EP0472057B1 - Arrangement and process to obtain a cathodic protection - Google Patents

Description

The invention relates to an arrangement and a method for achieving cathodic corrosion protection for a surface of a substrate containing metallic aluminum that can be washed around by an electrolyte (heat transfer medium, cooling medium).

From GB-A-2 132 226 it is known to achieve the cathodic protection of aluminum by periodically interrupting the cathode current. The cathodic potential of the substrate is observed with respect to a reference electrode and - as soon as the cathodic potential comes into an area where the substrate begins to corrode - it is connected directly to an electrical source and switched off again from the source as soon as the cathodic potential becomes significant has lowered. With this switching mode, the possibility of a sudden increase in the control voltage is not taken into account, so that under certain circumstances corrosion phenomena can occur, for example if the control voltage potential is suddenly changed during certain changes over time.

Electrochemical corrosion protection methods are extremely difficult to handle if the substrate to be protected is made of aluminum or an aluminum-based alloy. In principle, aluminum is a relatively base metal, and corrosion protection with "sacrificial anodes" is more difficult. The described "cathodic corrosion protection" using an external voltage source cannot be used without further ado. As is known, aluminum in an oxygen-containing environment is soon covered with a relatively dense oxide layer, which is physically very stable, but due to the amphoteric nature of the aluminum oxide it is attacked not only by acids but also by alkalis. The aluminum oxide layer is only stable in an electrolyte whose pH is between about 4.5 and about 8.5; too strongly acidic or too strongly basic electrolytes attack metal. There is indeed a risk of corrosion from alkaline media when using the usual cathodic corrosion protection for aluminum or aluminum-based alloys; due to the electrolysis required for cathodic corrosion protection, the cathode is loaded with the the electrolysis reduced cations from the electrolyte;
This creates an alkaline liquid boundary layer in the vicinity of the surface of the object to be protected, which under certain circumstances can lead to alkaline corrosion of the aluminum.

If an object made of an aluminum-containing material is to be cathodically protected against corrosion, the chemical conditions prevailing on the object must be monitored very carefully in order to exclude the occurrence of alkaline corrosion with certainty. A modification of the method of cathodic corrosion protection, with a certain degree of control at least of the electrical conditions, is proposed in DE-U 8900911. This document relates to the cathodic corrosion protection of a steel pipe,
which envelops an electrical high-voltage line for underground laying, and whose counter electrode is the earth of a connection station of the pipe. A negative voltage is applied to the tube against the mass; However, in order to maintain the relevant protective regulations, the height of the pipe against the mass must be reliably limited. It is proposed to do this using a number of anti-parallel diodes. However, this is not suitable for monitoring the electrochemical conditions at the cathode; in the electrolysis taking place between the anode and cathode, not only is the cathode changed; the cathode is loaded with reduced cations from the electrolyte and the anode is loaded with oxidized anions.

As a result of the electrolysis both electrodes change their electro-chemical potential against the electrolyte; The voltage between the electrodes is therefore not a measure of the conditions at the electrode, but rather is a measure of the total change at both electrodes and is therefore not to be used to reliably assess the conditions at a single electrode, be it cathode or anode.

Accordingly, the invention is based on the object of the method of cathodic corrosion protection for a surface facing an electrolyte of a metallic, especially an aluminum-containing Specify substrates, the electro-chemical conditions on the surface to be protected can be precisely monitored and controlled to avoid excessively alkaline boundary layers.

• a) mindestens eine von dem Elektrolyten umspülbare Gegenelektrode, die durch den Elektrolyten nicht oder nur in unwesentlichem Maße korrodierbar ist;
• b) eine zugehörige, von dem Elektrolyten umspülbare Bezugselektrode, die gegenüber dem Elektrolyten ein konstantes elektro-chemisches Potential aufweist;
• c) einen zugehörigen Potentiostaten, der mit dem Substrat und der Gegenelektrode elektrisch verbunden und durch den in dem Elektrolyten eine Elektrolyse bewirkbar ist, wobei zwischen dem Substrat als Kathode und der Gegenelektrode als Anode eine elektrische Elektrolysierspannung eingeschaltet wird;
• d) eine zugehörige Regeleinrichtung, die mit dem Substrat, der Bezugselektrode und dem Potentiostaten elektrisch verbunden ist, durch die eine elektrische Kontrollspannung zwischen der Bezugselektrode und dem Substrat meßbar sowie die Elektrolysierspannung ein- und ausschaltbar sowie regelbar ist.

