Tin-plated copper bar

as the current carrying element

is used widely in primary power equipment. Its reliability is related to the stable and safe operation of the primary power equipment. In this paper

a case of discoloration（yellowing

blackening）and pitting of tinned copper bar in a project discovered during the annual inspection（after 5 years of operation）is introduced. The causes of the discoloration are analyzed in combination with site conditions and surface state of copper bar. The discoloration layer is characterized by means of such means as scanning electron microscope（SEM）

energy dispersive spectrometer（EDS）

X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）. The results show that the composition at the blackening and yellowing parts of ti

n plating layer is SnO

2

. The causes of the discoloration are the surface oxidation corrosion due to overheating at the overlap of copper bars and defects of tin plating layer

and the relevant improvement measures are given.