US3759704A - Corrosion resistant alloys - Google Patents

Abstract

AN ALLOY WHICH IS RESISTANT TO CORROSION BY BOTH OXIDIZING AND REDUCING SULFURIC ACID SOLUTIONS OVER A WIDE RANGE OF ACID STRENGHTS. THE ALLOY CONSIST OF BETWEEN ABOUT 36% AND ABOUT 46% BY WEIGHT NICKEL, BETWEEN ABOUT 33% AND ABOUT 42.06% BY WEIGHT CHROMIUM, BETWEEN ABOUT 2.94% AND ABOUT 7.84% BY WEIGHT MOLBYDENUM, BETWEEN ABOUT 1.95% AND ABOUT 5.18% BY WEIGHT COPPER, UP TO ABOUT 0.12% BY WEIGHT CARBON, UP TO ABOUT 1.50% BY WEIGHT SILICON, UP TO ABOUT 2.50% BY WEIGHT MANGANESE, UP TO ABOUT 2% BY WEIGHT TITANIUM, UP TO ABOUT 4% BY WEIGHT NIOBIUM PLUS TANTALUM, UP TO ABOUT 0.01% BY WEIGHT BORON, AND THE BALANCE ESSENTIALLY IRON. MOST OF THE ALLOYS OF THE INVENTION ARE READILY WELDABLE, MACHINABLE, AND WORKABLE. THOSE WHICH CANNOT BE READILY WORKED ARE GENERALLY HARD AND WEAR-RESISTANT.

Description

United States Patent US. Cl. 75-122 Claims ABSTRACT OF THE DISCLOSURE An alloy which is resistant to corrosion by both oxidizing and reducing sulfuric acid solutions over a wide range of acid strengths. The alloy consists of between about 36% and about 46% by weight nickel, between about 33% and about 42.06% by weight chromium, between about 2.94% and about 7.84% by weight molybdenum, between about 1.95% and about 5.18% by weight copper, up to about 0.12% by weight carbon, up to about 1.50% by weight silicon, up to about 2.50% by weight manganese, up to about 2% by weight titanium, up to about 4% by weight niobium plus tantalum, up to about 0.01% by weight boron, and the balance essentially iron. Most of the alloys of the invention are readily weldable, machinable, and workable. Those which cannot be readily worked are generally hard and wear-resistant.

BACKGROUND OF THE INVENTION This invention relates to corrosion-resistant alloys and more particularly to weldable, machinable and workable alloys which are resistant to corrosion by both oxidizing and reducing sulfuric acid solutions over a wide range of acid strengths.

Sulfuric acid is an ubiquitous industrial reagent which is generally very corrosive to most metals. The corrosivity of sulfuric acid to any given metal, however, varies widely with the strength of the acid, the temperature of the acid environment, and the nature and concentration of various contaminants. Because of the wide ranging uses for sulfuric acid, industrial process streams may be found which run the gamut of sulfuric acid concentrations; which must be handled from temperatures below room temperature up to the boiling point of the acid; and which contain an extensive variety of contaminants, e.g., other acids and salts.

For purposes of analyzing and predicting their corrosive effect on metals, acids and other corrosive agents are commonly classified as either oxidizing or reducing." A reducing medium is generally defined as one which includes no component more oxidizing than the hydrogen ion or hydronium ion while an oxidizing medium is one which does contain such a component. Sulfuric acid, along with such other common materials as hydrochloric acid, acetic acid, phosphoric acid, aluminum chloride, hydrobromic acid, and hydrofluoric acid, is normally a reducing medium. At concentrations above approximately 85% by weight, however, sulfuric acid becomes an oxidizing agent. If its temperature is elevated, sulfuric acid may be oxidizing at even lower concentrations. Thus, a 60% by weight sulfuric acid solution becomes oxidizing at temperatures in excess of 150 F. Even lower concentrations of sulfuric acid can be moderately to strongly oxidizing when they contain various oxidizing acids and salts. Among the most common solutions of this type are the so-called mixed acids, which are mixtures of sulfuric acid and nitric acid used in organic nitration processes. Other oxidizing materials, some of which may be found in industrial sulfuric acid streams, include hydrogen peroxide, ferric sulfate, silver nitrate, potassium nitrate, sodium nitrate, copper sulfate,

ice

potassium permanganate, sodium dichromate, chromic acid, calcium chloride, mercuric chloride, aqua regia, sodium hypochlorite, ferric chloride, and cupric chloride.

