WO1993009900A1

WO1993009900A1 - Storage of metal particles

Info

Publication number: WO1993009900A1
Application number: PCT/US1992/010012
Authority: WO
Inventors: Irving William Wolf; Daniel Ting-Ho Lee
Original assignee: Ampex Media Corporation
Priority date: 1991-11-22
Filing date: 1992-11-20
Publication date: 1993-05-27
Also published as: DE4294047T1; JPH07503499A

Abstract

Disclosed are a method and an arrangement for storing metal particles having a sufficient level of metal oxide coating on the surface thereof in order to impart oxidative stability to the metal particles. The method and arrangement employs an oxygen/inert gas storage atmosphere having a ratio of oxygen to inert gas selected such that sufficient oxygen is present to maintain the level of metal oxide coating on the surface of the metal particles and yet is sufficiently low to prevent undesired oxidation of the metal particles.

Description

STORAGE OF METAL PARTICLES

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method of storing metal particles having a sufficient surface layer oxide coating so as to impart oxidative stability to the particles, and more particularly relates to the storage of such metal particles under conditions to prevent degradation of the desirable properties of the metal particles. Suitable metal particles employed herein include magnetic metal particles for use in magnetic recording materials, magnetic printing materials, and the like.

Description of the Related Art

Metal particles, such as finely divided iron metal, have numerous uses such as in magnetic recording media, ferrofluids, and the like. However, use of such metal particles, especially when finely divided into a powder, is impeded because of their pyrophoric characteristics, presumably due to their large surface area compared with their volume. In order to avoid the pyrophoric nature of such particles and to allow for their safe handling, such metal particles have heretofore been stabilized against undesirable oxidation, including spontaneous ignition, by passivating the particles with a thin oxide layer. See for instance U.S. Patent Nos. 3,520,676; 4,197,347; 4,318,735 and 4,420,330. After stabilization such particles can be immediately employed for their intended use. For example, when the metal particles are employed in magnetic recording media, the metal particles are mixed with a curable resin and then the mix is coated onto a non-magnetic substrate whereupon the resin is cured to bind the particles to the substrate. After curing the metal particles are generally stable to undesirable oxidation. On the other hand, until such particles are used, it is common practice to store the stabilized particles in an inert and stable environment in order to prevent undesirable oxidation and thereby preserve their desired properties. For example, one storage technique previously employed involves adding the particles to an organic solvent, such as toluene, to prevent exposure to the open atmosphere which is an oxidizing environment. However, the use of such solvents is costly and requires the use of special handling techniques when placing the particles into the organic solvent and when removing the solvent from the particles prior to their use. Moreover, this technique is practically undesirable due to the frequently inconsistent formulations of magnetic particle coatings obtained with particles stored in such solvents. This is believed to be caused either by the difficulty in removing all of the solvent from the particles before use in a magnetic coating formulation or by an irregular chemical or physical interaction of the solvent with the particles.

In another technique, the stabilized pyrophoric particles are stored in an inert atmosphere, such as a nitrogen atmosphere, which prevents their exposure to an oxidizing environment. However, when so stored, safe handling procedures are again required. A hazard occurs because there is a potential for an undetectable loss of the protective metal oxide layer during storage. It has been found that the loss of the protective layer is both time and temperature dependent. That is to say, the loss of the protective metal oxide layer, which is believed to be chemical in nature, is an increasing function of both time and temperature. Consequently as a result of such loss, there is a risk of spontaneous ignition when the previously stabilized particles are suddenly exposed to the open atmosphere, especially if the particles had been stored at high ambient temperatures and/or for long periods of time.

