EP3140438B1 - Zinc-plated workpiece with improved adhesion for cover layers - Google Patents

Description

The invention relates to a galvanized workpiece having polycrystalline zinc at least on parts of the workpiece surface. The invention further relates to a method for producing a galvanized workpiece with improved adhesion for cover layers.

As is well known, workpieces made of red or iron or steel, e.g. Body parts of automobiles, extensively coated with zinc metal, in short: galvanized to prevent rusting. The zinc metal, which is inherently base, quickly forms on its surface under ambient air conditions a passivation layer of zinc oxide or zinc carbonate, which adheres well to the zinc metal and significantly increases the chemical stability of the galvanized surface compared to an untreated workpiece surface.

However, the protective passivation layer has the disadvantage of reduced adhesion to polymers, paints or other metallic films to be applied as a cover layers on a galvanized workpiece surface.

Adhesion can be improved by providing adhesion promoting chemicals, either as a separate interlayer or as a component of the overcoat to be applied. But in addition to the cost of the primer usually additional work steps are required, and the coating result is not always satisfactory. For example, delamination may occur over time when the thermal expansion coefficients of the workpiece and the paint are very different and the painted workpiece is subject to high temperature variations.

It is generally known that a roughening of the metallic workpiece surface can be helpful in improving the adhesion of cover layers. In particular, trenches, pores or local elevations on the workpiece surface can be structures which can be penetrated, filled or surrounded by a flowable polymer or lacquer, so that the subsequently curing polymer or lacquer film is fixed as a result of mechanical anchoring. Even very poorly adhering polymers such as polytetrafluoroethylene (PTFE) can be so mechanically bondable to metal surfaces.

For example, the DE 1811264 A1 (1968 ) to disclose how a metallic piston of a cryogenic expansion device is coated with PTFE by electrochemically etching a large number of holes or grooves in a chromium layer initially deposited on the piston, so that the subsequently applied PTFE fills and mechanically anchors at least these recesses. In the DE 1811264 A1 It is about a functional coating, which should reduce the friction of the piston and avoid its tilting in the cylinder. The etched chromium layer plays the role of a bonding agent. Preferably, contiguous network patterns of grooves are to be produced, as can be seen in FIGS. 6 and 7 of the document. It may be assumed that the deposited chromium layer is polycrystalline and that the grooves form by etching along the grain boundaries.

Intergranular etching of metal surfaces to improve the adhesion of polymers is also the subject of DE 601 28 364 T2 , which also proposes an immersion plating step.

The galvanizing of a workpiece can be done by galvanic deposition, flame spraying or by dip coating in a molten zinc - also: hot dip galvanizing. Dip coating is the most widely used industrial process for corrosion protection of steel. He will both on finished parts - galvanizing with coating thicknesses in the range 50-150 micrometers - as well as on endless steel strips - strip galvanizing with coating thicknesses up to about 20 micrometers - applied. Strip-galvanized steel sheets are generally still cut after cutting, punched and formed, whereby the coating is damaged in places. It is therefore common to provide such galvanized workpieces with other cover layers, not least also to restore the corrosion protection.

In the present specification, the term galvanized workpiece is also used for such workpieces that attain their final shape only after being coated with zinc. It is only important that at least parts of their surface have a zinc layer.

Typically, the coating is a polycrystalline zinc layer on the workpiece surface, whereby the zinc crystallites can reach particle sizes of millimeters to centimeters. The etching of depressions along the grain boundaries does not provide sufficient space for a robust anchoring of cover layers on the galvanized workpiece with very large particle sizes.

The invention therefore has the task of improving the adhesion of outer layers on galvanized workpieces.

The object is achieved by providing a galvanized workpiece, which has polycrystalline zinc with particle sizes greater than 20 micrometers, at least on parts of the workpiece surface, characterized by irregularly distributed, penetrating into the zinc grains, different diameter, conical pores.

The subclaims are directed to advantageous embodiments of the galvanized workpiece and to a method for its production.

A conical pore in the sense of the present invention is a pore whose pore diameter at the pore approach - on the workpiece surface - assumes a maximum and decreases monotonically towards the pore tip - in the interior of the zinc layer. The pore diameter never increases with the etching depth, but it decreases or at best remains piecewise constant.