According to the invention, an arrangement for achieving cathodic corrosion protection for a surface that can be flushed with an electrolyte and a metallic substrate containing aluminum is specified, which has the following components:

• a) at least one counterelectrode which can be rinsed by the electrolyte and which is not or only insignificantly corrodible by the electrolyte;
• b) an associated reference electrode which can be flushed by the electrolyte and which has a constant electrochemical potential compared to the electrolyte;
• c) an associated potentiostat, which is electrically connected to the substrate and the counterelectrode and through which electrolysis can be effected in the electrolyte, an electrical electrolysis voltage being switched on between the substrate as the cathode and the counterelectrode as the anode;
• d) an associated control device which is electrically connected to the substrate, the reference electrode and the potentiostat, by means of which an electrical control voltage between the reference electrode and the substrate can be measured and the electrolysis voltage can be switched on and off and regulated.

An essential element of the invention is to provide, in addition to the protective electrode given by the substrate to be protected and the counter electrode, a further electrode, namely a reference electrode with constant electrochemical potential with respect to the electrolyte. Electrolysis is operated between the protective electrode and the counter electrode via the potentiostat, the voltage source. Between the protective electrode and the A control voltage is measured for the reference electrode; this control voltage corresponds to the difference between the electrochemical potentials of the reference electrode and the protective electrode, and it faithfully reproduces the electrochemical conditions at the protective electrode, since the electrochemical potential of the reference electrode is not changed by the electrolysis and the electrochemical potential of the Counter electrode contributes nothing to the control voltage. Therefore, the provision of the reference electrode first enables the measurement of the electrochemical potential of the substrate to be protected.

According to a further element of the invention, the control voltage characterizing the electrochemical conditions on the substrate to be protected is used to control the electrolysis; both the electrolysis can be switched on and off, and the electrolysis itself can also be influenced, for example by regulating the electrolysis voltage.

In the context of the invention, those counter electrodes are particularly suitable which have a surface layer which can be rinsed by the electrolyte, which cannot be attacked by the electrolyte and which is not dissolved by the electrolysis; Precious metals, especially platinum, platinum-coated or titanium, gold, silver, nickel, or other metals or electron conductors which are difficult or impossible to corrode, and also carbon, are particularly suitable. Commercial reference electrodes can be used as reference electrodes. As a rule, these are not coated. However, reference electrodes made of titanium, which are coated with oxides of the platinum metals, can also be used.

One possible way of regulating the electrolysis in the context of the invention is to regulate the electrolysis voltage in such a way that the control voltage measured during the electrolysis is equal to a first limit voltage that can be predetermined by the control device. In the course of the electrolysis, the electrochemical potential of the substrate to be protected is established according to the electrolysis voltage; Accordingly, it is advisable to choose the electrolysis voltage so that the electrochemical Potential of the substrate to be protected does not become too negative, so that the formation of excessively alkaline boundary layers is reliably excluded. An inexpensive and easily realizable way of realizing this is to build the potentiostat with an operational amplifier with an inverting input, a non-inverting input, an output and a ground connection, the substrate with the ground connection, the reference electrode with the inverting input and the counter electrode is connected to the output, and further the first limit voltage is connected between the ground connection and the non-inverting input.

Switched in this way, the operational amplifier generally regulates the electrolysis voltage so that the control voltage becomes practically equal to the first limit voltage. Of course, the operational amplifier does not have to be the only active element of the potentiostat, it can of course still be used with the gem. expert knowledge required additional elements, such as voltage followers or power amplifiers.

A further control method which can advantageously be used within the scope of the invention is characterized, without prejudice to other developments of the invention, by a control device which can be given a limit voltage, the electrolysis voltage which is initially switched off being switched on when the control voltage becomes equal to the second limit voltage. This corresponds to switching on the electrolysis when the electrochemical potential of the substrate to be protected becomes equal to a limit potential; this limit potential is favorably the highest potential that the substrate to be protected may assume without anodic corrosion occurring. The selection of this value is - like the rest of the selection of the value of the first limit voltage - to be chosen according to the material of the substrate. Such a type of arrangement then works with a pulsating electrolysis; If the electrochemical potential of the substrate to be protected is shifted too strongly towards positive values, the electrolysis is switched on, whereupon the electrochemical potential becomes more negative again. After a The electrolysis is then switched off for a certain period of time, and the control device again monitors the control voltage and switches the electrolysis on again, if necessary.

