The Electrochemistry of Intergranular Corrosion and Sensitization in 5XXX Aluminum Alloys

The use of 5XXX aluminum alloys for hull and structural materials in future U. S. Navy ships is attractive due to their high strength-to-weight ratio, fabricability, cost, and availability.  These alloys have been routinely used for fast ferries and pleasure craft.  Such ships and craft are significantly smaller than Navay ships and have substantially different operational and maintenance cycels.  In addition, their planned service lives are much shorter than those anticipated by the Navy.  Thus demands on the long-term performance of the alloys have not been particularly high.  Sensitization to intergranular corrosion and cracking can develop in 5XXX alloys via the formation of beta phase at grain boundaries after exposure to temperatures as low as 70 C.  Such sensitization has been observed to occur in service ovber periods of 10 to 15 y.  Currently the only means to quantitatively assess the degree of sensitization (DOS) in 5XXX alloys is through the use of a destructive laboratory test, ASTM G-67.  This study investigates the use of electrochemical methods for detection of sensitization in AA5456.  The electrochemical behavior of the important metallurgical phases and their dependence on solution composition have been measured in order to develop a fundamentally based, robust, quantitative method for DOS determination.