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PINSTECH/NMD-116
IHTER6RAHULAR CORROSION OF INCOLOY 8 0 0 BY ELECTROCHEMICAL
POTERTIOKINETIC REACTIVATION AND HUEY TEST METHODS
SOHAIL UR RAHMAN
NAZAR HUSSAIN
K. A. SHAHID
(NUCLEAR MATERIALS DIVISION)
Pakistan Institute of Nuclear Science and Technology
Nflore, Islamabad.
March, 1986.
PINSTECH/NMD-116
INTERGRANULAR CORROSION OF INCOLOY 800 BY ELECTROCHEMICAL
POTENTfOKINETIC REACTIVATION AND HUEY TEST METHODS
Sohail ur Rahman
Nazar Hussain
K.A. Shahid
Nuclear Materials Division
Pakistan Institute; of Nuclear Science and Technology
Mi lore, Islamabad.
CONTENT
Page
S/NO.
Description
1.
ABSTRACT
2.
INTRODUCTION
1
3.
EXPERIMENTAL
1
4.
RESULT fc DISCUSSION
3
5.
CONCLUSION
6
6.
BIBLIOGRAPHY
7
7.
TABLES
8.
FIGURES
8-il
ABSTRACT
To measure the degree of sensitization (DOS)
quantitatively in stainless steels an Electro­
chemical Potentiokinetic Reactivation (EPR) method
was developed. Here in this report the reliability
and reproducibility of this technique was studied.
The test was also extended to quantify the DOS
in Incoloy-800 and efforts made to correlate it
with ASTM A 262-Practice C; Huey Test, the
investigation was performed by annealing the
specimens at different temperatures around and
within the sensitization range of material and
testing by both the methods.
The results measured by potentiokinetic
reactivation method and the Huey test were not
in correspondence with each other. The reproduci­
bility of EPR method was found to be good.
1.
INTRODUCTION
A technique was developed * 'to measure the degree of
sensitization quantitatively in thermally treated stainless
(2)
steels of type 304 t> 304 L. Elsewhere
, the method was
correlated and compared to ASTM Procedure A-262 Practice E.
Sensitized nickel-rich chromium-bearing alloys behave,
similar to the sensitized austenitic stainless steels, according
to the generally accepted model for intergranular corrosion
involving precipitation of Cr-rich carbides accompanied by narrow
Cr-depleted zones adjacent to the grain boundaries.
The objective of the present investigation is to establish
a relationship between the Huey Test and the new EPR technique
with respect to Alloy 800-H. In relation to the electrochemical
evaluation of the degree of sensitization, efforts have been
made to correlate the parameters of the part of polarization
curve of Incoloy 800 in sulphuric acid with intergranular
corrosion. A summary of different methods is found in Ref.
(3). To quantify the degree of sensitization through Huey
Test, the material exhibiting high corrosion rates with
increasing tendency in later periods is usually rejected.
2.
EXPERIMENTAL
Composition of the material used in this study is shown
in Table 1. Only one heat of alloy-800 was employed for the
purpose of investigation. The specimens were annealed in the
range of 500°C - 1000°C for one and two hours.
EPR TEST
The arrangement for EPR testing consisted of a Wenklng ST
72 potentiostat, a Wenking SNP 72 stepping potentiometer
coupled to Houston Instruments 2000 X-Y-recorder. Measure­
ments on flat specimens about 10 x 10 mm were carried out
-2 in the type of one liter, 5 neck flask specified in ASTM
G 5-72 * . An AISI 304 SS wire, 1.0 mm thick and 300 mm
long was soldered to the side opposite the test surface.
The entire assembly was then mounted in Epokwick resin
10-15 mm deep by placing a glass tube in a 25 mm diameter
mold in the fashion as shown in figure 1. The specimens
were sequentially polished to a final 1 um finish and
rinsed with triple distilled water and acetone just before
testing. Masking tape was then applied to the surface of
2
the specimen so as to allow only 19.63 mm of the area
exposed to the solution. For each test, a fresh solution
4
of IN H SO with 0.97 g/1 KSCN was used. Two high density
graphite auxiliary electrodes, necessary wiring and a
saturated calomel electrode completed the cell. An electric
hot plate was used to maintain the temperature of solution
at 30°C ± 1°C. The solution was purged with nitrogen or
argon and stirred magnetically before and during the run.
