ORNL-2156.txt

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Metallurgy and Ceramics

TID-4500 (13th ed.)

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CORROSION PRODUCTS FORMED IN THE REACTION
BETWEEN FUSED SODIUM HYDROXIDE AND IRON-RICH

ALLOYS OF IRON, CHROMIUM, AND NICKEL

 
 

G. P. Smith
E. E. Hoffman

 

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METALLURGY DIVISION

CORROSION PRODUCTS FORMED IN THE REACTION
BETWEEN FUSED SODIUM HYDROXIDE AND IRON-RICH

ALLOYS OF IRON, CHROMIUM, AND NICKEL
G. P. Smith

E. E. Hoffman

DATE ISSUED

APR 261957

 

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OMATITZMAIPOIVAOINIMALCAOIOCSE-MBO-EE-NOS

ORNL-2156
Metallurgy and Ceramics
TID-4500 (13th ed.)

. J. Skinner

. Boyd
. Smith
Frye, Jr.
. Parkinson
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. Adamson, Jr.
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Hikido

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UNCLASSIFIED
UNCLASSIFIED

CORROSION PRODUCTS FORMED IN THE REACTION BETWEEN FUSED SODIUM
HYDROXIDE AND IRON-RICH ALLOYS OF IRON, CHROMIUM, AND NICKEL

G. P. Smith

E. E. Hoffman

ABSTRACT

A study was made of the microstructures of corrosion-product layers formed by the action of

fused sodium hydroxide at 815°C on types 304 ond 347 stainless steel and on four high-purity

iron-chromium-nickel alloys with nominal compositions of 80% Fe-20% Cr, 80% Fe-~10% Cr~

10% Ni, 74% Fe~18% Cr—8% Ni, and 60% Fe~20% Cr-20% Ni.

Each corrosion-product layer was

found to consist of a nonmetallic network threcding through o metallic matrix and to resemble

similar layers formed by the action of hydroxide melts on Inconel.

 

INTRODUCTION

This report is one of a series describing
reacfions between fused sodium hydroxide and
alloys composed of elements which have widely
differing reactivities in a hydroxide melt. Such
reactions were found to involve complex solid-
state processes which brought about the selective
eaching of the more reactive alloy components.
The decrease in volume of the alloy which ac-
companied this leaching process was accomplished
by the retreat of the alloy surface along preferred
paths to form a network of deep pits or channels
extending inward from the alloy-hydroxide inter-
face. For some alloys which have been studied!
these channels were filled with corroding medium,
while for the alloy Inconel? {nominal composition:
78% Ni, 15% Cr, and 7% Fe) the channels were
filled with reaction products. The metallographic
evidence presented in this report indicates that
the reaction between fused sodium hydroxide and
iron-rich alloys of iron, chromium, and nickel is
similar to the reaction between sodium hydroxide
and Inconel,

1,2

 

1G. P. Smith and E. E. Hoffman, The Action of Sodium
Hydroxide Melts on Allo:i/s of Nickel, Molybdenum, and
Iron at 815°C, ORNL.-2131 (Oct. 23, 1956).

2G. P. Smith, M. E. Steidlitz, and E. E. Hoffman,
Two-Phase Product Formed irn the Reaction Between
F_itse]g S;;dium Hydroxide and Inconel, ORNL-2129 {(March
11, 1957).

Previous studies of the corrosion of alloys of
iron, chromium, and nickel by fused sodium hy-
droxide, other than the above-mentianed work on
Inconel, have given little information on micro-
structures. Williams and Miller® studied reactions
with type 304 stainless steel and with iron-nickel
alloys which contained 36 and 64% nickel. They
reported that the mechanism seemed to involve
severe penetration of the metal, but no metallo-
graphic dota were given, Craighead, Smith, and
Jaffee? carried out reactions of sodium hydroxide
with a variety of iron-chromium-nickel alloys for
24 hr at 538°C under a 90% nitrogen—10% hydrogen
blanketing atmosphere. In general they found a
layer of scale with intergranular penetration and
pit formation beneath. However, the metallo-
graphic data that they reported were very meager.

