CRACK INITIATION IN THE CORROSION-FATIGUE OF STRUCTURAL-STEELS IN SALT-SOLUTIONS

The growth of short cracks in a structural steel 3.5% NaCl has been studied using smooth bending specimens. The influence of sodium nitrite additions has also been investigated. Cracks initiated at non-metallic inclusions, primarily oxide inclusions in air, and sulphides in the corrosive environments. At lower stresses pitting at inclusions preceded the formation of identifiable cracks. Different behaviours of ferrite and pearlite were detected, with the ferrite appearing more susceptible to cracking in the absence of corrosion, while cracking in the pearlite became significant at lower stresses in the corrosive environment. The observed behaviour has been related to corrosion processes in the region of the crack tip, with some evidence of corrosion-product induced crack closure in the presence of the sodium nitrite.