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Electrochemical Crack Size Effect in Stress Corrosion Cracking and Corrosion Fatigue

The growth rate of small and long stress corrosion and corrosion fatigue cracks in 12Cr steam turbine blade steels in low conductivity water containing 35 ppm Cl- (simulating upset steam condensate chemistry) showed a significant dependence on crack size for the same mechanical driving force.

Product Number: 51317--8853-SG
ISBN: 8853 2017 CP
Author: Alan Turnbull
Publication Date: 2017
Industry: Process Industries
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The growth rate of small and long stress corrosion and corrosion fatigue cracks in 12Cr steam turbine blade steels in low conductivity water containing 35 ppm Cl- (simulating upset steam condensate chemistry) showed a significant dependence on crack size for the same mechanical driving force. However, the crack-size effect disappeared in lower conductivity solution, 300 ppb Cl- and 300 ppb SO4 2- (corresponding to normal steam condensate chemistry). Furthermore, corrosion fatigue long crack growth rates were the same in aerated and in deaerated solutions for the two environments but stress corrosion cracks arrested in deaerated solution. An explanation for these varied results is presented based on the concept of the solution-conductivity dependent crack size effect and its impact on potential drop and hence the crack-tip potential. To underpin this conceptual idea and to explore further the scale of this effect for varied crack size and solution conductivity combinations, modelling of crack electrochemistry is being undertaken and the preliminary results are described.

Key words: small crack, martensitic stainless steel, crack electrochemistry, stress corrosion cracking, corrosion fatigue

 

The growth rate of small and long stress corrosion and corrosion fatigue cracks in 12Cr steam turbine blade steels in low conductivity water containing 35 ppm Cl- (simulating upset steam condensate chemistry) showed a significant dependence on crack size for the same mechanical driving force. However, the crack-size effect disappeared in lower conductivity solution, 300 ppb Cl- and 300 ppb SO4 2- (corresponding to normal steam condensate chemistry). Furthermore, corrosion fatigue long crack growth rates were the same in aerated and in deaerated solutions for the two environments but stress corrosion cracks arrested in deaerated solution. An explanation for these varied results is presented based on the concept of the solution-conductivity dependent crack size effect and its impact on potential drop and hence the crack-tip potential. To underpin this conceptual idea and to explore further the scale of this effect for varied crack size and solution conductivity combinations, modelling of crack electrochemistry is being undertaken and the preliminary results are described.

Key words: small crack, martensitic stainless steel, crack electrochemistry, stress corrosion cracking, corrosion fatigue

 

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