WO2008011627A2

WO2008011627A2 - Method and device for controlling the results of deposition on substrate surfaces

Info

Publication number: WO2008011627A2
Application number: PCT/US2007/074126
Authority: WO
Inventors: Andreas Mobius; Axel Fuhrmann; Karl-Heinz Wandner; Rüdiger Dreissig
Original assignee: Enthone Inc.
Priority date: 2006-07-21
Filing date: 2007-07-23
Publication date: 2008-01-24
Also published as: WO2008011627A3; EP1881091A1

Abstract

The present invention relates to a method and a device for monitoring and/or controlling the result of the deposition of a metal or metal alloy layer on a substrate surface, and to methods for deposition of a metal or metal alloy layer onto a substrate surface. For example, the invention relates to a method and a device for the determination of the result of the deposition during the treatment of metallic surfaces, including aluminum or magnesium surfaces or surfaces from aluminum or magnesium alloys by means of socalled zincate baths. In one embodiment a zincate bath (5) is prepared in a suitable container (1). The free corrosion potential is determined by electrically connecting a reference electrode (2) and a base material (3) with a measuring device (4).

Description

METHOD AND DEVICE FOR CONTROLLING THE RESULTS OF DEPOSITION ON

SUBSTRATE SURFACES FIELD OF THE INVENTION

[0001]The present invention relates to a method and a device for monitoring and/or controlling the result of the deposition of a metal or metal alloy layer on a substrate surface, and to methods for deposition of a metal or metal alloy onto a substrate surface. For example, the invention relates to a method and a device for the determination of the result of the deposition during the treatment of metallic surfaces, including aluminum or magnesium surfaces or surfaces from aluminum or magnesium alloys by means of so-called zincate baths. The present invention is further directed to methods for deposition of metals such as, for example, tin onto substrates including, for example, copper.

BACKGROUND OF THE INVENTION

[0002]Aluminum and magnesium substrate surfaces, as well as surfaces containing their alloys, typically contain oxide layers at their surface. Prior to a further galvanotechnical surface treatment (e.g., metal plating), these oxide layers are typically removed so that the oxide layer does not interfere with the further processing. To prevent formation of new oxide layers once the original oxide layer is removed, the corresponding substrate surfaces may be treated to deposit a protective layer; such methods are often referred to as an "immersion process." This deposition may be carried out using electrolytes for the deposition of tin, silver, gold, copper, zinc and zinc alloys. Electrolytes used for deposition of zinc are commonly referred to as zincate solutions.

[0003] Zincate solutions are typically alkaline, zinc-containing solutions. For example, at the contacting of an aluminum surface, or corresponding alloy surface, with a zincate solution, zinc will become deposited on the substrate surface according to the following formula

3 Zn (OH)4^2" + 2 Al → 3 Zn + 2 Al (OH)₄ ^" + 4 OH^" [0004]Aluminunn oxide (magnesium oxide in the case of a magnesium substrate) and also aluminum (magnesium in the case of a mangesium substrate) are dissolved on the surface and zinc is deposited by charge exchange.

[0005] In terms of the invention the term zincate layer is understood to be a deposited zincate layer.

[0006] In order to attain a sufficient adhesion of the deposited zinc on the surface it is common in prior art to remove the deposited zinc layers by a suitable reagent (e.g., a solution containing nitric acid) and to contact the surface again with the zincate solution. The thus deposited layers are referred to as zincate layer. Depending on the number of zinc deposition operations, these processes are commonly referred to in the art as "double zincate" or "triple zincate" processes.

[0007] In addition to the deposition of pure zincate layers it is also known from prior art to deposit zinc alloy layers on aluminum- and/or magnesium-containing substrate surfaces. Alloyed metals can be for instance nickel or iron, and binary Zn/Ni or Zn/Fe alloys and/or also ternary Zn/Fe/Ni alloys can be deposited. For the deposition of corresponding zinc alloys the zincate solutions normally include suitable nickel and/or iron compounds. The zincate solutions that are known from prior art can be cyanide-containing or can also be used as a cyanide-free system. To assist in formation of hydroxo complexes, the metals contained in the zincate solutions may be complexed by suitable complexing agents.

[0008] Zincate solutions for deposition onto magnesium-containing substrates are typically acidic. Acidic zincate solutions are typically based on pyrophosphate.

[0009] The formation of a zincate layer generally depends on one or more of a variety of factors including, for example, the free hydroxide concentration, the pre- treatment of the surface, the composition of the surface to be plated, the foreign metal content of the zincate bath, as well as the complexing agent content of the zincate bath. In dependence of these parameters, which will change in the course of the plating process, strongly adhering compact or porous to spongy layers can be deposited on the substrate surfaces having an excellent to bad or even no adhesion. The result of the plating depends on one or more of the aforementioned parameters, but may also depend on operating parameters such as temperature and duration of the plating process. [0010]Since the zincate plating of an aluminum- and/or magnesium- containing surface frequently forms the basis of a subsequent galvanization with a nickel- or copper-containing layer in an electrochemical or autocatalytic plating process, a poorly deposited zincate layer, and especially a poorly adhering zincate layer, may lead to insufficient plating results. Such defects very often appear only at the time of storage or during tempering of the finally plated substrate surface or of the finally plated part where surface defects such as lumps and blisters may occur. Particularly based on the late occurrence of detection, these defects result in a high quantity of rejects, and are associated with high costs. In the worst case the damage caused by a poor-quality zincate layer occurs only in the finally installed part which fact leads to considerable costs for replacing such parts. Thus, a method for early prediction of deposition quality would be desired to avoid these costs associated with later-discovered defects and the problems associated therewith.

[0011] Moreover, further immersion processes are known from prior art.

[0012] Immersion tin, often referred to as "immersion tin electrolytes" is typically deposited as Hot Air Solder Leveling (HASL) substitute on printed circuit boards. In the same manner silver from immersion electrolytes is increasingly used. In the semiconductor industry gold electrolytes are found which are also based on a charge exchange. A classical technique of the immersion process is the copper plating of wires from hot sulfuric copper sulfate solutions.

[0013] Plating defects may occur during deposition of metal or metal alloy layers other than zinc-containing including, for example, copper-, nickel- or chromium-containing layers or alloys. It is to be noted that the present invention is directed to an improved method for deposition of these metals as well as a method to deposit zinc.

[0014]This analogously applies also to all the other layer systems that follow after a zincate layer.

[0015] Thus, the p r o b I e m to be addressed by the present invention is the need for a method and a device which make it possible during the plating process to determine in particular a sufficient adhesion of the deposited metal or metal alloy layer, in order to be able to control the plating process in dependence thereof. That is, there remains an unfulfilled need for a simple, cost-effective method for deposition of a well-adhered zincate layer that allows for further treatment of the substrate. SUMMARY OF THE INVENTION

[0016] Among the objects of the invention, therefore, is to provide a method for depositing a zincate layer onto a metal substrate that is suitable for metal plating thereon.

[0017] Briefly, therefore, the present invention is directed to methods for depositing a metal or metal alloy layer onto a substrate surface. In various embodiments, the method comprises immersing the substrate into a deposition bath containing a source of ions of the metal or metal alloy to be deposited; depositing the metal or metal alloy onto the substrate surface; continuously or intermittently measuring a free corrosion potential between the substrate surface being plated and a reference electrode during the depositing, to yield a measured free corrosion potential; comparing the measured free corrosion potential to a predetermined value of free corrosion potential, wherein the predetermined value of free corrosion potential corresponds to a free corrosion potential between a control electrode of the metal or metal alloy to be deposited and the reference electrode; and controlling the deposition by continuing or discontinuing the deposition on the basis of the comparing.

[0018] In various other embodiments, the method comprises immersing the substrate into a deposition bath containing a source of ions of the metal or metal alloy to be deposited; depositing the metal or metal alloy onto the substrate surface; continuously or intermittently measuring a free corrosion potential between the substrate surface being plated and a reference electrode during the depositing, to yield a measured free corrosion potential; and discontinuing the deposition when the measured free corrosion potential approaches or achieves steady state as a function of time.

[0019] In still further embodiments, the present invention is directed to a method for controlling the result of the deposition of a metal or metal alloy layer onto a substrate surface, the method comprising measuring a free corrosion potential between the substrate surface to be plated and a reference electrode during deposition, and comparing the free corrosion potential to a predetermined free corrosion potential between the substrate surface and reference electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Fig. 1 shows a schematic of a device suitable for carrying out the process of the present invention.

