Caustic Stress Corrosion Cracking of Mild Steel

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I.

INTRODUCTION

R E V I E W S 1'2'3 on the subject of stress corrosion cracking (SCC) of mild steel in hot, aqueous solutions of NaOH show that cracking may be encountered in the range of 4 to 75 pct by weight of NaOH, with concentrations near 35 pct being the most severe. Several workers have shown that the electrochemical potential regime for SCC is associated with a small range of potentials near the active-passive transition. 1'3-8 In simple NaOH-H20 systems, the range is centered near -0.76VsHE in - 1 2 pct NaOH 7'8 and near --0.70VsnE in 35 pct NaOH. 5'6'9The range tends to broaden as the concentration of NaOH increases.3 Cracking has been reported to occur in the potential region where Fe304 films form and HFeO2- ions are stable. 1,6 Crack path is predominantly intergranular near the active-passive transition, although some transgranular cracking may be observed. 1.5.7.8 Several mechanisms for caustic cracking have been proposed. These include crack tip advancement via localized electrochemical dissolution in the presence of a film covered crack surface, where deformation processes at the crack tip produce localized rupture of passive films to expose the metal to dissolution. Parkins 3'~~has provided considerable support for this general model and further support has been obtained by the authors. 8 Newman 11 obtained quantitative agreement between theory and experiment for a film rupture-repassivation model based on passage of a constant coulombic charge density. In contrast, Perdieus et a1,12 proposed a hydrogen embrittlement model based on the observed effect of potential on crack path and the detection of hydrogen in specimens subjected to SCC (also detected by Dahl et all3). Mazille and Uhlig 9 proposed an adsorption model of cracking whereby damaging ions adsorb on, and weaken, strained metal bonds in a critical potential range. Quantitative kinetic studies of caustic crack propagation are scarce. Time to failure for smooth specimens has been

the usual parameter studied. 2'3'5'9It has produced data corresponding to an apparent activation energy (Q) for cracking in the range of 44 kJ/mol 9 to 75kJ/mol s for mild steels and 73 kJ/mol 9 for a martensitic 0.24 pct C steel. Crack growth rate data obtained from sectioned specimens of a heat treated 3 pct Cr-Mo steel gave a Q value of 88kJ/mol. H With the exception of preliminary work by the authors on mild steel 8 and others on low alloy s t e e l s 2A4'15 n o studies have been reported in the open literature on the effect of opening mode stress intensity (Kt) on crack propagation rates. Fractographic studies of caustic SCC of mild steels have rarely been conducted, 8 due to the presence of corrosion products on fracture s u r f a c e s , 16A7 and only one previous study has attempted to measure crack liquid pH. 8 The present study was designed to enhance the mechanistic understanding of caustic SCC of mild steels by utilizing the techniques of fracture mechanics. Quantitative kinetic data were obtained on the effect of Kt and temperature (T) upon crack pr