Classifications

• • 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.

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• Chemical & Material Sciences (AREA)
• General Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Chemical Treatment Of Metals (AREA)

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• 1984
  • 1984-04-13 DE DE19843413905 patent/DE3413905A1/de not_active Withdrawn
• 1985
  • 1985-04-04 EP EP85200521A patent/EP0162489A1/fr not_active Withdrawn
  • 1985-04-11 AU AU41034/85A patent/AU4103485A/en not_active Abandoned
  • 1985-04-12 JP JP7822785A patent/JPS60251280A/ja active Pending
  • 1985-04-12 GB GB8509374A patent/GB2157446B/en not_active Expired

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