High Temperature Nitridation Resistance of Several Modern High Temperature Ni- and Co-Based Alloys

High temperature nitridation is a common corrosion issue in the processes ranging from those containing air to those containing more specific gases, such as nitrogen or ammonia. High temperature alloys could provide good nitridation resistance, but the nitridation data of commercial alloys in nitrogen or ammonia at high temperatures, such as 871°C and above, is limited. The paper studies the high temperature nitridation resistant behaviors of six modern commercial high temperature Ni- and Co-based alloys, which include four chromia forming alloys, N06230, R30188, 625 and N06617, and two alumina forming alloys, N07214 and a recently developed NiCoCrMoAl alloy. The high temperature nitridation tests were conducted for 1,008h cycled weekly in ultra-high purity nitrogen gas at 871 and 982°C. The test results showed different nitridation resistant behaviors of the chromia and alumina forming alloys. Either chromia or alumina scales formed on these alloys to provide nitridation protection in the nitrogen atmosphere. Alumina scales exhibited excellent nitridation resistance at both temperatures, and pre-oxidation to form an alumina scale was proven to be very effective to improve nitridation resistance. Chromia scale showed less protection, particularly at 982°C. The nitridation resistant behaviors of these alloys are analyzed and discussed.

Nitrogen or nitrogen-containing gases are widely used or present in industry. Some industry processes utilize nitrogen gas or nitrogen-rich atmosphere to produce specific products at high temperatures, such as use of nitrogen gas (N₂) to protect components from oxidation in heat treating or sintering operations and the use of ammonia (NH₃) for nitriding purpose during alloy heat-treatment. In the chemical processing industry, ammonia is produced from the reaction of hydrogen and nitrogen with a catalyst under high temperature and pressure conditions. With the presence of nitrogen in these processes, nitrogen molecules can react with alloying elements to form nitrides. In other applications, nitrogen also exists in many combustion processes that use air in the fuel mixture. Although high temperature oxidation is a major material issue in combustion processes, alloys could suffer the formation of internal nitrides when oxide scales do not provide sufficient protection, especially those containing strong nitride forming elements, such as aluminum (Al) and titanium (Ti). The formation of internal nitrides could cause severe material issues as it not only reduces the effective thickness of an alloy component, but also causes embrittlement.¹ The type of high temperature corrosion mainly involving nitride attack mechanism is referred as nitridation, and it is very important for high temperature alloys to resist nitridation attack for long-term operation.