Crack Tip Microsampling and Growth Rate Measurements in Low-Alloy Steel in High-Temperature Water

The importance of the dissolution of manganese sulfide (MnS) inclusions in environmentally assisted cracking (EAC) of pressure vessel steels in high-temperature water is well recognized. However, no direct measurement of the crack tip chemistry that develops during EAC has been performed, and only estimates exist for the dissolution rate of MnS and the resulting sulfur (S) levels in the crack. In the present work, microsampling of the crack tip solution in ASTM A533(B) low-alloy steel (0.013% S) exposed to constant and cyclic loading in 288°C water under various test conditions was used to measure the crack tip S concentration directly, while simultaneously monitoring crack length. A reversing direct current (DC) potential drop method was used to monitor crack length continuously. Ion chromatography (IC) and inductively coupled plasma (ICP) were used to measure the dissolved S species in the microsampled solutions. Most experiments involved varying the corrosion potential by changing the dissolved oxygen (O₂) concentration in high-purity water. At 10 ppm O₂, high crack growth rates were observed, and the microsampled solutions contained between 1 ppm and 2 ppm S, ~ 10 times higher than at 0 ppm O₂, where crack growth rates were low. Measurements in room-temperature solutions showed most S was present as sulfate (SO₄²⁻), although in the high-temperature deaerated water in the crack, MnS undoubtedly dissolved to form hydrogen sulfide ions (HS⁻) and hydrogen sulfide (H₂S). Decreasing the loading frequency also lowered the crack growth rate and the crack tip S concentration. Very high microsampling rates effectively flushed the crack tip chemistry, decreasing the crack tip S content and crack growth rate.