High chromium nickel base alloys

Disclosed is a nickel-base alloy which provides excellent corrosion resistance to a variety of severe environments, especially hot phosphoric acid. The alloy preferably contains, in weight percent: about 30 chromium, about 4 molybdenum, about 2 tungsten, about 1 Cb/Ta, about 1.5 copper, about 14 iron and the balance nickel plus the impurities and modifying elements usually found in alloy of this class. The alloy is eminently suited for use as articles in chemical processing apparatus in the manufacture and/or containment of phosphoric acid and sulfuric acid.

This invention relates to corrosion-resistant nickel-base alloys and, more 
particularly, to Ni-Cr-Fe alloys containing molybdenum, tungsten and 
copper which are corrosion resistant in a variety of severe environments 
especially phosphoric acid. 
Nickel-base alloys containing chromium have been used as corrosion 
resistant articles for many years. For example, U.S. Pat. No. 873,746 
granted to Elwood Haynes on Dec. 17, 1907, disclosed a nickel-base alloy 
containing a total of 30 to 60% chromium, molybdenum, tungsten and/or 
uranium that is resistant to boiling nitric acid. 
For over seventy years since the Haynes disclosure, continuous research and 
development has been done to find specific nickel base alloys that are 
resistant to a variety of corrosive media. Certain alloys especially 
resistant in one type of acid are usually not resistant in another type of 
acid. 
Thus, the research and development goes on to discover "ideal" alloys that 
more nearly approach resistance to various media of oxidizing and reducing 
acid environments. This is of particular interest to The Chemical Process 
Industries, where the move is toward more efficient processes involving 
high temperatures and concentrations of various corrosive process media. 
One typical corrosive medium in chemical processing, and perhaps the most 
severe, is phosphoric Acid (P.sub.2 O.sub.5). 
In general, it is accepted that alloys with high nickel content, i.e. 
nickel base alloys, exhibit the best corrosion resistance in phosphoric 
acid media. Some of these nickel base alloys are disclosed in Table I. 
These alloys are representative of this crowded art and the subtle degree 
of advancement that each novel alloy represents. A study of the most 
recent patents in this art reveals that the new alloys generally contain 
the same basic elements i.e. (Ni-Cr-Mo-Cu) in various amounts and some 
elements may be in certain proportions to each other. 
U.S. Pat. No. 3,203,792 discloses a NiCrMo alloy commercially known as 
C-276 alloy in Table 1. This alloy is especially resistant to 
intergranular corrosion, especially after welding. 
U.S. Pat. No. 2,777,766 discloses the NiCrFeMo alloy commercially known as 
Alloy G in Table 1. Alloy G is generally considered the standard in 
resistance in many acids including hot sulfuric and phosphoric acids. The 
alloy resists stress corrosion cracking and pitting. 
U.S. Pat. No. 3,160,500 discloses a NiCrMoCb alloy commecially known as 
Alloy 625 in Table 1. This alloy has a good combination of properties at 
temperatures up to about 1500.degree. F. 
Alloy 690, as defined in Table 1, was disclosed as an experimental alloy. 
The alloy has a high degree of wet corrosion resistance in acid and 
caustic solutions. U.S. Pat. Nos. 3,573,901 and 3,574,604 describe alloys 
of this general class. 
After much experimentation, it was found that none of these commercial 
alloys offers adequate resistance to high concentration phosphoric acid at 
elevated temperatures, i.e., conditions encountered in the production of 
superphosphoric acid. None of the prior art patents teach how to obtain 
alloys with high degree of corrosion resistance to phosphoric acid. 
It is the principal object of this invention to provide an alloy highly 
resistant to a variety of acids, especially phosphoric acid. 
Other objects will be apparent to those skilled in this art. 
These objects and other benefits are provided by the invention of the alloy 
as defined in Table II. Both molybdenum and tungsten must be in the alloy. 
