A Fracture Mechanics Study of Caustic Cracking of Mild Steel in Hot NaOH / Na2 S  Solutions

Abstract

Fracture mechanics techniques were employed to study stress corrosion cracking (SCC) of AISI C-1018 mild steel in hot (92 ~ solutions of3.35m NaOH and 2.5m NaOH + 0.423m Na2S. Tests were conducted under potentiostatic conditions at the most susceptible SCC potentials near the active-passive transition, using fatigue precracked double cantilever beam speci-mens. The solution pH within the crack was measured and the fractography was studied by scanning electron microscopy. Crack rates (v) are related to the stress intensity (K~) and are lower in the sulfide solution at comparative K ~ levels. The mechanism ofcracking is consistent with an anodic dissolution process involving rupture of passive films at he crack tip. The lower crack rates in the sulfide solution are discussed in terms of faster repassivation kinetics and the effect of S ~- on iron dissolution rates. The widespread use of mild steel in contact with hot, industrial, alkaline sulfide solutions in the Kraf t pro-cess for converting wood chips to pulp prompted some recent studies on the behavior of mild steel in synthetic solutions of similar composition (1-3). Employing hot (92 ~ NaOH and NaOH + Na2S solutions, it was