Critical pitting temperature defined

Pitting tendency increases with increasing temperature, with the result that the critical pitting potential decreases as temperature increases. The critical pitting temperature (CPT) is defined as the temperature below which an alloy does not... 

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. 

The critical pitting temperature (CPT) is defined at what temperature pitting occtus. A common range for stainless steels is 10-100 C and obviously depends on the alloy composition. [Pedrazzoli Speidel, 1991] has reported that the critical temperature for crevice corrosion (CCT) is appirox. 20 ° lower compared to pitting, see fig.l6 17 for details. 

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