Furthermore, it is preferred that molybdenum exceeds tungsten within the 
ranges Mo:W=1.5:1 and 4:1. 
In superalloys of this class molybdenum and tungsten are generally 
considered to be equivalents. This is not true in the alloy of this 
invention. Although the exact mechanism is not completely understood, it 
is believed that the content of more molybdenum than tungsten effects an 
unexpected improvement in high chromium nickel base alloy containing 
critical contents of copper, iron, and columbium and/or tantalum. 
Nickel base alloys of this class may be produced by a variety of 
metallurgical processes--for example: hot-rolled plate sheet, cold rolled 
sheet, casting, wire for weld overlay and powder metallurgy. 
The alloy of this invention may be produced by several well-known methods 
as practiced in this art. There is no unusual problem in the production of 
this alloy since the basic elements are well known to those skilled in the 
art. 
The test examples of the alloy of this invention were produced as sheet and 
plate by conventional melting, casting, forging and rolling methods.

CHROMIUM CONTENT 
The need for high chromium content in an alloy to resist phosphoric acid 
was demonstrated in the test results given in Table III. The compositions 
for each of the alloys tested are essentially as shown as "typical" alloy. 
The corrosion rate is given in mils per year (Mpy). The specimens were 
tested in 46% phosphoric acid at 116.degree. C. These data suggest that 
the corrosion resistance is directly related to the chromium content and 
that there is a need for a 30% Cr to provide good resistance to phosphoric 
acid. 
MOLYBDENUM CONTENT 
The effect of molybdenum in this class of alloys was demonstrated in the 
test results given in Table IV. The specimens were tested in 52% 
phosphoric acid at 149.degree. C. Alloy 690 is molybdenum-free while alloy 
G-30A contains 4% molybdenum. Alloy G-30A clearly has improved corrosion 
resistance to phosphoric acid over the molybdenum-free alloy. 
TUNGSTEN CONTENT 
The criticality of tungsten content was demonstrated in the test results 
given in Table V. The specimens were tested in 54% phosphoric acid at 
149.degree. C. Both alloys had compositions essentially as shown for G-30 
alloy in Table II except Alloy G-30A was tungsten free. In this test, both 
alloys contain about 30% chromium; and 4% molybdenum; however, Alloy G-30, 
containing an additional 2% tungsten, had a more favorable corrosion 
resistance to the superphosphoric acid. Molybdenum must always exceed the 
tungsten content. 
Finally, the alloy of this invention, alloy G-30, and alloy G were tested 
for corrosion resistance in other acid media, specifically in reducing 
sulfuric acid and in oxidizing sulfuric acid. Data are given in Table VI. 
Compositions of the alloys were essentially as given in Table I and Table 
II for alloy G and alloy G-30; respectively. 
While the corrosion resistance of alloy G to sulfuric acid is known to be 
outstanding in this art, the results from Table VI clearly show the 
advantages of alloy G-30 over alloy G in providing excellent resistance to 
sulfuric acid media. 
In the production of nickel base alloys of this class, impurities from many 
sources are found in the final product. These so-called "impurities" are 
not necessarily always harmful and some may actually be beneficial or have 
an innocuous effect, for example, boron, aluminum, titanium, vanadium, 
manganese, cobalt, lanthanum and the like. 
Some of the "impurities" may be present as residual elements resulting from 
certain processing steps, or adventitously present in the charge 
materials: for example, aluminum, vanadium, titanium, manganese, 
magnesium, calcium and the like. 
In actual practice, certain impurity elements are kept within established 
limits with maximum and/or minimum to obtain uniform cast, wrought or 
powder products as well known in the art and skill of melting and 
processing these alloys. Sulfur and phosphorus must be kept at the lowest 
possible level. 
Thus, the alloy of this invention may contain these and other impurities, 
within the limits as usually associated with alloys of this class. 
