FIELD OF THE INVENTION The present invention relates to methods and products useful for inhibiting the oxidation of easily oxidized metals and, in particular, relates to the use of metal fluoroborate salts for in situ generation of boron trifluoride gas to prevent oxidation of molten magnesium and magnesium base alloys, and to a system for in situ generation of boron trifluoride from metal fluoroborate salts.

BACKGROUND OF THE INVENTION It is well known that the metals of the alkali and alkaline earth groups are attacked by air under ordinary conditions, resulting in the formation of various non-metal products such as oxides. The severity of this attack increases with increasing temperature, and has been recognized as a particularly serious problem in operations where such metals are exposed to air in a molten state, for example during the operations of melting, alloying, transferring and casting of such metals.

In particular, molten magnesium and magnesium base alloys react with oxygen in air to form a thin layer of magnesium oxide on the surface of the molten metal. However, the oxide layer formed by reaction with air is porous and is not sufficiently protective to prevent further oxidation of the molten metal. Due to its exothermic nature, the oxidation reaction may also create local over-heating of the molten metal, resulting in accelerated oxidation and formation of oxide"blooms"which spread out on the melt surface.

The presence of magnesium oxide is particularly undesirable in products which are produced from molten magnesium and magnesium alloys. For example, the presence of oxide particles may interfere with machining operations, produce discontinuities in the metallic structure which may be a source of weakness and serve as focal points for corrosive attack, and affect the appearance of the final product.

Numerous solutions have been proposed to reduce or eliminate the formation of oxides in easily oxidizable metals. However, none of these methods of protection is completely free from disadvantages. Early approaches to this problem included addition of an alloying element to the molten metal, covering the surface of the molten metal with a salt-based flux, or protecting the molten metal under a blanket of inert gas. The disadvantages of these methods are well known and have been partially overcome by the use of reactive gas systems, in which a chemical reaction between a gas and the molten metal produces a thin, highly elastic protective film on the molten metal surface.

Examples of reactive gas systems which have been used include carbon dioxide, which reacts with molten metals to produce a highly protective oxide skin, described in U. K. Patent No.

437,572, issued to Aluminium Limited on October 31,1935; sulfur dioxide in air; and fluorine- containing gases such as BF3 and SF6, as disclosed in U. S. Patent No. 1,972,317 issued to Reimers on September 4,1934 and U. S. Patent No. 3,400,752 issued to Unsworth on September 10,1968. Fluorine-containing gases react with the molten metal to form a protective film consisting of oxide and/or fluoride on the surface of the molten metal.

Fluorine-containing gases have become the preferred reactive gas systems, and SF6 has been particularly preferred at least partially due to the fact that it is effective when used in low concentrations in air or carbon dioxide (U. S. Patent No. 4,089,678 issued to Hanawalt on May 16,1978), and is relatively non-toxic. However, it has been recently discovered that SF6 is a "greenhouse"gas, with 1 kilogram of SF6 in the atmosphere having approximately the same greenhouse effect as 24 metric tons of carbon dioxide. Furthermore, the life expectancy of SF6 in the atmosphere is estimated to be about 3,200 years. Therefore, it is likely that the use of SF6 will be restricted in the near future, requiring the development of alternate reactive gas systems for protection of molten metals.

Reactive gas systems which do not utilize SF6 are known in the prior art. For example, the above-mentioned Reimers patent discloses a number of compounds which are liquids or solids at ambient temperatures, but which produce fluorine-containing gases when heated to a sufficient temperature. Among such compounds listed in the Reimers patent are ammonium fluoroborate (NH4BF4), fluoroboric acid (HBF4) and diazonium fluoroborate (C6H5N2BF4). It is known that when such compounds are heated to a temperature at which they decompose, boron trifluoride

(BF3) gas is produced. It is generally recognized that boron trifluoride is at least as effective as SF6 with respect to protection of molten metals, but that SF6 is less toxic and therefore has been preferred for environmental reasons. However, the fluoroborate compounds disclosed by Reimers have been found to provide improper protection for molten metals.

