Detrimental Effect of Surface Mechanical Grinding on Stress Corrosion Behavior of Monel 400 Alloy in Hydrofluoric Acid Vapor

Abstract

Surface finish or integrity is always an important problem threatening the safety service of components. In this study, the stress corrosion cracking (SCC) and crack initiation behavior of Monel 400 alloy with different surface working treatments (mechanical grinding TD specimen [the grinding direction is perpendicular to the loading direction] and PD specimen [the grinding direction is parallel to the loading direction]) and electrolytic polishing [EP specimen]) in hydrofluoric acid (HF) vapor were compared by conducting slow strain rate test (SSRT) and constant loading test (CLT). Although SSRT can evaluate the enhancement effect of mechanical grinding on SCC susceptibility, it cannot quantify its contribution to the crack initiation stage. The CLT shows the cracking initiation rate of different specimens in order of: EP specimen < TD specimen < PD specimen. For mechanical grinding specimens, local corrosion and cracks are inclined to nucleate at the groove shoulder zone rather groove bottom, which is mainly ascribed to the accumulation of higher corrosion degradation and higher defects at the groove shoulder. As the shoulder edge on the PD specimen is less constrained, it is easier to induce cracking under the synergistic action of stress concentration and preferential corrosion dissolution. The shoulder edge of the TD specimen is not a stress concentration position under tensile loading, and the microcracks formed by grinding are parallel to the loading direction, which is not conducive to the development of microcracks. Therefore, the crack initiation sensitivity of the TD specimen is lower than that of the PD specimen.