Size effect of scratches on the degradation behavior of alloy 690TT in high temperature caustic solution

Abstract

Size effect of scratches on the mechanical properties and degradation behavior of Alloy 690 T T were investigated in high temperature caustic solution using scanning electron microscopy (SEM) with energy dispersive X-ray (EDS) detector and electron back-scattered diffraction (EBSD) detector, and a quantitative dependent relationship of corrosion and stress corrosion cracking (SCC) behavior of scratched surface on the scratch size factor α (the ratio of scratch depth to scratch tip radius) was established. The results showed that the corrosion behavior of Alloy 690 T T was aggravated by the accelerated growth of inner oxide layer on scratched surface, which was due to the high strain zone formed by scratching process and tension stress. The thickness distribution of inner oxide layer on scratched surface was approximately described by the Gaussian function, while that of surface oxide layer followed a linear distribution. Additionally, the inner oxide layer thickness at scratch tip increased linearly with the increase of α, while that of surface oxide layer wasn’t related to α. The SCC at the scratch tip did not take place until α increased to a critical threshold value. The surface oxide particles near the crack mouth became sparse and the corresponding inner oxide layer became thinner, which was mainly affected by the low stress zone formed near the crack mouth.