Device For The Application Of A Metal Coating On The Internal Cylindrical Surface Of A Hollow Body

Abstract

In a device for the application of a metal coating on the internal surface of a hollow metal element in the form of a cylinder of revolution there is provided a sealed enclosure adapted to be placed on to said hollow element, a pumping set for generating a high vacuum in said enclosure which is provided with a hole cooperating with an orifice in said hollow element, electrically conductive elements passing through said hole of said enclosure from which they are insulated, a source of current connected to said conductive elements and a vapour emitter connected to said conductive elements and adapted to be introduced into said hollow metal element.

Description

The present invention relates to devices for the application of a metal coating on the internal surface, having the form of a circular cylinder, of a hollow unit.

Steel vessels, and bottles or cylinders in particular, are used at the present time for conveying and containing various compressed or liquefied gases.

The vessels, which consist of thick-gauge steel, are exposed to corrosion caused by the gases or liquids they contain. As a result of moisture present in the said gases or liquids, rust may form in the vessels which is liable to contaminate their contents.

On the other hand, some gases, such as hydrogen in particular, diffuse into steel to cause embrittlement of the latter, which may give rise to serious accidents during storage in tanks or during conveying in steel pipelines or ducts. The risk of such accidents increases as the storage pressures in the tanks and bottles and the pressures in the conveying pipelines are increased.

However, there is a trend towards a substantial increase in gas pressure in the bottles and the degree of acceptable corrosion is thus reduced considerably, so that it is necessary to incorporate an internal protective coating.

To eliminate the shortcomings caused by corrosion and by the decrease in the strength of the steel, it is known to protect the internal surface of the walls of vessels, particularly bottles or cylinders, made of steel, by covering these with a plating which commonly consists of a thin-gauge sheet of stainless steel applied in the course of production. In consequence of this covering, the production cost of a vessel thus equipped, is high. This is due to the fact that the material employed for the plating is not only expensive but also implies high costs since considerable labour is required to achieve the covering.

Moreover, it may occur that the coating or plating may crack or weaken during the life of the vessel; it is then impossible to perform any kind of restoration or repair, rendering it unusable for its original purpose, at least.

The known methods for the application of a coating do not offer complete satisfaction in respect of coating strength and, by their effect on purchase price, they are likely to restrict the use of gases in bottles.

The fundamental problem of the invention consists in producing a device which renders it possible to apply an adherent metal coating and which may easily undergo restoration or repair if need be, on the internal surface, having the form of a circular cylinder, of a hollow element, in easy, reliable and economical manner.

One solution to the problem raised by the application of a metal coating on the cylindrical internal surface of a hollow metal element is achieved by means of a device according to the invention, which is characterised in that it comprises a sealed enclosure adapted to be placed over the said hollow element, a pumping set connected to the said sealed enclosure for the purpose of generating a high degree of vacuum therein, a hole in the sealed enclosure co-operating with an orifice on the axis of revolution of the cylindrical internal surface of the said hollow element, electrically conductive elements passing through a wall of the enclosure from which they are insulated, a source of current connected to the external ends of the said conductive elements, and an emitter of metal vapours connected to the internal ends of the said conductive elements and arranged to pass through the said hole and the said orifice and into the hollow element.

According to one feature, the metal vapour emitter consists of a steel wire coated with a layer made of a vapourisable metal intended to form the said coating. This wire forming the emitter may be positioned along the longitudinal axis of the internal cylindrical surface of revolution which is to be coated.

The invention will be further described in connection with the accompanying drawings which show two examples of embodiment of the invention, solely by way of illustration.

IN THE DRAWINGS:

FIG. 1 is a diagrammatical view in section of a device according to the invention, employed for the application of a coating on the internal surface of a gas bottle.

FIG. 2 is a diagrammatical view, to a different scale, of another form of sealed enclosure.

FIG. 1 shows a sealed enclosure 2 of cylindrical form equipped with a lateral pipe 3 leading to a vacuum pump 4. This pump evacuates the air from the enclosure 2 and generates a vacuum of approximately 10.sup.-.sup. 4 Torr in the same. In the base of the enclosure 2 there is an orifice 5 closed off by a hermetic clack-valve or the like 6; around the orifice 5, on the outer surface, there is a hermetic seat 7 against which is thrust a neck 8a of a steel bottle 8 which is to have its internal surface provided with a protective metal coating, for example of nickel. This bottle 8 is placed on a carrying plate 9. The clack-valve 6 is opened by a suitable mechanism, which is not shown, which places the inside of the bottle 8 in direct communication with the inside of the enclosure 2; a vacuum of approximately 10.sup..sup.-4 Torr is thus also established within the bottle 8, and the enclosure 2 is firmly thrust on to the bottle 8 in this manner.

The time required to obtain this vacuum depends on the power of the vacuum pump 4. As soon as the appropriate vacuum is reached within the enclosure and the bottle, a control which is not shown causes a metal vapour emitter 10 to slide on a guiding bar 11, so as to enter the bottle 8 along the axis 8b of the same. This emitter comprises a wire 10 of a diameter of approximately 1 mm consisting of a refractory metal such as tungsten, which is wound in coil form and sheathed with a layer of a thickness of approximately 0.1 mm consisting of the vapourisable metal which is to form the internal coating of the bottle. A rigid bar 12 of electrically conductive metal is carried by arms 15 sliding on the guiding bar 11 up to the edge of the neck; the length of the wire 10 wound in coil form and the length of the bar 12 are a little greater than the length of the axis 8b of the bottle. The plating metal may be nickel, chromium, aluminium, cadmium, cobalt, zinc, or an alloy of these metals.

