GB2455802A - Method and device for increasing corrosion resistance of a steel component - Google Patents

Abstract

A method of increasing corrosion resistance of a nitrocarburised or nitrocarburised and oxidised surface of a steel component 2 comprises an initial step of degassing the surface of the component in a vacuum at a temperature at least equal to 25{ C. In a subsequent step an anti-corrosion agent 14 is applied to the surface of the component for sealing thereof. The anti corrosion agent can be an oil, wax or polymer. A device 1 for carrying out this method comprises a heatable vacuum container 3 in which the component can be exposed to a vacuum at the stated temperature and an immersion container 13 which contains an anti-corrosion agent and in which the degassed component surface can be brought into contact with the agent. The immersion container can be situated within or in close proximity to the vacuum container.

Description

METHOD AND DEVICE FOR INCREASING CORROSION RESISTANCE OF A

NITROCARBURISED OR NITROCARBURISED AND OXIDISED

SURFACE OF A STEEL COMPONENT

The present invention relates to a method and a device for increasing the corrosion resistance of a nitrocarburised or nitrocarbunsed and oxidised surface of a component consisting of steel.

Nitrocarburising has been used on a large scale on a worldwide basis for decades to increase wear protection of steel components subjected to strong loading. In methods of that kind, which are described in, for example, Liedtke, D. et al, Wármebehandlung von Eisenwerkstoffen -Nitrierten und Nitrocarbuneren', Expert-Verlag, Renningen, 2006, a wear-resistant iron nitride layer of approximately 20 micrometres thickness is produced on a component by treatment of the component in a gas giving off nitrogen, usually an ammonium atmosphere, in a plasma containing nitrogen or in molten salts giving off nitrogen, usually melts of potassium cyanate, sodium cyanate and soda, at temperatures around 580° C. This so-called white layer consists of the iron nitrides FeRN, wherein x is 2 to 3 (epsilon-nitride) and Fe4N (gamma-nitride). In the case of alloyed steels the white layer also contains the nitndes of the alloying elements in submicroscopic fine distribution, such as, for example, chromium nitride or vanadium nitride. The white layer is extremely hard and resistant to wear, particularly to adhesive wear, and has less a metallic character and more the character of a nitride ceramic. The outer boundary zone of this wear inhibiting layer always includes, in a region of 10 to 50% referred to the total layer thickness, a border of pores penetrating the nitride. The creation of this pore border is mentioned in, for example, Prenosil, BI., Härterei Technische Mitteilungen 23', (1968), pages 216 -226; Mittemeijer, E., Somers, M., Verbindungsschichtbildung beim Nitrieren und Nitrocarbuneren', Hârterei-Techn. Mitt. 51, (1996), 3, pages 162 -169; and Liedtke, D, et al, Warmebehandlung von Eisenwerkstoffen -Nitrieren und Nitrocarburieren', Expert-Verlag, Renningen, 2006, pages 21 -26. A schematic illustration of a nitrocarbunsed layer on steel is illustrated in Fig. 1.

Nitrocarbunsed layers on unalloyed steel are somewhat more corrosion-resistant than the base material, but the increase in corrosion resistance is not significant. In a standard salt spray test with diluted cooking salt solution according to DIN 50 021 [DIN 50 021, Beuth-Verlag, Burggrafenstr. 6, 10787 Berlin (1988)] it can be assumed that, for example, components of the steel 42CrMo4 are rusty even after the shortest period of time, generally I to 4 hours. Nitrocarbunsed and steel without further treatment would begin to rust after approximately 12 to 24 hours.

Some considerable time ago it was discovered that by an additional treatment of nitrocarburised components in an oxidising salt melt, for example, on the basis of alkaline nitrite, alkaline nitrate, alkali hydroxide or alkali carbonate, an oxidation of the nitrocarburised layer occurred in the region of the porous zone. Oxidation can be camed out in a gas atmosphere with superheated steam or with carbon dioxide. In that case black iron oxide Fe304 which fills out the pore border and causes passivation of the surface forms. In this manner a significant increase in corrosion resistance is achieved additionally to wear resistance and the components gain a quite decorative black surface.

