Influence of nitrogen on stress corrosion behavior of S32707 in simulated deep-sea environment
ZHANG Binbin
ZHANG Shucai
ZHOU Jie
SUN Wenchang
LI Huabing
JIANG Zhouhua
Abstract：[Objective]The behavior of stress corrosion cracking(SCC)in super duplex stainless steel S32707 under deep-sea environments is a critical issue that significantly influences its engineering reliability.As a vital alloying element in steel,nitrogen(N)needs in-depth exploration regarding its role in regulating stress corrosion resistance.Revealing how N influences the evolution and mechanisms of stress corrosion in S32707 steel under simulated deep-sea environments,characterized by high pressure and chloride ion levels,will provide theoretical basis for developing better SCC-resistant solutions for deep-sea engineering materials.[Methods]Slow strain rate tension testing was adopted to analyze the tensile strength,yield strength,and elongation after fracture of S32707 with various N contents in air,simulated sea level,and deep-sea environments.Meanwhile,the SCC sensitivity was combined to evaluate the inhibitory influence of N content on SCC sensitivity,with the scanning electron microscope(SEM)adopted to characterize fracture morphologies and examine the crack propagation path.Furthermore,the stability of the passive film and corrosion kinetics were analyzed by employing potentiodynamic polarization curves.[Results]In simulating deep-sea environments,the mechanical properties of S32707 steel gradually improve with the increasing N content.The tensile strength rises significantly,while yield strength shows a slight decrease,and elongation after fracture increases obviously.The SCC sensitivity of S32707 steel drops from 16.8％to 10.3％with a decrease amplitude of 6.5％,which is significantly higher than the decrease amplitude of SCC sensitivity(1.8％)during simulating sea level environment.This means N improves the stress corrosion resistance of S32707 steel.Additionally,as N content increases,the electrochemical behavior of the steel gradually improves,shown by a decrease in pitting current density from 505.0 nA/cm2 to 341.6 nA/cm2 and an increase in the pitting potential from 60.2 mV to 101.0 mV,which helps inhibit cathodic reaction.Furthermore,as the N content increases,the microstructure exhibits gradual improvement,the area of the quasi-cleavage zone at the steel fracture decreases,quasi-cleavage characteristics diminish,and the number of cracks and crack length decrease,the section shrinkage rate increases,and the crack propagation degree gradually diminishes,suggesting that N suppresses crack initiation and propagation by enhancing fracture toughness.[Conclusions]N enhances the deep-sea stress corrosion resistance of super duplex stainless steel via various mechanisms.It reduces SCC sensitivity,boosts fracture toughness,and inhibits stress corrosion cracking.It can increase self-corrosion potential,decrease pitting current density,inhibit cathodic reaction,and slow local corrosion,thus improving the pitting-resistant performance of S32707 steel.Solid-solution N consumes H+in the pitting corrosion pit and generates NH4+,inhibiting pit acidification and hydrogen evolution corrosion.Therefore,the results show that adjusting N content is an effective method to enhance the stress corrosion resistance of S32707 in high-pressure deep-sea environment,thus providing a theoretical basis for the composition design and engineering application of highly corrosion-resistant duplex stainless steel.
Keywords：super duplex stainless steelstress corrosion crackingdeep-sea environmentslow strain rate tension testingcrack propagation pathpassive filmcorrosion kineticselectrochemical behavior
Publication Date：2025-11-25
Online Publishing Date：2025-12-29(First online date of this platform, not the publication date of the document)
Pages：8( 751-758 )