Activated Tungsten Inert Gas (A-TIG) welding has emerged as a promising
technique for improving weld penetration and
overall productivity,
particularly in austenitic stainless steels that are commonly employed in
highly corrosive environments
such as chemical processing, marine applications, and nuclear industries. This study investigates the influence of
oxide-based activating
f
luxes— specifically titanium dioxide (TiO2
) and zinc oxide (ZnO)—on the corrosion
behavior of A-TIG
welded austenitic stainless steel.The experimental procedure
involved welding using conventional Gas Tungsten Arc Welding
(GTAW), A-TIG
with TiO2
flux, A-TIG with TiO2
flux applied with a 4 mm flux gap, A-TIG with ZnO
f
lux, and ZnO flux with a
4 mm flux gap. The welded specimens were then subjected
to corrosion testing in a sulfuric acid (H2
SO4
) environment using
potentiodynamic
polarization techniques. Microstructural analysis of the weldments was performed
using optical microscopy to
evaluate grain structure and surface morphology. Among
the tested fluxes, TiO2
demonstrated superior corrosion resistance
compared to ZnO,
likely due to the formation of a more stable and adherent passive oxide film on the
weld surface. The findings
of this study underscore the importance of flux selection
in enhancing both the weld quality and corrosion resistance of stainless
steel joints,
particularly for service in aggressive and corrosive operating conditions.
Keywords: Austenitic stainless steels, Activated tungsten inert gas welding,
Titanium dioxide, Zinc oxide flux, Potentiodynamic polarization.
INTRODUCTION
Austenitic stainless steels, particularly grade 304, are
extensively used in industries such as nuclear power, chemical
processing, and food equipment manufacturing due to their
excellent corrosion resistance, strength, and ductility. Despite
these advantages, welding of 304 stainless steel presents
challenges because of its high coefficient of thermal expansion
and low thermal conductivity, which can lead to microstructural
changes, residual stresses, and distortion in the welded
joints.
Since welding is the primary fabrication method for stainless
steel components, developing techniques that ensure
sound
weld quality while minimizing defects is of great importance.
Gas Tungsten Arc Welding (GTAW) is the most widely
adopted technique for joining stainless steel due to its ability to
produce cleanand high-quality welds. However, conventional
GTAW is limited by shallow penetration, making it less efficient
for thicker plates and often requiring edge preparation and multipass welding. To overcome these drawbacks, the
Activated
TIG (ATIG) process has been developed, in which
oxide-based fluxes such as titanium dioxide (TiO2
) and zinc
oxide (ZnO)
are applied on the surface before welding.
These
fluxes modify weld pool dynamics through mechanisms such as
Marangoni convection reversal and arc constriction,
resulting
in deeper penetration in a single pass while also influencing the
microstructure and mechanical properties of the joint.
In the present work, 5 mm thick 304 stainless steel plates
were welded using the ATIG process with self-developed
TiO2
and
ZnO fluxes. The effect of these fluxes on weld bead
geometry, hardness, microstructural evolution, and corrosion
resistance
was systematically investigated. The outcomes of
this study aim to provide insights into the role of oxide fluxes
in enhancing
the weldability and performance of austenitic stainless steel for advanced industrial applications.
RESEARCH SIGNIFICANCE
The study addresses two critical aspects of austenitic
stainless steel welding: achieving adequate penetration and
ensuring
long-termcorrosion resistance. Conventional GTAW
often requires multiple passes and may compromise corrosion
performance
due to microstructural changes. By applying
oxide-based activating fluxes, particularly TiO2
, the work
demonstrated improved
weld penetration and the formation of
a stable passive film that enhances corrosion resistance. These
findings provide valuable
insights for optimizing A-TIG welding
parameters, guiding flux selection, and extending the service life
of stainless steel weldments
in highly aggressive environments
such as chemical, marine, and nuclear industries.
MATERIALS AND METHODS
Austenitic stainless steel 304 grade plates (size 80mm x
50mm x 5mm) were used as base material. The welding trials
were
conducted usingboth conventional GTAW and A-TIG with
TiO2
and ZnO fluxes. Flux pastes were applied with and without
a
4 mm gap using a brush.
