NO321356B1 - Compressible explosive composition - Google Patents

Description

The present invention relates to compressible aluminum-containing explosive compositions. More specifically, the invention relates to low-sensitivity compressible aluminum-containing explosive compositions and a method for producing them.

Background

In recent times, there has been an increased interest in aluminum-containing explosive compositions because this gives an explosive with increased blast pressure (so-called "enhanced blast explosives" or also called "thermobaric explosives"). The reason for this increased interest is that there is a greater need for charges that can have an effect against bunkers and tunnels. For this, you need explosives that can release energy over a longer period than what is achieved with traditional compositions with pure high explosives. The use of aluminum to increase the effect in explosives has been known for a long time and was patented for the first time already in 1900 by G. Roth (DE 172,327).

There has also been a lot of focus in recent years on insensitive ammunition ("Insensitive Munition" also called simply "IM") - The requirements for IM are based on the fact that it must be safe to store and handle the ammunition in all situations both in peace and in strife Much work has therefore been aimed at improving the stability of the explosive used in the ammunition.

Pressable explosive compositions containing aluminum and high explosives such as 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX) and 1,3,5-trinitro-1,3,5-triazacyclohexane ( RDX) has been known for quite some time. These have mainly been explosive compositions with wax as a binder. However, such wax-based compositions do not satisfy the requirements for explosives for low-sensitivity ammunition.

Traditionally, compressible explosive compositions can be made in an aqueous environment using what a person skilled in the art knows as the water slurry process. For safety reasons, aluminum-containing compositions will not normally be made in water, as aluminum reacts exothermically with water to form aluminum hydroxide and hydrogen gas. Therefore, it has been more common to mechanically mix aluminum into this type of composition. However, this means that you can get a mixture of explosives and aluminum that is not homogeneous in the charges after they have been pressed.

Known technique

There are several varieties of explosive compositions with aluminium, which a specialist in the field calls cast-cured or melt-pourable compositions. These compositions are suitable for producing large charges with a diameter of over 100 mm. Smaller charges can also be made with these compositions, but this is a relatively expensive process and for smaller charges a pressing process is far more economical. However, there are far fewer compressible aluminum-containing explosive compositions than there are cast/hardenable and cast/fusible compositions. Of compressible aluminum-containing compositions, almost all, with a few exceptions, are produced by a mechanical mixing of aluminum.

Steven L. Jones et.al presented a compressible aluminum-containing composition (called PBXIH-18) at a symposium in Florida in 2003 (2003 Insensitive Munitions and Energetic Materials Technology Symposium 10-13 March 2003, Orlando USA). This composition consists of HMX, aluminum, Di-2-ethylhexyl adipate (also called Di-octyl adipate or simply DOA) and HyTemp 4454 (Polyacrylic binder marketed by Zeon Chemicals). The composition of. the composition in terms of weight % HMX, aluminum and binder was not stated and is still not known. This composition was developed as an intended replacement for a composition consisting of Comp A3 with 30% aluminum added by weight (Comp-A3 is a well-known RDX composition with wax as a binder). This Comp-A3/Aluminium composition is produced by mechanically mixing aluminum with Comp-A3 and has been used in large quantities in weapon systems. A disadvantage of such a mechanical mixture is that the aluminum and the explosive are not mixed well enough and after pressing, charges are obtained where the aluminum and the explosive are not homogeneously distributed. Another disadvantage of this Comp-A3/aluminium composition is that it does not meet the requirements for use in low-sensitivity ammunition. Therefore, PBXIH-18 was developed to satisfy the requirements for the explosive. The process for making PBXIH-18 uses a slurry of explosives and aluminum in a perfluorinated solvent PF-5080 (essentially fully fluorinated octane compounds marketed by 3M). The process is similar to what one skilled in the art calls a water slurry process. However, the water has been replaced with PF-5080 because the reactivity of aluminum towards water is feared.

