Nitric Acid Loop for Long Term Corrosion Evaluation

SCIENCE-54
Nitric Acid Loop for Long Term Corrosion Evaluation
n EXECUTIVE SUMMARY
A 400 liter capacity dynamic nitric acid loop has been designed and constructed for evaluating the corrosion performance
of AISI type 304L SS over long operating periods under plant simulated conditions. Corrosion resistance of the materials
in different metallurgical conditions, namely, cold worked, solution annealed, sensitized, alloy composition, etc. will be
evaluated in this loop in flowing nitric acid at different flow velocities (upto 1.55 m/s) and temperatures ( 313, 333, 353,
380 K and vapour phase). The data generated over a long period of exposure (up to 10,000 h) would be used to arrive at an
acceptable corrosion rate of AISI type 304L SS used for applications in nitric acid service. The availability of such a reliable
and useful data will be helpful in predicting the remnant life of the components used in the reprocessing plants by
developing suitable analytical and modeling tools of the corrosion processes
n OUTLINE
The corrosion evaluation of conventional AISI type 304L
SS used for the reprocessing plants is presently evaluated
as per ASTM A262 practice C (HUEY test) (ASTM, A262-91,
1991). The test is also recommended by the United States
Nuclear Regulatory Commission (USNRC) for evaluating
the materials intended for nitric acid service in
reprocessing plants (USNRC Regulatory Guide 3.37,
1975). It is an accelerated test involving boiling nitric acid
of 65% Nitric acid and the experiments are conducted by
immersing the test coupon for five times of 48 h periods in
the solution. A small volume of the acid is used in a 1-L
flask (350 ml) and every 48 h fresh solution is used. The
major drawbacks of this test procedure are that the
specimens are tested in highly concentrated medium in
static condition, the total test period is shorter, and it is
specific to evaluate intergranular corrosion only. The
testing procedure assesses resistance to intergranular
corrosion, and does not simulate the plant conditions. It is
well known that AISI type 304L SS used in nitric acid
medium undergoes several types of corrosion other than
intergranular corrosion, namely, end-grain attack,
transpassive dissolution, vapour phase attack etc. So, it
is evident that comprehensive corrosion assessment of
AISI type 304L SS by ASTM A262 practice C test is
impossible, taking into consideration the ranges of
concentration of nitric acid used, temperature of
operation, impurities and redox chemicals present, and
the metallurgical state of the materials used in the plant.
The limitations of the current approach motivated us to
design a dynamic nitric acid loop made of AISI type 304L
SS, with flowing nitric acid at different temperatures to
evaluate the corrosion performance of materials used in
reprocessing plants (Fig. 1).
A loop has been designed with 400 L capacity for
evaluating the corrosion performance over long operating
periods under plant simulated conditions. Corrosion
resistance in different metallurgical conditions, namely,
cold worked, solution annealed, sensitized, alloy
composition etc. will be evaluated in flowing nitric acid at
different flow velocities (up to 1.55 m/s) and temperatures
(313, 333, 353, 380 K and vapour phase). The data
generated over a long period of exposure (up to 10,000 h)
would be used to arrive at an acceptable corrosion rate of
type 304L SS. The availability of such a data will be helpful
in predicting the remnant life of the components used in
the plants by developing suitable analytical and modeling
tools of the corrosion processes. Currently, the loop is
operating with 304L SS samples of solution annealed,
sensitized (923 K/L h) and welded conditions, used in
demonstration fast reactor reprocessing plant. Results
obtained after seven campaigns of cumulative 4850 h of
operation indicated insignificant corrosion rates (<1 mpy)
upto 353 K, beyond which it increased with time as shown
in Figure 2. Non-destructive testing using eddy current
and ultrasonic methods and surface examination by SEM,
in addition to in service inspection of the loop components,
are also being carried out. Presently, the loop is operating
for more 1000 h operations. The long-term operation of the
loop, and the resultant corrosion data of the materials, will
provide valuable information on the life and performance
of the actual components in the operating plants, and in
qualifying materials under realistic conditions for
reprocessing plant applications.