Because of this variety in the character of various industrial sulfuric acid streams, there are relatively few metals available which can be said to be generally useful in sulfuric acid service. For example, a metal which quite satisfactorily resists the corrosive effect of reducingtype sulfuric acid solutions may fail rapidly if a small proportion of an oxidizing agent is present, or if the temperature of the system is elevated well above room temperature. Many alloys which resist dilute sulfuric acid solutions are completely unsuitable for sulfuric acid solutions having concentrations in excess of 60% or 70% by weight. Certain other alloys are available which are highly resistant to a wide range of sulfuric acid solutions, including concentrated sulfuric acid but, for the most part, such of these alloys as have been available have suffered from undesirable mechanical or other properties. Common drawbacks of such alloys have been poor machinability and weldability, with poor workability being an almost universal problem with these alloys, i.e., essentially none of these alloys can be feasibly produced in wrought form.

SUMMARY OF THE INVENTION Among the several objects of the present invention, therefore, may be noted the provision of novel alloys which are resistant to sulfuric acid over a wide range of concentrations; the provision of such alloys which are resistant to sulfuric acid up to 200 F. or higher; the provision of such alloys which are resist-ant to sulfuric acid solutions containing oxidizing contaminants; and the provision of such alloys which are weldable, machinable and workable. Other objects and features will be in part apparent and in part pointed out hereinafter.

The present invention is therefore directed to an alloy resistant to corrosion by both oxidizing and reducing sulfuric acid solutions over a wide range of acid strengths, consisting essentially of between about 36% and about 46% by weight nickel, between about 33% and about 42.06% by weight chromium, between about 2.94% and about 7.84% by weight molybdenum, between about 1.95% and about 5.18% by weight copper, up to about 0.12% by weight carbon, up to about 1.50% by weight silicon, up to about 2.50% by weight manganese, up to about 2% titanium, up to about 4% by weight niobium plus tantalum, up to about 0.01% by weight boron, and the balance essentially iron.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The alloys of the present invention are suitable as materials of construction for equipment used in reducingtype sulfuric acid solutions at temperatures up to the boiling point. As a result of the high proportion of chromium contained in these alloys, the alloys also exhibit good corrosion resistance to high temperature concentrated sulfuric acid and to sulfuric acid contaminated with oxidiz ing agents such as nitric acid. Except in the relatively narrow composition ranges where the alloy structure is not sufficiently austenitic, the alloys of the invention are readily weldable and machinable and, most importantly, are possessed of sufiicient ductility to be shaped and processed by hot or cold-working. The alloys of the invention which are not readily workable generally possess the alternative advantageous properties of high hardness and wear resistance. In either case, these alloys are adapted for use as materials of construction for a wide variety of chemical and other industrial process equipment.

The essential constituents of the alloys of the invention are:

Percent by weight Nickel 36 to 46 Chromium 33 to 42.06 Molybdenum 2.94 to 7.84 Copper 1.95 to 5.18

Carbon Up to 0.12 Silicon Up to 1.50 Manganese Up to 2.50 Titanium Up to 2 Niobium+tantalum Up to 4 Boron Up to 0.01 Iron Balance lybdenum and other constituents for a particular alloy are established, the balance of the alloy is desirably constituted by iron, since it is abundant, inexpensive, and generally lends good mechanical properties to the alloy. If enough iron is included so as to modify the austenitic structure of the alloy, however, fabricability is adversely affected. It is, therefore, preferred that the iron content be maintained below the proportion which would introduce non-austenitic characteristics to the alloy microstructure.

The alloys of the invention are prepared by conventional methods of melting and no special conditions such as controlled atmospheres are required. In preparing the alloys, the constituents of a melting furnace charge need not be of any particular type. Thus, raw materials such as remelt scrap, copper scrap, ferro alloys such as ferrosilicon and ferromanganese and other commercial melting alloys may be used.

The following examples illustate the invention.

