US3345278A - Anodic passivation of metals - Google Patents

Description

Oct. 3, 1967 MEKJEAN ANQDIC PASSIVATION OF METALS Filed March 25, 1963 United States Patent This invention relates to the prevention of corrosion of metals. More particularly, it relates to the prevention of corrosion of metals in contact with a molten salt system and to a passivation system relating thereto.

Molten salt systems have great utility throughout the chemical industry. They may be employed as fairly high constant temperature heat sources for reactions, usually for cracking, reforming, disproportionating, halogen-ating, dehydroha'logenating, and catalytic reactions, including specific organic transformations, metatheses and recombinations. These systems a'hve also been found useful in electrolytic reductions of refractory metal salts to the metal, such as in the production of titanium, zirconium, tantalum, and niobium.

The alloy steel industry makes use of molten salt systems to assist in the conversion, and subsequent removal, of heat scale developed in annealing and hot rolling mill operations. These systems are generally termed descaling baths which fall into three main classifications. They are classified as (a) oxidizing, (b) reducing, and (c) neutral baths. These descaling baths are usually alkaline baths, e.g., molten caustic baths, and the type of additives incorporated with the caustic mel-t determines the classification of the bath. If sodium nitrate, an oxidizing agent, is added to the caustic melt, an oxidizing bath is formed, whereas a reducing agent additive produces a reducing bath. If a neutral agent (one neither oxidizing nor reducing in action) is added, e.g., sodium sulfate, a neutral bath is formed.

' New uses for molten salt systems are also being developed in the growing of crystals and of gem stones, for end uses in the fields of electronics, optics, electronic analyzers, jewelry, machine tool tips, phonograph styli and abrasives.

' The use of molten salt systems as a heat capacity heat sink or storage unit for home, office, or industrial heating is another area which presents unusual possibilities. By using lower cost electric power during periods of low demand, a caustic molten salt system may be capable of storing the total heat required for each 24-hour day. Using this system, the stored energy may be transferred to and then conveyed by hot water pipes, hot air ducts, or hot water radiators to the desired area.

In any molten salt system, however, an ever present problem is the corrosion of the component elements, especially those made of metal. The frequent replacement of equipment necessitated by this problem is not only costly, but at times is extremely inconvenient.

It is, therefore, an object of the invention to provide a method of protecting metals, in situ, from the corrosive action of molten salt systems.

It is a further object of this invention to create and maintain a substantially insoluble surface'film on metallic objects and equipment to protect them from corrosive media to which they are subjected.

It is still a further object of this invention to use a novel type of electrolytic passivation'to protect metals from cor rosion in an anhydrous molten salt system.

In accordance with this invention, it has been found that the metals in a system which are in contact with a substantially anhydrous molten corrosive salt bath may be passivated by applying electrical potential below the decomposition voltage of the molten salt system on the metal to be passivated and thereafter maintaining the voltage below the said decomposition voltage of the electrolytes present.

The various aspects of the invention will be apparent from the following description, taken with the drawing illustrative of an embodiment of the invention, in which:

FIGURE 1 is a vertical section of the apparatus of the invention along 11, and

FIGURE 2 is a horizontal view of the apparatus.

Referring to FIGURE 1, metal container 10 containsv a molten salt bath 12, sludge pan 14, gas fired immersion tubes 16, fired by gas manifold 18 and air manifold 20. The container 10 is equipped with jigs 22 to hold articles to be inserted into the molten salt bath. During operation, the molten salt bath is maintained at the desired temperature by the gas fired immersion tubes. Cathode 28 is inserted into the molten salt bath. Direct current is supplied to cathode 28 and anode'contact means 30 by a source of electric current 32, the electrical lines being schematically illustrated.

A metal is said to be passivated when its surface has been rendered comparatively, substantially, and preferably, completely inactive to its environment. It has been found that metals in contact with a substantially nonaqueous molten salt system may be passivated according to this invention in either one of two ways, depending on the compostiion of the molten salt system and the metal incontact with the system; The metal may be made the anode, causing oxidation to take place at its surface to form a substantially insoluble protective film on the metal, or the metal may be made the cathode, causing reduction to take place at its surface and preventing the repeated development of soluble films on the metal. This latter procedure passivates certain metals, which, if made the anode, might have a soluble film formed on its surface. Such a film could continually dissolve in the molten salt system, causing the metal of the container or other part to corrode at an accelerated rate. If the metal is'maintained cathodic, the continual formation of a soluble film is prevented, thus passivating the metal to the molten salt system.

