Background of the Invention
[0001] This invention relates to the field of corrosion-resistant alloys and more particularly
to low strategic metal content, nonmagnetic, workable alloys resistant to both oxidizing
and reducing sulfuric acid solutions over a wide range of acid concentrations and
temperatures.
[0002] It is now well recognized that alloys of the iron-nickel-chromium-molybdenum-copper
types, which offer good resistance to both oxidizing and reducing solutions of sulfuric
acid, are also quite resistant to rusting and to a wide variety of corrosive media
and conditions. Such resistance has, in fact, been demonstrated for alloys of practically
no iron content up to alloys of considerable iron content. For example, an alloy known
commercially as Illium R, consisting nominally of 68% nickel, 21% chromium, 5% molybdenum,
3% copper,
1% iron and small amounts of other elements has such corrosion resistant properties.
Another alloy commercially known as Carpenter 20, consisting of nominally 29% nickel,
20% chromium, 2.5% molybdenum, 3.5% copper, 42% iron, and small amounts of other elements
has been very widely and successfully applied in all sorts of corrosive conditions
and exhibits relatively good resistance to the various sulfuric acid solutions noted
above. There are a number of other alloys ranging in iron content between these two
with similar corrosion resistance properties.
[0003] I have noted these same characteristics in a number of alloys of my own invention
which are of the same type, namely, iron-nickel-chromium-molybdenum-copper base with
additions of certain other elements. It is known that such alloys generally tend to
be substantially insensitive to sea water, to salt air, and to industrial atmospheres
and a variety of other corrosive atmospheres.
[0004] While various stainless steels resist general attack in certain ranges of sulfuric
acid concentration and other corrosive media, a majority of them are susceptible to
a pitting type attack, crevice corrosion, and stress corrosion cracking failures in
chloride media such as sea water or the aqueous fluids often encountered in solar
heating systems. Such fluids typically contain 180-1200 ppm Cl- and 5-400 ppm Cu
++. It is now well known that additions of the order 0.5 to 3% by weight molybdenum
can alter the properties of standard stainless steels in a manner that remarkably
increases their resistance to pitting, crevice corrosion, and stress corrosion cracking
failures. This has been demonstrated repeatedly for many stainless steels. The most
successful stainless steel at present for solar heating applications contains nominally
18% chromium, 2% molybdenum and small amounts of other elements with the remainder
consisting of iron. But this alloy is of ferritic crystal structure and therefore
strongly magnetic.
[0005] For various services, particularly certain military and naval applications, it is
highly desirable that construction materials be substantially nonmagnetic. Thus, for
example, to provide immunity to magnetic detection, it is of critical importance to
construct missile-firing submarines from non-magnetic alloys. Fixed site missiles
can, of course, be located, and the detection of surface vessel movements is fairly
readily achieved by satellite reconnaissance. The chief advantage of the missile-firing
submarine is that it is movable and capable of evading detection. Available locating
techniques for submarines include sonar, thermal detection systems, sea-bed sensors
and magnetic anomaly detection.
[0006] Thermal systems are useful only in shallow water. Sea-bed sensors may be evaded.
Sonar has already been developed to near the limit of practical possibility. Present
sonar transmitters and receivers are powerful enough already to be limited by turbulence
and temperature effects in the ocean. However, unless an ordinary steel submarine
is very deep and stationary, the presence of so large a mass of moving magnetic metal
causes a disturbance in the earth's magnetic field which can be detected by sensitive
apparatus. As this equipment is refined, magnetic steel submarines will become more
and more vulnerable to attack from airborne missiles or from enemy killer submarines.
[0007] Substantially non-magnetic reactor materials such as aluminum, zirconium, titanium
and others are well-known, but the greatest metallic mass of the hull, decks, bulkheads
and structural parts are generally not fabricated at present from such materials.
For example, titanium alloys have remarkable resistance to sea water, but would be
virtually impossible to use in hull construction at anything but enormous cost, if
at all. Integrity of welds in so vast a structure as an atomic submarine is not feasible
at present.
[0008] Austenitic alloys, that is alloys of face-centered- cubic crystal structure, may
and generally do posses relatively low magnetic permeabilities of the order of 1.2
gauss/ oersted or less, as compared to maximum permeabilities for various iron materials
ranging from about 100 to 15,000. Ferritic alloys generally exhibit relatively high
permeabilities. While alloys of magnetic permeabilities of the order of 1.003 to 1.007
at 200 oersteds can be considered substantially nonmagnetic, permeabilities of abut
1.01 characterize very weakly magnetic materials and are probably still very much
below the tolerance levels for use in such applications as the construction of naval
vessels such as mine-sweepers or atomic missile-firing submarines.
