Method for passivating a surface of a workpiece and apparatus for passivating workpieces

In order to create a method for passivating a surface of a workpiece, said method enabling a complete and sufficient passivation of the surface of the workpiece in narrow cavities of the workpiece, even in the case of workpieces with complex geometry, it is proposed that the method comprises the following:          introducing at least one workpiece into a treatment chamber;     providing a passivating agent in the treatment chamber;     cyclically changing the pressure in the treatment chamber while the passivating agent is located in the treatment chamber.

FIELD OF THE DISCLOSURE

The present invention relates to a method for passivating a surface of a workpiece, said method comprising the following:introducing at least one workpiece into a treatment chamber; andintroducing a passivating agent into the treatment chamber.

Such a method is known from DE 10 2007 022 033 A1.

In this known method, the interior space of a treatment chamber is flooded with a passivating agent multiple times, the passivating agent being completely discharged from the treatment chamber after each flooding.

If the workpiece to be passivated has constrictions such as bores with a small diameter, capillaries, and small lumens, the penetration of the passivating agent into such constrictions is impeded by the surface tension of the passivating agent.

An exchange between air present in the workpiece and the passivating agent occurs only to an insufficient extent. Workpieces with complex geometry are therefore passivated incompletely and insufficiently, at least in such constrictions. Also pre-processes such as, e.g., electropolishing cannot be performed completely for these reasons.

In accordance with an embodiment of the invention, a method is created for passivating a surface of a workpiece of the kind stated at the outset, said method enabling a complete and sufficient passivation of the surface of the workpiece in narrow cavities of the workpiece, even in the case of workpieces with complex geometry.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention, a method for passivating a surface of a workpiece is provided, said method having the features of the preamble of claim1, wherein the method further comprises the following:cyclically changing the pressure in the treatment chamber while the passivating agent is located in the treatment chamber.

The concept underlying the invention is to generate cyclical pressure changes in the closed treatment chamber, wherein the treatment chamber is preferably evacuated to a lower pressure value and then ventilated up to an upper pressure value.

Due to these repeated pressure changes, the passivating agent is also reliably introduced into narrow cavities of a workpiece to be passivated.

If another fluid is contained in cavities of the workpiece due to pre-processes, this fluid is thus reliably exchanged for passivating agent through the pressure changing operations. As a result, it is ensured during the passivating process that the passivating agent can securely coat all surfaces of the workpieces and the formation of a gapless passivation layer can reliably take place.

The provision of the passivating agent in the treatment chamber can take place by the passivating agent being introduced into the treatment chamber after the introduction of the workpiece into the treatment chamber. This is preferably the case with chamber systems for treating workpieces.

Alternatively hereto, provision may also be made that the passivating agent is already located in the treatment chamber before the introduction of the workpiece into the treatment chamber. In this case, provision may also be made that the passivating agent remains in the treatment chamber after the passivation treatment. This is preferably the case with in-line immersion systems for treating workpieces.

Passivation of a workpiece refers to the removal of free iron molecules and the targeted creation of a protective layer on a metallic workpiece, the oxygen corrosion of the base material of the workpiece thereby being prevented or slowed down considerably.

The standards ASTM A380-06 and ASTM A967-05 are relevant for the passivation of surfaces of a workpiece. Furthermore, the specifications ISO 16048, AMS 2700 and QQ-P-35 are applicable.

The workpiece to be passivated may be made, for example, entirely or partially of a stainless steel material, which has a chromium content of at least 11% by weight.

The workpiece to be passivated may be produced, in particular, in an additive manufacturing process.

The workpiece to be passivated may be a workpiece from the field of medical technology, for example an implant or an instrument.

The workpiece to be passivated may have, e.g., a structured, in particular biocompatible, surface.

Such a surface may be provided, in particular, to intergrow with bone structures.

The workpiece to be passivated preferably has narrow cavities such as, for example, bores or capillaries into which liquids are not easily able to penetrate from the outside.

In a preferred embodiment of the invention, provision is made that the maximum pressure pois less than the ambient pressure (about 1.0 bar) during the cyclical changing of the pressure in the treatment chamber.

Furthermore, it is favorable if the minimum pressure puis less than half the maximum pressure poduring the cyclical changing of the pressure in the treatment chamber.

Furthermore, it is advantageous if the minimum pressure puis less than the vapor pressure of the passivating agent during the cyclical changing of the pressure in the treatment chamber.

Due to the cyclical pressure change in the treatment chamber, cavitation bubbles are created directly on the surfaces of the workpiece. A portion of these cavitation bubbles is stable and changes its volume in the course of the pressure change; the volume of the cavitation bubbles decreases when the pressure rises and increases when the pressure falls.

Due to these volume changes, micro-flows are created in the bath of the passivating agent and, in particular, in cavities of the workpiece to be passivated.

In a low pressure phase, the stable cavitation bubbles expand in the capillaries of the workpieces, fluid thereby being ejected from the capillaries.

In a subsequent phase with higher pressure, the stable cavitation bubbles contract, fluid thereby being sucked into the capillaries.

Another portion of the cavitation bubble is not stable, but instead is formed as transient cavitation bubbles. These transient cavitation bubbles implode when the pressure in the treatment chamber is increased, which leads to very high flow speeds on the surface of the workpiece.

The micro-flows in the bath of passivating agent bring about a very effective exchange of the fluid on the surfaces of the workpiece, particularly on the surfaces of bores and/or capillaries of the workpiece.

A high concentration of oxygen on the surface of the workpiece to be passivated facilitates the formation of oxides and the formation of a passivation layer.

In a particular embodiment of the passivation method, the oxygen concentration in the passivating agent within the treatment chamber is therefore detected by at least one measuring sensor.

When the oxygen concentration in the passivating agent falls below a lower limit value, the oxygen concentration is raised by supplying air into the treatment chamber.

In order to enable such a supply of air into the passivating agent within the treatment chamber, provision may be made that the treatment chamber is provided with at least one bead nozzle.

The limit value of the oxygen concentration below which a supply of oxygen to the passivating agent is performed is preferably at least 5 mg/l and/or preferably at most 8 mg/l.

The average cycle duration ti of the changing of the pressure in the treatment chamber is preferably at least 1 second, in particular at least 3 seconds, particularly preferably at least 5 seconds.

Furthermore, the average cycle duration ti of the changing of the pressure in the treatment chamber is preferably at most 30 seconds, particularly preferably at most 10 seconds.

