GB2153854A - Automatically controlling the phosphate coating of metals - Google Patents

Abstract

Apparatus for automatically controlling processes for the phosphate coating of ferrous metals or galvanized iron and comprises a cell 7 having a silver anode and a mercury drip cathode enabling the concentration of the zinc ion contained in the liquid used as phosphate coating agent to be measured polarographically, a first pump 2 for taking samples of the phosphate coating solution and sending such sample to the measuring cell, a polarographic current recorder 6 for measuring the strength of the measuring cell current and a timed electronic system 9 which compares the current measured by the measuring unit with predetermined limit values and controls the operation of a second pump 11 for supplying any necessary corrective additions to the phosphate coating solution. <IMAGE>

Description

SPECIFICATION Apparatus and method thereof for automatically controlling processes for the phosphate coating of metals This invention relates to an apparatus providing automatic control of a process for phosphate coating metal surfaces of the ferrous type.

More particularly, the invention relates to an apparatus enabling measurement and control of the Zn + concentration in a phosphate coating solution containing Zn and other metals and used to provide a protective coating on metal surfaces of the ferrous or galvanised iron kind.

The use of phosphate coating processes for corrosion protection of ferrous metal or galvanised iron surfaces is familiar in the art, being widely used in the production of motor vehicles, domestic electric appliances and other metal articles having a large superficial area.

Aqueous solutions of zinc phosphates are normally used as phosphate coating agents.

These solutions may contain other metal elements such as Ni or Mn.

The baths containing these elements other than the zinc are those used at present.

Phosphate coating baths can be accelerated by the presence of exiding anions such as for example nitrite or chlorate.

However, in both ordinary and accelerated baths the zinc is the element deposited on the metal surface in the form of a phosphate which forms the protective layer.

Phosphate coating solutions can be used for dip treatments, spray treatments and mixed spray and dip treatments at acid pH values at temperatures of from ambient temperature to about 80"C and with Zn concentrations of from 1 to 10 g/l of solution.

In all cases a particular combination of parameters helps to provide the required kind of coating (crystalline grain, sticking to the metal surface, thickness of coating).

Clearly, therefore, the control and maintenance of the parameters at the required values is one of the main problems in industrial phosphate coating processes.

The conventional procedure is to use determinations of the pH and of the oxidation-reduction potential of the phosphate coating solution to control the additions of reagents consumed in the process. Periodically, complexometric measurements are also made of the quantity of Zn present in the solution in order to control concentration to the required value.

As a rule, the analyses are made periodically at predetermined intervals by taking samples of the phosphate coating solution, whereafter appropriate additions of the various reagents are made in dependance upon the sample values.

Endeavours have been going on for some years to try to provide continuous adjustment of a phosphate coating process by means of facilities giving automatic determination of the values of the parameters of interest.

For instance, apparatuses have been proposed for continuously determining the pH value and oxidation reduction potential of a phosphate coating solution, the analyzing facilities controlling the operation of pumps for supplying replenishment substances to the solution, as in the case of Uster's CORROSTATR apparatus.

Another proposal has been to control the concentration of nitrite ion (which is one of the most commonly used ingredients in accelerating compound mixtures for phosphate coating solutions) by sending a partial flow of the solution to an anion exchanger, for instance, as proposed by U.K. Patent Specification No. 1,518,534.

The methods and apparatuses proposed, although giving better results than the conventional manual control systems, do not solve the problem completely satisfactorily because they need to be integrated with the manual analytic determination of the concentrations of the other ingredients of the phosphate coating solution, more particularly of the zinc.

It has now been found that the problem of controlling phosphate coating processes can be solved completely satisfactorily by controlling the Zn+ + ion content of the solution by means of a very simple apparatus providing very accurate evaluation of the Zn ++ ion content automatically and virtually in real time and adapted to actuate a pump which introduces the replenishment solution into the phosphate coating solution in use.

According to the invention, in the apparatus of the kind hereinbefore outlined, the cell for measuring the concentration of zinc in solution is a polarographic cell having a mercury droplet electrode and a silver anode which is supplied with a support electrolytic solution mixed in appropriate proportions with the solution sampled from the phosphate coating bath it is required to control.