The loss of a protective metal oxide layer on pyrophoric metal particles can also be very troublesome when the particles are shipped by a common carrier, particularly when vibration occurs during shipment or under shipping conditions of high ambient temperature. It is quite customary for such metal particles to be shipped in a closed container under an inert atmosphere. Similar to the situation when the particles are stored, exposure to high ambient temperatures during transit will accelerate the rate of loss of the passivation layer. As an additional complication, however, movement or vibration during shipment will cause the particles to rub against each other with the possibility that a portion of the oxide layer can be scraped or worn off. This loss of the oxide layer due to vibration has the same deleterious effect, i.e., spontaneous ignition, when the container is opened under normal atmospheric conditions. Accordingly, if the bulk shipping material is not properly stabilized to reduce the vibration, the entire mass can ignite when exposed to an oxygen rich atmosphere.

In view of the above, methods are required which will ensure that stabilized pyrophoric metal particles retain their stability during storage and particularly when such storage is prolonged. Moreover, it is most desirable that such methods should be safe, convenient to use and cost effective.

Accordingly, it is an object of the present invention to provide an environment for storing pyrophoric metal particles which will substantially maintain the oxidative stability of stabilized pyrophoric particles and thus reduce the risks associated with the conventional metal particle storage techniques currently in use. It is a further object of this invention to provide a method for transporting pyrophoric metal particles which maintains the oxidative properties of the particles during shipment. These and further objects will become apparent from the following description of this invention.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, these and other objectives are achieved through a method of storing pyrophoric metal particles having a sufficient level of metal oxide coating on their surface layer that imparts oxidative stability thereto, which method preserves the oxidative stability and other properties of the particles. Metal particles for which the method of this invention has particular advantage include magnetic metal particles, especially finely divided magnetic metal particles for magnetic recording media, magnetic printing compositions, and the like. In one aspect of tius invention, the stabilized metal particles are confined in an atmosphere of a particular oxygen/inert gas mixture. The ratio of oxygen to inert gas is selected such that sufficient oxygen is present to maintain the stabilizing metal oxide surface layer while at the same time being insufficient to cause excessive oxidation of the metal particles while confined in the atmosphere of oxygen/ inert gas mixture. In another aspect of the present invention, it is directed to a technique of stabilizing and storing pyrophoric particles having either no oxide coating on the surface layer or an insufficient level of oxide coating of the parti¬ cles required to impart oxidative stability to the metal particles. In such storage technique, we

a) confine the pyrophoric particles in a desired gas mixture of oxygen and inert gas. The desired ratio of oxygen-to-inert gas is selected so that sufficient oxygen is present to cause the particles to develop a desired level of oxide coating, thereby imparting oxida¬ tive stability to the particles, but also insufficient oxygen to cause excessive oxidation of the metal particles while so confined. The time and temperature is also controlled so as to cause formation of the desired metal oxide coating.

b) After formation of the desired stabilizing level of metal oxide coating, we adjust the oxygen content of the oxygen/inert gas atmosphere to a level that is sufficient to maintain the level of metal oxide coating on the surface layer of the particles produced in a) above, but is insufficient to cause excessive oxidation of the metal particles.

In another aspect, this invention is directed to a storage arrange¬ ment comprising a container, metal particles confined in the container wherein the metal particles have a sufficient level of metal oxide coating on their surface layer so as to impart oxidative stability to the metal particles, and an oxygen/inert gas atmosphere within the container wherein the ratio of oxygen-to-inert gas within the container is selected such that sufficient oxygen is present to maintain said level of metal oxide coating on the surface layer of the particles while confined in the container and such that insufficient oxygen is present to cause excess oxidation of the metal particles while confined in the container.

It is particularly surprising that the presence of a specified amount of oxygen in the storage environment maintains the stability of the pyrophoric metal particles having a metal oxide coating on their surface because the purpose of such storage is to prevent further oxidation of such metal particles. Accordingly, heretofore such storage environments invariably did not contain oxygen. DETAILED DESOUFπON OF THE PREFERRED EMBODIMENT

The present invention is a technique that provides a surprising improvement in the storage of pyrophoric metal particles having a metal oxide coating by effiάently and conveniently preventing detrimental changes in the stabilization characteristics of the metal partides. In particular, storage of such metal partides involves the use of an atmosphere containing a specified amount of oxygen, i.e., an amount at least suffident to maintain a level of metal oxide coating on the surface of the partides so as to impart oxidative stability to the metal partides, but less than that which would cause excessive/oxidation, as runaway oxidation, of the partides.

