DE10300388B4 - A method of protecting a weathered metal-containing structure applied to a surface of a substrate against corrosive attack and methods of operating a moisture sensor disposed on a surface of a substrate facing the weather - Google Patents

Abstract

method to protect one on a surface a substrate applied, exposed to the weather metal-containing Structure, in particular an electrical conductor structure, against corrosive, especially against electrocorrosive attacks, thereby characterized in that the structure at least temporarily with a electrical passivation voltage applied in the area the passivation of the respective conductive material is, wherein the voltage either between two on the substrate surface itself or otherwise closely adjacent, not galvanic with each other connected electrical conductors is applied.

Description

The The invention relates to a method for protecting metal-containing substrates applied to substrates Structures, in particular of electrical conductors, against corrosion.

It is well known that on vehicle windows, which is mostly made of glass, but increasingly also of plastics (eg polycarbonate) are manufactured, for different purposes structures of electrical Conductor tracks or fields are applied. They are called antennas, Heaters, sensors and the like used. Also in the construction sector, especially in roof glazing, rain sensors state of the art. On prestressed glass panes such structures z. Belly used as break sensors (quiescent current loops) for indoor applications.

The said structures are usually on a large industrial scale Glass substrates by screen printing a high silver content stoving paste created and burned. The baking is usually accompanied with heating the glass sheet for bending and then tempering, if it is a monolithic glass pane.

are such conductor structures on the outside of the windowpane arranged, as is the case in particular with humidity or rain sensors the case is, it may take longer währendem exposed use under weather conditions to corrosive phenomena come. It has already been proposed various protective measures.

So describes DE-OS 2 231 095 applying a dielectric material (varnish) over conductor patterns used on a glass sheet surface as a heating conductor. DE-C1-100 15 430 describes a capacitive sensor for detecting condensates on a glass pane surface, on whose electrodes a dielectric passivation layer is applied. The application of an additional layer targeted over the already baked structure, however, is a very cumbersome, time-consuming and labor-intensive intermediate step in the production process, especially since it must be performed with high precision. Is such a sensor z. B. in the wiping field of vehicle windows wiping, so the protective layer wears off over time and must be renewed if necessary.

It is known in itself that metals against electrocorrosion by applying can be effectively protected with an electrical voltage. Under the Internet link http://docserver.bis.uni-oldenburg.de/publikationen/dissertation/2000/ducper00/pdf/kap02.pdf (As of November 2003) a documentation on this topic is available. These Source is an excerpt (Chapter 2) from DE dissertation "Periodic and Chaotic Oszillationserscheinungen on metal electrodes and Electrochemical Model Experiments on Nerve Excitation "/ Matthias Ducci - 2000. - IX, 268 P. + Video sequences on CD-ROM. - Univ. Oldenburg, 2000.

As a result, it is found there for the corrosion protection of iron, that the metal is set by applying a sufficiently strong electrical external voltage, a mixed potential, which is above a to be determined for the material Passivierungspotentials. Once passivation has occurred, this state can be maintained with very low current density. The passive current density is comparable to the corrosion current density and is 10 μA / cm ² for iron, while the passivation current density is about 0.2 A / cm ² .

WO-A1-01 / 07 683 describes a corresponding application for the protection of concrete reinforcements made of steel against corrosion. One feeds through an anode system controlled low DC voltage in the steel reinforcement to Differences of the surface potential pick up and a steady potential to provide corrosion, which prevents corrosion.

In Other known applications become an AC voltage for passivation of metals proposed against corrosion. However, it was added toughen observed that the corrosion with a passivation AC voltage progressed faster than when using DC voltage. this will with a degradation of the passive surface layer by the AC voltage explained.

It However, it was also observed that with increasing frequency of AC voltage the corrosion tendency of the structure thus acted upon decreases or the protective effect is improved. This is explained by the fact that the polarity change the current direction is faster as the diffusion of the corrosive charge carriers through the passive layer.

The level of the passivation voltage must be determined individually for the material to be protected against corrosion. It is usually possible to determine a pronounced passivation range as a function of the level of the external or passivation voltage, in which the corrosion current (proportional to the rate of metal dissolution) is minimized, possibly going to zero, which means that corrosion no longer takes place. at too small external stresses no sufficient corrosion-inhibiting effect is achieved ("active" range), while too high voltages (above the "breakdown potential") enters a so-called "transpassive" state in which the protective effect fails and the corrosion current increases again significantly.

application These electrical passivation are essentially for steel structures known in construction.

