DE102004003412A1 - Process for high rate electrodeposition on metals and substrates by a pulsed stream technique to avoid dendritic crystal growth useful for corrosion protection of components in the automobile industry - Google Patents

Abstract

Process for high rate electrodeposition on metals and substrates by a pulsed stream (technique) to avoid dendritic crystal growth using cathodic and anodic current pulses (12,14) at cathodic and anodic pulse times -\60 milliseconds.

Description

The This invention relates generally to a process for high speed galvanic deposition of Metals according to the preamble of the independent claim. specially The invention relates to such a method using the Pulsed power technology.

The Depositing metals on substrates and in particular the Galvanizing of components is predominant in conventional electroplating as rack or drum product performed. By the unspecific flow conditions and electrochemical conditions, the coating may be slow, that is, with low current densities. When galvanizing but is it theoretically possible with very high current densities with appropriate adjustment of the electrolyte or the process conditions the speed of deposition to increase extremely. Compared to a conventional galvanic deposition process Thus, the deposition rate of about 1 micron / min can be increased to over 1 micron / s.

With however, there are some problems with increasing the current density et al the occurrence of dendritic crystal growth at local current density peaks. The dendritic crystal growth reduces the current efficiency of the Coating process and requires the removal of the dendrites.

Become simple coating geometries are used, such as at the strip galvanizing, The dendrites occur in the edge areas of the bands and can through easy cutting of the edges be removed. But if complicated geometries are used, such as. Brake parts, removal of the dendrites is not economical and not process-safe feasible.

By the possible Use of additives to the electrolyte can cause the appearance of dendrites dependent on of current density, component geometry and flow conditions prevented become. In the high-speed separation their use appears However, due to the limited effectiveness or the effort to Post-dosing and process control uneconomical.

The US 6,402,924 B1 describes a programmed galvanic pulse method in which the ratio of anodic and cathodic current density is continuously varied to improve the surface appearance, grain structure and leveling of the deposited metal layer. In this way, a uniform metal distribution on the substrate is ensured when deposited at high current densities. Current densities between 5 and 200 ASF (equivalent to 0.54 and 21.52 A / dm ² ) are used in this process. The pulse times for the "forward" pulse pulses (forward pulse time) are between 1 and 50 milliseconds, those for the "reverse" pulse pulses (reverse pulse time) in the range of 0.1 to 4 milliseconds. An avoidance of dendritic crystal growth is not mentioned anywhere in this document.

aim It is therefore the object of the present invention to overcome the disadvantages of the prior art to avoid the technique and a process for galvanic high-speed deposition of metals available to provide a dendritic crystal growth prevents.

Advantages of invention

The inventive method for high-speed galvanic deposition of metals across from the prior art has the advantage that a dendritic crystal growth, that significantly reduces the current efficiency of the coating process, can be avoided.

Farther is advantageous that the adjustment by the inventive method the desired Metal layer thickness can be achieved in a simple way.

advantageous Further developments of the invention will become apparent from the mentioned in the dependent claims Activities.

Short description the drawings

embodiments The invention is illustrated in the drawings and in the following Description closer explained. Show it

1 schematically the cathodic and anodic current pulses generated by a pulse rectifier; and

2 schematically a typical for the inventive method pulse pattern.

embodiments

The core of the invention is a process for high-speed galvanic deposition (HGA) of metals based on pulse current technology. In particular, this method can be applied to the Verzin kung of substrate surfaces are applied.

With the exact combination of parameters that define a pulse pattern, is the avoidance of dendritic growth in the zinc HGA possible without the use of additives. This is fast, cost effective Coating processes for Various coating geometries can be realized reliably.

From this results, for example, the advantage that by increasing the deposition speed shorter process times as well as a more compact system technology and thus additional Cost savings can be achieved. Furthermore, this is the prerequisite for a possible integration of the coating process fulfilled in a production line, what in terms of production flow and flexibility, warehousing, logistics costs etc. offers further cost saving potential. Due to the wide distribution of zinc coatings as corrosion protection for Steel and castings is thus clearly in this technology Potential for cost savings compared to conventional coating processes.

In order to avoid the formation of dendrites in the HGA process, a pulse rectifier is used which provides the required current for the deposition, as in 1 shown not only as DC 10 but also in the form of rectangular pulses 12 . 14 (Current pulses). The pulse rectifier must be able to withstand cathodic pulses 12 also anodic pulses 14 To generate (reverse rectangular current pulses, pulse reverse technology).

The Pulse-current technology used here is based on the use of defined Pulse patterns (cathodic or anodic current phases). While in The cathodic phase of zinc is deposited during the Anodic phase previously deposited zinc, and here in the first place the germinal sites of dendritic growth, dissolved. Also by the anodic Phases, previously deposited zinc is redissolved, however mainly the deposited layers at the exposed sites (areas with very high current density) of the components with complicated geometry, and less in the other areas that are shielded, for example, corners and / or holes (Areas with relatively low current density). Exactly the opposite happens at the cathodic pulses, at those at the exposed Areas gradually Dendrites arise. The use of a combination of cathodic and anodic pulses thus brings a leveling of the surface yourself.