When the free lower end 12a of the bar 12 comes into abutment against the inside of the base of the bottle, this closes an electric circuit 13 connected by the corresponding conductors 13a to the wire 10 and to the bar 12, an electric current of approximately 60 amps, having a voltage of approximately 24 volts, which is supplied by a source of current 14, flows through the bar 12 and the wire 10; this latter forms an electric resistance and undergoes heating to a high degree together with the metal sheathing the same, which latter is rapidly brought to a temperature exceeding its melting point; this metal is then volatilised and deposited over the entire internal surface of the wall of the bottle 8 in an even and homogenous layer.

The operation is completed after a period lasting from 3 to approximately 30 seconds; if it is wished to obtain a thick coating on the inside of the bottle, the operation may be repeated several times if required. When the final coating has been obtained, it is then possible to expose the bottle, with its internal coating, to a tempering operation for the purpose of engendering a definite degree of diffusion of the plating metal and secure a perfect bond.

Another form of embodiment of the enclosure, which is a removable head unit, is shown in FIG. 2. This head unit comprises a box 16 closed off by a cover 17 held in position by tightening nuts 18a screwed to screw-threaded bolts 18 hinged on the said box; the hermeticity of the cover is secured by means of a gasket 19. The box has lateral pipe 16a which is connected by means of a reinforced flexible pipe, which is not shown, to a vacuum pump which is not shown. In the cover 17 are set, insulated by means of plugs 17a, two electrically conductive elements 20 whereof the external ends 20a are connected to a source of current, which is not shown, whereas the internal ends 20b are connected to a metal vapour emitter analogous to that illustrated in FIG. 1; the emitter also comprises one branch consisting of a straight rigid bar 21 and a branch formed by a steel wire 22 wound in coil form and coated with a layer of the metal which is to be deposited. These two branches 21 and 22 pass through a hole 23 in the bottom of the box 16. The source of current supplies a current of high intensity and low voltage, which may amount to 62.5 amps under 24 volts, for example.

When the head unit is placed on an axial orifice in a hollow steel element such as a cylindrical gas bottle, and if the vacuum pump is then placed in operation, a vacuum of approximately 10.sup..sup.-3 to 10.sup..sup.-4 Torr is then rapidly generated within the head unit and the bottle. If the wire coil 22 is supplied with a zinc coating of approximately 250 grs per m.sup.2 or approximately 2 grs of zinc for the surface of the wire, and if a current of approximately 60 amps is then allowed to flow through for approximately 10 seconds, the zinc is vapourised almost totally as a result of the heating of the wire 22; the vapours it produces are condensed on the internal surface of the wall of the bottle, in such manner that if the latter has a height of 80 cms and an internal diameter of 20 cms, a coating of a thickness of approximately 0.5 micron is deposited.

If it is wished to increase this thickness, it is sufficient to repeat the operation several times, corresponding to the thickness required; the operation is thus performed four times to obtain a layer of 2 microns. The coating obtained is of dark grey appearance and offers excellent protection against corrosion.

It is also possible to perform this operation whilst employing a metal other than zinc and such as, for example, cadmium, chromium, nickel, aluminium; moreover, after depositing the coating, it is possible to expose it to a tempering operation intended to cause its diffusion; it is apparent that the conditions of application vary according to the nature of the metal to be deposited, which are determined in each particular case; it is thus that the vacuum within the hollow element, the intensity of the current, and the voltage will vary according to the metal to be deposited.

Claims

What I claim is:

1. Apparatus for the application of a metal coating to the internal cylindrical surface of a steel bottle for compressed or liquefied gases, said bottle being of the type having a reduced neck that opens along the axis of said cylindrical surface, said apparatus comprising a removable sealed enclosure adapted to be placed on said neck, said enclosure being cylindrical and having a cylindrical side wall and opposite end walls, means carried by one of said end walls for registry with a said neck of a said bottle and defining an opening through said one end wall, pumping means connected to an opening into said sealed enclosure through said cylindrical side wall for generating a high vacuum of approximately 10.sup..sup.-4 torr in said enclosure and in said steel bottle, electrically conductive elements passing through and insulated from a wall of said enclosure, a source of current connected to the external ends of said conductive elements, a vapor emitter comprising a refractory steel wire coated with a layer of vaporizable metal to be deposited as the coating and wound in coil form and connected at one end to an end of an electrically conductive bar, said wire and bar lying generally along the axis of said bottle and connected at their opposite ends to the internal ends of said conductive elements, said wire and bar being adapted to pass through said hole and said neck for introduction into said steel bottle, means mounting said bar and refractory steel wire for sliding movement between a position in which said wire is entirely disposed within said enclosure and a position in which said wire extends through said opening and neck into said bottle, and means responsive to contact between said bar and the inside of the base of said bottle to cause an electric current to flow from said source of current through said wire to heat said wire to emit said vapor.