What Are the Causes of Intergranular Corrosion?

Stainless steel resists corrosion well because the chromium in the alloy forms a passive oxide film. Under the wrong conditions it can still corrode, and pipe and structures made from it can be badly damaged. Intergranular corrosion is a common kind of corrosion, and one of the harder ones to spot. It attacks steel along the boundaries between its grains, so it can penetrate well below a surface that still looks sound.

Below, we explain what intergranular corrosion is and what causes it. After that we cover prevention, which comes down to the alloy you choose and how it’s heat treated and welded.

What’s Intergranular Corrosion?

Intergranular corrosion, also called intercrystalline corrosion, attacks a metal along its grain boundaries and the narrow zones right next to them, while the grains themselves stay largely intact. (In cast metal, a similar attack follows the segregation pattern left as the metal solidified, and it’s called interdendritic corrosion.) Intergranular corrosion is hard to spot. In most cases you can’t confirm it without metallographic examination, which means looking at the microstructure under a microscope. Whether a given steel is susceptible is something you test for in the lab, using standard practices such as ASTM A262 for austenitic stainless steels.

Why Does It Happen?

Intergranular corrosion is a form of localized corrosion caused by local differences in composition. In stainless steel, the difference is a loss of chromium right next to the grain boundaries, left behind by chromium carbide precipitation at the boundaries themselves. The chromium doesn’t vanish. It’s tied up in the carbides, so a thin band of metal along each boundary ends up with less chromium in solution than a passive film needs. That minimum is generally taken as about 12 percent. The depleted band is anodic to the grains around it, and its area is very small next to the cathodic grain faces, so it corrodes fast. In severe cases, intergranular corrosion undercuts the grains until they drop out of the surface.

That carbide precipitation is what’s called sensitization. It happens when austenitic stainless steel is held in the sensitizing range of roughly 800 to 1,500 °F (425 to 815 °C). Steel can end up in that range during heat treatment or slow cooling, in service at temperature, or in the heat-affected zone of a weld. How badly it’s sensitized depends on both the temperature and the time spent there. Once sensitized, the steel corrodes preferentially along its grain boundaries, and intergranular corrosion is one of the more damaging structural effects of corrosion.

How Can I Prevent It?

Like most corrosion, intergranular corrosion is preventable, but the fix has to be in the metallurgy of the base metal. You can prevent intergranular corrosion by:

• Choosing a low-carbon grade of stainless steel (304L, 316L)
• Using a stabilized grade alloyed with titanium or niobium (321, 347)
• Solution annealing sensitized material to put the grain-boundary carbides back into solution, then quenching it rapidly
• Controlling weld heat input and cooling rate, so the metal spends less time in the sensitizing temperature range

Even stabilized grades aren’t always immune to intergranular corrosion. Certain weld thermal cycles can still leave a narrow band of attack right beside the weld, known as knife-line attack.

Surface treatments such as hot-dip galvanizing or zinc phosphate priming won’t prevent intergranular corrosion. The coating covers the steel, but the chromium-depleted grain boundaries are still in the base metal underneath, and they’re attacked wherever the coating is breached or the surface is machined. There’s a separate problem with zinc and austenitic stainless steel. Molten or vaporized zinc in contact with stainless at elevated temperature can cause liquid metal embrittlement and intergranular cracking. That can happen during welding or hot forming, or in a fire, so zinc contamination on stainless surfaces has to be controlled.

Which control works depends on what’s causing the corrosion. Intergranular corrosion is a metallurgical condition, so it’s handled through alloy selection and thermal processing. Coatings and cathodic protection don’t address it. Buried and submerged structures that are losing metal to their environment are a different case. Dreiym Engineering designs cathodic protection systems for them and tests existing systems for cathodic protection interference. If that’s your situation, contact us about a new system or an interference test on the one you have.