FRACTURE OF AISI 4340 STEEL IN CONCENTRATED SODIUM HYDROXIDE SOLUTION

摘要

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. The results clearly demonstrate that positive current flows from the crack to the external cathodes (through the solution) during crack growth of AISI 4340 steel in concentrated (6 to 12 M) sodium hydroxide solution at 70 deg 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 the mean current increasing with crack growth rate. The characteristic shape of the individual transients in the noise at lower SCC crack growth rate is rapid drop followed by slow recovery. The form of the noise in the coupling current during SCC at high NaOH concentration (8 M and 12 M) is attributed to overlap of many cracks propagating simultaneously through micro fracture events along grain boundaries. The discrete events 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. Measurement of the electrochemical noise is shown to be capable of detecting and distinguishing between uniform corrosion and stress corrosion cracking in the steel/NaOH system.