High-Temperature Oxidation and Corrosion Behaviours of Aluminized Nickel-based IN-718 Superalloy and Titanium-based Ti-6Al-4V Alloys

• Main Author: Pius Kibet Koech
• Other Authors: Chaur-Jeng Wang
• Format: Others
• Language: en_US
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/u3f5f7

博士 === 國立臺灣科技大學 === 機械工程系 === 107 === Inconel 718 and Ti-6Al-4V alloys are commonly used in aerospace, nuclear, biomedical and chemical industries due to their excellent corrosion resistance with good mechanical properties at elevated temperatures. However, these alloys among other metallic materials are susceptible to high temperature oxidation due to cracking and spallation of the scales especially under thermal-cycling conditions which resulted in the failure of components. In the present work, effects of water vapour, NaCl salts, and thermocycling commonly prevailing on the morphology and composition of alumina formed during high-temperature cyclic-oxidation were studied using two types of aluminide coatings. The aluminide hot-dip and plasma spray coatings prepared on inconel 718 and Ti-6Al-4V alloys were cyclically oxidized in horizontal tube furnace at 650 and 750 °C respectively. Test were done both in dry and moist air conditions by passing dry air through hot water bath at a flow rate of 200 cc/min. The exposure period was 10 hours in hot the chamber and 30 minutes at 25 C in room temperature per cycle. After recording mass changes, analyses of the composition, microstructure characterization, and morphology of the scales were conducted. Uncoated materials were also tested for comparisons. A Cr2O3 layer formed on uncoated inconel 718 alloy oxidized in dry air. Small amounts of Fe and Ni were also detected on the surface. However, porous oxide scale of Fe2O3 formed on the surface of uncoated inconel 718 substrate exposed to thermal-cycle in NaCl salt with extensive corrosive attack and scale spallation. Oxide of Fe2O3 formed on top of Cr2O3 oxide layer due to high volatilization of iron-chloride in respect to chromium-chloride. On the other hand, the scales formed on uncoated Ti-6Al-4V alloy in dry air consisted mainly TiO2 and minor oxide of Al2O3. The hot-dip coating provided a better oxidation resistance of both IN 718 and Ti-6Al-4V. However, spallation of Al2O3 oxide scale and formation of Kirkendall voids in the aluminide layer increased under thermal-cycle exposure. In presence of NaCl salts, the hot-dip coating improved corrosion resistance of the alloy compared to uncoated material. However, degradation of aluminide layer due to formation of cracks and voids allowed NaCl ions to diffuse into the substrate which resulted in an extensive corrosion. The mass change results show that aluminide hot-dip coating had the lowest value while both the plasma spray coating and uncoated specimens had an increase in mass then slightly decreased probably due to spallation of the oxide layer. The results also show that the oxide scale formed on the surface of the hot-dip coating has an inherently different morphology and growth rate compared to those formed on the plasma spray coating. Such discrepancies were also noted in coatings oxidized in dry air compared to those oxidized in moist air. High oxidation rate, severe spallation, extensive blistering and large voids with little protective alumina scale were observed in wet air. This can be attributed to decreased interfacial toughness of alumina oxide in the presence of water. Thus, it was concluded that the hot-dip coating has a better oxidation resistance compared to plasma spray coating.