ASSESSMENT OF SCC SUSCEPTIBILITY OF SUPERMARTENSITIC STAINLESS STEEL THROUGH SLOW STRAIN RATE TESTS

摘要

The 13%Cr supermartensitic stainless steels (SMSS) are a new group used as pipelines for oil and gas transport. The optimized microstructure of these materials offers good corrosion resistance in environments containing CO_2 and H_2S. In addition to reducing carbide precipitation, the low carbon content (about 0.01 wt-%) of supermartensitic steels SMSS provides good weldability with conventional arc welding techniques. The susceptibility to stress corrosion cracking (SCC) through slow strain rate tests (SSRT) of SMSS and its comparison with a Hastelloy C-22 in substitute ocean water was carried out. Tests were supplemented by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The SSRT were performed in air and in a chloride solution at 5, 15, 25 and 35℃. The susceptibility to SCC was evaluated considering the reduction in area ratio and time to failure ratio. According to these considerations it is clear that all the specimens tested in the chloride solution exhibited high resistance to chloride SCC. The specimens tested in airrnexhibited a ductile fracture. In the corrosive solution, the fracture was a combination of both, ductile and brittle fracture with a transgranular appearance. An evident temperature effect on the steel was observed. The major susceptibility to SCC was presented to the lower temperature (5℃). According to EIS evaluations, the corrosion rate increased with the temperature, which promoted an increase in the SCC susceptibility at a low temperature. The corrosion mechanism observed on the material under dynamic stress was a combination of high activation resistivity in combination with a diffusive process on the metallic interface given by a thin film of a protector layer of chromium oxide.