Examples thereof are given in DE 25 14 398, DE 19607 369 Al, EP 667 401 Al, EP 638 661 Al and EP 524 037 BI. For example, a component of the steel 42CrMo4, which was nitrocarburised and oxidised and the surface of which is dosed and not disrupted by fissures, faults or distortions, survives without rusting in the salt spray test for over 500 hours, usually even more than 1,000 hours.

Nitrocarburisation with postoxidation for increasing wear resistance and corrosion resistance is used on a large scale under the designation Q method or QPQ method (registered trade mark of the company Durfemt CmbH) for treatment of many components liable to wear or corrode, for example gas compression springs, hydraulic cylinders, ball pins, screen wiper axles, valves, weapon components such as locks, runners and much more. This is described in more detail in Wahl, G., Verbesserung der Bauteileigenschaften durch Nitrieren im Salzbad, Zwischenbearbeiten und Oxidieren', Tech. Communications, Durferrit GmbH, Mannheim/Hanau, 1982; WahI, G., Korrosionsbestandige Oberflächen durch Salzbadnitrieren', in Fachberichte Huttenpraxis -Metallweiterverarbeitung, Vol. 12, 1981.

Passivation of the component surface by the black iron oxide Fe304 is, however, always subject to small errors in practice. For example, the oxidation may not be perfect at individual locations of the component surface or small cracks form during cooling of the components in water or filling of the pore border with iron oxide (Fe304) is not complete or, apart from Fe304, the undesired Fe203 -which is porous and does not offer any corrosion protection -forms. In addition, attention has to be given to the surface property of the base material. Thus, punched, cold-formed or only coarsely ground components have a large number of surface defects, so that components of that kind usually cannot be rendered sufficiently corrosion-proof by nitrocarburisation and subsequent oxidation alone.

The described faults are in general statistically distributed over the surface of the treated components and are, for example, visible as spot corrosion when testing the component in a corrosive environment such as, for example, the salt spray test according to DIN 50021.

However, there is now a technical requirement for every component in the course of the salt spray test to exceed a specific minimum test duration which in practice corresponds with a specific minimum service life of the component under conditions of use in daily life.

There have therefore been experiments to conceal or remedy the inevitable fault locations on nitrocarburised and postoxidised components by additional treatment with hardenabie polymers, waxes, oils, plastics and the like, such as described in, for example, EP 524 037, EP 497 663 and EP 472 957. All of these methods can be regarded as embraced by the general designation of subsequent sealing of fault locations. The starting point of these methods is that the corrosion-protective substances, such as polymers, waxes or oils, are mechanically applied by immersion or spraying or by means of brush, cloth or the like. An improvement in corrosion resistance is also achieved by the application of anti- corrosion agents to nitrocarbunsed and oxidised layers. However, it is disadvantageous that the substances have to be applied in a substantial amount in order to even be effective for corrosion protection. As a consequence, a number of other disadvantages can anse. Thus, for example, waxy or highly viscous anti-corrosion agents are indeed effective, but form on the components a closed film of several micrometres thickness which in most applications is rapidly abraded away or is even unacceptable with respect to the use of the components, for example because the substance is incompatible with sealing materials (for example in gas compression springs, hydraulic cylinders, etc.).

Other oils, waxes and polymers produce sticky, unattractive surfaces when applied In the amount necessary for enhanced corrosion protection. Moreover, other agents harden relatively satisfactorily, but form only thin films and can be wiped off easily or run off the component surface by themselves and de facto produce no or at least no appreciable improvement in the corrosion resistance of the component.

The invention therefore has the object of providing a method and a device by which the corrosion resistance of nitrocarburised or nitrocarbunsed and oxidised surfaces of steel components can be increased.

