Note: Descriptions are shown in the official language in which they were submitted.
1 3 B4685
DESCRIPTION
CORROSION RESISTAI`IT GLASSY_MEl'AL ALLOYS
BACKGROUND OF THE INVENTION
Field of the Invent_on
This invention relates to corrosion resistant
glassy metal alloys.
5Description of the Prior Art
The corrosion resistance of any given metal or
alloy in a reducing mediura is often sharply different
from its corrosion resistance in an oxidizing medium,
with some metals and alloys being more resistant to
reducing media and others to oxi~izing media. These
differences in behavior are thought to be attributable
to differences between the corrosion mechanism in a
reducing medium and the corro~ion n,echanism in an
oxidizing medium. I'hus, corrosive attack by a reduciny
acid is generally considered to involve attack on the
metal by hydrogen ions, resulting in the oxidation of
metal to soluble ions and release of hydrogen gas.
Metals of relatively high nobility, therefore, as in-
dicated by their positions in the galvanic series are
generally resistant to corrosion by reduciny acid.
Attack by oxidizing media, on the other hand, does not
involve release of hydrogen but commonly results in the
forlilation of metal oxides or other metallic compounds at
the metal surface. Unlike the situation with reducing
acids, a favorable position relative to hydrogen in the
electromotive series provides no insurance that a metal
;will not be rapidly attacked by an oxidizing medium.
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However, certain elements, such as chromium, aluminum
and silicon, form tough insoluble oxide ~ilms upon
initial contact with an oxidizing medium, and such films
serve as barriers against further reaction between ~he
medium and the metal to prevent further corrosion from
taking place.
Sulfuric acid solutions are not only very cor-
rosive generally, but the nature of their corrosion
properties varies markedly with both acid concentra~ion
and tempera~ure. This variability relates at least in
part to sulfuric acid's ambivalent assumption of both
reducins and oxidizing properties as its concentration
temperature and the nature and proportion of various
contaminants are altered. As a consequence of this
variability in its corrosive properties, few materials
are available which are reasonably resistant to s~l~uric
acid solution over a wide range of concentrations and
temperatures.
Of the ]snown alloys which are demonstrably
effective over wide ranges of sulfuric acid concentra-
tions, many contain re~atively high proportions of
nickel and chromium and are, thus, rather expensive.
Corrosion resistant crystalline alloys are
well known and are exemplified by stainless steels, for
example. Corrosion resistant glassy metal alloys are
also well known, see, for e~ample~ U.S. Patent
3,856,513, which discloses a corrosion resistant glassy
y, ~oNi38P14B6A12 (the subscripts are in
atom percent), as being several orders of magnitude less
reactive than stainless steels with concentrated hydro-
chloric acidO Other prior art corrosion resistant
glassy metal alloys include iron-nickel-chromium-
phosphorus-carbon alloys~ However, these alloys
evidence stress corrosion cracking and thus are not
suitable in many applications, even though their
corrosion resistance is superior to many other glassy
metal alloys. -
; A continuing need exists for corrosion resis-
g ~ ~
;~ -
tant alloy~ having a reï~tively low expensive metal
content. In particular, a need has existed for such
alloys in which the nickel and chrorniuln content is
relatively low, since these are both expensive mate
rials. At the same time, there is a need for such
alloys which are not only low in nickel and chromiulTI~
but also have low proportions of other expensive compo-
nents such as ~nolybdenum.
Summary of the Invention
In accordance with the invention, a metal
alloy is provided that is substantially glassy and
resistant to corrosion in acid media. The glassy metal
alloy consists essentially of from about 0 to 18 atom
percent nickel, from 7 to about 21 atom percent
chromium, from 0 to abou~ 8 atom percent molybdenum,
from about 13 to 18 atom percent of at least one ele-
ment selected from the group consisting of phosphorus,
carbon and boron, other than phosphorus plus carbon,
and the balance essentially iron with the proviso that
the ratio of phosphorus to the sum of phosphorus, boron
and carbon present is greater than or equal to 0.64.
Brief Description of the Drawings
FIG. 1, on coordinates of potential and cur-
rent density, is a plot of a typical curve for an
active-passive alloy; and
FIG. 2, on coordinates of potential and cur-
rent density, is a plot of typical alloys of the inven-
tion compared with prior art stainless steel and prior
art glassy metal alloys.
