There has been a lot of discussion in the marine industry over the past year with regard to "bad" aluminum. To briefly recap, a number of boat builders purchased aluminum from a supplier that met the requirements of ASTM 5083 H321. This alloy is accepted by both Lloyds Register (Lloyds) and Det Norske Veritas (DNV) for building aluminum vessels. After the aluminum was used to construct some vessels, the operators began to observe surface pitting corrosion. Upon investigation, it was discovered that the purchased plate had been treated differently during manufacturing, with the result that manganese nodules were precipitating out of the alloy, thus making it susceptible to inter-granular corrosion. The result was that newly-built vessels had to be rebuilt and their original hulls scrapped.