Understanding Corrosion in Water Pipelines: A Guide for Pipeline Designers


Grain Boundary (GB)

Last updated: September 30, 2020

What Does Grain Boundary (GB) Mean?

A grain boundary (GB) is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material.

A grain boundary is a general planar defect that separates regions of different crystalline orientation (such as grains) within a polycrystalline solid. Grain boundaries are usually the result of uneven growth when the solid is crystallizing. Grain sizes vary from 1 µm to 1 mm.

Most grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. They are also important to many of the mechanisms of creep. On the other hand, grain boundaries disrupt the motion of dislocations through a material, so reducing crystallite size is a common way to improve the strength of a material.


Corrosionpedia Explains Grain Boundary (GB)

The juncture between adjacent grains is called a grain boundary. The grain boundary is a transition region in which some atoms are not exactly aligned with either grain.

Therefore, the grain boundaries are:

  • Where grains meet in a solid
  • Transition regions between the neighboring crystals
  • Where there is a disturbance in the atomic packing

Characterization of grain boundaries is of critical importance in materials studies. The properties of grain boundaries often determine the grains':

  • Formation
  • Evolution
  • Stabilization (or dissolution)

Grain boundaries have two types, as per their orientation:

  • Low-angle grain boundaries are those with a misorientation less than about 11 degrees.
  • High-angle grain boundaries are whose misorientation is greater than about 11 degrees.

High-angle boundaries are considerably more disordered, with large areas of poor fit and a comparatively open structure. The mobility of low-angle boundaries is much lower than that of high-angle boundaries. Both low- and high-angle boundaries are retarded by grain refinement, to minimize or prevent recrystallization or grain growth during heat treatment.

The grain-boundary atoms are more easily and rapidly dissolved, or corroded, than the atoms within the grains. Grain boundaries will oxidize or corrode more rapidly, usually referred to as grain-boundary penetration or intergranular attack. In under-deposit corrosion and hydrogen damage, grain boundaries are the site at which the methane collects that leads to the intergranular cracking characteristic of hydrogen damage.


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