PROCEDURE

The ‘‘capsule test technique’’ previously de-
scribed® was used to study the action of fused

 

3D. D. Williams and R, R. Miller, Thermal and Related
Phbysical Properties of Molten Materials, Part 1. High
Temperature Reactions of Sodium Hydroxide, WADC-TR-
54-185 (Part 11) (Feb. 1955).

ic. M. Craighead, L. A, Smith, and R. |, Jaffee,
Screening Tests on Metals and Alloys in Contact with
lsgd{z);m Hydroxide at 1000 and 1500°F, BMI-706 (Nov. 6,
S -

5G. P. Smith, M. E. Steidlitz, and E. E. Hoffman,
Experimental Procedures Used for the Measurement

of Corrosion and Metal Transport in Fused Sodium
Hydroxide, ORNL-2125 (Sept. 25, 1956).

UNGLASSIFIED ,
sodium hydroxide on the commercial alloys types
304 and 347 stainless steel and on four special
alloys with the following nominal compositions
(by weight): 80% Fe-20% Cr, 80% Fe-10% Cr—
10% Ni, 74% Fe-18% Cr~8% Ni, and 60% Fe~20%
Cr-20% Ni. These special alloys were prepared
as 30-Ib ingots from high-purity elemental metals
by vacuum melting and casting. The ingots were
extruded into rods, which were machined into
capsules and specimens for testing. The compo-
sitions of these special alloys were checked by
chemical analysis, and the results are reported in
Table 1. Contaminants, it will be noted, were
found to be present only in small amounts.

Corrosion tests were run at 815°C for 100 hr,
Duplicate tests were run on each alloy. After
test the capsules were cut open, the hydroxide
was dissolved in water, and the test specimens
were weighed and prepared for microscopic ex-
amination. All photomicrographs in this report
show samples cut perpendiculor to a corroded
surface and left in the as-polished condition. As
a result of the corrosion products reacting slowly
with moisture and carbon dioxide in the air, the
specimens had to be examined immediately after
polishing.

RESULTS

All specimens were found to have corroded by
the formation at their surfaces of a mixed layer
of metallic and nonmetallic phases. A more or
less typical layer is shown in cross section in
Fig. 1 ot a magnification of 250X. The alloy
illustrated consisted of 74% Fe, 18% Cr, and
8% Ni. At a higher magnification (1000X) the

corrosion-product layer was found to be a network

of nonmetallic, acicular particles within a metallic
matrix as seen in Fig. 2. Somewhat to the left
of center and running from top to bottom of Fig. 2
may be seen a wide stringer of nonmetallic phase
passing along what had been a grain boundary
of the parent alloy. Other intergranular regions
are occupied by the metallic phase interlaced by
relatively large particles of the nonmetallic phase.
Within what had been individual grains of the
parent alloy, the acicular crystals showed a
marked tendency to lie along preferred orien-
tations.

Intergranular attack at the bottom of the two-
phase corrosion-product layer is shown at 1000X
in Fig. 3. It will be seen that limited frans-
granular attack branched out from a substantial
portion of the affected intergranular regions.

Another alloy, composed of 80% Fe, 10% Cr, and
10% Ni, had particularly long and slender non-
metallic particles in transgranular regions, These
are illustrated in Fig. 4, ot 1000X, which also
shows intergranular attack at the bottom of the
corrosion-product layer.

The corrosion-product microstructures shown in
Figs. 1 through 4 are more or less typical of all
specimens tested except for the specimens of the
80% Fe~20% Cr alloy. The microstructures of
the corrosion-product layer of this alloy are shown
in Figs. 5 and 6 at 1000X. Figure 5 shows the
boundary region between the corrosion-product
fayer and unattacked metal. An attacked grain
boundary passes approximately down the center
of the photomicrograph. Although there was a
tendency for corrosion to advance more rapidly
along the intergranular region, this tendency was
not pronounced and the intergranular layer of
corrosion product come to an abrupt end rather

Table 1. Composition of Alloys Tested

 

Nominal Composition

Composition* (wt %) Determined by Analysis

 

 

(wt %) Fe Ni Cr c Si S
80 Fe-20 Cr 80.91 19.14 0.017 0.060 0.021
80 Fe=10 Cr=10 Ni 79.60 10.40 9.93 0.018 0.030 0.012
74 Fe=18 Cr—8 Nj** 73.50 8.11 18.70 0.022 0.030 0.019
60 Fe-20 Cr-20 Ni 61.43 19.88 19.66 0.005 0.040 0.013

 

*Manganese was less than 0.002% for all compositions.
**The analyzed composition of this alloy falls within the

specifications for AlS) type 304L stainless steel.
 