To complete the control of the electrolysis, the control device is designed in such a way that an electrolysis time can be predetermined for it and the electrolysis is carried out for a time interval equal to the electrolysis time. The changes in the surface to be protected which are caused by the electrolysis can thus be precisely characterized, and the alkalinity of the boundary layer on the surface remains limited.

• a) Durchführung der Elektrolyse für eine vorgegebene Zeitdauer, dann Abschaltung der Elektrolyse;
• b) Beobachtung der Kontrollspannung, und Einschaltung der Elektrolyse, wenn die Kontrollspannung gleich einer vorgegebenen zweiten Grenzspannung ist.

The invention also relates to a method for operating an arrangement according to the invention, the following steps being carried out recursively:

• a) performing the electrolysis for a predetermined period of time, then switching off the electrolysis;
• b) observation of the control voltage, and activation of the electrolysis when the control voltage is equal to a predetermined second limit voltage.

This method, which ultimately leads to pulsating electrolysis to effect cathodic corrosion protection, has, in addition to the advantages already mentioned, the advantage of a significantly reduced energy requirement compared to continuous electrolysis; electrolysis is only carried out when it is actually necessary, and accordingly the energy expenditure is limited to those cases in which it is really necessary. Of course, the control device and potentiostat also require a certain amount of energy; With commercially available, extremely energy-saving electronic components (CMOS logic), however, this energy expenditure can be kept to a practically negligible level.

As already mentioned, the regulation of the electrolysis voltage makes sense during the electrolysis in such a way that the control voltage is equal to a predetermined first limit voltage, or that the electro-chemical potential present at the cathode during electrolysis remains limited to a corresponding limit value in the negative direction. This limit value, and thus the first limit voltage, must be selected according to the substrate and the electrolyte. The limitation makes it possible to reliably prevent the formation of excessively alkaline boundary layers and thus the risk of alkaline corrosion.

In a particularly advantageous further development of the method according to the invention, a control period and a critical voltage, which lies between the first limit voltage and the second limit voltage, are additionally specified for monitoring the control voltage when the electrolysis is switched off, and the electrolysis is switched on again immediately after the control period has elapsed since it was switched off if the control voltage becomes equal to the critical voltage during the control period. As part of this training, in addition to observing the level of the control voltage, there is an observation of its development over time; the fulfillment of the condition mentioned means that the electrolysis is already switched on again even if the control voltage has not yet reached the second limit voltage, but has a change over time that exceeds a certain predetermined limit. This is the case if an electrolysis cycle could only have a relatively small influence on the surface to be protected; This additional switch-on criterion further improves the reliability of the method. To a certain extent, the modification implements a PD controller for switching the potentiostat.

The invention is explained in more detail below with reference to a preferred embodiment of an arrangement and a method for achieving cathodic corrosion protection for a metallic aluminum alloy with reference to the drawings.

Figur 1

Ein Blockschaltbild der Anordnung zur Erzielung eines kathodischen Korrosionsschutzes für das aus einer metallischen Aluminiumlegierung bestehende Substrat, dessen Oberfläche durch einen Eleketrolyten umspült ist und in einer Elektrolyse die Kathode gegenüber einer als Anode fungierenden Gegenelektrode bildet. Letztere ist dabei aufgrund des kathodischen Korrosionsschutzes aus einem Material gebildet, welches durch den Elektrolyten nicht korrodiert. Durch eine Regelvorrichtung in Form eines Triggers ist der Potentiostat ein- und ausschaltbar, so daß eine Regelung der potentiostatischen Impulse ihrer Höhe nach erfolgen kann und insofern durch kurzzeitige Potentialabsenkung eine geeignete kurzzeitige kathodische Polarisation des Substrates erreichbar ist;

Figur 2

Ein Stromdichte-Potential-Diagramm des Substrates während der Durchführung einer Elektrolye unter Darstellung des "Passivbereiches", in dem eine Korrosion nicht auftritt sowie der Bereich in anodische Richtung und in kathodische Richtung, wobei dort eine Lochkorrosion oder eine flächenmäßige Abtragung des Aluminiums unter Aluminatbildung auftritt.

