Dear  Ineeass

You are describing the phenomenon of sensitization of the stainless steel which leads eventually to intergranular  corrosion.   For a steel alloy to become a stainless steel, the steel alloy should have at least 12% Cr.   The 12% of Cr  is an essential percentage for the stainless steel alloy to attain passivity against a chemical attack, i.e., corrosion, in the surrounding environment. Therefore, if the stainless steel is reheated, between  950 to 1450 0C, for some reasons, i.e.,  annealing, welding , and so on, and the 12% Cr has been depleted ,as a chromium carbide, from the grain boundaries of the stainless steel to  adjacent areas of  grain boundaries.   Consequently, the grain boundaries become more susceptible to corrosion than the adjacent areas of grain boundaries, in the surrounding environment.    One can think of the above as a Galvanic coupling corrosion occurs between  the grain boundaries ,depleted of chromium carbide, of the stainless steel and   adjacent areas , rich in chromium carbide, of grain boundaries.          

Dear Ineeass,

This is a known phenomenon in welded stainless steel. Usually carbon is bonded to different steels' stabilizers such as Ti or Nb. However, when sufficient heating appear, such as in the case of welding, chromium carbides can be formed, leading to depletted zones of chromium, especially near the fusion line. This event may caused intergranular corrosion. Cr is responsible for the passivation layer (stainless) of the steel.

For preventing the corossion product, a heating treatment needs to perform usually to about 1065oC. 

Thank you 

You may find the following links useful to study Intergranular corrosion

Microstructure and intergranular corrosion of the austenitic stainless steel 1.4970 (PDF Download Available)

https://www.researchgate.net/publication/222429590_Microstructure_and_intergranular_corrosion_of_the_austenitic_stainless_steel_14970

Inter granular corrosion of duplex stainless steel

http://www.arv-offshore.com/downloads/arv09.pdf

Intergranular corrosion - facts and how to reduce the risk — Sandvik Materials Technology

http://smt.sandvik.com/en/materials-center/corrosion/wet-corrosion/interngranular-corrosion/

Article Microstructure and intergranular corrosion of the austenitic...

Dear  Ineeass

You are describing the phenomenon of sensitization of the stainless steel which leads eventually to intergranular  corrosion.   For a steel alloy to become a stainless steel, the steel alloy should have at least 12% Cr.   The 12% of Cr  is an essential percentage for the stainless steel alloy to attain passivity against a chemical attack, i.e., corrosion, in the surrounding environment. Therefore, if the stainless steel is reheated, between  950 to 1450 0C, for some reasons, i.e.,  annealing, welding , and so on, and the 12% Cr has been depleted ,as a chromium carbide, from the grain boundaries of the stainless steel to  adjacent areas of  grain boundaries.   Consequently, the grain boundaries become more susceptible to corrosion than the adjacent areas of grain boundaries, in the surrounding environment.    One can think of the above as a Galvanic coupling corrosion occurs between  the grain boundaries ,depleted of chromium carbide, of the stainless steel and   adjacent areas , rich in chromium carbide, of grain boundaries.          

Dear Ineeass

Sensitization is explained by Khaled but you may find the following links useful. 

John 

https://www.corrosionpedia.com/definition/1334/sensitization-stainless-steel

The complete range of different stainless steel grades are shown in the link below.

http://www.outokumpu.com/sitecollectiondocuments/outokumpu-steel-grades-properties-global-standards.pdf