 

UNCLASSIFIED
Y.1930 001

 

 

 

 

 

 

 

010

 

Ll

 

Relk

 

013

 

 

 

 

 

 

 

Fig. 1. Corrosion-Product Layer Formed on 74% Fe-18% Cr—8% Ni Alloy. Unetched. 250X.

than narrowing down to invisibility as it did with
other alloy compositions. Transgranulor attack
also ceased along o well-defined front, as con-
trasted with that illustrated in Figs. 3 and 4, The
nonmetallic phase in this transgranular attack
showed no sign of the acicular particles found for
all other alloys studied, but consisted instead
of a network of irregular shapes. The particle
size in the two-phase layer increased continvously
from the lowest line of attack, shown in Fig. 5,
up to the surface. The microstructure near the
middle of the film is shown in Fig. 6. The ratio
of volume of nonmetallic constituent to volume
of metallic constituent increased with increasing
particle size, so that the relative amount of metal
near the external surface was small,

For all alloys it was found that the volume of
the corrosion-product layer was significantly
greater than the volume of the alloy consumed.

The more gquantitative aspects of the corrosion
data are presented in Table 2. Each measurement
in this table is an average of duplicate tests. It
will be noted that all specimens which were
weighed gained in weight by an amount which
increased with increasing thickness of the cor-
rosion-product layer. Because of the small number
of specimens tested, these numerical data should
be taken as only an approximate indication of
resistance to attack.

In all tests except those invelving the two more
corrosion-resistant alloys (marked by an asterisk
in Table 2), a gquantity of solidified droplets of
 

 

SUNCLASSIEIED
Y.19305 |

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1000 X

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Fig. 2. Network of Nonmetallic Particles Within the Corrosion-Product Layer Shown in Fig. 1. Unetched.

1000X.

Table 2. Corrosion Data of Iron-Base Alloys Exposed
to Sodium Hydroxide for 100 hr at 815°C

 

Nominal Composition

 

(wt %) Weight Gain Film Thickness
(g/in.2) (in. x 10°%)
Fe Cr Ni
80 20 0.76 23
80 10 10 0.04 6*
74 18 8 0.1 13
60 20 20 0.04 5*
Type 304 stainless 20
steel
Type 347 stainless 17
steel

 

*No metallic sodium found.

metallic sodium was found on the inside capsule
walls at the completion of testing.

DISCUSSION

The corrosion-product layers described in this
report consisted of nonmetallic networks threading
through metallic matrices. These layers resembled
in many ways the corrosion-product layers formed
on Inconel by reaction with fused sodium hy-
droxide.® |t was shown that the reaction between
Inconel and fused sodium hydroxide consisted of
the selective leaching of iron and chromium from

 

6G. P. Smith, M. E. Steidlitz, and E. E. Hoffman,
Two-Pbase Product Formed in the Redction Between
I;_its?gs%r;dium Hydroxide and Inconel, ORNL-2129 (March
 

 

JUNCLASSIFIED!
. Y-19304

.002

 

003

 

1000 X

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B

 

 

 

Fig. 3. Intergranular Attack at Bottom of Corrosion-Product Layer Shown in Fig. 1. Some transgranular attack

can also be noted. Unetched. 1000X.

their solid solution with nickel to form a network
of oxides and oxysalts of chromium and iron within
a metallic matrix containing about 97% nickel
(balance iron plus chromium). This behavior was
attributed to the great difference in relative re-
activity between nickel on the one hand and iron
and chromium on the other. It seems reasonable
to assume that the two-phase corrosion products
found for iron-rich alloys of iron, chromium, and
nickel as described in this report were also formed
by a selective leaching process.

The gaseous oxidation of nickel-iron alloys at
elevated temperatures is known to give two-phase

 

7). Benard and J. Moreau, Rev. Met. 47 (M), 317
{1950).

corrosion products consisting of oxides of the
more reactive metal, iron, embedded in metal
enriched in the less reactive metal, nickel. Behard
and Moreau” assumed that the mechanism of this
attack involved the dissolution of oxygen in the
alloy lattice with a subsequent precipitation of
oxides such as is known to occur in the so-cailed
**subsurface scale formation” of a number of
alloys. However, Wagner® has shown that two-
phase corrosion-product layers may, under certain
conditions, be formed by a buckling of the oxide-
metal interface rather than by precipitation of
initially isolated particles of oxide within the
metal.

