US3473922A - Corrosion-resistant alloys - Google Patents

Description

US. Cl. 75ll71 7 Claims ABSTRACT OF THE DISCLOSURE A castable, weldable, machinable, air-meltable alloy which is highly resistant to corrosion under widely divergent conditions in various corrosive media including oxidizing media and reducing or non-oxidizing media. The alloy is composed essentially of about 14.7 to 18.0 percent by weight chromium, about 3.5 to 9.0 percent by weight tantalum, about 15 to 18 percent by weight molybdenum, up to about 0.15 percent by weight carbon, up to about 1.50 percent by weight silicon, up to about 1.50 percent by weight manganese, up to about 5.25 percent by weight tungsten, up to about 7.0 percent by weight iron, up to about 0.35 percent by weight vanadium, up to about 10.5 percent by weight cobalt, up to about 4 percent by weight copper, up to about 4 percent by weight columbium, and the balance nickel.

Background of the invention This invention lies in the field of alloys and more particularly in the field of corrosion-resistant alloys which display a high degree of resistance to corrosion in various corrosive media and which are weldable, machinable and may be produced in wrought or cast form.

In recent years, the rapidly expanding chemical and petrochemical industries have created a demand for metals or alloys suitable for use in environments destructive to the standard types of stainless steels. Corrosive media encountered in industrial equipment and processes may, in general, be of the oxidizing type or non-oxidizing (or reducing) type. By an oxidizing environment is meant one more oxidizing than the hydrogen ion. By a reducing environment is meant one which does not contain any substance more oxidizing than the hydrogen ion.

Among oxidizing agents which have caused corrosion problems in the past may be mentioned nitric acid, ferric chloride, aqua regia, cupric chloride, hydrogen peroxide and sodium hypochlorite. Non-oxidizing or reducing media include those which do not afford the chemical passivity well known in the industry to be associated with oxidizing media and include such media as hydrochloric acid, concentrated sulfuric acid, phosphoric acid, oxalic acid, acetic acid, and aluminum chloride. In general, the aggressive agents of this group readily atack and destroy metallic alloys which are virtually immune to many severely-oxidizing media. Conversely, many oxidizing agents readily attack those metallic alloys which are most resistant to aggressive non-oxidizing media. The presence of halide ions, particularly chloride ions, further aggravates attack on most alloys in many applications. Thus, for example, standard stainless steels resist most concentrations of nitric acid very well but are completely dissolved in time by aqua regia (one part nitric acid and three parts hydrochloric acid).

Metals or alloys which are resistant to corrosion under both oxidizing and reducing conditions are highly desirable in the industrial equipment field inasmuch as they afiord the designer and equipment builder a greater degree of latitude and render the equipment more flexible in use. Further, since process conditions may unexpectedly change the environment involved from reducing to oxidiz nited States Patent 3,473,922 Patented Oct. 21, 1969 "ice the resistance of titanium to corrosion is considerably en-;

hanced, particularly in a reducing environment in which unalloyed titanium may fail rapidly. However, such titanium alloys cannot be furnished in cast form and must be vacuum melted and prepared from very high purity materials. Also, such alloys must be welded in an inert gas atmosphere. Similarly, an alloy consisting of 50 percent by weight vanadium and 50 percent by weight columbium has been found to have appreciably greater ability to withstand oxidizing or reducing acids in the presence or absence of chlorides over pure vanadium, which resists reducing agents with or without chlorides but does not resist oxidation. Here again, however, such an alloy is available only in wrought form, not in cast form, is quite expensive and must be vacuum melted from high purity stock. It is also welded only with extreme difiiculty. There has remained, therefore, an unfulfilled need for alloys which are resistant to corrosion under both oxidizing and reducing conditions, with or without the presence of halide ions and which may be prepared from lower cost materials and supplied in either cast or wrought form.

Summary of the invention Among the several objects of the invention may thus be noted the provision of novel alloys which are highly resistant to corrosion under widely varying conditions, including oxidizing and non-oxidizing environments; the provision of such alloys which may be provided in either cast or wrought form; the provision of alloys of this type which are weldable, machinable and air-meltable; the provision of such alloys which need not be subjected to high temperature solution heat treatment for service in most corrosive media; and the provision of alloys of thi character which may be readily prepared by conventional methods from lower cost materials. Other objects will be in part apparent and in part pointed out hereinafter.

