Electrochemical critical temperature

Temperature has been used in conjunction with electrochemical control to quantify the resistance of materials to localized corrosion. Kearns (26) has reviewed the different critical temperature tests in some detail. Electrochemical critical temperature testing consists of holding a material exposed to a solution of interest potentiostatically at a potential in its passive region while increasing the temperature of the solution either intermittently (54) or continuously (55). An example of the results of the latter type of testing is shown in Fig. 48. In this... 

Electrochemical critical temperature (ECT) testing has been evolving for more than two decades [48,50,64,73-75,79, 91-94,101] The programmed changes in applied potential and temperature that constitute the ECT method Eire schematically illustrated in Fig. 4. (Note that the corresponding changes in current density are not illustrated in Fig. 4.)... 

Electrochemical studies of redox systems in solution using superconducting electrodes at temperatures below the critical temperature are still very much in their infancy. 

Important differences also exist between plasmas and electrolyte solutions. In the latter, below the critical temperature (374°C for water), the density is not an independent variable at constant temperature, except when the system is pressurized, and even then the density can be varied only over a narrow range. Above the critical temperature, the density can be varied over a wide range by changing the volume, but, except for the work by Franck (18) and by Marshall (79), for example, on ionic conductivity, these systems are unexplored. This is particularly true for electrode and electrochemical kinetic studies. In the case of plasmas, the density may be varied under ordinary formation conditions over a wide range and, as shown in Figure 6-2, this also results in the unique feature that the temperatures of the electrons and the ions may be quite different. Another important difference between electrolytes and plasmas is the fact that free electrons exist in the latter but not in the former (an exception is liquid ammonia, in which solvated electrons can exist at appreciable concentrations). Thus, interfacial charge transfer between a conducting solid and a plasma is expected to be substantially different from that between an electrode and an electrolyte solution. The extent of these differences currently is unknown. 

G150-99, Standard test method for electrochemical critical pitting temperature of stainless steel. Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 638, Vol. 3.02. 

ASTM Standard G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels. 

ASTM G 150 (Standard Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels) describes in detail how to perform such experiments. The solution prescribed is 1 M NaCl, the suggested applied potential is +700 mV(SCE) and the starting temperature is deemed to be 0°C, with the temperature being increased at l°C/min. The CPT is defined as the temperature at which the current exceeds 100 pA/cm and remains so for greater than 1 min. The test method clearly states that alternative potentials can be used if they are within the range for which the CPT is potential independent. 

Critical pitting temperature can be rapidly obtained using electrochemical equipment to maintain a preset potential, increase temperature and detect the onset of corrosion by monitoring corrosion current [24], In ASTM G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels, a pre-set potential provides consistent conditions and a potential-independent critical pitting temperature. Sensitive detection of attack using the current allows an accelerated evaluation. 

Wang et al. [96] constructed a Na/S battery with a sodium metal anode, liquid electrolyte, and a sulfur (dispersed in polyacrylonitrile) composite cathode and tested its electrochemical characteristics at room temperature. The charge/discharge curves indicated that sodium could reversibly react with the composite cathode at room temperature. Average charge and discharge voltage was 1.8 and 1.4 V, respectively. Similar to lithium batteries, dendrite formation was noted as a critical problem for these cells. 

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