GB2117793A - Corrosion resistant nickel base alloy - Google Patents

Abstract

A nickel base alloy is provided having excellent hot and cold workability and superior corrosion resistance to a variety of media including deep sour gas well environments. The alloy consists of 27 to 33% by weight of chromium, 8 to 12% by weight of molybdenum, 0 to 4% by weight of tungsten, up to 1.5% by weight iron, up to 12% by weight of cobalt, up to 0.15% by weight of carbon, up to 1.5% by weight of aluminium, up to 1.5% by weight of titanium, up to 2% by weight of columbium, and the balance nickel.

Description

1 GB 2 117 793 A 1

SPECIFICATION

Corrosion resistant nickel base alloy This invention relates to a corrosion resistant nickel base alloy, and more particularly to an improved hot and cold workable nickel base alloy which has excellent corrosion resistance under a broad range of corrosive conditions, and which is particularly suited for use in highly corrosive deep sour gas well applications.

Many of the alloys used commercially in applications requiring good corrosion resistance are nickel base alloys. Such alloys generally contain relatively large amounts of chromium and molybdenum, and usually also contain substantial proportions of iron, copper or cobalt. Alloy C- 276 for example, a well known corrosion resistant nickel base alloy used in a variety of corrosive applications, has a nominal composition of about 15.5% chromium, 15.5% molybdenum, 3.5% tungsten, 6% iron, 2% cobalt and the balance nickel. Other known corrosion resistant alloys include alloy B-2, which has a nominal composition of about 28% molybdenum, 1% chromium, 2% iron, 1 % cobalt, and the balance nickel; alloy 625, which contains about 21.5% chromium, 9% molybdenum, 4% iron, 3.6% columbium, and the balance nickel; and alloy 718, which 1 contains about 19% chromium, 3% molybdenum, 19% iron, 5.1 % columbium, and the balance nickel.

Perhaps one of the most severely corrosive environments for a corrosion resistant nickel base alloy is found in deep sour gas well operations, where casing, tubing and other well components are subjected to high concentrations of hot wet hydrogen sulfide, brine and carbon dioxide under conditions of high temperature and pressure. Heretofore, the industry has relied on commercially available corrosion resistant 20 nickel base alloys such as those noted above, which were developed for other, less severe applications. However, these alloys have been less than fully satisfactory in the severe conditions found in sour gas well operations. While certain alloys having high corrosion resistance have have been developed, such alloys are high in cobalt, and are therefore significantly more costly.

We have now discovered a nickel base alloy having outstanding corrosion resistance over abroad range of 25 corrosive conditions ranging from oxidizing conditions to reducing conditions, and which performs particularly well in tests designed to simulate the extremely severe corrosive environment found in deep sour gas well operations. Additionally, this alloy exhibits excellent hot and cold workability, and has a relatively low content of expensive alloying elements.

These and other advantageous properties are obtained in accordance with the present invention in a nickel 30 base alloy having a critical balance of chromium, molybdenum, and tungsten within the following weight percentage ranges:

Chromium about27-33 Molybdenum about 8 - 12 35 Tungsten about 0 - 4 Nickel base alloys having this critical balance of chromium, molybdenum and tungsten exhibit superior corrosion resistance in a variety of solutions when compared to other commercially available corrosion resistant alloys, including alloy C-276, alloy B-2, alloy 718 and alloy 625. Further, based upon the cost of the 40 metals contained therein, alloys in accordance with this invention are less expensive than certain other commercial nickel base alloys which have poorer corrosion resistance. Alloys of the invention are easily hot workable so that they can be formed into various desired shapes, and also exhibit excellent cold workability so that high strength can be imparted to the final product by cold working.

In carrying the invention into practice, advantageous results are obtained when the alloy consists 45 essentially of about 27 - 33% chromium, about 8 - 12% molybdenum, about 0 - 4% tungsten, up to about 1.5% iron, up to about 12% cobalt, up to about.1 5% carbon, up to about 1.5% aluminum, up to about 1.5% titanium, up to about 2% columbium, and the balance nickel. By the term "consisting essentially of" we mean that in addition to the elements recited, the alloy may also contain incidental impurities and additions of other unspecified elements which do not materially affect the basic and novel characteristics of the alloy, 50 particularly the corrosion resistance of the alloy.

