Microchemistry of Ti-Carbonitrides and Their Role in the Early Stage of Pit Initiation of Alloy 600

Abstract

The microchemistry of Ti-carbonitrides in Alloy 600 (UNS N06600) was analyzed using transmission electron microscopy and scanning Auger electron spectroscopy. Pit initiation testing was conducted to clarify the preferential pitting sites of Alloy 600 in a deaerated 10,000-ppm chloride solution at 90°C. It was found that sulfur/phosphorus was segregated on the surfaces of the Ti-carbonitrides and that (Mg,Ca)S sulfide particles adhered to them. The pit initiation site was the Ti-carbonitride/matrix interface, which was attributed to preferential dissolution of the (Mg,Ca)S particles and the sulfur/phosphorous-segregated layer, followed by dissolution of the matrix.