[0009] Lange, Howells, and Bukowski at the U.S. Naval Research Laboratory, as far back as
1958, reported development of various alloys that finally led to employment of alloy
steels of about 0.4% carbon, 18% manganese, 4% chromium, and 0.1% nitrogen in actual
submarine construction. However, such alloys have proven unsatisfactory due to cracking
in service.
[0010] A long list of precipitation-hardening stainless steels is now available. These include
alloys of good mechanical and fabricating properties but all exhibit substantial magnetism.
[0011] Most standard grades of wrought stainless steels develop permeabilities up to 5,
10, even 20 gauss/oersted, during rolling or cold working as the result of structural
instability leading to the formation of ferrite, the amounts of which depend upon
the grade of stainless and the degree of cold working. Even where not forged or wrought,
standard grades of cast stainless steels may display considerable magnetism due to
the presence of amounts of ferrite or carbides and nitrides or other compounds in
their structure.
[0012] Post and Eberly reported in the transactions of the American Society for Metals of
1947 mathematical relationships between the elements in austenitic stainless steels
such that they remained structurally stable and substantially nonmagnetic for alloys
containing iron, nickel, chromium, molybdenum, carbon and manganese. Schaeffler in
1948 extended the knowledge to include silicon, niobium, and nitrogen, and subsequent
workers have recognized that up to a few percent of copper is mildly austenitizing.
Franks, Binder and Thompson extended the limits to include up to about 22% manganese
in reports in various publications including the 1954 Transactions of the ASM and
U.S. Patent No. 2,225,440.
[0013] Thyssen Rohrenwerke Aktien-gesellschaft revealed in British Patent No. 1,062,658,
alloys containing iron, nickel, chromium, manganese, molybdenum, carbon, silicon,
nitrogen and niobium and mathematical relationships between these elements to maintain
nonmagnetisability.
[0014] In 1924, Gustav Tammann of Germany suggested his rule of eights describing the effects
upon corrosion resistance of additions of atomic fractions of 1/8, 2/8, 4/8, or 7/8
of a noble or resistant element alloyed with a baser or less noble or non-resistant
element in various media. Under this concept, one expects the very poor resistance
in a given corrosive medium for lesser or non-resistant elements, but there would
be stepwide reductions in corrosive attack when the more noble or more resistant element
additions reach amounts corresponding to specific atomic percentages. Gradual increases
between these amounts have virtually no effect, but sudden drops in corrosion rates
are expected at several if not all of these concentrations.
[0015] In practice there are very few alloys of iron and nickel base which contain over
about 38% chromium by weight, for higher chromium levels tend to result in severe
embrittlement problems. Also, there is generally little if any improvement in resistance
to corrosion in most media for additions of chromium above this level. Indeed, there
are few conditions in which chromium levels over about 22 to 24% are warranted, though
levels up to 27 or 28% by weight are regularly specified because effective chromium
is somewhat depleted in the formation of carbides, nitrides, or other compounds and
to provide some range of tolerances for production purposes.
[0016] At the other end of the chromium range, the atomic fractions of 1/16 or 1/8 provide
only enough passivating protection to be useful in very special or relatively weak
corrosive media. About 4 to 6% by weight chromium was once used in cutlery grades
to resist the mild action of fruit and other food juices. However, even the 11 to
12% chromium range is not efficacious in most severely corrosive substances. The major
drop in corrosive rate in most highly corrosive substances takes place at chromium
levels corresponding to the 3/16 or 1/4 atomic fraction levels of chromium.
[0017] In U.S. Patent Nos. 3,759,704 and 3,893,851 I disclosed alloys of excellent general
corrosion resistance but of particularly good resistance to wide ranges of sulfuric
acid concentrations and temperatures, with chromium levels of 33 to 42% by weight,
or at approximately the 3/8 atomic fraction. Johnson U.S. Patent No. 3,758,296 also
teaches sulfuric acid resistance alloys of this general level of chromium. Later I
disclosed alloys for the same service but of the lower chromium levels of 23.3 to
30% by weight in Culling U.S. Patent Nos. 3,947,266 and 4,135,919. The latter was
actually superior to the former despite its reduced strategic metal content and resultant
permissible increase in iron content.