Chromium is an essential element in the alloy of the present invention because of the added corrosion resistance that it contributes. It appears from testing that the corrosion resistance is at an optimum when the chromium is at about 31 % of the composition. When the chromium is raised above about 33%, both the hot workability and the corrosion resistance worsen. Corrosion resistance also worsens below about 27% 55 chromium.

The presence of molybdenum provides improved pitting corrosion resistance. An optimum content of about 10% molybdenum appears to yield the lowest corrosion rate in the solutions tested. When the molybdenum content is decreased below about 8%, the pitting and crevice corrosion increases significantly.

The same occurs when the molybdenum is increased above about 12%, and in addition, the hot and cold 60 workability decrease noticeably.

Tungsten is not generally included in commercial alloys developed for corrosion resistance applications.

This element is usually provided in applications where enhanced strength, particularly at high temperature, is of primary concern, and is not generally thought to have any beneficial effect on corrosion resistance.

However, in the alloys of this invention, the presence of tungsten has been found to significantly enhance the 65 2 GB 2 117 793 A 2 corrosion resistance. Corrosion testing shows that the absence of tungsten results in a significantly higher corrosion rate, while a tungsten content in excess of about 4% causes the material to corrode at a higher rate in certain solutions, as well as making the alloy more difficult to hot work. The optimum tungsten content at the 10% molybdenum level appears to be about 2%, although replacement of some or all of the tungsten with additional molybdenum, for example, provides good corrosion resistance in some test media (see Table 1, alloy M).

The alicywill normally also contain carbon at a level of up to 0.15% byweight, either as an incidental impurity or as a purposeful addition for forming stable carbides. Preferably, the carbon level should be maintained at a level up to a maximum of about 0.08% by weight, and most desirably to about 0.04%.

Cobalt and nickel are generally regarded as being interchangeable and provide similar properties to the 10 alloy. Tests have shown that the substitution of cobalt for a portion of the nickel content does not adversely affect the corrosion resistance and workability characteristics of the alloy. Therefore cobalt may be included in the alloy if desired, even at levels up to about 12% by weight. However, because of the present high cost of cobalt, substitution of cobalt for nickel would not be economically attractive.

Aluminum may be present in small amounts to serve as a deoxidant. However, higher additions of aluminum adversely affect the workability of the alloy. Preferably, aluminum is present in amounts up to about 1.5% by weight, and most desirably up to about 0.25%.

Titanium and columbium may also be present in small amounts to serve as carbide formers. These elements are included at levels preferably up to about 1.5% by weight of titanium and about 2% by weight of columbium, and most desirably up to about 0.40% by weight. However, addition of significantly larger 20 amounts of these elements has been found to have deleterious effects on hot workability.

Alloys in accordance with this invention may.also contain minor amounts of other elements as impurities in the raw materials used or as deliberate additions to improve certain characteristics as is well known in the art. For example, minor proportions of magnesium, cerium, lanthanum, yttrium or misch metal may be optionally included to contribute to workability. Tests have shown that magnesium can be tolerated up to 25 about 0.10% by weight, preferably 0.07%, without significant loss of corrosion resistance. Boron may be added, preferably up to about.005%, to contribute to high temperature strength and ductility. Tantalum may be present at levels up to about 2% by weight without adversely affecting the corrosion resistance or workability, but the presence of tantalum at these levels has not been observed to benefit these properties of the alloy. Similarly vanadium can be present up to about 1% and zirconium up to 0.1% by weight.

Iron in significant amounts lowers the corrosion resistance of the alloy. Iron can be tolerated at levels up to about 1.5% by weight, but the corrosion resistance drops quite significantly at higher levels. Copper, manganese, and silicon, when present in small amounts or as impurities, can be tolerated. However, when added in significant amounts as alloying elements to the basic composition of this alloy, the elements have been found either to lower the corrosion resistance orto decrease the workability of the alloy or a combination of both. For example, the corrosion resistance of the alloy worsens significantly when copper is present at levels of about 1.5% by weight or greater, or manganese is present at levels of about 2% by weight or greater. Silicon is preferably maintained at levels less than 1%.

Alloys in accordance with the invention are produced by introducing into a furnace metallic raw materials containing nickel and the other specified metallic elements within the percentage ranges stated. Heating the 40 raw materials to form a melt, and pouring the melt from the furnace into a mould for solidification. Preferably, the melting is carried out under vacuum conditions. If desired, the thus formed alloy ingot may be further refined by remelting under vacuum conditions.

