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01237 ENVIRONMENT ASSISTED CRACKING OF STEAM TURBINE STEELS - MODELLING OF CRACK CHEMISTRY

Picture for 01237 ENVIRONMENT ASSISTED CRACKING OF STEAM
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Product Number: 51300-01237-SG
ISBN: 01237 2001 CP
Author: Alan Turnbull
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A mathematical model has been developed to predict the chemistry and electrochemistry in a crack in a steam turbine disc steel exposed to an external environment of low conductivity water of varying pH and chloride concentration. The model incorporates mass transport by diffusion and ion migration, anodic and cathodic reactions in the crack, hydrolysis reactions in solution, and precipitation of ferrous hydroxide. The potential drop external to the crack was also evaluated with attention given to the influence of a condensate layer. The compression of the field lines to a thin liquid layer increases the potential drop in the external solution, a factor which is usually overlooked when conducting full immersion tests in simulated condensate fluid. As predicted in other work, the expectation is that the influence on crack tip electrochemistry of dissolved oxygen in the external solution will be significant only when the external solution is sufficiently conducting. The ability to polarise the crack tip anodically is also predicted to decrease with increasing crack depth, primarily because of the increase in net current from the deeper crack and the increased potential drop in the external environment.
A mathematical model has been developed to predict the chemistry and electrochemistry in a crack in a steam turbine disc steel exposed to an external environment of low conductivity water of varying pH and chloride concentration. The model incorporates mass transport by diffusion and ion migration, anodic and cathodic reactions in the crack, hydrolysis reactions in solution, and precipitation of ferrous hydroxide. The potential drop external to the crack was also evaluated with attention given to the influence of a condensate layer. The compression of the field lines to a thin liquid layer increases the potential drop in the external solution, a factor which is usually overlooked when conducting full immersion tests in simulated condensate fluid. As predicted in other work, the expectation is that the influence on crack tip electrochemistry of dissolved oxygen in the external solution will be significant only when the external solution is sufficiently conducting. The ability to polarise the crack tip anodically is also predicted to decrease with increasing crack depth, primarily because of the increase in net current from the deeper crack and the increased potential drop in the external environment.
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