Austenitic activity measurement

An analysis of the carbon activity measurements in the y austenite made by [1992Ball] was carried out by [2004Sho] for modelling the "y(Fe,Ti)C sohd solution. The model in which Ti atoms are bonded to C was proved to be more realistic than that in which Ti atoms are randomly distributed over the metal sublattice and more in agreement with Ti-C interactions stronger than C-Fe interactions. 

The stabilised austenitic stainless steels for cladding contain an alloying element (niobium), which forms stable grain boundary carbides. This prevents chromium depletion along the grain boundaries and makes the material immune to stress corrosion cracking. Non-stabilised material was used for the first layer because the thermal expansion coefficient of the material is closer to that of the low-alloy pressure vessel material. The presence of niobium in the second layer allows performance of so-called retrospective dosimetry in the RPV inner surface by machining out some scraps for further chemical separation and activity measurement to determine real neutron fluence on the RPV inner surface. 

Reactions of considerable extent occur even in very dilute solutions of non-metals in alkali metals. Small concentrations of dissolved non-metals also influence physical properties of the molten metals. Very exact analyses are necessary to define the chemical potentials of non-metals in alkali metals. Oxygen can be removed from sodium, for instance, to such a degree, that only 0.1 to 0.01 wppm remain in solution. Electrochemical cells have the ability to estimate such extremely low concentrations. Carbon in liquid alkali metals, which are in contact with austenitic stainless steels, is in the same range of concentrations. Thus, only activity meters, based on gasanalytical devices or electrochemical cells, are able to measure such low carbon concentrations. There is still need for the development of analytical procedures to estimate nitrogen in alkali metals with the same sensitivity and accuracy. 

Pil] Pillai, S.R., Mathews, C.K., Measurement of the Carbon Activity of 18/8 Austenitic Steel and the Free Energy of Formation of the Metal Carbides Fe Cr23 cC6 , High Temp.-High Pressures, 20(3), 263-270 (1988) (Thermodyn., Experimental, 12)... 

H2-CH4 gaseous mixtures may be used to measure carbon activity in steels [1969Bun] by equilibrating the carbon potential in gaseous and solid phases. The kinetics of carburization and decarburization of steels with methane according to the reaction CH4 C (dissolved in Fe) + 2 H2 was measured at 928°C by [1965Gra] on austenite and at 786°C by [1973Gra] on the ferrite. The kinetics of the carburization at... 

Numerous investigations have been done regarding the liquidus surface, die isothermal sections and the vertical sections in the stable and metastable systems. The other investigations on die ternary system concern the solubility measurements of carbon in the "y and liquid phases which go always widi activity measme-ments, the determination of the phase diagram under high pressures and die kinetics studies of die austenite transformation in martensite or bainite because these phases are important in die forecast of mechanical properties of steel. The main experimental investigations on crystal structure, phase equilibria and thermodynamics are gathered in Table 1. 

As mentioned before, austenitic stainless steels are susceptible to IGC due to sensitization caused by exposure to high temperatures (450-850 C). The IGC of austenitic stainless steel can also be characterized by normalized classical tests ASTM G28, ASTM A262-86, SEP 1877, AFNOR A05-159 and AFNOR A05-160, currently known as the Strauss, Huey and Streicher tests [54-57]. These methods however are destructive, difficult to perform on site and require sampling that can be harmful to the integrity of materials during service. For this reason, the electrochemical, non-destructive tests commonly known as EPR (electrochemical potentiokinetic reactivation) and DL-EPR (double loop electrochemical potentiokinetic reactivation) were developed to measure the sensitivity of austenitic stainless steels to IGC [58-66]. However, EPR and DL-EPR are based on measurements of characteristic potentials and currents of passive/active zones on potentiody-namic curves in an aqueous solution (linear voltammetry curve from oxygen to hydrogen evolution in the... 

Active Peak Current as a Function of pH Several authors [3,103,108] use the measurement of the active peak current below the activation pH to compare the propagation kinetics of crevice corrosion on different materials. Figure 42 shows an example of this kind of measurement. Notice that the peak current of the duplex stainless steels (UR 47, 31803, 31603) is much higher at low pH values than that of austenitic grades (304 and 904L). This is consistent with the beneficial effect of nickel on the crevice propagation rate mentioned earlier. 

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