The process described by Jones et. al is economically unfavorable compared to a standard water slurry process since it uses PF-5080 which is far more expensive than water. It is true that PF-5080 can be recovered, but this requires additional purification steps in the process before it can be used again. Being able to recover 100% is also completely unrealistic. Environmentally, PF-5080 is very unfriendly. Such perfluorinated compounds are particularly stable and are therefore very little degradable in nature. The manufacturer 3M therefore recommends avoiding the release of PF-5080 into nature. This process is also disadvantageous in that it requires the use of dry crystals of the explosive. This is not very beneficial in terms of safety, as it is associated with a far greater risk of handling dry explosive crystals than water-moistened ones. Explosive crystals of HMX or RDX cannot be transported dry (must contain a minimum of 15% water) and from the production process they come out directly as water-moistened. Pre-drying the explosive crystals is not very economically beneficial since this requires both time, energy and equipment.

Joseph Turci et al (US 5,472,531) describes an aluminum-containing composition which is made by grinding explosives, aluminum and binder together with a volatile solvent in a kneader to then extrude the product which is cut up to make a powder of the composition. This process has several disadvantages, including the need for expensive kneading machines and extrusion equipment. A more risky disadvantage is that explosives that have been dried in advance must also be used in this process. As previously mentioned, transport and handling of dried HMX or RDX is associated with a risk. In addition to the increased risk aspect, there is also an economic disadvantage of having to dry the explosive in advance, as this requires time, energy and equipment.

Steven M. Nicolich et.al presented examples of compressible aluminum-containing compositions at a symposium in Florida in 2003 (2003 Insensitive Munitions and Energetic Materials Technology Symposium 10-13 March 2003, Orlando USA). These compositions are based on CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane) as explosive and CAB (cellulose acetate butyrate) as polymer with BDNPA/F (1:1 mixture of Bis(2,2-dinitropropyl)acetal and Bis(2,2-dinitropropyl)formal) as softener. These use a liquid-slurry process to make the compositions where the liquid can be water. However, they emphasize that the aluminum quality used cannot be in water above 30 °C. They must therefore remove the solvent by a method other than distillation, how they do this is not described. Whether the aluminum quality is passivated at all, or possibly what it is passivated with, say Nicolich et al. nothing about. The described compositions contain CL-20 as an explosive which is known to a person skilled in the field as a significantly more sensitive explosive than HMX or RDX. Therefore, a person skilled in the field will be careful not to use this type of composition in low-sensitivity ammunition. CL-20 is also very expensive as a result of the expensive process for its manufacture.

Dyno Nobel ASA has for a number of years made compositions based on RDX, wax and aluminium. This has been carried out with passivated aluminum where the process is based on an old French patent (FR 2,031,677). The French patent describes a process whereby, by using passivated aluminium, compositions can be made with wax and other binders as long as the binder contains an acidic group such as a carboxylic acid group or an acidic hydroxyl group as in, for example, phenol. The acidic group was necessary in order to be able to coat aluminum with wax and thus to be able to create a homogeneous granule of RDX, aluminum and binder. Without these acidic groups, the explosive could only be coated with aluminum dust all around. The passivation of aluminum in this patent is based on heating a suspension of aluminum in an aqueous solution of potassium bichromate or in an aqueous solution of monosodium phosphate and boric acid. The former, so-called bichromatised aluminium, was for a number of years used as passivated aluminium. However, due to environmental reasons, there were restrictions on producing bichromated aluminum a number of years ago. This has meant that today it is very difficult to obtain large quantities of this, and that what can be obtained has a very high price. Of course, it is also highly undesirable for environmental and health reasons to use this bichromated aluminum quality. For several years before the restrictions came for bichromated aluminium, Dyno Nobel worked with alternative passivation methods. It was thought that aluminum could be pre-coated with, for example, a wax or other binders. After a number of trials in collaboration with the aluminum suppliers, it was found that by pre-coating aluminum with 0.3% by weight isostearic acid, but not limited to isostearic acid or the amount of 0.3% by weight, it was achieved that aluminum was sufficiently stable above water so that could safely use it in a water slurry process.

