rust-like patches on shells

[lulu.shell]

Thank you so much for all your kind advice. I could not rightly say when the contamination with rust occurred. These shells were given to me by an old lady (a sought of aunt from my childhood)and I have had them in a plastic container away from all my other shells for years (probably a decade). Now, I am trying to organise my collection because I want to make an archive of what I have, I want to transfer them to little plastic bags. I will make sure I keep them separate from other shells.