Detailed Description of the Invention
Potentiostatic anodic polarization measure-
ments are performed by immersing a metallic electrode in
an electrolyte solution and varying its potential in a
stepwise manner with a special feedback power supply
(i.e.l a potentiostat). If the current corresponding
to each potential is recorded, an anodic polarization
curve can be constructed. A typical curve for an
active-passive alloy is shown in FIG. 1. Potential is
1 31 6~B85
ploted on the ordinate and the logarithm of the current
density on the abscissa. The current density, which is
equivalent to the alloy dissolution rate, at first in-
creases with increasing potential (active region 10).
At more noble (positive) potentials, the dissolution
- current density decreases and then remains at a low
value (passive region 11). At very positive potentials,
the dissolution rate again increases with increasing
potential (transpassive region 12). The current maxi-
mum which occurs at the primary passiv~ potential ~pp is
termed the critical anodic current density Ic. The
passive region beyins at the passive potential Ep and is
characterized by the passive current density Ip
Potentiostatic anodic polarization curves may
be viewed as plots of solution oxidizing power (E)
versus corrosion rate (current density). Thus, a typi-
cal active-passive alloy possesses maximum corrosion
resistance under moderately oxidiziny conditions. The
corrosion resistance is lo~er under reducing conditions
and in the presence of very strong oxidizers which cor-
respond to the active and transpassive states, respec-
tively. Passivation becornes easier as Epp becomes more
negative and as Ic decreases. Therefore, it i5 possible
to compare the ease of passivation of alloys on the
basis of their anodic polarization curves, Also, corro-
sion rates in the passive state can be directly compared
on the basis of Ip values. Finally, the range of useful
corrosion resistance can be estimated by noting the
width of the passive region and the potential at whicn
the transpassive region begins. Although actual anodic
polarization curves sometimes deviate from the schematic
illustration of FIG. 1, they can be compared on the same
basis.
The corrosion resistant alloys of the inven-
tion consist essentially of from about 0 to 18 atom per-
cent nickel, from 7 to about 21 atom percent chromium,
from 0 to about 8 atom percent molyb~enum, from about
13 to 18 atom percent of at least one element selected
-~ 1 3 ~68~
-5-
froln the group consisting of phosphoru~, carbon and
boron, other than phosphorus plus carbon, an~ the
balance essentially iron with the proviso that the
ratio of phosphorus to the sum o~ phosphorus~ carbon
- 5 and boron is greater than or equal to 0.64.
Glassy metal alloys of the invention are
forlned by cooling a melt of the desired composition at a
rate of at least about 10 DC/sec. A variety of rapid
quenching techniques, well known to the glassy metal
alloy art, are available for producing glassy metal
powders, wires, ribbon and sheet. Typically, a partic-
ular composition is selected~ powders or granules of the
requisite elements in the desired portions are melted
and homogenized, and the molten alloy is rapidly
quenched on a chill surface, such as a rapidly rotating
cylinder, or in a suitable fluid medium, such as water.
Under these quenching conditions, a metastable, homo-
geneous, ductile material is obtained. The metastable
material may be amorphous or glassy, in which case there
is no long range order. X-ray di~fraction patterns of
glassy metal alloys show only a diffuse halo, similar to
that observed for inorganic oxide glasses.
The amorphous metal alloys are at least 50%
amorphous, and preferably at least 80% amorphous, as
measured by X-ray diffraction. However, a substantial
degree of amorphousness approaching 100~ alrlorphous is
obtained by forming these amorphous metal alloys in a
partial vacuum. Ductility and corrosion resistance are
thereby improved, and such alloys possessing a substan-
tial degree of amorphousness are accordingly preferred.
Examples
A number of iron-based glassy metal alloys
were studied and compared with prior art glassy metal
alloys and crystalline stainless steel alloys. Exposed
electrode areas for as-cast glassy metal ribbons ranged
from 0.3 to 1.0 cm . The ribbons were mounted (dull
side out) on lucite rods. The plastic~lnetal interfaces
were coated with lacquer to prevent crevice effects.
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Stainless steel standards were in rod form wi~h ~n
exposed area of about 5 cm2. The experiments were per-
formed in nitrogen or hydrogen satura~ed electrolyte
prepared from distilled water and reagent-grade chemi-
cals. The electrodes were inserted into a Princeton
Applied Research polarization cell and pre-exposed until
the corrosion potential became nearly constant. Polari~
zation measurements were conducted in the noble direc-
tion using a Potentiodyne. The scanning rate for glassy
metal alloys was generally 1.5 V/hrO
Polarization curves for all alloys were
obtained in lN H2S04 at 22C. Ma~erials exhibiting in-
teresting charac~eristics were studied in other
environments such as lN H2SO4 plus 5 percent NaCl,
lON H2SO4, and 38 percent ~Cl. In addition, weight
losses were determined in 6% FeC13 at 60C for 20 hours.
