EP0162489A1 - Process for controlling fluoride-containing baths for metal surface treatment - Google Patents

Process for controlling fluoride-containing baths for metal surface treatment Download PDF

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Publication number
EP0162489A1
EP0162489A1 EP85200521A EP85200521A EP0162489A1 EP 0162489 A1 EP0162489 A1 EP 0162489A1 EP 85200521 A EP85200521 A EP 85200521A EP 85200521 A EP85200521 A EP 85200521A EP 0162489 A1 EP0162489 A1 EP 0162489A1
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European Patent Office
Prior art keywords
fluoride
activity
bath
solution
baths
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EP85200521A
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German (de)
French (fr)
Inventor
Joachim Albrecht
Klaus Bittner
Klaus Dr. Wittel
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GEA Group AG
Continentale Parker Ste
Continentale Parker SA
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Metallgesellschaft AG
Continentale Parker Ste
Continentale Parker SA
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C22/73—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals characterised by the process
    • C23C22/77—Controlling or regulating of the coating process

Definitions

  • the invention relates to a method for maintaining the fluoride activity in fluoride-containing baths for the surface treatment of metals by determining the fluoride activity by means of a fluoride-sensitive electrode and supplementing the baths with fluoride ions, and its application to chromating baths.
  • fluoride-containing solutions such as chromating solutions or phosphating solutions
  • fluoride-containing solutions such as chromating solutions or phosphating solutions
  • the etching effect of a fluoride-containing solution on glass is measured by weighing a glass sample before and after treatment with the solution and calculating the fluoride activity from the weight loss of the glass.
  • the process is reliable, however time consuming and very difficult to automate and can be used for continuous automatic control.
  • DE-AS 12 91 919 describes a method for measuring the fluoride activity of acidic aqueous fluoride solutions, in which the current strength in an electrolysis cell at constant voltage is taken as a measure of the fluoride activity of the solution contained in the cell.
  • the process works with an inert cathode and an anode, which consists of p-doped silicon.
  • Defects of this method are the non-linearity of the relationship between fluoride activity / current strength, the failure of the method at ph> 4 and the direct dependence on the surface of the measuring electrode and its properties in all amperometric methods, which in practice only obtain reliable, reproducible values after complex, careful cleaning cycles leaves.
  • DE-PS 11 57 610 and DE-AS 12 91 919 have in common that it is not the concentration or the activity defined in the context of thermodynamics that is linked to the concentration by the activity coefficients that is measured, but rather an “activity” in relation to a special reaction.
  • the situation is similar with the anode reaction of the p-doped silicon in the case of DE-AS 12 91 919.
  • a method for measuring and regulating the fluoride activity is desirable, which is not due to the difficult to reproduce and complex material properties of the Measuring device is defined, but the direct measurement of the activity of fluoride ions, as defined in chemical thermodynamics, and thus allows a much better transferability of the values.
  • fluoride-sensitive electrodes for measuring fluoride activity which contain a crystal of lanthanum fluoride as an essential component, have been developed. They are just as suitable for direct measurement of fluoride activity as glass electrodes, for example, for pH. Since the degree of dissociation of acids depends on the pH value and hydrofluoric acid as a moderately weak acid (pK a approx. 3.5) dissociates only weakly at low ( ⁇ 2) pH values, the measurement with fluoride-sensitive electrodes in buffered solutions is carried out at pH -Range 5 - 8 recommended, as the fluoride activity in this range is only slightly dependent on the pH value.
  • a disadvantage of this apparently attractive method of fluoride ion activity measurement directly in the treatment solution is that the fluoride-sensitive electrode cannot be used directly for continuous measurement of fluoride activity, for two reasons in particular: firstly, the electrode changes over the course of a few hours when it is in solutions is immersed, such as those used for chromating zinc.
  • these solutions can contain the following ions: CrO 4 -, Cr +++ , SO 4 Zn ++ , NO 3 - , SiF 6 - , TiF 6 H 2 PO 4 - , HPO 4 - .
  • Probably a layer forms on the surface of the LaF 3 crystal used for the measurement, which falsifies the measurement of the fluoride activity.
  • the object of the present invention is to provide a method for maintaining the fluoride activity in fluoride-containing baths for the surface treatment of metals, which does not have the aforementioned disadvantages, is reliable and thereby e.g. with an even, i.e. non-varying layer formation is connected, and is still easy to carry out.
  • the object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the fluoride-containing working bath is at least controlled in a controlled manner with a solution whose pH deviates from the working bath by a maximum of one pH unit diluted to five times its volume, then measures the fluoride activity and determines the fluoride activity of the working bath from the measured value.
  • Deviation by a maximum of one pH value unit should express that at a pH value of the working bath of e.g. 1.5 dilution with a solution with a pH of 0.5 or 2.5 is just permitted.
  • the fluoride-containing bath is preferably diluted to 20 to 200 times its volume.
  • the degree of dilution is capped by the risk of insufficient dilution, capped by the desire for the lowest possible consumption of dilution solution, and the content of traces of F - in the dilution solution.
  • the fluoride-containing bath is diluted with a buffer solution that absorbs such fluctuations, so that even small deviations in the pH of the treatment bath do not lead to undesired changes in the pH of the measurement solution and thus make fluoride control more difficult .
  • the HCl / KC1 system contains a buffer solution that is particularly suitable for dilution.
  • the deviation of the actual value determined from the measured value from the target value required for the surface treatment process serves to regulate the addition with fluoride-containing solution.
  • the addition which has to take into account both the active substances consumed by reaction with the metal surface and by removal with the treated parts and by overflow from the treatment bath, is usually carried out by two supplementary concentrates.
  • the addition of the concentrate which is fluoride-free or contains only a little fluoride, is carried out by known titrations or other measurements appropriate to the respective surface treatment process, e.g. electrical conductivity.
  • the need for fluoride that goes beyond this regulated addition is supplemented with a fluoride solution by the method according to the invention.
  • the first supplementary concentrate must be composed in such a way that a state of excessive fluoride activity in the treatment bath is never reached.
  • the second, mainly fluoride-containing supplement concentrate can also contain other components.
  • the determination of the fluoride activity according to the invention can advantageously be used to regulate the entire supplement.
  • the method according to the invention can be used universally and is used in particular to maintain the fluoride activity of chemical or electrolytic chromating baths, preferably for chromating zinc and / or aluminum surfaces.
  • the basis weight of the chromate layer in the green chromating of aluminum depends essentially on the fluoride activity. If different basis weights are required for different productions, a separate fluoride supplement is required.
  • the advantages of the invention are that surface coverings and other changes in the measuring crystal which impair the measurement are effectively eliminated.
  • the measuring range is not restricted, since the measuring range for the fluoride-sensitive electrode comprises many powers of ten and is essentially limited by the content of fluoride traces in the solution used for dilution.
  • the fluoride-sensitive electrode used (Orion, type 96-09-00) was immersed in the individual solutions of 20 ° C. for two minutes and then a measured value of + 190 mV for solution 1 and a measured value of + 134 mV for solution 2.
  • the measured value was recorded after 2 minutes of immersion at a solution temperature of 20 ° C.
  • a chromating solution was prepared to chromate hot-dip galvanized steel sheets that had previously been cleaned in a strongly alkaline cleaner and rinsed with water contained and had a pH of about 0.9.
  • a bath sample was made 100 times with a buffer solution contained in deionized water, diluted.
  • the measured value obtained was + 157 mV, which corresponded to the target value on the previously determined calibration curve.
  • the first step was with a containing solution added to constant chromic acid (8.0 g / 1). Then a bath sample of the supplemented solution was diluted 100 times with the abovementioned buffer solution in order to determine the F - activity. The resultant measured value was + 160 mV (20 ° C). Taking the calibration curve into account, the measured value corresponded to a fluoride activity in the chromating solution of 0.35 g / l. The chromating solution was then supplemented with 50 mg / 1 F - . A control measurement showed that the original measured value of + 157 mV (20 °) corresponding to a fluoride activity in the chromating solution of 0.40 g / 1 was reached again.

