GB2157446A - Control of fluoride in solutions for treatment of metals - Google Patents

Description

1 GB 2 157 446A 1

SPECIFICATION

Control of fluoride in solutions for treatment of metals It is known that fluoride-containing solutions such as chromatising solutions or phosphating 5 solutions can be used for the surface treatment of metals and that it is necessary to keep the concentrations of fluoride and of the other bath components as constant as possible. Various ways are known for measuring fluoride activity, so as to permit appropriate accurate replenish ment of fluoride.

The effect of a fluoride-containing solution on glass is measured according to DE-PS 11 57 610 by weighing a glass sample before and after treatment with the solution and by calculating the fluoride activity from the weight loss of the glass. Although the process is reliable, it is timeconsuming and very difficult to automate for use in continuous automatic control.

DE-AS 12 91 919 describes a process for measuring the fluoride activity of acidic aqueous fluoride solutions in which the current intensity of an electrolysis cell having a constant applied voltage is used as a gauge of the fluoride activity of the solution contained in the cell. The process operates with an inert cathode and an anode composed of p-doped silicon. Unfortunately the relationship between fluoride activity and current intensity is not linear and the method fails at pH > 4. Also in this and all amperometric methods, the direct dependance between the surface of the test electrode and the composition thereof necessitates very thorough 20 (and expensive) cleaning if reliable reproducible values are to be achieved.

The methods in DE-PS 11 57 610 and DE-AS 12 91 919 do not measure the concentration or the activity defined in the context of thermodynamics and linked with concentration by the activity coefficient, but measure an -activity- with respect to a specific reaction. For example in DE-PS 11 57 610 it is assumed that the material removed from the glass is dependent upon the 25 (thermodynamic) fluoride activity in the same manner as the chromatising reaction. The same applies to the anode reaction of the p-doped silicon in the case of DE-AS 12 91 919.

In view of the generally higher requirements for controlling the composition of baths for the surface treatment of metals, it is desirable to find a method for measuring and controlling the fluoride activity which does not involve measuring devices having complex material properties 30 which are difficult to reproduce but which allows direct measurement of the activity of fluoride ions in the manner defined in chemical thermodynamics and thus makes it substantially easier to transfer the values.

Fluoride-sensitive electrodes containing a crystal of lanthanum fluoride as essential constituent have been developed for measuring fluoride activity and are just as suitable for direct measurement of the fluoride activity as, for example, glass electrodes are for the pH value. As the degree of dissociation of acids depends on the pH value and hydrofluoric acid is a moderately weak acid (pK. about 3.5) which only dissociates weakly at high acidity (pH below about 2), measurement using fluoride-sensitive electrodes in buffer solutions in a pH range of from 5 to 8 would appear suitable as fluoride activity is only slightly dependent on the pH in 40 this range.

Unfortunately this process for measuring fluoride ion activity directly in the treatment solution has the disadvantage that the fluoride-sensitive electrode cannot be used directly for continuous measurement of fluoride activity, in particular for two reasons. On the one hand, the electrode changes in the course of a few hours if it is immersed in solutions of the type used for forming 45 surface coatings on metals, such as for the chromating of zinc or for phosphating metals (e.g., zinc phosphate solutions). Such solutions can contain the following ions, among others, in addition to fluoride: Cr04- -, CrI + 1, SO,- -, Zn' +, N03, SiF,- -, TiF6- , CA-,- + +, H2P04-, HP04- -. A layer which distorts measurement of fluoride activity probably forms on the surface of the LaF3-crystal of the electrode. For instance layers are probably formed from phosphating 50 solutions. On the other hand, it is not always possible to keep the pH of the treatment solution constant. As the dissociation equilibrium H F = H + + F - is dependerit on the pH, undesirable reactions with respect to the fluoride control cycle could occur as a result of unsatisfactory pH control of the bath.

It would therefore be desirable to be able to provide a process that is reliable, easy to operate 55 and permits accurate control of the fluoride activity of fluoride- containing baths for the surface treatment of metals, so as to permit uniform coating formation on the metals.

In the invention the fluoride activity of the fluoride-containing 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 activity of the diluted 60 sample using a fluoride-sensitive electrode, calculating the fluoride activity of the bath from the measured value, and replenishing the bath with fluoride ion in response to the calculated value.

The process of the invention can be conducted by performing the measuring, calculating and replenishing operations separately. For instance the fluoride activity of the diluted solution may be measured and this value used, in combination -with the volume of diluent, to calculate the 65 2 GB2157446A 2 fluoride activity of the bath. This calculated value may then be subtracted from the theoretical optimum and the difference utilised to determine the amount of replenishment that should be added. However some or all of these operations may be conducted automatically and it is not necessary, for instance, to generate a precise value for the calculated fluoride activity. For instance it is possible to control the supply of replenishment directly in response to the fluoride 5 measurement of the diluted solution if, for instance, the volume of diluent was predetermined.

The sample that is used for measuring the fluoride activity must be diluted to at least 5 volumes with a diluent solution, that is to say it must be diluted to at least 5 times its volume by the diluent. Preferably it is diluted to from 20 to 200 times its volume. If the degree of dilution is inadequate the electrode may operate less satisfactorily. If the degree of dilution is too 10 high there will be excessive utilisation of diluent and the results may be distorted by traces of fluoride in the diluent. It is of course essential that the degree of dilution is known in order that it is possible to use the measured value to determine the replenishment that is required.

The diluent must have a pH within 1 pH unit of the pH of the operating bath. For instance if the pH of the operating bath is 1.5 the diluent must have a pH in the range 0.5 to 2.5. 15 To prevent small differences in the pH of the treatment bath from leading to undesirable pH variations in the test solution, thus leading to difficulties in the fluoride control, the diluent is preferably a buffer that will absorb such variations. A preferred buffer contains HCI/KCI.

