To test the resistance of materials to intergranular corrosion, metal specimens can be subjected to various intergranular corrosion test methods, chosen according to the type of alloy. The most common tests include:

• Strauss Test
• Huey Test
• Streicher Test

These tests are described in major international standards such as ASTM A262, ASTM G28, and ISO 3651-1 and 2. The test duration, solution type, method, and heat treatment, if any, are selected according to the type of material and specifications provided by the customer.

Intergranular corrosion

Intergranular corrosion is a phenomenon that occurs at the edges of the grains that make up metallic materials, due to the formation of precipitates or damaging phases during the various production steps, particularly during heat treatments. This type of corrosion is particularly relevant in austenitic steels, nickel alloys containing significant amounts of chromium and, in general, in all those alloys where heat treatments lead to the precipitation of carbides at the grain boundaries.

Stainless steels can be subject to intergranular corrosion when exposed to temperatures between 500 and 1000°C. Such thermal conditions can cause “sensitization” of the material, making it vulnerable to corrosion. This can occur during hot forming processes such as forging, stamping, rolling, as a result of improper solution treatment or during welding operations.

Nickel alloys with the presence of chromium can be subject to intergranular corrosion under aggressive service conditions. Corrosion, caused by precipitation phenomena at grain boundaries, can be a consequence of the chemical composition of the material or the thermomechanical transformation processes undergone.

Evaluation of intergranular corrosion

The presence of intergranular corrosion can be assessed by various methods:

• Observation of cracks after bending or crushing
• Measurement of penetration rate based on weight loss
• Metallography of the cross section of the corroded specimen and/or the specimen after heat treatment

Heat treatments and resistant materials

For materials whose composition makes precipitation of harmful phases difficult, aging heat treatments aimed at forcing their precipitation can be performed. “L”-type (low-carbon) materials or those stabilized with elements such as titanium or niobium tend not to precipitate carbides at grain boundaries and are therefore more resistant to intergranular corrosion than their non-stabilized counterparts.
The application of a sensitization heat treatment allows the material’s behavior to be observed under particularly severe conditions, where these phases could develop despite compositional arrangements. Solubilized materials can also be tested to verify the effectiveness of the solubilization treatment.