The passivation process, which comprises numerous pressure change cycles, is performed during a treatment time of preferably at least one minute, in particular at least 5 minutes, particularly preferably at least 10 minutes.

Furthermore, the passivation process is preferably performed during a treatment time of at most 60 minutes, in particular at most 30 minutes, particularly preferably at most 20 minutes.

In a preferred embodiment of the method in accordance with the invention, provision is made that the treatment chamber is connected to a negative pressure source during the changing of the pressure at least at times.

Such a negative pressure source may comprise, for example, a vacuum pump, preferably a positive displacement vacuum pump.

Provision may further be made that the negative pressure source comprises a negative pressure store.

Such a negative pressure store is preferably separable from the interior space of the treatment chamber by a valve or a valve arrangement.

Furthermore, the negative pressure store is preferably connectable to a vacuum generator, for example a vacuum pump, in order to lower the pressure in the negative pressure store.

During phases of the passivation process during which the negative pressure store is separated from the treatment chamber by the valve or the valve arrangement, the internal volume of the negative pressure store can be evacuated by means of the vacuum generator to a pressure that is significantly lower than the pressure in the treatment chamber.

Due to a subsequent pressure equalization between the negative pressure store and the interior space of the treatment chamber, the pressure in the interior space of the treatment chamber can then be lowered significantly in a simple manner, preferably below the vapor pressure of the passivating agent that is located in the interior space of the treatment chamber. It is thereby possible to increase the depth of action of the evaporation of the treatment agent in the interior space of the treatment chamber.

Here, the depth of action is the distance (taken in parallel to the direction of gravity) from the surface of the bath of the passivating agent up to which an evaporation process takes place in the passivating agent.

Furthermore, it is possible to provide a mechanism in the negative pressure store for separating liquid droplets entrained with the gas stream from the interior space of the treatment chamber. Such a mechanism may comprise, e.g., a wire mesh mist eliminator, an impact separator, and/or a condenser, which preferably has a cooled face.

Furthermore, a mechanism may be provided in the negative pressure store that precipitates vapor of the treatment agent entrained with the gas stream from the interior space of the treatment chamber, said precipitation taking place actively or passively through condensation.

Such a mechanism may comprise, e.g., a condenser, which preferably has a cooled face.

It is particularly favorable if the negative pressure store is provided with a heat exchanger.

In such a heat exchanger, which may act, in particular, as a condenser, heat from the gas stream reaching the negative pressure store from the treatment chamber can be transferred to a heat removal medium. If a vapor of the passivating agent condenses on the heat exchanger, in particular, the condensation heat thereby released can be transferred to the heat removal medium.

The heat removal medium may be a treatment agent to be supplied to the treatment chamber, for example a passivating agent or a rinsing liquid.

The heat removal medium may also serve to indirectly heat a treatment agent to be supplied to the treatment chamber, for example a passivating agent or a rinsing liquid.

For this purpose, it is particularly favorable if the heat removal medium is used as a coolant of a heat pump.

Furthermore, it is possible to heat a treatment agent of another treatment process or to heat a building by means of the heated heat removal medium.

The passivating agent is preferably an aqueous solution to which at least one acid is added.

For example, provision may be made that the passivating agent comprises citric acid.

Here, the proportion of the citric acid may be preferably at least 4% by weight and/or preferably at most 10% by weight.

Alternatively or in addition hereto, provision may be made that the passivating agent comprises phosphoric acid and/or nitric acid.

For example, an aqueous solution of nitric acid may be used as a passivating agent, said solution containing nitric acid in a proportion of at least 20% by volume and at most 55% by volume, in particular at most 45% by volume, particularly preferably at most 25% by volume.

Alternatively hereto, provision may be made that the passivating agent contains phosphoric acid in a concentration of at least 1.5% by volume and/or at most 3% by volume.

In a preferred embodiment of the invention, provision is made that the passivating agent is an aqueous solution of phosphoric acid and nitric acid, wherein the passivating agent contains phosphoric acid in a concentration of 1.5% by volume to 3% by volume and nitric acid in a concentration of 0.1% by volume to 0.5% by volume.

Furthermore, the passivating agent contains non-ionic surfactants in a concentration of preferably 0.05% by volume to 0.5% by volume.

A basis of the aqueous solution is preferably demineralized water having an electrical conductivity of at most 10 μS/cm.

In order to facilitate the creation of micro-flows directly on the surface of a workpiece to be passivated in combination with the cyclical pressure changes and to thus obtain an even greater exchange of the fluids in cavities of the workpiece, provision may be made that the passivating agent and/or the at least one workpiece is/are acted upon with ultrasound during the changing of the pressure.

Here, the ultrasonic frequency is preferably at least 20 kHz, particularly preferably at least 25 kHz.

Furthermore, the ultrasonic frequency is preferably at most 120 kHz, particularly preferably at most 80 KHz.

The ultrasonic power coupled into the interior space of the treatment chamber is preferably at least 5 watts per liter of passivating agent in the treatment chamber, particularly preferably at least 8 watts per liter of passivating agent in the treatment chamber.

Furthermore, the ultrasonic power coupled into the treatment chamber is preferably at most 20 watts per liter of passivating agent in the treatment chamber, particularly preferably at most 15 watts per liter of passivating agent in the treatment chamber.

By modulating the amplitude of the ultrasonic vibrations and/or by modulating the ultrasonic frequency (so-called “sweep” function), the effect of the action on the bath of passivating agent and/or the workpiece to be passivated can be further intensified and the effect of the application of ultrasound can be distributed uniformly over all surfaces of the workpiece to be passivated.

In a preferred embodiment of the method in accordance with the invention, provision is made that a workpiece to be passivated is introduced into a plurality of passivation stations one after the other, which contain different passivating agents.

Because a plurality of passivation stations are provided in which different workpieces to be passivated are passivated simultaneously, the throughput of a passivation system used for the passivation method can be increased.

The bath of the passivation station into which the workpiece is first introduced is most heavily contaminated and the bath of the passivation station into which the workpiece is last introduced is least contaminated. As a result, the surface quality and the cleanliness of the passivated workpiece are further improved.

For processing the passivating agent, the passivating agent can be conveyed in a cascade partially from the passivation state located further back in the treatment sequence into the passivation station located further ahead in the treatment sequence.

The passivating agent from the passivation station located first in the treatment sequence can be partially discarded.

In the multi-stage passivation method, the treatment temperature and/or the concentration of the acid in the respective passivating agent may be different in the different passivation stations.

Here, it is particularly favorable if in the passivation state located last in the treatment sequence the concentration of the acid in the passivating agent is lowest, such that less acid is carried over from this passivation station, for example into a subsequent rinsing station.