Example 1 100 lb. heats of seven different alloys were prepared in accordance with the invention. Each of these heats was then melted in a 100 lb. high frequency induction furnace. The compositions of these alloys are set forth in Table 1, with the balance in each case being iron.

TABLE 1.PERCENTAGE BY WEIGHT OF ALLOYING ELEMENTS Cr M0 Cu O Si Mn Ti N b+Ta B Silicon enhances the corrosion resistance of the alloys to all strengths of sulfuric acid, and a small proportion of silicon is desirably present. However, large proportions of silicon tend to render the alloys hard and brittle, with poor welding and machining characteristics. It is preferable, therefore, to maintain the silicon content below about 1.50% to insure the workability of the alloys.

Manganese is desirably present because of its deoxidizing capabilities. Up to 8-l0% of manganese can be tolerated in these alloys without adverse effect on either corrosion resistance or mechanical properties. If more than about 2.50% by weight manganese is present, however, enough manganese oxide may be present to cause attack on a furnace lining or a molding sand. Thus, the maximum manganese content of the alloys should not exceed about 2.50% by weight.

A small amount of boron improves the fabricability of the alloys. Thus, as much as 0.01% boron may be present. Higher percentages of boron are undesirable, since significantly higher proportions tend to harden the alloys or to adversely aifect their corrosion resistance.

Up to about 2% titanium and up to about 4% niobium plus tantalum are useful in stabilizing any excess carbon that may be present in the alloys. These elements are also relatively corrosion-resistant and contribute as such to the corrosion resistance of the alloys. Titanium additionally contributes to the fabricability of the alloys.

A preferred embodiment of the alloys of this invention contains between about 40% and about 44% by weight nickel, between about 33% and about by weight chromium, between about 3% and about 4% by weight molybdenum, and between about 3% and about 4% by weight copper. Alloys in this range combine particularly high corrosion resistance with a high degree of toughness, ductility and workability.

Normally, the alloys of the invention include a certain proportion of iron, up to about 25% by weight. Once the appropriate proportions of nickel, chromium, copper, mo-

Two standard physical test blocks and three corrosion test bars were prepared from each heat. One of the physical test blocks from each alloy was solution-annealed at 1950 F. for three hours and then oil quenched. The physical properties of the alloys, in both the annealed and the as-cast state, were measured. The as-cast physical properties of a representative number of these alloys are set forth in Table 2, and the physical properties of two of the same alloys after annealing are set forth in Table 3. The magnetic permeability of these alloys is generally less than about 1.02, in either as-cast or annealed condition.

TABLE 2.PHYSIOAL PROPERTIES OF ALLOYS, AS-CAST Tensile Yield strength, strength, Percent Brinell Alloy p.s.i. p.s.i. elongation hardness TABLE 3.PHYSICAL PROPERTIES OF ALLOYS, AFTER SOLUTION ANNEALING The corrosion test bars were also annealed for thirty minutes at 1950 F. and oil quenched prior to machining into 1 /2 diameter x A high discs having a diameter hole in the center. Twelve to fourteen discs Were obtained for each alloy. As machined, these discs had a surface roughness of about 32 microinches. A number of the discs of each alloy were electropolished to reduce surface roughness to about 4 microinches.

Both rough and electropolished discs were used in the comparative corrosion tests described hereinafter, comparing the performance of the alloys of the invention with a number of commercially available alloys. Because of the relatively short duration of the corrosion tests, significant differences were expected between the observed corrosion rates of rough and polished sample discs, and this is the reason for which both types of surfaces were tested. It is well-known in the art that rough surfaces often have substantially higher initial corrosion rates than do polished surfaces, especially those which are electropolished. This phenomenon is variously ascribed to the higher actual contact area presented by a rough surface, the higher chemical activity of a rough surface, and the greater difficulty in forming a protective film on a rough surface, where corrosion resistance depends on the presence of such a film. It has also been posited that machining operations, such as those involved in preparing sample discs, can cause work-induced phase changes at the metal surface. In the case of the alloys of this invention, such a phenomenon may alter the austenitic character of the alloy surface and thus reduce surface corrosion resistance. Electropolishing removes this work-transformed margin and exposes the unaifected subjacent area.

The compositions of the commercially available alloys which were used in the following corrosion tests and the respective trade designations under which they are marketed are set forth in Table 4.

