Phenomenological investigation of the influence of Cathodic protection on corrosion fatigue crack propagation behaviour, in a BS 4360 50D type structural steel and associated weldment microstructures, in a marine environment.

Abstract

The influence of Cathodic Protection potential upon corrosion fatigue crack propagation rates in a medium7strength ferritic-pearlitic structural steel (ES 4360 grade 50D) and associated weldment microstructures in simulated sea-water was studied and the results were presented in bi-modal da/dN vs AK curves. Above transition propagation rate data was satisfactorily described by the Paris relationship da/dN = C. AKm and a relationship of the formin = Aln C+D between Paris exponent m and constant C was confirmed. In all microstructures the influence of cathodic protection on crack propagation rate was found to be dependent upon the level of cathodic potential applied and crack depth. A. critical crack depth of approximately 5.0mm was identified. Optical crack monitoring combined with graphical data processing, was found to be particularly effective for the characterisation of corrosion fatigue crack propagation behaviour and resulted in minimal data scatter. The phenomena of transition which was evident in the bi-modal da/dN vs LK curves was found to be associated with secondary or branched-crack activity. Back-extrapolation of below transition data was used to estimate Arm values and showed that the weldment beat-affected zone microstructure was a major potential source of fatigue or corrosion fatigue fracture. The effect of a single cycle of simulated overload, to stormload levels, on both fatigue and corrosion fatigue crack propagation behaviour in parent plate material was studied and the phenomenon of retardation was confirmed