Figur 3

Ein Potential-Zeit-Verlauf des Redoxpotentials des Substrates im Elektrolyten bei Durchführung eines aktiven Korrosionsschutzes für das Aluminiumsubstrat unter Anwendung einer Folge von potentialgeregelter kathodischer Spannungsimpulse, welche das Redoxpotential des Aluminiumsubstrates in die Nähe des untersten zulässigen negativen Potentialwertes des "Passivbereiches" absenken, wobei während der Dauer dieser Impulse eine kathodische Polarisation des Substrates eintritt und dabei während einer gewissen Ausschaltdauer des Potentiostaten noch gegeben ist.

The drawings show:

Figure 1

A block diagram of the arrangement for achieving cathodic corrosion protection for the substrate consisting of a metallic aluminum alloy, the surface of which is washed by an electrolyte and which forms the cathode in an electrolysis in relation to a counter electrode functioning as an anode. The latter is formed from a material which is not corroded by the electrolyte due to the cathodic protection against corrosion. The potentiostat can be switched on and off by a control device in the form of a trigger, so that the level of the potentiostatic impulses can be regulated and a suitable short-term cathodic polarization of the substrate can be achieved by briefly lowering the potential;

Figure 2

A current density-potential diagram of the substrate during the implementation of an electrolyte, showing the "passive area" in which corrosion does not occur, as well as the area in the anodic direction and in the cathodic direction, where pitting corrosion or surface removal of the aluminum occurs with formation of aluminate .

Figure 3

A potential-time curve of the redox potential of the substrate in the electrolyte when performing an active corrosion protection for the aluminum substrate using a sequence of potential-controlled cathodic voltage pulses, which lower the redox potential of the aluminum substrate in the vicinity of the lowest permissible negative potential value of the "passive area", while the duration of these pulses a cathodic polarization of the substrate occurs and is still present during a certain switch-off time of the potentiostat.

In the block diagram of the arrangement (1) for achieving cathodic corrosion protection according to FIG. 1, a galvanic element (2) is indicated in the middle. The aluminum substrate (3) to be protected against corrosion, which consists of an AlMgSi 1 alloy and the surface of which is bathed by the electrolyte of the galvanic element, is arranged in this element.

The aluminum substrate can be a solid semi-finished or finished part, such as. B. profiles, plates, sheets, foils, containers, tanks, etc., or an aluminum coating, z. B. by roll or drawing plating, fire or spray aluminum and alitation, on other materials such as. B. be steel.

To carry out the cathodic corrosion protection, an electrolysis is carried out between this aluminum substrate and a counter electrode (4) arranged at a distance from it. In this electrolysis, the aluminum substrate is switched as a cathode, the counter electrode (4) being the anode. In this way, the surface to be protected of the aluminum substrate with the aluminum having a strongly negative character is still exposed to the corrosive electrolyte, but because electrons are added to the aluminum substrate as the cathode during electrolysis, the potential of its metal surface is shifted in the cathodic direction and the corrosion rate that occurs is reduced so much that there is practically no aluminum removal. This applies to the present aqueous electrolyte, however, only in a pH range from 4.5 to 8.5, in which the solubility of the aluminum oxide layer is low and if, according to. FIG. 2 shows the polarization potential that occurs on the surface of the aluminum substrate in the “protective potential range” (passive range), that is to say neither pitting corrosion, nor — due to the formation of alkaline liquid boundary layers — surface corrosion.

Via the potentiostat (5) shown in the left part of the drawing, which with the aluminum substrate (3) and the counter electrode (4) is electrically connected, the necessary electrolysis voltage is switched on between these probes, which voltage can also be regulated and also switched off via an operational amplifier (6).

The aluminum substrate is kept at a constant cathodic potential within the protective potential range by the electrolysis. During a certain switch-off period of the electrolysis, a "control voltage" is measured between a reference electrode (7) which is surrounded by the electrolyte and has a constant electrochemical potential, and the aluminum substrate (3), which is a direct measure of the redox potential on the surface of the Aluminum substrates in the electrolyte.

