DE3404283A1 - DEVICE AND METHOD FOR AUTOMATICALLY MONITORING THE PHOSPHATING OF METALLIC SURFACES - Google Patents

Description

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QH U H

Device and method for automatically monitoring the Phosphating of metallic surfaces

The invention relates to a device and a method for automatically monitoring the phosphating of metallic materials Surfaces, in particular made of iron, steel or galvanized iron, using aqueous solutions of zinc phosphates.

The use of phosphating to protect ferrous metals or galvanized iron surfaces from corrosion is known and is widely used in the manufacture of motor vehicles, household electrical appliances and other metal objects that have large surface areas. Aqueous solutions of zinc phosphates are usually used as a phosphating bath. These baths can contain other metal elements such as nickel or manganese. Phosphating baths with these additional elements, besides zinc, are those that are used today. Phosphating baths can be accelerated by the presence of stimulating anions, such as nitrite or chlorate. In Both baths, the normal and the accelerated baths, zinc is the element that is on the metal surface in the form of a Phosphates is deposited, which forms the protective layer. Phosphating solutions can be used for immersion treatment, spray treatment or mixed spray and immersion treatments at acidic pH values, used at temperatures from ambient to about 80 ° C and with zinc concentrations of 1 to 10 g / l in the solution will. In all cases, a special combination of parameters helps to apply the required type of coating (grain size crystallization, adhesion to the metal surface, thickness of the coating) . It is clear that the monitoring and maintenance of the parameters on the required values of one of the Main problems in the industrial phosphating process is.

The usual offense is to make determinations of pH and to use the reduction potential of the phosphating bath, to control the addition of reagents used up in the process. Complexometric measurements of the Zinc present in the solution carried out to increase the concentration to the desired value.

As a rule, these analyzes are carried out periodically at predetermined time intervals by taking samples of the phosphating solution carried out, after which suitable additions of various reagents are made depending on the values of the sample.

For several years efforts have been made in an attempt to continuously regulate the phosphating process by means of possibilities that give an automatic determination of the values of the parameters of interest.

For example, apparatus have been proposed for continuously determining the pH and reduction potential of a phosphating solution. The devices control the actuation of pumps that deliver refreshing substances to the solution, as is the case with the KORROSTAT ^R device from Uster.

Another suggestion was to increase the concentration of nitrite ions (which is the most common additive in accelerating mixtures for phosphating solutions) by adding a partial flow of the Solution is sent through an anion exchanger, as proposed for example in GB-PS 15 18 534.

The proposed method and device, although better results than the usual hand-operated monitoring systems give, do not solve the problem completely satisfactorily, since they involve a manual, analytical determination of the concentrations dor other ingredients of the phosphating solution, in particular of Zinc, must be connected.

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The invention is based on the object of a device and a method for continuous automatic monitoring of the To create phosphating process.

This object is achieved by the device according to patent claim 1 and by the method according to patent claim 6.

It has been found that the problem of controlling the Phosphating process can be solved completely satisfactory can, by monitoring the zinc ion content of the solution by means of a very simple device, which enables a very precise evaluation the zinc ion content is carried out automatically and in virtually a short time and which is designed so that it operates a pump, which introduces the replenisher into the phosphating solution in use.

In an embodiment of the invention, the device has a polarographic device which measures the concentration of zinc in the solution Measuring cell which has a mercury drop cathode and a silver anode and which is supplied with a carrier electrolyte which is mixed in suitable proportions with the solution that is taken as a sample from the phosphating bath, which should be monitored.

If a suitable support electrolyte is used, there will be interference with atmospheric air and especially oxygen which could influence the measurement results, the limiting diffusion current density of the zinc ions can be very precisely at the mercury drop electrode can be measured, the value of this current being strictly proportional to the zinc ion concentration in the checked liquid.

In fact, every metal ion has its characteristic reduction potential, so that, under suitable conditions, the limiting diffusion current density of the zinc ions can be precisely measured can, even in the presence of other metals.

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The discharge potential, i.e. the potential difference between the both electrodes of the measuring cell, whichever one is in the solution existing ion changes into the metallic state, is half characteristic of each type of ion present in the tested solution.

