Effect of Chloride on Environmentally Assisted Cracking of Low Alloy Steels in Oxygenated High Temperature Water

Abstract

The aim of this thesis was to derive a better understanding with regard to the effects of chloride on the general corrosion behaviour of low-alloy steels (LAS) in oxygenated high-temperature water (HTW) and to investigate the underlying mechanisms for crack initiation and propaga-tion due to chloride assisted environmentally assisted cracking (EAC). Therefore, systematic investigations on the effect of chloride on the EAC behaviour of LAS were performed to un-derstand and elucidate the underlying mechanisms.The overall thesis is divided into three parts focussing on the effect of chloride on:i) general corrosionii) crack initiation, andiii) crack growthof low-alloy steels in oxygenated high-temperature water.Studies on the effect of chloride on the general corrosion behaviour were performed by immer-sion tests that were evaluated using electrochemical monitoring techniques and different post-test investigation methods like SEM, ToF-SIMS, and others. From the performed investiga-tions it is concluded that the presence of small amounts of chloride in oxygenated HTW causes an incorporation of chloride into the oxide layer, a thinning of the oxide layer thickness, and pronounced pitting.The crack initiation susceptibility of LAS was investigated using CERT tests. These tests showed an increased number of crack initiation locations and a decrease of the elongation at fracture with increasing chloride concentrations.Crack growth rate tests clearly demonstrated that not the increase in the chloride concentration per se, but the conjoint occurrence of an active or dormant crack and increased chloride con-centration causes an increase in the observed crack growth rates.For practical applications of LAS in oxygenated HTW this means that short term transients seem to be not harmful regarding component integrity, but long term increased chloride con-centrations should by prohibited since they cause increased general corrosion of LAS. Taking crack initiation and crack growth into consideration, the conjoint occurrence of increased chlo-ride concentrations and mechanical straining at stress levels above the yield strength should be avoided.