 

8C. Wagner, |. Electrochem. Soc. 103, 627 (1956),
 

Fig. 4.
Cr—10% Nt Alloy. Unetched. 1000X,

For most of the alloys described in this report,
corrosive penetration proceeded more rapidly along
grain boundaries than through grains, as evidenced
by the regions of intergranular attack beneath the
regions of transgranular attack. A similar sensi-
tivity of grain boundaries to selective attack was
noted for Inconel exposed to sodium hydroxide
under certain conditions® and for nickel-iron
alloys exposed to gaseous oxygen.’

One of the most striking microstructural features
ot corrosion-product layers formed on all but one
of the alloys studied was that a substantial portion
of the inorganic reaction product was contained
in the form of very slender particles. In the study
of corroded Inconel such shapes were also found,
and speculation as to their origin was presented.®
The present research shows that such shapes are

 

002

 

003

 

1000 X

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&

 

 

 

Network of Nonmetallic Particles Near Bottom of Corrosion-Product Layer Formed on 80% Fe=10%

not peculiar to an alloy of the composition of
Inconel but are of more general occurrence.

The metallic sodium found on the inside capsule
walls following test deserves special mention.
Smith? has outlined the known reactions between
metals and fused sodium hydroxide. In brief the
primary reactions are as follows. For all reactions
the metal acts as a reducing agent. Hydrogen is
generally produced, although very reactive metals
such as zirconium will simultaneously generate
metallic sodium with ease. The action of weak
reducing agents such as nickel on sodium ions
is brought to equilibrium by a very small concen-
tration of sodium; therefore the generation of

 

%. P. Smith, Corrosion of Materials in Fused Hy-
droxides, ORNL-2048 (March 15, 1956).
 

Fig. 5.
Cr Alloy. Unetched.

1000X.

appreciable quantities of sodium is possible only
under conditions in which the alkali metal can
escape from the reacting mixture, as, for example,
by distillation. Consequently, weak reducing
metals generally react preferentially with hydroxyl
ions, and not until this reaction has virtuvally
ceased are significant quantities of sodium pro-
duced. For metals intermediate in reducing
strength between zirconium and nickel, the re-
duction of sodium ions to form metallic sodium
is complicated by a competing reaction in which
metallic sodium reduces hydroxyl ions. The role
of this competing reaction is obscure because of
the lack of knowledge of both its kinetic and
equilibrium aspects. However, Villard!? reported

 

194, P. Villard, Compt. rend. 193, 681 (1931).

 

.002

 

003

 

1000 X

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&

 

 

Network of Nonmetallic Particles Near Bottom of Corrosion-Product Layer Formed on 80% Fe-~20%

the production of sodium by the action of nickel,
iron, and chromium on fused sodium hydroxide,
while Williams and Miller,'! who also studied
these reactions, reported that sodium metal was
produced only after the reduction of hydroxyl ions
had ceased.

In the experiments described in this report,
sodium was not removed from the field of reaction
but remained in the confined space immediately
above the melt and at the same temperature as
the melt. At the test temperature (815°C) scdium
was near its normal boiling point (889°C), so that
a substantial pressure of gaseous sodium was in
contact with the melt. At the completion of

 

nD. D. Williams and R. R. Miller, Thermal and Re-
lated Physical Properties of Molten Materials. Part Il

High Temperature Reactions of Sodium Hydroxide,
WEDC-TR—54-|85 (Part 11) (Feb. 1255).

 
. UNGLASSIFIED

 

Fig. 6.
80% Fe-20% Cr Alloy. Unetched. 1000X.

testing, the melt contained considerable uncon-
sumed hydroxide. These results attest to the
strong reducing action that the alloys of iron,
chromium, and nickel have on sodium hydroxide
at 815°C, and, when compared with the data of
Williams and Miller,’! raise questions concering
the kinetics of sodium ion reduction,

 

 

 

UNCLASSIFIED
Y.19302
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Network of Coarse-grained Nonmetallic Phase Near Middle of Corrosion-Product Layer Formed on

ACKNOWLEDGMENT

H. Inouye and T. K. Roche made the special
alloys used in this study, and R. J. Gray and
N. M. Atchley metallographically prepared the
samples and took the photomicrographs.

; UNGLASSIFIED