Protective Coating For Plasma Apparatus

Abstract

A plasma nozzle and process for treating plasma nozzles used in operations with reactive gases in which a face of the nozzle, preferably the front face, is provided with means to protect against corrosion from reactive gases, the means consisting of a coating of a metallic element or alloy within the group consisting of gold, platinum, palladium, rhodium, iridium, osmium and or ruthenium.

Description

Extensive use is being made of plasma torches in which a gas stream is projected through an orifice in a nozzle, a direct current establishes an arc to form a plasma, and an alternating current is superimposed on the plasma to further heat it.

Examples of apparatus for creating a plasma of this character are shown in J. A. F. Sunnen U.S. Pat. Nos. 3,205,338 granted Sept. 7, 1965, and 3,248,513 granted Apr. 26, 1966, both for Equipment for Forming High Temperature Plasma. The intention is to relate the present invention to any of such plasma torches and to the protection of the faces of the nozzles used therein, especially the exposed faces surrounding the orifice through which the plasma issues and encountered by any reactive gas employed with the plasma, called the front faces.

When gases which are reactive at high temperature are used in plasma-producing apparatus which superimposes an alternating current on a plasma created by a plasma-sustaining direct current, and circulating between two or more electrodes (nozzles), the front faces of nozzle or nozzles, which are in contact with any high temperature reactive gases employed with the plasma may become deeply corroded. For example, where the reactive gases are oxidizing, the front face of the nozzle which is likely to be made of a metal such as copper, becomes oxidized, and the oxidized areas from impact points for the superimposed alternating current arc, creating instability of the superimposed alternating applied voltage after several hours of operation, and localized extensive corrosion of the nozzle.

One of the major purposes of the present invention is to overcome the instability of the alternating current voltage and to almost eliminate such instability.

In accordance with the present invention nozzles used in plasma-producing apparatus operating with reactive gases as above described are provided with protective coatings on the front faces against the attack of the reactive gases.

The protective means used consists of metallic elements which have an electrochemical potential lower than -0.80 volt, or of alloys thereof, and which are applied by a suitable means to the faces of the nozzles, for example by plating. Within the limit referred to, the affinity of these elements for reactive gases is extremely weak.

In the drawings I illustrate one only of the numerous embodiments in which the invention may appear, selecting the form shown from the standpoints of convenience in illustration, satisfactory operation, and clear demonstration of the principles involved.

FIG. 1 is a diagrammatic axial section of plasma torches according to the invention.

FIG. 2 is an enlargement of a portion of FIG. 1.

FIG. 3 is a section on the line 3-3 of FIG. 2.

FIG. 4 is a section on the line 4-4 of FIG. 2.

In order to illustrate a system of torches to which the present invention is applicable, I show a torch system from Sunnen and Schoumaker U.S. Pat. application Ser. No. 788,209, filed Dec. 31, 1968, for Heating a Reactive Fluid to High Temperatures.

There are three plasma torches 20, 21 and 22, whose projections in plan view are set at 120.degree. to one another in FIG. 1, and oriented in fact along the generatrices of an inverted cone having its apex directed downward and located at the zone of convergence of plasma jets 23, 24 and 25 which respectively emerge from these three torches. For the sake of simplicity, the torches are shown as if they all lie in one plane. Each of said torches comprises a central solid electrode 26, 27 or 28, and a coaxial hollow electrode 30, 31 or 32. The torches 20, 21 and 22 are fed respectively from sources of direct current 33, 34 and 35 to sustain a direct current arc between the solid electrode and the hollow electrode of each torch. Means to inject a gas such as an inert gas, for example argon or nitrogen is suggested by arrows 36 (FIG. 1) through the interior of the hollow electrode from a suitable gas conduit at the back of the hollow electrode so as to create the plasmas 23, 24 or 25.

The hollow electrodes 30, 31 and 32 are each connected to one distinct phase of a source of three-phase alternating current 37 whose neutral point 38 is at the same potential as the center junction 40 of the zone of convergence of the plasma jets. The apparatus shown in FIG. 1 also includes a vertical tube 41 located above the zone of convergence of the plasma jets through which a material 39 is introduced to enter the plasma jets and to cause or undergo any suitable reaction or action. A reactive gas suggested by arrow 53 reacts chemically with the material 39 and first comes into contact with the plasma near the front faces of the nozzles 30 to pick up the superimposed alternating current, which they later discharge to the neutral point or center junction 40. Due to the rotating magnetic field created, the plasma jets and the reactive gas are subjected to swirling action.

It will be evident that the reactive gas by being brought into contact with the front faces of the nozzles of the plasma torches, creates a hazard from the standpoint of corrosion which will impair the stability of the alternating voltage.