After precisely six hours, the sample discs were removed from the boiling acid solution and cleaned of corrosion products. Most samples were cleaned sufficiently with a small nylon bristle brush and tap Water. Those samples on which the corrosion product was too heavy for removal with a nylon brush were cleaned with a 1:1 solution of hydrochloric acid and water. After the corrosion products had been removed, each disc was again weighed to the nearest 10,000th of a gram. The corrosion rate of each disc, in inches per year, was calculated by the following formula in accordance with ASTM specification G1-67.

where R-, =corrosion rate in inches per year W =original weight of sample W =final weight of sample A=area of sample in square centimeters T=duration of test in years D=density of alloy in g./ cc.

Results of this corrosion test are set forth in Table 5.

TABLE 4.COMMERCIAL ALLOYS UTILIZED IN COMPARATIVE CORROSION TESTS Ni G! M0 Cu Si W 0 Mn 00 Others Hastelloy A... 0.70 0. 08 1. 00 Hastelloy B 0.70 0. 04 0. 70 Hastelloy C 0.70 0.11 0. 70 Hastelloy D 9. 0 0.10 1. 00 Hastelloy 0. 50 0. 04 1. 50 Illium G 0.65 0.20 1.25 Illium IL.-- 0. 70 0.05 1. 25 Illium 98..-- 0.7 0. 05 1. 25 Worthite-- 3. 50 0. 07 1. 00 Inconel 625 0. 0.08 0.30 Duriron 14. 5 0.50 0.50

RIIA 0.70. 0.04 1.00 Ni-O-N e1 0. 0. 05 0. 50 Marker SN 42- 0. 70 0. 05 0.70

F 8M 1.00 0.05 1.00 CF 8 0.50 0.07 0.50 Monel. 0.10 0.15 1. 00 Inconel 0. 25 0. 08 0. 25 Stellite No. 25--. 0. 50 0.10 0.70 Carp enter 20 4 Carpenter 20 Cb3 32. 5 20 2.5 35 0.03 0.05 CD4M Cu 25 2 3 0. 03 0.5

Example 2 TABLE 5 Comparative corrosion tests were conducted in a boiling mixed acid solution containing 5% by weight nitric acid and 10% by weight sulfuric acid.

Disc samples of Hastelloy B, Carpenter 20, nickel and Monel were prepared having the same dimensions as the discs prepared in Example 1. Residual machining oil and dirt were removed from all of the sample discs by cleaning them with a small amount of carbon tetrachloride. The discs were then rinsed in Water and dried. Surface roughness of these discs was on the order of 4-10 microinches.

Each disc was weighed to the nearest 10,000th of a gram and suspended in a beaker containing a sufiicient amount of boiling 10% sulfuric/ 5% nitric acid solution so that the entire sample was surrounded. The sample was suspended by means of a thin platinum wire hooked through the center hole of the disc and attached to a glass rod which rested on the top of the beaker. To insure the exposure of the discs to mixed acid solutions of substantially constant strength, frequent substitutions of beakers containing fresh boiling acid were made.

[Corrosion rates in boiling 10% H2304 plus 6% HNOa solution] Loss in inches I Not resistant.

Example 3 Comparative corrosion tests were conducted in boiling 10% sulfuric acid solution. Sample discs were prepared and tested in the manner described in Example 2, except that the test solution was boiling 10% sulfuric acid. The results of this test are set forth in Table 6.

TABLE 6 [Corrosion rates in boiling 10% H2804 solution] Loss in inches of penetration per year, i.p.y.

Surface roughness Alloy number:

1,025 32 microin.

971d- I do 4-10 microin.

Example 4 TABLE 7 [Corrosion rates in 65-68% HNO; solution at 150 F.]

Loss in inches of penetration per year, i.p.y.

Surface roughness Alloy number:

Hastelloy A- Hastelloy B.

I No resistance.

Example Comparative corrosion tests were conducted in 10% sulfuric acid solution at 176 F. Sample discs were prepared and tested in the manner described in Example 4, except that a 10% sulfuric acid solution was utilized and the temperature was maintained at 176 F. The results of this test are set forth in Table 8.

TABLE 8 [Corrosion rates in 10% H28 04 solution at 176 F.]

