What is alloy sensitization?

What alloys are prone to sensitization and how to avoid it?

Sensitization refers to the precipitation of carbides at grain boundaries in a stainless steel or alloy, causing the alloy to be susceptible to intergranular corrosion.

Certain alloys, when exposed to a temperature characterized as a sensitizing temperature, become particularly susceptible to intergranular corrosion. In a corrosive atmosphere, the grain interfaces of these sensitized alloys become very reactive and intergranular corrosion results. This is characterized by a localized attack adjacent to grain boundaries with relatively little corrosion of the grains themselves. Once the area around the grains has corroded, the grain will literally fall out. 

The figure on the left is the normalized microstructure and the one on the right is the sensitized structure and is susceptible to intergranular corrosion or intergranular stress corrosion cracking.          

The figure on the left is the normalized microstructure and the one on the right is the sensitized structure and is susceptible to intergranular corrosion.

 

Intergranular corrosion is caused by the segregation of impurities at the grain boundaries or by enrichment or depletion of one of the alloying elements in the grain boundary. In the case of austenitic stainless steels, when they are sensitized by heating to about 900° to 1500°F, depletion of chromium in the grain boundary occurs.  This results in susceptibility to intergranular corrosion.

Several methods have been used to control or minimize the intergranular corrosion of susceptible alloys, particularly of the austenitic stainless steels.

 

  • High-temperature solution heat treatment, commonly called solution-annealing, has been used. The alloy is heated to a temperature of about 1950° to 2050°F and then water or air cooled. 

  • Another control technique for preventing intergranular corrosion involves incorporating strong carbide formers or stabilizing elements such as niobium or titanium in the stainless steels. Such elements have a greater affinity for carbon than chromium does.  Carbide formation with these elements reduces the carbon available in the alloy for formation of chromium carbides.  Examples of stabilized grades include 321H and 347H stainless. 

  • A third option is the stainless steel have reduced carbon content below 0.03 percent so that insufficient carbon is available for carbide formation. Many stainless grades have a low carbon version, such as 304L and 316L.  The low carbon is attractive to avoid sensitization, but lower carbon versions have lower strength requirements.  Luckily many stainless steels are dual certified as 304/304L or 316/316L showing that they meet the low carbon limits of chemistry and the high strength of the straight grade.  In addition to meeting the strength and chemistry requirements it is also common to see an intergranular corrosion test, such as ASTM A262 Practice E on the material test report.  This corrosion test starts with a sample that has seen temperatures within the sensitization range and then submitted to a corrosion test.  This ensures that if the material sees elevated temperature in service or during welding it shouldn’t become sensitized. 

 

Zach Schulz