The passivating system disclosed herein may be useful in passivating metals in contact with an alkali metal hydroxide salt bath containing oxidizing agents, reducing agents, neutral agents, or mixtures thereof. It may also be useful in passivating metals in contact with various molten anhydrous salt bath mixtures, such as alkali metal molten salt systems, e.g., sodium and potassium fluoride or other alkali metal anhydrous salt systems; alkaline earth metal salt system-s, e.g., calcium chloride, magnesium sulfate; and any combination of salts useful as a molten salt system, e.g., sodium sulfate and barium chloride. It is even within the contemplation of the invention that a metal in contact with pure molten metal such as sodium in the presence of an anion selected from the group consisting of hydrides, sulfates, nitrates, persulfates, permanganates, sulfides, phosphates, carbonates, peroxides, chromates, sulfites, thiosulfates, halides, etc., may be passivated by forming a substantially insoluble film on its surface by the technique described herein. The alkali metals which may be suitably utilized in the invention are sodium, potassium, lithium, rubidium, cesium and francium. The suitable alkaline earth may beselected from the group consisting of beryllium, magnesium, calcium, strontium, barium, and radium.

The passivation system as described herein is preferably utilized in a molten salt system that has, as its major constituent, sodium hydroxide with the remaining proportion of the system made up of anions containing electrolytically available oxygen. The anions which may be utilized in the preferred salt system in combination with Patented Oct. 3, 1967- hydroxyl ions may be selected from the group consisting of nitrates, persulfates, permanganates, sulfides, sulfates, phosphates, carbonates, peroxides, chromates, sulfites, thiosulfates, hydrides, etc., all of the combinations of which have electrolytically available oxygen so that an oxidation reaction may take place at the surface of the metal in contact with the molten salt system to form a substantially insoluble film on the metal surface.

Although electrolytically available oxygen is preferred in forming the insoluble film on the metal surface, it may be desirable in a salt system to require the formation of a substantially insoluble film which is not an oxide of the metal. For example, in a salt system where the available anion is fluoride ion, it is preferred to form an insoluble metal fluoride film on the surface of the metal and thus, passivate the metal.

In the preferred molten salt system of sodium hydroxide and sodium sulfate, the preferred proportion of sodium hydroxide to sodium sulfate may be 55 to 99 percent by weight sodium hydroxide and 1 to 45 percent by weight of sodium sulfate, with more preferred proportions being from 80 to 99 percent by weight sodium hydroxide and 1 to 20 percent by weight sodium sulfate. The most preferred proportion is about 90 percent sodium hydroxide and 10 percent sodium sulfate.

All metals used in the molten salt system, such as the container pot, the heating elements, sheathing of the heaters and probes, heating tubes, thermocouple probes, bafile plates, pumps, sludge pans, agitators, heat exchangers, electric resistance heaters, immersion rollers, quiet zones or sludge settling areas, sparger tubes and submerged gas lines, and any other equipment exposed to the molten salt system, will be passivated by the technique of the invention. The preferred metal used in molten salt systems varies with the salt system utilized. Therefore, each system should be evaluated prior to determining the metal to be used in the system. The determination is one which includes economic considerations and the result desired to be achieved. A technique that may be used is maintaining all the metal elements anodic, thus forming a protective, substantially insoluble film, usually oxide, on the surface of the metal, or alternatively, where suitable, the metal elements may all be maintained cathodic or some may be maintained anodic and others cathodic. Metals which are generally used in molten salt systems may be used in practicing the invention, such as nickel, silver, low carbon steel, e.g., American Iron and Steel Institute 1010, other ferrous alloys, such as stainless steel types 302, 304, 310, 316, 405, 410, and 416, Hastelloy B and Carpenter 20, and the rare metals, e.g., titanium, zirconium, hafnium, niobium. For the preferred molten salt system of sodium hydroxide and sodium sulfate, the metal preferred to be utilized in the system is a low carbon steel. The passivation technique preferred for this particular system is the formation of a substantially insoluble oxide film on its surface.

