CRACK-TIP MICROSAMPLING AND GROWTH-RATE MEASUREMENTS IN LOW-ALLOY STEEL IN HIGH-TEMPERATURE WATER

The importance of the dissolution of manganese sulfide (MnS) inclusion s in environmentally assisted cracking (EAC) of pressure vessel steels in high-temperature water is well recognized. However, no direct meas urement of the crack tip chemistry that develops during EAC has been p erformed, and only estimates exist for the dissolution rate of MnS and the resulting sulfur (S) levels in the crack. In the present work, mi crosampling of the crack tip solution in ASTM A533(B) low-alloy steel (0.013% S) exposed to constant and cyclic loading in 288 degrees C wat er under various test conditions was used to measure the crack tip S c oncentration directly, while simultaneously monitoring crack length. A reversing direct current (DC) potential drop method was used to monit or crack length continuously. ion chromatography (IC) and inductively coupled plasma (ICP) were used to measure the dissolved S species in t he microsampled solutions. Most experiments involved varying the corro sion potential by changing the dissolved oxygen (O-2) concentration in high-purity water. At 10 ppm O-2, high crack growth rates were observ ed, and the microsampled solutions contained between 1 ppm and 2 ppm S , similar to 10 times higher than at 0 ppm O-2, where crack growth rat es were low. Measurements in room-temperature solutions showed most S was present as sulfate (SO42-), although in the high-temperature deaer ated water in the crack, MnS undoubtedly dissolved to form hydrogen su lfide ions (HS-) and hydrogen sulfide (H2S). Decreasing the loading fr equency also lowered the crack growth rate and the crack tip S concent ration. Very high microsampling