The test was started by immersing the mounted specimen
taking care not to expose the S.S wire to the solution.
After 2 minute immersion to establish corrosion potential,
which is near -350 mV Vs SCE, the potential was scanned
to
200 mV and held there for 2 minutes. The potential
was then traversed back to the original corrosion potential
at a rate of 6V/h.
4 )
2
HUEY TEST
This widely used test consists of exposure of samples to
boiling 65% nitric acid for five 48 hour periods. The
samples used in this study were fabricated from 1 mm
thick sheet of the material under investigation and had
the following dimensions; 50 x 20 millimeters. A 4 mm
diameter hole was provided to facilitate its suspension
from glass cradles. A 2 liter round bottom flask fitted
with a reflux condenser, contact thermometer and heating
mantle constituted the experimental set up. 65% nitric
acid of Analar grade was used for experiments. Specimens
after each period of exposure were washed, dried and
weighed. Data was established and samples were studied by
metallographic examination.
3 -
RESULTS AND DISCUSSION
EPR TEST
Typical polarization curve of alloy 800 can be seen in
Pigure 2. The test conditions of the EPR method are given
in Table II. Idea behind the EPR method is to judge the
extent of impoverishment in chromium, it is therefore
necessary to choose a comparative measurement from which
the reduction of the total activation as a result of the
formation of a weak passive layer would follow. Polarization
and reactivation curves at various heat treatments are
shown in Figure 3a & b. The practical absence of the
reactivation peaks in Figure 3a (i) and its propagating
height with increasing degree of sensitization can be
clearly observed. In order to numerically express the
suscepitibility to intergranular corrosion or the degree
of sensitization, a number of writers have proposed various
methods (5-8). For our evaluation of results we used the
reactivation charge Pa for different temperatures. Pa was
measured from time-current curve. Sensitized specimens
are easily activateu and show high value of Pa, Table V.
Grain boundary penetration can also be seen on the specimen
surface after exposure, figure 4. Values of current above
10 uA only were taken into account to calculate the quantity
Pa. The heat treatment experiments were carried out in
the furnace of a dilatometer which had to be modified for
this purpose. Argon was used to displace the air present
in the furnace. With the behaviour observed in case of
specimens between 650°C and 850°C, it was thought that
something must have gone wrong with these heat treatments
because in case of these few specimens the reactivation
peaks were almost of the height of active peaks. A delay
in arresting the required temperature was thought to be
the offender. So in order to make sure of correct heat
treatments the whole set of experiments was repeated.
Reproduction of sensitized specimens was conducted in a
different tubular furnace* The specimens were then subjected
to EPR tests under conditions shown in Table II. The
results deduced from this second set of tests were in
excellent agreement with the previous one.
-4 -
The ASTM grain size number was not decided practically
but the value of 4.5 given by the manufacturer was relied
upon. The maximum value of Pa is for specimens sensitized
from 700°C and 8OO C. The EPR method moves the peak of
sensitization curve towards higher temperatures.
0
HUEY TEST
The effect of the acid on the material was measured by
determining the loss of weight of the specimen after each
period and the total of the test periods. The corrosion
rate pertaining to each specimen was calculated from the
following equation;
R
=
KW
DAT
where,
R = the rate in mpy, mro/yr, in, penetration per
month.
K = constant, depends upon units of penetration
desired.
W * Weight loss in milligrams.
A = Total surface area in square inches.
D * Density of the material in g/cc.
T = Duration of the test period in hours.
Table 111 shows the tabulation of sensitization treatments
at different temperatures. The results of Huey tests are
given in Table IV. The ASTM A 262-C does not give criterion
to enable determination of whether the test results indicate
intergranular corrosion susceptibility of material being
-5 tested. When the data shows increasing weight loss for
successive 48 hr test periods, this should indicate
intergranular corrosion. This test readily detects
sensitization effects due to carbide precipitation. However,
for certain grades of stainless steels this test is sensitive
to a phase which does not cause intergranular corrosion.