In contrast to the prior art, the structures according to the invention are not created by intergranular etching, but by etching into the zinc crystallites, including zinc grains. The pores penetrate into the zinc grains and penetrate into the interior of the zinc grains. This is only possible and expedient because it has surprisingly been found that etching conditions can be found in which, firstly, the pore etching speed along a crystal axis differs approximately ten times from the etching speeds along a crystal axis - this can be termed the "fast etching direction" of the zinc grain secondly, crystal defects such as grain boundaries and dislocation nests are not preferentially etched.

The prior art knows deep, largely constant diameter in the micrometer range having channel-like or conical pores, for example, of monocrystalline semiconductors, in particular silicon wafers. These so-called macropores can penetrate almost the entire wafer if, for example, they are electrochemically etched in n-type silicon from one wafer side, while the other wafer side is illuminated. The illumination serves to provide sufficient free charge carriers at the pore tips. By contrast, the pore walls are hardly etched because there are no free charge carriers available there. The pore wall stability is therefore most likely to be found in semiconductors and also not along arbitrary crystal axes. In metals with freely mobile electrons, the ability to etch deep conical pores in crystallites is unexpected to those skilled in the art, as it can initially assume no pore wall stability. However, it does show up in the case of zinc to the extent that the etching rates vary greatly along different crystal directions.

Within a single grain of zinc, the pores that penetrate into this zinc grain are parallel. The fast etching direction is a property of the zinc grain, but can also be affected by the etching electrolyte. However, it is not readily foreseeable in any of the zinc grains which direction the fast etching direction will face. During galvanizing of the workpiece, zinc grains with different orientations and thus also a different direction of the rapid etching direction with respect to the workpiece surface form. Typically, adjacent zinc grains have quite different crystal orientations in the finished zinc layer. Therefore, the fast etch directions with respect to the workpiece surface at quite different angles are also with respect to the surface of the workpiece. In particular, the fast etching direction is almost never perpendicular to the workpiece surface.

The pore structure of the galvanized workpiece according to the invention thus typically comprises side by side in a random arrangement in the wall of the same zinc grain penetrating, parallel, conical pores, of which typically only a few are directed perpendicular to the workpiece surface. The diameter of the pores, which has its maximum at the pore approach and should preferably be in the interval between 100 nanometers and 15 microns, more preferably between 1 and 5 microns, varies along the wall of each individual zinc grain, ie each grain is both of wide and deep as well as at the same time interspersed by less wide and less deep pores. If the depth to diameter ratio - also: aspect ratio - exceeds the value of 3, the pores penetrate so deeply into the grain, which is a part of the pore space of the zinc grain wall on which the pores attach, hidden. In other words, the pore tip is no longer visible from the surface of the workpiece formed by the zinc grain walls because the pore has grown obliquely. This is certainly the case with larger aspect ratios, preferably greater than about 10.

With increasing etch depth, the proportion of hidden pore space increases. Hidden pore space undercuts the workpiece surface, thereby providing greater resistance to peeling, e.g. a polymer film, which is applied in liquid form to the galvanized workpiece, also penetrates into the pore space and then cured.

Of particular importance here is that the direction in which the pores undercut the workpiece surface varies from zinc grain to zinc grain. A covering layer which is toothed with the zinc layer locally in different directions along the surface of the material in particular sets forces which are suitable for displacing the covering layer on the surface to be of greater resistance. Among other things, temperature-induced expansion changes of the workpiece relative to the cover layer at many points of the workpiece surface lead to increased adhesion of the cover layer, namely where the locally acting forces press the cover layer deeper into the hidden pore space. By virtue of its anchoring in the conical pores, which are not perpendicular to the surface of the workpiece, the cover layer has barbs in practice, and these have local directions along the entire workpiece surface in each case in a direction dependent on the zinc grain present there.

Fig. 1

Elektronenmikroskop-Aufnahmen einer porösen Zinkfolie in der Aufsicht in zwei Vergrößerungen;

Fig. 2

a) Schematisch senkrechter Schnitt durch die poröse Zinkfolie, b) Elektronenmikroskop-Aufnahmen einer porösen Zinkfolie in der Aufsicht und c) eine Elektronenmikroskop-Aufnahme eines senkrechten Schnittes durch eine poröse Zinkfolie.

