A. Ul-Hamid

Diverse scaling behavior of the Ni-20Cr alloy

Abstract A study was undertaken to investigate the isothermal oxidation behavior of the Ni–20Cr binary alloy at 750 and 1000xa0°C. The samples were exposed in air for 6 and 50xa0h. The oxide phases formed on the alloy were α-Cr2O3, NiCr2O4 and NiO. Microstructural examination of the alloy perpendicular to the plane of oxidation revealed co-lateral formation of diverse scale configuration at its surface. The thickness of the scale formed on the alloy surface varied from 1.5 to 18xa0μm. The protection imparted was due mainly to α-Cr2O3; however, various regions of the alloy surface constituted predominantly of NiCr2O4.

The corrosion behavior of scandium alloyed Al 5052 in neutral sodium chloride solution

Alloying with scandium has a strong influence on the strengthening and weight saving characteristics of Al–2.5Mg alloys. Scandium addition (0.1–0.3 wt.%) to Al–2.5Mg alloys does not introduce any appreciable loss in their resistance to corrosion in 3.5 wt.% NaCl. The corrosion behavior of these alloys is not significantly affected by age hardening. Because of a unique combination of outstanding mechanical properties and a good resistance to corrosion Al–Mg alloys containing scandium represent a major improvement over the more familiar Al–Mg alloys.

A TEM study of the oxide scale development in Ni-Cr-Al alloys

Abstract In the present study the isothermal oxidation behaviours of Ni–10Cr–5Al, Ni–20Cr–5Al and Ni–30Cr–5Al alloys were investigated. The alloys were oxidised in air for 50 h at 1000 °C. Analytical transmission electron microscopy was used to characterize the morphology, structure and composition of the oxide scale. The oxide formed adjacent to the alloy was α-Al2O3 such that the higher was the Cr content of the alloy the easier was its formation. The Ni–30Cr–5Al alloy formed a complete layer of α-Al2O3 in the initial stages of oxidation through ‘oxygen gettering’ by Cr. A decrease in scale thickness and an increase in scale adherence were observed with an increase in Cr content from 10 to 30 wt.%.

A TEM study of the oxide scale development in Ni‐Cr alloys

A study was undertaken to investigate the isothermal oxidation behaviour of Ni‐Cr binary alloys with 10, 20 and 30u2009wt per cent Cr exposed in air for 50u2009h at 1,000°C. Analytical transmission electron microscopy along with light microscopy and X‐ray diffraction were used to characterise the oxide scale. It was observed that the scaling behaviour exhibited by the Ni‐Cr alloys changes from generally “non‐protective” to “protective” as their Cr content increases from 10 to 30u2009wt per cent. The Ni‐10Cr alloy formed a continuous network of NiO leading to scales of high thickness while Ni‐30Cr exhibited only α‐Cr2O3 at its surface. The Ni‐20Cr alloy exhibited all three oxide phases and an intermediate scaling behaviour.

Surface mechanical characterization of sputtered nickel using nanoindentation

The surface of carbon steel (CS) samples was deposited with pure Ni for the durations of 2, 5 and 10 minutes using DC magnetron sputter deposition process. The aim was to examine the microstructure and surface mechanical properties of Ni coatings. Field emission scanning electron microscope coupled with an energy dispersive x-ray spectrometer and x-ray diffractometer were used to undertake materials characterization. Instrumented nanoindentation hardness, elastic modulus, adhesion and coefficients of friction of coatings were evaluated. Nickel coatings obtained were relatively uniform, continuous and adherent for all deposition times. Thickness of Ni coatings increased with deposition times. Coatings with lower deposition times showed relatively higher nanohardness, elastic modulus and creep which is thought to be due to its lower thickness. Coatings were found to crack and delaminate at relatively low applied normal force during micro-scratch testing. Coefficient of friction values of coatings was comparable with that found in the literature.

Cyclic oxidation behaviour of a Ni‐Mo‐Cr alloy at 800°C

The oxidation behaviour of a wrought Ni‐Mo‐Cr alloy was studied under thermal cyclic conditions in air at 800°C for exposure periods of up to 1,000u2009h. The morphologies, microstructures and compositions of the oxide scales were characterized by scanning electron microscopy, energy dispersive X‐ray spectroscopy and X‐ray diffraction. Oxidation kinetics were determined by weight gain measurements. Results show that steady‐state oxidation was achieved within 1u2009h of exposure while partial scale spalling was observed after 400u2009h. The alloy grain boundaries intersecting the alloy surface showed preferential oxidation. They became depleted in Ni and enriched in Mo and Cr during transient oxidation. The scale initially formed at the surface was NiO which grew outwardly and laterally to cover the entire alloy. Upon continued oxidation, the scale developed into an outer NiO layer and an inner Cr2O3 layer while the presence of NiMoO4 was also observed within the scale.

