Caustic Cracking of AISI 4340 Steel

Abstract Fracture of AISI 4340 steel in concentrated sodium hydroxide solution has been monitored by measuring the coupling current that flows between the crack and the external surfaces and by using the Acoustic Emission Technique (AET). The result clearly shows that a positive current flows from the crack to the external cathodes (through the solution) during crack growth of AISI 4340 steel in concentrated sodium hydroxide solution at 70°C. The (electron) coupling current contains periodic noise that is attributed to fracture events occurring at the crack front, with the amplitude of the noise and mean current increasing with crack growth rate. The characteristic shape of the individual transients in the noise at lower SCC crack growth rate is a rapid drop followed by slow recovery. A study of the kinetics of repassivation of the individual micro-fracture events indicates that the current relaxes in a first order fashion with a rate constant of 0.029s-1. The discrete events, which have a dimension of about 49μm, are postulated to be hydrogen induced, and the mechanism of caustic cracking of AISI 4340 steel is considered to be hydrogen embrittlement along grain boundaries. Simultaneous monitoring of the crack growth rate using the Acoustic Emission Technique has helped in validating the data measured using the electrochemical noise technique. Viability of using the Direct Current Potential Drop (DCPD) method as a tool for monitoring the insitu crack length in stress corrosion cracking studies is also being explored. Measurement of the electrochemical noise is shown to be capable of detecting and distinguishing between uniform corrosion and stress corrosion cracking in steel/NaOH system.