US 6,114,863 describes a surface-mounted humidity sensor whose two electrodes are mounted on an insulator material of a water-cooled generator. Between them, an electric field is generated, which is changed by moisture penetrating into the substrate, these changes being detected and evaluated. However, it is less a surface moisture measurement than a "penetrating" detection of (increasing) the presence of water in the insulating material.

To the Feeding the electric field between the two electrodes of the known humidity sensor is a voltage range between 0.1 and about 2 volts, preferably 0.5 to 1 volt, with a (from the monitored Material dependent) Frequency between 10 kHz to about 100 MHz proposed. It will however, expressly found that a particular voltage for the electric field is not very special important. A secondary effect "corrosion protection" of this voltage range does not disclose the document.

Of the Invention is based on the object, a method for protecting the weather-exposed metal-containing structures on substrates, in particular on glass panes, to indicate against weather-related corrosion, that an additional passivating Can spare coating the electrically conductive structures.

These Task is according to the invention with the Characteristics of claim 1 solved. The features of the dependent claims advantageous developments of this method.

Of the Invention is the consideration on the basis that also discussed at the outset conductive surface structures With metals, especially silver, passivatable systems could be through Apply a suitable electrical voltage to protect against corrosion could.

In In fact, a number of experiments have found that industrially for structures printed on glass or plastic disks, such as moisture sensors, Antenna and heating conductor used material, namely a screen printing paste of a glass frit with a high silver content, by applying both DC voltage and AC voltage can be effectively protected against rapid corrosion. However, it is not absolutely necessary, the passivation voltage constantly to the To let electrodes lie.

critical is the arrangement of the conductor structure. For electrical passivation and thus the active corrosion protection is a potential difference between two on the substrate surface itself or in another Way closely adjacent, not galvanically interconnected electrical ladders in height the passivation voltage required. For capacitive working Sensors this is particularly easy to implement. But others too Use cases, z. B. antenna structures, which are also capacitively coupled can, are at a suitable spatial Arrangement to a counter pole with the method described here passivated. Thus, e.g. a system with a signal conductor, the parallel to a ground rail (ground or + 12V), by selecting a suitable signal amplitude and possibly frequency be passivated.

So far it is usual and (in accordance with certain manufacturer test standards) for the performance of the Salt spray tests according to DIN 50021 on vehicle glass panes to be tested Covered printed circuit structures, so they are not the artificial Aggressive weathering, because you have to assume that these structures in the aggravated, corrosive effects for the entire component life simulating test conditions are safely destroyed.

To execution said test at a number of during the execution of the test were subjected to a Passivierungsspannung applied test patterns in visual assessment, even after 240 hours dwell time only to detect relatively low corrosion phenomena. This corrosion led but not to a complete malfunction the structure concerned.

The possible electrical passivation by applying a relatively low electrical (alternating) voltage opens up the possibility of being able to inexpensively use silver-containing conductor structures produced by screen printing on substrates, in particular on glass, also in such outdoor applications where previously either the known measures against corrosion were necessary or these structures have been dispensed with in favor of other solutions (eg optical or capacitive sensors behind a pane of glass). The protective effect by applying an electrical voltage consumes only very little energy, so that in this respect only negligible additional operating costs. With measured current densities of <10 μA / cm ² , quiescent currents occur in the passivation mode which are orders of magnitude below the permissible values of 1.5 mA in the automotive sector.

in the Automotive sector can now silver-containing conductor structures on the outside the windowpanes for Sensors or other uses in the wet area without cover will be realized. In the construction sector, the application of printed rain or fracture sensors on the outer disks z. B. of skylights possible. The price for the loading of the structures with the protective voltage is comparatively low.

you may possibly refrain from burning in printed structures, that usually their mechanical and chemical resistance increase should. Then the use of substrates other than glass, z. As plastic discs, simplified.

Of the Operation of sensor structures can be done with the electrical passivation be combined very advantageous, if one the already required sensor operating voltage or measuring voltage in the range of the passivating voltage. So far you have the one discussed here Disregarded connection and the sensors with the usual available electronics operated at a voltage of about 3 V ~. This voltage value but has no protective, passivating effect. Also, the usual frequencies for these AC measurement voltages below the optimal frequencies. The tentatively determined Passivation range is at voltage values of significantly less than 3V. Optimum (minimum corrosion current) was 1.1V and a frequency of 3000 Hz found at sinusoidal voltage waveform and statistically secured.