After the electrolysis has been switched off after a certain electrolysis time, this control voltage is now observed and fed to the control device for the electrolysis voltage, the redox or polarization potential of the aluminum substrate thereby being regulated. The control device is constructed from the operational amplifier (6) and a window discriminator (8) and a timer (9) for switching the potentiostat (5) on and off. The potentiostat is switched on and off by the voltage states available at the outputs of the window discriminator. For this purpose the timer (9) is connected. After falling below the necessary negative protection potential, the timer (multivibrator) supplies an output signal which, as such, puts an "astable multivibrator" into operation. This can be used to set the desired polarization period (t _p ) and the switch-off period (t _a ) for the electrolysis.

The control device switches the electrolysis on again after a certain switch-off period if the control voltage is equal to a predetermined limit voltage U ' _s at the anodic limit of the protective potential range. As a result, a constant cathodic potential is applied to the Aluminum substrate applied so that the redox potential is in turn at the lower cathodic limit of the protective potential range.

The lowered "cathodic potential" is generated by potential-controlled cathodic voltage pulses which polarize the surface of the aluminum substrate in areas in which the "passive behavior" of aluminum is given as long as possible according to the protective potential area. This lower negative limit voltage U ' _s (first limit voltage) can be predetermined for the operational amplifier (6), the electrolysis voltage being adjustable during the electrolysis in such a way that the control voltage between the comparison electrode (7) and aluminum substrate (3) is equal to this first limit voltage U' _s will.

As shown in the potential-time profile of FIG. 3, the potential-controlled cathodic voltage pulses are always only specified for a relatively small time interval, the same applies to the electrolysis time and polarization time t _p on the surface of the aluminum substrate. For a certain time interval t _a , the potential drop is interrupted by negative cathodic voltage pulses and also the electrolysis itself.

This has the advantage that a cathodic overpolarization which is inherently possible with an unfavorable geometric arrangement of the substrate is avoided or at least minimized.

Compared to ongoing cathodic corrosion protection, the required protective current requirement is also considerably lower. Since the lower and upper potential limits (U ' _S , U _S ) of the protective potential range are only exceeded after a surface-specific induction time, it makes more sense compared to continuous cathodic corrosion protection to protect the aluminum substrate "only when required", ie when the upper potential limits ( 2. Limit voltage U _S ) can be achieved by polarizing a cathodic voltage drop.

This protection method based on the potential-controlled pulse method is cheaper in terms of energy consumption than permanent polarization. It is suitable e.g. For use with aluminum materials in the maritime sector or also for drinking water tanks and tanks.

As shown in FIG. 3, the regulation of the electrolysis voltage and the cathodic voltage drop also takes place as a function of the steepness of the course of the redox potential of the aluminum substrate after the electrolysis has been switched off.

If, after deactivation of the electrolysis within a certain control period, for example, the off-time t _a, a critical voltage U _i, which is between the first threshold voltage U _'S and the second threshold voltage U _S, positive by corresponding to the redox potential of the aluminum substrate expectant control voltage is exceeded, the electrolysis is switched on again, so that cathodic voltage drop and polarization to the level of the first limit voltage again result for the duration of the voltage pulse.

If, on the other hand, the critical voltage is not exceeded by the increasing control voltage during the switch-off time t _a , the voltage is not reduced until the control voltage has risen to the second limit value U _S. A potential indicator is used to determine the slope. In this respect, this is used to model a PD controller and is not shown in the drawings.

The counterelectrode (4) consists of platinum or another metal that cannot be corroded, or is not corrodible, or another electron conductor. This material is inert to the electrolyte. The reference electrode, which is arranged in the vicinity of the surface of the aluminum substrate in the electrolyte and has a constant electro-chemical potential with respect to the electrolyte, consists of a cylindrical hollow body made of glass, organic plastic or another insulating material and is made with a special shaped tip potential sensor. The reference electrode contains a diaphragm and, thanks to its special construction, enables potential tapping near the wall of the aluminum substrate to be protected. Hg / Hg₂Cl₂, Ag / AgCL or suitable noble metals can be used as the reference system of the 1st half cell in their aqueous solution or in a fixed bed. The reference electrode in the protection system has the function of detecting the redox potential (corrosion potential) that occurs on the wall of the aluminum substrate and this "control voltage" as an electrical voltage signal both for the potentiostat (5) for controlling the electrical currents and for determining the steepness of the To supply the potential indicator.