Figure 6, a graph of loss of weight in milligrams versus
the number of periods of test, depicts the behaviour of
alloy at various heat treatments. From the results in
Table IV the sensitization range for one and two hour
treatments is about 600°C-700°C. Figures 4 & 5 show the
micro-strucu*._e of the alloy at various heat treatment
after exposure to 65% boiling nitric acid. A temperature
- corrosion rate graph for the alloy is shown in figure
7. Rates in mils per year are plotted for various
temperatures on semilog paper because the range of rates
represented is over 100 fold. Certain tests were disconti­
nued after specimen failed during examination, therefore
these points are connected by dotted lines. The temperature
for maximum sensitization for one hour exposure is about
620°C. It should be noted ' ' that Huey test does not
assess the resistance of material to pure boiling nitric
acid, but rather to the acid plus corrosion products. The
standard test is, therefore self accelerating and becomes
more selective as corrosion rate increases. Because of
the absence of molybdenum from the alloy matrix, the high
corrosion rates leading to failure of certain specimens
can only be attributed to the presence of chromium ion in
the solution. These appear to have been admitted as corrosion
products of heavily sensitized specimens. The chromium
ions are known to accelerate the rate of attack to an
appreciable extent
. it must also be noted that at
potentials greater than 1.2V SHE the addition of Cr to Fe
decreases resistance to dissolution. However, below 1.2V
SHE the reverse is true. Because the boiling nitric acid
test considers the extent of grain boundary attack, which
occurs in Cr-depleted areas, in an aggressive corrodent
at a potential set by the solution and the potential of
Huey test lies in a range which is very near to 1.2V SHE,
it gives us a picture that it may be responsible for high
attack on some samples.
4.
CONCLUSION
According to the test results the alloy 800-H seems to be
prone to intergranular corrosion attack in sensitized condition
both by the standard boiling 65% nitric acid test and by the
electrochemical potentio kinetic reactivation test. However,
the EPR method tends to move the sensitization peak towards
higher temperatures. The two methods could not be correlated
because of their difference in detecting the peak sensitization
of the material. Nevertheless, the reproducibility of EPR method
has been found good.
B I B L I O G R A P H Y
W.L.Clarke, N.M. Romero, and J.C. Oanko "Delection of
sensitization in stainless steel using electrochemical
techniques". General Electric Company Report GEAP-21382,
August 1976 and NACE Corrosion Conf. Paper No. 180,
SanPranciso CA, March, 1977.
W.L. Clarke, V.M. Romero, "Detection of sensitization
stainlles steel: 11. EPR Method for nondestructive field
tests". General Electric Company Report CEAP-1297. February,
1978.
Cowan, R.L, & Tedmon, Jr. C.S., Advances in Corrosion
Science and Technology, Vol. 3, 1973, PP-293-395 (New
York:Plenum Press).
ASTM GS-72 standard Reference Method for Haling Potentiostatic and potentio dynemic, Anodic Polarization Measure­
ments. ASTM, Philadelphia, Pennsylvania.
Sohail Rehman, Study of intercrystalline corrosion Internal
Report PINSTECH/NM0-116.
W.L. Clarke, R.L. Cowan, W.L. Walker, "Comparative Methods
Measuring Degree of Sensitization in S.S." Intergrar.ular
corrosion of stainless Alloys ASTM STP 656, R.F.
Steigerwald, Ed, ASTM, Philadelphia, Pennsylvania, P. 99,
1978.
W.B. Delong, American Society for Testing Materials, Special
Technical Publication No. 93.
UK.C-1
Owrtcol CoomiUoa of Test *>teriol {Alloy 9BO-H)
lest
Qiocnsfons
Nsey-Test
SO x 30 « 1 M
crt-rest
.1963 Cm row*
Eleae»t
mmml « S
Ti
ill
Cr
«i
C
F»
Co
0.05 B.S 32.1 0.38 0 . M
0.38
•5.87 0.083
TABUS-11
BPR Test Conditions
Electrolyte
0.5* H S 0
Temperature
30»C i 1>C
Specinen Surface
Finish.
1 u* (diaeond paste)
Initial potential
B corr. VS SCE.
Vertex Potential
• 200 «V .
Vertex Delay
2 win.
Scan rate
l.tt •V/sec.
Dn-aeration
Argon 1 litre/ain.
2
4
S
* 0.97 9/1
»
.33/
- * -
TABLE-III
Sensitizat ion Annealing Tines and Temperatures
For Huey Test Specimens.