The temperature range at which the described invention may be employed is from 25 degrees centigrade to 3,000 degrees centigrade, which in all cases will be below the melting point of the metal being passivated. A preferred temperature is from 100 degrees centigrade to 1,000 degrees centigrade, and a more preferred range is between 250 degrees centigrade and 800' degrees centigrade. In the preferred salt system, the temperature may be between about l70 degrees centigrade and about 650 degrees centigrade. Variation in temperature within this range has no adverse effect upon the passivated metals.

It is to be understood that for the process and passivat-' ing system of this invention to be effective, the molten salt system is substantially anhydrous. A slight amount of water present would be decomposed by electrolysis to hydro gen and oxygen gases and removed as such from the system. This may also be true when the molten salt system picks up water from the atmosphere or by condensation.

when the temperature of the bath is lowered.

4 The maximum current required to passivate a metal in contact with a molten salt system depends on the decomposition voltages of the molten electrolyte or electrolytes present. The electrical resistance of the molten salt system and the distance between the electrodes determine the current that flows in a system. The decomposition voltage of each component may be determined by utilizing the Gibbs-Helmholtz equation:

J=electrical equiv. of heat (the Joule constant) z=valence of the metal F =free energy of formation T=degrees Kelvin (absolute temperature) or by referring to known decomposition tables, as for example, those illustrated at pages 242 and 243 of Electrochemistry, volume 1, Creighton and Koehler, fourth edition (John Wiley & Sons), 1951. The current density on the metal to be passivated may be varied between 0.0025 ampere .per square foot to a maximum of about 10 amperes per square foot in the preferred sodium hydroxide-sodium sulfate molten salt system. The maximum current density, as stated above, will depend primarily on the decomposition voltage of each molten salt system and the electrode material utilized. In the preferred system, about 5 amperes per square foot of current are applied, when utilizing a zirconium cathode with a low carbon steel container as the anode. At first, the current may floW readily, but within 2 to 10 seconds, it may decrease to almost zero. At this time the current density may be reestablished by increasing the voltage until a steady-state equilibrium situation is achieved, i.e., the current flow remains constant. At this point, it is found, in the preferred embodiment of the invention, that a voltage of 2.6 volts maintains a current of 5 amperes. It was found that by maintaining such a current in the preferred embodiment the low carbon steel container walls are very well passivated. Other current flows at voltages below the decomposition point of the bath are also useful to obtain this effect. It is believed that the initial surge of current creates a monomolecular film of ferric oxide on the total surface of the anode immersed in the molten salt system. As the current continues to flow, the oxide film builds up to a thicker and thicker layer, creating a greater and greater resistance, which continues to limit the current flow. Once the thickness of the oxide film is established, the current flow, if desired and if the decomposition voltage of the ionic liquid is not exceeded, may be increased by increasing the voltage. In the preferred embodiment of this invention, a voltage of from about 2.4 to about 2.8 volts was found to be suitable to maintain the passivating film on the surface of the vessel.

However, voltages between about 0.005 volt and 3.7 volts may be usefully employed, depending on the molten salt system, the temperature and the electrodes utilized. Direct current or rectified alternating current is utilized in the practice of the invention.

As mentioned above, in the preferred embodiment of the invention, zirconium was used as a long-lived sacrificial cathode. Other cathode materials, however, may be selected, according to their efficiencies, life in the molten salt system utilized, and cost. Among the metals which may be used as a cathode and which exhibited superior electrical and corrosion-resistant properties, are copper, nickel, silver, molybdenum, their common alloys, and copper or silver sheathed in zirconium.

These are the initial reactions and thereafter there are essentially no reactions at the anode or cathode, but the potential applied maintains the passive film. The

Foo F6 created at the anode is a substantially insoluble film which passivates the metal against the corrosive environment.