Metal partides useful in the present invention indude any metal partides which have been (or can be) stabilized against oxidation by forming a passivating metal oxide coating on the surface layer thereof. The metal oxide forming the passivating surface coating should not exceed about 60% of all of the metal atoms, induding both metal reacted (oxides) and unreacted, in the metal partides. Metal partides requiring a metal oxide coating wherein the metal oxide exceeds about 60% of all of the metal atoms, including both metal reacted (oxides) and unreacted, in order to impart oxidative stability to such partides are not considered metal partides within the scope of this invention. Passivated metal partides for whidi the present invention is particularly suited indude magnetic metal partides, espedally magnetic metal partides for use in magnetic recording media. Such passivated metal partides indude diromium, iron, nickel, zinc, cobalt, and the like. Passivated magnetic metal partides useful in magnetic recording media indude nickel, cobalt, and iron, induding iron metal powder as well as iron metal powder compositions containing alloys of iron and nickel and/or cobalt. Preferred magnetic metal partides for magnetic recording media are iron metal powder, and typically they are finely divided adcular ferromagnetic iron powder. The partides forming the finely divided adcular ferromagnetic iron powder, generally have a size of about 2000 A (Angstroms) or less (measured along the principle particle axis).

The pyrophoric metal particles to which the storage aspects of this invention are espedally suited are partides that have been stabilized against tmdesired oxidation, especially spontaneous ignition, by forming a metal oxide coating on their surface layer. In practice, a sufficient amount of metal oxide is formed on the surface of the metal partides so as to stabilize them against undesirable oxidation by temporarily exposing them to oxidizing conditions, e.g., by momentary exposure to the open atmosphere. It is preferable that the metal oxide coating on the surface of the metal partides not be any thicker than is necessary to ensure the requisite stability to oxidizing conditions for the metal particles. In fact, when the metal oxide forming the protective surface coating exceeds about 60% of all the metal atoms, including both metal reacted (oxides) and unreacted, in the metal partides, such metal particles are considered to have excess oxidation. While the thickness of the metal oxide coating necessary to impart oxidative stability to the metal partides will vary from metal to metal, an appropriate thickness for a particular metal can be readily determined empirically by the skilled artisan. For example, iron metal powder having an iron oxide coating on its surface layer of at least about 20 A thick is stable against further oxidation, i.e., possesses oxidative stability. Preferably, iron metal powder has a passivating iron oxide coating on its surface layer of from about 20 A to about 60 A. Moreover, in the case of iron metal powder used in magnetic recording media, oxidative stability is not enhanced by increasing the thickness of the iron oxide coating on the surface of the particles of greater than about 60 A. At the same time increasing the thickness above 6θA has the detrimental result in the undesired loss of magnetization. Accordingly, any incremental increase in the iron oxide coating on the surface of the iron partides greater than about 60 A is also considered as excess oxidation and should be avoided. The thickness of the passivating metal oxide coating on the surface of the magnetic partides is conveniently deteπnined by comparison of the residual magnetization of the metal partides before and after formation of the metal oxide coating. However, it is understood that other methods for determining such thickness may provide slightly different results. Accordingly, thicknesses of somewhat less than 20 A or of somewhat greater than 60 A as determined by these other methods would be considered within range of about 20 A to 60 A specified herein, if the preferred residual magnetization comparison technique provides a thickness for the metal oxide layer of from about 20 A to about 60 A. The metal oxide thicknesses on the surface of non-magnetic partides can be determined by any appropriate technique, such as with an electron microscope and the like.