Therefore, a system for protecting molten metals from oxidation is desired which achieve a similar level of protection as SF6, but which does not cause damage to the ozone layer.

SUMMARY OF THE INVENTION The inventors have discovered a method for preventing oxidation of an easily oxidizable, molten metal which utilizes a protective agent comprising a solid metal fluoroborate salt which, when caused to decompose, produces in situ boron trifluoride gas which effectively prevents oxidation of the surface of the molten metal.

The inventors have found that prior art fluoroborates, such as those disclosed in the above-mentioned Reimers patent, are unsuitable for use in protecting molten metals by reason that the hydrogen and/or nitrogen atoms contained in these compounds react with molten metals such as magnesium to produce hydrides and nitrides, respectively. The formation of magnesium hydride causes blistering and burning of the surface of molten magnesium, and magnesium nitride causes undesirable sludge formation in the molten magnesium. In addition to these disadvantages, ammonium fluoroborate has the additional disadvantage that little decomposition to boron trifluoride gas is observed when ammonium fluoroborate is heated to its sublimation temperature. Sublimation is undesirable since it does not result in production of boron trifluoride gas and may cause deposition of solid ammonium fluoroborate in portions of the apparatus which are at a temperature cooler than the sublimation temperature. Therefore, use of ammonium fluoroborate may result in clogging of apparatus lines etc.

The inventors have found that the above-discussed disadvantages of prior art fluoroborates are overcome by the use of solid metal fluoroborate salts. The most preferred metal fluoroborate salts for use in the process of the present invention are sodium fluoroborate

(NaBF4) and potassium fluoroborate (KBF4). These compounds are solid at ambient temperature, and are therefore more easily handled than liquid or gaseous compounds. Also, these compounds do not contain hydrogen, nitrogen or other elements which react in a detrimental way with molten metals such as magnesium. Further, in situ decomposition of these compounds in the presence of molten magnesium and magnesium alloys results in the formation of protective boron trifluoride gas.

Therefore, in one aspect, the present invention provides a method for preventing oxidation of an easily oxidizable, molten metal having a surface exposed to an oxygen-containing atmosphere. A quantity of a solid metal fluoroborate salt is placed in an enclosed chamber heated to a temperature sufficient to cause the solid metal fluoroborate salt to decompose to produce boron trifluoride gas and a metal fluoride. The boron trifluoride gas is then mixed with an oxygen-containing gas to produce a boron trifluoride-containing gas mixture, which is then transferred to an enclosed vessel containing the molten metal in contact with the oxygen- containing atmosphere.

In another aspect, the present invention provides a system for preventing oxidation of an easily oxidizable, molten metal having a surface exposed to an oxygen-containing atmosphere.

The system comprises a source of solid metal fluoroborate salt and a source of an oxygen- containing gas connected to a decomposition chamber, heating means for heating the decomposition chamber to a temperature at which the solid metal fluoroborate salt decomposes to produce boron trifluoride gas and a metal fluoride, a closed vessel for containing the molten metal in contact with the oxygen-containing atmosphere which is connected to the decomposition chamber by a gas conduit.

In preferred aspects of the invention, the system may additionally comprise drying means to dry the oxygen-containing gas before it is mixed with the boron trifluoride, and an additional conduit which is adapted to combine the boron trifluoride gas mixture with additional oxygen- containing gas, and a scrubber connected to an outlet of the vessel In yet another aspect, the present invention provides a method for preventing oxidation of an easily oxidizable, molten metal having a surface exposed to an oxygen-containing atmosphere.

