Corrosion fatigue initiation in stainless steels : The scanning reference electrode technique.

The early stages of damage by corrosion fatigue in austenitic and duplex stainless steel were studied using a novel scanning reference electrode technique (SRET). Emphasis was made on the role played by corrosion pits, which can act as stress concentrators and promote fatigue crack nucleation. SRET measurements conducted during the potentiostatic generation of pits on the austenitic stainless steels in artificial seawater showed that the pit current density increases with the time of application of anodic polarisation. The data of the volume of metal dissolved calculated from pit current density obtained by SRET agreed well with the measurements of pit profiles. The austenitic and duplex stainless steels showed high resistance to pitting corrosion in artificial seawater at free corrosion potential. The use of a 0.05 M FeCl3 solution as the electrolyte promoted severe localised attack in 304 and 316L stainless steel specimens. SRET tests carried out simultaneously with the application of cyclic stress to the specimens permitted the evolution of the electrochemical activity of corrosion pits to be followed. Transformation from pitting to fatigue cracking was observed. In this work it is proposed that a decrease of the pit electrochemical activity in terms of pit current density, is related to the nucleation of the corrosion fatigue crack. From SRET measurements a critical pit depth was calculated for the nucleation of the fatigue crack. A threshold stress intensity factor range was then calculated, which was found to be similar to the values reported in the literature.It is suggested at the end of this thesis that parallel studies and measurements of the pit-to-crack transition can be conducted in order to corroborate the reliability of SRET measurements to assess semi-quantitatively the threshold conditions for the transition. The scanning reference electrode technique showed advantages over other electrochemical methods used to assess the damage induced by localised corrosion in that it provides in-situ, spatially resolved, real-time electrochemical activity measurements.