Moreover, when magnetic metal partides preferred for magnetic recording media are so stabilized, such partides should retain after stabilization a residual magnetization of at least 90 emu/g, and preferably a residual magne¬ tization of at least 106 emu/g.

As described above, during storage in an inert atmosphere, such as nitrogen, the surface oxidation characteristics of stabilized metal particles can change to the point where the partides become unstable in air at room temper- ature. Sudi occurrence can pose a hazard. Even where the oxidation change is not to the point where the partides become hazardous, the quality of the partides can diange to the point that they are unsuitable for their intended use.

Without being limited by any theory, it is believed that oxygen on the surface of the metal partides diffuses or migrates into the interior of the partides during storage. Sudi diffusion or migration results in diminishment of metal oxide on the surface of the partides. As metal oxide diminishes on the surface of the metal partides, the metal partides gradually lose the oxidative stability provided by the metal oxide surface. Continued diminishment eventually results in loss of oxidative stability of the metal particles which can lead to spontaneous ignition of the particles when exposed to an oxygen containing atmosphere. As described above, this process is both time and temperature dependent.

While the susperted internal migration or diffusion of the oxygen in the partides is not fully understood, the present invention is based in part on the discovery that maintaining the oxygen level or oxygen partial pressure within certain ranges during storage either prevents the suspected migration or diffusion, or it replenishes or replaces the oxygen which does so migrate or diffuse.

The oxygen partial pressure, or the ratio of oxygen-to-inert gas present, in the containment atmosphere must be maintained at a level, prefer¬ ably 3% O₂ for many particles, which is suffident to maintain an oxidatively stable level of metal oxide on the surface layer of the metal partides, but below a level which can cause excess oxidation of the metal particles. As described above, in the case of iron metal powder, excess oxidation, i.e., an iron oxide layer greater than about 60 A thick or oxide exceeding about 60% of all metal atoms in the partides, causes loss of magnetization while not providing any additional benefit to the iron metal powder. Moreover, at higher oxygen concentrations, undesired runaway oxidation of the metal particles can occur. While the spedfic amount of oxygen employed to maintain the above described level of metal oxide on the surface of the metal particles will vary from metal to metal depending on the rate of reaction between the metal and the oxygen and the desired thickness of the metal oxide coating, the amount of oxygen employed in the storage atmosphere for a particular metal can readily be determined empirically by the skilled artisan. For example, for iron metal powder, the mole ratio of oxygen-to- inert gas will generally be in the range of from about 1:2000 to about 1:20, and preferably will be in the range of about 1:1000 to about 1:20. It is understood that if the storage atmosphere is static, i.e., is not changed during the storage of the metal partides, a higher initial oxygen partial pressure may be required to maintain the oxygen level above the necessary minimum level during the entire storage period. For example, in a static storage atmosphere, the mole ratio of oxygen to inert gas will generally be in the range of from about 1:100 to about 1:20

K the storage atmosphere is not static, then the oxygen partial pressure can be maintained throughout the storage period by, for example, periodic replenishment of the storage atmosphere. In such an embodiment, the storage atmosphere can be either periodically or continuously flushed and replaced with an atmosphere having the requisite oxygen to inert gas ratio. It is also contemplated that oxidizing materials equivalent to oxygen can be used in place of oxygen in the storage atmosphere. Such oxidizing materials indude, for example, organic peroxides, hydrogen peroxides, or various oxides of nitrogen and the like. It is further contemplated that in such an embodiment, an appropriate solid or liquid oxidizing material, such as a quasi-stable metal oxide, can be placed within the container in which the metal partides are stored. The metal oxide or other oxygen source selected is to provide a constant partial pressure of the oxidizing agent in the storage atmosphere.

