Metal dusting of nickel-base alloys

• Main Author: Di Gabriele, Fosca
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2005
• Subjects: Materials science
• ISBN: 9780355977707; 0355977702

Metal dusting is usually defined as catastrophic carburisation in conditions of high carbon activity and low oxygen partial pressure, in the temperature range between 450° and 800°C. The phenomenon produces filamentous carbon growth from the metal and detachment of particles of metal, oxides and carbides from the base alloy. The damage can be in the form of pitting or general metal wastage. The performance of a wide range of nickel-base alloys has been investigated at 650°C in a 20vol%H2 - 80%CO gas mixture that can cause severe metal dusting damage to many alloys. A review of the relevant literature in the field of oxidation/carburization at high temperatures has been carried out and the main findings and hypotheses on the mechanisms of the phenomenon have been reviewed, with the aim of highlighting the current understanding and aspects that require further investigation. It is well known that the deposition of carbon onto metallic surfaces is promoted by the catalytic decomposition of carbon-rich gases by iron, nickel and cobalt. Marked improvements in the resistance of the alloys can be achieved by isolating these elements from the gas environment by an inert oxide layer. The alloys considered in this study have the potential to develop protective chromia scales and (where the relevant elements are present) alumina or silica subscales. Nevertheless, many of the alloys underwent damage in this environment. The experimental conditions had a considerable influence on the performances of the alloys; in particular, the specimen holders had an important role on the onset of carbon transfer from the gas to the alloy surfaces. Specimens of most alloys exposed to the gas mixture while suspended from quartz hangers were shown to have relatively good resistance to the environment for periods up to 5000h. They developed protective scales, although, in few cases, the specimens underwent some damage in localised areas. Metal dusting was initiated more rapidly on Alloy 600 than on the other alloys, due to its relatively low chromium content. This was insufficient to heal any defects that developed in the initially-protective chromia-rich scale. Hence, specimens of this alloy formed metal dusting pits after short exposure times (about 100h); however, the Abstract eventual pit depths were generally smaller than those formed on other alloys that were richer in iron. The high content of nickel in this and other alloys is believed to be beneficial in slowing down the rate of damage to specimens following failure of the oxide scale. On the other hand, specimens of these alloys exposed while located in porous refractory ceramic holders gained large amounts of coke and underwent relatively severe damage. The holder was shown to affect the gas composition and possible causes of the increased aggressivity have been proposed. For instance, increased production of CO2 (due to the Boudouard reaction 2CO = CO2+C) has an effect on the protectiveness of the oxide scale. Here, internal precipitation of oxides was observed, at least for aluminium-containing alloys, resulting in the generation of internal stresses that could damage the outer scale. Contaminants evolved from the refractory material also have a strong influence on the performance of the alloys. The combined effect of these factors is believed to give a partial explanation for the enhanced rates of damage in the presence of this material. Thermodynamic calculations, together with TEM observations, EPMA analyses, SEM observations and EDX analyses, have been used to define possible mechanisms for the metal dusting damage on the nickel-base alloys in this research. The roles of spinel oxides and selective oxidation of alloying elements are discussed. Comparisons with the current hypotheses in the field have been used to highlight and summarise the main findings of this project.