The series of alloys made use of is sensational: weathering steel for patina, marine-grade aluminum for weight financial savings
https://www.mediafire.com/file/4aie19pn43rpjw9/pdf-72924-48992.pdf/file
The range of alloys used is spectacular: weathering steel for aging, marine-grade aluminum for weight savings, and duplex stainless for deterioration resistance in extreme climates.