[0021] Fig. 2 shows electric potential measurements collected as described in Example 1.

[0022] Fig. 3 shows electric potential measurements collected as described in Example 2.

[0023] Fig. 4 shows electric potential measurements collected as described in Example 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] This application claims priority from EP 06015270.9 filed July 21 , 2006, the entire disclosure of which is incorporated herein by reference.

[0025] Concerning the method, problems encountered by methods of the prior art are addressed, and preferably solved, by a method for controlling the results of the deposition of a metal or metal alloy layer deposited on a substrate surface, wherein the free corrosion potential between a substrate surface to be plated and a reference electrode is determined at least temporarily during the plating process and the determined (i.e., measured) value is compared to a predetermined value, and the predetermined value is selected so that if the measured value and the predetermined value agree a sufficient result of the deposition will be obtained. In various embodiments, the present invention is directed to a process for deposition of zinc onto an aluminum or magnesium-containing substrate. The present invention is also directed to processes for deposition of various other metals (e.g., copper, tin, silver, gold, and combinations thereof) onto substrates comprising one or more of aluminum, magnesium, steel, copper, brass, and bronze.

[0026] The present invention involves monitoring and/or controlling the deposition of a metal (e.g., zinc) or an alloy onto the surface of a substrate. In particular, the present invention includes measuring the electric potential between the substrate and a reference electrode during processing, and comparing this potential to a predetermined, target value. As detailed elsewhere herein, this predetermined target value is selected as an indicator of deposition of a layer that properly adheres to the substrate and is generally free of defects observed with conventional methods. Typically, the predetermined value is represented by a time derivative of the free corrosion potential. For example, this fixed, or predetermined value may be selected based on results of prior plating operations. In various embodiments, suitable plating adhesion and quality have been observed to be associated with a curve of time versus electric potential nearing, or becoming, substantially horizontal. Thus, the predetermined value may be selected to be at or near the value where the time/potential curve for a combination of substrate and electrode begins to become horizontal, or does in fact become horizontal. The predetermined value in one embodiment is selected to be a value at which the coating is expected to be adherent and substantially non-porous.

[0027] Suitable plating may also be determined by testing the quality of the plating using various methods known in the art. One such method includes heat treating the plated substrate (e.g., heating the substrate to a temperature of approximately 25O⁰C). Another method involves alternately contacting the plated substrate with a relatively hot liquid and a relatively cold liquid (e.g., contacting the substrate with water at a temperature of approximately 100⁰C, followed by contact with water at room temperature or below). Corrosion potential measurements for operations that provide suitable plating as determined by such methods may then be used to provide the predetermined value for further operations.

[0028] Known disadvantages of the prior art are avoided by the process of the present invention. In particular, the process has been observed as providing a manner to detect when plating operations have provided a layer of metal or alloy that is well-adhered to the substrate, and substantially, preferably entirely, void of surface defects associated with conventional processes. Furthermore, the present process provides an economic benefit over conventional plating processes since the method allows the plating operation to be discontinued once suitable deposition is achieved, thereby avoiding excess operation and the associated costs. Moreover, in various embodiments, the process of the invention may provide a suitable metal layer such that removal and re-deposition of the metal layer may be unnecessary, providing further economic benefit. But it should be understood that use of the process of the present invention in "double zincate" or "triple zincate" processes is also advantageous. [0029] It is currently believed that the result of the deposition of, for example, zincate layers on aluminum- or magnesium-containing substrate surfaces not only may depend on the composition of the zincate solution and the set conditions of the process but may also depend on the composition and quality of the substrate surface on which a corresponding metal or metal alloy layer is to be deposited.

[0030] In this connection it turned out that the determination of the free corrosion potential between the substrate surface to be plated and the reference electrode is suited for monitoring the process of the deposition on the substrate surface.

[0031] As described herein, free corrosion potential refers to the electrode potential of metal or an alloy in an attacking medium without the influence of external currents, measured against an unpolarized reference electrode.

[0032] It has been observed that in the course of the plating process the free corrosion potential approximates a limit value. This limit value may be dependent on the composition of the plating solution, for example a zincate solution, and/or may also depend on the composition and/or quality of the substrate surface to be plated.