The reference electrode is almost without current during the electrolysis. A current flows in the electrolyte only between the aluminum substrate (3) and the counter electrode (4), this being regulated by the operational amplifier (6) in such a way that the potential applied to the current-carrying circuit, which is connected as the cathode, is reduced by the potential drop in the potentiostat Aluminum substrates (3) (control voltage U _ist ) follows the specified 1st limit voltage (U ' _s ) and is kept constant at its instantaneous value regardless of electrochemical processes.

An operational amplifier (6) is preferably used as the potential-regulating unit of the potentiostat (5) shown in FIG. 1 with respect to its basic circuit, and because of its high input resistance (FET input stage) and its low quiescent input current, neither the comparison electrode (7) nor the target voltage source (U _should ) of the amplifier. Since the operational amplifier only supplies a maximum output current of +/- 20 mA, a power amplifier is connected downstream of it, which, depending on the electrical requirements, can, for example, generate a maximum output current based on the aluminum substrate of +/- 200 mA and more.

Further requirements for performing an active corrosion protection by means of potential-controlled cathodic impulses for aluminum parts are a possible maximum control of the counter electrode (4) to + 12 V, a manual one Adjustment of the 1st limit potential (U ' _S ) at the setpoint voltage source from 0 ... / 2000 mV, adjustability of the protective potential U _S (2nd limit voltage) to a value between 0 ... / 2000 mV, adjustability of the critical voltage of the potential indicator (U _I ) to values between 0 ... / 2000 mV and an adjustability of the polarization duration t _p between 1 min ... 10 min and the switch-off duration t _a between 1 min ... 10 min.

The critical voltages U ' _S and U _S are chosen depending on the limit potentials for pitting corrosion (U _L ) and surface corrosion (U _A ). However, U _A and U _L are not constants, but must be determined electrochemically for the respective substrate material and corrosion medium. This is usually done by recording and evaluating current density-potential diagrams in accordance with FIG. 2. Here U _A and U _{L result} as limit potentials of the passive region, pitting corrosion occurring above U _L and surface corrosion occurring below U _A.

The critical voltages U _S and U ' _S are preferably set approximately 30 to 50 mV away from the limit potentials U _A and U _L , ie U' _S is approximately 30 to 50 mV above U _A and U _S is approximately 30 to 50 mV below U _L.

(SCE = bezogen auf gesättigte Kalomelelektrode)
Für die Legierung AlMgSi 1 in künstlichem Meerwasser werden z. B. folgende Werte erhalten: $$\begin{array}{c}\begin{array}{c}{\text{U}}_{\text{L}}{\text{= - 750 mV (SCE) U}}_{\text{S}}\text{= - 780 mV (SCE)}\\ {\text{U}}_{\text{A}}{\text{= - 1330 mV (SCE) U'}}_{\text{S}}\text{= - 1300 mV (SCE)}\end{array}\end{array}$$

Die frei wählbare Spannung U_I wird zwischen den Werten von U_S und U'_S eingestellt. Sie dient als Indikator, um zu bestimmen, ob die vorherige Polarisationsdauer bei der Spannung U'_S ausreichend war. Wird die Spannung U_I nach Abschalten der Elektrolyse innerhalb einer vorgegebenen Ausschaltdauer t_a nicht überschritten, wurde eine ausreichende Polarisation erzielt. Andernfalls wird die Polarisation unmittelbar wiederholt.For the alloy AlMgSi 0.5 in artificial sea water there are z. B. the following values: $$\begin{array}{c}\begin{array}{c}{\text{U}}_{\text{L}}{\text{= - 740 mV (SCE) U}}_{\text{S}}\text{= - 790 mV (SCE)}\\ {\text{U}}_{\text{A}}{\text{= - 1470 mV (SCE) U '}}_{\text{S}}\text{= - 1420 mV (SCE)}\end{array}\end{array}$$

(SCE = based on saturated calomel electrode)
For the alloy AlMgSi 1 in artificial sea water z. B. get the following values: $$\begin{array}{c}\begin{array}{c}{\text{U}}_{\text{L}}{\text{= - 750 mV (SCE) U}}_{\text{S}}\text{= - 780 mV (SCE)}\\ {\text{U}}_{\text{A}}{\text{= - 1330 mV (SCE) U '}}_{\text{S}}\text{= - 1300 mV (SCE)}\end{array}\end{array}$$

The freely selectable voltage U _I is set between the values of U _S and U ' _S. It serves as an indicator to determine whether the previous polarization period at the voltage U ' _{S was} sufficient. If the voltage U _{I is} not exceeded within a predetermined switch-off time t _a after the electrolysis has been switched off, sufficient polarization has been achieved. Otherwise the polarization is repeated immediately.

