Failure analysis of crevice corrosion on 304 stainless steel tube heat exchanger

The cause of the premature failure of 304 stainless steel tube heat exchanger was investigated. The unique skeleton structure inside the leakage point reveals that this is a new damage mechanism caused by a δ + γ two-phase structure and crevice corrosion. The three-dimensional structure of the leakage point was demonstrated using X-ray diffraction topography. The results of scanning electron microscope examination show the microstructure of the weld to be columnar and dendritic. It is found by electron probe microscope analysis and transmission electron microscopy that columnar dendrites consisted of γ-dendrite and an amount of δ-ferrite phases at the dendrite trunk. Simulated corrosion test results confirmed that the corrosion medium was the chloride ion. Crevice corrosion of chloride ions occurred at weld defects on the inner wall thus forming a concentration cell. Grains of columnar dendrites were then corroded by chloride ions and δ-ferrite phases on the grain boundaries were retained, which formed the particular skeleton corrosion structure. As a result, it led to leakage when the corrosion of weld occurred from the inner wall to the outer wall.