From the above description, it is readily seen that a low carbon steel container may be passivated in a corrosive molten salt system comprising 90 percent sodium hydroxide and 10 percent of another salt, such as Na SO Na S O Na S O Na S, and Na S 0 or mixtures thereof. (This is a reducing system.) The vessel is made so the anode and zirconium (or other suitable material indicated above) may be utilized as the cathode. The reactions illustrated in Equations 1, 2, and 3 are believed to be substantially the reactions which take place in this system to passivate the metal container.

An aluminum container holding a mixture of sodium hydroxide and sodium sulfate may also be passivated, following the description of this invention, by making the container wall anodic. A surface him may be created on the aluminum which is substantially insoluble. The reactions which are thought to take place at the anode and cathode of this system, utilizing a zirconium cathode, are as follows:

At the cathode:

In this case, the decomposition voltage is not exceeded and molecular oxygen is not released. A substantially insoluble aluminum oxide and/ or sulfate film is created and maintained at the anode, that passivates the metal and acts as an electrical insulator. Thus, little current is required to maintain the film after the initial flow of current. This electrolytically-created and maintained oxide and/or sulfate film is substantially non-soluble in the molten salt system, preserving the aluminum container from catastrophic corrosion which would normally be encountered.

The low carbon steel walls of a vessel with a molten salt alkaline hydroxide oxidizing bath therein may also be passivated and maintained in a passive state in accordance with this invention. A molten salt system of sodium hydroxide, sodium nitrate and sodium nitrite, although essentially not exceedingly corrosive to steel, gradually undergoes chemical change, absorbing CO from the atmosphere and creating sodium carbonate at elevated temperatures such as those greater than 700 degrees centigrade. The sodium nitrate decomposes to the nitrite and oxygen, which in turn continues to decompose to sodium monoxide and nitrous oxide. While the nitrate is present, it chemically creates a passive, insoluble film on the surface of steel. When the nitrate-nitrite mix is gone, and replace by sodium monoxide in the melt, a very corrosive system is established. By imposing an anodic current on the steel container, at an elec-tromotive force not exceeding the decomposition voltages of NaO NaNO or 6 NaNO the components of the mix, a passive iron oxide film is created and maintained, protecting the vessel.

As mentioned hereinabove, these are some molten salt systems and container combinations wherein the film formed on the container walls, if they are made anodic, is more soluble in the melt than the base metal itself. For example, a caustic alkali melt containing an oxidizing agent in a silver vessel will form a soluble film on thesilver surface and cause corrosion of the base metal. To prevent the formation of a soluble film the container walls may be made cathodic and a current applied, in accordance with the description of this invention, to prevent the formation of such a soluble film. The base metal in this instance would'have the greater resistance to dissolution than the metal with the oxide film forming thereon.

It will be recognized by those skilled in the art that various modifications within the invention are possible, some .of which are referred to above. Therefore, the invention is not to be interpreted as limited to the following examples. All values in the following examples are by weight and temperatures are by degrees centigrade unless otherwise stated.

Example I To a low carbon steel vessel was added 3,000 parts of sodium hydroxide and 570 parts of sodium sulfate. The container was made anodic and a zirconium electrode was made cathodic and immersed in the salt mixture. The mixture was then heated to about 360 degrees centigrade. The current was initially established at 5 amperes at 2.6 volts, impressed and was maintained for 432 hours. At that time, the current was deliberately reduced to 2.5 ampers at 2.2 volts, and this was continued for an additional 215 hours. At the end of this period, after a total elapsed time of 647 hours, there was essentially no evidence of corrosion. A thin film .of approximately 4 inch thickness was noted on the inner walls of the container and the weight of the container had decreased, from the 3445 parts it initially weighed, by only 7.5 parts.

Example II The experiment of Example I was followed up by reestablishing all the identical conditions of salt composition and temperature in the same steel pot container, but without any of the present electrolytic passivation techniques being applied. After 187 hours, the evidence of corrosion was so apparent that the experiment was terminated and the situation evaluated. The container was found to have lost weight equivalent to 65.5 parts from an initial weight of 3445 parts. Had the experiment been allowed to continue for the same total time as that of Example I, or 647 hours, and assuming that the rate of corrosion was uniform, the container would have lost 227 parts by weight, or over 30 times the weight loss suffered without passivation. It may be seen, therefore, that the electrolytic passivation technique employed in Example I is effective in reducing corrosion by a favorable factor of at least 30 to 1.

