Defect Reason on Outer Surface of 1Cr18Ni9Ti Stainless Steel Oil Return Tube

During the routine ground inspection of multiple aircrafts, yellow markings were found on the unpainted parts of the outer surface of 1Cr18Ni9Ti stainless steel oil return tubes. Physical and chemical tests such as morphology inspection, chemical composition analysis, microstructure morphology inspection, and micro-hardness test were performed on a defective tube, and simulation tests were performed on the defective tube and its raw materials to analyze the causes of the defects. The results show that the defects on the outer surface of the tube belonged to localized shallow corrosion under the influence of the external environment, which was related to the poor outer surface condition of the raw material of the tube (there was a surface fine-grained area).