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 SO42- (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.

Subject
Pits, External stress corrosion cracking, Crack growth, Corrosion fatigue, Intergranular stress corrosion cracking, Corrosion rate, Chloride ion concentration, Fatigue crack growth rate, Crack length, Corrosion potential, Stress corrosion cracking, Conductivity, Chloride stress corrosion cracking
Keywords:
small crack, martensitic stainless steel, crack electrochemistry, stress corrosion cracking, corrosion fatigue
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2017
Association for Materials Protection and Performance (AMPP)
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