US2871175A

US2871175A - Electropolishing metal

Info

Publication number: US2871175A
Application number: US667821A
Authority: US
Inventors: Donald R Zaremski
Original assignee: Allegheny Ludlum Steel Corp
Current assignee: Allegheny Ludlum Steel Corp
Priority date: 1957-06-25
Filing date: 1957-06-25
Publication date: 1959-01-27
Anticipated expiration: 1976-01-27

Description

Jan. 27, 1959 D. R. ZAREMSK! 2,871,175

ELECTROPOLISHING METAL Filed June 25, 1957 INVENTOR. Donald R.Zoremsk| ATTORNE United States Patent ELECTROPOLISHIYG METAL Donald R. Zaremski, Cheswick, Pa., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., :1 corporation of Pennsylvania Application June 25, 1957, Serial No. 667,821

9 Claims. (Cl. 204-1405) This invention relates to the method of electropolishing metals and alloys and particularly metals of the class titanium, titanium base alloys, zirconium and zirconium base alloys.

In the manufacture and processing of metals and metal products it is frequently desirable to obtain a bright and lustrous surface. Such a-surface is usually obtained by mechanical polishing and bufling or by electrochemical treatment such as electroplating or electropolishing. Electropolishing has found wide application because it is economical and provides a highly satisfactory bright and.

lustrous surface on most metals and alloys. This method of surface treatment is highly popular in modern manufacturing. Electropolishing consists of immersing the metal object to be polished as an anode in an electrolyte and passing a current through the electrolyte between the object being polished and another object or cathode. The electrolytes generally employed are aqueous acid solutions. Some metals and alloys are difficult to electropolish because of their inability to pass current as an anode in an aqueous electrolyte. This is particularly true of the metals titanium, titanium base alloys, zirconium and zirconium base alloys. Although it has been possible to electropolish these metals on a laboratory scale with aqueous type electrolytes, these methods have proved to be impractical when applied commercially It has now been found that metalsand alloys, and par ticularly titanium, titanium base alloys, zirconium and zirconium base alloys may be electropolished in molten caustic salt baths without employing the usual impressed electrical currents.

It is therefore an object of the present invention to provide a method of electropolishing the surface of metals and alloys.

Another object of this invention is to provide for the electropolishing of metals that cannot be satisfactorily electropolished in aqueous electrolytes.

A more specific object of the present invention is to provide a method of electropolishing metals selected from the group consisting of titanium, titanium base alloys,

zirconium and zirconium base alloys.

Other objects and advantageous features may be best understood by the'following description when taken in conjunction with the accompanying drawing in which:

Figure 1 is a view, partially in section, of an electropolishing cell such as is employed in practicing this invention, and

Fig. 2 is a view in section of the electropolishing cell takenalong the lines II.-II of Fig. 1.

' In practicing this invention the metal to be polished is immersed in a molten caustic salt bath and is coupled to a metal also in directcontact with the bath that is more noble in the caustic salt bath than the metal being treated. Reactive metals of the class titanium, titanium base alloys, zirconium and zirconium base alloys, and particularly such metals in the form of sheet'products, are benefited by being connected to another metal 2,871,175 Patented Jan. 27, 1959 object in the bath so that the' reactive metal is provided with an electrical potential and is anodic with respect to the other metal object in the bath. In practicing the invention, an external electrical connection is made between the metal being treated and another metal that is chemically electronegative, in the molten caustic salt bath, to the metal being treated. Thus there i created a simple electrolytic cell in which the molten caustic salt is an electrolyte, the metal object being treated is an anode, and a second metal object is a cathode. The use of impressed currents or providing electrical current from an outside source is neither necessary nor desirable. It is preferred to employ the electrochemical reaction obtained only when a second metal that is electronegative in the caustic salt bath to the metal being treated is employed in order to provide a cell type reaction.

Referring to the drawings, this invention is illustrated by reference to the immersion of a sheet 11 of titanium or the like and a sheet 12 in a bath 13 of molten caustic salt contained in a steel tank 14. The sheet 12 may be of any metal that is electronegative with respect to the metal of sheet 11 in the caustic salt bath 13. In practice the material of the construction of the tank 14 is not critical and may be any metal or material satisfactory for holding a molten caustic bath under the conditions employed.