The method comprises placing a quantity of a solid metal fluoroborate salt in proximity to the

surface of the molten metal, and heating the salt to a temperature at which it decomposes to produce boron trifluoride gas. Preferably, the solid alkali fluoroborate salt is sodium fluoroborate or potassium fluoroborate, and the molten metal comprises magnesium or a magnesium base alloy. In a preferred embodiment of this method, the solid alkali fluoroborate salt is heated by the molten metal to a temperature at which it decomposes to produce boron trifluoride gas.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be more fully described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration, partly in cross-section, of a preferred gas generating system for use in the process according to the present invention; Figure 2 is a graph of boron trifluoride partial pressure versus decomposition temperature for potassium fluoroborate; and Figure 3 is a graph of boron trifluoride partial pressure versus decomposition temperature for sodium fluoroborate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The present invention provides a method and a system for preventing oxidation of easily oxidizable metals such as magnesium, aluminum, beryllium, and alloys thereof. These metals are readily oxidized by contact with gases of the atmosphere, particularly oxygen. However, the term"oxidation"as used herein is intended to mean not only the reaction of oxygen with a readily oxidizable metal but also the chemical reaction of such a metal with nitrogen, carbon dioxide, carbon monoxide, sulfur dioxide or any other gases which may be constituents of the atmosphere under which these metals are maintained in their molten condition.

The process of the present invention can be used in a wide variety of industrial processes

involving molten metals, for example cold and hot chamber die-casting machines, holding furnaces in sand-foundries/sand-casting, ingot casting, sand mold purging, pouring into molds, and direct chill casting.

The preferred metal salts for use in the process of the present invention are alkali metal and alkaline earth metal fluoroborate salts which are solid at ambient temperatures. More preferably, the alkali metal fluoroborate salts are selected from the group comprising lithium fluoroborate, sodium fluoroborate, potassium fluoroborate, rubidium fluoroborate, cesium fluoroborate, and the alkaline earth metal fluoroborate salts are preferably selected from the group comprising calcium fluoroborate, strontium fluoroborate and barium fluoroborate.

For preventing oxidation of molten magnesium and magnesium base alloys, the most preferred metal salts for use in the process of the present invention are the alkali earth metal fluoroborate salts, more preferably sodium fluoroborate and potassium fluoroborate. As shown in Figures 2 and 3, both potassium fluoroborate and sodium fluoroborate decompose when heated to a temperature at or above the melting point of magnesium (651 °C). However, potassium fluoroborate is preferred at least for the reason that it forms a eutectic composition with its decomposition product, potassium fluoride (m. p. 875°C), at a temperature of about 460°C. The KF: KBF4mole ratio at the eutectic point is 25.5: 74.5. This permits easy removal of the potassium fluoride decomposition product from the crucible by decanting the liquid eutectic mixture.

The exact amount of boron trifluoride gas fed to the surface of the molten metal can be controlled by controlling the decomposition of the metal fluoroborate salt. This can be achieved in a number of ways. For example, the compound can be fed to a high temperature decomposition chamber maintained at a sufficiently high temperature to cause decomposition of the fluoroborate compound. The boron trifluoride gas produced by the decomposition can be then mixed with air or another appropriate carrier gas to achieve the desired concentration necessary for protection of a molten metal.

A boron trifluoride atmosphere can also be created by dropping, dusting or otherwise placing, at suitable intervals of time, small quantities of the metal fluoroborate salt on the surface of the molten metal, or on a hot surface immediately out of contact with the molten metal. For

example, where the molten magnesium is contained in a covered crucible, the solid fluoroborate compound may be placed in a cage or basket within the crucible to constantly purge the interior of the crucible of air and to replenish any boron trifluoride gas which may escape the crucible.

The cage or basket may for example be suspended from the underside of the crucible cover and be secured by means of a clamp on the upper flange of the crucible. Alternatively, the metal fluoroborate salt can be directly placed within an empty crucible, and molten magnesium may be poured into the crucible directly over the metal fluoroborate salt.

However, it is preferred that boron trifluoride be generated in a decomposition chamber prior to being fed to the crucible. The use of a separate decomposition chamber allows the amount of boron trifluoride being fed to the crucible to be precisely controlled, thereby ensuring that a sufficient, but not excessive, amount of boron trifluoride enters the crucible. The use of a system containing a separate decomposition chamber also permits the temperature of the gas to be adjusted to a temperature which is the same as or close to the temperature of the molten metal, thereby avoiding thermal effects. In addition, generation of boron trifluoride in a decomposition chamber avoids problems such as flash decomposition which could occur when a solid compound is contacted with the surface of the molten metal.

