Bill, that's a beautiful M.34. They were good little pistols and favored by the Germans (M.35 in 7.65). Beretta made good stuff, from those to the MABs.
I believe Beretta used steel with a high nickel content, which I understand will also cause this. Their SMG barrels have a high nickel content and resist corrosion. The slides on those pistols would be heat treated and hardened as would the barrels.