Stainless steel alloys owe their anti-corrosive properties to chromium, which grants a passivation effect to both ferrite and austenite steels when found in sufficient concentrations. In the case of austenite steels that make up the majority of the stainless steel family, the minimum chromium concentration is 16%, while ferrite steels require at least 12.5%, and martensitic steels 10.5% for a passive oxide layer to form on the surface of the material.
When it comes to engineering materials in general, one of the major issues is their non-homogeneous nature, which is in this case augmented by the relationship between chromium and carbon, since chromium carbides do not provide the same passivation effect as pure chromium does.
While this in itself might not present a significant problem because these carbides can sometimes improve the mechanical properties of the material, they tend to form at grain boundaries, leaving the intergranular space partially or completely depleted of chromium.
This in turn has a two-fold effect:
It makes these intergranular spaces particularly vulnerable to corrosion, and it also leads to a difference in electric potential between the depleted and carbide area, which leads to galvanic coupling and subsequent galvanic corrosion.
Corrosion Mechanism and Sensitization Temperature
When an austenite stainless steel alloy is heated to high temperatures of 1000°C (1832°F) or higher, the distribution of carbon is uniform. If we, however, heat the alloy to lower temperatures, between 425 – 875°C (797 - 1607°F), then carbon tends to diffuse along the grain boundaries, increasing the local concentration. (For more information about the characteristics of stainless steel, see An Introduction to Stainless Steels.)
Due to this, precipitation of chromium carbides tends to occur in these carbon-rich pockets, leaving the surrounding area with a reduced chromium concentration. The most common carbide formed is Cr23C6 (M23C6 to be more precise, where M represents chromium with traces of iron), which means that even for low concentrations of carbon there can be a significant reduction in chromium concentrations in adjacent areas.
The localized depletion of chromium can easily leave the affected area with concentrations of chromium lower than the minimum required for the creation of a passive protective layer.
This phenomenon occurs along the grain boundaries or in the immediate vicinity of grain boundaries, leading to a localized corrosion attack, further sped up by the potential difference. Namely, metallic grains represent large cathodic areas, while grain boundaries become the opposing anodic areas that are much smaller in surface area. This leads to a very intense damage mechanism that can act quickly, and allow corrosion to penetrate deep into the interior of the material.
Ferrite stainless steels can also be affected by intergranular corrosion, but sensitization temperatures are different. In ferrite steels, chromium depletion occurs at temperatures over 925°C (1697°F), while the temperatures associated with austenite sensitization are in this case beneficial to the material – if a ferrite alloy is subjected to temperatures of 650 – 815°C (1202 – 1499°F) for a short period of time (less than an hour) it regains its corrosion immunity.
To complicate matters further, even non-sensitized alloys can experience intergranular corrosion due to the creation of sigma phase, a brittle and hard intermetallic compound that can impair a material’s corrosion resistance in certain environments, like the ones that contain nitric acid.
The Effect of Intergranular Corrosion
It is important to note that the mechanical properties of the material do not change significantly due to sensitization. Since the reduction in the chromium concentration is followed by the precipitation of carbides, there is a slight increase in hardness and a decrease in ductility, which is to be expected when such elements are introduced into the alloy.
However, if the construction or affected member is in a corrosive environment the damage can be severe. This type of corrosion can greatly reduce the local mechanical properties and cause failure in a fairly short period of time.
For example, if there is a case of intergranular corrosion in a mildly corrosive environment, such as a marine environment, it could take up to several months for a failure to occur. But if the corrosive environment is more aggressive and temperatures higher, it can happen in the span of several hours.
Since intergranular corrosion is tied to short-term heat treatments, welding makes for a particularly problematic process. (Find out more in the article Causes and Prevention of Corrosion on Welded Joints.) Even though the temperature of arc welding is significantly higher than the sensitization temperature of austenite steels, the same cannot be said for the temperature at the heat affected zone (HAZ).
In the case of ferrite alloys, intergranular corrosion occurs near the weld metal or even in the weld metal, due to the higher sensitization temperatures, while austenite alloys experience it in the HAZ nearer to the base material.
Prevention and Treatment of Intergranular Corrosion
Even though this is a very dangerous damage mechanism, there are several methods or factors that can prevent or remedy it. They must be applied differently to austenitic and ferritic steels due to their vastly differing behavior in this situation.
Heat treatment can negate or at least greatly reduce the effect. Austenitic steels should be heated to 1050 − 1100°C (1922 − 2012°F) and then quenched, or rapidly cooled. This way, formed carbides dissolve due to the temperature, and rapid cooling doesn’t allow for their reformation.
Ferritic steels should be heated to 650 – 815°C (1202 – 1499°F) and held at that temperature for several minutes. This way, due to the high chromium diffusion rates in these steels, the chromium concentration evens out, allowing for regeneration of the passive layer at the previously depleted area.
These heat treatments should be done whenever there is a possibility of intergranular corrosion, particularly after welding.
Interestingly enough, intergranular corrosion is largely dictated by the amount of carbon in the alloy, not chromium, meaning that both low and high chromium alloys perform similarly in this regard if they have the same percentage of carbon.
Austenitic steels see a large increase in intergranular corrosion resistance in low-carbon alloys. If the carbon content is reduced below 0.03%, their susceptibility to this damage mechanism is greatly reduced. Of course these alloys are expensive, and they are still not immune, only more resistant.
In the case of ferritic stainless steels, sensitivity to the carbon concentration is much greater, and alloys that had as little as 0.009% carbon were still found to be susceptible. That being said, there are reports that extremely low-carbon alloys (0.002% carbon) can be immune.
Introducing other alloying materials that have a higher affinity to carbon than chromium is another fairly common prevention practice. Adding titanium or niobium can prevent formation of chromium carbides as they have a higher affinity to carbon and will bind with it first.
It should be noted that intergranular corrosion can happen in other materials, such as aluminum, but it is not tied to chromium in that case.