According to a first aspect of the present invention there is provided a method of increasing the corrosion resistance of a nitrocarburised or nitrocarburised and oxidised surface of a component consisting of steel, comprising the steps of degassing the surface of the component in a vacuum at a temperature at least equal to 25° C and subsequently applying an anti-corrosion agent to the surface of the component for sealing thereof.

According to a second aspect of the invention there is provided a device for increasing the corrosion resistance of a nitrocarburised or nitrocarbunsed and oxidised surface of a component consisting of steel, comprising a heatable vacuum container in which the surface of the component can be degassed in a vacuum at a predetermined temperature and an immersion container which contains an anti-corrosion agent and in which the surface of the component after degassing can be brought into contact with the agent in vacuum.

In the method according to the invention an increase in corrosion resistance of nitrocarbunsed or nitrocarbunsed and oxidised surfaces of steel components is achieved.

For this purpose initially the surfaces of the components are degassed in a vacuum at temperatures greater than or equal to 25° C. In a subsequent step a corrosion protection agent is coated on the surfaces of the component for sealing thereof. The device for performance of this method comprises a heatable vacuum container in which the components are arranged at predetermined temperatures in a vacuum for degassification of the surfaces thereof. In addition, the device comprises an immersion or dip container which contains a corrosion protection agent and in which the components can be brought into contact, by surfaces degassed in vacuum, with the corrosion protection agent.

The corrosion resistance of steel components having either nitrocarburised or nitrocarbunsed and oxidised surfaces can unexpectedly be significantly increased by means of the method or device according to the invention.

In the case of components with nitrocarburised and oxidised surfaces the corrosion resistance thereof is already increased by the oxidation of the surfaces. This already increased corrosion resistance can be still further significantly increased by the method according to the invention.

Moreover, the method according to the invention can also be used generally with steel components having surfaces which are solely nitrocarburised, thus not oxidised. In this instance the method represents an alternative to increasing corrosion resistance by oxidation of the surfaces. Oxidation in this manner of the components may not be desired if, for example, the black surface coloration caused by oxidation is to be avoided.

It is important for increase in the corrosion resistance of the components by the method that the components are exposed to a vacuum at elevated temperature before the respective anti-corrosion agent is applied to the component surface or surfaces. Oil, wax or, especially, liquid polymer is preferably used as anti-corrosion agent.

The components are advantageously treated in a vacuum at pressures of 0.01 to 800 millibars, preferably 0.1 to 5 millibars, and at temperatures of 15 to 300° C, preferably 50 to 110° C, for a time period of 5 to 120 minutes, preferably 20 to 30 minutes, and only thereafter is the oil, wax or low viscosity polymer as anti-corrosion agent applied by dipping, spraying or mechanical coating.

The reason for the increase, which is achieved by the method, in the corrosion resistance of steel components having nitrocarbunsed or nitrocarburised and oxidised surfaces is a displacement mechanism.

In the case of steel components with nitrocarburised surfaces the boundary zones of these surfaces generally include a pore zone or margin, wherein the iron oxide (Fe304) formed in the pores of the nitride layer itself still has a residual porosity. These and other pores which were not filled by iron oxide always contain gases or liquids from the original nitrocarburisation process, primarily moisture residues.

Corresponding conditions are also present with steel components with nitrocarbunsed and oxidised surfaces. During oxidation of nitrocarburised components in a gas atmosphere the oxidation is undertaken in the pore border by application of steam and CO2. The subsequent cooling is carried out under protective gas, in air or in an individual case in special quenching oils, aqueous polymer quenching media or in water. In the case of salt-bath nitrocarbunsation with subsequent oxidation in a nitrate melt the components are finally quenched in water and washed. In all these processes water inevitably penetrates into the fine pores not only In the ferrous oxide, but also in cracks, fault locations or free pores of the nitride layer.

