Intergranular corrosion for Stainless Steel

Intergranular corrosion for Stainless Steel

Austenitic Stainless Steel

For austenitic stainless steel, the cause of intergranular corrosion is precipitation of chromium carbides. At temperature above 1035^oC, chromium carbides are completely dissolved in austenitic stainless steel. However, the steels are slowly cooled to 425~815^oC, chromium carbides are precipitate at grain boundary. This causes the depletion of chromium content in the matrices. The depleted zone is more susceptible to corrosion, which is termed as sensitization. This leads to integranular corrosion of austenitic stainless steel.  The sensitization can be catalysed with cyanide, thiosulfate ions.

Heat treatment with fast cooling below 425^oC can prevent the precipitation of chromium carbides, which make it exemption to the integraular corrosion. However, stress relieving heat treatment, which reheats alloy to 425~815^oC, will precipitate the carbides and result in integranular corrosion. For Austenitic stainless steel, intergranular corrosion at some distance from the weld, since the peak temperature of welding is around 675^oC.

Ferritic Stainless Steel

For Ferritic stainless steel, the cause of intergranular corrosion is precipitation of chromium nitride as well as chromium carbides. Due to the low solubility of nitrogen in ferritic stainless steels, the depletion of chromium can be caused by the precipitation of chromium nitrides as well. The sensitization temperature for normal ferritic stainless steel is much higher than that of austenitic stainless steel ( 925^OC vs. 425~815^oC). This explains that  the fusion zone and the weld itself of ferritic stailess steel are the most susceptible to the integranular corrosion. The normal heat treatment at 700~925^OC can re-diffuse the chromium back into depleted zone, which will de-sensitize the ferrtic stainless steel.

Intergranular corrosion Control

Austenitic Stainless Steel

• Restrict carbon content to less than 0.03%

This will limit the amount of chromium carbides. However, this is not effective for long term exposure at 425~815^oC service temperature.

• Microalloy with titanium and/or niobium

Titanium and niobium are introduced to form more stable carbides to prevent the depletion of chromium.

• Anneal the austenitic stainless steel prior to welding to remove cold work stress, which accelerates the precipitation of carbides
• High cooling heat during welding by lower heat input and /or water cooling
• Solution heat treat after welding

Ferritic Stainless Steel

• Lower interstitial levels
• Microalloy with titanium and/or niobium
• Proper welding procedures
• Proper PWHT procedures.