As a result, the potential difference between the two electrodes can be varied in such a way that either a qualitative determination of the type of ion present (based on the observed discharge potential) or its quantitative determination are carried out can (based on the measured current strength under the conditions of the limiting diffusion current density). This is the system that usually is used for polarographic analysis.

In the preferred embodiment of the invention, a constant potential, i.e. there is a potential difference between the two electrodes of the measuring cell on one held constant value, which is the limiting diffusion current density of the zinc ion. A current is therefore obtained which includes the portions of metals that have a more positive reduction potential than zinc, while the elements that have a more negative reduction potential have no influence.

The phosphating solutions commonly used in industry contain other metal ions in addition to the zinc ions, which are either used to accelerate the reaction substances added to the bath or from partial detachments of the metal alloys that make up the articles that the Be subjected to phosphating treatment.

As a rule, such metal ions are still present in amounts that are well below the amounts of zinc, so that their Proportion of the measured and used to determine the zinc ions Electricity is practically negligible.

In all cases it is possible to use a value of the limiting diffusion current density that can be used to monitor the phosphate bath,
as far as changes with respect to a given optimal value can be traced back to the changes in the zinc ion concentration in the
testing solution are.

In a preferred embodiment of the invention, the j

the sample of the bath to be analyzed mixed carrier electrolyte from \

an aqueous ammonia solution of the metal ion-binding sub- |

punch, such as tartrates or citrates or their Mi- j

mixtures or their double salts, from wetting agents, preferably j

non-ionic, substances to eliminate dissolved oxygen-. i

ren and so a degassing of the mixture sent to the measuring cell to j

avoid, such as from sulfites, and other additives such as] Chlorine of potassium, which is particularly recommended if a metallic!

Silver electrode is used to keep it active. j

The use of the carrier electrolyte and its composition

are very important when expected and reproducible results f

should be preserved. I.

The various components of the supporting electrolyte may be present in a \ large quantity range. %

In preferred embodiments of the invention, the following percentages

composition ranges have been found very advantageous: From 1.5 i

up to 5 g / l ammonia, from 10 to 30 g / l complexing agent, from 10. to J

50 mg / 1 wetting agent, from 5 to 15 g / l oxygen-absorbing sub- |

punch and add from 3.5 to 10 g / l additives to keep the silver \

electrode. 1

In the accompanying drawings i

Fig. 1 is a diagram in which the values of the boundary diffusion |

current density (ordinate) versus zinc ion concentration!

of the phosphating bath (abscissa) for different ver |

ratios of phosphating bath sample to carrier electrolyte |

are applied, I.

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2 shows a schematic representation of a device according to the invention for monitoring the phosphating process and

Fig. 3 shows an embodiment of a measuring cell of the invention Contraption.

Curves A, B and C in Fig. 1 show the values of the diffusion current density (in microamps) depending on the zinc ion concentration (in g / l) in a phosphating bath based on zinc phosphate at different dilutions with the carrier electrolyte. The curve A corresponds to a 1:10 dilution with respect to the volume between the bath sample and the carrier electrolyte, while the Curve B for a ratio of 1:25 and curve C for a ratio 1: 40 applies. The value of the zinc ion concentration is the value of the tested sample before dilution with the carrier electrolyte. The measurements were carried out at 25 ° C. The aqueous carrier electrolyte used contained 15 ml of a concentrated aqueous electrolyte per liter Ammonia solution (d = 0.892), 10 g sodium sulfite, 20 g dihydric tartrate of soda, 7.5 g of potassium chloride and 1.5 ml of a two percent aqueous solution of a non-ionic wetting agent.

The dilution ratio between the phosphating bath and the carrier electrolyte, which are mixed, is preferably and fed to the measuring cell, in terms of volume 1:10 to 1:40.

The best measurement results are obtained with dilution ratios of Obtained 1: 2C to 1:30. As is known, the zinc concentration in phosphating baths can vary between 1 and 10 g / l, however For the more frequently used baths, the limit ranges from 1.5 to 3.5 g / l.

