PSI - Issue 13

M. Newishy et al. / Procedia Structural Integrity 13 (2018) 353–360 M. Newishy / Structural Integrity Procedia 00 (2018) 000–000

359

7

Fig 10. The result of EDS analysis of point 5 and 6 on Fig. 7.

The pattern of the branched tree cracks observed in the microstructures of the received sheets is a regular multi branched intergranular crack of chloride stress corrosion cracking (CLSCC) as shown in Figures 11, 12. The (CLSCC) is one of the most common reasons of failures of austenitic stainless steel. This type of failure requires residual or applied stress and chloride ions, while its onset at the localized pitting corrosion initiated at the outer surface of the sheets. Therefore, cracks propagate at stress levels much lower than those required to cause normal tensile failure. In addition, the crack propagation makes almost right angle to the direction of the stresses. The pitting resistance of stainless steel in a chloride-containing environment is primarily determined by its composition while the elements that have significant effects are chromium, molybdenum, and nitrogen. The pitting resistance can be evaluated as a function of these elements by; PREN = 1 × %Cr + 3.3 × %Mo + 16 × %N (1) where, PREN is the pitting resistance equivalent number that reflects the relative pitting corrosion resistance of stainless steels. When PREN exceeds 32, the steel is considered to be chloride pitting corrosion resistant and the higher the PREN value, the more corrosion resistant the steel is 6. Conclusions and Recommendations Based on the results obtained in this investigation, the packing sheets failed due to intergranular chloride stress corrosion cracking. The root cause of the failure of the received packing sheets could be mainly attributed to the material residual shear bands after sheet manufacturing process (lack of solution annealing after cold deformation process) and presence of chloride irons (probably from water). Therefore, corrosion in this condition is mainly controlled by chloride irons and residual stresses (remaining of shear bands). It is recommended to look for a better quality material within the same grade free from residual stresses (applying solution annealing heat treatment after cold deformation of the sheets) and removing of the chloride ions by controlling the inlet water).