Example III Two steel heating tubes were immersed in a substan tially anhydrous fused salt mixture containing 2160 parts of sodium hydroxide, 216 parts of sodium sulfate, and 24 parts of a combination of Na S O Na S O Na S O Na SO and N18. The fused salt mixture was heated to a temperature of 425 degrees centigrade. One of the steel heating tubes was made anodic while the other was not connected to any electrical system. These tubes remained immersed in the fused salt mixture for a total of about 1400 hours. About 17 percent of this time, the bath was held at temperatures between about 625 and 675 degrees centrigrade. For most of this time, the voltage impressed on the anodic steel tube was 1.5 volts at a rectifier employed and 0.9 volt between the tube and cathode, resulting in a current of 2.5 amperes. A zirconium cathode was utilized in this example. For a part of the time, the voltage applied to the passivated tube was 0.05 to 0.1 volt. The average rates of corrosion were then measured for the passivated and unpassivated tubes. The average metal loss on the passivated tube from .one wall was 0.00011 7 Example IV A salt bath melt having the composition of Example III was again utilized. This melt was held at 380 degrees centigrade. A steel tube was immersed in the bath and 2.7 volts was applied, making the tube anodic. A copper cathode was utilized in this system. The pot walls were made anodic and a nickel diaphragm was placed in the pot, but was not electrically connected to the passivation system. After 196 hours, the system was shut down and the nickel diaphragm was examined. It was observed that corrosion of the diaphragm was extensive but the pot walls showed no visible evidence of corrosion.

A new nickel diaphragm was then installed in the melt and connected to the passivation circuit of the pot. A voltage of 1.2 volts was maintained between electrodes. A current density of 0.00508 ampere/square inch was employed.

After a total of 362 hours for the pot, and 166 hours for the new diaphragm, the nickel diaphragm and the pot wall-s were visually examined. Neither the pot walls nor the diaphragm was visibly attacked by the melt..

Example V A salt melt of the composition set forth in Example III was again charged to a steel pot and a zirconium diaphragm was connected to the anodic passivation circuit. A copper cathode was utilized in the melt to complete the circuit.

The temperature of the melt was varied between 450 and 480 degrees centigrade, and 3 volts were impressed, which maintained 2 amperes in the passivation circuit. An average current density of 0.002015 ampere/square inch was employed. After 21 days, the steel pot showed no visible evidence of corrosion. The zirconium diaphragm also did not show any visible evidence of corrosion. Upon a weighing of this diaphragm, the visible observation was confirmed, as the weight remained constant.

Example VI A composition containing 95 percent NaOI-I and 5 percent Na POgwas charged into a steel pot. A steel baffie plate, a steel housing and a sludge pan with lifting rods connected thereto, were immersed in the melt. The temperature of the melt was held at between about 450 and 480 degrees centigrade. The baflie plate was completely corroded after 28 days. At this time, the other equipment was visually examined. All elements showed signs of ex cessive corrosion and a large volume of oxide was collected within the sludge pan, indicating rapid corrosion of the elements.

' Example VII Example VI was repated, utilizing a nickel diaphragm and a passivation circuit in addition to the equipment utilized in Example VI. The anodic passivation circuit was connected in parallel to all the elements, and a copper cathode completed the circuit. Initially 3.75 volts was applied between the electodes but this was gradually decreased to 0.9 volt. After six and half months, the system was shut down and all elements were visually inspected. There was no evidence of corrosion of any of the metallic elements connected to the passivation circuit.

8 Example VIII Equipment similar to the equipment utilized in Example VI was placed into a molten salt composition of V percent sodium hydroxide, 10' percent potassium chloride, and 5 percent sodium phosphate. A current of 3 amperes was applied to the passivation circuit with 1.8 volts at the rectifier, and 0.9 volt across the electrode.

After about three months, the salt was pumped out and the equipment was found to be in perfect condition, with no visible evidence of corrosion.

Example IX Example VIII was repeated utilizing a molten salt composition of percent sodium hydroxide, 1 percent sodium chloride, 1 percent sodium carbonate and 8 percent sodium nitrate. The same current and voltage of Example VIII were again applied for about three months. Again there was no visible evidence of corrosion of the elements.

