Patent Application
20170029954
The invention relates to a method of activating metal surfaces, in particular of coated steel sheet, preferably galvanized steel sheet, before a phosphating process, in which the metal surface is brought into contact with an activating bath containing inorganic-metallic activating particles, preferably based on phosphate and/or titanium, dispersed in water.
Zinc phosphate layers are used in the prior art for surface treatment of galvanized fine steel sheet in order to improve surface-relevant properties of the galvanized fine steel sheet. These include, in particular, increasing the corrosion resistance and improving the formability and adhesion of surface coatings.
It has been found by the applicant that, in past years, not periodic, always recurring surface coating adhesion problems occurred on, for example, electrolytically galvanized and phosphated metal strip, in particular steel strip (fine sheet).
Proceeding therefrom, it was an object of the present invention to provide a method by means of which the problems of poor adhesion of surface coatings to metal strip can be considerably reduced or even avoided.
In a method of the type mentioned at the outset, this object is characterized by at least one additive which suppresses or at least slows agglomeration of the activating particles being added to the activating bath.
The inventors have examined the mechanisms of activation, nucleation and growth of the zinc phosphate crystals on the zinc coating. They have established that agglomerates of activating particles are formed with increasing time of operation of the activating bath. In addition, they were able to recognize an adverse effect of the increasing particle sizes in the activating bath on phosphating and adhesion of surface coatings.
The addition according to the invention of an additive which suppresses or at least significantly slows agglomeration of the activating particles enables the problems of poor adhesion of surface coatings to phosphated metal strip, in particular galvanized, phosphated steel strip, to be considerably reduced or even avoided.
The additive used for stabilizing the activating bath can be, in particular, one or more of the following materials:
Preferred and advantageous embodiments of the method of the invention are indicated in the dependent claims.
An advantageous embodiment of the method of the invention is characterized in that polyethylene glycol (PEG) and/or sodium stearate is added to the activating bath as additive for suppressing or slowing agglomeration of the activating particles. These two materials have each been found to be very effective in experiments.
To slow the agglomeration of the activating particles in the activating bath, it is also advantageous for, according to a further preferred embodiment of the method of the invention, the activating bath to be agitated continuously or discontinuously by stirring and/or pump circulation and/or introduction of ultrasound. In this way, the operating life of the activating bath can be increased further. The intensity of bath agitation (by stirring and/or pump circulation and/or introduction of ultrasound) should, however, not be too high since otherwise agglomeration of the activating particles in the activating bath may be promoted. The activating bath is preferably stirred by means of at least one mechanical stirrer.
A further preferred embodiment of the method of the invention is characterized in that the particle size distribution of the activating particles present in the activating bath is determined and in that the activating bath is replaced or taken out of operation as a function of the particle size distribution of the activating particles. In this way, critical or excessive deposition (adhesion) of agglomerated activating particles on the preferably electrolytically galvanized metal sheet can be very largely avoided and defect-free adhesion of surface coatings can thus be achieved.
In this context, it is advantageous for, according to a preferred embodiment of the method of the invention, the particle size distribution of the activating particles to be determined at regular intervals or continuously by means of dynamic light scattering (photon correlation spectrometry) during operation of the activating bath. As an alternative or in addition, the particle size distribution of the activating particles can be determined at regular intervals or continuously by means of nanoparticle tracking analysis (NTA) during operation of the activating bath. These two measurement methods are each particularly useful and reliable at the particle sizes and distribution widths relevant here. The measurement can be carried out in each case on separate, limited samples of the activating bath or alternatively by means of at least one flow-through measurement cell.
However, other measurement methods can also be employed for determining the particle sizes and particle size distribution of the activating particles in the method of the invention. For measurement in liquid, for example on separate, limited samples and also in a flow-through measurement cell, the following measurement methods are, for example, also conceivable here:
As an alternative or in addition, measurements can, in this context, be carried out on suitable supports or substrates using electron-microscopic methods, for example:
With regard to effective activation, nucleation and good growth of the zinc phosphate crystals on the zinc coating, it is additionally advantageous for the activating bath to be adjusted, according to a further preferred embodiment, in such a way that it has an activating particle concentration in the range from 0.1 g/l to 10 g/l, in particular from 0.5 g/l to 3 g/l.
The invention will be illustrated below with the aid of a drawing and a number of working examples. The single figure schematically shows a process flow diagram of continuous electrolytic galvanizing and phosphating of (rolled) steel strip.