The treatment temperature in a passivation station located further ahead in the treatment sequence may be at least 10% higher and/or at most 20% higher than the treatment temperature in a passivation station located further back in the treatment sequence (the percentages referring to the absolute temperature of the respective passivating agent).

It is further favorable if the concentration of the acid in the passivating agent in the passivation station located further ahead in the treatment sequence is at least 50% higher than the concentration of the acid in the passivating agent in the passivation station located further back in the treatment sequence.

Provision may further be made that the concentration of the acid in the passivating agent in the passivation station located further ahead in the treatment sequence is at most 80% higher than the concentration of the acid in the passivating agent in the passivation station located further back in the treatment sequence.

If a workpiece to be passivated is treated in two separate treatment chambers one after the other, provision may thus be made, for example, that the workpiece in the first passivation station is immersed in the passivating agent and that the workpiece in the subsequent passivation station is sprayed with the passivating agent and/or the passivating agent is poured thereover.

In accordance with an embodiment of the invention, an apparatus is created for passivating workpieces, said apparatus enabling a complete and sufficient passivation of the surface of a workpiece to be passivated, even when the workpiece has a complex geometry and/or narrow cavities.

This object is achieved, in accordance with the invention, in an apparatus for passivating workpiece, said apparatus having the features of the preamble of claim14, in that the apparatus comprises a pressure changing apparatus for cyclically changing the pressure in the treatment chamber while passivating agent is located in the treatment chamber.

Particular embodiments of such an apparatus in accordance with the invention for passivating workpieces have already been explained above in connection with particular embodiments of the method in accordance with the invention for passivating a surface of a workpiece.

In a particular embodiment of the apparatus in accordance with the invention, provision is made that the apparatus comprises at least two treatment chambers, wherein a workpiece is treatable with a passivating agent in the first treatment chamber and a workpiece is rinsable with a rinsing liquid in the second treatment chamber simultaneously.

The apparatus in accordance with the invention for passivating workpieces is suitable, in particular, for performing the method in accordance with the invention for passivating a surface of a workpiece.

The method in accordance with the invention for passivating a surface of a workpiece is preferably performed by means of the apparatus in accordance with the invention for passivating workpieces.

For many applications of a workpiece to be passivated, in particular for applications as a medical technology product, the workpieces passivated by means of the method in accordance with the invention or the workpieces passivated by means of the apparatus in accordance with the invention are permitted to have no residue of the passivating agent on their surfaces after the passivation process. In particular when a passivated workpiece has a cavity, it must be ensured by means of a suitable process that the passivating agent is rinsed out of such a cavity.

Therefore, a rinsing process may follow the passivating process.

The rinsing process may be performed in the same treatment chamber as the passivation process.

Alternatively thereto, provision may be made that the rinsing process is performed in a different treatment chamber or in an open treatment basin.

For example, demineralized water is used as a rinsing liquid.

The workpiece to be rinsed is preferably immersed completely or partially into the rinsing liquid.

When the rinsing process is performed in a closed treatment chamber, the pressure in the treatment chamber may thus also be cyclically changed, as has been described in connection with the passivation process.

Due to the cyclical pressure changes, the rinsing liquid is then flushed into constrictions and capillaries of the passivated workpiece, the passivating agent thereby being flushed out at the same time.

The rinsing effect can be facilitated by acting upon the bath of the rinsing liquid and/or the workpiece in the treatment chamber or in the open treatment basin by means of ultrasound, for example by an ultrasonic oscillator.

Further features and advantages of the invention are subject matter of the subsequent description and the graphical representation of exemplary embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The same or functionally equivalent elements are provided with the same reference numerals in all Figures.

An apparatus, schematically depicted inFIG.1and denoted as a whole with100, for passivating workpieces102comprises a treatment chamber104for accommodating at least one respective workpiece102in an interior space106of the treatment chamber104, said interior space106being surrounded by a wall108of the treatment chamber.

In order to be able to introduce a workpiece102into the interior space106of the treatment chamber104, provision is preferably made that the treatment chamber104comprises a tank110(seeFIG.8), which is fillable with a liquid112, for example a passivating agent114, up to a highest level118and is closeable by means of a lid116.

In order to be able to fill the treatment chamber104with treatment agent, in particular passivating agent, the interior space106of the treatment chamber104is connected to a storage container122by way of a supply line120.

The supply line120is openable or closeable by means of a valve124arranged therein.

The supply line120opens in a clean region126of the storage container122, which is filled with a treatment agent, in particular a passivating agent, which is prepared for a treatment operation in the treatment chamber104.

The clean region126is separated from a dirty region130of the storage container122by a separating wall128.

Located at the upper rim of the separating wall128is an overflow132by way of which treatment agent is able to travel from the clean region126into the dirty region130.

The treatment agent located in the dirty region130is prepared for use in the treatment chamber104by being transferred from the dirty region130of the storage container122into the clean region126of the storage container122by way of a filtration line134.

A filtration pump136, a filter138, and a flow restrictor140are arranged in the filtration line134.

An emptying of the treatment agent, in particular the passivating agent, from the interior space106of the treatment chamber104is possible by means of an emptying line148, which is preferably connected at a lowest point of the interior space106of the treatment chamber104and which opens into the dirty region130of the storage container122.

The emptying line148is openable and closeable by means of a valve150arranged therein.

For monitoring the treatment process in the treatment chamber104, the treatment chamber104is provided with different sensors, in particular with a pressure sensor152, a temperature sensor154, and a lower level sensor156and/or an upper lever sensor158.

By means of the lower level sensor156, it is determined whether the interior space106of the treatment chamber104is completely emptied of treatment agent.

With the upper lever sensor158, it is determined whether the interior space106of the treatment chamber104is completely filled with liquid treatment agent, in particular passivating agent.

In order to be able to cyclically change the pressure in the treatment chamber104, the apparatus100for passivating workpieces102further comprises a pressure changing apparatus160, which in the depicted embodiment comprises an evacuation valve296, a separator166, and a vacuum generator168.

The evacuation valve296is connected to a pressure change outlet174of the treatment chamber104by way of a pressure change line172.

Furthermore, the evacuation valve296is connected to an inlet180of the separator166by way of an evacuation line178.

Gas stemming from the interior space106of the treatment chamber104, said gas being loaded with a vapor of the treatment agent and/or with droplets of the treatment agent, in particular the passivating agent, travels through the inlet180into the interior space182of the separator166, which may be configured, for example, as a cyclone in order to separate droplets contained in the entering gas stream from the gas stream by centrifugal action.