The caustic bath 13 is maintained in a molten state. It should be noted that the upper edges of

sheets

11 and 12 extend above the upper level 15 of salt bath 13 and that in this area of

sheets

11 and 12, represented in Fig. 1 as 16, there is an external electrical connection 17 which in this embodiment is a wire cable. Wire connection 17 may be clamped, welded or otherwise attached to

sheets

11 and 12 in any conventional manner so long as an electrical coupling is made. In the present embodiment there is shown a substantially nonconductive insulating material 18 which may be made of any nonelectrical conductive substance which is used to prevent either of

sheets

11 or 12 from touching 'the steel tank 14 to avoid electrical currents caused by potential differences between the metal of

sheets

11 and 12 and the steel of the tanks from flowing between the sheets and the tank. However, should the tank be made of a substantially nonconductive material, such insulation as is provided by 18 would be unnecessary.

Reference is particularly directed to Fig. 2 wherein is shown the electric cell type of reaction employed. Sheet 11 is anodic in the salt bath 13 with respect to sheet 12 which functions as the cathode. Since the sheets are provided with an external electrical connection 17,

l a natural flow of electrons and ions takes place between the two sheets. The anode 11 dissolves and titanium ions leave the surface of the metal, leaving behind electrons. The titanium ions thus take on a positive valence. These particles then tend to migrate towards the cathode or sheet 12. The negatively charged particles or electrons on anode 11 flow through the external electrical connection 17 to the cathode sheet 12, thus increasing the attraction of the cathode for the dissolved positively charged ions of titanium. It is to be noted that the flow of electrons through the external connection is in a direction opposite to the conventionally assumed flow of electrical current.

It can bereadily observed from the above that the anode dissolves and looses metal from its surface. By such a process the anode surface is evenly attacked in the molten caustic bath, leaving a bright and lustrous surface.

The caustic bath employed may be any of the known com'me'rcialsalt baths which usually have an alkali metal base and arepreferably heated, and in use are preferably maintained at a temperature in the neighborhood of 875 F. to 975 P. so that the salt bath is in a molten state and will readily act as an electrolyte for the electropolishing of the anodic metal. The compositions of these baths consist essentially of alkali metal hydroxides and preferably sodium and potassium hydroxides, plus minor additions of inhibitors, oxidizing agents, etc. While many variations in the composition of the caustic bath are possible, a typical example of a suitable caustic bath, is that of the commercially known Virgo salts having about 88.55% sodium hydroxide, about 7.66% sodium nitrate, 1.75% sodium chloride and 0.53% sodium carbonate with traces of manganese dioxide present. The Kolene salts formed of alkali metal hydroxides, nitrates and chlorides as de scribed in Patent No. 2,458,661 are also effective in carrying out the metal treatment of the present invention.

The magnitude of the current necessary to carry out the electrochemical treatment of the present invention is relatively small. The cathode must be a metal that is electronegative to the metal or alloy being treated. For example, it is preferable to use iron or steel as the cathode when treating titanium. Other metals that are cathodic to titanium in molten caustic Virgo are aluminum, molybdenum, chromium and, of course, alloys of these metals. The electrical potential created by employing the cell type of reaction provides the necessary low amperage currents that effectively electropclish the anodic metals. In electropolishing titanium the potential that exists between titanium and steel electrodes in Virgo baths generates sufficient electrical current to effectively electropolish the surface of the titanium. As shown by Table 1 below, the current density in such a cell drops off rapidly relative to time. When the electrical connection is first made, a current density of slightly less than 2 amperes per square inch exists; however, after 10 minutes the current has decreased to about ampere per square inch. This reduction in current density does not materially reduce the electropolishing action of the bath. Some electropolishing Will be accomplished so long as the metal being treated is maintained anodic, regardless of how low the current density may become.