Based on aluminum that has been passivated with 0.3 wt% isostearic acid, Dyno Nobel ASA has produced large quantities of compositions with RDX, aluminum and wax using a so-called standard water slurry process. The knowledge that isostearic acid can passivate the aluminum sufficiently to be used in the water slurry process has been kept within the company as a production secret. Moreover, these compositions do not satisfy the current requirements for explosives for use in low-sensitivity ammunition (IM requirements). The market therefore wants new compressible compositions based on high explosives with aluminum and binders that satisfy today's IM requirements.

Aim of the invention

It is therefore a goal of this invention to provide compressible high explosives with aluminum and binders that satisfy the current requirements for low-sensitivity explosives (IM requirements).

It is also a goal of this invention to provide a method for producing such compressible high explosives with aluminum and binders that satisfy today's requirements for low-sensitivity explosives based on the industry-friendly and relatively low-risk method known as the water slurry process.

Description of the invention

The aim of the invention can be achieved by the features that appear from the following description and attached patent claims.

The present invention relates to compressible explosive compositions which contain between 5% by weight and 55% by weight aluminum and between 45% by weight and 95% by weight HMX and/or RDX, the remainder being a binder. The binder used is of such a type that the explosive composition will be able to satisfy requirements for use in low-sensitivity ammunition (IM requirements). The binder may be, but is not limited to, a mixture of a polyacrylic elastomer such as HyTemp 4454 (marketed by Zeon Chemicals) and a plasticizer. The plasticizer may be, but is not limited to, 2-diethylhexyl adipate (also known as plastomoll, dioctyl adipate, or DOA). It will be clear to a person skilled in the art that the compositions in the present invention will, as a result of the aluminum content, give an explosive with increased blast pressure ("enhanced blast explosive"). At the same time, a person skilled in the field knows that the use of elastomeric binders, such as HyTemp 4454 / DOA mixtures used in the present invention, gives explosive compositions with suitable properties so that they can be used in low-sensitivity ammunition. HMX- and RDX-based compositions with a binder mixture of HyTemp 4454 and DOA have been in focus for the last 10-20 years and have proven to have very good IM properties. Compositions with this binder system have therefore been widely used in various weapon systems.

With the present invention, compared to the state of the art, charges can be produced in a press operation that have increased explosion pressure and can be used in low-sensitivity ammunition. For a professional in the field, there are known several advantages to being able to use a pressing operation rather than a casting process, either cast/hardenable or cast/fusible. One of these is that you can have a much higher production pace of the charges as curing/solidification processes are time-consuming. At the same time, pressing equipment is far cheaper than casting equipment. For a person skilled in the field, it will therefore be far more economically advantageous to press charges rather than to cast them, as long as the charges do not have too large a diameter, which is limiting for pressable charges.

The present invention relates to a method for producing the aforementioned compositions in what is known to a person skilled in the field as the standard water slurry process. In this process, the explosive crystals and passivated aluminum powder are slurried in water and heated before a solution of a binder system is added. After the addition, the mixture is further heated to 100 °C by distilling off the solvent so that the binder precipitates out and settles on the particles of explosive and aluminium, binding these together into granules. The distilled solvent is directly reused after water has been separated from it. Granules of the coated product are isolated by filtration. For a person skilled in the field, it is very surprising that such compositions containing aluminum can be safely produced in such a water slurry process with a temperature of up to 100 °C because it is a safety risk to mix aluminum powder with water at high temperatures. Based on the article by Steven L. Jones et al. presented in Florida in March 2003, it is being told by those skilled in the art that aluminum-containing compositions cannot be made in a standard water slurry process because of this safety risk. Steven M. Nicolich et al. presented at the same symposium that, for safety reasons, it is impossible to use the water slurry process at temperatures above 30 °C when manufacturing aluminum-containing explosive compositions.

With the present invention compared to the state of the art, clear advantages are achieved in that the compositions can be manufactured in a water slurry process. Rather than a mechanical dry mixture of a composition with explosives and aluminum powder, the water slurry process produces a much more homogeneous granule where aluminum and explosives are evenly distributed in each grain. This is for a professional in the field of great importance since an even homogeneous distribution of explosive, aluminum and binder is essential in the finished charges since otherwise they will not have the same effect in terms of safety and performance.