The bath has to be replenished both for active material consumed by the metal coating process and for material carried from the bath on coated parts and by overflow. It is generally 20 preferred to replenish the bath using two replenishment concentrates or other replenishment solutions. One is free of fluoride or contains only a small amount of fluoride and its replenishment may be controlled by known titration or other control methods, for instance electrical conductivity. If this replenishment does contain fluoride the concentration must be such that it never results in full replenishment of the fluoride in the bath. The second replenishment provides most or all of the fluoride and is added in response to the fluoride determination described above. This fluoride replenishment may serve solely to replenish fluoride or it may contain other constituents that are to be replenished into the bath.

However in some instances, for instance if there is constant throughput of constant material, it may be permissible to use a single replenishment containing fluoride and all the other necessary 30 replenishment chemicals, and control the addition of this replenishment by the described method.

The invention is of value in all processes where it is necessary to control the fluoride content of bath for the surface treatment of metals but is of special value when the baths are to be used for chemical or electrolytic chromating, in particular for chromating zinc and/or aluminium 35 surfaces.

In one particular use, alternating portions of steel and aluminium surfaces are subjected to zinc phosphating. layers of tertiary zinc phosphate form on both surfaces, but only the aluminium surfaces lead to a significant consumption of fluoride, for example according to the reaction AI + + + + 6 F - + 3 Na ±Na3A1F6 with which the unwanted substance in the bath, aluminium, is effectively removed from the treatment solution. Zinc, phosphate and accelerator are supplemented on the basis of titration 45 operations, possibly automatic, and fluoride is supplemented with a second concentrate containing predominantly fluoride, for example in the form of NI-1,HF2. With alternating portions of treated aluminium surfaces, supplementation is effected with varying quantities of fluoride containing concentrate controlled by the process according to the invention.

In another use zinc is subjected to acidic chromating. The treatment solution is enriched with 50 Zn+ + and with CrI 1 + ions formed by the reduction of hexavalent chromium. The latter probably react slowly with fluoride and thus reduce the content of free fluoride. After standing times, for example overnight, therefore, fluoride has to be supplemented separately.

In another use, aluminium is subjected to green chromatising. The weight per unit area of the chromate layer during the green chromatising of aluminium depends substantially on the fluoride activity. If different area weights are required for different production processes, separate fluoride supplementation is necessary.

The advantages of the invention reside in the fact that surface coatings and other changes in the measuring crystal which impair measurement are effectively eliminated. The measuring range is not restricted as the measuring range with the fluoride- sensitive electrode coves many 60 powers of ten and is limited substantially by the content of fluoride traces in the solution used for dilution.

The invention is described by way of example and in more detail with reference to following Example.

3 GB 2 157 446A 3 Example To produce a calibration curve, two solutions were used which contained 3.73 9.1 KCI 5.47 9/1 HCI 0.001 9/1 F- (as NaF) Solution 1 and 3.73 9/1 KC] 5.47 9/1 HCI 0.010 9/1 F- (as NaF) Solution 2 dissolved in completely desalted water in each case and had a pH of about 0.9.

The fluoride-sensitive electrode used (Orion, type 96-09-00) was immersed into the individual 15 solutions each at 20C for two minutes and a measured value of + 190 mV was then determined for solution 1 and a measured value of + 134 mV for solution 2.

During further measurement, the measured value was determined after an immersion period of 2 minutes at a solution temperatures of 2WC.

Achromatising solution containing 8.0 9/1 Cr03 9.0 g/] N03 (introduced as HN03) 0.40 9/1 F- (introduced as HF) and having a pH of about 0.9 was used for chromatising hot galvanised sheet steel which had previously been cleaned in a strongly alkaline cleaner and had been purged with water.

To determine the measured value of the F- activity of the freshly applied solution, a sample of bath was diluted 100 fold with a buffer solution containing 3.73 9/1 KCI and 5.47 9/1 HCI dissolved in fully desalted water. The measured value obtained was + 157 mV and this 35 coincided with the desired value on the previously determined calibration curve.

Once 0.66 M2 surface area per litre of solution had been passed through the chromatising solution defined above, the solution was supplemented with a solution containing 8.0% by weight Cr03 and 40 13.5% by weight HN03, to constancy of chromic acid (8.0 g/I). To determine the Factivity, a bath sample of the supplemented solution was then diluted 100 fold with the above-mentioned buffer solution. The measured value then determined was + 160 mV (20'C). According to the calibration curve, the measured value corresponded to fluoride activity in the chromatising solution of 0.35 9/1. The 45 chromatising solution was then supplemented with 50 mg/l of F-. Control measurement showed that the original measured value of + 157 mV (20'C) corresponding to fluoride activity in the chromatising solution of 0.40 g/1 had been achieved again.

Claims (8)

1. A process for maintaining the fluoride activity of a fluoridecontaining bath for the surface treatment of metals and comprising 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 fluoride activity of the diluted sample using a fluoride-sensitive electrode, calculating the fluoride activity of the bath from the measured value and replenishing the bath with fluoride ion in response to the calculated value.

2. A process according to claim 1 in which the sample of the bath is diluted to from 20 to volumes.

3. A process according to claim 1 or claim 2 in which the diluent is a buffer solution.

4. A process according to claim 3 in which the diluent is a HCI/KCI solution.

5. A process according to any preceding claim in which the fluoridesensitive electrode includes a lanthanum fluoride crystal.

6. A process according to any preceding claim in which the bath is a chemical or electrolytic chromating bath.

7. A process according to any preceding claim in which the bath is a bath for chromating 65 4 GB2157446A 4 zinc or aluminium surfaces.

8. A process according to claim 1 substantially as herein described.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A l AY, from which copies may be obtained.