Degradation of stainless steel grids in chemically aggressive environment

The present study concerns the failure analysis of a perforated austenitic stainless steel grid, operating in a shell-and-tube heat exchanger of a petrochemical industry. Macroscopic examination of the grid indicated extensive friability and severe cracking in a direction perpendicular to its normal loading, while both grid surfaces as well as the interior of the filtration holes were covered significantly by decayed deposits. Microscopic examination of selected grid areas, after the surface deposits removal, indicated severe cracking exhibiting multiple branching, which advocates for stress corrosion cracking. Besides the extensive cracking areas, voids surrounded by twinning and slip bands were observed. Elemental microanalysis carried out in the areas around voids indicated the presence of iron and chromium at proportions that can be correlated to the formation of σ-phase. The detection of oxygen, iron and chromium within the cracks is attributed to corrosion products consisting of a mixture of iron and chromium oxides. The premature catastrophic failure of the stainless grid occurred as a synergistic effect of these distinct root-causes. Potential substitution of the currently used stainless steel with another alloy of higher resistance in stress corrosion cracking and microstructure stability at high temperatures is suggested.