Abstract

The fluoride activity of a bath for the surface treatment of metals is maintained by diluting a sample of the bath to at least 5 volumes with a diluent solution having a pH within 1 pH unit of the bath measuring the fluoride content of the diluted solution using a fluoride-sensitive electrode, and utilising the measured value to replenish the bath with fluoride ion. Preferably the dilution is to 20-200 volumes with a HCl/KCl buffer solution.

Description

Die Erfindung betrifft ein Verfahren zur Aufrechterhaltung der Fluoridaktivität in fluoridhaltigen Bädern zur Oberflächenbehandlung von Metallen durch Ermittlung der Fluoridaktivität mittels einer fluoridsensitiven Elektrode und Ergänzung der Bäder durch Fluoridionen sowie dessen Anwendung auf Chromatierbäder.The invention relates to a method for maintaining the fluoride activity in fluoride-containing baths for the surface treatment of metals by determining the fluoride activity by means of a fluoride-sensitive electrode and supplementing the baths with fluoride ions, and its application to chromating baths.

Es ist bekannt, fluoridhaltige Lösungen, wie Chromatierungslösungen oder Phosphatierungslösungen, zur Oberflächenbehandlung von Metallen einzusetzen. Damit die durch den Fluoridgehalt erzielten Effekte beibehalten werden, ist es erforderlich, die Konzentration von Fluorid wie auch die der anderen Badkomponenten möglichst konstant zu halten.It is known to use fluoride-containing solutions, such as chromating solutions or phosphating solutions, for the surface treatment of metals. In order to maintain the effects achieved by the fluoride content, it is necessary to keep the concentration of fluoride as well as that of the other bath components as constant as possible.

