|
(11) | EP 0 091 308 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
|
|
|
|
|||||||||||||||||||||||||||
| (54) | Corrosion resistant nickel base alloy |
| (57) 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 essentially 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. |
[0001] 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.
[0002] 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 contains about 19% chromium, 3% molybdenum, 19% iron, 5.1% columbium, and the balance nickel.
[0003] 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 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 been developed, such alloys are high in cobalt, and are therefore significantly more costly.
[0004] We have now discovered a nickel base alloy having outstanding corrosion resistance over a broad range of 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.
[0005] These and other advantageous properties are obtained in accordance with the present invention in a nickel base alloy having a critical balance of chromium, molybdenum, and tungsten within the following weight percentage ranges:
[0006] 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 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.
[0007] In carrying the invention into practice, advantageous results are obtained when the alloy consists 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 .15% 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, particularly the corrosion resistance of the alloy.
[0008] 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% chromium.
[0009] 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 workability decrease noticeably.
[0010] Tungsten is not generally included in commercial alloys developed for corrosion resistant 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 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 4X 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 I, alloy M).
[0011] The alloy will normally also contain carbon at a level of up to 0.15% by weight, 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%.
[0012] Cobalt and nickel are generally regarded as being interchangeable and provide similar properties to the 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.
[0013] 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%.
[0014] 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 amounts of these elements has been found to have deleterious effects on hot workability.
[0015] 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 about 0.107; 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.
[0016] 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 or to 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%.
[0017] 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 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.
[0018] 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 a better understanding of the invention, but are not intended to be understood as limiting the invention.
Example 1
[0019] Developmental heats of several alloy compositions in accordance with the invention were produced, and the chemical compositions of these alloys are set forth in Table I as alloys A - M. The percentages set forth in Table I 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.
[0020] 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 50°C. 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 10% sodium chloride and 5% ferric chloride for 24 hours. Test 3 is a standard test method for detecting susceptibility to intergranular 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.
[0021] 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 I.
[0022] 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 alloys listed above.
Example 2
[0023] 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 forth below in Table II:
[0024] These tests clearly indicate that the alloy of this invention has a corrosion resistance in the test solutions considerably superior to alloy C-276 when compared in the cold reduced condition.
Example 3
[0025] 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: 31X Cr, 10% Mo, 2% W, .40% Cb, .25% Ti, .25% Al, .001% max B, .10% max Fe, .10% max Cu, .04% C, .015% max S, .25% max Co, .015% max P, .10% max Ta, .10% max Zr, .10% max Mn, .01% max V, .25 max Si, balance nickel.
[0026] For purposes of comparison, specimens of alloy C-276 were evaluated under similar conditions. All three materials were studied in the 500°F (260°C) aged and unaged conditions following unidirectional cold working.
[0027] The mechanical properties of the three alloy test specimens are set forth in Table III below.
[0029] 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 yield strengths. Samples which had been aged at 260°C 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.
[0030] Test A - Stress Corrosion Cracking in NACE Solution (5 percent NaCl + 0.5 percent CH3COOH; Saturated with 100
Percent H2S gas) at 24°C.
[0031] 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 NACE Solution at 24°C.
[0032] Beam specimens stressed to 80 or 100 percent of yield strength were fitted with steel couples and placed in NACE solution for 28 days. All the beams were recovered unbroken.
[0034] Nickel-chrome wire was spot welded to the ends of beams stressed to 80 or 100 percent of yield 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 day test.
[0035] Test D - Weight-Loss Corrosion in "Green Death" Solution (Boiling 1% H2SO4 + 3% HC1 + 1% FeCl + 1% CuCl3) 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.
[0036] These tests indicate that the performance of the alloy of this invention under simulated oilfield hydrogen sulfide environments equals or surpasses that of alloy C-276 and that the corrosion resistance of the alloy under conditions of the simulated scrubber environment ("Green Death") test is clearly superior to that of alloy C-276.
Example 4
[0037] 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:
31X Cr, 10% Mo, 2% W, .02% C, .25% Ti, .25% Al, .40% Cb, balance Ni. For each of the
elements chromium, 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 f 3. The results are shown in Table V.
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 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.
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.