Intergranular corrosion (Topic 11006)
Intergranular corrosion proceeds along the grain boundaries. This form of corrosion can be very dangerous because the attack can proceed
undetected through the material and may deteriorate the mechanical properties
of the metal and cause fracture without any visible exterior signs of corrosion.
Susceptibility to intergranular corrosion is mainly dependent upon alloy composition and heat-treatment. If there is any risk of intergranular corrosion
occurring, the choice of alloy and the heat-treatment must be controlled carefully.
The alloys in the 6000-group are normally very resistant to intergranular
corrosion as long as the Mg/Si ratio does not increase too much above the
stoichiometric ratio of 1,73, the copper content should be kept at a low level and correct
heat-treatment procedures should be adhered to. In addition the heat affected zone of
a weld that is not properly executed, may be a starting point for
intercrystalline corrosion. Internal weld zones in the profile may also be a starting point
for preferential corrosion attacks if there is associated segregation of
alloying elements. This is quite similar to intergranular corrosion and these welds
should be placed in non-critical areas of the profile.
An intergranular attack is shown below.
References [35, 38]