Stress Corrosion Cracking Mechanism Of Nb/Sb-Microalloyed High Strength Steels In Polluted Marine Atmosphere

The study object of this work is the stress corrosion cracking(SCC)issue of high strength low alloy(HSLA)steels in marine atmosphere environment,and the method of microalloying design based on SCC mechanism is conducted to reduce the SCC risk of steels in marine engineering.HSLA steels with various amounts of Nb and Sb were first prepared by vacuum melting and thermal mechanical control processing(TMCP)techniques,and then examined with microstructural analysis,electrochemical and wet-dry cycling tests for evaluation of microstructure and corrosion resistance.The SCC behavior and mechanism were investigated using U-bend specimen exposure and slow strain rate tension(SSRT)tests in the SO2-polluted marine atmosphere,and the impact of Nb and Sb on the SCC mechanism was explored and clarified by kinds of advanced characterization methods.At last,electrochemical hydrogen charging tests were carried out to investigate the resistance of the microalloyed steels to SCC in a SO2-polluted marine atmosphere in presence of hydrogen charging.The results showed that the addition of Nb and Sb elements had no negative effect on the microstructure and corrosion resistance of the prepared steels,which met the prerequisite for enhancing the SCC resistance in the SO2-polluted marine atmosphere.Specifically,Nb could retard the cathodic hydrogen evolution process and inhibit the formation of corrosion pits at the initial stage.In addition,the selective dissolution of microstructure at the pit bottom was weakened.However,in the late corrosion period,Nb had marginal effect on corrosion rust properties.Sb could promote the deposition process of Cu and its oxides and generate an enriched film consisting of Sb2O3,Sb2O5,Cu and its oxides.This synergy of Sb and Cu could restrain both cathodic and anodic processes and accelerate the transformation from ?-FeOOH to ?-FeOOH in the later period,which enhanced the protectiveness of rust.As a result,the enrichment degree of Cl-and SO42-was reduced,which improved corrosion resistance of HSLA steels.A synergy of Nb and Sb addition could inhibit the anodic dissolution(AD)and hydrogen embrittlement(HE)mechanisms of the microalloyed HSLA steels in the SO2-polluted marine atmosphere,which decreased electrochemical activity,improved rust layer property,and declined the susceptibility of microstructure to hydrogen,leading to the improvement of resistance to SCC.Nb addition helped optimize microstructure and decrease hydrogen evolution current density through the formation of NbC precipitates,which reduced the hydrogen-induced cracking(HIC)susceptibility of primary austenite grain boundary(PAGB)and lath bainite boundary.Meanwhile,the hydrogen-enhanced localized plasticity mechanism was repressed and the proportion of(111)crystal planes around the crack tip increased after Nb addition,which decreased the crack propagation power.Sb could not only inhibit the anodic active dissolution and cathodic hydrogen evolution processes,but also enhance the rust protectiveness under the synergy of Cu and Sb by generating insoluble oxides and aggregating in the inner layer of rust,which improved the distribution of Cl-and SO42-.Thus,the localized AD and HE effect were slowed down,which restrained the initiation and propagation of microcracks.After strengthening the role of hydrogen in SCC,Sb only had little effect on the hydrogen behavior and SCC susceptibility in the SO2-polluted marine atmosphere after hydrogen charging.In contrast,Nb played an important role in hydrogen action in steels,which varied with precipitation temperatures.Evenly distributed NbC nano-precipitates formed in the HSLA steels precipitated at 920? in comparison with 750?,which reduced the PAGB size and increased the low angle grain boundary(LAGB)proportion,causing the increase in the number of reversible and irreversible hydrogen traps in steels.The concentration of diffusible hydrogen atoms was decreased,and the initiation and growth of hydrogen-induced cracks were accordingly inhibited.Finally,the resistance of the microalloyed steels to SCC was significantly enhanced even under hydrogen charging.By the way,at 750?,the generated martensite/ferrite interface tended to be the initiation sites and preferred propagation path of hydrogen-induced cracks,which decreased SCC resistance.The addition of 0.06%Nb+0.1%Sb and appropriate microstructural regulation are verified to improve the resistance to atmospheric corrosion and SCC of HSLA steel in the SO2-polluted marine atmosphere.Thus,microalloy design and microstructural regulation based on SCC mechanism contribute tremendously to reducing SCC risk,and are of great significance for solving the SCC problems of HSLA steels in marine environment.