The person skilled in the art will now see how a galvanized workpiece provided with the features according to the invention enables a better anchoring and thus adhesion of cover layers on the zinc coating. In the following, the production of a galvanized workpiece according to the invention is described, and a porous zinc foil produced in the laboratory is presented. The following figures are also used for this purpose. Showing:

Fig. 1

Electron micrographs of a porous zinc foil in the plan view in two magnifications;

Fig. 2

a) Schematic vertical section through the porous zinc foil, b) electron micrographs of a porous zinc foil in plan view, and c) an electron micrograph of a vertical section through a porous zinc foil.

The pore etching takes place in an electrochemical etching cell with a potassium chloride-containing, aqueous electrolyte. The temperature is controlled and kept constant by a thermostat. A square wave voltage is applied between the zinc layer and the electrode in the electrolyte, i. a temporal voltage curve, which is described by a rectangular function.

The square-wave voltage is periodic and preferably has a predetermined number of periods. In this case, a first voltage is applied for a first time interval and a second voltage for an immediately subsequent second time interval, the time period from the beginning of the first to the end of the second time interval being the period of the periodic square voltage.

It is very advantageous to initiate pore etching with a nucleation step that creates the first pits in random distribution on the workpiece surface. The nucleation of the pores should preferably be done by applying a third voltage once for a third time interval before applying the periodic square wave voltage.

The pore etching process takes place in each zinc grain predominantly along the aforementioned rapid etching direction of the zinc grain. A major reason why this is possible with the zinc etch presented here is the periodic switching from cathodic to anodic stresses, which in the choice of the electrolyte to form a well-defined zinc oxide coverage on Pore walls and areas with crystal defects that protect them against a preferential etching.

As an exemplary embodiment, a polycrystalline, rolled, 100 micron thick zinc foil (Zn content 99.95%) is etched. The electrolyte contains 0.1 mol / liter of potassium chloride. The temperature is chosen to be 50 ° C. The first voltage is 0 V, the length of the first time interval is 4.6 s, the second voltage is -1 V, the length of the second time interval is 0.8 s, the third voltage is 0.5 V, and the length of the second third time interval has been determined to be 1 s. The total etching time is 10 min.

In the case of a polycrystalline zinc layer having many smaller zinc grains and thus many grain boundaries, the etching parameters must be adjusted so as to suppress preferential etching of the grain boundaries. This can e.g. be achieved by extending the second time interval compared to the first time interval. In the case of large zinc grains, preferential etching of the grain boundaries plays a negligible role.

Besides the preferential etching of grain boundaries, it may e.g. for mechanically strongly deformed zinc surfaces also leads to a preferential etching of defect structures, e.g. come from dislocations / Versetzungsnestern etc. This could be suppressed in the same way as in the previously described case. In the case of zinc grains in centimeters or, in extreme cases, monocrystalline zinc, the number of nucleation nuclei may be too small for the growth of pores. An increase in the number of nucleation germs could e.g. be achieved by increasing the third voltage.

Therefore, the specific etching parameters must be adapted and optimized to the specific zinc etch incident, but this is within the skill of the artisan and given him as necessary experimentation to find a favorable Is to expect working point. Usually, such tests are only to be carried out once for series production.

Fig. 1 shows the achieved pore structure. Out Fig. 1 a) It can be seen that the pores have been formed in a random distribution and with different pore diameters. Since all pores should become deeper and wider at the same time as the etch time increases, it can be assumed that not all visible pores are nucleated at the same time, ie during the third time interval, but some may only be nucleated during any period of the subsequent square wave. However, shows Fig. 1 a) also a random distribution of pore density along the surface. Closely adjacent pores do not inevitably grow together, but can certainly interfere with each other in their broadening, as can be seen from the excerpt from the enlargement Fig. 1 b) can suspect. In Fig. 1 b) In any case, the conical rejuvenation of the pores from the outside to the inside is easy to recognize. In addition, one sees the stepped course of the pore walls, in particular along a plane which runs from bottom left to top right in the image and is visible in both pores shown. Apparently, this is a crystal plane, which offers the etching attack particularly high resistance.

In connection with the stepped pore walls, it should be noted that the steps are subsequently simply wet-chemical with a weak acid, e.g. Citric acid, dissolve. They do not consist of zinc oxide, but of zinc metal, which is oxidized on its surface - by ambient air. This is a useful distinguishing feature of the prior art. Namely, it is well known to etch in zinc surfaces by first oxidizing the zinc and then etching into the semiconductor zinc oxide. The zinc oxide can then be completely or partially removed with a weak acid, for example, corresponding steps on the pore walls.

