The Russian steel story is a myth, there was (and still is) a steel plant at Taranto, only 200 miles away from Pomigliano d’Arco so the Italians had no reason to source steel from elsewhere. The main reasons for rust on Alfasuds were that there were many interruptions to production and bare metal unfinished bodyshells were often left outside. The factory is in an agricultural area and workers often just failed to turn up as they were working on the harvest. When the bodyshells were finally painted and assembled, they received little corrosion protection and cavities were filled with expanding foam which later trapped water and caused cars to corrode from the inside out. This YouTube video explains everything in detail:
