CA1211961A - Corrosion resistant nickel base alloy - Google Patents

Abstract

CORROSION RESISTANT NICKEL BASE ALLOY
Abstract of the Disclosure 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 about 27 to 33% chromium, about 8 to 12% molybdenum, about 0 to 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.

Description

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CORROSION RESISTANT NICREL BASE ALLOY
Thi~ invention relate~ to a corrosion resi~ant nlckel base alloy, and more particularly to an improved hot and cold workable nickel base alloy which has excellent corrosion resi~tance ~nder a broad range of 5 corrosive conditions, and which is particularly suited for use in highly corrosive deep sour gas well applications~
Man~ of the alloy~ used commercially in applica-tion~ requiring good corrosion resistance are nickel base.
alloy~. Such alloys generally contain relatively large amounts of chromium and molybdenum 9 and usually also con-ta~n æubstantial-proportions of iron, copper or cobalt.
- Alloy C-~76 or example, a well known corrosion resistant : nickel base alloy used in a variety of corrosive applications, ha~ 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 resis~ant alloys include alloy B-2, which has a nominal composltion of about 28% molybdenum, 1% chromium, 2% iron~
1% c~balt., and the balance nickel; alloy 625, which con-~0 tains about 21.5% chromium, 9% molybdenum9 4~ iron~ 3.6%columbi~m, and the balance ~ckel, and alloy 718, which cont~ins about 19% hromium 9 3% molybdenum, 19% iron, 5.1%
columbium, and the balance nickel.
~ ~ Perhap~ one of the mo~t severely corrosive 25~ environments for a c~rros.ion resi~tan~ nickel ba~e alloy ` i8 f~und in deep sour gas well operation , where caælng, ~ubing and other well componen~s are subjected ~o high concentra~ions of hot we~ hydrogen sulfide, brine and carbon dioxide under conditiono of high temperature ' ~ 3~

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 applica-tions. However, these alloys have been less than fully satisfac-tory in the severe conditions Eound 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.
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.
; In accordance with the present invention there is provided a nickel base alloy having excellent ho-t and cold wor]cability and superior corrosion resistance to a variety of media including deep sour gas well environments, the alloy having a chromium content of about 27 to 33%, a molybdenum content of about 8 to 12~ and a tunysten content of 1 to about 4%.
The present invention can also be defined as a nickel base alloy having excellent hot and cold workability and superior corrosion resistance to a variety of media including deep sour gas well environments and consisting essentially of about 27 to 33% chromium, about 8 to 12% molybdenum, about 1 to 4% tungsten, up to about 1.5% iron, up to about 1.5% copper, 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.
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 com-mercially available corrosion resistant alloys, including alloy C-276, alloy B-2, alloy 718 and alloy ~25. Further, based upon the cost o~ the metals contained therein, alloys in accordance with this invention are less expensive than certain other commercial - 2a -..,~,~

~ 3 nickel base alloys which have poorer cor~osion resist-ance~ Alloys of the invention are easily hot workable ~o that they can be formed in~o various desired shapes, and also exhibit excellent cold workability so ~hat high 5 strength can be imparted to the final product by cold orking .
In carrying the invention into practic~, advan-~ageous results are obtained when ~he alloy consist.s essentially of about 27 - 33X chromium, about 8 - 12%
molybdenum~ about 0 - 4% tungsten, up to about 1O5% iron, - up to abou~ 12% cobalt, up to about .15% carbon~ up to about 1.5% aluminum, up to about 1~5% tita~ium9 up to about 2% columbium~ and ~he balance nickel. .By the term "consisting essentially of" we mean that in addition to the elements recited, the alloy may also contain inci-dental impurities and additions of other unspecified elements which do not materially affect th~ basic and novel characteristics of the alloy, particularly the corrosion resistance of the alloy.
Chromium is an essential element in the alloy of the present invention because of the added corrosion resis~ance that it contribu~es. It appears from tes~ing that the corrosion resistance i8 at an optimum when the chromium is at about 31% of the composition. When the . 2S chromium is raised above about 33%, both the hot workabi-l~ty and the corrosion re~istance worsen. Corrosion ~esistance also worsens below about 27% chromium.
The presence of molybdenum provides improved pitting corrosion resistance~ An optimum content of about ~0% molybdenum appears to yield the lowest corrosisn rate in the solutions tested. When the mol~bdenum content ls decrea~ed below about 8~, the pit~ing and crevice corrosion increases significantly. The same occurs when the molybde-n~m is increased above abvut 12æ, and in addition, the hot and cold workabllity decrease noticeably.
Tungsten is not generally included in commer~
cial alloys developed for corroslon re istant applicationsO

