Sensitisation

What is Sensitisation in Stainless Steel?

Sensitisation in stainless steel refers to the formation of chromium carbides at grain boundaries when the alloy is exposed to elevated temperatures for extended periods, typically between 425°C and 870°C. This reaction depletes chromium in the areas adjacent to the boundaries, thereby reducing the material’s corrosion resistance.

How Sensitisation Occurs in Stainless Steel?

Sensitisation happens when stainless steel is heated within a certain temperature range, roughly between 425°C and 870°C. In this range, conditions allow carbides to form along the grain boundaries of the metal. When the temperature is lower than this range, the reaction occurs very slowly, and when it is higher, the carbides break apart and blend back into the metal.

Even small amounts of carbon, such as 0.03%, can significantly impact sensitisation. Higher carbon levels increase the likelihood of chromium carbide formation because more carbon atoms are available to migrate to grain boundaries. This is why low-carbon stainless steel grades (e.g., 304L, 316L) are specifically designed to minimise carbide precipitation during welding and heat exposure.

Key factors contributing to sensitisation include:

1. Intergranular corrosion: Once chromium depletion occurs at the grain boundaries, the corrosion resistance will be lowered in those regions and become preferential sites for attack when exposed to corrosive environments.
2. Heat exposure: Long periods at 425°C–870°C enable carbide formation.
3. Welding sensitisation: Localised heating during welding can initiate carbide precipitation.
4. Carbide precipitation: Occurs when carbon combines with chromium, reducing corrosion resistance.

Importance of Preventing Stainless Steel Sensitisation

Preventing sensitisation is essential to maintaining stainless steel’s durability and metallurgical integrity. Effective measures include:

1. Solution annealing: Heating above 1000°C and rapidly cooling the alloy to dissolve chromium carbides.
2. Using low-carbon grades: Restricting carbon contents up to 0.03% in grades such as 304L or 316L, which reduces the tendency for carbide formation.
3. Stabilised alloys: Adding titanium or niobium to form stable carbides instead of chromium carbides.
4. Controlled welding techniques: Limiting heat input to minimise welding sensitisation.

Applications and Risks in Stainless Steel Components

Sensitisation poses a risk to components exposed to high temperatures or corrosive environments. Common examples include:

1. Chemical equipment, such as reactors and heat exchangers operating under thermal stress.
2. Piping systems and tubes in refineries and chemical plants are prone to intergranular attack.
3. Pressure vessels and structural components involving welding are subjected to repeated heating cycles.
4. Industrial applications where prolonged exposure leads to chromium depletion and corrosion at grain boundaries.

Sensitisation can be assessed using standardised corrosion tests, for example, ASTM A262 Practices A–E (for Austenitic Stainless steels). These tests are commonly applied in industries where material integrity against intergranular corrosion is critical, such as petrochemical and power generation applications.