Dr. Dmitri Kopeliovich

Intergranular corrosion is an electrochemical oxidation-reduction (redox) process, which is localized in the zones adjacent to the grain boundaries.

• Mechanism of intergranular corrosion
• Intergranular corrosion of stainless steels (sensitization)
• Intergranular corrosion of aluminum alloys

Intergranular corrosion is caused by Microsegregation of impurities and alloying elements on the grains boundaries.
The driving force of intergranular corrosion is the difference between the Electrode potentials of the grain boundary and the grain itself, which form a galvanic cell in presence of an electrolyte.

If the phases segregated at the grain boundaries have lower value of electrode potential (higher position in the table of Electrochemical series) they will oxidize (anodic reaction) and the grain metal having higher value of electrode potential will provide cathodic reaction (reduction).
Dissolution of anodic grain boundaries starts from the surface and advances along the grains interfaces. The process results in deterioration of the bonding between the grains and drop of mechanical properties.

If the precipitates at the grain boundaries have higher electrode potential the grains will dissolve (anodic reaction). In this case the grain boundaries will not be attacked.

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Intergranular corrosion is a typical defect of austenitic and Nickel alloys. At the temperatures 900-1400ºF (482-760ºC) chromium carbides Cr₂₃C₆ form along the austenite grains. This causes depletion of chromium from the austenitic grains resulting in decreasing the corrosion protective passive film. The grain boundaries are anodically attacked in presence of an electrolyte.
This effect is called sensitization. It is also called weld decay since it usually happens during welding process when the zone around the weld is heated.

The chromium carbides form mainly at the grain boundaries and not within the grains. This effect is caused by different diffusion rate of atoms of chromium through the grains volume and along the boundaries saturated with crystal lattice imperfections.

Means of preventing sensitization:

• Solution heat treatment: heating to a temperature above 1900ºF/1038ºC followed by quenching (rapid cooling) in water or quenching oils. During the heating stage the carbides dissolve and their formation is suppressed by fast cooling.
• Lowering concentration of carbon. Sensitization is depressed in low carbon (max.0.03%) stainless steels designated with suffix L (304L, 316L).
  
• Stabilization by carbide forming elements. Formation of chromium carbides is avoided in stabilized austenitic stainless steels (321, 347) containing carbide forming elements like titanium, niobium, tantalum, zirconium. Stabilization heat treatment of such steels results in preferred formation of carbides of the stabilizing elements instead of chromium carbides.
  

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Aluminum alloys containing magnesium (Mg), zinc (Zn), copper (Cu), iron (Fe) are susceptible to intergranular corrosion:

• Wrought aluminum-copper alloys (2xxx) and copper containing Wrought aluminum-zinc-magnesium alloys (7xxx) (eg. 7001, 7072, 7178). Precipitation of Al₂Cu causes depletion of copper from the grain boundaries, which become anodes in presence of an electrolyte.
• Wrought aluminum-magnesium alloys (5xxx). The intermetallic inclusions Mg₅Al₈ precipitating at the grain boundaries cause their corrosion.
• Wrought aluminum-zinc-magnesium alloys (7xxx). Intergranular corrosion of these alloys is the result of segregation of MgZn₂ in the region of grain boundaries, which become anodic.

The corrosion products just below the uncorroded surface of a rolled aluminum part may cause a pressure resulting in formation of blisters. This phenomenon is called exfoliation.

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• Electrode potentials
• Polarization
• Pourbaix diagrams
• Galvanic corrosion
• Pitting corrosion
• Crevice corrosion
• Selective corrosion (dealloying)
• Stress corrosion cracking
• Corrosion fatigue
• Cathodic protection
• Corrosion protection coatings
• Corrosion inhibitors
• Volatile corrosion inhibitors (VCI)
• High temperature oxidation of metals

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to Corrosion and oxidation