The separator166may further contain a condensation apparatus184, which comprises, e.g., impact plates and/or a demister and/or a cooled heat exchanger on which vapor of the treatment agent, in particular the passivating agent, condenses and is thus able to be separated out of the gas stream flowing through the separator166.

The gas stream, freed of droplets and/or vapor of the treatment agent by means of the separator166, travels through a suction line186to a suction-side inlet188of the vacuum generator168.

The vacuum generator168may be configured, e.g., as a vacuum pump190, preferably as a positive displacement vacuum pump.

The gas stream then travels via a pressure-side outlet192of the vacuum generator168into a pressure line194and from there, for example, into the environment of the apparatus100.

The pressure changing apparatus160further comprises a ventilation valve298.

The ventilation valve298is connected to the pressure change outlet174of the treatment chamber104by way of the (branching) pressure change line172.

Furthermore, the ventilation valve298is connected to a ventilation line198, by means of which ambient air is suppliable to the ventilation valve298.

The pressure changing apparatus160is switchable from an evacuation state, in which the evacuation valve296connected to the evacuation line178is open and the ventilation vale298connected to the ventilation line198is closed, into a ventilation state, in which the ventilation valve298connected to the ventilation line198is open and the evacuation valve296connected to the evacuation line178is closed.

Furthermore, the pressure changing apparatus160can be switched into a holding state in which both the evacuation valve296and the ventilation valve298are closed, such that the pressure in the treatment chamber104is held substantially constant.

The switching of the evacuation valve296and/or the ventilation valve298between the open and the closed state may, in principle, take place in any manner, for example mechanically, electromechanically, pneumatically, hydraulically, or electromagnetically.

All switchable valves of the apparatus100and the sensors, for example the pressure sensor152, the temperature sensor154, the lower level sensor156, and the upper level sensor158, are connected by means of signal and control lines (not depicted) to a control apparatus (not depicted) of the apparatus100for passivating workpieces102, such that the control apparatus is able to receive and process signals from the sensors and is able to switch the switchable valves from one state into the other state.

The control apparatus of the apparatus100is preferably programmable, such that a control program for controlling a method for passivating workpieces102is performable by means of the control apparatus and the sensors and actuators controlled thereby.

In the operation of the apparatus100, condensate collected in the separator166can be removed from the interior space182of the separator166by means of a condensate trap200.

The condensate trap200comprises a first sluice valve204connected at a condensate outlet202of the separator166, a second sluice valve206arranged downstream of the first sluice valve204, and a sluice space208arranged between the first sluice valve204and the second sluice valve206.

The condensate deposited in the separator166travels into the sluice space208of the condensate trap200by opening the first sluice valve204while the second sluice valve206is closed at the same time.

After the sluice space208is filled with condensate, the first sluice valve204is closed and the second sluice valve206is opened.

The second sluice valve206is connected to the dirty region130of the storage container122by way of a condensate line210, such that the condensate deposited in the separator166travels via the condensate trap200into the storage container122.

By means of the apparatus100described above for passivating workpieces, a method for passivating workpieces is performed as follows:

Before the passivation of a workpieces102, all surfaces of the workpiece102must be freed of all filmic, particulate, and other contaminants and residue of cleaning agents. Therefore, an intensive cleaning of the workpiece102with subsequent rinsing of all surfaces of the workpiece102takes place before the passivation process.

The treatment chamber104of the apparatus100is provided with a closure device, for example with a lid116.

This closure device enables an air-tight closure of the treatment chamber104.

When the closure device is open, in particular when the lid116is raised, the workpiece102to be treated is introduced into the interior space106of the treatment chamber104.

The workpiece102may be held on a workpiece receptacle212.

After the introduction of the workpiece102into the interior space106of the treatment chamber104, the treatment chamber104is closed in an air-tight manner by means of the closure device.

If no bath of the treatment agent, in particular the passivating agent, is present in the interior space106of the treatment chamber104, a desired amount of treatment agent is sucked from the storage container122through the supply line120. For this purpose, the valve124in the supply line120is opened.

The pressure changing device160is first in the evacuation state in which the evacuation valve296is open and the ventilation valve298is closed. The vacuum generator168, in particular the vacuum pump190, is in operation and sucks gas, which may be loaded with vapor of the treatment agent and with droplets of the treatment agent, through the pressure changing line172, the evacuation line178, the separator166, and the suction line186to the suction-side inlet188of the vacuum generator168.

Here, droplets of the treatment agent entrained with the suctioned gas are separated in the separator166. Furthermore, the vapor of the treatment agent entrained with the gas condenses in the condensation apparatus184.

The treatment agent collected in the lower region of the separator166can be fed via the condensate trap200and the condensate line210to the dirty region130of the storage container122when a predetermined fill level is reached or after a predetermined operation time interval.

The separator166may also serve as a negative pressure store.

In this way, the pressure in the interior space106of the treatment chamber104is reduced from atmospheric pressure (about 1.0 bar) to a lower pressure value pu. After reaching a lower pressure value pu, the pressure in the treatment chamber104is cyclically changed during a passivation duration, i.e., increased from the lower pressure value puup to an upper pressure value poand then lowered again to the lower pressure value pu(seeFIG.2).

This process can be repeated, in particular periodically.

Between the pressure increase phases and the pressure reduction phases, the pressure in the treatment chamber104can remain at the lower pressure value puor at the upper pressure value poby the pressure changing apparatus160being switched into the holding state, in which the evacuation valve296and the ventilation valve298are closed.

To increase the pressure in the treatment chamber104, the pressure changing apparatus160is switched by the control apparatus into the ventilation state, in which the evacuation valve296is closed and the ventilation valve298is open, such that ambient air travels through the ventilation line198and the pressure change line172into the interior space106of the treatment chamber104.

The lower pressure value puis preferably at least 20 mbar and/or preferably at most 500 mbar, in particular at most 300 mbar.

The upper pressure value puis preferably at least 700 mbar and/or preferably at most 1 bar.

The cycle duration ti of a complete pressure cycle is preferably at least 1 second, in particular at least 3 seconds, particularly preferably at least 5 seconds.

Furthermore, the cycle duration ti of a complete pressure cycle is preferably at most 30 seconds, particularly preferably at most 10 seconds.

The workpiece102is arranged in the interior space106of the treatment chamber104such that it is immersed at least partially or completely in a bath of the treatment agent, in particular the passivating agent.