[0033] It has been observed that free corrosion potentials correlate with the adhesive strength of the deposited metal or metal alloy layers on the substrate surface.

[0034] Without being bound to a particular theory, it is currently believed that the measured free corrosion potential reaches a limit value as soon as the deposited layer on the substrate surface is completely closed (non-porous) (e.g., all or substantially all of the substrate surface is coated with the metal) and accordingly exhibits good adhesion properties.

[0035] Accordingly, by determining the free corrosion potential between a substrate surface to be plated and a counter electrode and by comparing the determined free corrosion potential with a fixed limit value it is possible to determine independently of the further plating parameters the point of time by which a metal or metal alloy layer will have reached a sufficient result of deposition on the substrate surface. The value to which the determined potential value is compared can be selected in dependence of the desired deposition result. [0036] Therefore the determination of the free corrosion potential allows for the first time the direct control of the deposition result already during the plating process. This results in very good possibilities especially with regard to quality control which leads to a clear reduction of rejects and accordingly to a clear optimisation of costs.

[0037] Typically, zinc to be deposited at the surface of the substrate is derived from a source selected from the group consisting of, for example, zinc oxide, zinc sulfate, and combinations thereof. In the case of processes for deposition of tin, the source of tin is typically, for example, tin chloride, tin sulfate, other conventional tin sources, and combinations thereof. Suitable sources of silver include, for example, silver nitrate, silver chloride, other conventinal silver sources, and combinations thereof. The concentration of metal to be deposited in the plating solution is typically from about 1 to about 100 grams per liter (g/l), more typically from about 5 to about 50 g/l and, still more typically, from about 10 to about 30 g/l.

[0038] As noted, the present invention is suitable for deposition of alloys at the surface of the substrates including, for example, alloys containing nickel and/or iron alloyed with zinc, tin, or silver. For example, iron may be provided by, for example, iron halides (e.g., FeCI₃), iron sulfates (e.g., FeSO₄ or Fe₂(SO₄)₃), and combinations thereof. Nickel may be present in the plating solution by virtue of inclusion of a source selected from the group consisting of nickel sulfate, nickel chloride, nickel acetate, and combinations thereof.

[0039] Typically, ions of any such metals to be alloyed and/or the source of such metal are present in the plating solution at a concentration of at least about 0.1 g/l, at least about 0.2 g/l, or at least about 0.5 g/l (e.g., from about 0.1 to about 1 g/l, or from about 0.25 to about 0.75 g/l).

[0040] The plating solution may also include a complexing, or chelating agent. Generally, agents commonly known in the art are suitable for use in the process of the present invention and include, for example, tartrates, acetates, citrates, lactates, maleates, and combinations thereof. In various embodiments, the complexing agent comprises Rochelle salt (i.e., potassium-sodium tartrate). It is to be noted that the concentration of complexing agent in the plating bath is not narrowly critical, but may be in the range of from about 5 g/l to about 200 g/l, from about 25 g/l to about 175 g/l, from about 50 g/l to about 150 g/l, or from about 75 g/l to about 125 g/l. [0041]The metal plating process of the present invention proceeds generally in accordance with means known in the art but, as previously noted, both the substrate and a reference electrode are contacted with the plating solution. As a reference electrode for the determination of the free corrosion potential a Calomel electrode is generally preferred. But any other type of reference electrode (e.g., a Ag/AgCI electrode) is also suitable for use as a counter electrode in the method according to the invention. During the plating operation, the electric potential of the substrate versus the reference electrode is measured.

[0042] Since the direct contacting of substrate surfaces to be plated for the determination of the free corrosion potential may be difficult during plating operations, in accordance with the present invention the free corrosion potential may, additionally or alternatively, be determined between a measuring electrode and a reference electrode during plating operations. In this sense, the measured potential is not directly from the substrate, but it at least corresponds to the substrate. In accordance with various embodiments, the electric potential may be determined between a measuring electrode identical, or substantially identical, in composition with the material of the substrate surface to be plated. It is possible thereby to use corresponding measuring electrodes that can be electrically contacted and which are subject to the plating process together with the substrates to be plated.

