EP1581670A1

EP1581670A1 - Method for protecting metal-containing structures deposited on substrates against corrosion

Info

Publication number: EP1581670A1
Application number: EP04700465A
Authority: EP
Inventors: Helmut Maeuser
Original assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Current assignee: Saint Gobain Glass France SAS; Compagnie de Saint Gobain SA
Priority date: 2003-01-09
Filing date: 2004-01-07
Publication date: 2005-10-05
Also published as: WO2004070084A1; JP2006518458A; MXPA05007048A; CN1723298A; KR20050097939A; DE10300388B4; DE10300388A1

Abstract

The invention concerns a method for protecting metal-containing structures, in particular electrically conductive structures, deposited on a substrate, against corrosive attacks, in particular electrocorrosion attacks. Said method is characterized in that it consists in applying at least temporarily to the structure a passivation electric voltage in the passivation range of the conductive material concerned.

Description

METHOD FOR PROTECTING STRUCTURES INCLUDING METAL DEPOSITED ON SUBSTRATES AGAINST CORROSION

The present invention relates to a method for protecting structures comprising metal, in particular electrically conductive tracks, deposited on substrates, against corrosion.

It is generally known that structures of electrically conductive tracks or fields intended for various uses are deposited on vehicle windows, which are most often made of glass, but also increasingly of synthetic materials.

(e.g. polycarbonate). They are used as antennas, heating fields, sensors and others. Also in the building industry, in particular for roof glazing, rain sensors are part of the state of the art. Such structures are for example used on tempered glazing as breakage sensors (quiescent current loops) for indoor applications.

Said structures are generally produced in large series by screen printing of a cooking paste with a high silver content on glass substrates. The cooking takes place most of the time at the same time as the heating of the glass for its bending, followed by tempering, if it is a monolithic glass.

In the case where such conductive structures are arranged on the outside of the glass, as is particularly the case for humidity or rain sensors, corrosion phenomena may appear after prolonged use with exposure to the weather. Different protective measures have been proposed for this.

Publication DE-OS 2 231 095 describes the deposition of a dielectric material (lacquer) on structures. conductors used as heating conductors on the surface of a window. The publication DE-C1-100 15 430 describes a sensor operating in a capacitive manner to detect condensates on the surface of a glass pane, on the electrodes of which a dielectric passivation layer is deposited. The targeted deposition of an additional layer on the already baked structure, however, represents a very troublesome, long and tedious intermediate step in the production process, because it must be carried out with great precision. If such a sensor is, for example, within the range of the wipers, the protective layer wears out over time and must, if necessary, be renewed.

It is known per se that metals can be effectively protected against electrical corrosion by applying an electrical voltage thereto. Documentation on this subject is available at the following Internet address: http: //docserver.bis .uni-oldenburg.de / publikationen / dissertation / 2000 / ducper00 / pdf / kap02.pdf. It is an extract (chapter 2) from the German thesis "Periodische und chaotische Oszillationserscheinungen an Metallelektroden und elektrochemische Modellexperimente zur Erregungsleitung am Nerven" / Matthias Ducci. 2000. - IX, 268 S. + video sequences on CD-ROM. - Univ. Oldenburg, 2000. The conclusion of this document notes, for the protection of iron against corrosion, that the application of a sufficiently strong external electric tension regulates in the metal a compound tension higher than a potential of passivation to be determined for the material . Once passivation has been achieved, this state can be maintained using a very low current density. The passivation current density would be comparable to the corrosion current density and is, for iron, around 10 μA / cm ² , while the passivation current density is approximately 0.2 A / cm ² .

The publication O-Al-Ol / 07 683 describes a use in this sense for the protection of steel concrete reinforcements against corrosion. The steel reinforcement is supplied with a continuous low voltage controlled using an anode system to cancel the differences in surface potential and create a uniform potential, which prevents corrosion.

In other known uses, an alternating voltage is proposed for passivating metals against corrosion. However, it has been observed, for steels, that corrosion progresses more rapidly with an alternative passivation voltage than in the case of the use of a direct voltage. This is explained by a degradation of the passive surface layer by the alternating voltage.

However, it has also been found that increasing the frequency of the alternating voltage reduces the corrosion tendency of the structure to which it is applied, respectively improves the protective effect. This is explained by the fact that the change of the polarity of the direction of the current takes place more quickly than the diffusion of the corroding charge carriers through the passive layer.

