Grain boundary corrosion is a type of localized corrosion. The grain boundary has a higher energy level than the grain itself and is therefore more chemically active and more susceptible to corrosion. In this case, the materials present at the grain boundaries act as the anode (with a smaller surface area) and the grains themselves (with a larger surface area) act as the cathode.

In grain boundary corrosion, attacks are concentrated at the grain boundaries and, along with relatively minor grain corrosion, cause the grains to separate from each other, which ultimately reduces the strength, flexibility and ductility of the metal.

The most important causes of grain boundary corrosion can be considered to be the accumulation of impurities at the grain boundaries of the metal and the depletion of the boundary from some alloying elements.

To better understand intergranular corrosion, consider the following examples:

1. Depletion of the grain boundaries of stainless steel from the alloying element chromium facilitates the deposition of chromium carbide (Cr23C6) at the boundaries, thus making the boundaries susceptible to intergranular corrosion.
2. Zinc separation in brass alloys in the boundary areas exacerbates intergranular corrosion.

Stainless steels have a high chromium content. In these types of steels, the chromium metal prevents corrosion of the rest of the metal by forming a protective layer, but at the grain boundaries, carbon may form chromium carbide with chromium. The deposition of chromium carbide at the grain boundary causes the chromium around the grain boundary to become depleted. If this deposition and depletion is relatively continuous, it makes stainless steel susceptible to intergranular corrosion. In this case, the corrosion attack will start and continue at the grain boundary.

Intergranular corrosion reduces the ability of a metal or alloy to undergo plastic deformation before failure. This type of corrosion, in more severe cases, also significantly reduces the tensile properties of the metal.

Intergranular corrosion occurs in two forms:

Weld line corrosion

One of the welding problems of stainless steels that we often encounter during welding operations is intergranular corrosion, which occurs more in the heat-affected zone and the weld zone, and this has caused a lot of damage.

When 8-18 stainless steel is heated for a period of time in the temperature range of 450-950 degrees Celsius, chromium carbide precipitates at the grain boundaries, and thus the grain boundaries are depleted of chromium. This phenomenon causes stainless steel to become sensitive.

Destruction and damage to welded steel structures along and near the weld line is called weld line corrosion.

Methods for preventing sensitization of austenitic steels

a) Reducing the carbon content in the steel to less than 0.03%, this will prevent the formation of sufficient carbides to cause intergranular corrosion.

b) Using temperature-time sensitization curves, it is possible to prevent sensitization and observe the effect of carbon content on this phenomenon. The amount of carbon can affect the degree of sensitization. Sensitization in 304 stainless steel occurs much faster with increasing carbon content. Studies show that under a given welding energy and speed, weld corrosion in 304 stainless steel increases with increasing carbon content.

c) Using stable steels. These stainless steels contain titanium or niobium, which tend to combine with carbon and easily form carbides, which allows chromium to remain in the solvent even when exposed to sensitization temperatures for a long time, minimizing the formation of chromium-rich grain boundary carbides.

The only solution to repair sensitized austenitic stainless steels is to anneal them.

Pitting Corrosion

Although stable austenitic stainless steels are not susceptible to weld pitting, they can be susceptible to a different type of intergranular corrosion attack called pitting. Like weld pitting, pitting corrosion is caused by the deposition of chromium carbide at grain boundaries. This type of corrosion is different from pitting corrosion. The differences are:

• Pitting corrosion occurs in a narrow zone very close to the weld metal.
• Pitting corrosion occurs in stable stainless steels.

Methods for preventing pitting corrosion

a) Post-weld heat treatment in the temperature range of 1000 to 1100°C causes the dissolution of chromium carbide.

b) The use of low-carbon alloy steels reduces the likelihood of chromium carbide precipitation.

c) Sometimes pitting attack can be prevented by modifying the welding procedure or program.