A value of U _I = - 900 mV (SCE) is preferably selected for the AlMgSi 1 alloy.

Claims (3)

1. An assembly (1) for achieving a cathodic corrosion protection for a surface of a metallic, aluminium-containing substrate (3) which may be encircled by an electrolyte (heat carrier or cooling medium), which assembly comprises the following constituents:

   a) at least one counter electrode (4) which may be encircled by electrolytes and which is not, or only slightly, corrodable by the electrolyte;

   b) at least one associated reference electrode (7) which may be encircled by the electrolyte and which comprises a constant electro-chemical potential relative to the electrolyte;

   c) an associated potentiostat (5) which is electrically connected to the substrate (3) and the counter electrode (4) and by means of which it is possible to effect an electrolysis in the electrolyte, with an electric electrolysing voltage being switched on between the substrate (3) as the cathode and the counter-electrode (4) as the anode;

   d) an associated control device (6, 8, 9), which is electrically connected to the substrate (3) of the reference electrode (7) and the potentiostat (5), by means of which it is possible, between the reference electrode (7) and the substrate (3), to measure an electric control voltage constituting a direct measure for the redox potential on the surface of the aluminium substrate in the electrolyte and by means of which the electrolysing voltage may be switched on and off as well as controlled;

   e) that a first limit voltage (U'_s) may be predetermined for the control device (6, 8, 9), with the electrolysing voltage being controllable during the electrolysis in such a way that the control voltage equals the first limit voltage (U'_s);

   f) that a second limit voltage may be predetermined for the control device (6, 8, 9), so that the electrolysing voltage is switched on when the control voltage equals the second limit voltage (U_s);

   g) that an electrolysing period t_p may be predetermined for the control device (6, 8, 9) and that the electrolysis may be effected for a time interval whose length equals the electrolysing period.
2. A process for achieving a cathodic corrosion protection for a surface of a metallic, aluminium-containing substrate (3) which may be encircled by an electrolyte (heat carrier or cooling medium), using an assembly according to claim 1,
   characterised in
   that

   a) an electrolysis is carried out for a predetermined period of time and then switched off;

   b) a control voltage is observed between a reference electrode (7) and the substrate (3) and that the electrolysis is switched on when the control voltage equals the predetermined second limit voltage;

   c) during the electrolysis, the electrolysing voltage between the substrate (3) as the cathode and a counter electrode (4) as the anode is controlled in such a way that the control voltage equals a first limit voltage;

   d) there are pretermined a control period and a critical voltage ranging between the first limit voltage and the second limit voltage, and that after elapse of the control period, the electrolysis, after having been switched off, is switched on again if, during the control period, the control voltage becomes equal to the critical voltage;

   e) that the first limit voltage constitutes a predeterminable lower negative voltage value which is greater than the limit potential which occurs in the cathodic direction in the current density/potential diagram and at which the cathod reduction of oxygen in water takes place;

   f) the second limit voltage is smaller than/equal to the pitting potential which occurs in the anodic direction of the current density/potential diagram and which, when exceeded, causes localised aluminium dissolution.
3. A process according to claim 2,
   characterised in
   that the first limit voltage (lower negative value of the redox potential of the substrate) is removed in the anodic direction away from the limit potential at which the cathodic reduction of oxygen occurs and that the second limit voltage (upper limit value of the redox potential held towards the positive potential) is removed in the cathodic direction away from the pitting potential - in both cases far enough to ensure that, especially when short-term reductions in potential are effected for generating a short-term cathodic polarisation of the substrate by means of potential-controlled cathodic polarising pulses, the control voltage range of the substrate (3) is narrowed (shorter polorisation period and shorter period of disconnection).

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