The influence of the individual parameters in the HGA was investigated. The parameters to be varied are accordingly the values of the respective current amplitudes as well as the duration and frequency of the pulse phases. The current amplitudes (in A) or current values depend on the surface of the component. Therefore, the current density is given (A / dm ² ), which is independent of the surface. The current densities usable for the present invention are in the range of 2 to 200 A / dm ² . Particularly good results are achieved in the range of 10-70 A / dm ² , in particular in the range of 50-70 A / dm ² . The duration of the pulse phases is indicated in the pulse pattern (cf. 2 ). The frequency of the pulse phases gives the desired duty cycle (see below).

The investigations have shown that the ratio of pulse duration (anodic pulses in the present example) and pulse pause (cathodic pulses in the present example) has a great influence, in particular for the occurrence or avoidance of dendritic formation. The ratio of pulse duration to pulse pause is referred to as duty cycle γ and calculated as follows: γ = t on / t off where t _on denotes the anodic time or pulse time, t _off the cathodic time or pulse pause, I _k the current amplitude of the cathodic pulses and I _a the current amplitude of the anodic pulses.

2 schematically shows a typical for the inventive method pulse pattern. The pulse time 15 (t _on ) in this example is 500 ms and the pulse pause 16 (t _off ) 100 ms, the ratio t _on / t _off thus 5: 1. Particularly good results are achieved with this value, however, ratios between 2: 1 and 10: 1 are also possible, depending on the geometry of the components. This ratio remains constant throughout the deposition process. The pulse pattern thus repeats itself continuously, so that in the present example a "forward" pulse (cathodic pulse) is followed by a "backward" pulse (anodic pulse) of 100 ms every 500 ms. The amplitude of the current is dependent on the component size, so that no exact value is given here.

With a specific duty cycle γ, different layer thicknesses and distributions can be achieved by varying the ratio of anodic to cathodic current. The lower the anodic current, the higher the layer thickness. This is true up to a certain limit where again dendrite formation occurs because the anodic phase is too small in relation to the cathodic phase. Conversely, the lower the cathodic current, the smaller the layer thickness. Again, there is a limit below which the component is no longer coated, and the surface is unusable. Thus it is at a fixed duty cycle (ie, at fixed times for anodic and cathodic pulses) possible within the limits specified to adapt the anodic and cathodic currents to each other so that, depending on the geometry of the components, a desired layer thickness can be achieved.

Consequently the pulse parameters have a considerable influence on the formation the dendrites and the thickness and distribution of the deposited zinc layer. Due to this considerable influence, there is a very narrow parameter window, in the framework of which the formation of dendrites and the desired zinc layer thickness are avoided can be achieved. It has been shown that a dendritic Crystal growth can be largely avoided if the duty cycle constant that is, no variation in the ratio of the pulse times of cathodic and anodic pulses during of the deposition process, and the pulse times of the cathodic and anodic current pulses 60 ms and more, in particular 100 ms and more.

With a defined parameter window, the reliable prevention of the occurrence of dendrites in the zinc HGA is thus realized on components and achieved the setting of the desired zinc layer thickness. In this case, current densities in the range of 0 to about 200 A / dm ² can be used, wherein the range of about 10 to about 70 A / dm ² appears particularly interesting due to the production cycle. Particularly good results are achieved at values between 50 and 70 A / dm ² .

Claims (9)

Process for the high-speed galvanic deposition of metals on substrates using pulse current technology to prevent dendritic crystal growth, wherein alternating cathodic and anodic current pulses ( 12 . 14 ), characterized in that the pulse times of the cathodic and anodic current pulses ( 12 . 14 ) ≥ 60 milliseconds. Method according to claim 1, characterized in that that the ratio the pulse times of cathodic and anodic pulses during the entire deposition process is kept constant. Method according to claim 1 or 2, characterized in that the pulse times of the cathodic and anodic current pulses ( 12 . 14 ) ≥ 100 ms. Method according to one of claims 1 to 3, characterized that zinc is used as the metal. Method according to one of claims 1 to 4, characterized in that the current density is in the range of 2 A / dm ² and 200 A / dm ² . A method according to claim 5, characterized in that the current density is 10-70 A / dm ² , in particular 50-70 A / dm ² . Method according to one of the preceding claims, characterized in that the ratio of the pulse times of the cathodic and anodic pulses ( 12 . 14 ) ranges from 2: 1 to 10: 1. Method according to claim 7, characterized in that that the ratio the pulse times is 5: 1. Use of the method according to one of the preceding claims for anti-corrosion coating of components in the automotive industry.