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Fig. 1 : The flow diagram of Nitric Acid Loop.
Fig. 2 : Corrosion rate of samples, and concentration of ions in
HNO3 after seven campaigns of cumulative 4850 h
operation.
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n ADDITIONAL INFORMATION
AISI type 304L stainless steel used for the reprocessing plants is presently evaluated as per ASTM A262 practice C (HUEY
test) which is recommended by the United States Nuclear Regulatory Commission (USNRC) for evaluating the materials
intended for nitric acid service in reprocessing plants. It is an accelerated test involving boiling nitric acid of 65%
concentration, and the experiments are conducted by immersing the test coupon for five times of 48 h periods in the
solution. The major drawbacks of this test procedure are that the specimens are tested in highly concentrated medium in
static condition, the total test period is shorter, and it is specific to evaluate inter granular corrosion. Comprehensive
corrosion assessment of 304L SS by ASTM A262 practice C test is difficult, taking into consideration the ranges of
concentration of nitric acid used, temperature of operation, impurities and redox chemicals present, and the metallurgical
state of the materials used in the reprocessing plants. This motivated to design a dynamic nitric acid loop made of type
304L SS, with flowing nitric acid at different temperatures to evaluate the corrosion behaviour of materials used in
reprocessing plants.
n GENERAL EXPLANATION RELATED TO THE DESCRIPTION
Austenitic stainless steels used in the reprocessing
plants are qualified as per ASTM A262 practices
which are basically accelerated corrosion testing
methods. Long term evaluation of the corrosion
behaviour under plant operating simulated
conditions are necessary to qualify the materials for
plant applications as well as to predict the corrosion
behaviour of components used in the plant. A
dynamic nitric acid loop of 400 litres capacity of 6N
HNO3 was thus designed, constructed and operated
with samples of AISI type 304L SS used in the
reprocessing plant. The tested samples would also be
examined by nondestructive testing methods like
eddy current and ultrasonics, as well as the loop
components undergo inservice inspection. SEM
examination of the samples, and the chemical
analysis of loop acid also provide valuable
information. Currently, the loop has operated seven
campaigns with 4850 h of cumulative operation, and
has produced valuable information on the corrosion
behaviour of type 304L SS used in the plant. Further
campaigns up to 10,000 h are planned to generate
additional data on the performance of the type 304L
SS used in the plant.
Fig. 3 : Photograph of Nitric Acid Loop
n BRIEF DESCRIPTION OF THEORETICAL BACKGROUND
Selection of materials for reprocessing and waste management applications is a challenging task as no failure of any
nature is acceptable for these applications. Extensive laboratory scale R&D has been carried out to evaluate materials for
specific applications. The materials qualification for reprocessing plant components was carried out so far as per ASTM A
262 Practice C which is a short term accelerated test in concentrated nitric acid. Though these data give a broad based
guidance in the selection of materials for different applications, the confidence level in choosing a particular material for a
specific application can only be increased by carrying out long term/accelerated tests under static/dynamic conditions as
the case may be. In this direction, a dynamic nitric acid loop to study the corrosion behaviour of austenitic stainless steels
in simulated reprocessing plant conditions has been designed, built and operated.
n ACHIEVEMENT
Design, construction and operation of a dynamic nitric acid loop of 400 litres capacity for conducting long term corrosion
evaluation of stainless steels used for reprocessing applications. For the first time such a loop has been used to generate
long term corrosion rate data to study the corrosion behaviour of stainless steels used in reprocessing applications. The
loop simulates the plant operating conditions, and provides a test bed for qualifying the stainless steels for critical
applications in reprocessing plants.
n PUBLICATIONS ARISING OUT OF THIS STUDY AND RELATED WORK
Baldev Raj and U. Kamachi Mudali, Progress in Nuclear Energy, 48 (2006) 283.
Further inquiries:
Dr. U. Kamachi Mudali, Corrosion Science and Technology Division,
Metallurgy and Materials Group, IGCAR, e-mail: [email protected]
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