[0043] The electric potential of the substrate and/or measuring electrode versus the reference electrode may be measured intermittently or continuously throughout the plating operation. Regardless of any interval of potential measurement, metal deposition generally continues until the value is at, or within a certain range of, a target, or predetermined value of electric potential of substrate versus reference electrode. For example, metal deposition typically continues until the potential is within about 40%, within about 30%, within about 20%, or within about 10% of the predetermined value. For example, in the case of a target, predetermined value of -1400, terminating metal deposition at a potential of from approximately -1540 eV to approximately -1260 eV represents termination within 10% of the predetermined value. Typically, plating is discontinued prior to reaching the predetermined value, but plating may also continue until after the predetermined value is reached. While the former is preferred due to the attendant economic benefits, operation in either manner provides for control over the plating operation and represents an advance over conventional methods. Stated another way, deposition is discontinued when a difference between the measured free corrosion potential and the free corrosion potential between the control electrode of the metal or metal alloy to be deposited and said reference electrode is less than about 40%, 30%, 20%, or 10% of the free corrosion potential between the control electrode of the metal or metal alloy to be deposited and the reference electrode. An alternative characterization of this same concept is to select a predetermined value which is within less than about 40%, 30%, 20%, or 10% of the free corrosion potential between the control electrode of the metal or metal alloy to be deposited and the reference electrode. This demonstrates that the concept is the same whether, semantically, the "predetermined value" is used to refer to the point at which the curve becomes substantially horizontal, or some target point short of the point at which the curve becomes substantially horizontal.

[0044]The concept of the invention in one aspect applies where the predetermined value of free corrosion potential corresponds to a free corrosion potential characteristic of an adherent and substantially non-porous coating of said metal or metal alloy onto the substrate surface. Alternatively, the predetermined value of free corrosion potential may be selected to correspond to a free corrosion potential between a control electrode of the metal or metal alloy to be deposited and said reference electrode.

[0045] These target values of electric potential may be determined by monitoring the electric potential between various substrates and reference electrodes, and noting the electric potential associated with sufficient adhesion of the metal layer and formation of a metal layer substantially free of defects. For example, these target potential values may be approximated and/or determined for various substrate/reference electrode combinations based on corrosion potential measurements collected, for example, in accordance with the methods described in Examples 1-3 set forth below.

[0046]As noted, it is currently believed that sufficient deposition may be represented by the curve of time versus free corrosion potential becoming horizontal. Without being bound to a particular theory, it is believed that change in the slope of this curve represents change in the composition of the substrate surface (e.g., a change from the free corrosion potential such as aluminum to the free corrosion potential of the metal to be deposited such as zinc). Deposition is believed to be completed (e.g., an aluminum surface is coated with zinc) once the free corrosion potential is no longer varying to any significant degree as evidenced by the curve becoming horizontal.

[0047] As noted, the temperature and/or duration of contact of the substrate and plating solution may generally affect metal deposition. Typically, the plating operation is conducted at a temperature of from about 5⁰C to about 100⁰C, from about 10⁰C to about 75⁰C, or from about 2O⁰C to about 5O⁰C. It is to be further noted that the duration of contact is typically at least about 10 seconds, at least about 20 seconds, or at least about 30 seconds (e.g., from about 10 to about 120 seconds, from about 20 to about 90 seconds, or from about 30 seconds to about 60 seconds). As noted above, metal oxide layers are typically removed from the substrate prior to metal deposition. These metal oxides may be formed during preparation of the substrate, and may be removed by an etching process that proceeds in accordance with methods generally known in the art. For example, the oxide surface may be contacted with an etchant containing, for example, nitric acid, sulfuric acid, phosphoric acid, caroate, sodium peroxomonosulfate, sodium peroxodisulfate, or a combination thereof. The concentration of additive(s) in the etching solution is not narrowly critical, and generally can be from about 10 g/l to about 50 g/l.

[0048] Concerning the device the problem on which the invention is based is solved by a device for controlling the results of the deposition of a metal or metal alloy layer deposited on a substrate surface, which device includes apparatus for determining the free corrosion potential between a substrate surface to be plated and a counter electrode as well as apparatus for comparing the determined free corrosion potential to a predetermined value.

