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Acoustic emission signals were continuously monitored during fatigue crack propogation for two kinds of low strength steel placed in either laboratory air or 3.5% NaCl solution. The plastic deformation of the materials at the crack tips were compared by determining the changes in specimen geometry at the fracture surface in the thickness direction while the hydrogen distribution from the crack tip material was analysed by SIMS. The results showed that acoustic emission was less active in an aqueous solution than in air and that the plastic zone size was reduced in a corrosive medium. It was considered that the entrance of hydrogen into the crack tip during corrosion fatigue results in a decrease of material ductility, which lead to a low activity in acoustic emission.