If an appropriate support electrolytic solution is used which obviates the interference of atmospheric air, more particularly of oxygen which might interfere with measurements, the limit diffusion current of the Zn+ + ion can be measured very accurately at the mercury droplet electrode, the value of such current being strictly proportional to the Zn + + concentration of the solution being tested.

Of course every metal ion has its characteristic oxide reduction potential, and so in appropriate conditions just the limit diffusion current of the Zn+ + ion can be measured accurately even in the presence of other metals.

The discharge potential-i.e., that potential difference between the two electrodes of the measuring cell at which the ion present in solution changes over to the metal static is therefore characteristic for every kind of ion present in the solution being examined.

Consequently, the potential difference between the two electrodes can be varied so as to provide either qualitative detection of the ion types present (based on the observed discharge potential) or their quantitative determination (based on the strength of the current measured in conditions of limit diffusion current).

This is the system normally used for polarographic determinations.

In the preferred embodiment of this invention working proceeds at a constant potential .e., maintaining the potential difference between the two electrodes of the measuring cell at a constant value such as will maintain the limit diffusion current of the zinc ion. A current strength is therefore obtained which covers the contributions of the metals having a more positive discharge potential than zinc while the elements having a more negative discharge potential do not interfere.

The phosphate coating solutions used in industry normally contain other metal ions in addition to Zn + + and such ions may derive either from substances added to "accelerate" the action of the bath or from partial dissolution of the metal alloys forming the articles being given phosphate coating treatment.

As a rule, such metals are still present in quantities appreciably below the quantities of the zinc and their contribution to the current which is measured and used for zinc determination is substantially negligible.

It is possible to have in all cases a limit diffusion current value usable for controlling the phosphate coating bath inasmuch as the variations relatively to the predetermined optimum value are reducible to the variations of the zinc ion concentration in the solution examined.

In a preferred form of the invention, the support electrolyte solution mixed with the bath sample to be analysed consists of an aqueous ammonia solution of substances complexing the metal ions, such as tartrates or citrates or their mixtures or their double salts surfactants, preferably of the non-ionic kind, substances for eliminating dissolved oxygen, such as sulphites, thus obviating deaeration of the mixture sent to the analysis cell, and other additives, such as potassium chloride, which is particularly advisable when a metal silver electrode is used in order to keep it active.

The use of the support electrolyte solution and its composition are very important if expected and reproducible results are to be obtained.

The various ingredients of the support electrolyte solution can be present in a wide variety of quantities.

In preferred embodiments of the invention the following composition ranges have been found very advantageous: from 1.5 to 5 g/l of ammonia, from 10 to 30 g/l of complexing substances, from 10 to 50 mg/l of surfactant, from 5 to 1 5 g/l of oxygen-absorbing substances, and from 3.5 to 10 g/l of additives to keep the silver electrode active.

In the accompanying drawings: Figure I is a diagram in which limit diffusion current values (ordinate) are plotted against the zinc ion concentration of the phosphate coating solution (abscissa) for different solution/support electrolyte ratios; Figure 2 is a diagrammatic view of an embodiment of the apparatus for controlling the phosphate coating process in accordance with this invention, and Figure 3 shows an embodiment of the analysis cell of the apparatus according to this invention.

Curves A, B and C in Fig. 1 show the values of the diffusion current (in microamperes) in dependence on zinc ion concentrations (in g/l) present in a zinc-phosphate-based phosphate coating bath for different dilutions with the support electrolyte solution. More particularly, curve A corresponds to a 1:10 dilution by volume between the bath sample and the support solution while curve B is for a 1: 25 ratio and curve C for a 1 :40 ratio. The value of the zinc ion concentration is the value of the sample examined before dilution with the support electrolyte.

The measurements were made at 25on. The aqueous support electrolyte solution used contained 1 5 ml of a concentrated aqueous ammonia solution (d = 0.892), 10 g of sodium sulphite, 20 g of sodium dihydrate tartrate, 7.5 g of potassium chloride and 1.5 ml of a 2% aqueous solution of a non-ionic surfactant per litre of solution. Preferably, the dilution ratio between the phosphate coating solution and the support electrolyte solution which are mixed and sent to the measuring cell is from 1:10 to 1 :40 by volume.