~ 4-This element is usually provided in applications where enhanced strength, particularly at high temperature, is of primary concern, and is no~ generally thought to have any beneficial effect on corrosion resistance~ However, in 5 the alloys of this invention, the presence of tungsten ha~ been found to significan~ly 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 4% causes the 1~ material to corrode at a higher rate in certain solutions, as ~eil as making the alloy more difficult to hot work.
The optimum tungsten content at the lQ% molybdenum level appears ~o be about 2% 9 although replacement of some or all of the tungs~en with additional molybdenum 9 for example, provides good corrosion resistance in some test media (see Table I, alloy M).
The alloy will normally also contain carbon at a level of up to about .15% 9 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 wei~ht, and most desirably to about 0~04%0 Cobalt and nickel are generally regarded as being interchangeable and provide ~imilar properties to the alloy. Tests have shown that the substitution of ~obalt for a portion of the nickel content does not adversely affect the corrosion resi~tance and workability chara~ter-i~tics of the alloy. Therefore cobalt may be included in ~he 8110y if desired~ even at levels up to about 12~ by weight. However, because of the present high cos~ of cobalt, substitution of cobalt for nickel would not be economically attractive.
Aluminum may be present i~ small amounts to 8erve as a deoxidantO However, higher additions of alumi-num adversely affect the workability of the alloy. Pref-erably, aluminum is present in amounts up to about lr 5% by weight, and most desir~bly up to abou~ 0.25~.

Ti~anium and columbium may also be present in ~mall amounts to serve as carbide ~ormers. These elements are included at levels preferably up-~o about 1.5% titanium and about 2% columbium; and most desirably up to ~bout 0040%0 However, additiQn of significantly larger amounts of these elements has been found to have deleterious effects on hot workability.
Alloys in accordance with this invention may al~o contain minor amo~nts of other elements as impurities 10 in the raw materials used or as deliberate additions to improve certain characteristics as is well known in the artO For example, minor propor~ions of magnesium, cerium, lanthanum, yttrium or misch metal may be option-ally included to con~ribute to workabilityO Tests have shown that magnesium can be ~olerated up to about 0010%, preferably 0.07% without significant loss of corro-810n resistance. Boron may be added, preferably up to about ~005%, to contribute to high temperature strength and ductilityO Tantalum may be present at levels up to about ~0 2% without adversely affecting the corrosion resistance or workability, but the presence of tantalum at these levels ha~ not been observed to benefit these properties of the alloyO Sîmilarly vanadium can be present up ~o about 1% :
and zirconium up to ~1%.
Iron in significant amounts lowers the corrosion resistance of the alloy. Iron can be tolera~ed a~ levels up to about 1.5%, but the corrosion resistance drops qui.te significantly at higher levels. Copper, manganese, and 8ilicon, when present in small amounts or as impurities, can be tolerated. However, when added in significant amount9 as alloying elements to the basic composition of this alloy 9 the elements have been found either to lower . the corroslon resifitance or to decrease the workability of the alloy or a combination of bothO For example, the corrosion resistance of the alloy worsen~ significantly when copper ls presen~ at levels of about 1. 5% or greater, or ~anganese is present at levels of about 2% or greater.
~ili.con is preferably m~intained at levels less than 1%.
The following examples illustrate a number of specif;c alloy compositions in accordance with the present invention and compare the corrosion resistance ~hereof to other kno~n nickel base corrosion resistant alloys~ These examples are presented in order ~o give those skilled in the art a better understanding of the invention, but are not lntended to be understood as limiting the invention~
Example 1 Developmental heats of several alloy com-positions in accordance with the invention were produred, and the chemical compositions of ~hese alloys are set ~orth in Table I as alloys A - M. The percentages set orth 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 $or melting.
Col d worked and annealed tes specimens of the various alloys, approximately 4 square inches in surface area , were prepared, weighed, and subjec~ed ~o corrosion tests in various test solutions, after which the samples were dried 9 reweighed and the weight loss in grams was deter-mlned and converted to mils per year~ Test 1 is a standard ~est method for determining pitting and crevice corrosion resistanre by the use of a ferric chloride solution. The test specimens were immPrsed in a 10% by weight solution of ierric chloride $or 72 hour~ at 50C. 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 tes~
~0 2 the samples are immersed in a boiling aqueous solution o 10% sodium chloride and 5% ferric chloride for 24 hours.
Tes~ 3 is a standard tPst method ~or detecting susceptibil-l~y ~o intergranular attack in wrough~ nickel-rich chro-mium bearing alloys (ASTM Test ~ethod G 28-72). In this tes~ the samples are immersed in a boiling ferric sulfate 50X sulfuric acid solution for 24 hoursO In test 4 the samples are immersed in boiling 65% nitrlc acid for 24 hours.