When reactive metals such astitanium, titanium base alloys, zirconium and zirconium base alloys are immersed in molten caustic salt baths such as Virgo and are insulated from any electrical connection or cathodic member, chemical and cell type electrochemical attack frequently takes place. The reactive metal does not dissolve evenly and thus the metal surface is subjected to pitting and cavitation. This is believed to be caused by the presence of hydrogen in the caustic bath which replaced titanium, causing gas evolution in the vicinity of the metal being treated. The dangers of hydrogen are eliminated when the metal or alloy being electropolished is made anodic to one or more objects in the molten salt bath since hydrogen tends to migrate to the cathode. When the reactive metals titanium, zirconium or their alloys are immersed by themselves in a molten caustic salt bath, hydrogen gas evolution may be visibly observed. However, when such metals are made anodic with respect to other objects in the bath there is no 'gas evolution. The reaction is definitely electrochemical and the anodes dissolve evenly leaving a smooth, bright and lustrous surface.

The time required to effect electropolishing does not appear to be critical. However, excellent results have been obtained on titanium when immersed in the caustic salt bath while employing steel as cathodes for a time of from 5 to 7 minutes.

Excellent results were obtained on titanium samples as follows: 2" x 3 sample plates were constructed of various steels, commercial grade titanium and titanium alloys. One sample of each type of titanium alloy and one sample of titanium were electrically connected at one end to a like sample of plain carbon steel. Another sample of each type of titanium alloy and one sample of commercial grade titanium were electrically connected in a like manner to a Type 316 stainless steel sample. In each case the electrical connection was made external of the bath and each pair of samples was immersed in molten Virgo salts with the upper edges of the samples and the electrical connections free from the molten salt. The titanium grades tested were Ti 75A (commercial grade titanium), 6% aluminum4% vanadiurn-Ti alloy and 2% iron2% chromium and 2% molybdenum-Ti alloy. The temperature of the Virgo was approximately 925 F. The time in solution was from 5 to 7 minutes. Upon removal from the electrolyte the titanium alloy samples were water quenched and then given a hot nitric acid dip to remove the adhering Virgo. Smooth, bright and lustrous electropolished surfaces were obtained on all the titanium and titanium alloy samples.

Similar samples of 2% iron-2% chromium and 2% molybdenum titanium alloy sheet were immersed in molten Virgo (925 F.) while being completely insulated from all other metal parts in contact with the Virgo bath. The samples were removed from the bath after a period of about 10 minutes and immersed in a nitric acid bath to remove the Virgo. All the samples exhibited badly pitted surfaces with the pits aligned in a direction at right angles to the plane of the bath surface. There was no surface brightening.

Similar samples of titanium Type Ti 75A (commercial grade titanium) were tested in Virgo while employing carbon steel and stainless steel Type 316 as the cathodes. The current densities were measured on these tests from the time of immersion and for 10 minutes. The results are as follows:

Table I Current densities mp Time Titaniurn/ Titanlum/ Virgo/carbon Virgo/stainsteel less 316 1. 669 1.785 1. 234 0. 492 0. 686 O. 481 0. 503 0. 400 0. 457 (I. 385 0. 407 0. 308 O. 389 l). 283 0. 366 0.277 0. 357 O. 262 0. 246 0. 288 O. 222

As can be seen in the above table the current density drops off rapidly in respect to time. The current drop is believed to be caused by cathodic polarization. Such drop in current does not materially reduce the electrochemical eifect of the treatment.

The process of the present invention may be applied to any metal object and is particularly applicable to objects made from reactive metals such as titanium, zirconium and alloys thereof. The treatment is particularly advantageous when used in connection with materials with fiat surfaces such as plates and sheets. The present application has described the batch type treatment such as is applicable to. sheets or plates; however, it is to be understood that modifications may be employed for the continuous treatment of strip material passing through a caustic bath. Strip material made of titanium,

zirconium or their alloys and which are continuously pickled in caustic salt baths may be treated by conducting a similar steel strip through the bath simultaneously with the reactive metal strip while maintaining a spaced relation between the two metals and providing an electrical connection therebetween. Where steel containers hold the molten caustic bath, it may be desirable to pro vide an electrical connection between the metal being treated and the steel container, thus making the container a cathode.

Although this invention has been described with reference to the preferred practices and by specific illustration, it is to be understood that the invention is susceptible to variation and extension and that the scope of the specification and claims is'to be interpreted broadly.