If, for the purpose of increasing corrosion resistance, anti-corrosion agents in the form of oils, polymers or waxes are now applied to the surfaces of these components by dipping or spraying these surfaces or by simple mechanical coating of material then there is as good as no substance transfer, since the gas or water contained in the pores is fixed by very high capillary forces. Even oils with very high creep capability cannot displace the water contained in the finest pores, so that only an insufficient sealing is necessarily achieved by the anti-corrosion agent.

In the case of a method exemplifying the invention, however, it is achieved by introduction of the components into a vacuum that the water escapes from the pores of the surface layers of the components.

Thus, through this treatment the pores of the nitrocarburised or nitrocarbunsed and oxidised surface layers of the components are, just as finest fissures and defect locations in the iron oxide, opened and the water and/cr adsorbed gases are extracted. The anti-corrosion agent can thereafter penetrate and produce the desired sealing of the surface.

As a consequence of penetration of the anti-corrosion agent into the pores of the pore border of the surface layers of the components there is not only improvement in the quality of sealing of these layers, but also creation of further advantageous effects.

A significant advantageous effect consists in that a wide spectrum of anti-corrosion agents can be employed for sealing the surface layers.

In particular, it is now possible to make use of anti-corrosion agents which also have other desired characteristics, thus, for example, those which form only a thin film capable of being wiped off or those which harden. It is even possible to wipe off or blow off -by means of compressed air -the anti-corrosion agent (oil, wax or thinly viscous polymer) from the surface after the treatment without thereby negating the additional corrosion protection.

In performance of a method exemplifying the invention it is important that the components after introduction into the vacuum are sealed by the anti-corrosion agent without the components being exposed beforehand to a moist or humid environment.

In that case it is possible, but not essential, to carry out the application of the anti-corrosion agent in vacuum, for example by lowering the charge into an immersion container which is accommodated in the interior of a vacuum container and which contains the appropriate anti-corrosion agent.

However, it is equally possible and in practice simpler to remove the vacuum by admitting clean dry air or nitrogen or argon into the vacuum container and immediately thereafter transferring the components to an external immersion container with the suitable anti-corrosion agent.

Examples of the method and embodiments of the device of the present invention will now be more particularly described with reference to the accompanying drawings, in which: Fig. I is a diagram showing the make-up of a nitrocarbunsed layer on steel; Fig. 2 is a schematic elevation of a first device embodying the invention in which a first example of a method exemplifying the invention can be carried out; Fig. 3 is a schematic elevation of a second device embodying the invention, in which a second method exemplifying the invention can be carried out; and Fig. 4 is a diagram showing the exposure time of screen wiper axles in a salt spray test according to DIN 50021.

Referring now to the drawings, Figs. 2 and 3 respectively show two embodiments of a device I for increasing corrosion resistance of nitrocarbunsed or nitrocarbunsed and oxidised surfaces of components 2 consisting of steel. The device according to Fig. 2 comprises a vacuum container 3 in the form of a vacuum retort. This vacuum retort is closable by a cover 4, the seam locations between the vacuum retort and the cover 4 being sealed by 0-rings 5. The thus-formed seal can, if needed, be cooled. A vacuum pump 6 coupled by way of a first blocking valve 7 with the vacuum container 3 is provided for generating a vacuum in the interior space, i.e. vacuum chamber, of the vacuum container 3. The pressure within the vacuum container 3 can be measured by a vacuum manometer 8. A safety valve 9 is provided as safety element for pressure anomalies in the vacuum container 3.

A retort oven 10 with heating coils is provided for heating the vacuum container 3.

Heating of the vacuum container 3 can generally be carried out electrically from outside by radiation heat, steam, heat carrier oil or the like. An internally located heating by heating coils, graphite films or metal films is also conceivable.

The components 2 to be treated are placed on a frame 11 within the vacuum container 3.

