Roof repair nails.

Use the right nails for the repair. This picture is of a couple of pieces of roof slate next to a valley flashing. These rusted and deteriorating nails were used to re install the slate after they had been removed by the previous roofer to replace the valley flashing. They are electroplated galvanized steel nails with a very thin coating of zinc. They are not good enough quality to stand up to the harsh conditions on a slate roof.
Copper slating nails or at the least hot dipped galvanized nails, a much better way to coat steel nails and also the original nails used, should have been used for the repair.