[0018] While there are ore deposits for molybdenum in the U.S., this metal is in such demand
relative to the supply that it is almost as strategically critical at times as are
nickel and chromium, both of which depend almost entirely upon imports. U.S. copper
deposits are virtually exhausted so that even this element is often in short supply.
Niobium is also an imported metal and metallurgically more desirable than titanium
and tantalum. Alloys of patent 4,135,919 were actually equal or superior to patent
3,947,266 despite effective reductions of about 5% in nickel content, 4% in chromium
content, 2% in molybdenum content and smaller amounts of copper and niobium.
[0019] There has remained the need to further reduce the proportions of strategic elements
in alloys of this type without significant loss in workability and weldability while
maintaining excellent corrosion resistance. The list of U.S. and foreign patents disclosing
alloys of about 20% or more chromium by weight to handle various sulfuric acid solutions
is very long, yet tests disclose many of them to be either quite inferior to my prior
inventions, or to contain extremely high proportions of strategic elements, or to
suffer severe mechanical limitations such as extreme brittleness, or to have all three
failings. Alloys of lower chromium contents have suffered even more drastically in
these deficiencies.
Summary of the Invention
[0020] Among the several objects of the present invention, therefore, may be noted the provision
of improved alloys resistant to both oxidizing and reducing sulfuric acid solutions;
the provision of such alloys which are resistant to sulfuric acid over a wide range
of concentrations and temperatures; the provision of such alloys which are resistant
to sulfuric acid containing oxidizing contaminants, such as nitric acid; the provision
of such alloys which can be cast or wrought; the provision of such alloys which have
low hardness and high ductility so that they may be readily rolled, forged, welded,
and machined; the provision of such alloys which may be economically formulated with
relatively low proportions of strategic metals such as nickel, chromium and molybdenum;
the provision of such alloys whose strategic metal content is sufficiently low so
that they may be formulated from such relatively low-cost raw materials as scraps,
ferro alloys or other commercial melting alloys; the provision of such alloys which
are substantially nonmagnetic, i.e. for military and naval applications such as submarines
and minesweepers; the provision of such alloys that do not require heat treatment
after welding to avoid intergranular attack; and the provision of such alloys which
resist pitting attack, crevice corrosion and stress corrosion cracking failures.
[0021] Briefly, therefore, the present invention is directed to an air-meltable, castable,
workable, non-magnetic alloy resistant to corrosion in sulfuric acid over a wide range
of acid strengths. The alloy consists essentially of between about 18 and about 22%
by weight nickel, between about 16.8 and 19.2% by weight chromium, between about 0.35
and about 1.95% by weight molybdenum, between about 2.5 and about 3.9% by weight copper,
between about 3.2 and about 4.7% by by weight manganese, between about 0.35 and about
0.80% by weight niobium; up to about 0.7% by weight titanium, up to about 0.4% by
weight tantalum, up to about 0.01% by weight boron, up to about 0.5% by weight cobalt,
up to about 0.7% by weight silicon, up to about 0.08% by weight carbon, up to about
0.6% by weight of a rare earth component selected from the group consisting of cerium,
lanthanum, and misch metal, up to about 0.15% by weight nitrogen, and between about
51% and about 58% by weight iron. The nickel content exceeds the chromium content
by between about 0.85% and about 5.2% by weight.
[0022] Other objects and features will be in part apparent and in part pointed out hereinafter.
Description of the Preferred Embodiment
[0023] In accordance with the present invention, alloys are provided whose proportions of
strategic metals are even lower than those of my earlier U.S. Patent No. 4,135,919.
However, despite-the low strategic metal content of the alloys of the
r invention, these alloys are highly resistant to corrosion by sulfuric acid over a
wide range of concentrations, both in the reducing and in the oxidizing ranges. These
alloys retain their corrosion resistance, even at elevated temperatures, and show
effective corrosion resistance in the presence of sulfuric acid concentrations of
20-70%, an environment in which rapid failure is frequently experienced in alloys
specifically designed for use in either dilute or concentrated acid. This strong resistance
to corrosion is retained moreover, even when the sulfuric acid solution contains oxidizing
agents, such as nitric acid.
[0024] It has also been found that the alloys of the invention exhibit very low magnetic
permeabilities and are thus uniquely suited for various military and naval applications
where non-magnetic character is important for purposes of evading detection or otherwise.
