GB1587472A - Processes for protecting chromium plated surfaces - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO PROCESSES FOR PROTECTING CHROMIUM PLATED SURFACES (71) We, VEREINIGTE OSTER REICHISCHE EISEN- UND STAHL WERKE - ALPINE MONTAN AKTIENGESELLSCHAFT, an Austrian Company, of Friedrichstrasse 4, A-1011 Vienna, Austria, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: Surfaces of steel articles are frequently chromium plated for achieving a protection against corrosion. This is the case, for example, with the surface of the inner piston of mine props which are guided in a telescoping manner within the outer cylinder with interposition of a sealing.In such cases, frequently a hard chromium plating is used and damaging of such a hard chromium plating is of particular disadvantage in view of impairing the tight seal of the inner piston within the outer piston which results in the danger of leakage of pressurized fluid and, as a consequenc, of a reduction in height of the mine prop. Such a reduction in height of mine props represents a severe danger because such a reduction in height of one or more mine props has as a consequence a higher roof pressure acting on the other mine props which then can become damaged by overload. Chromium platings, i.e.

bright chromium platings as well as hard chromium platings, do, however, not provide a complete protection against corrosion. It has been found that chromium platings which were produced to be free of cracks show, after some time, microcracks generated by mechanical or thermal stress.

Such cracks provide the areas of attack for corrosion of the base metal underlying the chromium plating. The result is so-called pitting of the base metal by galvanic local elements formed.

According to the invention, there is provided a process for protecting chromium plated surfaces of steel articles, particularly of mine props, against corrosion, characterized in that the chromium plated surfaces are impregnated under vacuum with a corrosion protection agent and are heated prior or during impregnating to a temperature of at least 100 C. A hard chromium plating contains hydrogen and this hydrogen does produce additional cracks by effusion and thus contributes to the corrosive attack. By heating the chromium plated article the hydrogen does diffuse out of the chromium plating. By such a heating the hardness will be reduced particularly when applying higher temperatures; such reduction in hardness does, however, only immaterially affect the wear properties.The formation of microcracks is promoted by stresses prevailing within the chromium plating. Such internal stresses become built up when using a chromium plated article, particularly a mine prop, after a more or less prolonged time interval. Chromium has a lower heat expansion than steel. The coefficient of heat expansion of chromium is 8.05x10-6 whereas the coefficient of heat expansion of a steel having, for example, a carbon content of 0.2 to 0.6 percent by weight is 12.6x10-6. By heating the chromium plated article the chromium plating is subjected to tension forces which produces at an early time those microcracks which would be formed at any case after a more or less prolonged time interval.By producing microcracks already when treating the chromium plated article (and not at a later time) there is provided the possibility that the corrosion protection agent enters the microcracks during the impregnating step and by applying reduced pressure the penetration of the corrosion protection agent into the microcracks is guaranteed or at least enhanced. By the stresses occurring during the subsequent operation the number of microcracks can not be increased and in view of these microcracks being filled with corrosion protection agent the danger of any corrosive attack on the base material via the microcracks is avoided.Heating shall conveniently be effected to a temperature within the range of 100 to 2500C, presently 100 to 110 C, because at these temperatues the hydrogen does diffuse for a major part out of the chromium layer and because the heat expansions occurring at these temperatures are sufficient to produce such a number of microcracks which is not or not substantially increased by the stresses under operating conditions. A more pronounced cracky chromium plating as produced at higher heat expansions would have an unfavourable effect. A vacuum of 0.01 to 0.005 bar shall preferably be applied.

For obtaining the effect to be achieved by the inventive process the chromium layer shall not be heated to a higher temperature than the steel material underlying said chromium plating. Therefore, and according to a preferred embodiment of the invention, heating is effected by induction heating or in the case of hollow articles carrying an outer chromium plating as is, for instance, the case with tubular mine props by introducing a heated fluid, particularly hot air, into the cavity.

The corrosion protection agent used is preferably a commercially available corrosion protection oil or wax based paraffin or silicone which has a viscosity of 100 cSt to 1000 cSt, preferably 200 cSt to 500 cSt (cSt = Zentistokes at 20"C).