Loss in inches Surface of penetration roughness per year, i.p.y.

Alloy number:

960 32 microin- 0. 00735 961 -d0 0. 00486 971 -do- 0. 00081 1.025 -(10. 0. 00540 CD4M Cu 4-10 microin- 0. 004 Hastelloy A .do- 0. 0036 Hastelloy B .110. 0.003 Hastelloy C .do- 0. 003 Hastelloy D d0 0. 005 Carpenter 20 .do. 0. 005 Carpenter 20Cb3 -410- 0. 0045 M el (1 0. 009 0. 012 0. 063 0.020 0. 0051 0. 004 4. 5 0. 197

Example 6 Comparative corrosion tests were conducted in boiling 25% sulfuric acid solution. Sample discs were prepared and tested in the manner described in Example 2, except that boiling 25 sulfuric acid was used as the test solution. The results of this test are set forth in Table 9.

TABLE 9 [Corrosion rates in boiling 25% H2804 solution] Loss in inches of penetration per year, i.p.y.

Surface roughness Alloy number:

32 microin- 0. 1620 1 About 1.0.

Example 7 Comparative corrosion tests were conducted in 25% sulfuric acid solution at 176 F. Sample discs were prepared and tested in the manner described in Example 4, except that a test solution of 25% sulfuric acid was utilized and temperature was maintained at 176 F. Results of this test are set forth in Table 10.

TABLE 10 [Corrosion rates in 25% H2804 solution at 176 F.]

Loss in inches Surface of penetration roughness per year, i.p.y.

Alloy number:

960 32 microin.. 0. 00378 96l d0 0.0000 971--.. -do- 0. 00108 972- d0 0. 00594 1, 0 --d0..- 0. 0041 Hastelloy A -IO mier01n 0. 020 Illium R-- 0. 007 Carpenter 20 0. 020 Carpenter 20Cb3 .do. 0. 011 -do- 0. 010

CD4M Cu do 0.200

Example 8 Comparative corrosion tests were conducted in 25% sulfuric acid solution at room temperature. Sample discs were prepared and tested in the manner described in Ex ample 4, except that 25 sulfuric acid at room temperature was used as the test solution. The results of this test are set forth in Table 11.

Example 9 Comparative corrosion tests were conducted in 93% sulfuric acid solution at 210 F. Sample discs were prepared and tested in the manner described in Example 4, except that 93% sulfuric acid solution at 210 F. was used as the test solution. Results of this test are set forth in Table 12.

TABLE 12 [Corrosion rates in 93% H2804 solution at 210 F] Loss in inches Surface of penetration roughness per year, i.p.y.

Alloy number:

960 32 microin..

Hastelloy F .I-

Example Comparative corrosion tests were conducted in 10% hydrochloric acid solution at room temperature. Sample discs were prepared and tested in the manner described in Example 4, except that 10% hydrochloric acid at room temperature was used as the test solution. The results of this test are set forth in Table 13.

TABLE 13 [Corrosion rates in 10% E01 solution at room temperature] Loss in inches of penetration per year. i.p.y.

Surface roughness Example 11 Comparative corrosion tests were conducted in 20% hydrochloric acid solution at room temperature. Sample discs were prepared and tested in the manner described in Example 4, except that 20% hydrochloric acid at room temperature was used as the test solution. Results of this test are set forth in Table 14.

TABLE 14 [Corrosion rates in 20% HO] solution at room temperature] Loss in inches Surface of penetration roughness per year. i.p.y.

Alloy number:

961..-"; 32 microin- 0. 0233 971.-. -do 0. 0187 Hastelloy D- 4-10 microin.-. 0. 026 8 do 1. 22

10 Example '12 Corrosion tests were conducted in 50% sulfuric acid solution at 176 F. Sample discs were prepared and tested in the manner described in Example 4, except that 50% sulfuric acid solution at 176 F. was used as the test solution. Results of this test are set forth in Table 15.

TABLE 15 [Corrosion rates in 50% H2SO4 solution at 176 F.]

Loss in inches of penetration per year, i.p.y.

Surface roughness:

Alloy number Example 13 Corrosion tests were conducted in 75% sulfuric acid at 176 F. Sample discs were prepared and tested in the manner described in Example 4, except that 75% sulfuric acid at 176 F. was used as the test solution. Results of this test are set forth in Table 16.

