SDR 510020164 | Cessna 208 Nacelle/pylon bulkhead/firewall—bulkhead corroded. Lower firewall bulkhead corroded. Found following removal/replacement of firewall lower forward doubler PNo 2653037-1. Lower LH engine mount PNo 2613087-7 also replaced due to corrosion damage. Aircraft had operated on floats at some time during the past. P/No: 26530371. TSN: 5020 hours/192 months.