With the present invention compared to the state of the art, the advantage is also achieved that the incoming crystals can be water-moistened before they enter the process. For a professional in the field, this will provide clear logistical and safety advantages since the explosive crystals are manufactured, stored and transported in a water-moistened state. In the case of a kneading process, as described by Joseph Turci et al (US 5,472,531), it is clear to a person skilled in the art that dry crystals are required to be used. Handling larger quantities of dry RDX and HMX crystals is for a person skilled in the field associated with a much higher risk than handling these in a water-moistened state. At the same time, you don't have to dry the crystals, which an expert in the field sees as economically very beneficial, as you don't need expensive drying equipment, energy and drying time.

With the present invention compared to the state of the art, aluminum-containing compositions can be produced in a standard water-slurry process by removing the solvent by a distillation as a result of heating to 100 °C, so that the binder precipitates and binds together explosives and aluminium. In the process presented by Nicolich et al, one cannot go above 30 °C when aluminum is present together with water. How Nicolich et al get the binder precipitated is not described.

With the present invention compared to the state of the art, aluminum-containing compositions can be produced in a standard water slurry process with aluminum that is passivated with isostearic acid rather than the far more environmentally and health-damaging and far more expensive aluminum passivated by bichromatization as described in the French patent (FR 2,031,677). Passivated means that the aluminum is coated with a substance/compound that will actively prevent the aluminum from reacting with water under the conditions that exist in a water slurry process for the production of explosives. This effect can be achieved by a coating that physically prevents the water from coming into contact with the aluminium, a coating that has an inhibitory effect on the reaction between water and aluminium, or a combination of these. It is preferred that the coating is formed from substances/compounds that have relatively little harmful effect on the environment beyond the intended explosive effect of the explosive, i.e. they should not leave toxic residues that can be absorbed into the natural cycle. The invention should therefore not be understood as limited to the preferred coating of isostearic acid, a person skilled in the art will understand that other types of wax or binder will have the same, or approximately the same, effect as the preferred isostearic acid.

A person skilled in the field will be able to obtain similar compositions covered by the present invention by using crystalline explosives other than RDX and HMX, such as e.g. 2,4,6,8,10,12-hexanitro-2,4,6,8, 10,12-Hexaazaisowurtzitane (CL-20), 2,2',4,4',6,6'-Hexanitrotransstilbene (HNS), 1,3,5-Triamino-2,4,6-Trinitrobenzene (TATB) and 3-Nitro-1,2,4-Triazole-5-one (NTO). Correspondingly, a specialist in the field will be able to obtain similar compositions covered by the present invention using other elastomers, for example styrene-butadiene or styrene-isoprene copolymers which are available from Kraton polymers, among others. Other examples are europrene and cyanacryl (trademarks from EniChem), Krynac (trademark from Bayer polymers), Nipol (trademark from Zeon Chemicals) and Noxtite (trademark from Nippon Mektron). In recent years, high-energy elastomers have been investigated for use in explosive compositions, but none of these are commercially available today. The use of such energy-rich elastomers for compositions is also expected to be able to be used in explosive compositions that are included in the present invention. 1 the present invention, HyTemp 4454 has been chosen because it has been used for a number of years in the explosives industry for compressible compositions and for the known property that HyTemp 4454 gives compositions with good properties for use in low-sensitivity ammunition. HyTemp is also known to have a good compatibility with the explosive, which is very important for this type of compound.

A person skilled in the art will be able to obtain similar compositions covered by the present invention by using other plasticizers. Alongside Dioctyl Adipate (DOA), plasticizers such as Dioctyl Sebasate (DOS) and Isodecyl Pelargonate (IDP) are also used together with HyTemp in explosive compositions (Amy J. Didion and K. Wayne Reed, 2001 Insensitive Munition & Energetic Materials Technology Symposium, Bordeaux, proceedings page 239). Other known plasticizers used in the explosives industry are, for example, dioctyl maleate (DOM), dioctyl phthalate (DOP), glycidyl acid polymer (GAP), BDNPA/F (1:1 mixture of Bis(2,2-dinitropropyl)acetal and Bis(2,2- dinitropropyl)formal) and N-alkyl-NitratoEthylNitramine (Alkyl-NENA). These plasticizers and other similar plasticizers will be able to function excellently in the present invention. Dioctyl adipate (DOA) has been chosen in the present invention as this compound is the most used together with HyTemp within explosive compositions. Dioctyl adipate (DOA) is also known to have good compatibility with the explosive and the binder.

In the present invention, HyTemp 4454 and DOA were used as a binder as this binder mixture is known to a person skilled in the art to provide compressible explosive compositions with good properties to be used in low-sensitivity ammunition. Previously, HyTemp 4054 and HyCar 4054 were used, which is the same polymer as HyTemp 4454, but is supplied in larger lumps compared to HyTemp 4454.

Detailed description of the invention

In order to further describe the invention, it will be illustrated by means of examples. These examples are intended only as examples of preferred embodiments, and should therefore not be construed as limiting the more general inventive idea of producing aluminum-based explosive compositions by a standard water slurry process.

Passivated aluminum in the examples below means that it is passivated with 0.3 wt% isostearic acid.

Example 1

In a 150 liter reactor, 13.05 kg of HMX was slurried in about 50 liters of water while stirring. The mixture is heated to about 60 °C and 0.75 kg of passivated aluminum is added to this mixture. An ethyl acetate solution of 0.3 kg HyTemp 4454 and 0.9 kg DOA is then added in a thin stream. The mixture was then further heated to 100 °C by distilling off the ethyl acetate-water azeotrope. The mixture is cooled and dropped into a filter where the product (approx. 15 kg) is filtered out.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 87.1% by weight HMX, 5.5% by weight DOA, 2.3% by weight HyTemp and 5.1% by weight aluminium.

Example 2

The same procedure as described in example 1, but the amounts used are as follows 12.3 kg HMX, 1.5 kg passivated aluminium, 0.3 kg HyTemp 4454 and 0.9 kg DOA.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 83.3% by weight HMX, 5.6% by weight DOA, 1.8% by weight HyTemp and 9.3% by weight aluminium.

Example 3

The same procedure as described in example 1, but the quantities used are as follows 10.06 kg HMX, 3.75 kg passivated aluminium, 0.3 kg HyTemp 4454 and 0.9 kg DOA.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 66.6% by weight HMX, 5.7% by weight DOA, 1.8% by weight HyTemp and 25.9% by weight aluminium.

Example 4

Same procedure as described in example 1, but the amounts used are as follows 9.3 kg HMX, 4.5 kg passivated aluminium, 0.3 kg HyTemp 4454 and 0.9 kg DOA.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 63.2% by weight HMX, 5.5% by weight DOA, 1.7% by weight HyTemp and 29.6% by weight aluminium.

Example 5

Same procedure as described in example 1, but the amounts used are as follows 6.3 kg HMX, 7.5 kg passivated aluminium, 0.3 kg HyTemp 4454 and 0.9 kg DOA.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 43.4% by weight HMX, 5.5% by weight DOA, 1.9% by weight HyTemp and 49.3% by weight aluminium.

Example 6

In a 150 liter reactor, 9.6 kg of RDX was slurried in about 50 liters of water while stirring. The mixture is heated to around 60 °C and 4.5 kg of passivated aluminum is added to this mixture. An ethyl acetate solution of 0.225 kg HyTemp 4454 and 0.675 kg DOA is then added in a thin stream. The mixture was then further heated to 100 °C by distilling off the ethyl acetate-water azeotrope. The mixture is cooled and dropped into a filter where the product (approx. 15 kg) is filtered out.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 63.5% by weight RDX, 4.4% by weight DOA, 1.4% by weight HyTemp and 30.7% by weight aluminium.

Example 7

In a 150 liter reactor, 9.0 kg of RDX and 1.5 kg of HMX were slurried in about 50 liters of water while stirring. The mixture is heated to around 60 °C and 3 kg of passivated aluminum is added to this mixture. An ethyl acetate solution of 0.375 kg HyTemp 4454 and 1.125 kg DOA is then added in a thin stream. The mixture was then further heated to 100 °C by distilling off the ethyl acetate-water azeotrope. The mixture is cooled and dropped into a filter where the product (approx. 15 kg) is filtered out.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 59.4% by weight RDX, 10.8% by weight HMX, 7.1% by weight DOA, 2.3% by weight HyTemp and 20.4% by weight aluminium.