The value of the passivation voltage must be determined individually for the material to be protected against corrosion. It is generally possible to determine a marked passivation range as a function of the value of the external voltage or passivation, in which the corrosion current (proportional to the rate of disintegration of the metal) is reduced to the minimum, or even tends towards zero, which means that no further corrosion takes place. In the case external voltages too low, a sufficient corrosion inhibiting effect is not obtained ("active" range), whereas in the case of too high voltages (greater than "ignition potential"), a state called " transpassive ”appears, in which the protective effect no longer acts and the corrosion current again increases significantly.

The uses of this electrical passivation are essentially known in the field of metal constructions of the building.

The object of this invention is to indicate a process for protecting against corrosion due to weathering of structures comprising metal deposited on substrates, in particular on glass panes, exposed to the weather, and which makes it possible to dispense with a passivating coating. additional on electrically conductive structures.

This object is achieved according to the invention by the features of claim 1. The features of the dependent claims indicate advantageous improvements of this process and its applications.

The invention is based on the reflection that the conductive surface structures comprising metals, in particular silver, mentioned in the preamble, could also constitute passivable systems which could be protected against corrosion by the application of an appropriate electrical voltage. .

It has actually been observed, during a series of tests, that the material used for structures printed industrially on glass panes or in synthetic material, such as sensors humidity, break detectors, antennas and heating conductors, i.e. a screen-printing paste composed of a glass frit with high silver content, can be effectively protected against rapid corrosion by application of both direct and alternating voltage. However, it is not absolutely necessary to apply the passivation voltage to the electrodes permanently.

The arrangement of the conductive structure is decisive. For electrical passivation, and therefore active protection against corrosion, a potential difference up to the passivation voltage is necessary between two electrical conductors arranged very close to each other, on the surface of the substrate itself or in another way, not galvanically connected to each other. In the case of sensors operating in a capacitive manner, this can be done in a particularly simple manner. But also other application cases, for example antenna structures, which can also be capacitively coupled, can be passive using the method described here, on condition of a spatial arrangement. appropriate in relation to an opposite pole. It is thus for example possible to passivate a system composed of a current signal conductor parallel to a ground rail (ground or + 12 V) by selecting a signal amplitude and, if necessary, a frequency signal adapters.

Until now, it is customary and (according to certain manufacturers' test standards) permissible, for carrying out the salt spray test according to DIN 50021, to cover the existing printed conductive structures on the windows of subject vehicles. to the tests so as not to expose them to aggressive artificial weather, because it is necessary admit that the conditions of the tests, of increased severity, simulating the corrosive influences undergone by the component during all its lifetime, would destroy with certainty these structures.

After carrying out the said test on a number of test samples to which a passivation voltage was applied during the carrying out of the test, the visual evaluation, even after a residence time of 240 hours, only revealed appearances relatively reduced corrosion. This corrosion did not, however, lead to a complete shutdown of the structure concerned.

The possibility of an electrical passivation by applying a relatively low (alternative) electrical voltage gives the possibility of economically implementing conductive structures with silver content produced by screen printing on substrates, in particular on glass, also for outdoor applications where until now it was necessary either to implement known corrosion protection measures, or to give up these structures to call on other solutions (for example optical or capacitive sensors behind a glass pane) . The protective effect by applying an electrical voltage consumes very little energy, which only causes negligible additional operating costs. With measured current densities <10 μA / cm ² , quiescent currents appear in the passivation mode, which are several orders of magnitude lower than the admissible 1.5 mA values in the automotive sector.

It is now possible, in the automotive sector, to produce, without coating, conductive structures with silver content on the outer face of the window panes for sensors or for other uses in wetlands. In the building, it is possible to install rain or breakage sensors printed on the exterior windows, for example skylights. The costs of supplying structures with the protection voltage are comparatively reduced.

If necessary, it is possible to dispense with the baking of printed structures, which is supposed, as a general rule, to increase their mechanical and chemical resistance. This also simplifies the implementation of other substrates than glass, for example plastic panes.

The operation of sensor structures can be very advantageously combined with electrical passivation, if the operating voltage or the measurement voltage of the sensor, which is in any case necessary, is shifted within the range of the passivation voltage. The relationship mentioned here has so far not been taken into account and the sensors have been supplied, with the electronics usually available, by a voltage of approximately 3 V ~. However, this tension has no protective passivation effect. Likewise, the usual frequencies for these alternative measurement voltages are below the optimal frequencies. The passivation range determined by tests is at voltage values significantly below 3 V. An optimal result (minimum corrosion current) has been determined and verified statistically for 1.1 V and a frequency of 3000 Hz, with a sinusoidal voltage curve.

