STUDY ON THE EFFECT OF TIOâ‚‚ AND ZNO FLUX on Corrosion Behavior of Activated TIG Welded Austenitic Stainless Steel
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.