Austenitic stainless steels passivity high-alloyed

An interesting aspect of the expression (10) concerns the case of metals and passive alloys because the real polarization potential exhibits a discontinuity around the zone of transition from active to passive state. In fact, if Ip denotes the passivity current density, the value of the discontinuity is of the same order of magnitude as R,IpS because during this transition the current intensity falls very rapidly. The discontinuity may be very pronounced because the values of Ip, which depend on the type of metal, the environment and temperature, may be very high. In the case of the AISI 321H titanium-stabilized, austenitic stainless steel in 1 M HCIO4 -1- 0.3 M NaCl solutions at 25 °C, the value of Ip depends on the thermal history of the specimen [50]. In meiny instances it was found to be about 10 mAcm . ... 

Fe-Cr and Fe-Cr-Ni alloys are of high technical importance, the main benefit for ferritic and austenitic stainless steels resulting from the excellent corrosion resistance of Cr203 layers. Figure 5.31 shows the polarization curve of Fe-15 Cr in 0.5 M H2SO4 [92]. Its characteristic features are determined by the electrochemical properties of the pure alloy components. Hydrogen evolution (with cathodic currents) is observed up to E = -0.2 V followed by the potential range of active dissolution of Cr and Fe + up to OV where passivity starts due to... 

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