So wird zur Messung der Fluoridaktivität gemäß DE-PS 11 57 610 die Ätzwirkung einer fluoridhaltigen Lösung auf Glas gemessen, indem eine Glasprobe vor und nach der Behandlung mit der Lösung gewogen und die Fluoridaktivität aus dem Gewichtsverlust des Glases berechnet wird. Das Verfahren ist zwar verläßlich, aber zeitraubend und nur sehr schwierig automatisierbar und zur kontinuierlichen automatischen Kontrolle einsetzbar.To measure the fluoride activity according to DE-P S 11 57 610, the etching effect of a fluoride-containing solution on glass is measured by weighing a glass sample before and after treatment with the solution and calculating the fluoride activity from the weight loss of the glass. The process is reliable, however time consuming and very difficult to automate and can be used for continuous automatic control.

Die DE-AS 12 91 919 beschreibt ein Verfahren zum Messen der Fluoridaktivität saurer wäßriger Fluoridlösungen, bei dem die Stromstärke in einer Elektrolysezelle bei konstant angelegter Spannung als MaB für die Fluoridaktivität der in der Zelle enthaltenen Lösung genommen wird. Das Verfahren arbeitet mit einer inerten Kathode und einer Anode, die aus p-dotiertem Silizium besteht.DE-AS 12 91 919 describes a method for measuring the fluoride activity of acidic aqueous fluoride solutions, in which the current strength in an electrolysis cell at constant voltage is taken as a measure of the fluoride activity of the solution contained in the cell. The process works with an inert cathode and an anode, which consists of p-doped silicon.

Mängel dieses Verfahrens sind die Nichtlinearität des Zusammenhangs Fluoridaktivität/Stromstärke, das Versagen der Methode bei ph>4 und die bei allen amperometrischen Methoden direkte Abhängigkeit von der Meßelektrodenoberfläche und deren Beschaffenheit, die in der Praxis nur nach aufwendigen, sorgfältigen Reinigungszyklen zuverlässige, reproduzierbare Werte erhalten läßt.Defects of this method are the non-linearity of the relationship between fluoride activity / current strength, the failure of the method at ph> 4 and the direct dependence on the surface of the measuring electrode and its properties in all amperometric methods, which in practice only obtain reliable, reproducible values after complex, careful cleaning cycles leaves.

Den Methoden gemäß DE-PS 11 57 610 und DE-AS 12 91 919 ist gemeinsam, daß nicht die Konzentration oder die im Rahmen der Thermodynamik definierte, mit der Konzentration durch die Aktivitätskoeffizienten verknüpfte Aktivität gemessen wird, sondern eine "Aktivität" in bezug auf eine spezielle Reaktion. Im Fall der DE-PS 11 57 610 bedeutet dies z.B., daß vorausgesetzt wird, der Beizabtrag auf Glas hinge in gleicher Weise von der (thermodynamischen) Fluoridaktivität ab, wie die Chromatierungsreaktion. Ähnlich verhält es sich mit der Anodenreaktion des p-dotierten Siliziums im Falle der DE-AS 12 91 919.The methods according to DE-PS 11 57 610 and DE-AS 12 91 919 have in common that it is not the concentration or the activity defined in the context of thermodynamics that is linked to the concentration by the activity coefficients that is measured, but rather an “activity” in relation to a special reaction. In the case of DE-PS 11 57 610 this means, for example, that it is assumed that the pickling removal on glass depends in the same way on the (thermodynamic) fluoride activity as the chromating reaction. The situation is similar with the anode reaction of the p-doped silicon in the case of DE-AS 12 91 919.

Im Hinblick auf die allgemein gestiegenen Anforderungen hinsichtlich Konstanz der Badzusammensetzung von Bädern zur Oberflächenbehandlung von Metallen ist eine Methode zur Messung und Regelung der Fluoridaktivität wünschenswert, die nicht durch schwer reproduzierbare und komplexe Materialeigenschaften des Meßgerätes definiert wird, sondern die direkte Messung der Aktivität von Fluoridionen, wie sie in der chemischen Thermodynamik definiert ist, und damit eine wesentlich bessere Übertragbarkeit der Werte gestattet.In view of the generally increased requirements with regard to the constancy of the bath composition of baths for the surface treatment of metals, a method for measuring and regulating the fluoride activity is desirable, which is not due to the difficult to reproduce and complex material properties of the Measuring device is defined, but the direct measurement of the activity of fluoride ions, as defined in chemical thermodynamics, and thus allows a much better transferability of the values.

