Dear Dr. Sky Xie,

the precipitation of such chromium-rich (or molybdenum-rich) particles (often carbides) usually also leads to regions depleted of Cr (or Mo) surrounding the precipitates. These regions will exhibit a lower resistance to the initiation of corrosion in the ASTM tests, and this is where the intergranular corrosion begins. Therefore, the presence of grain boundary precipitation can be elucidated from the occurrence of intergranular corrosion in the test environments. These environments are typically oxidizing and acidic e.g. ferric sulphate in sulphuric acid (ASTM G28 Method A), nitric acid (ASTM A262 Method C). However, the ASTM standards acknowledge that the environments that the material will experience in service are likely to differ from those of the test methods, in which case the corrosion performance could be different. It is possible to have grain boundary precipitation without the presence of chromium-depleted zones, and indeed such a microstructure has been shown to be beneficial in avoiding intergranular stress corrosion cracking (IGSCC) in high temperature water in nuclear industry applications, although clearly this cannot be generalized.

Source:

Applicability of ASTM G28 Testing

Briony Holmes (TWI Ltd) | Kasra Sotoudeh (TWI Ltd)

Document ID: NACE-2018-11243

Publisher: NACE International

Paper: CORROSION 2018, 15-19 April, Phoenix, Arizona, USA (2018)

Document Type: Conference Paper

Abstract available at: https://www.onepetro.org/conference-paper/NACE-2018-11243

Best regards, Pierluigi Traverso

thanks your help advice Pierluigi Traverso