The inert gas useful in this invention can be any desired inert gas which is normally used or has long term compatibility with the oxidized surface layer of the metal partides to be stored in accordance with the techniques of this invention. Suitable inert gases indude nitrogen, argon, neon, helium, carbon dioxide and the like. Because of cost and chemical properties, nitrogen is preferred. The storage atmosphere gas can contain water vapor. Generally, the amount of water vapor contained in the storage atmosphere will range from about 0% relative humidity (dry or anhydrous gas) to no more than about 40% relative humidity. In this regard, the storage temperature should not be allowed to drop to a point where the relative humidity is greater than 40%. Preferably, the storage atmosphere is anhydrous.

Storage of the oxidatively stable metal partides pursuant to the techniques of this invention are generally done for the sake of convenience under ambient temperature and pressure. However, because chemical reactions are slower at lower temperatures, lower storage temperatures are preferred. In general, suitable storage temperature generally range up to about 35°C. On the other hand, the particular storage pressure employed depends on the strength of the container employed and can generally range from about pounds per square inch (psi) atmospheric to about 100 psi or more.

When the oxygen/inert gas ratio is properly controlled, the storage techniques of this invention provides added benefits and advantages. For example, if it is desired to store metal particles which are not stabilized or which are not fully stabilized or which have suffered a loss in stability, such particles can be stored using the techniques of this invention. During such storage the metal oxide layer on the surfaces of the partides can be created or.restored to a level which imparts oxidative stability by first employing a suitable oxygen partial pressure in the storage environment until the desired level of oxidative stability has been achieved. For example, it is known in the art that an iron oxide coating can be generated in steps from iron metal powder (having no iron oxide coating) by first introdudng a small quantity of oxygen, e.g., about 5 parts per million (ppm), into the inert gas. After allowing a suffident period for the oxygen to react with surface of the iron metal powder, the oxygen content is then increased, e.g., to approximately 1000 ppm. By such carefully controlled oxidation, it is possible to form a metal oxide coating of desired thickness on the iron metal powder. For iron metal powder having an impaired metal oxide coating, i.e., a coating having lost a suffident quantity of metal oxide sudi that it no longer imparts oxidative stability to the metal particles, it is possible to start regeneration of the metal oxide coating of the partides at some intermediate oxygen content, e.g., approximately 500 ppm oxygen. The particular oxygen content employed in the generation/regeneration of the metal oxide coating will depend on the degree of metal oxide found on the surface of the metal partides prior to generation/regeneration and the particular metal partides employed and can be readily determined empirically by the skilled artisan.

In the generation/regeneration technique described above, the time and temperature of regeneration is not critical provided that such are suffident to cause formation/reformation of a suffident level of metal oxide coating on the surface layer of the metal partides so as to impart oxidative stability to the partides. Li general, regeneration times of from about 1 to about 5 hours are suffident for the regeneration whereas, regeneration temperatures are generally from about 15°C to 25°C. The generation/regeneration pressure at whidi generation/regeneration is conducted will effect the time required to do so, with the permitted pressure depending upon the strength of the container employed and can generally range from about 1 psi to about 100 psi or more. After generation/regeneration of the metal oxide coating on the surface of the metal partides, the oxygen partial pressure is then raised to the level described above that maintains the oxidative stability of the partides.

The metal partides can be stored within any suitable container which can retain the desired oxygen/inert gas atmosphere. Preferably, the container indudes a means of dianging and preferably also controlling the ratio of oxygen to inert gas such that the partial pressure of oxygen within the container is suffidently high to maintain or restore surface oxygen in the metal partides and yet is suffidently low to prevent undesirable excess oxidation, such as runaway oxidation, within the container. Such runaway oxidation can occur if, for example, air is used as the internal atmosphere for the container and the particles were not suffidently stabilized against oxidation in air. The particular container employed is not critical to this invention and can include containers having a flexible or non-flexible structure. For example, the container can be non-flexible drums or bottles suitable for shipping. Thus, the technique of the present invention is capable of providing a cost effective storage environment of magnetic partides having a metal oxide coating on their surface layer which eliminates those risks assodated with prior art environments.

