IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
Fe-0.22%, Cr-0.1%) in wrought
and welded forms in
comparison with other
candidate materials like CP Ti,
Ti-5%Ta and Ti-5%Ta-1.8%Nb.
Manual TIG welding and
electron beam welding were
carried out with Zircaloy-4, and
radiography was made for
choosing defect free regions for
making samples for corrosion
investigations. Three phase
corrosion testing in liquid,
vapour and condensate regions
was conducted in boiling
11.5M nitric acid using a
specially
designed
experimental set up. The
corrosion rates were evolved
based on the weight change
after the test for 240 h, and the
tested base samples in liquid
and condensate phases were
analysed for the nature of
surface films by using X-ray
photoelectron spectroscopy
(XPS), while that exposed to
liquid phase was analysed by
using secondary ion mass
spectroscopy (SIMS).
Potentiodynamic polarization
technique was used to study the
polarization behaviour in both
base and welded conditions in
11.5M HNO 3 solution at room
temperature.
The average corrosion rate
for five individual periods in
liquid, vapour and condensate
phases of boiling 11.5M nitric
acid for CP-Ti, Ti-5%Ta, Ti-
5%Ta-1.8%Nb, Zircaloy-4,
Zircaloy-4 TIG weld and EB
weld samples are shown in
Fig.1 (a). As the corrosion rates
are in close ranges the values
are also provided in Table
inserted in the figure. The
corrosion rates clearly
indicated superior corrosion
resistance of zircaloy-4 in both
wrought and welded conditions
as 'near-zero corrosion
material' in comparison to CP-
Ti, Ti-5%Ta, and, Ti-5%Ta-
1.8%Nb. The electron beam
welded samples did not show
any deterioration and
performed similar to wrought
alloy. The high corrosion
resistance of zirconium and its
alloys in highly oxidizing nitric
acid conditions is due to a
stable and adherent ZrO 2 film
formed on their surface. XPS
and SIMS spectra indicated the
formation of ZrO 2 protective
film on the surface after testing.
The binding energy of Zr3d 5/2
from high resolution spectra
indicated the presence of Zr in
+4 valency state in condensate
phase. SEM examination
revealed insignificant corrosion
attack of Zircaloy-4 and its
welds in all three phases in
comparison to CP-Ti, Ti-5%Ta,
and Ti-5%Ta-1.8%Nb after
testing in liquid, vapour and
condensate phases. However,
accelerated corrosion attack
was noticed for titanium after
testing in condensate phase.
Fig.2 Zircaloy-4 corrosion testing
system for three phase
corrosion evaluation in 11.5M HNO 3 .
The polarization curves for
Zircaloy-4 and its welds
exhibited a wide region of
stable passivation up to 1.5 V
(SCE) beyond which distinct
breakdown of passivity was
noticed. The welded samples
showed similar or better
corrosion behaviour (Fig.1 (b))
compared to wrought alloy in
11.5 HNO 3 M solution.
The results of microstructure,
microhardness, corrosion rate,
morphology of attack and
nature of surface film formed
clearly established that
zircaloy-4 is a candidate
material for application in
highly corrosion concentrated
nitric acid at high temperatures
in reprocessing plants. Based
on the results of the
investigation a zircaloy-4
testing system (Fig.2) with an
option to conduct long term
corrosion study in liquid,
vapour and condensate region
was fabricated, commissioned,
and evaluated for corrosion
behaviour.
108 FUEL CYCLE