The invention has been described with respect to preferred embodiments thereof, but is not to be construed as limited thereto. Variations of the invention may be made and equivalents may be substituted therein without going beyond the invention or the scope of the claims.

What is claimed is:

1. A method for anodically passivating a metallic element in contact with a substantially anhydrous molten com-position comprising contacting a substantially anhydrous molten composition comprised of at least 55 percent alkali metal hydroxide with a corrodible metallic element to be protected selected from the group consisting of steel, nickel, silver, titanium, zirconium, hafnium and niobium, contacting said anhydrous composition with a second metallic cathode element comprised of a metal selected from the group consisting of zirconium, copper, nickel, silver and molybdenum, applying a voltage of about 0.005 to 3.7 volts between said corrodible metallic element and said cathode element, said voltage being below the decomposition voltage of the anhydrous composition and sufficient to make the corrosive metal anodic and the second metallic element cathodic, thereby passivating said corrodible metallic element to the action of said substantially anhydrous composition.

2. The method of claim 1 wherein the temperature of the anhydrous composition is maintained between about the melting point and 3,000 degrees centigrade.

3. The method of claim 1 wherein the alkali metal hydroxide is sodium hydroxide.

4 The process of claim 1 wherein the cathode is zirconium.

5. The method of claim 1 wherein the metal to be protected is steel.

6. A method for anodically passivating a metallic element in a metallic container containing a substantially anhydrous sodium hydroxide comprising contacting a substantially anhydrous sodium hydroxide composition with a steel element to be protected at a temperature of 250 to 800 degrees centigrade, contacting said anhydrous composition with a zirconium cathode, applying a voltage of 0.005 to 3.7 volts between said steel element and said cathode, said voltage being below the decomposition voltage of said sodium hydroxide composition, thereby making said steel element anodic and passivating said steel element to the action of said substantially anhydrous sodium hydroxide compositions.

References Cited UNITED STATES PATENTS 476,914 9/1892 Bernard 204-196 641,438 1/1900 Darling 204l96 1,507,395 9/1924 Mershon 204-496 1,545,384 7/1925 Ashcroft 204196 2,311,257 2/1943 Sawyer et al 204196 2,485,276 10/1949 Gerbes 204-496 9 7/ 1951 Peyches 204-147 10/1959 Nies 204147 8/1965 MacNab 204147 FOREIGN PATENTS 2/1962 Great Britain.

10 OTHER REFERENCES

Claims (1)

1. A METHOD FOR ANODICALLY PASSIVATING A METALLIC ELEMENT IN CONTACT WITH A SUBSTANTIALLY ANHYDROUS MOLTEN COMPOSITION COMPRISING CONTACTING A SUBSTANTIALLY ANHYROUS MOLTEN COMPOSITION COMPRISED OF AT LEAST 55 PERCENT ALKALI METAL HYDROXIDE WITH A CORRODIBLE METALLIC ELEMENT TO BE PROTECTED SELECTED FROM THE GROUP CONSISTING OF STEEL, NICKEL, SILVER, TITANIUM, ZIRCONIUM, HAFNIUM AND NIOBIUM, CONTACTING SAID ANHYDROUS COMPOSITION WITH A SECOND METALLIC CATHODE ELEMENT COMPRISED OF A METAL SELECTED FROM THE GROUP CONSISTING OF ZIRCONIUM, COPPER, NICKEL, SILVER AND MOLYBDENUM, APPLYING A VOLTAGE OF ABOUT 0.005 TO 3.7 VOLTS BETWEEN SAID CORRODIBLE METALLIC ELEMENT AND SAID CATHODE ELEMENT, SAID VOLTAGE BEING BELOW THE DECOMPOSITION VOLTAGE OF THE ANHYDROUS COMPOSITION AND SUFFICIENT TO MAKE THE CORROSIVE METAL ANODIC AND THE SECOND METALLIC ELEMENT CATHODIC, THEREBY PASSIVATING SAID CORRODIBLE METALLIC ELEMENT TO THE ACTION OF SAID SUBSTANTIALLY ANHYDROUS COMPOSITION.

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