If the workpiece102has narrow tubes, so-called capillary tubes, it is thus favorable if said capillary tubes are oriented substantially vertically, wherein an end of each tube projects into the passivating agent and the opposite end of the tube projects into the region of the interior space106of the treatment chamber104that is filled with gas. Due to the hydrostatic pressure, the passivating agent is then sucked into the capillary tubes.

By means of this process, the fluid is reliably exchanged, said fluid being present on the surface of the workpiece102or in bores or capillaries of the workpiece102due to pre-processes.

As a result, during the passivating process, it is ensured that the passivating agent is able to completely coat all surfaces of the workpiece102, such that the formation of a passivation layer takes place on all surfaces of the component102.

The passivation process, which comprises numerous pressure change cycles, is performed during a passivation time of preferably at least 1 minute, in particular at least 5 minutes, particularly preferably at least 10 minutes.

Furthermore, the passivation process is preferably performed during a passivation time of at most 60 minutes, in particular at most 30 minutes, particularly preferably at most 20 minutes.

The temperature of the passivating agent during the passivation process is preferably at least 20° C., in particular at least 30° C., particularly preferably at least 50° C.

Furthermore, the temperature of the passivating agent during the passivation process is preferably at most 90° C., in particular at most 70° C., particularly preferably at most 65° C.

For performing the passivation process, a passivating agent is used as a treatment agent.

The passivating agent preferably has the following chemical composition:

A basis of the passivating agent is demineralized water having an electrical conductivity of at most 10 μS/cm.

The passivating agent further contains phosphoric acid in a concentration of 1.5% by volume to 3% by volume, nitric acid in a concentration of 0.1% by volume to 0.5% by volume, and non-ionic surfactants in a concentration of 0.05% by volume to 0.5% by volume.

Alternatively hereto, for example, an aqueous solution of nitric acid may be used as a passivating agent, said solution containing nitric acid in a proportion of at least 20% by volume and at most 55% by volume, in particular at most 45% by volume, particularly preferably at most 25% by volume.

Furthermore, an aqueous solution of citric acid may be used as an alternative passivating agent, wherein the proportion of citric acid is preferably at least 4% by weight and/or preferably at most 10% by weight.

The pH value of the passivating agent is preferably at least 1.8 and/or preferably at most 2.2.

Due to the cyclical pressure change in the treatment chamber104, cavitation bubbles are created directly on the surfaces of the workpiece102or the plurality of workpieces102. A portion of these cavitation bubbles is stable and change its volume in the course of the pressure change; the volume of the cavitation bubbles decreases when the pressure rises and increases when the pressure falls.

Due to these volume changes, micro-flows are created in the bath of the passivating agent and, in particular, in cavities of the workpiece102to be passivated.

In a low pressure phase, the stable cavitation bubbles expand in the capillaries of the workpiece102, fluid thereby being ejected from the capillaries.

In a subsequent phase with higher pressure, the stable cavitation bubbles contract, fluid thereby being sucked into the capillaries.

Another portion of the cavitation bubble is not stable, but instead is formed as transient cavitation bubbles. These transient cavitation bubbles implode when the pressure in the treatment chamber104increases, which leads to very high flow speeds on the surface of the workpiece102.

The micro-flows in the bath of passivating agent bring about a very effective exchange of the fluid on the surfaces of the workpiece102, particularly on the surfaces of bores and/or capillaries of the workpiece102.

A high concentration of oxygen on the surface of the workpiece102to be passivated facilitates the formation of oxides and thus the formation of a passivation layer.

Therefore, in a preferred embodiment of the passivation method, the oxygen concentration in the passivating agent within the treatment chamber104is detected by at least one measuring sensor.

When the oxygen concentration in the passivating agent falls below a lower limit value, the oxygen concentration is raised by supplying air into the lower region of the treatment chamber104. In order to enable such a supply of air into the passivating agent, provision may be made that the treatment chamber104is provided with at least one bead nozzle.

The limit value of the oxygen concentration below which a supply of oxygen to the passivating agent is performed is preferably at least 5 mg/l and/or preferably at most 8 mg/l.

In order to facilitate the creation of micro-flows directly on the surface of a workpiece102to be passivated in combination with the cyclical pressure changes and to thus obtain a greater exchange of the fluids, provision may be made that the apparatus100is provided with at least one ultrasonic oscillator214, by means of which the bath of the treatment agent in the interior space106of the treatment chamber104and/or the workpiece102is able to be acted upon with ultrasound.

Here, the ultrasonic frequency is preferably at least 20 kHz, particularly preferably at least 25 kHz.

Furthermore, the ultrasonic frequency is preferably at most 120 kHz, particularly preferably at most 80 KHz.

The ultrasonic power coupled into the interior space106of the treatment chamber104by means of the ultrasonic oscillator214or by means of a plurality of ultrasonic oscillators214is preferably at least 5 watts per liter of passivating agent in the treatment chamber104, particularly preferably at least 8 watts per liter of passivating agent in the treatment chamber104.

Furthermore, the ultrasonic power coupled into the treatment chamber104is preferably at most 20 watts per liter of passivating agent in the treatment chamber104, particularly preferably at most 15 watts per liter of passivating agent in the treatment chamber104.

By modulating the amplitude of the ultrasonic vibrations and/or by modulating the ultrasonic frequency (so-called “sweep” function), the effect of the action on the bath of passivating agent and/or the workpiece102can be further intensified and the effect of the application of ultrasound can be distributed uniformly over all surfaces of the workpiece102to be passivated.

In order to uniformly and completely passivate all regions of a workpiece102that is held on the workpiece receptacle212, or all workpieces102of a group of workpieces102that are held on the workpieces receptacle212simultaneously, it is favorable if the apparatus100comprises a rotating apparatus, by means of which the workpiece receptacle212and thus the workpiece102or the workpieces102are rotatable about a rotational axis during the passivation treatment.

The rotational axis is hereby preferably oriented substantially horizontally.

This rotational movement prevents air bubbles from collecting in the workpiece102or in the workpieces102and preventing a coating with the passivating agent.

Furthermore, by means of the rotating apparatus, it is possible to empty cavities of the workpiece102or the workpieces102when orifices of the cavities are located above the surface of the passivating agent bath in the treatment chamber104.

Alternatively or in addition to such a rotating apparatus, provision may also be made that the apparatus100comprises a vertical movement apparatus for the workpiece102or for a plurality of workpieces102. By means of such a vertical movement apparatus, the workpiece102or the workpieces102are moveable along the direction of gravity relative to the bath of passivating agent. The flow of the passivating agent caused by this relative movement facilitates the exchange of the passivating agent on the surfaces of the workpiece102to be passivated.