While that optimum stress level could be clearly defined in terms of the frequency can not be ruled out, that also at frequencies of more than 3 kHz, a similar protective effect or low corrosion currents to adjust.

to Preparation of practice-relevant tests, in particular the salt spray test according to DIN 50021, with corrosion-prone printed conductive surface structures was first determined in a series of experiments the passive range of the material. For this purpose, a series of sample electrodes was produced, in which the material for the surface structures flat was applied by screen printing on a substrate. The screen printing ink consists of a glass frit as support material, silver as electrical conductive metal with a share of 80%, and possibly dyes.

For the potentiodynamic experiments the following known construction was used:
A measuring cell comprises a container containing 5% saline. In the solution, a working electrode of the material to be examined, a counter electrode made of platinum and a reference electrode (silver / silver chloride electrode) immersed, wherein the potential at the reference electrode via a Haber-Luggin capillary is tapped. For the DC voltage and AC voltage tests respectively suitable devices (potentiostat for DC voltage, function generator for AC voltage) were used. Finally, a measuring computer with suitable software for signal evaluation was used.

The samples immersed in this measuring cell were initially taken with DC voltages in the range of 0 to 4 V (between the sample electrodes and the counter electrode) acted upon.

At first was a reasonable amount of time for to determine the passage of said voltage range. It showed that when passing through said too fast Voltage bandwidth (2 hours), although a slight decline in the Corrosion current at about 2 V = used, but no pronounced Passivierungsbereich was trained. In contrast, was at a maturity of 48 hours the corrosion even before reaching a passivation so far advanced or destroyed the material so far that also no Passivierungsbereich could be determined.

With a period of 12 hours to go through the voltage bandwidth from 0 to 4 V = finally became a pronounced Passivation range of the investigated material between about 0.75 and found 1.8 V DC.

you then examined the corrosion behavior when operating with an AC voltage in the found reduced passivation range between 0.75 and 1.8 V.

acted in the same way samples produced in the context of an industrial Manufacturing are considered to be identical to each other, with a mixed voltage (DC voltage with superimposed AC voltage), so the corrosion currents rise in principle significantly. Indeed an opposite trend was found at a frequency of 3000 Hz.

These was confirmed by further experiments with pure alternating voltage. Here showed that pure AC voltage basically better protection or caused a significant reduction of the corrosion current as equal or mixed voltage.

The Attempts have therefore been made with real design patterns, namely humidity sensors with comb electrodes printed on glass panes, while these are during of the salt spray test were applied with an AC voltage of 1.1 V at 3 kHz.

Of the The degree of corrosion of the sample structures increased steadily during the test period. The progress of the corrosion was even after 240 hours of residence not yet finished. Nevertheless it could be proven that the for the sensor function does not have important capacity of comb electrodes returned to useless values. That means the life of the conductive structures aside from a barely visible external corrosion the electrodes under normal weathering and the real conditions of use fully meet the expectations.

Claims (8)

Method for protecting a metal-containing structure, in particular an electrical conductor structure, applied to a surface of a substrate exposed to weathering, against corrosive, in particular against electrocorrosive attacks, characterized in that the structure is subjected at least temporarily to an electrical passivation voltage which is in the range of passivation of the relevant conductive material, wherein the voltage is applied either between two on the substrate surface itself or otherwise closely adjacent, not galvanically interconnected electrical conductors. Method according to claim 1, characterized in that that the electrical passivation voltage at the same time as a measuring voltage for one Sensor, especially for a capacitive humidity sensor with two on the surface of the Substrate arranged electrodes used. Method according to one of the preceding claims, characterized characterized in that the passivation voltage is a sinusoidally oscillating AC voltage used. Method according to claim 3, characterized that the amplitude of the passivation voltage between 0.75 V and 1.75 V, in particular at 1.1 V. Method according to claim 3 or 4, characterized that the frequency of the passivation voltage is above 2000 Hz, preferably between 2000 and 4000 Hz. Method of operating one in one of the weather facing surface a substrate arranged moisture sensor, the at least two electrodes from an electrically conductive, in particular comprises at least one metal-containing material, which are supplied with a supply or measuring voltage, characterized characterized in that the sensor with a supply or measuring voltage in the field of electrochemical passivation of the conductive material of the humidity sensor acted upon. Method according to Claim 6, characterized that one has a sensor from a silver-containing and by screen printing on a substrate, in particular on a window, applied Material with a range between 0.5 and 1.5 volts AC lying supply or measuring voltage with a frequency of more operates as 2 kHz. Method according to claim 6 or 7, characterized that the supply or measuring voltage is 1.1 V ± 0.2 V and their frequency 3000 Hz ± 100 Hz or an integer multiple of this frequency is.