| Alloy690is widely used for making steam generator (SG) heat exchange tube ofpressurized water reactor (PWR) nuclear power plants due to the combination of goodmechanical properties and corrosion resistance. However, under operating conditions,SG heat exchange tubes suffer from various corrosion failures, such as stresscorrosion crack (SCC), intergranular corrosion (IGA), and pitting. The relatedcorrosion mechanisms of the materials are still under discussion. This paper studiesthe relationship between the content of nonmetallic inclusions and the pittingcorrosion behavior of alloy690. Standard testing method for intergranular corrosioncombined with electron back-scatter diffraction (EBSD) was used to study the effectsof grain boundary characterization on the intergranular corrosion behaviors of alloy690. Electrical potential reactivation (EPR) method was used to evaluate theresistance to intergranular corrosion with different surface condition. The resistance tostress corrosion cracking of alloy690was evaluated using C-ring specimens inboiling concentrated sodium hydroxide solution.The results showed the positive correlation between the content of nonmetallicinclusions and the susceptibility to pitting corrosion. Therefore, the growth rate ofpitting cavities can be reduced by controlling the sizes and distribution of nonmetallicinclusions. The results also showed the negative correlation between the percentage ofspecial grain boundary and susceptibility to intergranular corrosion. Therefore, byoptimizing the grain boundary character distribution, the resistance to intergranularcorrosion of alloy690can be enhanced.The results of EPR tests show that thermal treatment at720C for10hoursimproves the resistance to intergranular corrosion. Tests in boiling50%NaOHsolution show that three kinds of alloy690can resist to stress corrosion cracking. |
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