 |
5- Intergranular Corrosion |
|
In most cases
of corrosion, including uniform corrosion, the grain boundaries
behave in essentially the same way as the grains themselves. However,
in certain conditions, the grain boundaries can undergo marked
localized attack while the rest of the material remains unaffected.
The alloy disintegrates and loses its mechanical properties.
This type of corrosion is due either to the presence of impurities
in the boundaries, or to local enrichment or depletion of one or
more alloying elements. For example, small quantities of iron in
aluminium or titanium (metals in which iron has a low solubility),
segregate to the grain boundaries where they can induce intergranular
corrosion. Certain precipitate phases (e.g. Mg5Al8, Mg2Si, MgZn2,
MnAl6, etc.) are also known to cause or enhance intergranular attack
of high strength aluminium alloys, particularly in chloride-rich
media.
The exfoliation corrosion phenomenon observed in rolled aluminium
alloys is usually, but not always, intergranular in nature. In
this case, the corrosion products occupy a larger volume than the
metal "consumed", generating a high pressure on the slivers
of uncorroded metal, leading to the formation of blisters.
Numerous alloy types can undergo intergranular attack, but the
most important practical example is the intergranular corrosion
of austenitic stainless steels, related to chromium depletion in
the vicinity of the boundaries, due to the intergranular precipitation
of chromium carbides (Cr23C6), during a "sensitizing" heat
treatment or thermal cycle.
|
  |
  |