[0025] The outstanding corrosion resistance of the alloys of the invention is attributable
in part to the fact that they are single-phase solid solutions having an austenitic
(face- centered cubic) structure. Attainment of this structure does not require heat
treatment but is realized in the as-cast condition of the alloy. These alloys not
only possess low hardness characteristics as-cast but also remain unaffected by precipitation
hardening or structrual transformation hardening treatments. Even if the alloy is
heat treated under conventional age hardening conditions, no precipitation, phase
changes or significant changes in hardness are observed.
[0026] The essential components of the alloys of the invention are:
These alloys are specifically designed to contain nickel, chromium, and molybdenum
contents below 23%, 20%, and 2% respectively by weight, the lower limits of present
successful commercial alloys for such service, and to contain iron of greater than
50% by weight. Normally, the alloys of the invention will also contain carbon, up
to a maximum of about 0.08% by weight.
[0027] Optionally, the alloys of the invention may further contain:
[0028] In the field of development of heat-resistant as well as of corrosion-resistant alloys,
it has been found that the leaner the alloys in more noble components, the more restricted
the allowable ranges of elements generally become for a given application. Stated
another way, as alloys come closer to their limits of application, the more carefully
their compositional ranges must be controlled. The balance of austenitizing and ferritizing
elements must be controlled carefully for corrosion resistance, and the ferritizing
elements minimized to limit magnetic permeability. To my knowledge, the alloys of
the present invention are the leanest ever devised that still maintain the degree
of corrosion resistance and workability shown.
[0029] The passivating effect of chromium in oxidizing media is widely recognized. In performing
numerous corrosion tests I have noted that the best results over a wide range of sulfuric
acid solutions are obtained when the relationship between nickel, chromium and molybdenum
are carefully controlled. In the present invention, these elements are balanced to
provide a very low chromium content that still results in superior corrosion resistance.
Test melts containing 13-14% by weight chromium resulted not only in lowered resistance
to oxidizing conditions as expected but also, rather surprisingly, to reducing conditions.
Since a chromium level of about 17.5 to 18% minimum is generally required in austenitic
stainless steels and of about 17.5% or more in higher alloys, it is noteworthy that
alloys of the present invention tolerate slightly lower minimum chromium content without
loss of properties. Higher chromium levels than about 19.2% not only result in some
loss in corrosion resistance in alloys of this invention, but such alloys also begin
to develop magnetism if other ferritizers are all to the high side of the ranges.
[0030] The range of chromium composition has also been determined to restrict the range
of nickel, which must always be higher than the chromium content by at least about
0.85% but by no more than about 5.2% to maintain the high level of corrosion resistance.
A nickel content higher than the chromium content is a characteristic notable in those
alloys more resistant to pitting attack, to crevice corrosion and to stress corrosion
cracking when the alloys are of the austenitic types. Those austenitic alloys which
are essentially standard stainless steels, or such steels modified by other additions
but still containing nickel levels quite a bit lower than chromium levels, are noted
for susceptibility to these types of attack in many media, especially those containing
halide ions. Especially favorable resistance to corrosion is achieved where the nickel
content of the alloys exceeds the chromium content by between about 0.87% and about
4.74%.
[0031] Molybdenum is also important for providing resistance to pitting and stress corrosion
cracking but, in the alloys of the invention, superior results are achieved at very
low levels of molybdenum. For service in halide ion and other highly corrosive environments,
it has been found perferable to restrict the ranges of chemical elements to the following
ranges:
[0032] For even better resistance to the widest range of corrosive conditions the components
of alloys of this invention should be even further restricted to the following ranges
of proportions:
[0033] In an especially preferred embodiment of the invention, the nickel content of the
alloy exceeds the chromium content by between about 2.18% and about 4.11% by weight
with other components in the following ranges of proportions:
[0034] A particularly advantageous alloy having optimum properties in various services has
the following composition:
[0035] Manganese, which has been disclosed in approximately the defined range in my prior
patents 3,947,266 and 4,135,919, is useful not only as an austenitizer but also as
an aid in corrosion resistance. Additionally, manganese is a deoxidizing element those
presence helps ensure the provision of gas- free sound metal ingots. In addition,
manganese in the defined range helps maintain non-magnetic properties even after cold
rolling or cold forming. However, when chromium content exceeds about 17%, manganese
content over about 8% tend to promote magnetism in these alloys during cold working.