A cold-rolled and optionally dressed steel strip (fine steel sheet) is provided as coil 1. The steel strip (fine steel sheet) 2 is unrolled from the coil 1 and welded onto the end of the previous strip. Since the subsequent electrolytic surface upgrading is a continuous process, the fresh strip entering the electrolytic upgrading plant is firstly passed into a strip loop storage 3 where it is stored in one or more loops so that the coating process does not have to be stopped when the beginning of a steel strip is welded onto the end of the previous steel strip.
In a first stage of the upgrading process (coating process), the strip surface is usually firstly mechanically and chemically cleaned. The strip surface is subsequently roughened in an acidic pickle before the strip 2 is passed through the electrolytic coating cells 4 and galvanized there. There, the steel strip 2 is dipped into a sulfuric acid zinc electrolyte and at the same time connected as cathode. In the case of soluble zinc electrodes, these are likewise dipped into the electrolyte solution and connected as anode. The zinc cations migrate from the anode through the electrolyte to the steel strip surface and are deposited cathodically there. In the case of insoluble anodes, on the other hand, the zinc is already present in solution in the electrolyte, and the anodes consist of appropriately more noble materials. The amount of zinc deposited on the strip surface depends in each case on the current density and the coating time. In order to achieve a zinc layer thickness of a few microns at a strip speed of, for example, 100 m/min, the steel strip 2 has to run through a plurality of coating cells 4 connected in series because of the relatively short coating time and accordingly low deposited amount in one electrolytic cell 4 at such a strip speed. In order to remove the electrolyte from the strip surface subsequently and thus avoid introduction of electrolyte into the next process step, the electrolytically galvanized steel strip 2′ is passed through a multistage rinsing apparatus 5.
A generally slightly alkaline activating bath 6 follows as pretreatment step for phosphating. Activating baths serve, in a phosphating process, to increase the number of nuclei and thus the phosphate crystals per unit area and thus increase the rate of crystal formation and increase the degree of coverage.
The activating bath 6 contains activating particles, generally particles based on phosphate and/or titanium or on metal oxides, dispersed in water. The activating particles which are, for example, obtainable in powder form are dispersed in water and form a colloidal solution with this. The activating bath 6 is, for example, adjusted so that it has an activating particle concentration in the range from 0.1 g/l to 10 g/l, in particular from 5 g/l to 3 g/l, preferably from 0.7 g/l to 1.5 g/l.
Suitable activating agents (activating particles) for the phosphating of electrolytically galvanized fine steel sheet 2′ are, for example, obtainable under the trade names SurTec® 145, SurTec® 610 V, SurTec® 615 V, SurTec® 616 V, Fixodine®X, Fixodine®50, Fixodine®50CF (now Bonderite® M-AC 50CF), Fixodine®950 (now Bonderite® M-AC 950), Fixodine®G 3039, Fixodine®C 5020 A, Fixodine®G 5020 B, Fixodine®C 9114, Fixodine®9112, Gardolene® Z26, Gardolene® V 6599, Gardolene® V 6560 A, Gardolene® V 6559, Gardolene® V 6526, Gardolene® V 6522, Gardolene® V 6520, Gardolene® V 6518, Gardolene® V 6513, Prepalene® X and Chemkleen® 163. Activating particles (activating agents) used for the pretreatment of metal surfaces to be phosphated, for example the fine steel sheet 2′, are usually Jernstedt salts or titanyl phosphates.
To maintain the dispersed state of the activating particles, the activating bath 6 is continuously or discontinuously stirred and/or circulated by pumping and/or treated with ultrasound. For example, the activating bath 6 is stirred by means of at least one mechanical stirrer 7.
After passing through the activating bath 6, the liquid film is squeezed or wiped off from the steel strip 2′ in order to avoid introduction of the possibly alkaline medium (liquid film) into the acidic phosphating solution. Drying of the steel strip surface can also be advantageous at this point. Accordingly, a hot air blower 8 is shown in the figure. In the phosphating stage 9, the phosphating solution is sprayed onto the activated strip surface.
This leads firstly to pickling of the zinc surface and secondly to growth of the zinc phosphate crystals on the activated regions. The remaining supernatant phosphating solution is subsequently squeezed off from the strip and the phosphated strip 2″ is then dried by means of a strip drier 10. In the last steps of this strip upgrading process, the phosphated steel strip 2″ is optionally oiled and rolled up to give a coil 11, so that it can be transported in readily handlable form to the customer.
At the customer's premises, for example an automobile manufacturer, plates are stamped from the phosphated steel strip and pressed to form components, for example bodywork parts. Since the forming of the plates by drawing and/or stretching of the material and also abrasion can result in damage to the phosphate layer, the metal surface is again activated and after-phosphated. The forming step is therefore usually followed by a degreasing step in a slightly alkaline solution and also rinsing-off of the cleaner in a multistage rinsing apparatus. Rinsing is followed by the renewed activation step and the after-phosphating.
