This specification covers an aluminum alloy in the form of honeycomb core in a non-hexagonal, flexible cell configuration, the core being treated for increased corrosion resistance and furnished only in the expanded form.This honeycomb core has been used typically in contoured sandwich construction for short-term service up to 350 mDF (177 mDC) or for long-term service up to 200 mDF (93 mDC), but usage is not limited to such applications.