At about 19% chromium manganese content above about 4.7% tend to do the same. The
manganese contents in alloys of this invention are always within the ranges that promote
austenite and preferably retard development of magnetism with cold working.
[0036] Copper is an essential component of the alloys of the invention in resisting sulfuric
acid corrosion but must not exceed about 3.9% or corrosion resistance begins to deteriorate,
particularly in pitting type attack.
[0037] Molybdenum levels below 2% are notable in the alloys of this invention, particularly
since contents of 2% to about 8.5% are specified in most other sulfuric acid resistant
alloys. Molybdenum content in the alloys of the present invention may be even slightly
lower than in patent 4,135,919. At least about 6.35% by weight molybdenum is beneficial
in resisting pitting attack, crevice corrosion, and stress corrosion cracking failures
in addition to being essential to sulfuric acid resistance. However, if molybdenum
exceeds about 1.95% by weight, resistance to various concentration of sulfuric acid
and to oxidizing solutions containing sulfuric acid begins to deteriorate. Molybdenum
is also a much stronger ferritizer and about seven times stronger than chromium in
producing magnetism, particularly under cold working, and must be restricted to the
specified range for that reason also.
[0038] Niobium is the carbide stabilizer of choice in alloys of this invention to prevent
intergranular attack even without heat treatment. It is effective in amounts over
about eight times the carbon content, but a niobium content over about 1% by weight
has an adverse effect on resistance to pitting in chloride-containing solutions. Niobium
is also a very strong ferritizer and is limited in alloys of this invention to maintain
nonmagnetic properties.
[0039] Tantalum is similar in effect to niobium as a carbide stabilizer but is much costlier,
scarcer and about half as effective as niobium on a weight percent basis. The allowable
tantalum content of about 0.'4% in alloys of this invention permits the use of ferro-alloys
that contain both niobium and tantalum, but maintains the latter at a sufficiently
low level to obviate difficulties in maintaining stable austenite, good workability,
and nonmagnetic properties.
[0040] Titanium is noted for its beneficial effect on workability in alloys of this type,
but it has an adverse effect on resistance to pitting when present above about 0.7%.
[0041] Although detrimental if present in excessive amounts, carbon is commonly present
as a component which can be tolerated to the extent of about 0.08% by weight. A small
amount of carbon may be beneficial in enhancing the fabricability of the alloy. Austenitic
steels with low carbon and nitrogen levels are highly resistant to stress corrosion
cracking, even though similar proportions of carbon and nitrogen are not beneficial
in ferritic stainless steels in that regard. The latter type of alloys are best at
about 0.02% carbon as compared to 0.08% permissible in alloys of this invention.
[0042] Nitrogen may also be present as an impurity in alloys of this invention prepared
by melting in the presence of air. The maximum of about 0.15% by weight is not difficult
to maintain with ordinary melting practice, and a small amount is actually beneficial
to the ductility and fabricability. Nitrogen is an austenitizing element, but carbides
and nitrides are magnetic and should not be present in great amounts. In the absence
of titanium, large amounts of nitrogen would also tend to impoverish the metallic
matrix in chromium content through formation of chromium nitrides.
[0043] Minor proportions of rare earth components such as cerium, lanthanum or misch metal
are optionally included in the alloys of the invention. Such proportions may contribute
to the fabricability of alloys. The rare earth component should not constitute more
than about 0.6% by weight of the alloy. Many of the rare earth metals are very strongly
paramagnetic, the gram-atomic susceptibilities being of the order of 1 x 10-
3 to 5 x 10-3, but the very small proportions of rare earths permitted in the alloys
of this invention have no significant effect on the magnetic properties of the alloys.
[0044] Silicon can be tolerated in the alloys of this invention up to about 0.7% by weight
without adverse effect on the corrosion resistance. Higher proportions of silicon
are undesirable since silicon is a hard, brittle, nonmetallic ferrite-forming element
which has a very adverse effect on the hardness, ductility and fabricability of the
alloy.
[0045] Within the limits specified, boron is beneficial for workability and thus may optionally
be included. However, when present at amounts significantly in excess of 0.01% by
weight, boron has the opposite effect, i.e. embrittling and damaging workability,
weldability and machinability.
[0046] Cobalt is typically present as an impurity in nickel sources. Accordingly, the alloys
of the invention may contain up to about 0.5% by weight cobalt.