The best measurement results are obtained with dilution ratios of from 1 :20 to 1 :30. As is known in the art the zinc concentrations in phosphating baths may vary from 1 to 10 g/l, although in the more commonly used baths the limits range from 1.5 to 3.5 g/l.

Once the operating conditions of the bath used (temperature, type of accelerator, method of treating the articles in the bath, treatment times, etc.) have been selected, the zinc concentration of the bath must be maintained in a very narrow range to obviate variations in the quality of the final protective coating. The apparatus according to this invention can control-i.e., measure and adjust,-the concentration of zinc in the bath completely automatically within a range of + 0.05 g/l of zinc. These values are sufficient to ensure protective coatings of high and constant quality.

Another advantage of automatic control of zinc concentration within the limits made possible by the apparatus according to this invention is the obviation of a useless excess of zinc in the phosphate coating bath, leading to a greater consumption of zinc per square metre of total superficial area.

As the curves of Fig. 1 show clearly, very small variations in the zinc ion concentration of the phosphate coating solution can be measured in that after dilution with the support electrolyte solution, variations of current strength of from 0.1 7 to 1.08 microamperes are detected (according to the dilution ratio used) for variations of zinc concentration in the bath specimen of 0.1 g/l. These currents are readily measurable by conventional commercially available potentiometric measuring facilities.

Normally, in the preferred embodiment of this invention it is preferred to work with a constant dilution ratio between the bath sample and the support electrolyte solution, the sensitivity of the potentiometric facility being adjusted with the normal sensitivity controllers associated with apparatus of this kind.

However, the invention can be carried into effect using measuring equipment not having sensitivity controllers, just by choosing the dilution ratios between the sample of phosphate coating solution and the support electrolyte solution such as give an optimum reading of the limit diffusion current values in the range of the available instrument.

Fig. 2 shows in diagrammatic form an apparatus according to this invention. A specimen of phosphate coating solution is taken from a phosphate coating bath 1 by means of a peristaltic pump 2 and, after being mixed in line 8 with the predetermined quantity of support electrolyte solution taken from a tank 3 by a peristaltic pump 4, goes to a measuring cell 7. An analyzer 6 measures the strength of the measuring cell current and records the values thereof. If the value detected by the analyzer 6 is below the value corresponding to the required Zn concentration, an electronic unit 9 triggers a pump 11 which intakes replenishment phosphate coating solution from tank 10 and supplies such solution to the bath 1.

The pump 11 operates for predetermined times, its stoppage being controlled by a timer 5.

The timer 5, in addition to controlling stoppage of the pump 11, controls the stopping and starting of the pumps 2 and 4 on a repetitive cycle.

By controlling the times of such cycle the bathcan be analyzed at constant time intervals and at the required frequency.

In view of the high sensitivity of the analysis provided by this invention, and the high frequency at which determinations can be made, the phosphate coating bath can be controlled and adjusted virtually in real time.

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

The body of cell 37 is made of glass or some other transparent material, for better observation of possible occlusions of the capillary 31, which permits the formation of the mercury droplet or drip or the like which forms the cell's cathode (the electrical terminations are not shown.) Tank 41 contains a reserve of pure mercury (bidistillate) for polarography and is connected to cell 37 through a polythene or rubber tube 40 and a capillary 31 secured to the cell 37 by means of a sealing-tight closure system 38.

The cell 37 also has an inlet tube 34 and an outlet tube 32, through which tubes the mixture of bath sample and support electrolyte flows. The outlet tube 32 has in it a silver electrode 33.

The mercury dripping through the capillary 31 accumulates at the base 35 of the cell and is maintained at a constant level there by means of a siphon 36, so that a sealing-tight closure of the cell is provided. Mercury is discharged from the cell through a tube 39 connected to the siphon 36.

The measuring cell is of very reduced volume in order to reduce waste of support electrolyte solution.

