Abstract: During the metallographic examination of a low carbon steel, it was found that the surface of ferrite grains turned gray. The reason for the surface graying of grains was studied by metallographic examination, corrosion test and scratch test. The results show that the reason for the graying of the grains was the residual polishing scratches on the surface of the sample, which increased the surface roughness of the sample and caused diffuse reflection phenomenon. The orientation of each grain was different, and the internal stress caused by polishing scratches was also different, resulting in different degrees of corrosion for each grain, which led to differences in surface morphology and ultimately led to graying of the ferrite grain surface.