The following examples illustrate a number of specific alloy compositions in accordance with the present invention and compare the corrosion resistance thereof to other known nickel base corrosion resistant alloys. These examples are presented in order to give those skilled in the art abetter understanding of the invention, but are not intended to be understood as limiting the invention.

Example 1

Developmental heats of several alloy compositions in accordance with the invention were produced, and 50 the chemical compositions of these alloys are set forth in Table 1 as alloys A - M. The percentages set forth in Table 1 are by weight, based on the total composition, and represent the nominal composition, i.e. the amount of each of the elements as weighed for melting.

Cold worked and annealed test specimens of the various alloys, approximately 4 square inches in surface area, were prepared, weighed, and subjected to corrosion tests in various test solutions, after which the samples were dried, reweighed and the weight loss in grams was determined and converted to mils per year. Test 1 is a standard test method for determining pitting and crevice corrosion resistance by the use of a ferric chloride solution. The test specimens were immersed in a 10% by weight solution of ferric chloride for 72 hours at WC. This test method is similar to ASTM Standard Test Method G 48-76, except that the ASTM test uses 6% by weight ferric chloride. In test 2 the samples are immersed in a boiling aqueous solution of 60 10% sodium chloride and 5% ferric chloride for 24 hours. Test 3 is a standard test method for detecting susceptibility to intergranuiar attack in wrought nickel-rich chromium bearing alloys (ASTM Test Method G 28-72). In this test, the samples are immersed in a boiling ferric sulfate - 50% sulfuric acid solution for 24 hours. In test 4 the samples are immersed in boiling 65% nitric acid for 24 hours.

1 W Alloy TABLE 1 Corrosion Rate Nominal composition in weight percent Test 1 Test2 Test 3 Test4 Cr Mo W Ni c Ti AI Cb Other (in grams) (im mils per year) A 31 10 2 Bal..02.25.25.40 -.0005 0.3 6.9 4.8 B 31 10 4 Bal..02.25.25.40 -.0013 38.0 8.3 6.2 c 32 10 2 Bal..01.20.20.20 -.0009 0.8 4.8 4.2 D 31 10 2 Bal..03.20.20.20 -.0000 1.3 4.7 4.5 E 32 9 2 Bal..01.20.20.20 -.0001 113.6 4.7 4.1 F 31 10 2 Bal..02.25.25.40 -.0006 2.1 9.1 8.4 G 31 10 2 Bal..02.25.25.40.10Mg.0000 1.3 8.7 nt H 31 10 2 Bal..01.20.20.20 4Co.0000 0.3 8.8 nt 1 31 12 2 Bal..03.20.20.10.025 Zr.0006 0.6 nt nt j 31 10 2 Bal..03.20.20.10.05 M isch.0007 1.6 nt nt K 31 10 2 Bal..07.70.25.70 -.0007 4.0 8.7 nt L 31 10 2 Bal..04.25.25.40 -.0010 0.7 9.0 nt M 31 12 0 Bal..02.25.25.40 -.0007 10.7 8.5 5.2 B-2 1 28 Bal..02 - - 2Fe, 1Co 3.6912 1955.8 671.0 nt C-276 15.5 15.5 3.5 Bal..02 - - 6Fe, 2Co.0020 4.8 221.5 242.1 718 19 3 - Bal..04 1.50 5.10 19Fe 1.9569 1577.0 18.5 nt 625 21.5 9 - Bal..05.30.30 3.6 4Fe.0833 nt nt nt Test 1 500C - 10% FeC13/72 Hrs. Test 2 - Boiling 10% NaCI + 5% FeC13/24 Hrs. Test 3 - Boiling 50% Solution of H2S04 + Fe2(S046/24 Hrs. Test 4 - Boiling 65% HN03/24 Hrs. nt = not tested constant sample size W 4 GB 2 117 793 A 4 For purposes of comparison, several commercially available corrosion resistant alloys (alloy B-2, alloy C-276, alloy 718, and alloy 625) were tested in the same manner, and these test results are also set forth in Table 1.

These tests indicate with very few exceptions that the alloy of this invention has superior corrosion resistance under these test conditions when compared to the commercially available corrosion resistant 5 alloys listed above.