Ta'~le I compares alloy performance ln
hydrogen-saturated 1~l E12SO4. Included are prior art
crystalline and glassy metal alloys, as well as glassy
metal alloys having compositions outside the scope of
the invention.
1 6~6~
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TABLE I
Values of IC ~ ~ 4
Composition, Atorn Percent Icc~2 Ip, 2
Fe Ni Cr P C B Other A/cm A/an
Prior Art Crystalline Allo~s
* 82 - 18 - - - - 4.0x10 2.4x10
~* 72 8 20 - - - - 5.3x10 8.5x10
***70 10 18 - - - 2 r~O 2.8x10 1.5x10
* 430 stainless steel
** 304 stainless steel
*** 316 stainless steel
Prior Art Amorphous Alloys
31.5 36 14.5 12.5 - 5.5 - 5.4x10 6 3.2x10 6
40 38 - 14 - 6 2Al
Amorphous Alloys of This Invention
_ -5 7
63. 7 - 20 16 - 0.3 - l.lx10 2.8x10
60. 7 - 21 18 - 0-3 ~ <1o-6 <1o-6
50. 217. 5 14 18 -` 0.3 - 9.0x10 7. 4xlO
60.2 - 14 18 _ 0.3 7.5 ~lo <10 7 <10-7
20 69.6 - 10 13 7 0. 4 - 1. 4xlO 8x10-7
59. 6 - 20 13 7 0. 4 - 1. lxlO 7 9. 5xlO 8
49.317.5 7 18 - 0.3 7.5 Mo <10 <10-7
Other ~rnorphous ~lloys
78.6 - 5 16 - 0.4 - 6.0xlO 4 5.8x10 5
25 74.7 - 7 18 - 0.3 - 2.8x10 3~6x10 6
73. 6 _ 10 16 - 0. 4 - 7. 6x10 5 2. 8xlO 6
72.1 2.5 5 13 7 0.4 - 6.9x10 4 3 . ox:L0~5
39.7 35 7 18 - 0.3 - 2.3x10 1.2x10
34 35.3 14 16 - 0.7 - 4.0x10 6 2.0x10 6
30 45.7 -- 21 17 -- 0.315 Mo
6 8 5
--8--
Table II cornpares weiyht loss of alloys in 6
FeC13 at 60C for 20 hours. Again, prior art crystal-
line and glassy metal alloys having compositions outside
the scope of the invention are included for comparison.
5~BLE II
Dissolution of Alloys in
. 6% FeCl , 60C, 20 hrs.
Alloy Con~osition, Atom Percent
10 Fe Ni Cr P C B Other % weight loss
_
82 - 18 - - - - 87
72 - 20 ~ 6
18 - - - - 1.5
Prior ALt Amorphous Alloys
15 31.5 36 14.5 12.5 - 5.5 - 0
40 38 - 14 - 6 2Al
Arnorphous Alloys of This Invention
-
63.7 - 20 16 - 0.3 - 0
60.7 - 21 18 - 0.3 - 0
2050.2 17.5 14 18 - 0.3 - 0
60.2 - 14 18 - 0~3 7.5 Mo 0
69.6 - 10 13 7 0.4 - 0
59.6 - 20 13 7 0.4 - 0
49.3 17.5 7 18 - 0.3 7.5 Mo 0
Other Amorpho~ Alloys
78.6 - 5 16 - 0.4 - 100
74.7 - 7 18 - 0.3 ~ 100
73.6 - 10 16 - 0.4 - 100
72.1 2.5 5 1?. 7 0.4 - 100
30 39.7 35 7 18 - 0.3 - 0
3435.3 14 16 - 0.7 - 0
45.7 - 21 18 - 0.3 15 .~o 0
On the basis of Ip values, the glassy metal
alloys of the invention evidence values less than lx10
A/cm , with many values less than 10 A/c~n . Further,
these alloys are stable in 6% Fe/C13. Alloys outside
the invention are seen not to possess this combination
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of corros ion res is tance .
Having thus descri~ed the invention in rakher
full detail~ it will be understood ~hat this detail need
not be strictly adhered to but that various changes and
modifications may suggest themselves to one skilled in
the art, all Ealling within the scope of the present
: invention as defined by the subjoined claims.
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