Hierzu sind fluoridsensitive Elektroden zur Messung der Fluoridaktivität, die als wesentlichen Bestandteil einen Kristall aus Lanthanfluorid enthalten, entwickelt worden. Sie sind genauso zur direkten Messung der Fluoridaktivität geeignet, wie beispielsweise Glaselektroden zu der des pH-Wertes. Da der Dissoziationsgrad von Säuren vom pH-Wert abhängt und Fluorwasserstoffsäure als mäßig schwache Säure (pKa ca. 3,5) bei niedrigen (<2) pH-Werten nur schwach dissoziiert, wird die Messung mit fluoridsensitiven Elektroden in gepufferten Lösungen im pH-Bereich 5 - 8 empfohlen, da in diesem Bereich die Fluoridaktivität nur noch geringfügig vom pH-Wert abhängig ist.For this purpose, fluoride-sensitive electrodes for measuring fluoride activity, which contain a crystal of lanthanum fluoride as an essential component, have been developed. They are just as suitable for direct measurement of fluoride activity as glass electrodes, for example, for pH. Since the degree of dissociation of acids depends on the pH value and hydrofluoric acid as a moderately weak acid (pK a approx. 3.5) dissociates only weakly at low (<2) pH values, the measurement with fluoride-sensitive electrodes in buffered solutions is carried out at pH -Range 5 - 8 recommended, as the fluoride activity in this range is only slightly dependent on the pH value.

Nachteilig bei diesem scheinbar attraktiven Verfahren der Fluoridionenaktivitätsmessung unmittelbar in der Behandlungslösung ist, daß sich die fluoridsensitive Elektrode insbesondere aus zwei Gründen nicht direkt zur kontinuierlichen Messung der Fluoridaktivität einsetzen läßt, : Zum einen verändert sich die Elektrode im Verlauf einiger weniger Stunden, wenn sie in Lösungen getaucht wird, wie sie z.B. zum Chromatieren von Zink eingesetzt werden. Diese Lösungen können neben Fluorid u.a. die folgenden Ionen enthalten: CrO4 --, Cr+++, SO4 Zn++, NO3 -, SiF6 --, TiF6 H2PO4 -, HPO4 --. Wahrscheinlich bildet sich auf der Oberfläche des zur Messung eingesetzten LaF3-Kristalls eine Schicht, die die Messung der Fluoridaktivität verfälscht. Dies gilt auch für fluoridhaltige Bäder, z.B. zur Zinkphosphatierung, wobei sich vermutlich Phosphatschichten bilden. Zum anderen gelingt es nicht immer, den pH-Wert der Behandlungslösung konstantzuhalten. Da das Dissoziationsgleichgewicht HF = H + F- pH-Wert-abhängig ist, könnte es bei ungenügender pH-Wert-Regelung des Bades zu unerwünschten Reaktionen bezüglich des Fluorid-Regelkreises kommen.A disadvantage of this apparently attractive method of fluoride ion activity measurement directly in the treatment solution is that the fluoride-sensitive electrode cannot be used directly for continuous measurement of fluoride activity, for two reasons in particular: firstly, the electrode changes over the course of a few hours when it is in solutions is immersed, such as those used for chromating zinc. In addition to fluoride, these solutions can contain the following ions: CrO 4 -, Cr +++ , SO 4 Zn ++ , NO 3 - , SiF 6 - , TiF 6 H 2 PO 4 - , HPO 4 - . Probably a layer forms on the surface of the LaF 3 crystal used for the measurement, which falsifies the measurement of the fluoride activity. This also applies to baths containing fluoride, for example for zinc phosphating, where phosphate layers presumably form. On the other hand, it is not always possible to keep the pH of the treatment solution constant. Since the dissociation equilibrium HF = H + F - pH-dependent, it could be inadequate pH control of the bath leads to undesirable reactions with regard to the fluoride control loop.

Aufgabe der vorliegenden Erfindung ist es, ein Verfahren zur Aufrechterhaltung der Fluoridaktivität in fluoridhaltigen Bädern zur Oberflächenbehandlung von Metallen bereitzustellen, das die vorgenannten Nachteile nicht aufweist, verläßlich ist und dadurch z.B. mit einer gleichmäßigen, d.h. nicht variierenden Schichtausbildung verbunden ist, und dennoch einfach durchführbar ist.The object of the present invention is to provide a method for maintaining the fluoride activity in fluoride-containing baths for the surface treatment of metals, which does not have the aforementioned disadvantages, is reliable and thereby e.g. with an even, i.e. non-varying layer formation is connected, and is still easy to carry out.