While the optimal voltage level has been unequivocally determined, it cannot be excluded with regard to frequency that a similar protective effect, respectively reduced corrosion, also appear for frequencies above 3 kHz.

The passive range of the material was first determined by a series of tests, for the preparation of practice-oriented tests, in particular the salt spray test according to DIN 50021 with conductive surface structures corrosion-sensitive prints. To this end, a series of test electrodes was produced, on which the material of the surface structures was applied in a thin layer by screen printing on a substrate. The screen printing ink consists of a glass frit serving as a base, silver acting as an electrically conductive metal in a proportion of 80%, and, where appropriate, dyes.

The following structure, known as such, was used for the potentiodynamic tests:

A measuring cell includes a container containing a 5% solution of cooking salt. A working electrode made of the material to be tested, a platinum counter electrode and a reference electrode (silver electrode / silver chloride) are immersed in the solution, the potential being noted on the reference electrode at l using a Haber-Luggin capillary tube. Appropriate devices have been used for DC and AC voltage tests (potentiostat for DC voltage and function generator for AC voltage). Finally, a measurement computer, with the appropriate software, was used to process the signal.

The test specimens immersed in this measuring cell were initially subjected to tensions continuous in the range of 0 to 4 V (between the test electrodes and the counter electrode).

It was first necessary to determine an appropriate duration for the scanning of the mentioned voltage range. It turned out that, in the case of a too rapid scanning of said voltage range (2 hours), a small decrease in the corrosion current certainly appeared around 2 V =, but that no clear passivation range never formed. On the other hand, for a period of 48 hours, the corrosion was already so advanced before obtaining a passivation, respectively the material was so destroyed that it was also not possible to determine a passivation range.

With a duration of 12 hours for scanning the voltage range from 0 to 4 V =, a marked passivation range for the material analyzed was finally found between DC voltages of approximately 0.75 and 1.8 V.

Then, the corrosion behavior in operation with an AC voltage was analyzed in the reduced passivation range found between 0.75 and 1.8 V.

If identical test pieces, which can be considered identical in the context of industrial manufacturing, are subjected to a compound voltage (alternating voltage superimposed on a direct voltage), the corrosion currents in principle increase markedly. However, it was discovered for a frequency of 3000 Hz an opposite development.

This was confirmed by additional tests with pure AC voltage. It turns out here that pure alternating voltage in principle ensures better protection, that is to say a marked reduction in the corrosion current, than a direct or compound voltage.

It is for this reason that the tests were continued with real production samples, that is to say humidity sensors provided with comb-shaped electrodes printed on glass panes, an alternating voltage of 1.1 V at 3 kHz being applied to them during the salt spray test.

The degree of corrosion of the test structures increased steadily over the duration of the test. Even after a residence time of 240 hours, the progress of corrosion was not yet complete. However, it has been possible to prove that the capacity of the comb-shaped electrodes, which is important for the function of the sensors, did not decrease until reaching unusable values. This means that the life of the conductive structures, apart from ^a barely visible external corrosion of the electrodes, fully meet the requirements in the case of normal weather and field conditions.

Claims

1. Method for protecting structures comprising metal, in particular electrically conductive structures, deposited on a substrate, against corrosive attacks, in particular electrocorrosive, characterized in that an electrical voltage of at least temporarily is applied to the structure passivation which is in the passivation range of the conductive material concerned.

2. Method according to claim 1, characterized in that the electric passivation voltage is used at the same time as the measurement voltage for a sensor, in particular for a humidity sensor operating in a capacitive manner.

3. Method according to one of the preceding claims, characterized in that an alternating voltage oscillating in a sinusoidal manner is used as the passivation voltage.

4. Method according to claim 3, characterized in that the amplitude of the passivation voltage is between 0.75 and 1.75 V, in particular at 1.1 V.

5. Method according to claim 3 or 4, characterized in that the frequency of the passivation voltage is above 2000 Hz, preferably between 2000 and 4000 Hz.

6. Application of the method according to any one of the preceding claims to structures comprising metal such as humidity sensors, case detectors, antennas and heating conductors.

7. Application according to claim 6, characterized in that said structures are deposited on glass panes or in synthetic material.