Assessment of Intergranular Corrosion of Heat Treated Austenitic Stainless Steel (AISI 316L Grade) by Electron Microscopy and Electrochemical Tests1
A. Kriaa, N. Hamdi, H. Sidhom
a Departement de Chimie, Ecole Superieure des Sciences et Techniques de Tunis,
Rue Taha Hussein-Montfleury Tunis, Tunisia
b Unite de recherche sur les materiaux, Institut National de Recherche Scientifique et Technique,
Tunis, BP 95-2050 Hammam lif, Tunisia
c Laboratoire de Mecanique et Materiaux de l’ESSTT, 5 avenue Taha Hussein 1008,
Montfleury, Tunis, Tunisia
Received June 23, 2007
Abstract — The resistance of austenitic stainless steel to intergranular corrosion (IGC) varies during the process of aging at temperatures between 500 and 700
C. This follows the well-known phenomena of precipitating of M23C6 chromium carbides and intermetallic phases (
, 
, 
). Consequently, this leads to significant Cr-depletion zones at grain boundaries responsible for material sensitization to IGC. The assessment of the sensitivity to IGC from the Strauss or equivalent tests requires cutting a sample off the material, which can be harmful to the integrity of the structure in service. Such a sampling is in essence only qualitative and insufficiently sensitive to the low widths of Cr-depletion at the beginning of precipitation. The DL-EPR method (Double-Loop Electrochemical Potentiodynamic Reactivation test) is known to be a non-destructive and quantitative test method of detecting relatively mild degrees of sensitization in austenitic stainless steel. The current ratios Ir/Ia > 1% (sensitization criteria) and, as a consequence, the electric charge ratios Qr/Qa > 1% of the degree of sensitization (DOS) to intergranular corrosion can be considered as good parameter values to differentiate materials with only difference in the DOS and to detect the fine precipitation responsible for the depletion in the elements of an alloy. This criteria is also valid for the detection of desensitization during aging for longer periods of time.
C. This follows the well-known phenomena of precipitating of M23C6 chromium carbides and intermetallic phases (, , ). Consequently, this leads to significant Cr-depletion zones at grain boundaries responsible for material sensitization to IGC. The assessment of the sensitivity to IGC from the Strauss or equivalent tests requires cutting a sample off the material, which can be harmful to the integrity of the structure in service. Such a sampling is in essence only qualitative and insufficiently sensitive to the low widths of Cr-depletion at the beginning of precipitation. The DL-EPR method (Double-Loop Electrochemical Potentiodynamic Reactivation test) is known to be a non-destructive and quantitative test method of detecting relatively mild degrees of sensitization in austenitic stainless steel. The current ratios Ir/Ia > 1% (sensitization criteria) and, as a consequence, the electric charge ratios Qr/Qa > 1% of the degree of sensitization (DOS) to intergranular corrosion can be considered as good parameter values to differentiate materials with only difference in the DOS and to detect the fine precipitation responsible for the depletion in the elements of an alloy. This criteria is also valid for the detection of desensitization during aging for longer periods of time.
PACS numbers: 81.65.Kn; 81.65.Rv
DOI: 10.1134/S0033173208050172