In a first method step the components 2 in the vacuum container 3 are exposed to a vacuum, wherein the pressure in the vacuum container 3 lies in a range between 0.01 and 800 millibars. The interior space of the vacuum container 3 is heated by the retort oven 10 to temperatures in a range between 25 and 3000 C, preferably between 50 and 1100 C. The components are exposed to the vacuum for a time period between 5 and 120 minutes, preferably between 20 and 30 minutes. As a result, water can escape from the pores of the pore zones of the surfaces of the components 2, such as illustrated in, for example, Fig. 1.

Thereafter, through opening a second blocking valve 12, gas, particularly dry air, argon or nitrogen, is introduced into the vacuum container 3 and the vacuum thus removed. This gas prevents renewed penetration of water into the pores of the pore zones of the components 2.

Immediately thereafter the vacuum container 3 is opened by removal of the cover 4 and the frame 11 with the components 2 is introduced by way of transport means (not illustrated), such as crane, into an immersion container 13 in which an anti-corrosion agent 14 is disposed. Oils, waxes or, in particular, liquid polymers can be used as anti-corrosion agent 14.

The surfaces of the components 2 are sealed by the contact with the anti-corrosion agent 14.

Fig. 3 shows a further embodiment of a device I for increasing the corrosion resistance of components 2 of steel with nitrocarburised or nitrocarburised and oxidised surfaces. The device 1 of Fig. 3 substantially corresponds with the embodiment according to Fig. 2 with respect to the construction of the vacuum container 3 and the heating thereof.

The device I of Fig. 3 differs from the embodiment of Fig. i in that the immersion container 13 is integrated in the vacuum container 3 itself. As apparent from Fig. 3, the lower part of the vacuum container 3 forms the immersion container 13. This is separated by a partition wall 15 from the upper part of the vacuum container 3, which forms the vacuum chamber. Fig. 3 shows the vacuum container 3 with the partition wall 15 closed.

The frame 11 with the components 2 in that case stands on the partition wall 15. The anti-corrosion agent is located in the immersion container 13 below the partition waIl 15.

The closed partition wall 15 forms a gas-tight separation between the vacuum chamber and the interior of the immersion container 13.

Analogously to the embodiment according to Fig. 2 a vacuum is generated in the vacuum chamber of the vacuum container 3 in a first method step. The devices provided for this purpose, namely the vacuum pump 6, vacuum manometer 8 and the safety valve 9 are constructed analogously to the embodiment according to Fig. 2. Similarly, the duration of generation of the vacuum, the pressure within the vacuum chamber and the heating of the vacuum chamber correspond with the embodiment according to Fig. 2.

After, through the vacuum treatment of the components 2, the water has been removed from the pores of the pore zones in the nitrocarburised or nitrocarburised and oxidised surface layers, the contacting of the components 2 with the anti-corrosion agent 14 is carried out for sealing the component surfaces. For this purpose the partition wall 15 is opened by means of a setting element and the frame 11 together with the components 2 is immersed in the anti-corrosion agent 14. In this case the application of the anti-corrosion agent 14 takes place still in vacuum. Thus, in the case of the device I according to Fig. 3 the blocking valve 2 required in the device I according to Fig. 2 is redundant, since gas does not have to be introduced for removing the vacuum in the vacuum chamber.

Examples of the method of the invention are described in the following: I0

Example 1:

Thirty-three items of hose clamps (hose springs) of the material Cf 76 were nitrocarburised for 90 minutes in a TFI (Registered Trade Mark) nitrocarbutising salt bath with a content of 36% cyanate, 27% carbonate and 3% cyanide as well as a ratio of cations K+ to Na+ of 80:20 at 580° C, subsequently oxidised for 20 minutes in an oxidising ABI salt bath consisting of 35% sodium nitrate, 2% sodium nitrite and the rest sodium carbonate and potassium hydroxide at 390° C, thereafter polished by means of glass-bead blasting and oxidised a further 15 minutes in the same bath, thereafter quenched in water and cleaned with tap water and finally with desalinated water. This manner of procedure is known as a QPQ method, which is described in Wahl, G., Verbesserung der Bauteileigenschaften durch Nitrieren im Salzbad, Zwischenbearbeiten und Oxidieren', Tech. Communications, Durfemt GmbH, MannheimlHanau, 1982, and Wahi, G., Korrosionsbestandige Oberflâchen durch Salzbadnitrieren', Fachbenchte Huttenpraxis -Metaliweiterverarbeitung, Vol. 12, 1981.

Eleven items of the thus-nitrocarburised workpieces without sealing were tested in the salt spray test according to DIN 50 021. All eleven hose clamps exhibited strong corrosion points already after 24 hours of test time.

Eleven items of the thus-nitrocarburised hose clamps were, after washing, dried by hot air and dipped into a water-displacing corrosion protection oil of the type Kaltol 4214 HFAF of the company Houghton, left to dry for 24 hours and subsequently tested in the salt spray test. Approximately half of the thus-treated components exhibited first corrosion points after approximately 48 hours; the mean test endurance time (attained test endurance time of all eleven components divided by number, here 11) was 72 hours.

Eleven items of the thus-nitrocarburised hose clamps were, after washing, dried and initially treated in vacuum in a metal retort at an absolute pressure of 0.2 millibars for 20 minutes at 80° C. Thereafter the vacuum was relieved by dry air, the components removed and then directly immersed in the same anti-corrosion agent (KaltoI 4214 IIFAF) and allowed to drip dry. Thereafter the surface was wiped free of oil residues by clean cloths and the thus-treated components were subsequently tested in the salt spray test.

The mean test endurance time achieved in that case (attained test endurance time of all eleven components divided by number, here 11) was in this case 185 hours.

Result: The achieved improvement in the service life of hose clamps in the salt spray test was 156% by comparison with the treatment without vacuum.

Example 2:

Twenty-five rods of Ck 15 with the dimensions 10 millimetres diameter, 120 millimetres length and a surface roughness of Rz = 0.5 to 1.15 micrometres were nitrocarbunsed for minutes in a TFI (Registered Trade Mark) salt bath at 580° C, thereafter oxidised in an oxidising ABI salt bath at 390° C for 20 minutes and quenched in water. The rods were subsequently blasted with glass beads, postoxidised again for 25 minutes in the ABI bath and cooled in water. This manner of procedure is known as QPQ method.

Five of the thus-treated rods were tested in the salt spray test according to DIN 50021.

The mean endurance time was 432 hours.

Ten of the thus-treated rods were immersed in the rust protection agent KaItol 4214 HFAF of the company Houghton, the oil was wiped off and the rods tested in the salt spray test.

The mean endurance time of the rods was 487 hours.

The remaining ten of the nitrocarburised and oxidised rods were degassed for 30 minutes in a metal retort at 0.2 millibars and at 100° C and only thereafter immersed in the rust protection agent Kaltol. The excess oil was stripped off and the components tested in the salt spray test according to DIN 50021. The endurance time of all ten rods was above 696 hours, at which point the test was broken off.

Result: The mean corrosion resistance was increased by the treatment in the method exemplifying the invention from 487 hours to above 696 hours.

Example 3:

Sixty items of drive axles for screen wipers were treated in a TF1 (Registered Trade Mark) salt bath with a cyanate content of 36.9%, a cyanide content of 4.9% and 100 ppm of iron for 90 minutes at 560° C. The parts were subsequently oxidised in an ABI bath with 14.6% sodium nitrate, 1.2% sodium nitrite and 32.7% sodium carbonate at 4000 C of 20 minutes. Quenching was carried out in water. This manner of procedure is known as a Q method.