Once the working conditions of the bath used have been determined (temperature, type of accelerator, type of treatment the objects in the bath, treatment time, etc.), the zinc concentration of the bath must be maintained in a very narrow range ' will be to change in .the quality of the final

Avoid protective coating. The device of this invention can monitor the concentration of zinc ions in the bath completely automatically in a range of + 0.05 g / l of zinc, i.e. Measure and adjust. These values are sufficient to create protective layers with high and constant quality.

Another advantage of the automatic monitoring and regulation of the Zinc concentration within that of the device according to the invention allowed limits is the avoidance of unnecessary increase of zinc in the phosphating bath, which leads to a greater consumption of Zinc per square meter of total surface area.

As the curve in FIG. 1 clearly shows, very small changes can be made in the zinc ion concentration of the phosphating bath can be measured by adding after dilution with the carrier electrolyte Changes in the zinc concentration in the bath sample of 0.1 g / l Changes in the current intensity from 0.17 to 1.08 microamps were found (according to the dilution ratio used). These currents can be measured immediately with commercially available potentiometric measuring equipment.

In the preferred embodiment of the invention is normally preferred to work with a constant dilution ratio between the bath sample and the carrier electrolyte, so that the Sensitivity of the potentiometric device with those of such Devices existing normal sensitivity controls can be set.

However, the invention can also be carried out using measuring devices which do not have any sensitivity classification have by choosing the dilution ratios between the sample of the phosphating bath and the carrier electrolyte so that an optimal reading of the limiting diffusion current density values in the range of the available measuring device is obtained.

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Fig. 2 shows a device accordingly in a schematic representation of an invention. A sample of the phosphating bath is drawn from a bath container 1 by means of a peristaltic pump 2 taken and after it in the line 8 with a predetermined Amount of a quantity of carrier electrolyte taken from a storage container 3 by means of a peristaltic pump 4 was mixed, a measuring cell 7 is supplied. An analyzer 6 measures the strength of the current in the measuring cell and records its values. If the from the value detected by the analyzer 6 is below the value corresponding to the required zinc concentration, actuates an electronic one Device 9, a pump 11, which refreshing phosphating solution removed from a tank 10 and supplied to the bath tank 1.

The pump 11 works for a predetermined time, stopping it is controlled by a timer 5. In addition to controlling the stopping of the pump 11, the timer 5 controls the stopping and starting pumps 2 and 4 according to a repetitive cycle.

By controlling the times of these cycles, the phosphating bath can be analyzed at constant time intervals and with the required frequency will.

Due to the high sensitivity of the created by the invention 'Analysis and because of the high frequency at which these analyzes can be carried out, the phosphating bath can be practical monitored and adjusted without any noticeable time delay will.

Fig. 3 shows an embodiment of the measuring cell according to the invention.

The body of the cell 31 is made of glass or another transparent one Material made to allow a better observation of possible clogging of the capillary 31, which the formation of Mercury drops or droplets or the like. allowed that the cathode the measuring cell (the electrical connections are not shown).

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A container 41 contains a supply of pure mercury (double distilled) for polarography and is connected to the cell 37 via a tube 40 made of polyethylene or rubber and a capillary 31, which is held on the measuring cell 37 by means of a sealing system 38.

The measuring cell 37 also has an inlet pipe 34 and an outlet pipe 32 through which the mixture of bath sample and carrier electrolyte flies. A silver electrode 33 is arranged in the outlet pipe 32.

The mercury dripping out of the capillary 31 collects in the bottom area 35 of the measuring cell and is there at a constant level Level held by means of a siphon 36, so that a tight seal of the measuring cell is provided. The mercury is taken from the 'j Cell discharged through a pipe 39 connected to the siphon 36; is.

The measuring cell has a very small volume for consumption of carrier electrolyte to reduce. The volume of the cell is preferably from 5 to 50 ml. Larger volumes are safely feasible, however, smaller volumes would lead to great difficulties in cell construction.

Since, among other things, the value of the diffusion current density depends on the surface area of the mercury droplet acting as cathode depends, the selected capillaries can contain mercury droplets of suitable dimensions emit signals in order to obtain current values in the required measuring range. Typically they have in the measuring cell according to the invention used capillaries an inner diameter of 0.05 to 0.10 mm.

The frequency of renewal of the cathode surface, i.e. the number of capillaries exiting in a given time Mercury drops is set in a simple manner by the height of the mercury container 41 with respect to the measuring cell after a certain diameter has been set for the capillary.