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~ or purposes of comparison, several commercially available corrosion resistant alloy~ (alloy B~2~ alloy C 276, alloy 718, and alloy 6253 were,,,tested in the sam manner, and these test results are also set forth in Table S I.
These test~ indi.cate with very few exceptions that ~he alloy of this invention has supPrior corrosion resistance under these test conditions when compared to the commeroially available corrosion resistant alloys listed 10 aboveO
Example 2 Two of the alloys of Example 1 were cold reduced 70% in cross~sec~ional area on a rolling mill~ A sample of alloy C~276 was similarly reduced~ These alloys were then ~ested in the test solutions~ and ~he-resultQ are set forth ~elow in Table II: .

TABLE II

ALLOY ¦ AVERAGE WEIGHT LOSS IN GRA~lS
~ Test 1 Test 2 Test 3 F .0000 .0~20 .0055 .~000 ~0016 .0101 ~-27~ ~0008 o00~2 .1926 These tests clearly indicate that the alloy of thi~ inventlon has a corrosion resistance in the te~t solution~ considerably superior to alloy C-276 whe~ com~
pared in the cold reduced condition~
Example 3 5pecimens of ~wo alloys in accordance with the pre~ent invention (alloy N and alloy 0) were subjected to corro ion studies designed for eval~ating per~orman e in corrosiv~ oilfield sour gas well hydrogen sul~ide environ-ment~ (Tests A~ B and C) and simulated scrubber environ-ments (Test D). Alloys ~ and O had a nominal chemical composition as follows: 31~ Cr, 10% ~o, 2% W, .40% Cb, ~:~9~
0~5% Ti, .25% Al, .001% max B, ~lO~o max Fe, .10% max Cu9 o~48h C, .015% max S9 o25% max Co, .015% max P, .10% max Ta, clO~ max Zr, ~10% m~x Mn9 .01% max V, .25 max Si, balance nickel.
For purposes of comparison 3 spe imens of alloy C~276 were evaluated under similar conditions. All three materials were s~udied in the 500F (260C) a~ed and unaged condltions following unidirectional cold workin~
The mechnical properties of the three alloy test ~0 ~pecimens are set forth in Table III below~

.... ._ .. ~ . _ _ TABLE III
Mechanical Properties o~ Materials Evaluated In Corro~ion Studie~