Preferably, the cell volume is from 5 to 50 ml; larger volumes are of course feasible but smaller volumes would lead to great difficulties in cell construction.

Since ceteris paribus the value of the diffusion current depends upon the superficial area of the mercury droplet acting as cathode, the capillaries selected can provide mercury drops of dimensions such as to have current values in the required measurement range.

Typically, the capillaries used in the measuring cell according to this invention have an internal diameter of from 0.05 to 0.10 mm.

The renewal rate of the cathode surface-i.e., the number of mercury drops issuing from the capillary in a given time-is adjusted simply by the height of the mercury tank 41 relatively to the measuring cell once a particular capillary diameter has been determined.

Typically, the drip time varies from 2 to 6 seconds .e., from 10 to 30 drops of mercury per minute.

For a better explanation of the results which can be provided by the apparatus according to the the invention, but without any limitative effect, reference may be made to the following examples: EXAMPLE 1 A phosphate coating solution having a zinc ion concentration of 1.7 g/l is prepared. The solution, accelerated with NaN O2 in a concentration of from 0.2 to 0.6 g/l, is applied to specimens made of Type FePO4-MB cold rolled steel.

The application is made by a sprayer capable of providing the following parameters: Temperature 25"C Time 2' Spraying pressure1.5 kg/cm2 Covering volume 160 It/min/m2.

The same apparatus was used previously to degrease the specimens with a slightly alkaline degreaser activated with titanium salts.

The phosphate coated specimens were covered by a fine and dense crystalline layer weighing approximately 1.6 g/m2.

Specimens were painted for immersed electrophoretic use with a grey primer conventionally used in the automotive industry.

The primer was about 18 microns thick.

A cross-shaped incision was made on the painted specimens and they were given a salt mist test to ASTM B 117-67.

After 600 hours of exposure rust began to penetrate below the primer from the cross-shaped incision and blistering was noted, in the values set out in Table 1.

EXAMPLE 2 A phosphate coating solution having a zinc ion concentration of 1.6 g/l is prepared. The solution, accelerated by NaNO2 in a concentration of from 0.2 to 0.6 g/l, is applied to specimens made on type FePO4-MB cold rolled steel.

The application is made in a spraying apparatus able to provide the following parameters: Temperature 25"C Time 2' Spraying pressure1.5 kg/cm2 Covering volume 1 60 I/min/m2.

The same apparatus was previously used to degrease the specimens with a slightly alkaline degreasing agent activated by titanium salts.

The phosphate-coated specimens are covered by an unhomogeneous crystalline and macrocrystalline layer weighting approximately 1.2 g/m2.

The specimens were painted for immersed electrophoretic application with a grey primer conventionally used in the automotive industry.

The thickness of the primer was approximately 1 8 microns.

A cross-shaped incision was made in the specimens and they were given salt mist testing to ASTM B117-67.

After 600 hours of exposure rust started to penetrate below the paint from the cross-shaped incision and blistering was found, in the values set out in Table 1.

TABLE 1 EXAMPLE 1 EXAMPLE 2 Depth of incision ASTM B117/67 0-0.5mm 3-4mm Blistering None M 4 ASTM D 714/74 EXAMPLE 3 A comparison of the results in Table 1 shows clearly the importance of the variation of even as little as 0.1 % in the zinc concentration of the phosphate coating bath in relation to the quality of protection of the treated metal.

When the control and adjusting apparatus shown in Fig. 2 was used on an industrial phosphate-coating line (using the same operating conditions as are described in Example 1-temperature, composition of the phosphate-coating solution, kind of application, spraying pressure and covering volume), the actuation of the electronic unit 9 was adjusted to a zinc concentration in the bath of 1.68 g/l, at which value the system triggered the pump 11 which supplied the bath with the replenishment solution until the zinc concentration of the phosphate coating solution in use rose to 1.7 g/l. The performance of the phosphate coating process was therefore always controlled by the apparatus according to the invention between values of 1.68 g/l and 1.7 g/l of zinc in the phosphate coating solution.

