Distributions of Cr Depletion Levels in Sensitized AISI 304 Stainless Steel and Its Implications Concerning Intergranular Corrosion Phenomena

Abstract

An electrochemical method has been employed to obtain the population distribution of minimum grain boundary Cr concentrations for sensitized AISI 304 stainless steel ($30400). The detailed information about the extent of Cr depletion provided by the distribution is compared to the average degree of sensitization (DOS) evaluated by the standard and a modified single loop-electrochemical potentiokinetic reactivation (SL-EPR) technique. The distribution of Cr levels on a grain by grain basis is shown to provide information about intergranular stress corrosion cracking (IGSCC) susceptibility that cannot be provided by EPR methods. Sensitized 304SS tested in an environment relevant to nuclear reactors has been shown to fail by IGSCC when the grain boundary Cr concentration is depleted to below a critical level of =13.5%. In a separate study, it has been shown that more than 23 % of the grain boundaries must be depleted to the relevant critical level in order to observe macroscopically brittle behavior. The present study combines these two criteria and defines a sensitized material's IGSCC susceptibility by revealing the extent of Cr depletion as well as the quantity of depleted grain boundaries.

Case reviews on the effect of microstructure on the corrosion behavior of austenitic alloys for processing and storage of nuclear waste

“…Only when the fraction of random grain boundaries increased beyond 75 pct was the beneficial effect in sensitization, IGC, and IGSCC observed. It has been shown by the bond percolation theory [29] that there is a continuous connectivity of grain boundaries of a particular type (or sensitized boundaries) when the fraction of that type of boundary increases beyond 75 pct. At 75 pct of the boundaries, the random boundaries are continuously connected together by random boundaries.…”

“…This was confirmed by measurements with a ferrite meter, which showed FN numbers greater than 30. It has been shown that depletion of chromium in alloy 800 [28] and in alloy 600 [29] makes the materials magnetic, and measurements of magnetic susceptibility of these alloys have been correlated [31,32] to degree of sensitization and also to levels of chromium in the chromium depletion zones at grain boundaries. These results show that chromium is selectively removed from the surfaces of alloy 690 in contact with molten glass at 950 °C to 1000 °C.…”

Alloy Design to Prevent Intergranular Corrosion of Low-Cr Ferritic Stainless Steel with Weak Carbide Formers

“…2,4,13,[26][27][28] The DL-EPR test was conducted in 0.5% H 2 SO 4 solution with 0.0013 wt% KSCN at 30…”

DEIS evaluation of the relative effective surface area of AISI 304 stainless steel dissolution process in conditions of intergranular corrosion

“…Intergranular corrosion of austenitic stainless steels has been explained by the chromium depletion theory [6,7]. It has been shown that the extent of chromium-depleted zones at grain boundaries is a direct indication of the material's susceptibility to IG process, and in the presence of residual stresses and strain to IGSCC as well in the final stage [8].…”

“…According to the ASTM standard, metallographic etching and weight-loss tests are regularly performed to assess degree of sensitization (DOS) of austenitic stainless steels (SS) [7][8][9][10][11][12][13][14][15]. These evaluation techniques suffer from either a long testing period or requirement of a large amount of etchant.…”