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Caustic cracking susceptibility of SAE A 516 Gr. 70 steel in alkaline sulphide solutions Ramakrishna, Shankar


The stress corrosion cracking (SCC) of A516 Gr. 70 steel was investigated in three solution composed of 3.35 m NaOH, 2.5 m NaOH + 0.42 m Na₂S, and 3.35 m NaOH + 0.02 m Na₂S. The electrochemical potential for maximum susceptibility was assessed by the slow strain rate testing technique (SSRT), and was found to reside in the active passive transition zone in each solution (-1 V[sub SCE] in the 3.35 m NaOH solution, and -0.88 [sub SCE] in the solution containing sulphide ions). Some secondary cracking was visible at potentials corresponding to the passive zone in the 3.35 m NaOH + 0.42 m Na₂S solution, indicating that the material was mildly susceptible to stress corrosion cracking at anodic protection potentials. Since most industrial failures have occurred, in the vicinity of welds, a series of tests with a weld incorporated in SSRT specimen was conducted to ascertain whether or not changes in microstructure affected stress corrosion susceptibility. The fusion zone of a single pass weld was found to be most susceptible to cracking in alkaline sulphide solutions, at potentials corresponding to the active-passive transition. The fracture mechanics technique, utilizing fatigue precracked to study the effects of stress intensity, electrochemical potentials, microstructure, and heat treatment, on crack propogation rates in the 3.35 m NaOH + 0.42 m Na₂S solutions. Both stress intensity dependent (regions I and III) and stress intensity independent (region II) cracking propogation behavior was observed. Region II crack velocities of the order of 4 x 10⁻¹⁰ m/sec were observed at -0.88 V[sub SCE], and 2 x 10⁻¹⁰m/sec at -0.75 V[sub SCE]. No significant change in region II crack velocity was observed when the base material was subjected to a simulated stress relief anneal (650°C for 1 hr.) and tested at -0.88 V[sub SCE]. The region II crack velocity through a material with a dendritic microstructure (fusion zone of a weld) was found to be approximately 1 x 10⁻¹⁰ m/sec. A mechanism for failure due to coalescence of cracks and not due to the penetration of a single crack through the wall, has been suggested. The applicability of anodic protection in prolonging the service life of digesters has been examined. Although no experiments were conducted to determine the mechanism of crack propogation, hydrogen embrittlement has been ruled out as a possible mechanism contributing to failure. The results obtained are expected to find applications in the pulp and paper industry.

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