Investigations Using Smooth and Notched Specimens into Validity of Caustic Cracking Susceptibility Diagram
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INTRODUCTION
STRESS corrosion cracking (SCC) is considered to be the most dangerous form of corrosion-assisted failure, which is caused by a simultaneous action of stress and corrosive environment for a material susceptible to SCC.[1] Corrosion cracks may propagate undetected to a leak or sudden failure. Undetected SCC of in-service components has been responsible for a long time for major safety concerns, loss in production time, and cost of the maintenance of materials in major industries, such as SCC in a caustic environment (which is called caustic cracking), in alumina processing, and in pulp and paper industries.[2–9] Caustic concentration can vary considerably depending on the application in different industries and, for that matter, at different processing steps of the same industrial application. Caustic cracking occurs across a wide range of caustic concentrations[1–13] and temperatures (373 K to 623 K (100 °C to 350 °C)).[14] Figure 1 shows a typical caustic cracking susceptibility (caustic concentration–temperature) map.[11,13,14] This map suggests that caustic cracking can occur at temperatures as low as 313 K to 323 K (40 °C to 50 °C) in highly concentrated (30 pct to 50 pct) caustic solutions. The susceptibility diagram in Figure 1 can be divided into the following parts: The hatched upper area defines the certain caustic cracking zone, the area below lower curve is the area of no caustic cracking, and the area between the lower curve and the hatched upper area defines the potential caustic cracking zone. However, the upper hatched area is suggested to have been developed based on laboratory test results,[11,14] whereas the lower curve is based on field experience.[13,14] Caustic cracking of R.K. SINGH RAMAN, Associate Professor, is with the Departments of Mechanical and Aerospace Engineering and Chemical Engineering, and SARVESH PAL, Postgraduate Student, is with the Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia. Contact e-mail: raman.singh@ eng.monash.edu.au Manuscript submitted November 28, 2009. Article published online June 2, 2010 2328—VOLUME 41A, SEPTEMBER 2010
in-service components is highly likely to be influenced by the sharp notches and other stress raisers that often are present in the fabricated components. Therefore, the lower curve in Figure 1, which was developed based on the field experience of caustic cracking, could have received contribution from the notches that are often present in the in-service components. However, it is important to note that the research programs for alumina and pulp-and-paper industries (where caustic cracking is a concern) have used both smooth and notched specimens.[9,15,16] It may be necessary to investigate the validity of the caustic cracking susceptibility diagram when smooth and notched specimens are tested, as reported in the current study.
II.
EXPERIMENTAL PRINCIPLES AND TECHNIQUE
Tests were conducted on rolled commercial mild steel (AS/NZ 3678-grade 250). The chemical composition and mechanical proper
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