TABLE 16 [Corrosion rates in 75% H2804 solution at 176 F.]

Loss in inches of penetration per year, i.p.y.

Surface roughness:

Alloy number Example 14 Corrosion tests were conducted in 93% sulfuric acid at 176 F. Sample discs were prepared and tested in the manner described in Example 4, except that 93% sulfuric acid at 176 F. was used as the test solution. Results of this test are set forth in Table 17.

TABLE 17 [Corrosion rates in 93% H2804 solution at 176 F.]

Loss in inches of penetration per year, i.p.y.

Surface roughness Alloy number Example 15 Comparative corrosion tests were conducted in boiling 40% sulfuric acid. Sample discs were prepared and tested in the manner described in Example 2, except that boiling 40% sulfuric acid was used as the test solution. Results of this test are set forth in Table 18.

TABLE 18 [Corrosion rates in boiling 40% H2804 solution] Loss in inches Surface of penetration roughness per year, i.p.y.

Alloy number:

960 4 microin 0.0000 961--. 0. 0043 971... 0. 0000 1, 2 0.- 0.0000 Carpenter 200133 4-10 micrcin- 0. 040

Example 16 Comparative corrosion tests were conducted in boiling 50% sulfuric acid. Sample discs were prepared and tested in the manner described in Example 2, except that boiling 50% sulfuric acid was used as the test solution. Results of this test are set forth in Table 19.

TABLE 19 [Corrosion rates in boiling 50% H2804 solution] Loss in inches In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above products without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An alloy resistant to corrosion by both oxidizing and reducing sulfuric acid solutions over a wide range of acid strengths, consisting essentially of between about 36% and about 46% by weight nickel, between about 33% and about 42.06% by weight chromium, between about 2.94% and about 7.84% by weight molybdenum, between about 1.95% and about 5.18% by weight copper, up to about 0.12% by weight carbon, up to about 1.50% by weight silicon, up to about 2.50% by weight manganese, up to about 2% by weight titanium, up to about 4% by weight niobium plus tantalum, up to about 0.1% by weight boron, and the balance essentially iron.

2. An alloy as set forth in claim 1 having a microstructure which is substantially austenitic so that the alloy is readily workable.

3. An alloy as set forth in claim 1 wherein the nickel content is between about 40% and about 44% by weight, the chromium content is between about 33% and about 35% by weight, the molybdenum content is between about 3% and about 4% by weight and the copper content is between about 3% and about 4% by weight.

4. An alloy as set forth in claim 3 containing about 42% by weight nickel, about 34% by weight chromium, about 3.2% by weight molybdenum, and about 3.6% by weight copper.

5. An alloy as set forth in claim 1 containing about 43% by weight nickel, about 42% by weight chromium, about 4.9% by weight molybdenum, and about 3.9% by weight copper.

6. An alloy as set forth in claim 1 containing about 46% by weight nickel, about 35% by weight chromium, about 7.8% by weight molybdenum, and about 5.2% by weight copper.

7. An alloy as set forth in claim 1 containing about 37% by weight nickel, about 33% by weight chromium, about 3.8% by weight molybdenum, about 3.3% by wei-ght copper, about 1.7% by weight silicon, about 2.0% by weight manganese, about 0.08% by weight titanium, and about 0.005% by weight boron.

8. An alloy as set forth in claim 1 containing about by weight nickel, about 41% by weight chromium, about 2.9% by weight molybdenum, and about 2.0% by weight copper.

9. An alloy as set forth in claim 1 containing about 43% by weight nickel, about 38% by weight chromium, about 3.6% by weight molybdenum, and about 3.2% by weight copper.

10. An alloy as set forth in claim 1 containing about 44% by weight nickel, about 34% by weight chromium, about 3.4% by weight molybdenum, and about 2.6% by weight copper.

References Cited UNITED STATES PATENTS 3,552,950 1/1971 Rundell -171 2,553,330 5/1951 Post 75-122 3,356,542 12/1967 Smith 75-122 3,574,612 4/1971 Maness 75-171 3,582,318 6/1971 Szumachowski 75-171 HYLAND' BI ZOT, Primary Examiner US. Cl. X.R.