US2245155A - Process for the production of substantially stable fused saltpeter baths - Google Patents

Description

Patented dune i, 3941 this PROCESS 50R THEPRQDUCIEQN (BF STAN'gEALLY STABLE FUSED fiAK. a",

BATH

No Drawing. Application June 22, 1939, Serial- Nc. 280,572. In G June 2?, 1938 (or. its-2n 4 Claims.

This invention relates to a process for the production of substantially stable i'usedsaltpeter baths, consisting substantially of sodium nitrate or potassium nitrate or mixtures thereof.

In the heat treatment of light metals, use is frequently made of baths consisting substantially of fused saltpeter. In using such baths it has often been observed that the metals under treatment and also the metal containers-containing the baths become corroded in course of time, leading to a local staining of the surface of the workpieces under treatment and the progressive formation of a highly inconvenient sludge consisting of solid reaction products. Recent investigations have shown that these phenomena are substantially caused by a progressive decomposition of the saltpeter baths which initially leads to the formation of nitrate, accompanied by liberation of oxy en: said nitrite subsequently decomposes to form free alkali. In order to prevent the corrosion of the work= pieces in the bath, to add chromates to the saltpeter baths. While such additions may succeed in suppressing the action of the decomposition products of the saltpeter baths on the metal undergoing treat-- meant, they do not go to the root of the trouble since they do not prevent the decomposition, as such, of the baths.

It has now been ascertained that equilibria, dependent on the temperature, exist between saltpeter and its decomposition products, and

that the progressive decomposition oi saltpeter baths is caused by their tendency to attain the state of equilibrium at the actual working temperature of the bath. By incorporating at the outset in the iiundecomposed) fused saltpeter bath, such amounts of the decomposition products as are required to establish the equilibrium at the bath temperature, the decomposition ofthe bath can thus be avoided.

metals, especially it has already been proposed The decomposition of the saltpeter proceeds in accordance with the ibllowing equations:

the bottom,- as sludge, in the course of time. Since the saltpeter decomposition products thus continuously separate out from the bath, the decomposition practically never ceases.

Based on the foregoing discoveries, the present invention provides a process for the production ofa substantially stable fused saltpeter bath, which comprises adding to fused saltpeter at least such an amount "of alkali metal nitrite as will establish eqiulibrium between said saltpeter and said nitrite at the temperature at which said bath is intended to be used.

In carrying out the invention, it is preferable to add a slight excess of alkali metal nitrite over that corresponding to the equilibrium at the temperature at whichthe bath is intended used.

Since the addition, of the alkali metal nitrite in accordance with the invention considerably lowers the liquidus point of the saline mixture, the fluidity of the bath at a given temperature, is increased. In the heat treatment of metals, such high fluidity of the saline bath is important, inasmuch as the losses of bath material, due to its adhesion to the workpieces on their. removal from the bath, are correspondingly reduced. Moreover this reduction of the melting point of the swine baths occasionally enables them to be employed at temperatures at which baths consisting solely of saltpeter would already be solid.

In view of the relatively small quantities of nitrite required for stabilising the saltpeter baths, the fact that the nitrite itself decomposes into nitrogen trioxide and alkali metal oxide, is of small practical consequence. This decomposition of the nitrite can however be counteracted by incorporating a small proportion of an alkali (such as alkali metal carbonate) in the bath. In order to prevent corrosion of the metals by the alkali, said alkali is preferably added in the form of neutral or slightly alkaline reacting salts of heavy-metal oxyacids of the kind known to have a passivating action on metals, such as alkali metal monochromates, alkali metal tungstates, alkali metal vanadates, and also the less soluble chromates of the alkaline-earth metals and zinc.

The required proportions of these passivatlng additions vary within a very narrow range, e. g. between about 0.1% and 1% in the case of alkali metal monochromates. In the case of alkalineearth metal chromates or zinc chromate, the solubility of which in saltpeter melts at temperatures closely above the melting point of sallt= peter is very slight, but strongly increases with metal nitrite after in contact wlththesalinemeit oi portanceintheemploymenhortreatmentofiron and steel. because it prevents the formation of fled.

I claim:

l. A process for the production of a fused saltpeter bath substantially stable within a temperature range of between about 350 and 550' 0., which comprises adding to said fused saltpeter an amount of alkali metal nitrite approximately corresponding stoichiometrieally to the amount of nitrite found inthe saltpeter bath at the operating temperature without the addition of'alkali equilibrium is reached.

pound and slightly 2. A process for the production of a fused saitpeter bath substantially stable within a temperature range of between about 350 and 550 0., which comprises addina to said fused saltpeter an amount of alkali metal nitrite approximately corresponding stoichiometrlcally to the amount of nitrite found in the saltpeter bath at the operating temperature without the addition 0! alkali metal nitrite after equilibrium is reached. and further a small proportion of an alkali.

3. A process for the production of a fused saltpeter bath substantially stable within a temperature ranse of between about 350 and 550' 6.. which comprises adding to said fused aaltpeter an amount of alkali metal nitrite approximately corresponding stoichiometrically to the amount of nitrite found in the saltpetcr bath at the operating temperature without the addition of alkali metal nitrite after equilibrium is reached, and further a small proportion of at least one como! the group consisting of the neutral alkaline reacting heavy metal oxyacid salts having a passivating effect.

4. A process for the production of a fused saltpeter bath substantially stable within a'temperature range of between about 350 and 550' C which comprises addinl to said fused saltpeter an amount of alkali metal nitrite approxi-' mately corresponding stoichiometrically to the amount of nitrite found in the saltpeter bath at the operating temperature without the addition of alkali metal nitrite after equilibrium is reached, and further between 0.1 and 1% of an v alkali metal chromate.

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