Example 2

Two of the alloys of Example 1 were cold reduced 70% in cross-sectional area on a rolling mill. A sample of alloy C-276 was similarly reduced. These alloys were then tested in the test solutions, and the results are set 10 forth below in Table fl:

TABLE 11

Alloy Average Weight Loss in Grams Test 1 Test2 Test 3 F.0000.0020.0055 L.0000.0016.0101 20 C-276.0008.0062.1926 These tests clea fly i ndicate th at th e a 1 loy of th is i nventio n has a corrosion resista nce i n the test so] utions considerably superior to alloy C-276 when compared in the cold reduced condition.

Example 3

Specimens of two alloys in accordance with the present invention (alloy N and alloy 0) were subjected to corrosion studies designed for evaluating performance in corrosive oilfield sour gas well hydrogen sulfide environments (Tests A, B and C) and simulated scrubber environments (Test D). Alloys N and 0 had a nominal chemical composition as follows: 31 % Cr, 10% Mo, 2% W_40% Cb_25% Ti,25% Al,.001% max B, 30 10% max Fe,.1 0% max Cu_04% C_01 5% max S_25% max Co_01 5% max P_1 0% max Ta_10% max Zr, 10% max Mn_01% max V_25 max Si, balance nickel.

For purposes of comparison, specimens of alloy C-276 were evaluated under similar conditions. All three materials were studied in the 500'F (260OC) aged and unaged conditions following unidirectional cold working.

The mechanical properties of the three alloy test specimens are set forth in Table Ill below.

TABLE Ill

Mechanical Properties of Materials Evaluated In Corrosion Studies 40 0.2 Percent Tensile Offset Yield Strength Elongation Strength (ksi) (ksi) (percent) 45 Alloy N (the invention) Coldworked 128.4 155.1 17.6 (Aged) Coldworked + 260'C/50 h r 138.9 159.1 23.4 50 Alloy 0 (the invention) Coldworked 134.0 156.6 16.8 (Aged) Coldworked + 55 260'C/50 h r 136.3 160.7 17.4 Alloy C-276 (comparison) Coldworked 168.8 203.7 17.5 60 (Aged) Coldworked + 260'Cl50 hr 182.5 213.5 15.4 GB 2 117 793 A 5 The three materials were studied in four environments, as follows:

Test Aqueous Conditions Temperature A - Sulfide Stress Cracking NACE Solution 240C 5 B - Hydrogen Embrittlement NACE Solution 240C (steel couple) C - Hydrogen Embrittlement 5% H2S04 + As 240C 0 = 25mA/cM2) D Weight-Loss Corrosion "Green Death" Boiling 10 (7% H2S04,3% FICI, 1 % FeC13, 1 % CUC13) All the embrittlement tests were conducted using 4.375-inch x 0.25-inch x 0.094-inch beam specimens stressed in three point bending. The unaged materials were stressed to 80 and 100 percent of their respective 15 yield strengths. Samples which had been aged at 26WC for 50 hours were stressed to 100 percent of their yield strength. Unstressed creviced coupons measuring 2-inches x 0.625- inch x.062515-inch were used in the weight-loss corrosion tests. Tests A- C were run for 28 days. The coupons in test D were examined and weighed at the end of 24,72 and 168 hours.

TESTA Stress corrosion cracking in NACEsolution (5percent NaCI + 0.5percent CH3COOH, saturated with 100 percentH2S gas) at24oC Beam specimens stressed to 80 or 100 percent of yield were exposed for 23 days in NACE solution. All specimens were recovered unbroken with no visual signs of corrosion.

TEST B Hydrogen embrittlement in NA CE solution at 24C Bea m s peci mens stressed to 80 o r 100 percent of yiel d strength were fitted with steel co u p 1 es a nd p 1 aced i n NACE solution for 28 days. All the beams were recovered unbroken.

TEST C Hydrogen embrittlement in 501. H2S04 + 1 mgI 1 sodium arsenite at MC N ickel -ch ro me wi re was spot wel ded to th e en ds of bea m s stressed to 80 o r 100 perce nt of yiel cl strength. The beam specimens were then placed in the test solution and cathodically charged with hydrogen at a current of 25 mA/cM2. At the end of 13 days, alloy C-276 in the aged condition stressed at 100 percent of yield was found to have failed. Alloy C-276 in the unaged condition stressed to 100 percent yield strength failed after 21 days. Specimens of alloys N and 0 were retrieved unbroken at the end of the 28 daytest.

