Abstract: A crack appeared near the weld seam of a 7 series aluminum alloy component used in a certain marine installation. The causes of component cracking were analyzed by macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination. The results show that the paint on the surface of the component partially peeled off, causing direct contact with corrosive media and resulting in pitting corrosion. Chain like η-MgZn₂ phase precipitated along the grain boundary in the heat affected zone under welding thermal cycling, reducing the corrosion resistance of the grain boundary. There was also significant welding residual stress in this area, which led to crack initiation at the pitting corrosion pit and ultimately resulted in component cracking.