Die Aufgabe wird gelöst, indem das Verfahren der eingangs genannten Art entsprechend der Erfindung derart ausgestaltet wird, daß man das fluoridhaltige arbeitende Bad mit einer Lösung, deren pH-Wert maximal um eine pH-Wert-Einheit vom arbeitenden Bad abweicht, in kontrollierter Weise Mindestens auf das Fünffache seines Volumens verdünnt, dann die Fluoridaktivität mißt und aus dem Meßwert die Fluoridaktivität des arbeitenden Bades ermittelt.The object is achieved by designing the method of the type mentioned at the outset in accordance with the invention in such a way that the fluoride-containing working bath is at least controlled in a controlled manner with a solution whose pH deviates from the working bath by a maximum of one pH unit diluted to five times its volume, then measures the fluoride activity and determines the fluoride activity of the working bath from the measured value.

wIn kontrollierter Weise" bedeutet hierbei, daß der Grad der Verdünnung nicht beliebig ist, sondern auf ein bestimmtes Volumen erfolgt, um dann auf die Fluoridaktivität zurückrechnen zu können.In a controlled manner "means here that the degree of dilution is not arbitrary, but takes place on a certain volume in order to then be able to calculate back on the fluoride activity.

Abweichung um maximal eine pH-Wert-Einheit soll zum Ausdruck bringen, daß bei einem pH-Wert des arbeitenden Bades von z.B. 1,5 eine Verdünnung mit einer Lösung, deren pH-Wert 0,5 bzw. 2,5 beträgt, gerade noch zulässig ist.Deviation by a maximum of one pH value unit should express that at a pH value of the working bath of e.g. 1.5 dilution with a solution with a pH of 0.5 or 2.5 is just permitted.

Vorzugsweise wird das fluoridhaltige Bad auf das 20- bis 200-fache seines Volumens verdünnt. Der Verdünnungsgrad wird nach unten begrenzt durch die Gefahr ungenügender Verdünnung, nach oben durch den Wunsch nach möglichst niedrigem Verbrauch an Verdünnungslösung, sowie den Gehalt an Spuren von F- in der Verdünnungslösung.The fluoride-containing bath is preferably diluted to 20 to 200 times its volume. The degree of dilution is capped by the risk of insufficient dilution, capped by the desire for the lowest possible consumption of dilution solution, and the content of traces of F - in the dilution solution.

Damit auch kleine Abweichungen des pH-Wertes des Behandlungsbades nicht zu unerwünschten pH-Wert-Änderungen der Meßlösung führen und so die Fluorid-Kontrolle erschweren, wird in einer weiteren bevorzugten Ausgestaltung der Erfindung das fluoridhaltige Bad mit einer Pufferlösung, die solche Schwankungen auffängt, verdünnt.In a further preferred embodiment of the invention, the fluoride-containing bath is diluted with a buffer solution that absorbs such fluctuations, so that even small deviations in the pH of the treatment bath do not lead to undesired changes in the pH of the measurement solution and thus make fluoride control more difficult .

Eine zur Verdünnung besonders geeignete Pufferlösung enthält das System HCl/KC1.The HCl / KC1 system contains a buffer solution that is particularly suitable for dilution.

Die Abweichung des aus dem Meßwert ermittelten Ist-Wertes von dem für das Oberflächen-Behandlungsverfahren notwendigen Sollwert dient zur Regelung der Ergänzung mit fluoridhaltiger Lösung.The deviation of the actual value determined from the measured value from the target value required for the surface treatment process serves to regulate the addition with fluoride-containing solution.

Die Ergänzung, die sowohl die durch Reaktion mit der Metalloberfläche verbrauchten als auch durch Austrag mit den behandelten Teilen und durch Überlauf aus dem Behandlungsbad entfernten wirksamen Substanzen zu berücksichtigen hat, erfolgt üblicherweise durch zwei Ergänzungskonzentrate. Dabei wird die Ergänzung des einen - fluoridfreien oder nur wenig Fluorid enthaltenden - Konzentrates durch bekannte, dem jeweiligen Oberflächen-Behandlungsverfahren angemessene Titrationen oder andere Messungen, z.B. der elektrischen Leitfähigkeit, gesteuert. Der über diese so geregelte Ergänzung hinausgehende Bedarf an Fluorid wird nach dem erfindungsgemäßen Verfahren mit einer Fluoridlösung ergänzt. Hierbei muß das erste Ergänzungskonzentrat so zusammengesetzt sein, daß nie ein Zustand zu hoher Fluoridaktivität im Behandlungsbad erreicht wird. Mit entsprechenden Einschränkungen versehen, kann das zweite, überwiegend fluoridhaltige Ergänzungskonzentrat auch andere Bestandteile enthalten.The addition, which has to take into account both the active substances consumed by reaction with the metal surface and by removal with the treated parts and by overflow from the treatment bath, is usually carried out by two supplementary concentrates. The addition of the concentrate, which is fluoride-free or contains only a little fluoride, is carried out by known titrations or other measurements appropriate to the respective surface treatment process, e.g. electrical conductivity. The need for fluoride that goes beyond this regulated addition is supplemented with a fluoride solution by the method according to the invention. The first supplementary concentrate must be composed in such a way that a state of excessive fluoride activity in the treatment bath is never reached. Provided with corresponding restrictions, the second, mainly fluoride-containing supplement concentrate can also contain other components.