Figure 1 schematically illustrates an example of an in-situ gas generating system 10 for preventing oxidation of molten magnesium using the process of the present invention. In this preferred embodiment, potassium fluoroborate stored in a bulk container 12 is fed by a screw feeder 14 to a heated decomposition chamber 16. The temperature of the decomposition chamber 16 is such that the potassium fluoroborate will decompose to produce boron trifluoride gas and potassium fluoride, which is discharged from the decomposition chamber as a solid or a melt through discharge outlet 18.

Air stored in a cylinder 20 is passed through a conventional drying system 22 to produce a stream of dry air 24. The dry air stream 24 is then split into first and second dry air streams 26 and 28, respectively. The first dry air stream 26 is fed to the decomposition chamber 16 via an inlet valve 30 which adjusts the flow of dry air into the decomposition chamber 16. Inside the decomposition chamber 16, the boron trifluoride gas is mixed with dry air such that the gas stream 32 exiting the decomposition chamber 16 comprises a boron trifluoride/air mixture. Gas stream 32 from chamber 16 is then further mixed with the second dry air stream 28. An outlet

valve 34 is provided to control the flow of gas exiting the decomposition chamber 16, thereby controlling the amount of boron trifluoride in the mixed gas stream 36 which results from combining gas streams 32 and 28. As discussed above, the system 10 may further comprise heating means (not shown) to heat one or more of the dry air streams 24,26,28 and/or the boron trifluoride-containing gas streams 32,36 to a temperature at or close to the temperature of the molten metal, but preferably not greater than a temperature at which excessive decomposition of boron trifluoride occurs.

The mixed gas stream 36 containing boron trifluoride and dry air is fed into a magnesium melt furnace 38 which contains a covered crucible 40 containing a molten metal 42 such as magnesium or a magnesium base alloy, with an air space 43 being provided above the surface of the molten metal 42. Inside crucible 40, BF3 gas reacts with the surface of the molten metal 42 to produce a thin, highly elastic protective film 44 on the surface thereof. At low concentrations of boron trifluoride (e. g. < 0.1% by volume above the melt), the protective film 44 may be primarily comprised of magnesium oxide, similar to the oxide thin film that forms with air, but also containing absorbed fluorine and being smoother and more elastic, possibly due to controlled oxidation of the melt surface. At higher concentrations of boron trifluoride (e. g. > 0.1% by volume above the melt), the protective film 44 may be primarily comprised of magnesium fluoride (MgF2) formed by reaction of magnesium with boron trifluoride.

Preferably, the concentration of boron trifluoride in air, both in the gas stream and above the molten metal, is up to about 2 % by volume, and more preferably from about 0.05 to about 0.1 % by volume.

The temperature of the molten metal is preferably maintained in a range in which the molten metal is workable, the rate of boron trifluoride decomposition is not excessive, and in which an effective passivation layer is formed on the surface of the metal. For example, where the molten metal is magnesium, the temperature of the melt is maintained above the melting temperature of 651 ° C, and more preferably at or above 680°C. There will be some self decomposition of boron trifluoride at this temperature, such that about 5% of the boron trifluoride entering the crucible will be decomposed. When the temperature of the melt is raised to about 750°C, the amount of boron trifluoride decomposition is increased to about 10%, although this is considered by the inventors to be an acceptable amount of decomposition.

Therefore, the temperature of molten magnesium and magnesium alloys is preferably maintained in the range of from about 680 to about 750°C, and more preferably below about 700°C to generate an effective passivation layer on the surface of the molten magnesium.

Furthermore, it is preferred that the boron trifluoride/air mixture above the surface of the melt be periodically or continuously replenished. More preferably, the volume of the air space above the melt is changed at a rate of about 10 volume changes per hour.

Some of the reactions which occur when boron trifluoride comes into contact with air and molten magnesium are as follows: Mg + BF3 (g) + BF (g, unstable) Mg + BF3 (g) + 02 (air) (film) + MgF2 (film) + BOF + BF + F (g) Boron trifluoride therefore reacts directly with the molten metal to form a fluoride film and also interacts with oxygen to generate oxyfluoride gases such as BOF.