The present invention is therefore directed to a cor rosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, the alloy being composed essentially of from about 14.7 to 18.0 percent by weight chromium, about 3.5 to 9.0 percent by weight tantalum, about 15 to 18 percent by weight molybdenun, up to about 0.15 percent by weight carbon, up to about 1.50 percent by weight silicon, up to about 1.50 percent by weight manganese, up to about 5.25 percent by weight tungsten, up to about 7.0 percent by weight iron, up to about 0.35 percent by weight vanadium,

Description of the preferred embodiments In accordance with the present invention, I have found that alloys which are highly corrosion resistant in various environments may be formed from a solid solution composed essentially of 14.7 to 18 percent chromium, 3.5 to 9.0 percent tantalum, 15 to 18 percent molybdenum, and the balance nickel. The alloys of the invention may also contain up to about 0.15 percent carbon, up to about 1.50 percent silicon, up to about 1.50 percent manganese, up to about 5.25 percent tungsten, up to about 7.0 percent iron, up to about 0.35 percent vana dium, up to about 10.5 percent cobalt, up to about 4 percent copper and up to about 4 percent columbium, The

alloys may be prepared using conventional electric furnace, either arc or induction, procedures. The components of my alloys are employed in substantially pure form in preparing the alloys, but the usual impurities present in commercially available metals are acceptable and do not adversely affect the properties of the alloys produced. When iron is included in the alloys of the invention, then the tantalum can be added in the form of ferro-tantalum. Similarly, molybdenum may be added in the form of ferro-molybdenum provided the iron content of the alloy is not permitted to exceed about 7 percent. Silicon and manganese may be included as deoxidizing agents to avoid loss of tantalum from the melt but may be omitted when vacuum melting is employed. In order to attain a high degree of corrosion resistance, the following composition range has been found satisfactory and preferable:

Percentage by weight Chromium 14.7-18.0 Tantalum 3.5-9.0 Molybdenum 15-18 Carbon 0.02-0.15 Silicon 0.3-1.5 Manganese 0.3-1.5 Tungsten -5.25

Iron 0-7.0 Vanadium 0-0.35 Cobalt 0-10.5 Copper 0-4 Columbium 0-4 Nickel Remainder When iron is included, the alloys preferably contain about 3.3-3 :84 percent iron. When vanadium is included, about 0.20-0.22 percent may be present. While it is preferable to formulate my alloys without cobalt, the alloys may include up to 10.5 percent cobalt, preferably l.1-10.5 percent. In general, the copper content should be held below 4 percent since this component may be detrimental in some media. Similarly, the amount of cobalt should be held below about 10.5 percent, the amount of columbium below about 4 percent and the amount of tungsten below about 5.25 percent. While these elements may be tolerated up to the stated amounts, preferably they are omitted where feasible in the practice of the invention.

It is to be noted that the tantalum content of my alloys is greater than that required to merely combine chemically with and stabilize the carbon which has been the function of tantalum (or columbium) in certain prior art alloys. Thus, the tantalum content of the alloys of the invention exceeds the amount necessary for car-bide stabilization so that the excess amount remains in solid solution in the alloy and function essentially as a noble metal.

In addition to exhibiting remarkable resistance to corrosion in various corrosive media, including both oxidizing and reducing types of media, the alloys of the invention provide a number of significant advantages not pre- 'viously attainable with alloys possesing a high degree of corrosion resistance. Thus, my alloys may be produced in both wrought and cast form and may be heat treated in the conventional manner. In general, the alloys of the invention in a solution heat treated condition exhibit a higher degree of corrosion resistance in some media than do the alloys in an as-cast condition. In both forms however, the alloys possess improved corrosion resistance as demonstrated by the test work described hereinafter. Moreover, the alloys are weldable and machinable using conventional techniques. Also, the alloys are air-meltable and therefore need not be vacuum melted, and likewise need not be prepared from high purity materials. Finally, the alloys of the invention display a high degree of hardness and when tested, show Brinell hardness number values of 250 to 300. Accordingly, the alloys are usable in many 'H SO -'non-oxidizing medium with no halide ions 4 applications and environments in which currently available corrosion-resistant alloys are incapable of use or have serious drawbacks.

The following examples further illustrate the invention.

