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Corrosion fatigue and pitting behaviour of duplex stainless steels in chloride solutions Sriram, Rajagopal


The pitting and corrosion fatigue (CF) behaviour of two commercial duplex stainless steels (SS), one cast and other wrought, were studied in chloride solutions. One solution was a simple chloride (1M NaCl) and the other was a synthetic white water containing a lower concentration of chloride, together with oxidized sulphur species (thiosulphate). Differences in composition between the ferrite and austenite phases were determined by micro-analytical techniques. The pitting studies showed that the pitting potentials and the preferential pitting of the ferrite or austenite phases were dependent upon partitioning of the elements Cr,Mo and N between the two phases. Alloying considerations leading to improved pitting resistance were discussed and it was concluded that the beneficial effects of alloyed nitrogen were due to surface enrichment of N atoms. Potentiostatically controlled CF tests were combined with studies of repassivation kinetics to determine the mechanism of CF crack propagation. The crack tip chemistry was maintained under well characterized conditions by using high frequency fatigue testing to produce good mixing between the crack solution and bulk solution. Supplementary experiments confirmed that such mixing was achieved. Near-threshold CF propagation rates were studied with compact tension specimens as a function of cyclic stress intensity, ΔK, and electrochemical potential. The propagation of cracks was measured by optical microscopy and a back face strain gauge. Repassivation kinetics were studied as a function of potential by using potentiostatically controlled rapid scratch tests and monitoring anodic current transients with a fast response oscilloscope. The experiments showed that near-threshold fatigue crack propagation (FCP) rates in 1M NaCl were influenced by the applied potential. The FCP rates were faster at very cathodic potentials (-1.2Vsce) and very anodic potentials (+0.3 Vsce). At intermediate potentials (-0.4 Vsce) crack propagation rates were slower. However, there was little effect of potential on FCP in synthetic white water. The rapid scratch tests showed that the anodic potential where the fastest FCP rates were observed coincided with the potential at which the peak transient current density was highest and the repassivation rate was most rapid. Fractographic observations showed that at potentials where hydrogen evolution was not possible, the fracture surface features were independent of potential. At cathodic potentials where hydrogen evolution was possible, more interfacial fracture regions were seen. Cracking was completely transgranular at anodic potentials and opposing fracture surfaces contained fine scale (~2000A) interlocking ridges. It was concluded that FCP at anodic potentials was consistent with a restricted slip reversibility (RSR) model of cracking, where potential affects the rate of oxidation of freshly exposed surface which, in turn, controls the degree of slip reversibility at the crack tip. At cathodic potentials, where hydrogen is evolved during fatigue, it was concluded that hydrogen transport and embrittlement processes can increase the rate of fatigue crack propagation.

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