Effect of Y2O3 content on the oxidation behavior of Fe-Cr-Al-based ODS alloys

A study was conducted to investigate the cyclic oxidation behavior of two oxide dispersion strengthened (ODS) Fe-Cr-Al based alloys containing 0.17 wt.% and 0.7 wt.% Y2O3. The alloys were oxidized in air for 100 h at 1200°C based on a 24 h cycle period. X-ray diffraction (XRD) and analytical transmission electron microscopy (TEM) were used to characterize the structure, morphology, and composition of the oxide scales. Both alloys formed highly adherent and continuous layers of α-Al2O3 exhibiting a morphology indicative of inward scale growth. The role of Y2O3 was to promote adherence by segregating to the grain boundaries within the oxide. Concurrently, Y2O3 generated micro-porosity resulting in a scale of comparatively higher thickness in the alloy with 0.7 wt.% Y2O3.

Electrochemical Deposition of Ni on an Al-Cu Alloy

Metallic coatings can be used to improve the wear and corrosion resistance of Al alloys. In this study, Ni was used as a candidate material for such a coating which was applied on the surface of Al 2014 alloy using electrodeposition in a standard Watt’s bath. A two-step heat treatment procedure was employed that served to increase the adhesion as well as hardness of Ni. Deposition was undertaken for different durations using both galvanostatic and potentiostatic techniques. The effect of deposition parameters such as surface finish, current, potential, temperature, pH level and duration on the microstructure, adhesion, and surface properties of the Ni deposit was studied. Materials characterization was performed using scanning electron microscopy, atomic force microscopy, and x-ray diffraction. Cross-sectional scanning transmission electron microscope images revealed the fine-grained (10xa0nm) structure of Ni initially deposited at the Ni-Al alloy substrate. Microhardness, adhesion, and corrosion behavior of the Ni deposit were evaluated. Experimental results indicate that deposition by galvanostatic technique on 1xa0μm surface finish at 45xa0°C with a pH level maintained at 3.6 represented the optimum conditions to generate a uniform Ni deposit on Al 2014. It was concluded that Ni deposition can be used to improve the surface properties of Al alloys.

Failure of ethylene furnace outlet transfer line due to overheating

An outlet transfer line tube of an ethylene-cracking furnace failed after five years of service. The tube, made from Incoloy alloy 800H, developed cracks that penetrated the entire thickness. Optical and scanning electron microscopy, X-ray diffraction, and microhardness tests were used to evaluate the failed tube. It was found that the tube was considerably embrittled by high-temperature carburization, leading to intergranular fracture at the inner surface. Carburization appeared to have been caused by high-temperature exposure to the carbonrich atmosphere associated with the coke adhered to the inner surface. Additionally, the decrease in heat transfer due to the coke deposits caused the wall temperature to increase during service. However, at the outer surface, the tube fractured by a fatigue process that could result from the use of a counterweight to prevent sagging of the tube as a result of the coke deposition. Based on the results obtained, a short-term solution was to modify various process parameters to reduce the extent of coke deposition and/or increase the frequency of decoking and to discontinue the use of a counterweight. However, the long-term solution is to replace alloy 800H.

Failure analysis of catalytic steam reformer tubes

Purpose – To determine if the interim use of liquid waste as a fuel in a catalytic steam reformer unit had any deleterious effect on the long‐term life of the reformer tubes.Design/methodology/approach – Standard metallographic techniques were used to prepare representative samples obtained from various sections of the reformer tubes for metallurgical evaluation. Microstructural characterization was carried out in a scanning electron microscope equipped with an energy dispersive X‐ray spectrometer. Imaging and elemental analysis was used for the identification of the alloy material, corrosion products and other microstructural features.Findings – Hydrogen was produced in a catalytic steam reformer by cracking methane using natural gas as a fuel. Corrosion of reformer tubes occurred when natural gas fuel was replaced with a liquid waste. Use of liquid fuel waste accelerated the rate of oxidation at the outer tube surface. However, foreign species from the fuel were not transported into the tube material. T...