Crack Initiation Mechanism of Z3CN20.09M Duplex Stainless Steel during Corrosion Fatigue in Water and Air at 290

The crack initiation mechanism of a Z3CN20.09M duplex stainless steel （DSS） during corrosion fatigue （CF） in water and air at 290 ℃ was investigated by using a CF cracking machine and a scanning electron microscopy （SEM）. The cracks were initiated successively at the persistent stip bands （PSBs）, phase boundaries （PBs） and pitting corrosion points （PCPs） of the specimens when they were tested in water at 290 ℃, while in airat 290 ℃ the cracks were only initiated at the PSBs and PBs. And the cracks were found mainly to initiate at the PSBs and PBs when the specimens were tested in water and air at 290 ℃, respectively. The results also reveal that the cracks were likely to be initiated at the first 20% of fatigue life of the specimens tested in water at 290 ℃. However, the cracks were not found until 50% of fatigue life when tested in air at 290 ℃. Moreover, the crack numbers of the specimens tested in water at 290 ℃ were much more than those tested in air at 290 ℃.