Bei konstanten Verhältnissen bezüglich des behandelten Materials und der Durchsatzgeschwindigkeit kann es allerdings auch genügen, auf eine Korrektur der Fluoridaktivität mit einer separaten Lösung zu verzichten und mit nur einem Ergänzungskonzentrat zu arbeiten. In einem solchen Fall kann die erfindungsgemäße Ermittlung der Fluoridaktivität vorteilhaft zur Regelung der gesamten Ergänzung eingesetzt werden.With constant conditions regarding the treated mate rials and the throughput rate, it may also be sufficient to dispense with a correction of the fluoride activity with a separate solution and to work with only one supplementary concentrate. In such a case, the determination of the fluoride activity according to the invention can advantageously be used to regulate the entire supplement.

Das erfindungsgemäße Verfahren ist universell einsetzbar und findet insbesondere Anwendung auf die Aufrechterhaltung der Fluoridaktivität chemischer oder elektrolytischer Chromatierbäder, vorzugsweise zur Chromatierung von Zink- und/oder Aluminiumoberflächen.The method according to the invention can be used universally and is used in particular to maintain the fluoride activity of chemical or electrolytic chromating baths, preferably for chromating zinc and / or aluminum surfaces.

Beispiele für die Anwendung sind:

  • zinkphosphatierung von Stahl- und Aluminiumteilen
Examples of the application are:
  • zinc phosphating of steel and aluminum parts

Behandelt werden wechselnde Anteile von Stahl- und Aluminiumoberflächen. Auf beiden bilden sich Schichten aus tertiärem Zinkphosphat, aber nur die Aluminiumoberflächen führen zu einem wesentlichen Verbrauch von Fluorid, z.B. gemäß der Reaktion

Figure imgb0001
mit der das "Badgift" Aluminium wirksam aus der Behandlungslösung entfernt wird. Die Lrgänzung von Zink, Phosphat und Beschleuniger erfolgt aufgrund von gegebenenfalls automatischen Titrationen, die von Fluorid mit einem zweiten Konzentrat, das vorwiegend Fluorid, z.B. in Form von NH4HF2 enthält. Bei wechselnden Anteilen von behandelten Aluminiumoberflächen erfolgt die Ergänzung mit wechselnden Mengen fluoridhaltigen Konzentrats, die durch das erfindungsgemäße Verfahren gesteuert wird.Changing parts of steel and aluminum surfaces are treated. Layers of tertiary zinc phosphate form on both, but only the aluminum surfaces lead to a substantial consumption of fluoride, for example according to the reaction
Figure imgb0001
 with which the "bad poison" aluminum is effectively removed from the treatment solution. Zinc, phosphate and accelerator are supplemented on the basis of automatic titrations, if appropriate, of fluoride with a second concentrate which contains predominantly fluoride, for example in the form of NH 4 H F2 . In the case of changing proportions of treated aluminum surfaces, changing amounts of fluoride-containing concentrate are carried out, which is controlled by the method according to the invention.

Chromatierung von ZinkChromating of zinc

Bei der sauren Chromatierung von Zink reichert sich die Behand- lungslösung mit Zn - und mit Cr+++-Ionen, die aus der Reduktion von sechswertigem Chrom entstehen, an. Vermutlich letztere reagieren langsam mit Fluorid und reduzieren so den Gehalt an freiem Fluorid. Nach Stillstandszeiten, z.B. über Nacht, muß daher Fluorid separat ergänzt werden.The acidic chromating of zinc, the B accumulates ehand- development solution with Zn - and Cr +++ ions resulting from the reduction of hexavalent chromium to. Probably the latter react slowly with fluoride and thus reduce the free fluoride content. After downtimes, eg overnight, fluoride must therefore be added separately.

Grünchromatierung von AluminiumGreen chromating of aluminum

Das Flächengewicht der Chromatschicht hängt bei der Grünchromatierung von Aluminium wesentlich von der Fluoridaktivität ab. Werden für verschiedene Produktionen unterschiedliche Flächengewichte gefordert, so ist eine separate Fluoridergänzung erforderlich.The basis weight of the chromate layer in the green chromating of aluminum depends essentially on the fluoride activity. If different basis weights are required for different productions, a separate fluoride supplement is required.

