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°C) solutions of 3.35m normalNaOH and 2.5m normalNaOH+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 specimens. The solution pH within the crack was measured and the fractography was studied by scanning electron microscopy. Crack rates false(vfalse) are related to the stress intensity false(KIfalse) and are lower in the sulfide solution at comparative KI levels. The mechanism of cracking is consistent with an anodic dissolution process involving rupture of passive films at the crack tip. The lower crack rates in the sulfide solution are discussed in terms of faster repassivation kinetics and the effect of S2− on iron dissolution rates.

Investigations Using Smooth and Notched Specimens into Validity of Caustic Cracking Susceptibility Diagram

“…However, the results presented in Figure 5 unambiguously demonstrate an absence of caustic cracking when a smooth specimen was tested in a 30 pct caustic solution at a much higher temperature (373 K (100°C)), which clearly disregards the predictions of Figure 1. However, Tromans et al [2,15,27,28] have reported caustic cracking at 365 K (92°C) in 2 M to 5 M caustic solutions. The critical difference is that the specimens used in Troman's work were precracked, suggesting that preexisting crack tips can produce caustic cracking under conditions in which smooth specimens (used for generating the results presented in Figure 5) may not produce caustic cracking.…”

“…[17] Such specimens also have been used to determine K ISCC for caustic cracking. [2,6,15] Testing using traditional CT or DCB specimens may require prohibitively expensive machining and a relatively large amount of material.…”

“…However, it is important to note that the research programs for alumina and pulp-and-paper industries (where caustic cracking is a concern) have used both smooth and notched specimens. [9,15,16] It may be necessary to investigate the validity of the caustic cracking susceptibility diagram when smooth and notched specimens are tested, as reported in the current study.…”

Distinct Advantages of Circumferential Notch Tensile (CNT) Testing in the Determination of a Threshold for Stress Corrosion Cracking (KISCC)

“…For example, for extracting alumina from ore (bauxite), aggressive caustic solutions are extensively used at high temperatures and pressures in digesters, decomposer and precipitator in the Bayer process. Temperature and composition of caustic solutions vary vastly across these pieces of equipment in a given plant as well as across industry [ 34 , 35 , 36 , 37 , 38 ]. It is crucial to have plant-relevant data on the thresholds of caustic concentration and temperatures for causing caustic cracking to steels, since reaction vessels and pipes are most commonly constructed from steels [ 17 , 18 , 19 , 20 , 21 , 22 ].…”

SCC susceptibility of carbon steel radioactive waste packages exposed to concrete porewater solutions under anoxic conditions

“…It has been found that the presence of sulphides in alkaline solutions could significantly increase the SCC susceptibility of carbon and stainless steels. Singbeil and Tromans [27,28] have observed a 25-fold increase of the crack propagation rate in caustic solutions containing sulphides compared to sulphide-free solutions. Moreover, it was reported that temperature may strongly influence the SCC of mild steel in alkaline solutions.…”