Example 8

As example 7, but the amounts of RDX and HMX were respectively 6.0 kg and 4.5 kg otherwise the same as in example 7.

The product is a granule, examination in an optical microscope shows that aluminum and explosives are homogeneously distributed in each granule bowl. Composition analysis shows that the product contains 38.6% by weight RDX, 32.4% by weight HMX, 6.9% by weight DOA, 2.3% by weight HyTemp and 19.8% by weight aluminium.

The examples presented show that the water-slurry process works very well for a large range of aluminum contents. In the examples from 1 to 5, the content of aluminum is varied from 5% by weight to up to 50% by weight of aluminum. Example 6 shows that the process also works when HMX is replaced by RDX. Examples 7 and 8 show that it is also possible to produce granules with mixtures of RDX and HMX in different ratios together with aluminium.

Claims (19)

1. Compressible explosive composition with increased explosion pressure, characterized by; - that it contains between 45 and 95% by weight of explosive crystals and between 5 and 55% by weight of passivated aluminum powder, and - binder chosen so that the composition meets the requirements for low-sensitivity explosives (IM requirements). 2. Explosive composition according to claim 1, characterized in that the explosive crystals are HMX and/or RDX. 3. Explosive composition according to claim 1 or 2, characterized in that the aluminum is passivated by being coated with a coating that prevents the aluminum powder from reacting with water. 4. Explosive composition according to claim 3, characterized in that the coating is a wax. 5. Explosive composition according to claim 3, characterized in that the coating is isostearic acid. 6. Explosive composition according to any one of claims 1-5, characterized in that the amount of the binder in the composition is between 2% by weight and 15% by weight. 7. Explosive composition according to any one of claims 1 - 5, characterized in that the binder consists of polyacrylic elastomer, preferably HyTemp 4454 or HyTemp 4054. 8. Explosive composition according to claim 6 or 7, characterized in that the composition also comprises a plasticizer. 9. Explosive composition according to claim 8, characterized in that the plasticizer is selected from Dioctyl adipate (DOA), Dioctyl sebacate (DOS), isodecyl pelargonate (IDP), dioctyl maleate (DOM) or dioctyl phthalate (DOP). 10. Explosive composition according to claim 9, characterized in that the binder consists of HyTemp 4454 or HyTemp 4054 with dioctyl adipate as plasticizer. 11. Process for producing compressible low sensitivity explosive compositions, wherein the composition comprises; - between 45 and 95% by weight of explosive crystals, - between 5 and 55% by weight of passivated aluminum powder, - a binder chosen so that the mixture meets the requirements for low-sensitivity explosives (IM requirements), and optionally - a plasticizer, characterized by the fact that; - first the aluminum is passivated by coating it with a coating that prevents it from reacting with water, - then the explosive crystals and aluminum are slurried in water and heated, - then a solution of the binder and possibly the plasticizer is added, and - then heated to a temperature which results in the solvent being distilled out of the composition. 12.. Method according to claim 11, characterized in that the explosive crystals are RDX or HMX. 13. Method according to any one of claims 11 or 12, characterized in that the amount of the binder in the composition is between 2% by weight and 15% by weight. 14. Procedure according to claim 13, characterized in that the binder is a polyacrylic elastomer, preferably HyTemp 4454 or HyTemp 4054. 15. Procedure according to claim 13 or 14, characterized by the fact that one is added to the composition. 16. Procedure according to claim 15, characterized in that the plasticizer is selected from dioctyl adipate (DOA), dioctyl sebacate (DOS), isodecyl pelargonate (IDP), dioctyl maleate (DOM) or dioctyl phthalate (DOP). 17. Procedure according to claim 13, characterized in that the binder consists of HyTemp 4454 or HyTemp 4054 with dioctyl adipate as plasticizer. 18. Method according to any one of claims 11 - 17, characterized in that the passivation coating is a wax. 19. Procedure according to claim 18, characterized in that the passivation coating is isostearic acid.