INTERGRANULAR CORROSION I018
All austenitic stainless steels (the 300 series is one of them) contain a small amount of carbon in solution in the austenite.
Carbon is precipitated out at the grain boundaries of the steel in the temperature range of 1050-1600°F. (565-870° C.- Unfortunately this is a normal temperature range during the welding of stainless steel.
At these elevated temperatures the carbon combines with the chrome in the stainless steel to form chromium carbide, starving the adjacent areas of the chrome they need for corrosion protection.
In the presence of some strong corrosives an electrochemical action is initiated between the chrome rich and chrome poor areas with the areas being low in chrome becoming attacked.
The grain boundaries are then dissolved and become non-existent.
There are three ways to combat the formation of chrome carbide:
- Anneal the stainless after it has been heated to this sensitive range. This means bringing it up to the proper annealing temperature and then quickly cooling it down through the sensitive temperature range to prevent the carbides from forming.
- When possible use low carbon content stainless steel if you intend to do any welding on it. A carbon content of less than 0.3% will not precipitate into a continuous film of chrome carbide at the grain boundaries. 316L is as good example of a low carbon stainless steel.
- Alloy the metal with a strong carbide former. The best is columbium, but sometimes titanium is used. The carbon will now form columbium carbide rather than going after the chrome to form chrome carbide. The material is now said to be stabilized.