[0049] The measuring device may comprise, for example, a device for taking the potential measurements (e.g., an analog or digital multimeter, or a potentiostat) and a device for collecting and recording the data (e.g., a computer). In particular, the potential measurements may be taken by connecting the substrate to the inlet of the multimeter, which is connected to a computer to collect the data. In the case of an analog multimeter, an Analog-to-Digital (A/D) converted may be utilized as well. The data may also be taken, recorded, and collected utilizing a potentiostat in electrical communication with a computer. The data collection devices may be contacted to the support for the substrate and reference electrode.

[0050]The apparatus for the comparison of the determined free corrosion potential to the predetermined value (e.g., a computer in electrical communication with a multimeter or potentiostat) preferably is able to output a signal when the determined potential and the predetermined value agree, or the determined potential is near the predetermined value (e.g., at least 70% of the predetermined value). Advantageously, such a signal is integrated with a process control system that terminates or interrupts the plating process in response to the free corrosion potential nearing or reaching the predetermined value.

[0051] For example, the process control system may be adapted to interrupt the current supply to the substrate surfaces to be plated in the case of a galvanic plating process, or by terminating the contacting of the substrate surface to be plated with the electrolyte composition. For example, the process control system may be adapted to automatically remove the substrates to be plated from the electrolyte bath, which may be preferred in autocatalytic plating processes.

[0052] For the inventive determination of the free corrosion potential between a substrate surface to be plated and a counter electrode (e.g., a Calomel or Ag/AgCI electrode), an electrode serves as a measuring electrode for the determination of the free corrosion potential, which electrode may be identical in its composition with the material of the substrate surface to be plated.

[0053] For example, a piece of a material sample of the substrate surface to be plated can be adapted for being inserted in a suitable supporting device. Such a supporting device can be for instance a device for the electrical contacting of the material sample for tapping the free corrosion potential current. A supporting device which is designed in this way can then serve as a measuring electrode.

[0054] For carrying out the measurement a reference electrode is used.

[0055] In an advantageous embodiment of the device according to the invention a corresponding measuring electrode is fixed for instance on a support frame for substrates to be plated, whereby the measuring electrode can run through the plating process of the substrates together with the substrates. This makes sure that the measuring electrode is exposed to the same conditions as the substrate surfaces to be plated.

[0056] The support frame for the measuring electrode can also be configured as an electrode array and can receive in addition to the measuring electrode also the reference electrode. Advantageously, an electrode array may be configured totransmit in a wireless manner the determined potential value to the means for comparing the determined potential value to a predetermined value.

[0057] Being configured in such a way, the device according to the invention can be easily adjusted and universally used for the respective substrate surfaces to be plated or their composition.

[0058] In particular, the device according to the invention can be easily incorporated in existing plating systems, whereby the method according to invention can be used also in existing plating systems.

[0059] It is also possible to conduct the measuring of the free corrosion potential in a laboratory next to the installation. This enables a practically continuous control of the zincate solution and the immersion time. Usually one to two measurements are sufficient for each deposit (e.g., a plating operation that occurs over the course of, for example, eight hours). When using new alloys the same must be previously examined in the laboratory in order to determine the appropriate conditions. These conditions include: type of zincate solution, concentration of the components in the zincate solution, immersion time, temperature. If necessary, working with different plating solutions is possible.

[0060] The method according to the invention is suited also for the determination of the deposition result of chromizings on zinc layers.

[0061] Fig. 1 shows in a exemplary manner the experiment set-up for Example 1. A zincate bath 5 is prepared in a suitable container 1. The free corrosion potential is determined by means of a suitable reference electrode 2 such as for instance a saturated Calomel electrode between the same and the base material 3 to be plated, through a suitable measuring device 4. To this end, the reference eletrode 2 and the base material 3 are electrically contacted in a suitable manner with the measuring device 4.

[0062] Fig. 2 shows the measured free corrosion potential for different aluminum alloy compositions as a base material. The alloy compositions in zone A show a fast approximation to a limit value, whereas the alloy compositions in zone B show a clearly slower approximation to a limit value.

[0063] As noted, the substrate having a metal or metal alloy thereon prepared in accordance with the process of the present invention (e.g., zincate coated aluminum) may be subjected to a further metal plating operation. Metals that may be deposited include, for example, nickel generally in accordance with means known in the art (e.g., U.S. Patent No. 6,080,447, the entire disclosure of which is incorporated herein by reference for all relevant purposes).