TEST D Weight-loss corrosion in---GreenDeath -solution (boiling 1% H2S04 + 3% HCl + 1% F6C/3 + 1% CuCIA Weight-loss corrosion coupons of each material were weighed, creviced, and placed in the "Green Death" solution. The coupons were cleaned and reweighed at 24 hours, 72 hours, and 168 hours. The coupons of alloys N and 0 had significantly less corrosion weight loss than the coupons of alloy C-276, as shown in Table IV.

TABLE [V

Corrosion Rate (Mils per year) 50 24 hr 72 hr 168 hr Alloy N.27.15.7 Alloy 0 0.1.3.2 Alloy C-276 (Comparison).45.32.42 55 These tests indicate thatthe performance of the alloy of this invention under simulated oilfield hydrogen sulfide environments equals or surpasses that of alloy C-276 and thatthe corrosion resistance of the alloy under conditions of the simulated scrubber environment ("Green Death") test is clearly superiorto that of alloyC-276.

Example 4

A series of tests was carried out to investigate the effect of varying amounts of chromium, molybdenum, tungsten, copper and iron on corrosion resistance. The basic alloy composition (heat 367) was as follows:

31% Cr, 10% Mo, 2% W_02% C_25Ti_25% Al_40% Cla, balance Ni. For each of the elements chromium, 65 6 GB 2 117 793 A molybdenum, tungsten, copper and iron heats were prepared with varying amounts of that element while holding all of the other specified elements constant. Test specimens were prepared and tested as in Example 1 under the conditions of test # 2 and test # 3. The results are shown in Table V.

TABLE V

Corrosion Rate (milsperyear) HeatNo. Element % of Element Test2 Test3 367 Cu 0 0.3 6.9 10 850 Cu 0.5 1.2 nt 851 Cu 1 5.1 nt 852 Cu 1.5 659 nt 853 Cu 2 872 nt 15 854 Cu 5 1069 nt 367 Fe 0.0.3 6.9 821 Fe 0.5 1.4 12.1 822 Fe 1.0 3.1 18.9 823 Fe 1.5 653 9.0 20 824 Fe 2.0 879 12.5 392 Fe 5.0 2029 6.2 846 Cr 28 0.7 21.0 709 Cr 29 4.2 17.6 847 Cr 30 2.1 11.1 25 367 Cr 31 0.3 6.9 848 Cr 32 2.4 9.9 710 Cr 33 nt 19.3 849 Cr 34 nt nt 842 Mo 8 389 8.6 30 843 Mo 9 3.5 8.5 367 Mo 10 0.3 6.9 844 Mo 11 116 8.8 845 Mo 12 842 15.3 35 TABLE V Continued Average Weight Loss (milsperyear) 40 Heat No. Element % of Element Test2 Test 3 838 W 0 27.9 18.0 839 W 1 1.0 21.6 367 W 2 0.3 6.9 840 W 3 2.0 8.6 368 W 4 8.3 38.0 nt - not tested - unable to test-specimen split due to lack of workability 50 The present invention has been illustrated and described by reference to specific embodiments. However, those skilled in the art will readily understand that modifications and variations may be resorted to without departing from the spirit and scope of the invention.

Claims (6)

1. A nickel base alloy containing chromium and molybdenum and having excellent hot and cold workability and superior corrosion resistance to a variety of media including deep sour gas well environments, said alloy being characterised in that it consists essentially of 27 to 33% by weight of 60 chromium, 8 to 12% by weight of molybdenum, and 0-4% by weight of tungsten, with or without up to 1.5% by weight of iron, up to 12% by weight of cobalt, up to 0.15% by weight of carbon, up to 1.5% by weight of aluminium, up to 1.5% by weight of titanium and up to 2% by weight of columbium, the balance being nickel.

2. A nickel base alloy according to claim 1 further characterised in that the chromium content is substantially 31 %and the molybdenum content substantially 10% by weight.

i 7 GB 2 117 793 A 7

3. A nickel base alloy according to either of claims 1 or 2, further characterised in that the tungsten content is substantially 2% by weight.

4. A nickel base alloy according to claim 1 further characterised in that it contains substantially 12% by weight of molybdenum but no tungsten.

5. A nickel base alloy substantially as hereinbefore described with reference to the specific examples. 5

6. A method of producing a nickel base alloy substantially as hereinbefore described.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1983.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.