The techniques of the present invention and the advantages asso¬ dated therewith will be further appredated upon consideration of the following spedfic examples, it being understood that the same are intended only as illustrative and in no way should be construed as limiting.

EXAMPLES Example A Commerdal 1500 Oe iron particles, having about 64 percent by weight metallic iron in the particles and having a 20 A to 30 A metal oxide coating on the surface layer which imparts oxidative stability to the partides, were stored under accelerated nitrogen storage conditions set forth in Table I below. At the end of such storage, the partides were exposed to the atmosphere, and the percent iron in the partides measured. A lower percent iron in the particles reflects an increase in oxidation of the particles which in turn is related to loss of oxidative stability of the particles over such accelerated conditions. TABLE I

Weight % of Atomic % Sample Storage Storage Storage Metallic Iron of Metallic # Temp Humidity³ Time in Partide^b Iron^c

1 35°C 40% 1 month 63% 77%

2 35°C 40% 2 months 58% 72%

3 35°C 40% 3 months 56% 70%

4 50°C 40% 6 days 64% 78%

5 50°C 40% 13 days 64% 78%

6 50°C 40% 20 days 62% 76%

7 50°C 40% 27 days 0% 0%

a = Storage Humidity is reported as relative humidity; b = induding non-iron spedes; and c = Atomic percent of metallic iron refers to atomic percent metallic iron in iron containing spedes.

The above data demonstrates that initially, the partides show little loss in iron and were oxidatively stable over this short period. However, this data further shows that prolonged storage results in loss of oxidative stability as measured by loss in percent iron.

Example B Commerdal 1500 Oe iron partides, as described in Example A above, were stored over several months at 25°C and 10% relative humidity under a storage atmosphere ∞nta-ining a mole ratio of oxygen to nitrogen of about 3:100. The storage atmosphere was periodically replenished so as to maintain this mole ratio of oxygen to nitrogen. Storage under these conditions results in little or no loss in percent iron which reflects that the partides retain their oxidative stability under these storage conditions. Similarly, other metal particles, such as zinc particles, chromium particles, nickel particles, cobalt particles and the like, having a suitable metal oxide coating to impart oxidative stability can be stored under the conditions set forth in Example B above so as to retain their oxidative stability over prolonged storage conditions. Likewise, other suitable inert gases can be used in Example B above in place of nitrogen to provide the requisite oxygen/inert gas storage atmosphere which provides oxidative stability to the stored metal particles over prolonged storage conditions. Suitable inert gases include argon, helium, neon, carbon dioxide and the like.

While this invention is described in terms of various preferred embodiments, the skilled artisan will appredate that various modifications, substitutes, omissions and changes may be made without departing from the spirit thereof. Accordingly, it is intended that the scope of the present invention be limited solely by the scope of the following daims including equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A method of storing metal partides having a suffident level of metal oxide coating on their surface layer so as to impart oxidative stability to said metal partides, said method comprising:

confining said metal partides in an oxygen/inert gas atmosphere, and

maintaining in said atmosphere a ratio of oxygen-to-inert gas selected such that suffident oxygen is present to maintain said level of metal oxide coating on the surface layer of said partides while confined in said atmosphere and such that insuffident oxygen is present to cause nmaway oxidation of said metal partides while confined in said atmosphere.

2. The method according to Claim 1 wherein said metal partides are magnetic metal partides.

3. The method according to Claim 2 wherein said magnetic metal partides are iron metal powder.

4. The method according to Claim 3 wherein said iron metal powder is finely divided adcular iron metal powder.

5. The method according to Claim 1 wherein said inert gas is selected from the group consisting of nitrogen, helium, argon, neon and carbon dioxide.

6. The method according to Claim 5 wherein said inert gas is nitrogen.

7. The method according to Claim 3 wherein said oxygen /inert gas atmosphere contains a ratio of oxygen to inert gas from about 1:2000 to about 1:20.