Die Vorteile der Erfindung sind, daß Oberflächenbeläge und andere Veränderungen des Meßkristalls, die die Messung beeinträchtigen, wirksam ausgeschaltet werden. Eine Einschränkung des Meßbereiches ergibt sich dabei nicht, da der Meßbereich bei der fluoridsensitiven Elektrode viele Zehnerpotenzen umfaßt und im wesentlichen durch den Gehalt an Fluoridspuren in der zur Verdünnung verwendeten Lösung begrenzt wird.The advantages of the invention are that surface coverings and other changes in the measuring crystal which impair the measurement are effectively eliminated. The measuring range is not restricted, since the measuring range for the fluoride-sensitive electrode comprises many powers of ten and is essentially limited by the content of fluoride traces in the solution used for dilution.

Die Erfindung wird anhand des nachfolgenden Beispiels beispielsweise und näher erläutert.The invention is illustrated by way of example and in more detail using the example below.

Beispielexample

Zur Erstellung einer Eichkurve wurden zwei Lösungen angesetzt, die

Figure imgb0002
Figure imgb0003
 Two solutions were used to create a calibration curve, the
Figure imgb0002
Figure imgb0003

jeweils in vollentsalztem Wasser gelöst enthielten und einen pH-Wert von ca. 0,9 aufwiesen.each contained dissolved in deionized water and had a pH of about 0.9.

Die verwendete fluoridsensitive Elektrode (Orion, Typ 96-09-00) wurde zwei Minuten in die einzelnen Lösungen von jeweils 20 °C getaucht und dann für Lösung 1 ein Meßwert von + 190 mV und für Lösung 2 ein Meßwert von + 134 mV ermittelt.The fluoride-sensitive electrode used (Orion, type 96-09-00) was immersed in the individual solutions of 20 ° C. for two minutes and then a measured value of + 190 mV for solution 1 and a measured value of + 134 mV for solution 2.

Auch bei den weiteren Messungen wurde der Meßwert nach 2 Minuten Eintauchdauer bei einer Temperatur der Lösung von 20 °C erfaßt.In the further measurements, too, the measured value was recorded after 2 minutes of immersion at a solution temperature of 20 ° C.

Zur Chromatierung von feuerverzinkten Stahlblechen, die zuvor in einem stark alkalischen Reiniger gereinigt und mit Wasser gespühlt worden waren, wurde eine Chromatierungslösung angesetzt, die

Figure imgb0004
enthielt und einen pH-Wert von ca. 0,9 aufwies.A chromating solution was prepared to chromate hot-dip galvanized steel sheets that had previously been cleaned in a strongly alkaline cleaner and rinsed with water
Figure imgb0004
 contained and had a pH of about 0.9.

Zur Ermittlung des Meßwertes für die F -Aktivität der neu angesetzten Lösung wurde eine Badprobe auf das 100-fache mit einer Pufferlösung, die

Figure imgb0005
in vollentsalztem Wasser gelöst enthielt, verdünnt. Der erhaltene Meßwert betrug + 157 mV, was mit dem Sollwert auf der zuvor ermittelten Eichkurve übereinstimmte.To determine the measured value for the F activity of the newly prepared solution, a bath sample was made 100 times with a buffer solution
Figure imgb0005
 contained in deionized water, diluted. The measured value obtained was + 157 mV, which corresponded to the target value on the previously determined calibration curve.

Nachdem durch die vorstehend definierte Chromatierungslösung 0,66 m2 Oberfläche pro Liter Lösung durchgesetzt worden waren, wurde zunächst mit einer

Figure imgb0006
enthaltenden Lösung auf Chromsäurekonstanz (8,0 g/1) ergänzt. Dann wurde zur Bestimmung der F--Aktivität eine Badprobe der ergänzten Lösung auf das 100-fache mit der vorgenannten Pufferlösung verdünnt. Der daraufhin ermittelte Meßwert betrug + 160 mV (20 °C). Unter Berücksichtigung der Eichkurve entsprach der Meßwert einer Fluorid-Aktivität in der Chromatierungslösung von 0,35 g/1. Daraufhin wurde die Chromatierungslösung mit 50 mg/1 F- ergänzt. Eine Kontrollmessung ergab, daß der ursprüngliche Meßwert von + 157 mV (20 °) entsprechend einer Fluorid- Aktivität in der Chromatierungslösung von o,40 g/1 wieder erreicht war.After 0.66 m 2 of surface area per liter of solution had been passed through the chromating solution defined above, the first step was with a
Figure imgb0006
 containing solution added to constant chromic acid (8.0 g / 1). Then a bath sample of the supplemented solution was diluted 100 times with the abovementioned buffer solution in order to determine the F - activity. The resultant measured value was + 160 mV (20 ° C). Taking the calibration curve into account, the measured value corresponded to a fluoride activity in the chromating solution of 0.35 g / l. The chromating solution was then supplemented with 50 mg / 1 F - . A control measurement showed that the original measured value of + 157 mV (20 °) corresponding to a fluoride activity in the chromating solution of 0.40 g / 1 was reached again.