The phosphating solution is removed by a further multistage rinsing apparatus before a surface coating is applied to the component. Here, a primer is usually applied to the phosphated component surface by means of cathodic dip coating. The components with the still moist primer surface are conveyed into an oven, typically a flow-through oven, where the surface coating composition is crosslinked and cured at relatively high temperatures (e.g. about 180° C.). A filling coating and finally a topcoat is then optionally applied.
To avoid poor adhesion of the surface coating caused by activating particle agglomerates and to achieve good adhesion of the surface coating, at least one additive A which suppresses or at least slows agglomeration of the activating particles is, according to the invention, added to the activating bath 6 which precedes phosphating. The additive forms an envelope around the activating particles, by means of which agglomeration of the activating particles can be suppressed at least for some time compared to conventional activating baths. For this purpose, polyethylene glycol (PEG), for example, preferably PEG having molar masses below 6000 g/mol, in particular about 400 g/mol (known as PEG 400), is added as additive A to the activating bath 6. For example, from 1 to 200 g/l of PEG are added to the activating bath, with the activating bath 6 having an activating particle concentration in the range from 0.1 g/l to 10 g/l, in particular from 0.5 g/l to 3 g/l, preferably from 0.7 g/l to 1.5 g/l.
Instead of polyethylene glycol (preferably PEG 400), sodium stearate is, in a further working example of the method of the invention, added as additive A to the activating bath 6 preceding phosphating. Sodium stearate is the sodium salt of stearic acid and a basic constituent of many soaps. Sodium stearate is a water-soluble solid. For example, about 0.01 g/l to 100 g/l of sodium stearate is added to the activating bath, with the activating bath 6 having an activating particle concentration in the range from 0.5 g/l to 3 g/l, preferably from 0.7 g/l to 1.5 g/l.
In a further working example of the method of the invention, poly(oxy-1,2-ethanediyl)carboxylic ester, in particular sorbityl poly(oxy-1,2-ethanediyl)monododecanoate, is added as additive A to the activating bath 6 which precedes phosphating. This additive, which is generally also referred to as polysorbate 20 (trade name “Tween® 20”), is a nonionic surfactant. It acts as wetting agent. For example, from 0.01 g/l to 100 g/l of polysorbate 20 (“Tween®20”) are added per 1 l of activating bath having an activating particle concentration in the range from 0.1 g/l to 10 g/l, in particular from 0.5 g/l to 3.0 g/l, preferably from 0.7 g/l to 1.5 g/l. Instead of this additive, polysorbate 40, polysorbate 60, polysorbate 65 or polysorbate 80 (trade name “Tween® 80”) can also be added as additive A to the activating bath 6.
In a further working example of the method of the invention, alkyl polyethylene glycol ether, in particular isotridecyl polyethylene glycol ether, is added to the activating bath 6. This additive is a nonionic surfactant whose state of matter is liquid. It acts, in particular, as wetting agent and is obtainable in a variety of variants under the trade name MARLIPAL®O13, with the different variants differing in the number of ethylene oxide molecules included. For example, from about 0.1 to 10 ml of alkyl polyethylene glycol ether are added as additive A per 1 l of activating bath 6 which has an activating particle concentration in the range from 0.1 g/l to 10 g/l, in particular from 0.5 g/l to 3.0 g/l, preferably from 0.7 g/l to 1.5 g/l.
In an advantageous optional embodiment of the above working examples of the method of the invention, the particle size distribution of the activating particles present in the activating bath 6 is determined and the activating bath 6 is replaced or taken out of operation as a function of the particle size distribution determined. The measurement of the particle size distribution is carried out by means of dynamic light scattering. As an alternative or in addition, the measurement of the particle size distribution can also be carried out by means of nanoparticle tracking analysis (NTA). The measurement of the particle size distribution of the activating particles of the activating bath 6 is preferably carried out on separate samples (part volumes) of the activating bath 6 or by means of at least one flow-through measurements cell (not shown), with both sampling and the measurement preferably being carried out at regular intervals or continuously during operation of the activating bath.
The replacement or taking out of operation of the activating bath 6 as a function of the particle size distribution of the activating particles determined in the activating bath 6 is then preferably likewise carried out automatically. The phosphating process can thus be conducted more reliably.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 105 226.9 | Apr 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/057464 | 4/7/2015 | WO | 00 |
