Corrosion in the stainless steels and other alloys tends to begin at the sites of discontinuities of the passive film and propagate into stable pits, especially in the presence of chloride-containing environments. Under the test, the lowest temperature at which propagating pitting is observed under a constant anodic potential in a standard electrolyte is called the critical pitting temperature or CPT. The common test procedure ASTM G150 offers an electrochemical procedure to calculate CPT in a reproducible manner, which offers alloy developers and corrosion engineers an analogue measure of pitting resistance. The process is applicable to wrought or cast stainless steels (plate, sheet, tubing, bar, forgings, welds) possessing the corrosion-resistance of foundry-grade UNS S31600 down to UNS S31254 (6% Mo) and can be further widened to other alloys on a case-by-case basis by varying the parameters.
ASTM G150 Test Method
| Immersion and stabilization of the specimen | The prepared specimen is immersed in 1 M sodium chloride (NaCl) solution, which is maintained near 0 °C, and allowed to stabilize in the electrolyte in order for a uniform passive surface to be established before applying the potential. |
| Temperature ramp and potential application | The sample is usually exposed to a constant anodic potential, usually in the order of +700 mV vs SCE, for approximately 60 seconds before the temperature ramp starts. Subsequently, the temperature is raised with a controlled heating rate, typically around 1 °C per minute, while current density is recorded. |
| Determination of critical pitting temperature (CPT) | The CPT is defined as the temperature at which current density exceeds 100 µA/cm² and remains above the value for at least 60 seconds under the applied potential. The test concludes with a visual confirmation of stable pitting without the incidence of crevice corrosion. |