The process is preferably performed such that the chromium plated articles are put into a closed vessel and that the vessel is evacuated during or after heating and is connected to a vessel containing the corrosion protection agent at a location lying beneath the upper level of the corrosion protection agent. When heating of the chromium plated article is effected prior to evacuating, longer time invervals must be used and the time interval to be selected is primarily dependent on the thickness of the chromium plating. In general, a treating time of 6 to 24 hours will be sufficient for operating temperatures within the range of 100 to 250"C for chromium plating having a thickness of 50 to 100 Fm. This treating time can be reduced by effecting heating already under vacuum.Also in this case the required time is dependent on the thickness of the chromium plating. When applying the corrosion protection agent under vacuum, this can be, for example, also be effected by an immersion impregnation.

All corrosion protection agents which are suitable for the process according to the invention have the common property of a higher hydrophobic capacity and of a good adhesion property on metal surfaces. These conditions are fulfilled by a great number of corrosion protection oils and waxes. When using corrosion protection waxes, keresines and waxes obtained in coal refinement for impregnating the articles by immersion under vacuum, the impegnated props are freed of the adhering excessive wax after the impregnation step by a polishing operation performed with a cloth disc.

The invention will be further described, by way of example, with reference to the accompanying drawing showing an embodiment of a device for performing an embodiment of the process according to the invention.

The drawing schematically represents a plurality of tubular containers 1 adapted to be heated by induction. These containers 1 are provided at the upper end with cover lids 2 which enable the containers to be closed in an air-tight manner. At the lower ends of the containers 1, there are provided perforated plates 3 supporting the mine props 4 shown in the drawing. These perforated plates 3 allow the impregnating agent S contained within the storage tank 6 to move in upward direction via conduits 7 after having opened the valves 8 shown. The upper ends of the containers 1 are widening in upward direction for facilitating introduction of several props. Vacuum conduits 9 are connected to the upper ends of the containers 1 and connect the containers 1 via a magnetic valve 10 with a vacuum pump not shown. These vacuum pumps can be mechanical vacuum pumps, for example rotary piston pumps.The ends of a pressure equalizing conduit 11 are connected to the conically widening portion of each container for supplying air of atmospheric pressure or, if desired of increased pressure of for example 5 to 10 atmospheres into the evacuated containers after having actuated a magnetic valve 12. During equalizition of pressure, the valve 10 leading to the vacuum pump and the valves 8 leading to the storage container 6 for the corrosion protection agent S are closed.

The storage container 6 containing the corrosion protection agents is provided with an electrical heating means 13 so that the corrosion protection agent can also be kept at the desired temperature and can enter the containers via the conduits 7, the valves 8 and the perforated plates 3 in a liquid condition. The temperature maintained within the storage container 6 is dependent on the type of the corrosion protection agent used. When using corrosion protection oils, the impregnation step can principally be performed at room temperatures, i.e. without additional heating by the electric heating rods 13. For reducing the viscosity of the oil, also in this case a slightly increased temperature of, for example 40 to 70"C is preferred.When using corrosion protection waxes, the temperature used is preferably 20 to 300C above the flow point (pour point) of the wax.

After having opened the valves 8, the corrosion protection agent acts on the mine props heated to the impregnating temperature. For heating the mine props, the containers 1 are provided with induction coils 14 which, for example, are formed by hollow copper conductors though which cooling water is circulated and which are connected to the required alternating current source. The frequency of the alternating current is, as a rule, 50 cycles per second. The individual induction coils of adjacent containers can, as is shown in the drawing, be connected in series, noting that the current and, respectively, the voltage is determined by the required heating power.

After having obtained the heat treatment temperature and the impregnating temperature, respectively, the electrical heating is reduced or, respectively, interrupted.

For heat treating the mine props, the mine props can be enclosed in suitably shaped wire baskets and heated in forced air circulating ovens not shown in the drawing.

Of course, heating can also be effected within the container of the immersion impregnating device, noting that heating can be effected within the air-filled container or within the evacuated container.

If the mine props are heated within the forced air circulated oven it is required to immediately transfer the mine props after the selected treating time and thus with prevention of any substantial cooling into the containers 1 and to evacuate these containers. As soon as the desired final pressure of approximately 10-2 Torr has been attained, the magnetic valves 8 are actuated and corrosion protection agent is allowed to enter the containers until the mine props are completely immersed within the corrosion protection agent. Subsequently, the magnetic valves 8 are again closed.

