Revisiting Stress Corrosion Cracking of Steel in Caustic Solutions for Developing Cracking Susceptibility Diagrams for I
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INTRODUCTION
IN the Bayer process, steel is the commonly used material for construction of reaction vessels and pipes for different processing components for extraction of alumina from bauxite ores, such as digesters, decomposers, and precipitators.[1–11] Caustic embrittlement continues to be a concern for steel pipes and vessels in hot caustic service; however, low carbon steels is still the most frequently used material.[12] Berk and Waldeck[13] and Mazille and Uhlig[14] investigated the limits of caustic concentrations and temperatures that can cause embrittlement. For example, steels were found to show immunity to caustic embrittlement at 5 wt pct NaOH when temperatures were below 368 K (95 C), whereas this temperature limit was 313 K (40 C) at 50 wt pct NaOH. A caustic cracking susceptibility (CS) diagram (Figure 1)[12] is a plot of caustic concentration against temperature, based on industrial as well as laboratory tests for a maximum of 62 days. This diagram identifies different regions of CS. The upper hatched area is the severe cracking zone, whereas in the regime immediately below this area, the cracking may or may not take place. For example, the CS diagram suggests that the caustic concentration between 15 and 40 wt pct is most likely to SARVESH PAL, formerly Postdoctoral Student with the Department of Mechanical and Aerospace Engineering, Monash University, Clayton, VIC 3800, Australia, is now a Postdoctoral Fellow with the Department of Mechanical and Mining Engineering, The University of Queensland, Brisbane, OLD 4072, Australia, and is also with the CRC for Rail Innovation, Brisbane, OLD 4072, Australia. R.K. SINGH RAMAN, Professor, is with the Department of Mechanical and Aerospace Engineering, Monash University, and is also with the Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia. Contact e-mail: [email protected] R.N. IBRAHIM, Associate Professor, is with the Department of Mechanical and Aerospace Engineering, Monash University. Manuscript submitted July 15, 2011. Article published online January 27, 2012 1944—VOLUME 43A, JUNE 2012
cause caustic cracking at 373 K (100 C), whereas for the same concentration range, the caustic cracking may or may not take place if the temperature is below 353 K (80 C). The upper hatched area was developed in the laboratories,[12] whereas the lower curve is based on the information from field experience.[15] Understanding of CS is less clear in the regions between the upper hatched area and the lower curve. By developing an improved understanding of the effect of caustic concentration and temperature on caustic cracking, it may be possible to expand the limits for safe application of carbon steel in various steps of the Bayer process.[16] Caustic cracking of in-service components is highly likely to be influenced by the sharp notches and other stress raisers that are often present in the fabricated components. Therefore, to investigate the validity of the CS diagram, it may be more practical to test the notched and precracked s
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