I wonder, though- Ed never needed to know what kind of metal it was before, and nor did scar. It's possible that it's treated or tempered in such a way to make it resistant to alchemy, though. That would be huge, both in the brother's study of alchemy and in Ed's automail.
No, I don't think it has been treated to be resistant to alchemy, for two reasons :
--- the "pure" decomposition is something absolutly unknown among the alchemist, except for Scar and Edward. There is no sigil in Amestrian alchemy to destroy, there is only sigils to destroy then construct.
--- the reaction of Buccaneer when Ed tried to destroy his automail : he said "Did you try something?", because he didn't know what Ed tried to do. If his arm has been treated against alchemy, he would have known it and he would have said "That's useless!".
Personnaly I think that there is perharps a "skin" for the automail that is absolutly no metal, and as Ed tried to destroy this "skin", it was sure that he would fail...