To mitigate the risk of corrosion, tubes are often made out of corrosion-resistant alloys, including Inconel alloys and stainless steels. At elevated temperatures, however, sensitization can still occur—opening up your alloy to attack. Let’s explore what sensitization is, as well as some of the ways you can prevent it.

How Sensitization Occurs

At elevated temperatures, sensitization can occur in metals that contain chromium. During this process, chromium carbide precipitates form along the metal’s grain boundaries, or the interface between two crystals in a polycrystalline solid. This precipitation creates localized chromium depletion zones, which prevent the protective oxide layer from forming and expose the alloy to corrosive attack.

Precipitates form when chromium- and carbon-containing alloys are heated anywhere from 750°–1,500° F. Sensitization develops slowly as more carbides form and can take anywhere from 30 minutes to several hours before the material is significantly affected.

Sensitized grain structure of 316 stainless steel with carbides formed in the grain boundaries.

How To Prevent Sensitization

Inspection methods can also help you identify the signs of sensitization. When examining the grain mount of a sensitized metal under a microscope, the affected area will have thick, dark grain boundaries due to the presence of carbides. You can then test questionable specimens according to ASTM standard A262.Adding titanium and niobium to stainless steels and Inconel alloys can reduce the effects of sensitization. Because carbon prefers bonding with niobium and titanium over chromium, far less chromium is pulled out of the solution to form chromium carbide precipitates—keeping the oxide layer intact. Another strategy is to limit the alloy’s carbon content. When less carbon is present, less carbide precipitates form and more chromium is left to maintain the protective oxide layer.

To learn more, download our application note.

 

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