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Typical instructions, the drip varies for from 2 to 6 seconds, ie
from 10 to 30 drops of mercury per minute.

To better explain the results obtained by the ^{inventiveness:} modern device can be obtained, but without any

^{Intent of limitation, reference T} is made to the following examples

taken. ^:

Example 1:

A phosphating solution with a zinc ion concentration of 1.7 g / l] is prepared. The solution made with NaNO ₂ in one concentration
accelerated from 0.2 to 0.6 g / l, is applied to samples,
which are made of FePO., -MB cold rolled steel.

The application is carried out by a spray device that
is suitable to provide the following parameters:

temperature 25 ° C Time 2 minutes Spray pressure 1.5 kg / cm ² Cover volume 160 lt / min / m ²

The same device was previously used to take the samples with

a slightly alkaline and activated with titanium salts f

more fat to degrease. j

The phosphated samples had a fine and dense j

2 i

crystalline layer, which weighed about 1.6 g / m 2. j

The samples were for submerged electrophoretic use
painted with a gray primer commonly used in the automotive industry. The primer was about
18 microns thick.

A cross-shaped incision was made on the painted samples
that have been subjected to a salt spray test according to ASTM B 117-67.

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COPY ^%

After 600 hours of exposure, rust began to penetrate under the primer from the cruciform cut, and blistering was found in the values shown in Table 1.

Example 2:

A phosphating solution with a zinc ion concentration of 1.6 a / 1 is prepared. The solution, which is accelerated by NaNO ^ in a concentration of 0.2 to 0.6 g / l, is applied to samples made of FePO ₄ -MB cold-rolled steel.

The application is carried out with a spray device, the following parameters provides:

Temperature 25 ° C |

Time 2 minutes "j

2
Spray pressure 1.5 kg / cm

2
Coverage volume 160 l / min / m.

The same device was previously used to degrease the samples with a slightly alkaline degreasing solvent activated by titanium salts. .

The phosphated samples had an inhomogeneous crystalline

2 and covered macrocrystalline layer weighing about 1.2 g / m 2.

The samples were submerged for electrophoresis ^ application painted with a gray primer commonly used in the automotive industry. The thickness of the primer j mean was about 18 microns.

A cruciform incision was made in the specimens, which then were exposed to the salt spray test according to 2STMB 117-67.

After 600 hours of exposure, rust began to appear from the ' Cross-shaped incision to penetrate under the primer and

blistering was found in the values shown in Table 1.

Table 1

Depth of the cut ASTM B 117/67

Blistering ASTM D 714/74

example 1: example 2: O - 0.5 mm 3-4 mm no M 4

Example 3;

A comparison of the results in Table 1 clearly shows the importance of the ?; Change of even as little as 0.1 g / l in the zinc concentration of the “j Phosphating bath with regard to the quality of the protection of the treated | of metal. . Ü

The monitoring and control device according to FIG. 2 was in one industrial phosphating equipment used (using the same working conditions as described for example 1 - temperature, Composition of the phosphating solution, type of application, spray pressure and cover volume). The operation of the electronic Facility was set to a zinc concentration in the bath of 1.68 g / l set the value at which the system actuated pump 11, which added a replenisher to the bath until the zinc concentration rose to 1.7 g / l in the phosphating solution. The course of the phosphating process was therefore constantly changed by the method according to the invention Device regulated between values of 1.68 g / l and 1.7 g / l zinc in the phosphating solution.

Samples of the phosphating solution used were all from pump 2 Taken for 30 minutes and mixed in a dilution ratio of 1 to 30 with respect to their volume with carrier electrolyte, which by

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Mixing 15 ml of 30 percent ammonia (by weight), 7.5 g of potassium chloride, 10 g of sodium sulfite, 1.5 ml of a two percent aqueous solution Triton X-100 (Registered Trade Mark) and 20 g of dihydric tartaric acid sodium The whole thing was made up in one liter of twice distilled water.

Samples of the treated metal samples were taken every 10 man hours and subjected to ASTM B 177/67 and D 714/74 tests.

The samples taken and tested showed a coating, both in terms of crystallinity and density of the protective layer as well as was constant with regard to the layer weight.