0~2 Percent TensiIe Elonga~ion 15 Offset Yield Strength (percent) Strength (ksi) (ksi) Alloy N (the invention) Coldworked 128.4 15501 17~6-(Aged) Coldworked +
~0 260C/50 hr 138.9 159~1 2304 Alloy 0 (the invention) Goldwoxked 134.0 156c6 16.8 (Aged3 Coldworked ~
260C/50 hr 13603 16007 17~4 25 Alloy C~276 (comparison) Cold~orked 168 0 8 203 ., 7 17 . 5 (AgPd) Coldworked ~
260C/50 hr 182.5 213.,515.4 . , , . .~
The three materials were ~tudied in four 30 environments, as follows:
Test ~queou~ L~diei~n~ Temperature A Sulfide Stress Cracking NACE Solutlon 24 C
B ~ ~lydrogen Embrittlement NACE Solution 24~ C
(8teel couple) .

~l2~

. ~Q
C: ~ llydrogen Embrittle~ent 5% ~l2SO4 ~ As 24 C
25ml'./ cm2 ) ~3eight Loss Corrosion "Green Death" ~oilîng (7% H2SO~, 3~ HCl 9 1% FeC13, 1% CuC13) All the embrittlement tests were conducted using 4.375-inch x 0~25-inch x n . 094-inch beam specimens stressed in ` three point bendingO The unaged materials were stressed to 80 and 100 percent of their respective yield streng~hs.
Samples which had been aged at 260C for 50 hours were stressed to 100 percent of their yield strength. Un-stressed creviced coupons measuring 2 inches x 0.625~inch x .062515-inch were used in ~he weight-loss corrosion ~ests~ Tests A-C wer~ run ~or 28 daysO The coupons in 15 test D were examined and weighed at the end of 24, 72 and 168 hours4 Test A ~ Stress Corro~ion Cracking in NACE Solution (5 percent NaCl + 0~5 percent CH3COOH, Saturated with 100 2Q Percent H2S ~as) at 24C.
Beam specimens stressed to 80 or 100 percent of yield were exposed for 28 days in NACE solution. All specimens were recovered unbroken wîth no visual signs of corrosionO

est B - Hydrogerl Embrittlement in NACE Solution at 24~.
2S Beam specimens stressed to 80 or 100 percent of yield strength were fitted with steel couples and placed in NACE
~olution for 28 daysO All the beams were recovered unbro-kenO

Test C ~ ~ydrogen Embrittlement in 5% H2SO4 + 1 mg~l 3~ ~
Nickel-chrome wire was spot welded to the ends of beams stressed to 80 or 100 percent of yield ~trength. The beam specimens were then placed in ~he t~st solution and catho-dically charged with hydrogen at a current of 25 mA/cm2.
At the end of L3 days, alloy C-276 in the aged condition stressed at 100 percent vf yield was found to have failed.
Alloy C-~76 in the unaged condition stressed to 100 per cent yield strength failed after 21-~aysO Specimens of alloys N and O were retrieved unbroken at the end of the 28 day test.

Test D - Weight-Loss Corrosion in 'IGreen Death" Solution SO4 ~ 3~O HCl + 1% E'eC13_~ 1% CuC13) Weight-loss corrosion coupons of each material were weighed, creviced, and placed in the "Green Death"
1~ solution~ The coupons were cleaned and reweighed at ~4 hours, 72 hours, and 168 hours. The coupons of alloys N
and O had significantly less corrosion weight loss than the coupons of alloy C-276~ as shown in Table IV~

TABLE IV
~5 Corrosion Rate (~ils per year) 24 hr 72 hr 168 hr Alloy N ~27 ~15 .7 Alloy O 0~1 L3 ' ~2 Alloy C-276 (Comparison~ ~45 c32 ,42 These tests indicate that the performance of the alloy of this invention ~nder simulated oilfield hydrogen sulfide environments equals or ~urpasses that of alloy Ç-276 and that the corrosion resistance of the alloy under conditions of the simulated scrubber environment ~"Green Death") te~t is c~early superior to that of alloy C-276.
Example 4 A series of te~ts was carried out to investig~te the ef~ect of varying amounts of chromium, molybdenum, tungsten, copper and iron on corro~ion resistance. The ba~ic alloy ~omposition (heat 367) was a~ follows:
31Z Cr, 10~ Mo, 2% W, ~02~ C, .25% Ti7 ~5% Al, ~40~ Cb, balance Nio For each of the elements chromiu~, nolybdenum, .