Table 1: Chemical composition (wt. %) and mechanical properties of austenitic
stainless steel 304.
Figure 1: Schematic diagram of mixing and coating of flux on specimen
Welding parameters were kept consistent: current 90–125
A, voltage 20–25 V, and travel speeds ranging from 42 to 57
seconds
per 80 mmweld length. The welded samples were
prepared for macrostructure, microstructure, hardness testing,
and corrosion
testing. Potentiodynamic polarization tests were
performed in 0.1 N H2
S4
to assess corrosion behavior.
Table 2: Welding process variables
Figure 2: Weld surface appearance
TESTING AND EVALUATION (First level heading, Capital,
Arial, 12, Bold)
1. Visual Examination: To check for weld discontinuities
like cracks, porosity, undercut, incomplete fusion, spatter,
overlaps, and
surface irregularities.
2. Macrostructure Evaluation: To determine the weld
cross-section and determine the penetration, fusion
characteristics, bead
profile, and presence of any internal
defects in the
GTAW weld.
3. Microstructure Evaluation: Optical Microscope is used to
determine microstructure of the weld bead, heat-affected zone (HAZ),
and base material to study the grain structure,
phase transformation, and metallurgical changes that
occurred during welding.
• Etchant for Base Metal: 10% Oxialic Acid Electrolytic
solution.
• All samples were observed at 400X Magnification.
4. Hardness Test: Vickers hardness test was carried out
on the weld metal, heat- affected zone (HAZ), and base
material to
evaluate the hardness distribution and
mechanical property variation across the specimen.
5. Corrosion Testing: Potentiodynamic studies were carried
out on weld bead in H2SO4 solution as per ASTM G-5
standard, using
Potentiostat Gammry Reference 600.
Corrosion cell was consist of Calomel electrode as
reference electrode, graphite rod as
counter electrode
and test sample as working electrode.
Table 3: Operating Parameters of potentio- dynamic test
RESULTS AND DISCUSSION (First level heading, Capital,
Arial, 12, Bold)
Figure 3: Graph of hardness value
Table 4: Vicker Hardness Value
The hardness values in the weldment and heat-affected zone
(HAZ) were observed to increase under the TiO2
flux-bonded
condition, which can be attributed to the reduced presence
of ferrite islands compared to the ZnO condition. Conversely,
in
the case of ZnO flux-bonded welds, a decrease in hardness
was recorded due to the higher volume fraction of ferrite
islands
relative to TiO2
. Furthermore, the base metal exhibited
an increase in hardness with the application of flux-bonded
conditions for both TiO2
and ZnO.
Figure 4: Microstructure of Weld with ZnO
Figure 5: Microstructure of Weld withTiO2
2. Macrostructure of Penetration
Macrostructural analysis revealed that the conventional
plate showed a penetration depth of 2.95 mm. With TiO2
flux,
penetration increased significantly to 4.45 mm, whereas the
f
lux-bonded TiO2
condition showed a reduced depth of
2.88
mm. For ZnO2
flux, the penetration was 3.21 mm, but decreased
to 2.88 mm under the flux- bonded condition.
These results
indicated that weldment with TiO2
flux provided the highest
penetration, while flux-bonded conditions
for both TiO2
and
ZnO reduced penetration effectiveness.
3. Results of Potentiodynamic Test
Figure 11: Potentiodynamic scans of 304 plate welded with TiO2 flux and TiO2 FB
Table 5: Electrochemical Parameters of Potentio- dynamic study of 304
plate welded with TiO2
flux and TiO2FB
The weldment with TiO2
flux exhibited the most negative
corrosion potential (–573 mV) and the lowest corrosion
current
density (2.58 μA), confirming enhanced passivation.
However, the calculated corrosion rate was high (3.022 ×
10³ mpy),
suggesting film instability in the aggressive H2
SO4
medium. In the 4 mm flux gap condition, Icorr increased
sharply
(697 μA) with a less negative Ecorr (–394 mV),
indicating poor film protection and accelerated corrosion. The
weld without
flux showed intermediate behavior (Ecorr –556
mV, Icorr 198 μA, CR 296.7 mpy). These results highlighted
that uniform
TiO2
flux application promotes passivation,
whereas non-uniform flux distribution reduces
corrosion resistance.
