Metallographic Analysis of Filiform Corrosion

The surface line type defects on the can twist-off lids from lacquered tinplate material have been analyzed. The analyzed defects manifested themselves like the lines bulged above the surrounding surface. They were aligned roughly perpendicular to the lid circumference; they were mostly arranged on the lower part of lid circumference edge. The defects surface was fully covered by lacquer layer. After lacquer layer removal it was visible in the defective area that tin layer was damaged on some defect areas. The defect presented itself like filiform corrosion which occurred in conditions slightly above room temperature and high humidity under the originally continuous covering layer. According to the findings, corrosion occurred on the hidden sheet cut edges and then propagated first perpendicular to the circumference into the sheet. Then sometimes its direction changed into the sheet rolling direction along the lines with less but still suitable tin layer thickness. This defect can be controlled by special protecting of lid cut edge against corrosion or decreasing of the temperature and the humidity conditions in the stores.