[0064] The present invention is further illustrated by the following Examples. These Examples are not to be regarded as limiting the scope of the invention or the manner in which it may be practiced. In particular, the following embodiments show in an exemplary manner the inventive determination of free corrosion potentials during the plating process, but the idea of the invention cannot be limited to these exemplary alloy compositions and plating methods.

Example 1

[0065] Samples of base materials to be coated were embedded in a plastic ring with epoxy resin in such a way that one side remains open. This side is subject to a wet-grinding operation, finally using grain 500, rinsed and dried with filtering paper. Thereafter, these samples are immersed in a zincate bath of the type Enthone ALUMON EN, and the potential is measured using a Luggin capillary against a saturated Calomel electrode (SCE) and the progression of the potential at room temperature (23°C) was monitored and electronically recorded for 120 seconds (s) as shown in Fig. 1. The solutions are not tempered. A zinc/iron alloy was deposited on the substrate surface.

[0066] The progression in time of the potential in Fig. 2 shows differences between the individual alloy types.

[0067] In addition to the compositional detail for the substrates included in Fig. 1 , the substrates were of the following compositions (all values are percent by weight): Al 99.9 Mg 0.1 Al 98.5 Mg 1 Si 0.5 Al 98.5 Mg 1 Si 0.5 Al 98.5 Mg 1 Si 0.5 Al 89 Si 11 Al 88 Mg 1 Si 11 Al 98.3 Mg 1 Si 0.7

[0068] Group A shows a desired result. Within a relatively short time (e.g., approx 20 s), a dense strongly adhering zincate layer was formed. The curve becomes a horizontal after 20 seconds. Also, the substrates of group A did not show any blisters during tempering after deposition of a galvanic layer (composition: copper/ nickel/ chromium). The adhesion of the galvanic layers was excellent. The substrates of group A show dense but thin zincate layers with good adhesion.

[0069] Group B included alloys requiring approx 40 to 80 s until the oscillation of the values which occurs due to the development of gases terminates and the potential also remains constant at approx 100 s. All in all the progression of the curve with the surface not yet closed shows potentials which are more negative by 20OmV to 300 mV. The horizontal part of the curve is reached only after approx 80s. After this time the zincate layer is also dense but slightly thicker and more voluminous. The potential then approximates again the zone 1400 to 1450 mV/SCE. The adhesion is good and no blisters occur after the tempering. If the parts were removed from the bath after approx 60 s, adhesion problems would have to be expected, because the zinc layer has not yet fully formed.

[0070] One alloy (AISin/Mg) did not form completely dense zincate layers, even after 120 s. Although becoming smaller, the oscillation of the measuring values does not fully disappear. The layers can be wiped off with cellulose and they are obviously porous. This becomes evident also by the more negative potential of about 1550 mV/SCE after 120 s. In this case adhesion problems are to be expected if one does not carefully control the stripping of the first zincate layer and the formation of the second zincate layer. The size and shape of the silicon crystallites seems to have a considerable influence here.

Example 2

[0071] Specimens with freshly deposited copper were clamped in a specimen holder and connected to the input for the working electrode of a potentiostat. The Calomel reference electrode became immersed in the tin immersion solution Enthone STANNOSTAR GEM plus, which had a temperature of 68⁰C, already before the beginning of the measuring and was connected to the corresponding input of the potentiostat. The measuring operation of the potentiostat was started and one second later the specimen in the tin immersion solution. The corrosion potential was recorded for a 3 minutes duration, a plot of the measured free corrosion potential can be seen in Fig. 3.

Example 3

[0072] Specimens with freshly deposited bright copper were clamped in a specimen holder and connected to the input for the working electrode of a potentiostat. The Calomel reference electrode became immersed in the silver immersion solution Enthone ALPHASTAR SILVER, which had a temperature of 50⁰C, already before the beginning of the measuring and was connected to the corresponding input of the potentiostat. The measuring operation of the potentiostat was started and one second later the specimen in the silver immersion solution. The corrosion potential was recorded for a 3 minutes duration, a plot of the measured free corrosion potential can be seen in Fig. 4.