~2 ~ungsten, copper and iron heats were prepared with varying amounts of that element while holdin~ all of the other specified elements constantD Test specimens were prepared and tested as in Example 1 under the conditions of test 5 $ 2 and test ~ 3~ The results are shown in Table VO

TABLE V
~ ................. . , . , ~ , ~ . .

HEAT NOO ELEMENT æ OF ELEMENT ¦ CORROSION RATE
¦ ~ils per year~
I Test 2 Test 3 ~Q 367 Cu U 0.3 850 !' 0O5 1.2 nt 851 ~3 1 5O 1 nt 852 " 1 . 5 659 . nt 853 '~ 2 872 nt 854 ~t 5 1069 nt 367 F~ O OD3 609 ~21 " ~.5 1.4 12.1 - g22 " 1.0 311 18.9 ~3 tl 1~5 653 9.0 ~Q 824 '~ 2.0 B79 1205 392 '~ 5.~ 202g 6.2 846 Cr 28 0.7 21.0 709 " 2~ 4.2 17.6 847 ~I 30 201 llol 367 '~ ~1 0.3 6.9 848 '~. 32 2.4 9.9 710 l 33 nt 19.3 849 ~' 34 nt* nt*
842 Mo 8 389 8 . 6 ~43 '~ 9 . 3~5 8.5 367 " 10 0.3 609 ~44 " 11 116 8~8 ~45 " 12 842 15.3 ~13.-TABLE V Continued , _ _ ~EAT NO~ ELEMENT ~ OF EL~ME~T AVERAGE WEIG~T L~SS
(mils per year) Tes'c 2 l'est 3 _ ~,._ . . _ . . _ . .
5 838 W 0 27 D 9 18 ~ O
~39 '; 1 1., 0 21 . 6 - 367 ~3 2 0 ,. 3 6 . 9 840 " 3 2,.0 8.6 36~ ll 4 8~D~ 3800 10 nt ~ not tested * e~ unable to test- specimen split -due to lack of workability The present invention has been illustrated and described by reference to specifit~ embodiments. However, those skilled in the art will readily understand that .- modifirations and variations may be resorted to without departing from the spirit and scope of the invention.

?~;

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A nickel base alloy having excellent hot and cold workability and superior corrosion resistance to a variety of media including deep sour gas well environments, the alloy having a chromium content of about 27 to 33%, a molybdenum content of about 8 to 12% and a tungsten content of 1 to about 4%.

2. An alloy as set forth in claim 1 wherein the chromium is present at a level of about 31% and the molybdenum is present at a level of about 10%.

3. An alloy as set forth in claim 2 wherein tungsten is present at a level of about 2%.

4. A nickel base alloy having excellent hot and cold workability and superior corrosion resistance to a variety of media including deep sour gas well environments and consisting essentially of about 27 to 33% chromium, about 8 to 12% molybdenum, about 1 to 4% tungsten, up to about 1.5% iron, up to about 1.5%
copper, 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.

5. An alloy as set forth in claim 4 wherein tungsten is present at a level of about 2%.

6. An alloy as set forth in claim 5 wherein the molybdenum is present at a level of about 10%.

7. A nickel base alloy having excellent hot and cold workability and superior corrosion resistance to a variety of media including deep sour gas well environments and consisting essentially of about 31% chromium, about 10% molybdenum, about 2% tungsten, up to about 1.5% iron, up to about 4% cobalt, up to about .08% carbon, up to about .25% aluminum, up to about .40%
titanium, up to about .40% columbium, and the balance nickel.