a method of chemically converting metal surfaces of a combination of iron steel and / or galvanized steel and an aluminum alloy with a phosphate solution satisfying the following conditions : 2 . 0 ≦ na ion + k ion ≦ 15 . 0 1 . 0 ≦ mn ion + ni ion ≦ 5 . 0 1 . 6 - 0 . 02t ≦ zn ion ≦ 2 . 5 - 0 . 02t 3 . 2t - 1 ≦ free f ion ≦ 8 . 0t - 1 0 . 014t - 0 . 02 - 1 ≦ free acidity ≦ 0 . 027t - 0 . 02 - 1 where t represents a treatment solution temperature .