Figure 12: Potentiodynamic scans of 304 plate welded with ZnO flux
and ZnO FB
Figure 12: Potentiodynamic scans of 304 plate welded with ZnO flux
and ZnO FB
The weldment with ZnO2
flux (4 mm gap) showed Ecorr of –449 mV and a relatively low Icorr (36.20 μA), resulting in
the lowest corrosion rate (54.31 mpy) among the tested
conditions. In contrast, the uniform ZnO2
flux coating
exhibited
the most negative Ecorr (–573 mV) but a very high
current density (2.58 mA), leading to a severe corrosion
rate
(3.873 × 10³ mpy). The weld without flux presented
intermediate behavior (Ecorr –556 mV, Icorr 198 μA,
CR 296.7 mpy).
These results indicated that while ZnO2
influences the passivation potential, its effectiveness is highly
dependent on
coating uniformity; improper flux distribution (4 mm gap) provided better localized protection, whereas
uniform application
led to unstable passive film &
accelerated corrosion.
Fig 13 Potentiodynamic scans of 304 plate welded with ZnO flux and
ZnO FB, TiO2
flux and TiO2
FB and without flux.
Table 7: Electrochemical Parameters of Potentio- dynamic study of 304
plate welded with TiO2
flux and TiO2
FB, ZnO
and ZnO FB and
without flux
The electrochemical evaluation of TIG welded SS304 in H2
SO4
demonstrated that both flux chemistry and coating
uniformity
significantly influence corrosion performance. Uniform TiO2
f
lux application shifted Ecorr to more negative
values and
lowered Icorr, promoting passivation, although the passive
f
ilm showed instability at longer exposure.
Non-uniform TiO2
coating (4 mm gap) and uniform ZnO2
flux resulted in higher
corrosion rates due to poor film stability,
while the ZnO2
flux
with a 4 mm gap condition exhibited the lowest corrosion
rate, indicating localized protective effects.
The weldment
without flux showed intermediate behavior. Overall, TiO2
f
lux is more effective when applied uniformly,
whereas ZnO2
f
lux displays inconsistent protection, strongly dependent on
coating distribution.
Figure 14: Microstructure of plate welded without flux
Figure 14: Microstructure of plate welded with TiO2
flux FB
(4 mm gap)
Figure 15: Microstructure of plate welded with TiO2
flux
Figure 18: Microstructure of plate welded with ZnO flux FB
(4 mm gap)
Figure 18: Microstructure of plate welded with ZnO flux
Result of microstructural study
The base metal exhibited twin grains within the austenitic
matrix along with δ-ferrite, a feature also observed in the
HAZ, confirming structural stability without secondary phase
formation. The weld metal showed satisfactory penetration,
free from inclusions or defects, and consisted of austenite
with δ-ferrite. The presence of δ-ferrite improves resistance
to hot cracking but slightly decreases hardness compared
to the base metal, which explains the lower hardness values
measured in the weld region.
CONCLUSIONS
This study demonstrated that oxide-based activating fluxes
significantly affect the weldability and corrosion performance
of TIG welded SS304 in H2
SO4
environment. The application
of TiO2
flux improved weld penetration and promoted
passivation, as reflected by low Icorr values, but showed film
instability leading to a higher calculated corrosion rate.
In
contrast, ZnO2
flux exhibited inconsistent corrosion behavior;
while the uniform coating resulted in severe corrosion
due to
unstable passive film, the 4 mm flux gap condition provided
the best corrosion resistance with the lowest rate
(54.31 mpy).
Hardness testing revealed higher values for TiO2
weldments
and reduced hardness for ZnO2
weldments,
correlating with the volume fraction of δ-ferrite in the weld metal. Microstructural
observations confirmed twin grains within
the austenitic
matrix and δ-ferrite across the base, HAZ, and weld regions,
with δ-ferrite contributing to crack resistance
but lowering
hardness.
Overall, TiO2
flux is more effective when applied uniformly,
delivering improved penetration and acceptable corrosion
behavior, while ZnO2
flux demonstrated condition-dependent
protection. These findings highlight the critical role of flux
chemistry and application uniformity in optimizing the
performance of A-TIG welded stainless steel for service in
corrosive
environments.
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