickling diffusing into the metal and causing embritt1pm~ntJ and the rapid consumption of acid through the ol~ltion 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 aimwas to use electrolysis as a way to enh~nce convention~l acid piclding te~-hni~lues.
Consequently, the electrolytes used have always tended to be derivatives of the main mineral acids used during the conventional pickling plocesses (sulfuric and hydrochloric acid).
In electrolytic pickling, an applied electric potential causes a current to flowbetween a pickling solution and a metal surface. The current may be anodic or c~tho~ic and will typically be of a density of 1 to 200 amps dm-2.
Initially electrolytic pickling was introduced in order to make acidic pickling faster and more efflcient and therefore the pickling solutions used changed very little.
However, recently the advantages of lowering of acid concentration and tenlpeldtu,t; were fully recognised.

.. ..

CA 022~1782 1998-10-13 Electrolytic pickling has several advantages over acid pickling, namely shorter pickling times, minimi~tion of hydrogen embrittlement, and removal of heat scale without dissolution, with consequent lower consumption of pickling solution and less heavy metal co~t~min~tçd 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 Tnt~rn~tis)n~
June 1985, pages 62-67. An example of the use of electrolytic pic~ling 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 minimic~d when the scale is porous or cr~ d (either initially or following pickling). The piclcling time can also be lowered by agitation of the piclde liquor, as this can loosen insoluble scale and increase the rate at which solution at the scale surface is rep1eni~hPd.
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 ~;u~ tly fell~d method is generally wet shot blasting or acid pickling.
According to the present invention there is provided a process of desc~ling 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 ~git~tion, 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 internledi~te solution. It is advantageous that the body is not allowed to dry prior to ultrasonic agitation, as drying hardens the scale and hinders succç~ful desc~ling.
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.

~ f~ably the pulsed electric potential has a current density is in the range 0.1 to 10 amp cm-2, more p~efel~bly in the range 0.5 to 5 amp cm~2.
According to a first embodiment of the present invention, the electr~ is is carried out in a subst~nti~lly aggressive electrolyte bath (that is a bath cont~ g anions of a strong acid, such as a chlori~le, sulfate, nitrate or the like).
~feldbly, the aggr~ssi~e bath compri~es a solution of an ~......... o-.i.. or alkali metal chlorid~ or sulfate. In such salts, the alkali metal is typically so~ m.
It is ~f~lled that the electric po~ tial is predo.,.u-~n~ly anodic, with the anodic pulse duty cycle being preferably at least 67% (such as at least 75%). It is particularly prerc;l~ed 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 suhst~nti~lly non-aggressive electrolyte.
~ efelably~ the non-aggressive electrolyte compri~s a solutit)n of an~mmonillm or alkali metal tripolyphosphate; typically, the alkali metal is so-linm The electric ~otel.lial may be predominantly anodic or cathodic, typically with an anodic pulse duty cycle of S 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, thc.~for~ allowing the electrolytic soh~tinn to be re-used. The phosphate also forms a prote ~ e layer on the cleaned surface, which inhibits co~lusion.
Features of the invention will now be described, by way of e Y~mple only, with reference to the accompanying drawings, wherein:
Figure 1 is a sch~m~tic illustration of an exel-lpl~y process and ap~alus according to the invention;
Figure 2 is a diagr~mm~tir illustration of a typical pulsed current waveform used for electrolytic current in a process according to the invention (preferably in the first embo~im~-nt of the invention);
Figure 3 is a graph showing cle~ning 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;

