Why does Chloride stress corrosion cracking and PASCC only happens in Austenitic SS?
Answers
|
24/05/2016
|
A:
|
Ravi Kiran Kandikatla, Process Engineer, ravikiran204@gmail.com
Austenitic steels (general 300 series) contain around 18% Cr, 8% Ni and 0.03-0.1 wt% Carbon content. Due to high Cr and Ni contents, austenitic steels are highly resistant to corrosion. However, exposure to higher temperatures (more than 400°C) and presence of Carbon causes Chromium Carbide formation and leads to grain boundary precipitation. Depletion of Cr in steels promotes corrosion adjacent to grain boundaries. To avoid Chromium Carbide formation, Carbon content in steel is controlled to lower values, as low as 0.03 wt%. In stabilized grades of Austenitic steel, Titanium/Niobium are added which have stronger affinity to form Carbides than Chromium, thus protecting corrosion resistance. Presence of Cr, Carbon, Exposure temperatures and presence of Polythionic Acids play an important role in stress corrosion cracking.
However, to avoid SCC in Austenitic SS steels, exposure to conducive environment (Polythionic Acid formation in presence of Oxygen/Water/Chlorides along with Metal Sulfides/H2S) to be reduced by adopting mitigation methods such as Alkaline washing, dry Nitrogen purging etc. NACE Standard Practice-0170 will be helpful to understand the requirements of protection for Austenitic steels.
|