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 °F (177 °C) or for long-term service up to 200 °F (93 °C), but usage is not limited to such applications.