The filter138arranged in the filtration line134, by means of which contaminants are removed from the passivating agent that enters the dirty region130, may comprise a solids filter.

Furthermore, the filer138may also comprise a magnetic retaining apparatus, by means of which iron molecules are removable from the passivating agent.

Such a magnetic retaining apparatus preferably comprises at least one permanent magnet.

Such a magnetic retaining apparatus is advantageously integrated into a housing of the filter138.

When the passivating agent is reprocessed in the filtration line134, the concentration of the acid content in the passivating agent can also be monitored and, if necessary, brought into the desired range by adding acid or by adding water.

Such a concentration monitoring may take place, for example, by means of a conductivity measurement of the passivating agent or by means of a refractometer.

For many applications, the workpieces102passivated by means of the apparatus100are not permitted to have any residue of the passivating agent on their surfaces for reprocessing. In particular when a passivated workpiece102has a cavity, it must be ensured by means of a suitable process that the passivating agent is rinsed out of such a cavity to sufficient extent.

Therefore, a rinsing process may follow the passivating process described above.

For example, demineralized water is used as a rinsing liquid.

To perform the rinsing process, the passivating agent is emptied into the storage container122via the open valve150and the emptying line148.

The interior space106of the treatment chamber104is then filled at least partially with the rinsing liquid by means of a rinsing liquid supply (not graphically represented).

During the rinsing process, the pressure in the treatment chamber104may also be cyclically changed, as has been described in connection with the passivation process.

Due to the cyclical pressure changes, the rinsing liquid is flushed into constrictions and capillaries of the passivated workpiece102, the passivating agent thereby being flushed out at the same time.

The rinsing effect and thus the removal of the passivating agent from the surfaces of the workpiece102can be facilitated by acting upon the bath of the rinsing liquid and/or the workpiece102in the treatment chamber104with ultrasound, for example by the ultrasonic oscillator214, for example through the creation of micro-flows near the surface.

A second embodiment, depicted schematically inFIG.3, of an apparatus100for passivating workpieces102differs from the first embodiment depicted inFIG.1in that the pressure changing apparatus160of the apparatus100comprises a plurality, namely two in the embodiment depicted, negative pressure stores224, which are arranged between the treatment chamber104and the vacuum generator168with respect to flow, said vacuum generator168in this case being in the form of a vacuum pump190.

A first negative pressure store224ais connected by way a first supply line226ain which a first supply valve228ais arranged to a pressure change line172connected to the treatment chamber104.

Furthermore, the first negative pressure store224ais connected by way of a first discharge line230ain which a first discharge valve232ais arranged to an evacuation line178leading to the inlet180of the separator166. A suction line186leads from the outlet of the separator166to the suction-side inlet188of the vacuum generator168.

A second negative pressure store224bis connected by way of a second supply line226bin which a second supply valve228bis arranged to the pressure change line172.

Furthermore, the second negative pressure store224bis connected to the evacuation line178by way of a second discharge line230bin which a second discharge valve232bis arranged.

Each of the negative pressure stores224is coolable by means of a heat exchanger234that is able to be flowed through by a coolant or a heat removal medium during the operation of the apparatus100in order to condense out a vapor of a treatment agent, in particular a passivating agent, from the gas stream entering the respective negative pressure store224.

Furthermore, the pressure changing apparatus160in this embodiment comprises a ventilation line198, in which a ventilation valve236is arranged and which opens into the pressure change line172.

Furthermore, an adjustable flow restrictor238may be arranged in the ventilation line198, preferably downstream of the ventilation valve236.

The apparatus100further comprises a bypass line240, which connects the evacuation line178directly to the treatment chamber104.

Arranged in the bypass line240is a bypass valve242, by means of which the bypass line240is openable or closeable.

By means of the bypass line240, the interior space106of the treatment chamber104can be evacuated directly by means of the vacuum generator168, in the present case by means of the vacuum pump190, to the lower pressure value pu, for example at the beginning of the passivation process or at the beginning of a rinsing process, while circumventing the negative pressure store224.

In this embodiment, the negative pressure stores224are used for reducing the pressure in the treatment chamber104during the pressure reduction phases of the pressure change cycles of the passivation process and/or the rinsing process.

For this purpose, during the passivation process or the rinsing process, a respective one of the negative pressure stores224, for example the second negative pressure store224b, is evacuated by means of the vacuum generator168, while the respective other negative pressure store224, for example the first negative pressure store224a, is in fluidic connection with the interior space106of the treatment chamber104during one or more pressure reduction phases, such that a gas stream (which may be loaded with droplets of the treatment agent and/or with a vapor of the treatment agent) flows from the interior space106of the treatment chamber104into the interior space of the negative pressure store224a.

During this phase, the first supply valve228ais open and the first discharge valve232ais closed.

At the same time, the second supply valve228bis closed and the second discharge valve232bis open, such that the gas is sucked from the second negative pressure store224bthrough the evacuation line178and the interior space of the second negative pressure store224bis evacuated to a pressure that is preferably below the lower pressure value pu.

Due to the cooling by means of the heat exchanger234, a vapor of the treatment agent, in particular the passivating agent, entrained in the gas stream entering the first negative pressure store224ain a pressure reduction phase is condensed out of said gas stream.

When the pressure in the interior space106of the treatment chamber104has been reduced to the lower pressure value pudue to the fluidic connection with the first negative pressure store224a, the first supply valve228ais closed and the ventilation valve236in the ventilation line198is opened, such that ambient air travels through the ventilation line198and the pressure change line172into the interior space106of the treatment chamber104.

When the pressure in the interior space106of the treatment chamber104has increased to the upper pressure value po, the pressure changing apparatus160is switched back into the evacuation state in which the ventilation valve236is closed and the first supply valve228ais open.

In this way, a plurality of pressure reduction phases can be performed by means of the first negative pressure store224auntil the pressure in the negative pressure store224ahas increased to the lower pressure value puor to a higher pressure value, such that the interior space106of the treatment chamber104is no longer able to be evacuated through a fluidic connection with the first negative pressure store224a.

Starting with the next pressure reduction phase, the roles of the two negative pressure stores224aand224bare then switched. For this purpose, the second discharge valve232bis closed and the second supply valve228bis opened, such that a gas stream is able to travel from the interior space106of the treatment chamber104into the second negative pressure store224b.