Pigure 4 is a graph showing c1e~nin~ times for removal of oxide from heat scaled carbon steel wire as a function of pH and current density in a process, not accoldil~g to the invention, involving no ult~conic lre~ t;
Figure S is a 3--1imen.cional plot showing cl~ning times for the same wire in 10% aqueous NaCI solution at 65 C, as a function of the frequency and the anodic duty cycle;
Pigure 6 is a graph showing the percentage of scale (gl~hite) r~ e at the wire surface, as a function of time, for current ~lenciti~s of 0.5 and 2.5 amps cm-2, according to the first embodim~nt 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-~lepend~nt ~le~ni-lg at pH 7 with a current density of ~rc~ n ~e of scale (~-al-h;~-o) r~ ning at the wire s~ ce, as a function of time, for current ~len.ci~i-o.s of I amp cm2;
~ :igure 9 is a 3-dimen~ional surface plot of de~ling time as a function of current density and te~ d~ure in neutral sodium chloride solution with an anode duty cycle of 95%;
Figure 10 is a 3-~imtonsion~l surface plot of desc~ling time as a function of current density and pH in sodium chloride solution with an anode duty cycle of 95% and inlel~ ed ultrasound;
Figure 11 is a 3-dimencional surface plot of desc~ling time as a function of current density and ~en~ lu-c in neutral sodium chloride solution with an anode duty cycle of 95 ~o and inte~ ed ultrasound;
Figure 12 is a 3-~1im~-nsiQn~l surface plot of desc~lin~ time for removal of ~ hile scale as a function of anodic duty cycle and frequency at 60~C in neutrai sodium sulfate with a current density of 1 Acm~2;
Figure 13 is a 3-dimPnsional surface plot of desc~lin~ time for removal of rhi~. scale as a function of current density and pH at 60~C in sodium sulfate with an anode duty cycle of 95% and inlel~elsed ultrasound;
Figure 14 is a 3-~im~n~ional surface plot of desc~ling time for removal of gld~)hile scale as a function of current density and pH at 60~C and lHz in sodium sulfate with an anode duty cycle of 95% and continuous ultrasound;

~.. ~,,, Figure 15 is a 3-~limPn~ional surface plot of the desc~linE time for oxide heat scale as a function of the anodic duty cycle and frequency at 60~C and lHz in a 10% sodium tripolyphosphate solution at pH7 and 1.6 Acm~2 current density;
Figure 16 is a 3--limPn~iQnal surface plot of desc~lin~ time again for oxide heat scale as a function of current density and pH at 60~C and lHz in a 10% sodium tripolyphosphate solution with an anodic duty cycle of 95% and inte~ ed ultrasound;
Figure 17 is a 3-~lim~n~ional l~pl~S'-I~t~ti~ of the desc~inE time (again for oxide heat scale) vs electrolyte conce-ntration and solution te~ )elature~ with data collected under the contlitions of pH7, lHz at 95% anodic duty cycle; and Figure 18 is a 2--1imencion~1 graph of desc~ling time (again for oxide heat scale)vs anodic duty cycle at lHz frequency (for which the çle~ninE conditions were pH7, adiusted using orthophosphoric acid, 60~C, 10% sodium tripolyphosphate, samples being high carbon Si-Mn wire, piclcled 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 ll 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 ~;,pecLi~e electrodes 3 and 4.
Electrode 3 (in the form of wire) is the sadmple 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 cont~ininE water 10. The bath itself is thermost~tir~lly controlled in order to maintain a constant telllpeldture. 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 ~lleasure 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 ~ltern~tinE
current ~a typical waveforrn being shown schematically in Figure 2).