The specimen of phosphate coating solution in use was taken by the pump 2 every 30 minutes and mixed in the dilution of from 1 to 30 times its volume with a support electrolyte solution prepared by mixing 1 5 ml of 30% ammonia by weight, 7.5g of potassium chloride, 10 g of sodium sulphite, 1.5 ml of a 2% aqueous solution of Triton X-1 00 (Registered Trade Mark) and 20 g of sodium dihydrate tartrate, the whole made up to 1 litre with twice-distilled water.

Examples of treated metal specimens were taken every 10 hours of operation and given the ASTM B177/67 and D714/74tests.

All the examples taken and examined showed a coating which was constant as regards both crystallinity and density of the protective layer and as regards constancy of layer weight.

The values of the ASTM tests were identical for all specimens to those given for the specimens of Example 1.

Claims (14)

1. An apparatus for automatically controlling processes for the phosphate coating of metal surfaces of the ferrous or galvanized iron type by means of zinc-phosphate-based aqueous solutions, the apparatus comprising a first positive displacement pump for taking samples from the phosphate coating bath, a second positive displacement pump for taking a support electrolyte solution from a reserve tank, a measuring cell in liquid communication with the pumps, the cell having a metallic silver anode and a mercury droplet cathode for analyzing the specimen mixed with the support electrolyte solution, a unit for measuring the strength of the measuring cell current, and a timed electronic system which triggers the two pumps at an adjustable and predetermined frequency, compares the current measured by the measuring unit with predetermined limit values and controls the operation of a third pump supplying the solution for replenishing the phosphate coating bath.

2. An apparatus according to Claim 1, characterised in that the measuring cell has a volume of from 5 to 50 ml.

3. An apparatus according to Claim 1, characterised in that the measuring-cell temperature is controlled by thermostat means.

4. An apparatus according to Claims 1 to 3, characterised in that the measuring cell is made of glass or some other transparent material.

5. A method of automatically controlling a process for the phosphate coating of metal surfaces of the ferrous type by means of zinc-phosphate-based aqueous solutions, characterised in that a sample of the phosphate coating bath is taken automatically at predetermined time intervals and mixed with an aqueous solution of support-electrolyte and the resulting mixture is sent to an analysis cell having a silver anode and a mercury drip cathode measuring the strength of the limit diffusion current associated with the discharge of the zinc ion, comparing such value with a predetermined value corresponding to the optimum zinc ion concentration in the bath and, in the event of differences between these two values in excess of a predetermined amount, automatically actuating for a predetermined time a pump supplying the replenishment solution to the phosphate coating bath.

6. A method according to Claim 5, characterised in that the value of the limit diffusion current in the measuring cell is given by the sum of the current values of the zinc ion and of those of the other metal ions which may be present and which have a discharge potential below that of the zinc ion.

7. A method according to Claim 5, characterised in that the support electrolyte solution consists of an aqueous solution containing from 1.5 to 5 g/l of ammonia, from 5 to 1 5 g/l of a compound absorbing any dissolved oxygen in the sample sent to the measuring cell, from 10 to 30 g/l of one or more substances complexing the metal ions, from 10 to 50 mg/l of a surfactant and from 3.5 to 10 g/l of an additive for keeping the silver electrode active.

8. A method according to Claim 7, characterised in that the oxygen-absorbing substance is sodium sulphite.

9. A method according to Claim 7, characterised in that the complexing substance is sodium citrate, sodium tartrate and their double salts or mixtures.

10. A method according to Claim 7, characterised in that the surfactant is of a non-ionic kind.

11. A method according to Claim 7, characterised in that the additive is potassium chloride.

12. A method according to Claim 5, characterised in that the sample of the bath is mixed with the support electrolyte solution in a proportion by volume of from 1:10 to 1 :40.

1 3. A method according to Claim 5, characterised in that the zinc content of the bath is controlled in the range of from - 0.1 to + 0.1 of zinc relatively to the predetermined zinc concentration of the bath.

14. An apparatus for automatically controlling processors for the phosphate coating of metal surfaces substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

1 5. A method for automatically controlling processes for the phosphate coating of metal surfaces substantially as hereinbefore described with reference to the accompanying drawings.

1 6. A method as claimed in Claim 5 and substantially as described in the examples.