Subsequently the magnetic valve 12 is opened and the space above the mine props is filled with air via the pressure equalizing conduit 11. During such pressure equalization the oil or molten wax penetrates the network of cracks present in the chromium layers. This penetration can be further accelerated by suitably changing the position of the valve 12 for increasing the air pressure acting above the mine props to a value within the range of 5 to 10 atmospheres. After approximately 5 minutes, the containers 1 are pressure-released by establishing the original position of the valve 12.

After having released the pressure, the containers can be opened by removing the cover lids 2, and by actuating the valves 8 the corrosion protection agent can flow back into the storage container 6. Then, the prop is lifted out of the container by means of a crane and the corrosion protection agent adhering to the props is removed. For removing excessive corrosion protection agent absorbent paper can be used.

Chromium plated mine props of the same production series were, for comparison purposes, protected with a corrosion protection oil against corrosive attack by immersion under ambient air pressure and at room temperature on the one hand and by heat treatment and subsequent immersion impregnation on the other hand. Excessive oil was removed with absorbent paper and the props were subjected to a corrosion test according to Kesternich (DIN 50 018).

When comparing the samples it was found that the props protected against corrosive attack by mere immersion at ambient pressure showed rust stains or spots already after 6 to 8 Kesternich-cycles whereas the props additionally protected under vacuum were metallic bright even after 20 Kesternichcycles and showed only isolated rust spots after 25 Kesternich-cycles. Up till now, survival of 6 to 8 Kesternich-cycles was considered a sufficient corrosion protection.

It is somewhat difficult to conclude from the test according to Kesternich to the behavior of the mine props in mine operation and it has repeatedly be shown that survival of 6 to 8 Kesternich cycles is not in the least sufficient to prevent the formation of rust spots after a relatively short time of operation under severe mine conditions. Props which have been chromium plated according to the Duplex-process (two subsequent chromium layers) to a total thickness of the chromium layer of 50 llm frequently withstand only 6 to 8 Kesternich-cycles. Props being chromium plated with a chromium layer of 50 llm and having been treated according to the preferred process unobjectionally withstand 20 Kesternich-cycles and can thus be considered as being better protected against corrosive attack. Corrosive protection of a one-step chromium plating which has been provided with microcracks by a heat treatment and which has subsequently been impregnated by immersion under vacuum is, without a doubt, much cheaper than a twin chromium plating of equal thickness which has not been impregnated. The above comparison values clearly show that the preferred process is not only cheaper than the twin chromium plating process but is also superior to this twin chromium plating process (Duplexprocess).

WHAT WE CLAIM IS: 1.Process for protecting chromium plated surfaces of steel articles, particularly of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (11)

