There are dozen of alloys and also many different way to process them. I have no clue what they used, as I did not find any real specification sheet. But it loo

There are dozen of alloys and also many different way to process them. I have no clue what they used, as I did not find any real specification sheet. But it looks more like a purity problem than the "wrong" alloy, but many alloys will rust, in different ways. I have no experience in such large object as I work on micro mechanic. I would love to have an expert share a more informed comment.