At the same time, the first supply valve228ais closed and the first discharge valve232ais open, such that the gas collected in the first negative pressure store224band the condensate collected in said gas is able to be discharge through the first discharge line230aand the evacuation line178to the separator166and to the vacuum generator168.

In all other respects, the second embodiment of an apparatus100depicted inFIG.3for passivating workpieces102corresponds with respect to structure, function, and production method with the first embodiment depicted inFIG.1, to the preceding description of which reference is made in this regard.

A third embodiment depicted inFIG.4of an apparatus100for passivating workpieces102differs from the first embodiment depicted inFIG.1in that the pressure changing apparatus160in this embodiment additionally comprises a pressure reducing apparatus242, by means of which the pressure in the interior space106of the treatment chamber104is reducible in place of the vacuum generator168or in addition to the vacuum generator168.

In the embodiment depicted inFIG.4, the pressure reducing apparatus242comprises a working cylinder244, in which a piston246is arranged so as to be displaceable along a displacement direction248.

A gas connection250of the working cylinder244opens into a receiving space252of the working cylinder244, the volume of which is variable by displacing the piston246.

The gas connection250is connected to the pressure change line172by way of a supply line254in which a supply valve256is arranged.

Arranged in the supply line254is a first check valve258, which prevents fluid from flowing from the working cylinder244back into the interior space106of the treatment chamber104.

Furthermore, the gas connection250of the working cylinder244is connected to the storage container122, preferably to the dirty region130of the storage container122, by way of a discharge line260.

Arranged in the discharge line260is a second check valve262, which prevents fluid from flowing from the storage container122back into the working cylinder244.

The piston246in the working cylinder244can be retracted up to a separating wall264in order to increase the volume of the receiving space252in the working cylinder244.

On the side of the separating wall264facing away from the receiving space252, the piston246is provided with a collar266, which is able to be acted upon with a gaseous or liquid actuating fluid from its two different sides, such that the piston246is pneumatically or hydraulically displaceable.

The portion of the working cylinder244containing the receiving space252is preferably coolable by means of a heat exchanger268, which can be supplied with a coolant by a coolant line270.

As a result of the cooling by means of the heat exchanger268, a vapor of the treatment agent, in particular the passivating agent, which is entrained by the gas that has traveled from the interior space106of the treatment chamber104into the receiving space252, can be condensed out.

During a pressure reduction phase of the pressure change cycles in the treatment chamber104, the receiving space252in the working cylinder244is enlarged by displacing the piston246(upward inFIG.5), a negative pressure thereby being generated. The supply valve256is open, such that gas is sucked from the interior space106of the treatment chamber104due to the negative pressure in the working cylinder244. In this pressure reduction phase, the evacuation valve296and the ventilation valve298are closed.

In a subsequent pressure increase phase, the ventilation valve298connected to the ventilation line198is open, while the evacuation valve296and the supply valve256are closed.

Thus, in this pressure increase phase, the interior space106of the treatment chamber104is ventilated by the ventilation line198and the pressure change line172.

The piston246is displaced along the displacement direction248(upward inFIG.5) in order to reduce the volume of the receiving space252.

Here, the gas and condensate collected in the receiving space252is transferred through the discharge line260into the storage container122, preferably into its dirty region130.

In this embodiment of an apparatus100for passivating workpieces102, the vacuum generator168, which in this case is configured, e.g., as a vacuum pump190, and the separator166are optional and may serve, in particular, to evacuate the interior space106of the treatment chamber104from the atmospheric pressure to the lower pressure value puat the beginning of a passivating process and/or a rinsing process.

In all other respects, the third embodiment of an apparatus100depicted inFIG.4for passivating workpieces102corresponds with respect to structure, function, and production method with the first embodiment depicted inFIG.1, to the preceding description of which reference is made in this regard.

In a fourth embodiment, schematically depicted inFIG.5, of an apparatus100for passivating workpieces102, the apparatus100is configured as an in-line immersion system.

This system has a plurality of treatment stations, for example a cleaning station272, an intermediate rinsing station274, a first passivation station276, a second passivation station278, a first rinsing station280, a second rinsing station282, a third rinsing station284, and a drying station286.

The stations272to286are preferably arranged substantially linearly one behind the other along a transport direction288.

Each of the treatment stations is filled with a bath290of a treatment agent corresponding to the respective function, for example a cleaning liquid, a rinsing liquid, or a passivating agent.

By means of a transport apparatus292, the workpieces102are transportable from treatment station to treatment station, immersible in the respective bath290of a treatment agent, and removable from the bath290after completion of the treatment in the respective bath290.

The transport apparatus292may be provided with a rotating apparatus294for rotating a workpiece102arranged thereon.

Likewise, individual treatment stations or a plurality of the treatment stations may each be provided with a workpiece receptacle212, which is rotatable, preferably about a horizontal rotational axis, by means of a rotating apparatus (not depicted).

At least the passivation stations276and278are provided with a lid (not depicted inFIG.6) for closing the interior space106of the respective passivation station276and278, each configured as a treatment chamber104, in order to be able to perform the passivation method described above in connection with the first embodiment depicted inFIG.1of an apparatus100for passivating workpieces102in these passivation stations276,278, in which method the pressure in the interior space106of the respective treatment chamber104is cyclically changed.

Because a plurality of passivation stations276,278are provided in the passivation system, the throughput for the passivation of workpieces102can be increased.

Here, provision may be made that the entire passivation process is performed in only one of the passivation stations276and278and the workpieces102to be passivated are introduced into a respective one of these passivation stations276,278alternatingly.

Alternatively hereto, provision may also be made that each of the workpieces102to be passivated is introduced into a plurality of passivation stations276,278one after the other, such that a plurality of passivation processes are performed on each workpiece102in different passivation stations276,278.

Here, the bath290of the passivation station276into which the workpiece102is first introduced is most heavily contaminated and the bath290of the passivation station278into which the workpiece102is last introduced is least contaminated. As a result, the surface quality and the cleanliness of the passivated workpiece102are further improved.

For processing the passivating agent, the passivating agent can be conveyed in a cascade partially from the passivation station278located further back in the treatment sequence into the passivation station276located further ahead in the treatment sequence.

The passivating agent from the passivation station276located first in the treatment sequence is partially discarded.

In the multi-stage passivation process described above, the treatment temperature and/or the concentration of the acid in the respective passivating agent may be different in the different passivation stations276,278.

Here, it is particularly favorable if in the passivation station278located last in the treatment sequence the concentration of the acid in the passivating agent is lowest, such that less acid is carried over into the subsequent rinsing station280.

For example, provision may be made that the treatment temperature in the passivation station276located further ahead in the treatment sequence is at least 10% higher and/or at most 20% higher than the treatment temperature in the passivation station278located further back in the treatment sequence (the percentages hereby referring to the absolute temperature of the respective passivating agent).

Furthermore, it is favorable if the concentration of the acid in the passivating agent in the passivation station276located further ahead in the treatment sequence is at least 50% higher than the concentration of the acid in the passivating agent in the passivation station278located further back in the treatment sequence and/or if the concentration of the acid in the passivating agent in the passivation station276located further ahead in the treatment sequence is at most 80% higher than the concentration of the acid in the passivating agent in the passivation station278located further back in the treatment sequence.

Alternatively to the configuration as an in-line immersion system, a system for passivating workpieces102may also be configured as a single-chamber system with a plurality of storage containers for the different treatment agents such as, e.g., rinsing agent and passivating agent with different concentrations of acids.

An equalization of evaporation losses of the baths290in the passivation stations276and278can be equalized by the addition of rinsing liquid from one of the subsequent rinsing stations280,282, or284. These rinsing liquid are already enriched with passivating agent in low concentration due to carryover of passivating agent from one of the passivation stations276and/or278and are already tempered to the desired treatment temperature. By adding rinsing liquid from one of the rinsing stations280,282or284to the bath290of passivating agent in one of the passivation stations276or278, an energy savings and a saving of resources such as water and acid for the passivating agent is thus achieved.

The process stations of the apparatus100according to the embodiment depicted inFIG.5are each provided with a closed fluid circuit, which, in particular, comprises a pump for flowing through the respective treatment chamber, to avoid buildup.

In all other respects, the fourth embodiment of an apparatus100depicted inFIG.5for passivating workpieces102corresponds with respect to structure, function, and production method with one of the embodiments depicted inFIG.1,3, or4, to the preceding description of which reference is made in this regard.

A fifth embodiment, depicted schematically inFIG.6, of an apparatus100for passivating workpieces102differs from the embodiments depicted inFIGS.1,3, and4in that the apparatus100comprises two treatment chambers104, wherein the pressure changing apparatus160for both treatment chambers104comprises only one single vacuum generator168, for example in the form of a vacuum pump190, and a separator166.

Each of the treatment chambers104is connected by way of a respective evacuation valve296to an evacuation line178leading to the separator166and is connected by way of a respective ventilation valve298to a ventilation line198.

In the treatment chambers104, a respective passivating process or a respective rinsing process may be performed on a workpiece102, or a passivating process is performed in one treatment chambers104and a rinsing process is performed in the respective other treatment chamber104.

In all these cases, a cyclical pressure change operation is performed in each of the two treatment chambers104, the pressure reduction phases and the pressure increase phases of these cyclical pressure change operations being phase-shifted against one another such that the pressure in a respective one of the treatment chambers104is lowered by evacuating by means of the vacuum generator168, while the pressure in the respective other treatment chamber104is increased by ventilation by way of the respective ventilation valve298.

The treatment chambers104are thus alternatingly brought into fluidic connection with the separator166and the vacuum generator168by opening and closing their respective evacuation valves296in a time-offset manner. The vacuum generator168therefore always only has to evacuate one of the treatment chambers104, such that a cyclical pressure change in both treatment chambers104is made possible by means of only one single vacuum generator168.

In all other respects, the fifth embodiment of an apparatus100depicted inFIG.6for passivating workpieces102corresponds with respect to structure, function, and production method with the first embodiment depicted inFIG.1, to the preceding description of which reference is made in this regard.

Schematically depicted inFIGS.7and8is a particular embodiment of a treatment chamber104, which can be used in each of the embodiments described above of an apparatus100for passivating workpieces102.

The treatment chamber104comprises a tank110, which is fillable with a bath290of a treatment agent, in particular a passivating agent.

At its upper rim, the tank110is provided with a suction channel300, which is connected to the respective pressure change line172of the corresponding apparatus100.

The interior space of the suction channel300is connected to the interior space106of the treatment chamber104by way of a through-opening302.

The through-opening302may be configured, e.g., as a gap extending in a longitudinal direction.

The through-opening302may be closeable by means of a cover304.

The cover304may be pivotably arranged on a wall306of the suction channel300by means of a hinge.

The treatment chamber104may further comprise a guide308on which the lid116of the treatment chamber104is displaceably guided, preferably in the horizontal direction.

Provision may hereby be made that a lower rim region310of a substantially vertically oriented side wall312of the lid116is accommodated in an interior space314of the guide308.

The guide308may have a substantially U-shaped cross-section, for example.

In order to perform a validation of the passivation method, which is performed with any one of the embodiments described above of an apparatus100for passivating workpieces102, a test piece316can be used, which is depicted in a schematic longitudinal section inFIG.9.

The test piece316is affixed to a point of a goods carrier receiving the workpiece102to be passivated, which is representative for the effectiveness testing.

The geometry of the test piece316is selected such that it corresponds to the most difficult to passivate workpiece102of the workpiece variants of a production line that are to be passivated by means of the respective apparatus100for passivating workpieces102.

For this purpose, the test piece316is preferably provided with at least one cavity318and with at least one capillary320, preferably a plurality of capillaries320.

A measuring strip322is arranged in at least one of the cavities318and/or in at least one of the capillaries320.

The measuring strip322is provided with a sensitive surface324that reacts to the passivating agent (so-called “smart surface”).

The sensitive surface324of the measuring strip322changes due to contact with the passivating agent.

For example, the measuring strip322may be a pH value measuring strip, which changes its color through contact with the passivating agent.

Alternatively or in addition hereto, provision may also be made that the measuring strip comprises a stainless steel material, for example the stainless steel material with the workpiece number 1.4401. The passivating effect exerted on the stainless steel material by the passivating agent can be measured according to one of the methods described in the standard ASTM A 967 or alternatively by XPS measurement (x-ray photoelectron spectroscopy) of the chromium content and the iron content on the surface of the stainless steel material.

The effectiveness of the passivation method is tested after completion of the passivation treatment. For this purpose, the measuring strip322is removed from the test piece316, and the change of the sensitive surface324of the measuring strip322is evaluated by means of a suitable testing method.

In order to be able to insert the measuring strip322into a cavity318or into a capillary320of the test piece316in a simple manner, the test piece316is preferably of multi-part configuration, wherein at least two parts of the test piece316abut against one another along an abutment surface326when the test piece316is composed of a plurality of parts.