The crucible 40 has an outlet 46 through which gaseous byproducts of the process are fed to a scrubber 48 and subsequently exhausted to the atmosphere. Boron trifluoride gas and its byproducts are easily scrubbed and are freely soluble in water. For example, the solubility of boron trifluoride gas in water is 116 grams per 100 grams of water. Boron trifluoride reacts with hot water to generate boric acid and hydrogen fluoride which can be effectively neutralized.

Furthermore, boron fluoride dihydrate is a solid at 6°C, and therefore it may be possible to recover this byproduct as a solid.

The benefits of the process according to the present invention are further illustrated by the following examples.

EXAMPLE 1 The vapour pressures of potassium and sodium fluoroborate were determined by the transportation method, which essentially consists of passing an inert carrier gas over a sample of fluoroborate at a constant temperature. The vapour pressures were measured at various temperatures to generate graphs of boron trifluoride partial pressure versus temperature, shown in Figures 2 and 3 for potassium fluoroborate and sodium fluoroborate, respectively.

Potassium fluoroborate was observed to decompose in the range of about 350 to 800°C, the decomposition products being potassium fluoride and boron trifluoride gas. Preferably, the temperature should be lower than about 700°C in order to generate an effective passivation layer on the surface of molten magnesium. Gaseous byproducts of the reaction of boron trifluoride with molten magnesium in the presence of air may include small amounts of BOF and BF2.

Equations for reactions involving potassium fluoroborate decomposition in the presence of air and molten magnesium or molten magnesium/aluminum may be written as follows: KBF4 ^ KF + BF3 KBF4+MgoMgF2+KF+BF (g) KBF4 + Mg-AI * MgF2-AIF3 + KF + K3AIF6 + AIB2 KBF4 BF4+02 (air) + Mg w MgF2 + MgO + KF + K2O + BOF + BF3 Potassium fluoroborate begins to melt at around 570°C, while its decomposition residue potassium fluoride has a melting point of 875°C. However, in the presence of potassium fluoride there is a eutectic point of about 460°C corresponding to a composition of 25.5 mol% KF/74.5 mol% KBF4. The decomposition occurs gradually in the liquid and vapour phases.

The results of thermogravimetric analysis for several fluoroborates according to the present invention are presented below in Table I. <BR> <BR> <P>Table I

SaltTemp.TG/massTG/massDTGDTA*GaseousSolid range/° C loss/% loss/% Trna§ Tm ProductsProducts (obs.)(theor.) LiBF4160-35069.372.33340340d BF3LiF NaBF4450-700--650 240 p BF3NaF 370 m 650 d KBF4550-800-20.053.86 ~ 750 290 P BF3 KF,KBF4 550 m 700-100090.3-950 BF3 KF RbBF4550-1000100.0 980 240p BF3, RbBF4 550 m CsBF4550-100094.8-950 165 p BF3, CsBF4 CsF 530 m Ca(BF4)2 170-29061.063.47280285d BF3 CaF2 Sr(BF4) 2200-36050.951.91350350dBF3SrF2 Ba(BF4)2 270-42041.443.61405320 m BF3 BaF2 410 d * Abbreviations used in DTA column: d-decomposition, m-melting and p-polymorphic transformation.

All effects were endothermic.

EXAMPLE 2 This example demonstrates the effectiveness of potassium fluoroborate in protecting the surface of molten magnesium, using the apparatus shown in Figure 1. The crucible, containing 0.8 pounds of pure, molten magnesium had a cover comprised primarily of mild steel, but also having a portion comprised of a vycor (corrosion resistant ceramic) panel through which the molten metal could be continuously observed. A paddle was suspended in the melt with its handle extending outside the crucible, thereby permitting the surface of the metal to be vigorously

agitated and allow observations to be made as to whether the agitation induced any sparking, burning or formation of magnesium oxide powder.

Using a mass flow meter, a controlled amount of dry air was allowed to enter the crucible.

