Study of fatigue crack nucleation in 12% chromium stainless steel in sulphate and phosphate solutions

Abstract

An investigation of the morphology of fatigue crack nucleation has been conducted on 12%Cr stainless steel at a single stress level of 448 MN m⁻² in deaerated H₂SO₄–Na₂SO₄–NaOH ([SO²⁻ ₄] = 0·01M and 1M) and in H₃PO₄–NaOH ([PO³⁻ ₄] = 0·01M and 1M) solutions at pH 2, 7, and 10 at 100°C. The results of this investigation showed that the fatigue crack initiation processes are highly dependent on the corrosion behaviour of the steel. The corrosion fatigue life in sulphate solutions was shorter than that in phosphate solutions. In sulphate solutions at pH 2, the specimen suffered overall surface attack without the initiation of microcracks. The surface attack intensified at the prior austenite grain boundaries, martensite platelet boundaries, and non-metallic inclusions. In contrast, cracks initiated transgranularly in sulphate solutions at pH 7 and 10 with the development of selective attack at crack sites at later stages in the fatigue life. This attack was ascribed to local acidification as a result of hydrolysis of corrosion products extruded from inside the cracks. Long cracks with crack branching and debonded zones were observed in all phosphate solutions, except in 0·01M phosphate solution at pH 7. In this latter environment, short intrusion-extrusion microcracks were observed. This difference in behaviour was attributed to changes in the characteristics of the passive film as influenced by the local environmental conditions, mechanical rupture by plastic deformation, and electrochemical differences at the metal surface.