Drawbacks to the Application of Long-Term Degradation Tests, with Reference to Metal Dusting of Nickel-Base Alloys at High Temperatures

Abstract

Selection of alloys for components in industrial environments at high temperature is often based on long-term tests in environments that simulate as closely as possible the actual service conditions. However, particularly for alloys that rely on formation and retention of oxide scales for protection, such tests may not be sufficient for this purpose. Several nickel-base alloys have been exposed in an 80% CO-20% H2 gas mixture at 650oC for up to 5,000h, in order to give conditions similar to those that can occur in petrochemical plant where high carbon and low oxygen activities can lead to scale breakdown and the onset of metal dusting damage. The study has shown that small changes in test conditions, such as the introduction of contaminants, the addition of CO2, the proximity to alloy surfaces on which carbon deposition and metal dusting have already been initiated and the proximity to other surfaces that can initiate such deposition, can lead to very significant decreases in the induction period, prior to breakdown of the scale and the onset of dusting.