How to avoid intergranular corrosion? “L” or “Ti” Stainless Steel?
Despite of its name, stainless steel is a material that is susceptible to be corroded or oxidized. In short, what makes the stainless steel to get the “stainless” condition is the presence of at least 10 % chromium. A thin, continuous, transparent and self-healing chromium oxide film is formed on the surface of the steel, in such a way that it prevents oxygen penetrating, avoiding the progress of the oxidation. This phenomenon is called passivation.
This film is inert against many environmental conditions, while the passivation can be damaged by little changes in these conditions. One of the main problems of the austenitic stainless steel can be the intergranular corrosion. If a stainless steel sample is exposed for some time at high temperatures (between 450-800 ºC), chromium carbides are formed, precipitating at the grain boundaries and leaving the percentage of chromium in these areas below the minimum 10%, becoming the steel very sensitive to the corrosion process. This is known as “sensitization”. Moreover, under certain conditions (adverse environment or fluids, temperature, etc.), intergranular corrosion can occur.
For example, this range of temperatures is reached, and is exceeded, during the welding process. If the subsequent cooling is slow, an area can be “sensitized” in the HAZ or Heat Affected Zone in both sides of the weld seam. There are several solutions to “fight” against the described process, including:
To “stabilize” the steel by means of adding Titanium or Niobium
During the cooling process, carbon forms Titanium or Niobium carbides instead of chromium carbides, thus enabling that the chromium content is enough to form the protecting film. This solution is very common in some countries, as Germany, although it can also cause problems. As an example, under certain welding conditions and high cooling rates a phenomenon known as “knife life attack” can take place.
To bring down the percentage of carbon atoms available to react with the chromium
EThe below TTS diagram for a type 304 stainless steel shows as the time the steel has to be in the critical temperature range to produce sensitization, increases as the carbon content goes down.
In practice, if the carbon content is under 0.03 %, there will not be enough carbon atoms available to cause the precipitation of carbides. These steels are identified as “L”, “low carbon” stainless steels (316L, 304L).
In conclusion, we can state that the performance of both types of steels are similar. Nevertheless, if we consider the other benefits of the “L” stainless steels (as weldability, availability, price, etc.), this solution emerges, on balance, as the best one.
References: Aceros Inoxidables, Ing. Sergio G. Laufgang; Soldadura de los aceros, Manuel Reina Gómez; www.gewater.com