These findings suggest that the
failure mechanism is a combination of low-cycle
corrosion fatigue and stress-induced corrosion.
Extensive oxide formation relative to the depth
of cracking is a key feature. The formation of
oxide was associated with corrosion attack of
the ferrite phase. The lamellar pearlite phase
remained relatively intact and was contained
within the oxide product. The oxide itself exhibited
numerous cracks, allowing aqueous corrosion
of fresh metal to occur at the oxide-metal
interface. Mechanical or thermal stresses are
most likely responsible for this network of cracks
within the oxide product. The mechanism
appears to be stress-assisted localized corrosion.
Sharp, tight cracks were not found in fresh metal
beyond the periphery of the oxide corrosion
product. It therefore appears reasonable that
cracking could have occurred subsequent to corrosion
and within the brittle oxide.