8. The method according to Claim 3 wherein said metal oxide coating on the surface layer of said iron metal powder has a thickness of at least about 20 A.

9. The method according to Claim 8 wherein said metal oxide coating on the surface layer of said iron metal powder has a thickness of from about 20 A to about 60 A.

10. A method of stabilizing and storing metal partides having either no or an insufficient level of metal oxide coating on the surface layer of said particles required to impart oxidative stability to said partides, said method comprising:

a) confining said metal partides in an oxygen/inert gas atmosphere and initially maintaining said atmosphere of oxygen and inert gas with a ratio of oxygen-to-inert gas selected such that suffident oxygen is present so that a suffident level of metal oxide coating can be formed on the surface of said partides so as to impart oxidative stability to said particles and sudi that insuffident oxygen is present to cause excess oxidation of said metal particles while confined in said atmosphere;

b) retaining said metal particles in said atmosphere for a time and at a temperature selerted so as to cause formation of said metal oxide coating;

c) after formation of said suffident level of metal oxide coating on the surface of said partides, adjusting the oxygen content of the oxygen /inert gas atmosphere to a level that the ratio of oxygen-to-inert gas is such that suffident oxygen is present to maintain said level of metal oxide coating on the surface layer of said partides produced pursuant to steps a) and b) above and such that insuffident oxygen is present to cause excess oxidation of said metal particles produced pursuant to steps a) and b) above while confined in said container.

11. The method according to Claim 10 wherein said metal particles are magnetic metal partides.

12. The method according to Claim 11 wherein said magnetic metal partides are iron metal powder.

13. The method according to Claim 12 wherein said iron metal powder is finely divided adcular iron metal powder.

14. The method according to Claim 10 wherein said inert gas is selected from the group consisting of nitrogen, helium, argon, neon and carbon dioxide.

15. The method according to Claim 14 wherein said inert gas is nitrogen.

16. The method according to Claim 12 wherein after formation of said suffident level of metal oxide coating on the surface of said partides the oxygen content of said oxygen/inert gas atmosphere is adjusted to a ratio of oxygen to inert gas of from about 1:2000 to about 1:20.

17. The method according to Claim 12 wherein said metal oxide coating on the surface layer of said iron metal powder produced in step a) has a thickness of at least 20 A.

18. The method according to Claim 17 wherein said metal oxide coating on the surface layer of said iron metal powder produced in step a) has a thickness of from about 20 A to about 60 A.

19. A storage arrangement comprising a container, metal particles within said container and having a sufficient coating of metal oxide on their surface layer so as to impart oxidative stability to said metal particles, and an oxygen/inert gas atmosphere within said container having a ratio of oxygen to inert gas within said container selerted so as to have sufficient oxygen to maintain said coating of metal oxide on the surface layer of said particles while confined in said container and so as to have insuffident oxygen to cause excess oxidation of said metal partides while confined in said container.

20. The storage arrangement according to Claim 19 wherein said metal particles are magnetic metal particles.

21. The storage arrangement according to Claim 20 wherein said magnetic metal partides are iron metal powder.

22. The storage arrangement according to Claim 21 wherein said iron metal powder is finely divided adcular iron metal powder.

23. The storage arrangement according to Claim 19 wherein said inert gas is selected from the group consisting of nitrogen, helium, argon, neon and carbon dioxide.

24. The storage arrangement according to Claim 23 wherein said inert gas is nitrogen.

25. The storage arrangement according to Claim 21 wherein said oxygen/inert gas atmosphere contains a ratio of oxygen to inert gas of from about 1:2000 to about 1:20.

26. The storage arrangement according to Claim 21 wherein said metal oxide coating on the surface layer of said iron metal powder has a thick- ness of at least about 20 A.

27. The storage arrangement according to Claim 26 wherein said metal oxide coating on the surface layer of said iron metal powder has a thick¬ ness of from about 20 A to about 60 A.