Code of Specimens
Sensitized for
1 hr
1
2
1
4
S
t
7
t
»
10
11
12
1)
14
IS
1*
17
1»
1*
20
21
22
21
24
Sensitized for
2 hr
•
2
-
2
-
400
400
500
500
550
550
600
600
600
600
-
610
-
620
650
650
700
700
750
750
800
800
-
S-50-IA
S-50-2A
S-55-1A
-
S-40-lAj
S-40-2AJ
S-*0-lA
S-*0-2A
S-62-1A
S-62-2A
S-65-IA
S-*S-2A
S-70-1A
S-70-2A
S-7S-1A
S-7S-2A
-
•c
As received
H-A*-l
H-AK-2
-
Sensitization Heat
Treatment Temperature
S-45-1B
S-45-2B
S-50-1B
S-50-2B
S-55-1B
S-55-2B
S-60-1B
S-60-2B
S-65-IB
S-65-2B
S-70-IB
S-70-2B
S-75-1B
S-75-2B
S-BO-IB
S-80-2B
- 10 -
TABLE-IV
CORROSION RATE OF INCOLOY 800 BY HUEY TEST
Corrosion Rate at Time Periods, 48 hrs each
Sensitization Sensitization
Time (hrs.)
Temperature(C°)
(mpy)
5 th
1st
2nd
3rd
4 th
1.
2.
500
4
4
4
5
6
550
23
31
37
52
59
600
347
ND
ND
ND
ND
650
106
173
ND
ND
ND
700
5
6
8
9
10
800
5
5
7
8
9
26
50
55
62
ND
ND
ND
ND
5
500
I*
550
143
455
ND
ND
600
520
ND
ND
ND
650
375
522
ND
ND
700
43
304
498
ND
800
4
4
4
4
ND: Not determinable.
- 11 -
TABLE-V
The Reactivation Charge fa of the Specimens Sensitized
For One Hour Only in the EPtt-Test.
Code of Specimens
1)
2)
31
4)
5)
6)
7)
8)
91
101
11)
H-AR-I
S-50-1A
S-55-1A
S-60-1A
S-65-1A
S-70-1A
S-75-1A
S-80-1A
S-85-1A
S-90-1A
S-95-1A
Sensitization Heat
Treatment Temperature °C
As received
500
550
600
650
700
750
800
850
900
950
_
r
.
Reactivation Charge
Pa(mc/Cm )
0
41
77
69
220
440
420
364
294
264
161
Fig. 1 SPECIMEN ASSEMBLY FOR EPR TESTING
pr. ssi vi!
pesr.ive
transpassivr;
trar.spassive
U!:nV)
v
Fiij.2 TY iCAL ANODIC POLARIZATION PLOT OF ALLOY 800
trans passive"
trunspossivr
0-2V
0 3 5 V AS RECEIVED (1)
5-50
'J??.
1A ( 2 )
S-55-1AO)
FIG. 3 a
*-'•
-003V
OSV
,
•' .;V
(tf'/V
+0-2V
S - 6 0 - 1 A (4)
S-65-1A(5)
S-70-1AC6)
CHARACTERISTIC POTErjTIODYNAMIC CURVES OF
ALLOY 800 WITH VARYING DEGREES OF SFMSITIZ AT ION DRAWN IN ACCORDANCE WITH THE
DESCRIBED PROCEDURE
•
; . - \
-0.35V
U.?iV
035/
S-75-1AU)
5 - 8 0 - 1 A (8)
s-es-tAO)
FIG. 3 b
+
0.2V
-»0 ." V
-0;O\
S-9O-1AO0)
r
-0 - Y
5-95-lA(11)
+ \'
Q2
* 0 2V
02V
CHARACTERISTIC POTENTIODYNAMIC CURVES OF
ALLOY 800 WITH VARYING DEGREES OF SENSITI­
ZATION DRAWN IN ACCORDANCE WITH THE
DESCRIBED PROCEDURE.
v
590
o
550
600
600
iT-yfc
." ;
" *•!""•• -
500 Ih b
!
550 Ih b
]
FIG. U. MICROSTRUCTURES OF ALLOY 800 AFTER EXPOSURE
TO Q) EPR TESTING b) 65V. BOILING NITRIC ACID
r
620 l h
650 1h
-<r*
750 lh
FIG.5.MICR0STRUCTURES OF ALLOY 800 AFTER
EXPOSURE TO BOILING 65V. NITRIC ACIO
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FIG 7 EFFECT OF HEAT TREATMENTS ON EVALUATION
TEST RATE OF ALLOY 800-H
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