In FIGS. 2, 3 and 4 the construction of the individual torches is shown more in detail. Each of the torches has an anodic nozzle 42 suitably made of copper and having a cooling medium such as water circulating through passages 43. The nozzle is provided with a center orifice 44 through which the plasma 23 issues. The solid electrode 26 is axially in line with this orifice and is conveniently mounted in an opening 45 of a plate 46 suitably secured to a tubular housing 47 which is mounted on the back of the nozzle 30 and insulated from it by an electrically insulating ring 48. The means for introducing the inert gas to create the plasma is then a chamber 36 which receives supply of inert gas from a conduit 50 controlled by a valve 51. The front faces of the nozzle 30 (and all other nozzles) are coated with a protecting layer 52.

Protective metals used in the layer 52 may be gold, platinum, palladium, rhodium, ruthenium, iridium or osmium or alloys of these metals such as gold-platinum, platinum-palladium, platinum-ruthenium, palladium-iridium, platinum-osmium, etc. As shown in the table, these elements have an electrochemical oxidation potential of -0.80 volt or lower. ##SPC1## These elements, however, possess unequal resistances to attack by chemical agents and they have varying melting points, differing electrical conductivities and differing physico-chemical characteristics as set forth in the table which shows their constants. The table shows the constants also for silver and copper as a means of comparison. The elements have been listed in order of resistance to corrosion by acids, and in order of melting points within each category of acid corrodability.

It should be noted that gold and copper are very close in melting point and also have comparable resistivity.

Tests performed on three copper nozzles used according to FIG. 1, electrolytically plated with gold to a thickness of approximately 0.02 mm. on the front faces of the nozzles in contact with oxygen at a temperature of 1500.degree.--1800.degree. C. have shown a distinct improvement as measured by better stability of the superimposed alternating current arc voltage in a practical example of eliminating corrosion. Thicker and thinner layers may be used if desired.

It should be noted that electrical contact of the nozzle with the plasma is preferably made by the copper surfaces so that the electrical resistance is not increased by the coating.

Since platinum is much more refractory than copper and also has much higher resistivity, a copper nozzle has been plated with platinum and much better results obtained in stability of the superimposed alternating current are by avoiding corrosion of the nozzle.

Where, however, heating is accomplished under corroding conditions which are very extreme, the above composition can be further improved.

As will be seen from the table, platinum, ruthenium, rhodium and iridium are classified in respect to increasing melting point, but rhodium, ruthenium and iridium have the highest resistance to chemical corroding agents. The alloys of platinum, ruthenium, rhodium and iridium among themselves are particularly interesting for protecting the nozzle in accordance with the present invention against virulent chemical agents such as oxygen at elevated temperatures, since the corrosion resistances of the alloys are much greater than those of the individual elements. For example, the platinum-iridium alloy in which platinum makes up 90 to 95 percent of the alloy of weight and iridium composes the balance of 5-- 10 percent is resistant to chemical agents which will corrode platinum alone and is highly recommended for the protective layer 52.

The protective layer can be applied to the surface of the nozzle which will be in contact with the reactive fluid by electroplating, by explosion plating or by plasma spraying. Plasma spraying is described in Belgian Pat. No. 623,218 and in U.S. Pat. application of J. A. F. Sunnen, Ser. No. 742,051, filed July 2, 1968, for Method and Installation for Forming a Plasma Jet at High Temperature, and U.S. Pat. application of J. A. F. Sunnen and H. R. P. J. Schoumaker, Ser. No. 788,209, filed Dec. 31, 1968, for Heating a Reactive Fluid to High Temperature.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of my invention without copying the apparatus and process shown, and I therefore claim all such insofar as they fall within the reasonable spirit and scope of my claims.

Claims

I claim:

1. A high temperature heating apparatus comprising a first and second electrically conducting electrode, the second electrode including a nozzle, the second electrode being provided with a face adjoining the nozzle exposed to corrosion, means for projecting a gas through the nozzle in the second electrode, means for applying a direct current electric arc between the first and the second electrodes to create a plasma issuing through the nozzle of the second electrode, and means for superimposing an alternating current which flows through the plasma, characterized in a coating on said face of the second electrode composed of a material selected from a class consisting of metallic elements and alloys of said elements in which there is as a principal component an element present whose electrochemical oxidation potential with respect to hydrogen is lower than -0.80 volt.

2. A heating apparatus of claim 1, in which said coating is of the class which consists of elements and alloys containing an element which is gold, platinum, palladium, osmium, iridium, rhodium, and ruthenium.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

3. A heating apparatus of claim 1, in which said coating consists of an alloy of 90-- 95 percent platinum and 5-- 10 percent iridium by weight.