Effect of sulfate addition on stress corrosion cracking initiation of sensitized type 304 stainless steels in high temperature water

摘要

In order to study an effect of sulfate on stress corrosion cracking (SCC) initiation in primar y coolant environments of boiling water reactors, creviced bent-beam tests were conducted for sensitized Type 304 stainless steel specimens in sodium sulfate water at 288°. The addition of 30500 ppb sulfate ion to purity water increased both numbers of all cracks more than 1 μm in depth and steady-propagating cracks more than 50 μm in depth. Spatial distribution of initiated cracks deeper than 50 μm was approximated to a Poisson stochastic process under the condition of SO42- =100 ppb, whereas it was not approximated to the process under the condition of SO42- = 300 ppb. A new approximation method based on Poisson distribution was presented as an estimation method of cumulative probability for crack initiation and the availability of it was confirmed. Sulfate ion concentration dependency on lower-limit values, αest, for distribution of the SCC initiation life was given to be aest = 683/SO42- by an analysis based on the Poisson stochastic process model. For example, the addition of 40 ppb sulfate ion to purity water can shorten a lower-limit value of distribution of the SCC initiation life by one tenth without changing stochastic character of it. Shortening of aest by the addition of sulfate probably depended not on solution conductivity but on sulfate ion itself.