Experimental M~th~l The combined ultrasonic-electrolytic surface cle~nin~ of wires was carried out using the cle~nlng cell apparatus shown schem~tic~lly in Figure 1. The wire 3 to be cleaned was susp~Pn-lPcl in a volume of electrolyte 6 contained within a th~l..os~ ed ~
agitation bat,h 7. Electrolytic current was passed between the electrode wire 3 and a g~dl,lhle counter electrode 4; in all cases the area of the electrode wire exposed to electrolyte was determined, in order to c~lr,ul~t-P surface current density. The flow of electrolytic current was established using a voltage controlled current source ~galvanostat) 2 which was in turn ~rt-l~t~d by voltage waveform controller 11. The electrolytic current passed in the c1P~nir~g cell was usually in the form of a pulsatile alternating current and a typical current waveform is shown sch~-m~tir~lly in Figure 2, The ultrasonic agitation was either carried out continuously and simultaneously with the electrolytic current (referred to as simultaneous electrolysis-ultrasonication) or intermitt~ntly and in ~lt~ ti-ln with periods of electrolysis (referred to as intera~
electrolysis-ul~onic~tion): the object of these procedures was to detel,l.ine whether or not any syner~istic effects could be det~cted in the case of simultaneously applied ultrasound and electrolysis. The progress of çle~ning was followed by the periodic withdrawal of the wire sample and visual e-t~min~tion of the surface. Two types of estim~te of surface cle~nlin~
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 fr~ction of surface scale rçm~ining at the time; in this case the fraction of surface covere~ with scale was estim~te~ by viewing through a millimetre grid and the "%scale rem~inin~" 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-r~oni~tion~ the electrolytic current was inte~ ed and followed by a period ofIlltr~onir~tion to remove any loosened scale imrnetli~tely prior to visual evaluation of surface c1~nlinesc. Unless otherwise stated all experiments were conducted with a lHz (1 cycle per second) squarewave electrolytic waveform i.e. with an anodic duty cycle (as defined with reference to Figure 2) of 0.5.

F~r~mple 1 Oxide heat scale (aqueous so~ium chloride) Figure 3 shows cLP~ning 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 cl~P~nin~ times for the same system subject to ultrasound electrolysis. Figure 5 show clP~nin~ times for the same wire in 10% aqueous sodium chlori~1P, sol~lti. n at 65~C
as a function of frequency and anodic duty cycle (duty cycle shown as a per~l,~ge figure);
i~Pnti~l cl~P~ning times were meas.lred for the same system subject to int~
ultrasound-electrolysis. Using intel~l)el~ed ultrasound-electrolysis under neutral c~n-~iti (pH7) at 65~C, with a current density of 2 amp cm~2 and an anodic duty cycle of 95%, clP~nin~ was complete in apl,nJ~-imately twenty seconds.
it may be seen from Figures 3 and 4 that sle~ning time in the sulfate mPAi~lm dec,cases with increasing current density and decreasing pH; clP~nin~ times at pH3 were immP~ hly ;~ng ( > 30 minutes). Figures 3 and 4 also show that a synergistic effect exists between ultrasound and electrolysis, in that cle~ning times are about 30% shorta in the case of simult~npous electrolysis-ultrasonication.
It may be seen from Figure 5 that cleaning times in the chlori~P mPAi-)m are effectively indepPnd~Pnt of the frequency of the electrolytic current but decrease m~rlrPAly with increasing anodic duty cycle. The observation that there are negli~ihlP ~lirÇelcllces in clP~nin~ times for the cases of simultaneous and inte,~yel~cd ultrasoni~tion-electrolysis in neutra} rhlnride 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 signifir~nt 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. c1P~ling times with little chlorinP, evolution. It was concluded from these finrlin~ that:
1) The rate delellllining step for oxide scale removal was the anodic ~ $ollltion of underlying metal.
2) That the colnpe~ g 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 L~;Alul~d, metal surface is possible using combined ultrasound and anodic d.c. electrolysis in aqueous sodium sulfate solutions at pH~3.
S) Oxide scale removal, leaving a clean, satin textured, metal surface is possible using combined ultrasound and anodic d.c. electrolysis in aqueous sodium chlori~e solutions at pH7 but with ~ignifi~nt anodic chlorine evolution.
6) Pulsing the electrolytic current gives signific~ntly faster çlP~ning than the d.c. method and greatly reduced the amount of chlorine evolved in aqueous sodium chlt)ri-le.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.

F~? -- ~ I P 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 (gl~hite) rem~ining at the wire surface as a function of time, for current densitiçs of 0.5 and 2.5 amps cm-2, with and wilhou cimul~neous 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 ç1P~ning curve at pH7 with a current density of 1 amp cm-2.
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 cle~nin~ curves are different for the cases of simlllt~np~us electrolysis-ultMsonication and inte~ d electrolysis-ultrasonication, there is no significant influence of simultaneous ultr~onic~tion 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 o~ pH is reduced a higher current den~ities . .

