Effect of chloride transients on corrosion of low-alloyed steel under oxygenated high-temperature water conditions

Stress corrosion cracking (SCC) in low alloyed steels (LAS) has been extensively investigated during the last two decades. One finding from recent investigations with standard 1 CT specimen geometry is that even very small amounts (2.5 ppb) of chlorides increase tremendously the cracking susceptibility of LAS. However, no LAS cracking incidents in real plants have ever been attributed to a chloride transient. In the present work, the corrosion potential at the bottom of the crack tip of a 1 CT LAS specimen was calculated using a mixed-potential model for the corrosion reaction of low-alloyed steel at the metal/water interface allowing for active dissolution (mainly of iron), active-to-passive transition and dissolution in the passive state. The model was coupled to equations describing dilute solution transport for all the ionic and neutral species in the crevice associated with the crack tip. The chemical and electrochemical conditions at the bottom of the crevice, as well as an estimate of the enrichment factor of chloride in it, were obtained from the calculations. In the experimental part, the corrosion behavior of LAS in a crevice environment forming during a 50-ppb bulk water chloride transient was studied by in-situ electrochemical impedance spectroscopy (EIS) and mixed potential measurements, coupled to ex-situ characterization of the oxides by microscopic and surface analytical techniques as well as corrosion rate estimation from exposure coupons. The material studied was 20MnMoNi55 from the reactor coolant line of a German NPP. The general corrosion rate was found to increase several times when LAS was exposed to chloride from the start of the experiment, the effect vanishing after about 150 h. The EIS data revealed that the effect of chloride transients on an existing oxide film is moderate, concerns mostly the processes at the inner oxide layer/water interface and is to a major extent reversible. The SSRT experiments showed that LAS is susceptible to SCC in the crevice environment above a threshold potential of about -0.35 V (SHE). The model calculations revealed that the corrosion potential at the crack tip of a 1 CT specimen is about -0.24 V (SHE), almost irrespective of the crevice geometry. However, the chloride enrichment was found to depend strongly on the crevice geometry, increasing as the crevice width or depth increases. Thus, any limit concentration for chloride concentration based on 1 CT specimen laboratory SCC crack growth rate test results should be considered carefully and bearing in mind the possible differences in crevice width and depth between the 1 CT specimen and those of a realistic LAS flaw. Based on all the results obtained, it can be concluded that chloride transients up to 50 ppb in high temperature water do not result in any serious consequences for the corrosion of low alloyed steel with stainless steel cladding.