AE technique for detecting the delayed cracks under thermal sprayed coating

This study investigates how the sprayed coating protects metal substrates from delayed fracture. One surface of spring steel strips (0.6% carbon steel, 1.4-GPa tensile strength) was coated by flame spraying of 50Ni-50Cr and titania and exposed to cathodic charging or corrosion in strongly acidic solution under three-point bending load. AE technique was utilized to determine the threshold strain for the delayed fracture. We used stepwise strain increasing method (SSI) to separate AE signals from delayed crack and from coating damage. Here the AE detected during a strain-increasing stage was classified as coating damage and those during strain holding as the delayed crack in the strip. The threshold strain determined by AE technique was correlated with concentration of diffusible hydrogen in the strip. Coated strips shows lower hydrogen concentration and higher threshold strain than strips without coating. Coating was found to act as diffusion barrier and to reduce hydrogen concentration significantly. Even the coating damaged at higher applied strain can reduce the hydrogen concentration in limited area and convert the break-before-leak fracture to leak-before-break fracture.