| Economic savings are one of the routes that the development of duplex stainless steel(DSS)follows.As one relatively low cost DSS,LDX 2101 has been widely used in the petroleum,chemical,bridge construction and other fields.However,corrosion is a major factor that limits the use of DSS.Solid-state DSS with equal proportion of ferrite(?)and austenite(?)phase,uniform distribution of alloy elements and no deleterious phases has exhibited excellent comprehensive performance.However,in the practical applications,the dualphase balance is easily disturbed by the inappropriate heat treatment such as welding,which also cause the redistribution of the alloying elements.In addition,DSS may take place,especially stress corrosion cracking(SCC)during the longterm service of a sea structural materials.Therefore,it is necessary to explore the effect of temperature and stress on the corrosion resistance of LDX 2101.In this paper,combining with the previous researches,the relationship between the micro-sized ? and the corrosion resistance of DSS at different temperatures is still unclear and the effect of stress on corrosion resistance of DSS.Consequently,the optical microscopy(OM),scanning electron microscopy(SEM)and electrochemical techniques such as electrochemical impedance spectroscopy(EIS),potentiodynamic polarization and mott-schottky are used to explore the mechanism of the corrosion resistance of LDX 2101 in 3.5 wt.% NaCl solution under different temperatures and stresses.The volume fraction of austenite in LDX 2101 decreased and the ferrite increased as the annealing temperature increased from 950°C to1200°C.When the temperature increased from 950°C to 1100°C,pits preferentially ocurred at the ?/? phase boundaries or within the ferrite domains and then propagated into the ferrite phase.Moreover,at 1000°C,1050°C and 1100°C,pitting initiated along the ?/? boundaries that formed by the ? phases with the newly formed ferrite phases in austenite zones due to the dissolution of the ? phases.Once the annealing temperature reached 1150°C,The pitting resistance of LDX 2101 was the worst.At this condition,there were many micro-sized ? phases lied in the ferrite regions,which was also attributed to the not complete dissolution of ? phases with the increse of annealing temperature.Moreover,the two phase boundaries were close for the micro-sized ? grains.Therefore,the micro-sized ? phases could be removed easily.Continually rising the temperature to 1200°C,the micro-sized ? phases gradually dissolved to disappear and pit only nucleated within ferrite regions thereby improving the corrosion resistance.Moreover,the results of the samples annealed at 1150°C for different time further confirmed that when the temperature is relatively high,the micro-sized ? phases had great influences on the corrosion resistance of the LDX 2101.When the LDX 2101 was subjected to 0 MPa,89.5 MPa,101.3 MPa,139.4 MPa,150.8 MPa,172.6 MPa respectively,the doping concentration of the passivation film formed on the LDX 2101 in 3.5 wt.% NaCl solution increased with the tensile stress increased,which indicated that the stability of the passivation film decreased gradually with the increase of the tensile stress.However,the pitting potential(Epit)in the 3.5 wt.% NaCl solution was not affected by the tensile stress but depended on the inclusions in the surface or secondary surface layer,and the corrosion morphology presented the shape of the inclusions.When the corrosion reached to some extent,it is observed that the corrosion expanded along the rolling direction and exhibited selective corrosion of the ferrite phase and this phenomenon was more obvious with the tensile stress increasing to 172.6 MPa.Therefore,the tensile stress mainly influenced the propagation of corrosion.The corrosion was more likely to occur at the boundary and then propagated to the weak phase which was attributed to the tension in the phase boundaries. |
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