Effect of chloride transients on the corrosion behavior of low alloyed steel in cladding flaws of reactor pressure vessels under oxygenated high-temperature water conditions

Strain-induced corrosion cracking in low alloyed steels (LAS) has been extensively investigated during the last decades. One finding from recent investigations is that small amounts of chlorides tend to increase the cracking susceptibility of LAS. In order to evaluate this claim a demanding research program has been established. In light water reactors, the pressure vessel is made of low alloyed steel with a stainless steel cladding for corrosion protection. In structural failure assessments of primary circuits, penetrating cladding flaws that locally expose the underlying LAS to the cooling water have to be assumed. Due to the narrow opening of such defects, the composition of the aqueous solution in contact with LAS would differ from that of the bulk coolant. In this paper, calculations of the water chemistry in such conditions revealed that oxygen concentration decreases rapidly when going from the mouth to the bottom of the cladding flaw, bringing the redox potential to values, much lower than those in oxygenated high temperature water (HTW). Also, chloride was found to enrich at the cladding flaw bottom by a factor of 30. The material studied in these conditions was 20MnMoNi55 from the reactor coolant line of a German NPP. Based on the results obtained, it can be concluded that chloride transients up to 50 ppb in the bulk HTW, resulting in 1500 ppb of chloride at the bottom of the cladding flaw, do not result in any serious consequences for the corrosion of low alloyed steel.