List of reference numbers

1 container

2 reference electrode

3 base material

4 measuring device

5 zincate bath

Claims

WHAT IS CLAIMED IS:

1. A method for depositing a metal or metal alloy layer onto a substrate surface, the method comprising: immersing the substrate into a deposition bath containing a source of ions of the metal or metal alloy to be deposited; depositing the metal or metal alloy onto the substrate surface; continuously or intermittently measuring a free corrosion potential corresponding to a free corrosion potential between the substrate surface being plated and a reference electrode during said depositing, to yield a measured free corrosion potential; comparing said measured free corrosion potential to a predetermined value of free corrosion potential; and controlling said deposition by continuing or discontinuing said deposition on the basis of said comparing.

2. The method of claim 1 wherein said predetermined value of free corrosion potential corresponds to a free corrosion potential characteristic of an adherent and substantially non-porous coating of said metal or metal alloy onto the substrate surface.

3. The method of claim 1 wherein said predetermined value of free corrosion potential corresponds to a free corrosion potential between a control electrode of said metal or metal alloy to be deposited and said reference electrode.

4. The method of claim 1 , 2, or 3 wherein said controlling comprises discontinuing said deposition when a difference between said measured free corrosion potential and predetermined value is less than about 30% of said predetermined value.

5. The method of claim 1 , 2, or 3 wherein said controlling comprises discontinuing said deposition when said measured free corrosion potential reaches the predetermined value.

6. The method of claim 1 wherein said controlling comprises discontinuing said deposition when a difference between said measured free corrosion potential and said free corrosion potential between the control electrode of said metal or metal alloy to be deposited and said reference electrode is less than about 30% of said free corrosion potential between the control electrode of said metal or metal alloy to be deposited and said reference electrode.

7. A method for controlling the result of the deposition of a metal or metal alloy layer onto a substrate surface, the method comprising measuring a free corrosion potential between the substrate surface to be plated and a reference electrode during deposition, and comparing the free corrosion potential to a predetermined free corrosion potential between the substrate surface and reference electrode.

8. The method of claim 7, wherein the predetermined value is selected to indicate sufficient deposition of metal or metal alloy onto the surface of the substrate.

9. The method according to claim 7 or 8, wherein the free corrosion potential is determined continuously during deposition.

10. The method according to claim 8 or 9, wherein the free corrosion potential is determined intermittently during deposition.

11. The method according to any one of 7 through 10, wherein the predetermined value represents the free corrosion potential between a measuring electrode which consists of a material which is substantially identical in composition to the substrate and the reference electrode.

12. A method for depositing a metal or metal alloy layer onto a substrate surface, the method comprising: immersing the substrate into a deposition bath containing a source of ions of the metal or metal alloy to be deposited; depositing the metal or metal alloy onto the substrate surface; continuously or intermittently measuring a free corrosion potential corresponding to a free corrosion potential between the substrate surface being plated and a reference electrode during said depositing, to yield a measured free corrosion potential; and discontinuing said deposition when said measured free corrosion potential approaches or achieves steady state as a function of time.

13. The method of claim 1 wherein the substrate is an Al-Mg alloy containing at least 99% Al, wherein the metal to be deposited is a Zn-Fe alloy, and wherein the predetermined value is a potential between about -1500 mV and about -1300 mV versus the reference electrode.

14. The method of claim 1 wherein the substrate is an Al-Mg alloy containing at least 99% Al, wherein the metal to be deposited is a Zn-Fe alloy, wherein the predetermined value is a potential between about -1500 mV and about -1300 mV versus the reference electrode, and wherein said controlling comprises discontinuing deposition when the measured free corrosion potential is in the range of between about -1500 mV and about -1300 mV.

15. An apparatus for depositing metal or a metal alloy layer onto a substrate surface, wherein the apparatus comprises a sensor for determining the free corrosion potential between the substrate surface to be plated and a counter electrode as well and a device for comparing the determined free corrosion potential to a predetermined value of free corrosion potential.

16. The apparatus of claim 15 comprising a signal output connected to the device for comparing the determined free corrosion potential to the predetermined value of free corrosion potential.

17. The apparatus of claim 16, wherein the signal output is connected to a device suitable for terminating the deposition.

18. The apparatus of any of claims 15 to 17, comprising a measuring electrode comprising a material which is the same as the material of the substrate to be plated.

19. The apparatus of claim 18 wherein the measuring electrode is adapted for being fixed to a support frame to which one or more substrates are attached.