[0047] Despite their high iron content, the alloys of the invention have low magnetic permeabilities,
consistently below 1.003 in magnetizing field strengths from 5 oersteds up to 200
oersteds. Even with cold reductions in thickness of the order of 50%, the permeability
remains below 1.006.
[0048] While the goals of the work in developing the alloys of this invention were specifically
to obtain the desired properties with nickel, chromium and molybdenum contents below
the minimum contents of prior commercially successful alloys, no component may deviate
significantly from the specified range without major loss in corrosion resistance.
General effects of such deviations are noted as follows:
a) Low chromium and/or high molybdenum contents, depending upon the extent of deviation
from the specified ranges, cause moderate or severe losses in resistance in most concentrations
of sulfuric acid with our without nitric acid additions at 80°C or boiling temperatures.
b) Deleterious effects of low nickel contents are most apparent at higher temperatures
of the various corrosive agents, as demonstrated in the boiling solution tests reported
hereinbelow, and poor performance gen- erally over 60% H2S04.
c) Excessively high niobium behaves somewhat like chromium in its effect upon general
sulfuric acid resistance, while low niobium contents decrease resistance to hot acid
solution, especially at boiling temperatures, and in solutions of less than 40% sulfuric
acid at 80°C if absent altogether. Of course, niobium is advantageous in preventing
intergranular attack.
d) Low or high copper contents reduce resistance in concentrated acids and in very
hot or boiling solutions.
e) Low or high manganese contents are particularly bad in boiling solutions, while
the latter also damage resistance to concentrations below 50% sulfuric acid at lower
temperatures.
f) While the molybdenum content of the alloys of this invention is quite a bit lower
than those of most established or reported sulfuric acidresistant alloys, the above-defined
proportions of molybdenum are extremely important in all concentrations of sulfuric
acid except extremely dilute or cold solutions.
g) If the difference between nickel and chromium content does not fall within the
required range, resistance in any of the solutions at temperatures from 80°C to boiling
will suffer.
The following examples illustrate the invention.
Example 1
[0049] One hundred pound heats of several different alloys were prepared in accordance with
the invention. Each of the heats was air-melted in a 100-pound high frequency induction
furnace. The composition of these alloys is set forth in Table I, with the balance
in each instance being essentially iron.
[0050] Standard physical test blocks and corrosion test bars were prepared from each heat.
Using the as cast non-heat-treated physical test blocks, the mechanical properties
of each of these alloys were then measured. The results of these measurements are
set forth in Table II.
[0051] Without heat treatment, the corrosion test bars were machined into 3.81 cm diameter
by 0.635 cm thick discs, each having a 0.3175 cm diameter hole in the center. Care
was exercised during machining to obtain extremely smooth surfaces on the discs. Twelve
to 14 discs were obtained for each alloy.
[0052] These discs were used in the comparative corrosion tests, described hereinafter,
comparing the performance of the alloys of the invention with a number of alloys which
either conform to the prior art or which are similar to the alloys of the invention
but do not satisfy certain of the critical compositional limitations of the alloys
of the invention. The compositions of the comparative alloys used in the tests are
set forth in Table III.
[0053] In the above table, Carpenter 20 conforms to Parson's U.S. patent 2,185,987 and Carpenter
20Cb3 is a well-known commercial alloy which corresponds to Scharfstein U.S. patent
3,168,397.
Example 2
[0054] Using the disc samples of Example 1, corrosion tests were run in 10%, 25% 40%, 50%,
60%, 70%, 93% and 97% by weight sulfuric acid solutions at 80°C (176°F).
[0055] In carrying out these tests, each of the discs was cleaned with a small amount of
carbon tetrachloride to remove residual machining oil and dirt and the discs were
then rinsed in water and dried. Each clean, dry disc was weighed to the nearest 10,000th
of a gram and then suspended in a beaker by a piece of thin platinum wire hooked through
the center hole of the disc and attached to a glass rod which rested on top of the
beaker. Sufficient sulfuric acid solution was then added to the beaker so that the
entire sample was immersed. The temperature of the acid was thermostatically controlled
at 80°C by means of a water bath and each beaker was covered with a watch glass to
minimize evaporation.
[0056] After precisely 6 hours, the sample discs were removed from the sulfuric 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 products were
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
weighted 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 specifications
Gl-67.
where
Ripy = corrosion rate in inches per year
Wo = original weight of sample
Wf = 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 these corrosion tests are set forth in Table IV.