**WARNING** start of CLMS field may overlap end of DESC **. increased temperature of, for example 40 to 70"C is preferred. When using corrosion protection waxes, the temperature used is preferably 20 to 300C above the flow point (pour point) of the wax. After having opened the valves 8, the corrosion protection agent acts on the mine props heated to the impregnating temperature. For heating the mine props, the containers 1 are provided with induction coils 14 which, for example, are formed by hollow copper conductors though which cooling water is circulated and which are connected to the required alternating current source. The frequency of the alternating current is, as a rule, 50 cycles per second. The individual induction coils of adjacent containers can, as is shown in the drawing, be connected in series, noting that the current and, respectively, the voltage is determined by the required heating power. After having obtained the heat treatment temperature and the impregnating temperature, respectively, the electrical heating is reduced or, respectively, interrupted. For heat treating the mine props, the mine props can be enclosed in suitably shaped wire baskets and heated in forced air circulating ovens not shown in the drawing. Of course, heating can also be effected within the container of the immersion impregnating device, noting that heating can be effected within the air-filled container or within the evacuated container. If the mine props are heated within the forced air circulated oven it is required to immediately transfer the mine props after the selected treating time and thus with prevention of any substantial cooling into the containers 1 and to evacuate these containers. As soon as the desired final pressure of approximately 10-2 Torr has been attained, the magnetic valves 8 are actuated and corrosion protection agent is allowed to enter the containers until the mine props are completely immersed within the corrosion protection agent. Subsequently, the magnetic valves 8 are again closed. Subsequently the magnetic valve 12 is opened and the space above the mine props is filled with air via the pressure equalizing conduit 11. During such pressure equalization the oil or molten wax penetrates the network of cracks present in the chromium layers. This penetration can be further accelerated by suitably changing the position of the valve 12 for increasing the air pressure acting above the mine props to a value within the range of 5 to 10 atmospheres. After approximately 5 minutes, the containers 1 are pressure-released by establishing the original position of the valve 12. After having released the pressure, the containers can be opened by removing the cover lids 2, and by actuating the valves 8 the corrosion protection agent can flow back into the storage container 6. Then, the prop is lifted out of the container by means of a crane and the corrosion protection agent adhering to the props is removed. For removing excessive corrosion protection agent absorbent paper can be used. Chromium plated mine props of the same production series were, for comparison purposes, protected with a corrosion protection oil against corrosive attack by immersion under ambient air pressure and at room temperature on the one hand and by heat treatment and subsequent immersion impregnation on the other hand. Excessive oil was removed with absorbent paper and the props were subjected to a corrosion test according to Kesternich (DIN 50 018). When comparing the samples it was found that the props protected against corrosive attack by mere immersion at ambient pressure showed rust stains or spots already after 6 to 8 Kesternich-cycles whereas the props additionally protected under vacuum were metallic bright even after 20 Kesternichcycles and showed only isolated rust spots after 25 Kesternich-cycles. Up till now, survival of 6 to 8 Kesternich-cycles was considered a sufficient corrosion protection. It is somewhat difficult to conclude from the test according to Kesternich to the behavior of the mine props in mine operation and it has repeatedly be shown that survival of 6 to 8 Kesternich cycles is not in the least sufficient to prevent the formation of rust spots after a relatively short time of operation under severe mine conditions. Props which have been chromium plated according to the Duplex-process (two subsequent chromium layers) to a total thickness of the chromium layer of 50 llm frequently withstand only 6 to 8 Kesternich-cycles. Props being chromium plated with a chromium layer of 50 llm and having been treated according to the preferred process unobjectionally withstand 20 Kesternich-cycles and can thus be considered as being better protected against corrosive attack.Corrosive protection of a one-step chromium plating which has been provided with microcracks by a heat treatment and which has subsequently been impregnated by immersion under vacuum is, without a doubt, much cheaper than a twin chromium plating of equal thickness which has not been impregnated. The above comparison values clearly show that the preferred process is not only cheaper than the twin chromium plating process but is also superior to this twin chromium plating process (Duplexprocess). WHAT WE CLAIM IS:

1.Process for protecting chromium plated surfaces of steel articles, particularly of

mine props, against corrosion, characterized in that the chromium plated surfaces are impregnated under vacuum with a corrosion protection agent and are heated prior or during the impregnating step to a temperature of at least 100"C.

2. Process as claimed in claim 1, characterized in that the articles provided with chromium plated surfaces are heated to a temperature of 100 to 2500C, preferably 100 to 1100C.

3. Process as claimed in claim 1 oder 2, characterized in that a vacuum of 0.01 to 0.005 bar is applied.

4. Process as claimed in any of claims 1, 2 or 3, characterized in that heating is effected by induction heating.

5. Process as claimed in claim 1 or 2, characterized in that heating of hollow articles comprising an outer chromium plating is effected by introducing a hot fluid, particularly hot air, into the cavity.

6. Process as claimed in any of claims 1 to 5, characterized in that the chromium plated articles are put into a closed vessel and that the vessel is evacuated during or after the heating step and is connected to a vessel containing the corrosion protection agent at a location lying below the upper level of the corrosion protection agent.

7. Process as claimed in any of claims 1 to 6, characterized in that the corrosion protection agent used is a commercially available corrosion protection oil or wax based on paraffin or silicone and has a viscosity of 100 cSt to 1000 cSt, perferably 200 cSt to 500 cSt.

8. Process as claimed in any of claims 1 to 7, characterized in that for protecting a surface being provided with a chromium plating having a thickness of 50 to 100 llm, the cifromium plated articles are maintained for a period of 6 to 24 hours at a temperature of 100 to 250"C whereupon the chromium plated articles are immediately subjected to an impregnation by immersion under vacuum.

9. Process as claimed in any of claims 1 to 7, characterized in that the chromium plated articles are heated under vacuum to a temperature of 100 to 250"C for a period of less than 6 hours.

10. A process for protecting a chromium plated surface of a steel article, substantially as hereinbefore described with reference to the accompanying drawing.

11. A steel article having a chromium plated surface protected by a process as claimed in any one of the preceding claims.