In the case of the present invention, the etching is carried out directly in the metal. That is, on each free zinc metal surface quickly forms an oxide layer is inevitable and not further problematic. However, through-oxidation of the zinc layer to a predetermined depth to propel the pores to that depth does not occur. It would also be counterproductive to practically remove zinc plating for this purpose.

Incidentally, pore depths of between about 3 and 50 microns appear sufficient to provide good anchorage of cover layers. Of course, the conical pores at any point should not traverse the entire thickness of the zinc coating, because this would both weaken the anti-corrosion effect of the zinc layer and possibly lead to the replacement of the galvanizing of the workpiece.

A look at the sectional sketch in Fig. 2 a) are intended to illustrate the particularly favorable effect of the generated pore structure for the anchoring of cover layers. At the bottom (black) is the workpiece with a layer of zinc arranged on top of it, which here is very much made up of three zinc crystallites. The sketch represents a cross section to the galvanized workpiece surface. In the individual crystallites are each juxtaposed several parallel pores (recesses) etched into it. The pore shape is different in each crystallite, resulting in pores in all directions along the galvanized surface. In every crystallite the Fig. 2 a) there is covered pore space. If one thinks of the pore openings incident, parallel light in the sketch, so the hidden pore space would be the proportion in which the light could not occur due to shading. Material of a cover layer, which reaches the concealed pore space during application, usually in liquid form, forms barbs during solidification both against the removal and against the lateral displacement relative to the workpiece surface.

The real pore structure of a zinc workpiece is in Fig. 2b ) shown in supervision. You can see many individual areas parallel to each other grown pores. The orientation of the pores with respect to the surface of the zinc workpiece is very different in adjacent regions depending on the orientation of the zinc crystallites with respect to the surface of the zinc workpiece.

The Fig. 2c) Figure 3 shows the photograph of an actual vertical section through the porous zinc foil described above. This image is unexpectedly difficult to interpret, because you can barely make out the expected pore channels. This is due to the simple fact that it is very difficult to make the cut exactly along the fast etching direction of any zinc grain because it can not be determined beforehand. In contrast to monolithic semiconductors which typically cleave along certain planes, which afford a nice view of the etching geometry, are in Fig. 2c) at most difficult to identify small areas with pore walls of the etched pores in the image plane. Their parallel growth is, however, recognizable by the section.

Claims (9)

1. A zinc-plated workpiece which has at least on parts of the workpiece surface polycrystalline zinc having grain sizes of greater than 20 micrometres, characterised by irregularly distributed conical pores which penetrate into the zinc grain walls and are of different diameters.
2. A zinc-plated workpiece according to Claim 1, characterised in that the pores run in parallel within an individual zinc grain.
3. A zinc-plated workpiece according to one of the preceding claims, characterised in that the pores have a ratio of depth to diameter of at least 3, preferably of at least 10.
4. A zinc-plated workpiece according to one of the preceding claims, characterised in that the pores on the workpiece surface have diameters from the range of between 100 nanometres and 15 micrometres, preferably from the range of 1 micrometre to 5 micrometres.
5. A zinc-plated workpiece according to one of the preceding claims, characterised in that at least part of the pore space etched into the zinc grains is covered by the zinc grain walls which form the workpiece surface.
6. A method for producing a zinc-plated workpiece according to one of the preceding claims, characterised by electrochemical etching of the zinc-plated workpiece surface in an aqueous potassium chloride containing electrolyte at a predetermined temperature with the application of a periodic square-wave voltage.
7. A method according to Claim 6, characterised in that for a first time interval a first voltage and for a second, directly succeeding, time interval a second voltage, is applied, the amount of time from the beginning of the first until the end of the second time interval being the period of the periodic square-wave voltage.
8. A method according to one of Claims 6 or 7, characterised in that the nucleation of the pores takes place by one-time application of a third voltage for a third time interval prior to the application of the periodic square-wave voltage.
9. A method according to Claims 6 to 8, characterised in that the electrolyte contains 0.1 mol/litre potassium chloride, the predetermined temperature is selected to be around 50°C and the first voltage is determined at 0 V, the length of the first time interval at 4.6 s, the second voltage at -1 V, the length of the second time interval at 0.8 s, the third voltage at 0.5 V and the length of the third time interval at 1 s.

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