Abstract
The effect of grain size in the range 72 to 190 μm and carbon content in the range 0.105–0.073 wt.% on the intergranular corrosion of the austenitic stainless steel 301 has been investigated. Grain boundary chromium depletion has been studied directly using energy dispersive X-ray spectroscopy combined with scanning transmission electron microscopy and indirectly using double loop electrochemical potentiokinetic reactivation tests. In addition, chromium depletion has been modelled using the CALPHAD Thermo-Calc software TC-DICTRA. It is shown that the degree of sensitization measured using the double loop electrochemical potentiokinetic reactivation tests can be successfully predicted with the aid of a depletion parameter based on the modelled chromium depletion profiles for heat treatment times covering both the sensitization and de-sensitization or self-healing. Additionally, along with intergranular M23C6 carbides, intragranular M23C6 and Cr2N nitrides that affect the available Cr for grain boundary carbide precipitation were also observed.
Highlights
Austenitic stainless steels have excellent corrosion resistance
Heat treating austenite results in significant microstructural changes in the temperature ranges 500–900 ◦ C that depend on the chemical composition, grain size and the exposure time
An austenitic stainless steel with the composition Fe-16.80Cr-6.36Ni-0.105C-0.06N has been solution treated at 1100 ◦ C and 1200 ◦ C and quenched to produce various grain sizes in the range
Summary
Austenitic stainless steels have excellent corrosion resistance They are susceptible to intergranular corrosion, that is, sensitization when exposed to the temperature regime 500–850 ◦ C [1,2,3]. This is due to the grain boundary precipitation and growth of chromium carbides that cause the formation of Cr depleted zones, resulting in intergranular corrosion. The degree of sensitization is not directly correlated to the amount of carbide precipitation It depends on the details of the chromium concentration profiles in the vicinity of the grain boundaries. The composition of the electrolyte, temperature, potential limits and scan rate need to be determined for each alloy
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