CA 022F1782 1998- lo- 13 g Figure 8 shows that cle~ning 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 cle~ning time together with signific~nt amounts of anodic oxygen evolution due to water electrolysis.
Further results of des~ling of graphite scale are illustrated in Figures 9 to 14.
It was concluded from the results descibed above that:
1) The rate dt;~e.,-,ining step for graphite removal was the anodic ~ ol~ltinn of underlying metal.
2) That the co.l,peling 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 ~igniflc~nt concomitant chlorine evolution.
S) Combined ultrasound and anodic d.c. electrolysis in aqueous sodium sulfate solutions results in graphite removal, leaving a clean satin textured metal surface, with si~nific~nt concomitant oxygen evolution.
6) Pulsing the electrolytic current in aqueous sodium sulfate solutions, with ~lt~ g anodic and cathodic half cycles, gives cignifi~ntly faster cl~nin~ than the d.c.method with no si~nifie~nt concomitant oxygen evolution.
7) Electrolysis alone loosens the graphite layer but does not remove it.
8) Graphite ~et~hmPnt appears to occur by the anodic dissolution of a thin layer of the underlying metal.

Example 3 Oxide heat scale (,cor! ~ tripolyphos~te).
A 10% sodium tripolyphosphate bath adjusted to pH 7 and raised to 6~'C was set up. The current density for each sample was 1.6 Acm~2, representing lcm length of metal surface exposed for desc~ling. The electrical properties were methodically varied, the anodic city cycle adjusted from S to 95~ and the frequency of pulsed ranging from 0.3 to 1000 Hz.

CA 022F,1782 1998- lo- 13 Desc~lin~ times obtained were compiled and arranged into a 3-~1imPn~ior~1 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 condi~innc, 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-rlimPncional 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 Sy~lr~ tiC~lly from 0.5 to 2.5 Acm-2. Orthophosphoric acid was used to adjust the pH.
Lowest cies~ling times were clearly obtained at highest current density and lowest pH value of 3. At pH 12 where the solution is very ~Ik~line, desc~ling becomes slow and in~ffici~nt with cle~nin~ times reaching values of a several minutes as the current density is decreased below 2 Acm~2. Under neutral coll~itionC (pH7), desc~lin~ times are acceptably rapid, only a few se~Q~ds slower than under the more acidic conditions of pH3.
Figure 17 shows a 3-~im~ncional rc;~resenhtion of the desc~lin~ time results vs the tripolyphosphate concent~tion and solution te"~pe,dture. The tripolyphosph~tP
concentration was varied from 1-15% and the temperature of the bath adjusted at 20-6()~C.
The pH value was kept constant at 7 and the anodic duty cycle fixed at 95 ~o with a fre~uency of 1 Hz.
Figure 18 visually summarises results obtained on desc~lin~ times using sodium tripolyphosphate with varying heat scale thickness.
A furnace was allowed to reach the le~ elalule of 900'C before being filled with argon gas. Samples were laid out flat on a ceramic boat, s~yara~ed 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 s~mples 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 ten~pel~ule. 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 çle~ning solution was raised to 60~C and exposed to ultrasound for a minimum period of 15 minutes prior to experimentation. Electrical ~lo~ellies were set at lA and the current pulse fixed at 1 Hz. The anodic duty cycle was varied belween 5-95%
and its efficiency testing for the desc~ling of wire of various oxide thic~nps~es~
A general trend is evident with the dçsc~ling times at their lowest values when a 5% anodic duty cycle is used, irrespective of the scale thic~ne~c S~mrlPs oYiflised in air for a period of 1 to 15 ,..~ es show very similar ~nin~ time requirem~pnt~.
As the oxidation periods of the samples increase to 30 to 60 mimlt.~Pc~ sud-lP-nly a significant increase in cle~ning times is observed.
Optimum desc~ling conditions for fastest ~esc-~ling the metal s~mples were obtained at high electrolyte concPntrations (10-15%) and high te-"~ s of 50-6(PC.

Claims (17)

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 ultraconic 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 current density in the range 0.1 to 10 amp cm-2.

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.