Example 3
[0057] Since oxidizing contaminants are often present in commercial sulfuric acid streams,
the alloys of this invention were tested against the non-conforming alloys for resistance
to corrosion in such environments. Using the method described in Example 2, comparative
corrosion tests were conducted in 10%, 25%, 40%, and 50% sulfuric acid solutions,
each containing 5% nitric acid at 80°C. The results of these tests are set forth in
Table V.
Example 4
[0058] Using the method described in Example 2, comparative corrosion tests were conducted
in boiling 10%, 25%, and 40% sulfuric acid-water solutions, and in boiling 10% and
25% sulfuric acid-water solutions, the last two containing 5% nitric acid. Results
of these tests are set forth in Table VI.
[0059] In view of the above, it will be seen that the several objects of the invention are
achieved and other advantageous results attained.
[0060] 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.
1. An air-meltable, castable, weldable, nonmagnetic alloy resistant to corrosion in
sulfuric acid over a wide range of acid strengths, consisting essentially of between
about 18 and about 22% by weight nickel, between about 16.8 and about 19.2% by weight
chromium, between about 0.35 and about 1.95% by weight molybdenum, between about 2.5
and about 3.9% by weight copper, between about 3.2 and about 4.7% by weight manganese,
between about 0.35 and about 0.80% by weight niobium, up to about 0.7% by weight titanium,
up to about 0.4% by weight tantalum, up to about 0.01% by weight boron, up to about
0.5% by weight cobalt, up to about 0.7% by weight silicon, up to about 0.08% by weight
carbon, up to about 0.6% by weight of a rare earth component selected from the group
consisting of cerium, lanthanum and misch metal, up to about 0.15% by weight nitrogen,
and between about 51 and about 58% by weight iron, and wherein the nickel content
exceeds the chromium content by between about 0.85 and about 5.2% by weight (basis,
the entire alloy).
2. An alloy as set forth in claim 1 wherein the nickel content exceeds the chromium
content by between about 0.87 and about 4.74% by weight (basis, the entire alloy).
3. An alloy as set forth in claim 2 wherein the nickel content is between about 18.66%
and about 21.58% by weight, the chromium content is between about 16.84 and about
19.01% by weight, the molybdenum content is between about 0.52 and about 1.93% by
weight, the copper content is between about 2.51 and about 3.24% by weight, the manganese
content is between about 3.29 and about 4.54% by weight, the niobium content is between
about 0.45 and about 0.68% by weight and the iron content is between about 51.05 and-about
55.14% by weight.
4. An alloy as set forth in claim 1 wherein the nickel content is between about 18.66
and about 21.58% by weight, the chromium content is between about 16.84 and about
18.76% by weight, the molybdenum content is between about 0.52 and about 1.93% by
weight, the copper content is between about 2.82 and about 3.18% by weight, the manganese
content is between about 3.30 and about 3.70% by weight, the niobium content is between
about 0.56 and about 0.68% by weight, the iron content is between about 51.05 and
55.13% by weight and the nickel content exceeds the chromium content by between about
1.82 and about 4.11% by weight (basis, the entire alloy).
5. An alloy as set forth in claim 1 wherein the nickel content is between about 18.6
and about 21.6% by weight, the chromium content is between about 17.20 and about 18.76%
by weight, the molydenum content is between about 0.52 and about 1.93% by weight,
the copper content is between about 2.82 and about 3.17% by weight, the manganese
content is between about 3.30 and about 3.51% by weight, the niobium content is between
about 0.56 and about 0.66% by weight, the silicon content is between about 0.3 and
about 0.5% by weight, the carbon content is between about 0.03 and about 0.05% by
weight, the iron content is between about 51.05 and about 55.13% by weight, and the
nickel content exceeds the chromium content by between about 2.18 and about 4.11%
by weight (basis, the entire alloy).
6. An alloy as set forth in claim 1 wherein the carbon content does not exceed approximately
one-eighth of the niobium content.
7. An air-meltable, castable, weldable, nonmagnetic alloy resistant to corrosion in
sulfuric acid over a wide range of acid strengths, comprising approximately 21.0%
by weight nickel, approximately 17.5% by weight chromium, approximately 1.2% by weight
molybdenum, approximately 3.1% by weight copper, approximately 3.7% by weight manganese,
approximately 0.6% by weight niobium, approximately 0.3% by weight silicon, approximately
0.03% by weight carbon, and the balance essentially iron.