Corrosion behavior of austenitic alloy 690 under anodic and cathodic potentials

Abstract

The corrosion behavior of austenitic alloy 690 in a solution-annealed condition has been evaluated with the application of anodic as well as cathodic potentials in an acidic chloride solution at room temperature (RT). In a 0.5M H₂SO₄ + 0.5M NaCl solution, the alloy displayed active-passive pitting behavior with the application of an anodic potential. Surface films, formed at the onset and later stage of the passive region, were characterized using X-ray photoelectron spectroscopy (XPS). The XPS revealed that the surface film formed at the onset of passivity (+ 100 mV SCE) consisted of Cr(OH)₃, without any Fe⁺³/Fe⁺². The presence of nickel in the film was found in a transition state of Ni⁺² and Ni⁰. The passive film formed at the higher anodic potential (+ 700 mV SCE) consisted of Cr₂O₃ without any Fe⁺³/Fe⁺² or even Ni⁺²/Ni⁰. Microscopic studies of alloy 690 after anodic polarization in an acidic chloride solution revealed pitting, which was found to be initiated at large, faceted TiN-type inclusions. The susceptibility of the alloy to hydrogen embrittlement has been investigated by conducting cathodic charging of the tensile samples in a 0.5M H₂SO₄ solution at RT and by subsequent tensile testing of the charged samples in air at a strain rate of 1.3 × 10^-4 s^-1 up to fracture. An indication toward hydrogen-induced ductility loss was noticed for the samples of the alloy, which is believed to be attributable to a hydrogen-enhanced microvoid growth process. Since the microvoid growth process occurs at the last stage of fracture, the effect of hydrogen on the ductility of the alloy is little.