TABLE I 
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PRIOR ART ALLOYS 
COMPOSITION IN WEIGHT PERCENT wt/% 
Alloy C-276 Alloy G Alloy 625 Alloy 690 
Range Typical 
Range Typical 
Range 
Typical 
Range 
Typical 
__________________________________________________________________________ 
Cr 14-26 
15.5 18-25 22 20-24 
21.5 27.9-30.8 
30 
Mo 3-18 
16 2-12 6.5 7-11 
9 -- -- 
W 0-5 4 0-5 1 max 
0-8 -- -- -- 
Cu -- -- 0-2.5 
2 -- -- -- -- 
Cb/Ta 
-- -- .1-5 2 3-4.5 
3.5 -- -- 
Fe 0-30 
5 Bal-over 15 
20 20 max 
5 8.7-12.4 
10.5 
Ti -- -- -- -- Al + Ti 
.2 .16-.54 
.3 
.4 max 
C 0.1 max 
.02 max 
0.25 max 
.05 max 
.1 max 
.05 .01-.07 
.045 
Ni 40-65 
57 35- 50 
-- 55-62 
62 about 60 
59 
__________________________________________________________________________ 
TABLE II 
______________________________________ 
ALLOY OF THIS INVENTION 
IN PERCENT BY WEIGHT, wt/% 
Broad Preferred Alloy G-30 
______________________________________ 
Chromium 26-35 27-32 about 30 
Molybdenum 2-6 3-5 about 4 
Tungsten 1-4 1.5-3 about 2 
Cb + Ta .3 to 2.0 
.5-1.5 about 1 
Copper 1-3 1-2 about 1.5 
Iron 10-18 12-16 about 14 
Mn up to 1.5 
up to 1 about .6 
Si up to 1.0 
up to .7 about .1 
C .10 max .07 max about .04 
Al up to .8 
up to .5 about .25 
Ti up to .5 
up to .3 about .2 
Ni plus Bal Bal about 46 
impurities 
______________________________________ 
TABLE III 
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EFFECT OF CHROMIUM IN 
CORROSION RESISTANCE TO PHOSPHORIC ACID 
Corrosion Rates (Mpy) 
Alloys In 46%/--P.sub.2 O.sub.5 at 116.degree. C. 
______________________________________ 
C-276 (16Cr) 44 
G (22Cr) 16 
625 (22Cr) 18 
690 (30Cr) 5 
G-30 (30Cr) 4 
______________________________________ 
Increasing chromium content provides better resistance to phosphoric acid 
TABLE IV 
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EFFECT OF MOLYBDENUM IN 
THE CORROSION RATE TO PHOSPHORIC ACID 
Corrosion Rates (Mpy) 
Alloys In 52%/--P.sub.2 O.sub.5 at 149.degree. C. 
______________________________________ 
690 (30Cr-- 0-Mo) 
447 
G-30A (30Cr-- 4Mo) 
61 
______________________________________ 
As the concentration and temperature of P.sub.2 O.sub.5 increase, Mo 
alloying with is needed. 
TABLE V 
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EFFECT OF TUNGSTEN IN 
THE CORROSION RATE TO PHOSPHORIC ACID 
Corrosion Rates (Mpy) 
Alloys In 54%/--P.sub.2 O.sub.5 at 149.degree. C. 
______________________________________ 
G-30A (30Cr--4Mo--0W) 
165 
G-30 (30Cr--4Mo--2W) 
38 
______________________________________ 
Tungsten addition provides improved resistance to super phosphoric acid. 
TABLE VI 
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CORROSION RESISTANCE IN SULFURIC ACID 
Reducing Oxidizing H.sub.2 SO.sub.4 
Alloys 10% H.sub.2 SO.sub.4 
ASTM G-28 
______________________________________ 
G (22Cr-- 6Mo--0W) 
25 22 
G-30 (30Cr-- 4Mo--2W) 
12 8 
______________________________________ 
Excellent resistance to sulfuric acid media.