The alloys set forth inTable I below were prepared utilizing conventional electric furnace-procedures.

TABLE I Percentage by Weight Alloying Alloy No. 1 Alloy N o. 2 Alloy No. 3 Alloy No. 4 Element Chromium 16. 08 17. 35 17. 19 L4. 9

Tantalum 5. 76 4. 44 8. 93 ll. 79

Molybdenum... 17.45 16. 56 16. 40 t8. 0

3. 3. 84 3. 43 L ll. 30

Each of the above alloys was air melted and poured in batches of 500 pounds to 1,000 pounds using a high frequency induction furnace. Alloys from each of the heats were cast into well-risered cylinders of 5%." length and 2% diameter. Samples of each alloy were their solution heat treated by being brought up to 2225 F. for one hour, water quenched to 1000 F. and then air cooled.

The as-cast and heat treated cylinders were then cut into discs of approximately 1.750 inches diameter and 0.25 inch thickness, ground to a 600 grit and lightly polished with a diamond dust lapping paste to a fine micro-finish. Each sample disc was provided with a inch hole through the middle in order to facilitate sample suspension in the test solutions described below.

The chemical testing apparatus employed consisted of a water bath provided with a, heater, the temperature of .the Water being controlled to within plus or minus 3 F.- by abuilt-in thermostat. The entire unit was enclosed in a polyethylene hood which was provided witha small exhaust fan to remove corrosive gases.

The, test solutions were aerated by having air bubbled therethrough from individual glass .tube lines with each line being provided with a control stop cock. The flow of air was suflicient to keep the aerated solutions in constant agitation. r

The test disc. specimens were suspended by platinum wires hanging from glass hooks and immersed in watchglass covered beakers containing the various corrosive media employed in the tests. Liqiud media condensing on the respective Watch glasses dripped back into the beakers thereby preventing evaporation and changes in the concentration of the test solutions.

The following test solutions were employed in the test These corrosive mediawere selected to provide examples of each of the following widely divergent conditions:

- v Percent CuCl oXidizing medium with halide ions 30 HNO oxidizing medium with no halide ions 70 HCl-non-0xidizing medium with halide ions 37 25 The test solutions were maintained at a temperature of F. in carrying out the tests, and the test period was 24 hours. v

Samples of, commercially available alloys marketed under the trade designations Hastelloy C3 Hastelloy B," Durimet 20" and *Monel were prepared as described above and subjected to corrosion tests in certain of the corrosive media. The Monel alloy used consisted of 65 percent nickel, 30 percent copper, 2 percent iron, 1.5 percent silicon, 0.8 percent manganese and 0.2 percent carbon. The Durirnet alloy used consisted of 41 percent iron, 30 percent nickel, 20 percent chromium, 4 percent copper, 3 percent molybdenum, 0.8 percent silicon, 0.7 percent manganese and 0.05 percent carbon. The Hastelloy B alloy used consisted of 61 percent nickel, 28 percent molybdenum, 4 percent iron, 0.3 percent vanadium, 0.5 percent silicon, 0.5 percent manganese and 0.04 percent carbon. The Hastelloy C alloy used consisted of 52.6 percent nickel, 18 percent molybdenum, 16.2 percent chromium, 5.3 percent tungsten, 3.9 percent iron, 0.6 percent cobalt, 0.5 percent silicon, 0.3 percent manganese and 0.08 percent carbon. These alloys are used commercially in equipment for handling corrosive materials and exemplify alloys which resist corrosion in certain media but fail catastrophically in other media.

The results of the tests are set forth in Table 11 below.

3. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 14.9 to 17.35 percent by weight chromium, about 3.79 to 8.93 percent by weight tantalum, about 16.4 to 18.0 percent by weight molybdenum, about 0.06 to 0.13 percent by weight carbon, about 0.46 to 0.82 percent by weight silicon, about 0.3 to 0.44 percent by weight manganese, about 3.3 to 3.84 percent by weight iron, about 0.20 to 0.22 percent by weight vanadium, about 1.1 to 10.5 percent by Weight cobalt, and the balance nickel.

4. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 16.08 percent by weight chromium, about 5.76 percent by weight tantalum, about 17.45 percent by weight molybdenum, about 0.13 percent by weight carbon, about 0.52 percent by weight silicon, about 0.33 percent by weight TABLE II Solution lI- eat-Clreated3 Condition, Inches Per Year As-Cast Condition, Inches Per Year Corrosion Ol'IOS101'1 Alloy Unaerated Aerated Unaerated Aerated Unaerated Aerated Unaerated Aerated 30% CuCl 70% END; 37% HCl H SO 01.1015 70% NBC; 37% B01 25% H 804 Alloy No. 1 0. 0008 0. 0154 0. 0109 0. 0003 0. 020 0. 0170 0. 0369 0. 0081 Alloy No. 2. Nil 0. 0213 0. 0445 0. 0001 0. 0587 0. 0166 0. 0744 0. 0002 Alloy N0. 3. Nil 0. 0247 0. 0222 0. 0001 0. 7803 0. 0206 0. 0520 0. 0003 Alloy No 4 0. 0465 0. 0170 0. 0204 0. 0004 1. 062 0. 0194 0. 0245 0. 0048 Hastelloy C 0. 8484 0. 044 0. 0369 0. 006 1. 7588 0. 0451 0. 0283 0. 0235 Hastelloy B Bad 0. 002 Durimet 20 Bad 0. 005 Bad Monel Bad Bad Bad In the above tests, the legend Bad designates that the specimen failed catastrophically, either being completely dissolved or almost completely dissolved, so that no measurement could be made.

From the foregoing, it will be seen that the present invention provides alloys of improved corrosion resistance under varying corrosive conditions, such alloys being weldable, machina'ble, air-meltable and readily castable. While displaying these desirable properties, the alloys of the invention are nevertheless formed from readily avail- 'able and comparatively inexpensive non-precious metals of commercial purity and may be produced economically utilizing conventional techniques.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

What is claimed is:

1. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 14.7 to 18.0 percent by weight chromium, about 3.5 to 9.0 percent by weight tantalum, about 15 to 18 percent by weight molybdenum, up to about 0.15 percent by weight carbon, up to about 1.50 percent by weight silicon, up to about 1.50 percent by weight manganese, up to about 5.25 percent by weight tungsten, up to about 7.0 percent by Weight iron, up to about 0.35 percent by weight vanadium, up to about 10.5 percent by weight cobalt, up to about 4 percent by weight copper, up to about 4 percent by weight colurnbium, and the balance nickel.

2. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 14.7 to 18.0 percent chromium, about 3.5 to 9.0 percent by weight tantalum, about 15 to 18 percent by Weight molybdenum, about 0.02 to 0.15 percent by weight carbon, about 0.3 to 1.5 percent by weight silicon, about 0.3 to 1.5 percent by weight manganese, up to about 5.25 percent by weight tungsten, up to about 7.0 percent by weight iron, up to about 0.35 percent by weight vanadium, up to about 10.5 percent by weight cobalt, up to about 4 percent by weight copper, up to about 4 percent by weight columbium, and the balance nickel.

manganese, about 3.8 percent by Weight iron, about 1.1 percent by weight cobalt, and the balance nickel.

5. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 17.35 percent by Weight chromium, 4.44 percent by weight tantalum, about 16.56 percent by weight molybdenum, about 0.06 percent by weight carbon, about 0.82 percent by weight silicon, about 0.44 percent by weight manganese, about 3.84 percent by weight iron, about 0.20 percent by weight vanadium, about 8.36 percent by weight cobalt, and the balance nickel.

6. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 17.19 percent chromium, about 8.93 percent by weight tantalum, about 16.40 percent by Weight molybdenum, about 0.08 percent by weight carbon, about 0.46 percent by weight silicon, about 0.36 percent by weight manganese, about 3.43 percent by weight iron, about 0.22 percent by weight vanadium, and the balance nickel.

7. A corrosion-resistant, weldable, machinable alloy characterized by its ability to produce usable cast products, said alloy being composed essentially of from about 14.9 percent chromium, about 3.79 percent by weight tantalum, about 18.0 percent molybdenum, about 0.09 percent by weight carbon, about 0.6 percent by weight silicon, about 0.3 percent by weight manganese, about 3.3 percent by weight iron, about 10.5 percent by weight cobalt, and the balance nickel.

References Cited UNITED STATES PATENTS 2,777,776 1/1957 Binder -171 3,164,465 1/ 1965 Thielemann 75-171 3,203,792 8/ 196 5 Scheil et a1 75-171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75-134