Immersion critical pitting temperature

Determination of the critical pitting or critical crevice temperature is often used as an alternative to comparing the severity of attack on several samples [2f-23], Procedures for this testing are described in Methods C and D of ASTM G 48. Specimens are immersed for a given period (e.g., 72 h) at a temperature that is not expected to cause attack. Fresh specimens are then exposed at 5°C increments for the same duration until the temperature at which attack occurs (critical temperature) is determined. For many materials, the critical temperatures of replicate specimens fall within a 5°C variation. Retesting of the same specimen can affect the critical temperature [24]. Testing may be initiated just below the critical temperature, if it can be estimated. If not, this technique can require many specimens and a few weeks to find the critical temperature. 

The onset of pitting corrosion occurs suddenly If one performs electrochemical experiments with stainless steel, e. g. by applying a constant electrical potential to a sample immersed in dilute NaCl solution, the electrical current - which is an indicator for chemical activity (corrosion) on the metal surface - is low over a wide parameter range. But if critical parameters like temperature, potential, or electrolyte concentration exceed a certain critical value, the current rises abruptly and the metal surface is severely affected by pitting corrosion. The transition to high corrosion rates is preceded by the appearance of metastable corrosion pits. 

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