A controlled amount of fluoroborate was fed to the decomposition chamber maintained at an appropriate temperature to cause decomposition of the fluoroborate. The resultant boron trifluoride gas produced by decomposition by the fluoroborate was controlled, measured and intimately mixed with air in the decomposition chamber, and was passed over the surface of the molten magnesium in the crucible. The boron trifluoride gas concentration, both in the gas being fed to the crucible and in the protective atmosphere above the surface of the molten metal, was measured using an infra-red analyzer. The following parameters were held constant during the experiment: Surface area of molten ft2 Volume of gas above the surface of the molten ft3 Rate of gas flow into the ft3/min Leak rate of air into the protective ft3/min In order to obtain safe and efficient protection of the surface of the molten magnesium, a gas mixing unit having the ability to control both concentration and flow rate was employed. The gas mixing unit was also equipped with an air compressor and an integrated air dryer.

The gas was supplied to the crucible at high velocity through several nozzles, thereby providing a homogeneous concentration of boron trifluoride in the atmosphere above the surface of the molten magnesium. The nozzles were contained in a distribution tube which was fixed to the underside of the crucible lid. The velocity of the gas exiting the nozzle was selected to exceed or compensate the high buoyancy in the air above the melt.

The crucible was well sealed to minimize leakage of gas. This was done by using refractory seals between the lid and the crucible. In this example, the temperature of the molten magnesium was 750°C, and the concentration of boron trifluoride in the gas mixture entering the crucible was varied between 0 and 1% by volume. In each case, protection of the molten magnesium surface appeared to be excellent, with little or no sparking or burning being observed,

even with vigorous agitation. The oxide/fluoride skin on the surface of the molten magnesium remained bright and shiny when the boron trifluoride concentration was greater than 0.09% by volume. However, when the concentration was less than about 0.05% by volume, the skin or film turned a light tan color, but still provided effective protection of the melt surface.

It was also observed that the concentration of boron trifluoride gas which is metered into the crucible is slightly higher than that measured over the surface of the molten magnesium. The differences between these two measurements are generally larger than can be accounted for by air leakage into the protective atmosphere. It is believed that this difference is due to the reaction between the magnesium metal and the boron trifluoride gas.

The temperature of the molten magnesium is an important parameter in the protection of the surface, particularly where the metal fluoroborate salt is heated and thereby decomposed by the molten magnesium. For example, as the temperature of the molten magnesium was increased from 680°C to 750°C, the fraction of boron trifluoride decomposition was observed to double from 5 to 10%.

The moisture content of the air is also important, since boron trifluoride gas reacts with water. It has been observed that even small amounts of moisture in air significantly increases the rate of decomposition of the boron trifluoride gas. Therefore, it is preferred that the air used in the process of the present invention contains as little water as possible.

The results of this experiment are tabulated in the following Table II.

Table II: Effect of BF3 Concentration on Protection of Molten Mg at 750°C *BF3 Concentration(%byvolume)ObservedMetalProtection(withagitatio n) 0.8excellentprotection,bright,shinyfilm 0.5excellentprotection,bright,shinyfilm 0.1excellentprotection,bright,shinyfilm 0.05observedoccasionalbriefsparkswith vigorousagitation filmhasatan-whitecolour

* The gas metered into the protective atmosphere above the melt comprised extra dry air plus in- situ decomposed BF3 gas.

EXAMPLE 3 In this Example, pure magnesium was melted in a resistance electric furnace at 700 °C.

Potassium fluoroborate powder was placed in a stainless steel crucible at 825°C, resulting in the generation of boron trifluoride gas. The powdered fluoroborate was introduced into the decomposition chamber by using a venturi.

The BF3 gas was then carried into the melting furnace containing molten magnesium by dry air at a flow rate of 3700 cc/min.

It was shown in this experiment that a protective atmosphere containing about 1.0% by volume boron trifluoride prevented oxidation of the magnesium surface. In a similar experiment, a concentration of 0.5% by volume was used. The flow of gas to the melting furnace was stopped for approximately fifteen minutes, during which time the molten magnesium began to oxidize. The flow of protective gas was then resumed, and the oxidation reaction was observed to stop.