Intergranular Corrosion in AA5xxx: A Case for Continuous Attack with a Discontinuous Active Path

The US Navy has interest in the intergranular corrosion (IGC) and intergranular stress corrosion cracking of marine grade aluminum alloys (AA5xxx).  The service environment can age material in a detrimental fashion, causing the b phase to precipitate at grain boundaries.  IGC of AA5xxx alloys due to the precipitation of b phase on the grain boundaries is a well-established phenomenon.  At high degrees of sensitization, it is well established that the IGC path is a continuous active path.  At lower degrees of sensitization, the coverage at the grain boundaries may not be continuous. The question of why IGC still occurs when the b phase does not provide a continuous active dissolution path has yet to be addressed.  Theories of how IGC may occur given a discontinuous path are reviewed.  An additional theory based on cathodic protection by sacrificial anode is proposed.  Galvanic couple experiments were conducted to test this theory.