We investigated the cause of the whisker/discoloration which were found in the transistor lead of stocks (package type TO-18, low power use). In process of the investigation, we estimate two corrosion models that the first model is the remnant of sulfuric acid in cracks of the nickel-phosphorus plating layer in the transistor lead, the second model is the out-gassing or the dissolved ions from the stock container and conductive mat. As the results of the investigation which includes analyses of the whisker/discoloration cross section made by FIB (Focused Ion Beam), a reproductive experiment and so on, the whisker/discoloration were the corrosion reacted between the solder (Pb-Sn) on the transistor lead and SO42- ions of the stock container. We estimate that the new corrosion will not occur and grow in mounted devices because of rejecting the source of corrosion (stock containers). Further, in the worst case of the corrosion occurrence, protective coatings were applied to the mounted transistor lead, as the measure against falling away from the transistor lead.