Twenty items of the Q-treated axles were dried and thereafter subjected directly and without further subsequent treatment to the salt spray test according to DIN 50 021. All twenty rods exhibited initial rust points already after 24 hours.

Twenty items of the Q-treated axles were dried and sealed in an approximately 16% emulsion of the rust protection agent Corolac (Registered Trade Mark) of the company Castrol at 400 C for 20 minutes. After removal, the components were dried for 24 hours in air and subsequently subjected to the salt spray test. The mean endurance time of the thus-treated axles was 362 hours.

The remaining twenty items of the Q-treated axles were, prior to treatment with the rust protection emulsion Corolac (Registered Trade Mark), degassed in a steel retort at 12 millibars and 155° C for an hour. Sealing was subsequently carried out by Corolac (Registered Trade Mark) as described above. The mean endurance time of the thus-treated axles was 533 hours.

The endurance times of the individual axles are illustrated in Fig. 3.

Result: In the case of the Q treatment of screen wiper drive axles an improvement in corrosion resistance of 47% was achieved by vacuum degassing, by way of the method exemplifying the invention, prior to sealing with a rust protection agent.

Example 4

In a modification of Example 3 in which the drive axles for screen wipers were only nitrocarbunsed and quenched in water, but not postoxidised, an improvement in corrosion resistance of 10% was obtained by vacuum degassing by a method exemplifying the invention.

Claims (18)

1. A method of increasing the corrosion resistance of a nitrocarburised or nitrocarburised and oxidised surface of a component consisting of steel, comprising the steps of degassing the surface of the component in a vacuum at a temperature at least equal to 25° C and subsequently applying an anti-corrosion agent to the surface of the component for seating thereof.

2. A method as claimed in claim, wherein the component is exposed to the vacuum for a period of 5 to 120 minutes.

3. A method as claimed in claim 2, wherein the component is exposed to the vacuum for a period of 20 to 30 minutes.

4. A method as claimed in any one of the preceding claims, characterised in that the step of degassing comprises producing a vacuum at a pressure between 0.01 and 800 millibars.

5. A method as claimed in any one of the preceding claims, wherein the step of degassing is carried out at a temperature between 25 and 3000 C.

6. A method as claimed in claim 5, wherein the step of degassing is carried out at a temperature between 50 and 110° C.

7. A method as claimed in any one of the preceding claims, wherein the anti-corrosion agent comprises an oil, a wax or a polymer.

8. A method as claimed in any one of the preceding claims, wherein the step of applying the anti-corrosion agent comprises exposing the component thereto in a vacuum.

9. A method as claimed in any one of the preceding claims, comprising the further step between the steps of degassing and application of anti-corrosion agent of exposing the component to a gas atmosphere.

10. A method as claimed in claim 9, wherein the gas atmosphere comprises argon nitrogen or dry air.

11. A device for increasing the corrosion resistance of a nitrocarburised or nitrocarburised and oxidised surface of a component consisting of steel, comprising a heatable vacuum container in which the surface of the component can be degassed in a vacuum at a predetermined temperature and an immersion container which contains an anti-corrosion agent and in which the surface of the component after degassing can be brought into contact with the agent in vacuum.

12. A device as claimed in claim 11, wherein the immersion container is arranged within the vacuum container.

13. A device as claimed in claim 12, wherein the immersion container is separated from a vacuum chamber within the vacuum container by an openable and closable partition wall.

14. A device as claimed in claim 13, comprising setting means for opening and closing the partition wall.

15. A device as claimed in claim 13 or claim 14, wherein the vacuum chamber is gastightly separated from the interior of the immersion container when the partition wall is closed.

16. A device as claimed in any one of claims 13 to 15, wherein the partition wall when open permits movement of the component from the vacuum chamber to the immersion container.

17. A device as claimed in claim 11, wherein the immersion container is arranged outside the vacuum container.

18. A device as claimed in claim 17, comprising transfer means for transferring the component from the vacuum container to the immersion container.