The values of the ASTM tests were identical to those obtained by Example 1 for all samples taken.

Claims (14)

.- η at ε n.tan wä L-TS- ": DR.- SNG. H. H. WI LHE-L1V3 -" Da PL. ': 1 NG. H. DAUSTER D-7000 STUTTGART 1 · GYMNASIUMSTRASSE 31B -TELEFON (0711) 291133/29 28 Applicant: Stuttgart, February 07, 1984 D 6943 / 7a FOSFACOL S.p.A. 76, Via Caselle S. Lazzaro Di Savena (Bologna), Italy Da / Ei patent and protection claims 1. Device for automatic monitoring of phosphating of metallic surfaces, in particular made of iron, steel or galvanized iron, by means of aqueous solutions of Zinc phosphates, characterized in that a first extraction pump (2) for extracting samples from the phosphating bath (1), a second withdrawal pump (4) for withdrawing a carrier electrolyte from a storage container (3) and a Polarographic measuring cell (7) connected to the two extraction pumps (2, 4) for analyzing the zinc ion content are provided in the solution of sample and carrier electrolyte, as well as one of the two pumps (2, 4) with predetermined frequencies actuating electronic device (9) which compares the actual value measured by the measuring cell (7) with predetermined setpoints and which actuates a third to Refreshing the phosphating bath (1) serving pump (11) controls. 2. Apparatus according to claim 1, characterized in that the measuring cell (7) has a metallic silver anode (32) and one Has mercury drop cathode. -2- 3. Apparatus according to claim 1 or 2, characterized in that the measuring cell (7) has a volume of 5 to 50 ml. 4. Device according to one of claims 1 to 3, characterized in that that the measuring cell (7) is equipped with a temperature control device containing a thermostat. 5. Device according to one of claims 1 to 4, characterized in that that the measuring cell (7) is made of a transparent material such as glass or the like. 6. Process for the automatic monitoring of the phosphating of metallic surfaces, especially those made of iron or steel, by means of aqueous solutions of zinc phosphates, characterized in that a sample of the phosphating bath is automatically is removed at predetermined time intervals and mixed with a carrier electrolyte, then the mixture obtained a polarographic measuring cell is supplied, which determines the actual value of the zinc ion concentration of the phosphating bath via the Limiting diffusion current density dependent on the reduction of zinc ions measures which is compared with predetermined target values of the zinc ion concentration of the phosphating bath, where at A pump for deviations exceeding a predetermined amount between the actual values and the setpoint values automatically a predetermined period of time is actuated, which supplies the phosphating bath with refreshing solution. 7. The method according to claim 6, characterized in that the value the diffusion limit current in the measuring cell from the sum of the current densities of the zinc ions and other possibly existing ones Metal ions are determined which have a reduction potential below the reduction potential of the zinc ions to have. 8. The method according to claim 6 or 7, characterized in that the carrier electrolyte from an aqueous solution with 1.5 to 5 g / l ammonia, with 5 to 15 g / l of a substance absorbing oxygen in the sample fed to the measuring cell, with 10 COPY ~ ³ ~ up to 30 g / l of one or more complexing agents for the metal ions, with 10 to 15 mg / l of a wetting agent and with 3.5 to 10 g / l of an additive that keeps the Silbar anode active consists. 9. The method according to any one of claims 6 to 8, characterized in that that the oxygen absorbing substance is sodium sulfite. 10. The method according to any one of claims 6 to 9, characterized in that that the complexing agent is sodium citric acid, soda tartrate and their double salts or mixtures is. 11. The method according to any one of claims 6 to 10, characterized in that that the wetting agent is non-ionic in nature. 12. The method according to any one of claims 6 to 11, characterized in that that the addition is potassium chloride. 13. The method according to any one of claims 6 to 12, characterized in that that the sample of the phosphating bath is mixed with the carrier electrolyte in a ratio of 1:10 to 1:40 is. 14. The method according to any one of claims 6 to 13, characterized in, that the zinc content of the phosphating bath is within a tolerance range of - 0.1 to +0.1 g / l zinc with respect to the predetermined zinc concentration of the phosphating bath is regulated. -4-COPY