Claims (5)

1. Verfahren zur Aufrechterhaltung der Fluoridaktivität in fluoridhaltigen Bädern zur Oberflächenbehandlung von Metal- len durch Ermittlung der Fluoridaktivität mittels einer fluoridsensitiven Elektrode und Ergänzung der Bäder durch Fluoridionen, dadurch gekennzeichnet, daß man das fluoridhaltige arbeitende Bad mit einer Lösung, deren pH-Wert maximal um eine pH-Wert-Einheit vom arbeitenden Bad abweicht, in kontrollierter Weise mindestens auf das Fünffache seines Volumens verdünnt, dann die Fluoridaktivität mißt und aus dem Meßwert die Fluoridaktivität des arbeitenden Bades ermittelt.1. A method of maintaining the fluoride activity in fluoride containing baths for the surface treatment of M etal- len by determining the fluoride activity by means of a fluoride sensitive electrode and supplementing the baths by fluoride ions, characterized in that the fluoride-containing working bath with a solution whose pH value is maximum deviates from the working bath by one pH value, diluted in a controlled manner at least to five times its volume, then measures the fluoride activity and determines the fluoride activity of the working bath from the measured value. 2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß man das fluoridhaltige Bad auf das 20- bis 200-fache seines Volumens verdünnt.2. The method according to claim 1, characterized in that the fluoride-containing bath is diluted to 20 to 200 times its volume. 3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß man das fluoridhaltige Bad mit Pufferlösung verdünnt.3. The method according to claim 1 or 2, characterized in that the fluoride-containing bath is diluted with buffer solution. 4. Verfahren nach Anspruch 3, dadurch gekennzeichnet, daß man das fluoridhaltige Bad mit einer HCl/KCl-Lösung verdünnt.4. The method according to claim 3, characterized in that the fluoride-containing bath is diluted with an HCl / KCl solution. 5. Anwendung des Verfahrens nach Anspruch 1, 2, 3 oder 4 auf die Aufrechterhaltung der Fluoridaktivität chemischer oder elektrolytischer Chromatierbäder, insbesondere zur Chromatierung von Zink- und/oder Aluminiumoberflächen.5. Use of the method according to claim 1, 2, 3 or 4 to maintain the fluoride activity of chemical or electrolytic chromating baths, in particular for the chromating of zinc and / or aluminum surfaces.
EP85200521A 1984-04-13 1985-04-04 Process for controlling fluoride-containing baths for metal surface treatment Withdrawn EP0162489A1 (en)

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EP0261704A1 (en) * 1986-09-18 1988-03-30 Metallgesellschaft Ag Process for producing phosphate coatings on metal surfaces
WO1993014241A1 (en) * 1992-01-13 1993-07-22 Henkel Corporation A method for the acidic conversion treatment of aluminum containing metal
US5454882A (en) * 1992-01-13 1995-10-03 Henkel Corporation Process for controlling a fluoride containing conversion coating forming composition during its use for conversion coating aluminum containing metal

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JP2786075B2 (en) * 1993-04-23 1998-08-13 日本ペイント株式会社 Activity control device for zinc phosphate treatment solution

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JPS6022067B2 (en) * 1982-09-30 1985-05-30 日本パ−カライジング株式会社 Method for forming film on metal surface

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FR2120644A5 (en) * 1970-12-28 1972-08-18 Parker Ste Continentale
FR2208532A5 (en) * 1972-11-16 1974-06-21 Avesta Jernverks Ab Potentiometric determination of acid bath strength - using selective elec-trodes for F and H ions calibrated against KCL control electrode for pickling baths
JPS5599062A (en) * 1979-01-25 1980-07-28 Nippon Steel Corp Concentration measurement of pickling solution containing hydrofluoric acid

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US4849031A (en) * 1986-09-17 1989-07-18 Metallgesellschaft Aktiengesellschaft Process of producing phosphate coatings on metal surfaces
EP0261704A1 (en) * 1986-09-18 1988-03-30 Metallgesellschaft Ag Process for producing phosphate coatings on metal surfaces
WO1993014241A1 (en) * 1992-01-13 1993-07-22 Henkel Corporation A method for the acidic conversion treatment of aluminum containing metal
US5454882A (en) * 1992-01-13 1995-10-03 Henkel Corporation Process for controlling a fluoride containing conversion coating forming composition during its use for conversion coating aluminum containing metal

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