The effect of environment, chemistry, and microstructure on the corrosion fatigue behavior of austenitic stainless steels in high temperature water

Abstract

The effect of sulfur on the corrosion fatigue crack growth of austenitic stainless steel was evaluated under Light Water Reactor (LWR) conditions of 288°C deaerated (less than 5ppb O₂) water, to shed light on the accelerating effect of the LWR environment and to explore the effect of high sulfur content on the retardation of fatigue crack growth rates. Fatigue tests were performed using a trapezoidal loading pattern with rise times of 5.1, 51, 510, and 5100 seconds (fall time of 0.9, 9, 90, and 900 seconds), with Kmzx of 28.6 or 31.9 MPa[mathematical symbol]m and stress ratios (R, Pmin/Pmax) of 0.4 or 0.7. Two test materials were used to evaluate the effect of sulfur: (1) a low sulfur (<0.0025 wt%) stainless steel and, (2) a high sulfur (0.032 wt% stainless steel. The low sulfur stainless steel exhibited increasing crack growth rates from 9.4 x10-5 mm/cycle to 1.2x1 0-⁴ mm/cycle as rise times were increased from 5.1 to 5100 seconds with a stress ratio of 0.7. The high sulfur stainless steel exhibited decreasing crack growth rates from 1.4 x10-⁴ mm/cycle to 7.9 x10-⁵ mm/cycle as rise times were increased for a stress ratio of 0.4, and crack growth rates from 6.4 x10 5 mm/cycle to 3.6 x10-⁵ mm/cycle with increasing rise time at a stress ratio of 0.7. Evaluation of the crack growth rates showed environmental enhancement of the crack growth rates for the low sulfur stainless steel, while the high sulfur stainless steel showed retardation of environmental crack growth rates, likely due to the increased corrosion at the crack tip associated with the high sulfur content. The crack surfaces were characterized using Scanning Electron Microscopy (SEM). The low sulfur material showed a light layer of corrosion product that decreased in thickness as the testing progressed, and faceting on the surface was highly crystallographic. Faceting ran both perpendicular and parallel to the crack for the short rise time steps of the test, but fewer perpendicular facets were evident at the longer rise times. The high sulfur material was heavily corroded throughout the fracture surface, and crystallographic faceting was seen for stages of the test with R=0.4 For R=0.7, the heavy oxidation on the surface made the facets hard to resolve. Striations were apparent during the 5100 second rise time for the low sulfur material (where corrosion was almost nonexistent) and throughout the entirety of the crack surface for the high sulfur material. Materials were also characterized by optical microscopy. The low sulfur material showed pitting along the grain boundaries, due to the boron concentration in this material, which resulted in boron precipitates, while the high sulfur material showed pitting throughout the surface, due to the MnS inclusions. Electrochemical tests were also performed at room temperature on both materials in pH 4 (using H₂SO₄), 7, and 10 (using NH₄OH). Peaks in the passive region of the high sulfur material were seen at potentials of 160, 630, and 1400 mVSHE, due to dissolution of the MnS inclusions. The results suggest that the high sulfur material provides an increase in corrosion when exposed to the environment, which leads to the retardation of crack growth rates at the longer rise times due to prolonged exposure of the crack tip to the environment. At low stress ratios, the proposed mechanism for retardation of crack growth rates is crack tip closure, due to a buildup of corrosion product at the fracture surface, which lowers the effective load that the crack tip experiences. At high stress ratios, the proposed mechanism for retardation is an increased in injected vacancies and enhanced creep, which disrupt the slip bands ahead of the crack tip, reducing the crack tip stresses. Fractography of the fracture surface and crack growth rate comparisons of the low and high sulfur material provide supportive evidence for the proposed mechanisms, and further work is proposed to examine the effect of increased corrosion ahead of the crack tip.