Manufacturing technologies
Marin structure, Chemical plant

Initiation mechanism of pitting corrosion
in weld heat affected zone of duplex stainless steel
Department of Joining Metallurgical Evaluation, Joining and Welding Research Institute
Assistant Professor Hou, Yuyang
https://researchmap.jp/houyyjwri?lang=en

Abstract
The pitting corrosion resistance was investigated in duplex stainless steel welds performed by one pass high current gas metalarc welding, it was found that pitting corrosion mainly occurs inhigh-temperature HAZ (HT-HAZ), where peak temperature is heated to ferrite single phase range. Most of the pitting corrosion in HTHAZ is initiated from (Ti,Cr)N in the ferrite phase. Even in duplex
stainless steel with 50 ppm or less titanium, nitrides containing
titanium are precipitated in HT-HAZ, and no clear Cr-depleted zone
was confirmed around these nitrides, but it was confirmed that
almost no passive film was formed on the surface of (Ti,Cr)N. It isconsidered that the initiation of pitting corrosion is caused not onlyby the Cr-depleted zone formed around Cr nitride but also by thevulnerability or lack of passive film on the surface of (Ti,Cr)N.

Manufacturing technologies▍ Background & Results
Duplex stainless steel has a microstructure with a ferrite-to-austenite volume ratio of approximately 1:1, showing excellent corrosion resistance, mechanical properties, workability, and weldability.
It has numerous applications in critical areas, such as marine
constructions, seawater equipment, and various types of chemicalplants. Welding is an extremely important manufacturing technology for duplex stainless steel products. However, in the heat-affected zone (HAZ) of duplex stainless steel welds, excessive ferrite
forms owing to the high cooling rate and will easily deteriorate thepitting corrosion resistance. However, the pitting corrosion mechanism in duplex stainless steel welds is still unclear because of thecomplex microstructural changes caused by the welding thermal
cycle. There are very few analyses that consider precipitates as
the initiation sites of pitting corrosion. Therefore, it is impossible
to fully elucidate the relationship between precipitates and pitting
corrosion resistance. In this study, focusing on the HAZ of duplex
stainless steel, the initiation mechanism of pitting corrosion wasinvestigated using detailed measurements of the concentration dis-tribution near the pitting corrosion initiation site. As a result, it wasoriginally revealed that trace Ti content in the duplex stainless steel
will cause the precipitation of titanium-contained nitrides in the HAZ
region during gas metal arc welding (GMAW) of duplex stainlesssteel. The passive film was vulnerable or lacking on the surfaceof such nitrides containing titanium. The lack of passive film could
hardly protect the duplex stainless steel welds from the initiation ofpitting corrosion.

▍ Significance of the research and Future perspective
This research revealed an original mechanism of pitting corrosion
caused by precipitates in the HAZ of duplex stainless steel welds.
This result provides new prospects and considerations in the chemical compositional design and manufacturing process of the duplex
stainless steel to achieve a higher resistance to pitting corrosion.
This study will also ensure the safety of duplex stainless steel products for better environmental protection and energy saving.

Fig. 1 Identification of oxide accelerating TiN formation for δ-ferrite nucleation toimprove mechanical properties of stainless steel welds. (a) Nucleus characterization, (b) phase orientation relationship and nucleation mechanism.

Fig. 2 Initiation mechanism elucidation of pitting corrosion in HT-HAZ of duplex stainless steel welds. (a) Phase identification of (Cr,Ti)N, (b) vulnerability or lack of passive film on (Cr,Ti)N. (TEM-EDS line analysis.)
P a t e n t
Tr e a t i s e

Hou, Yuyang; Nakamori, Yudai; Kadoi, Kota et al. Initiation mechanism of pitting corrosion in weld heat affected zone of duplex stainless steel. Corrosion
Science. 2022, 201, 110278. doi: 10.1016/j.corsci.2022.110278
Hou, Yuyang; Kadoi, Kota. Nucleation of equiaxed δ-ferrite by accelerating TiN formation controlled by oxide during welding of ferritic stainless steel.
Materials Characterization. 2023, 204, 113212. doi: 10.1016/j.matchar.2023.113212

URL
K e y w o r d welding, metallurgy, microstructure, corrosion, defects