• Bojinov, Martin
• Mayinger, Wolfgang
• Sipilä, Konsta
• Selector, Maxim
• Saario, Timo
• Jäppinen, Essi

Abstract

Stress corrosion cracking (SCC) in low alloyed steels(LAS) has been extensively investigated during the lasttwo decades. One finding from recent investigations withstandard 1 CT specimen geometry is that even very smallamounts (2.5 ppb) of chlorides increase tremendously thecracking susceptibility of LAS. However, no LAS crackingincidents in real plants have ever been attributed to achloride transient. In the present work, the corrosionpotential at the bottom of the crack tip of a 1 CT LASspecimen was calculated using a mixed-potential model forthe corrosion reaction of low-alloyed steel at themetal/water interface allowing for active dissolution(mainly of iron), active-to-passive transition anddissolution in the passive state. The model was coupledto equations describing dilute solution transport for allthe ionic and neutral species in the crevice associatedwith the crack tip. The chemical and electrochemicalconditions at the bottom of the crevice, as well as anestimate of the enrichment factor of chloride in it, wereobtained from the calculations. In the experimental part,the corrosion behavior of LAS in a crevice environmentforming during a 50-ppb bulk water chloride transient wasstudied by in-situ electrochemical impedance spectroscopy(EIS) and mixed potential measurements, coupled toex-situ characterization of the oxides by microscopic andsurface analytical techniques as well as corrosion rateestimation from exposure coupons. The material studiedwas 20MnMoNi55 from the reactor coolant line of a GermanNPP. The general corrosion rate was found to increaseseveral times when LAS was exposed to chloride from thestart of the experiment, the effect vanishing after about150 h. The EIS data revealed that the effect of chloridetransients on an existing oxide film is moderate,concerns mostly the processes at the inner oxidelayer/water interface and is to a major extentreversible. The SSRT experiments showed that LAS issusceptible to SCC in the crevice environment above athreshold potential of about -0.35 V (SHE). The modelcalculations revealed that the corrosion potential at thecrack tip of a 1 CT specimen is about -0.24 V (SHE),almost irrespective of the crevice geometry. However, thechloride enrichment was found to depend strongly on thecrevice geometry, increasing as the crevice width ordepth increases. Thus, any limit concentration forchloride concentration based on 1 CT specimen laboratorySCC crack growth rate test results should be consideredcarefully and bearing in mind the possible differences increvice width and depth between the 1 CT specimen andthose of a realistic LAS flaw. Based on all the resultsobtained, it can be concluded that chloride transients upto 50 ppb in high temperature water do not result in anyserious consequences for the corrosion of low alloyedsteel with stainless steel cladding.

Topics

• surface
• stainless steel
• experiment
• crack
• steel
• forming
• iron
• electrochemical-induced impedance spectroscopy
• susceptibility
• stress corrosion
• laser absorption spectroscopy