Prediction of corrosion fatigue crack propagation life for welded joints under cathodic potentials

Abstract

Enhancement of corrosion fatigue crack growth rates by cathodic protection is observed below the optimum applied potential of the protection. An empirical formula for the effect of the protective potential below -0.8 V (Ag/AgCl₂) on the crack growth rates for some classes of shipbuilding steels tested in salt water has been derived for medium and high value ranges of the stress intensity factor. For the lower value range the formula reflects a relatively steep decrease of the crack growth rates (against the same values in air) along with decreasing the stress intensity factor range. A simple formula for the corrosion fatigue crack propagation life under cathodic potential has been derived for fillet welded joints in bending by integrating the corrosion fatigue kinetic characteristics. The new formula for the stress intensity factor range has been used for the bent joint. The predicted "S-Np" curves have been compared with experimental data, taken from literature, for two different values of both applied potential and the plate thickness. The predicted curves correspond approximately to lower bounds of the test results. The presented procedure can be applied to joints of higher strength steels (of the yield strength σ_Y= 315 and 355 MPa) fatigued at any applied cathodic potential below -0.8 V under sea loading of (0.05--0.2 Hz) frequency at (0--0.2) stress ratios.