Iclaim:

l. The method of electropolishing the surface of an object made of a metal selected from the group consisting of titanium, titanium base alloys, zirconium and zirconium base alloys which comprises, immersing said object in a molten bath that consists essentially of an alkali metal hydroxide, providing at least one other metallic object in said bath that is more noble in said bath to titanium, zirconium and alloys thereof and-providing an electric coupling between said objects so as to provide an electric current.

2. The method of electropolishing the surface of an object made from a metal selected from the group consisting of titanium and titanium base alloys, immersing said object in a molten bath that consists essentially of an alkali metal hydroxide, providing at least one other metallic object in said bath which is more noble in said bath to titanium and alloys thereof and providing an electrical coupling between said objects so as to provide a flow of electrons.

3. The method of electropolishing the surface of an object made from a metal selected from the group consisting of zirconium and zirconium base alloys, immersing said object in a molten bath that consists essentially of an alkali metal hydroxide, providing at least one other metallic object in said bath which is more noble in said bath to zirconium and alloys thereof and providing an electrical coupling between said objects so as to provide a flow of electrons.

4. The method of electropolishing a reactive metal of the class of titanium and titanium base alloys which comprises, immersing said reactive metal in a molten salt bath, the temperature of which is within the range of from 875 F. to 975 F. and the composition of which consists essentially of sodium hydroxide, providing an electric coupling between said reactive metal as the anode and at least one other object in said bath that is more noble than said reactive metal, so as to provide an electric current between said reactive metal and said object of from approximately 2 to A amperes per square inch.

5. The method of electropolishing a reactive metal of the class titanium and titanium base alloys which comprises, immersing said reactive metal in a molten salt bath, the temperature of which is within the range of from 875 F. to 975 F. and the composition of which consists essentially of sodium hydroxide, providing a terial in a molten salt bath that consists essentially of an alkali metal hydroxide, providing at least one other metallic object in said bath that is more noble in said bath to said sheet material, and providing an electric coupling external of said bath between said sheet material and said object so as to provide an electric current.

7. The method of electropolishing sheet material formed from refractory metal selected from the class of titanium, titanium base alloys, zirconium and zirconium base alloys which comprises; partially immersing said sheet material in a molten salt bath that consists essentially of an alkali metal hydroxide, partially immersing a like sheet in said bath of a metal that is more noble in said bath to said refractory metal sheet, maintaining said sheets in a spaced relationship to one another and providing an electric coupling external of said bath between said sheets so as to provide an electric current passing between said sheets, the refractory metal sheet being the anode.

8. The method of electropolishing a reactive metal selected from the group consisting of titanium, titanium base alloys, zirconium and zirconium base alloys which comprises, immersing said object in a molten bath that consists essentially of at least one alkali metal hydroxide selected from the group consisting of sodium hydroxide and potassium hydroxide, providing at least one other metallic object in said bath that is more noble in said bath to titanium, titanium base alloys, zirconium and zirconium base alloys, and providing an electric coupling between said objects so as to provide an electric current.

9. The method of electropolishing a reactive metal of' the class titanium and titanium base alloys, which comprises, immersing said reactive metal in a molten salt bath, the composition of which consists essentially of sodium hydroxide, providing at least one second metal to said bath of the class, aluminum, molybdenum, chromium, iron and alloys of aluminum, molybdenum, chromium and iron, providing an electric coupling between said reactive metal and second metal so as to provide an electric current.

References Cited in the file of this patent Metallurgia, vol. 49, No. 295, May 1954, page 256, by Durkin.

Claims

1. THE METHOD OF ELECTROPOLISHING THE SURFACE OF AN OBJECT MADE OF A METAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, TITANIUM BASE ALLOYS, ZIRCONIUM AND ZIRCONIUM BASE ALLOYS WHICH COMPRISES, IMMERSING SAID OBJECT IN A MOLTEN BATH THAT CONSISTS ESSENTIALLY OF AN ALKALI METAL HYDROXIDE, PROVIDING AT LEAST ONE OTHER METALLIC OBJECT IN SAID BATH THAT IS MORE NOBLE IN SAID BATH TO TITANIUM, ZIRCONIUM AND ALLOYS THEREOF AND PROVIDING AN ELECTRIC COUPLING BETWEEN SAID OBJECTS SO AS TO PROVIDE AN ELECTRIC CURRENT.